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The East African Rift System (EARS) provides an opportunity to constrain the relationship between magmatism and plate thinning. During continental rifting, magmatism is often considered a derivative of strain accommodation—as the continental plate thins, decompression melting of the upper mantle occurs. The Turkana Depression preserves among the most extensive Cenozoic magmatic record in the rift. This magmatic record, which comprises distinct basaltic pulses followed by periods of relative magmatic quiescence, is perplexing given the lack of evidence for temporal heterogeneity in the thermo‐chemical state of the upper mantle, the nonexistence of lithospheric delamination related fast‐wave speed anomalies in the upper mantle, and the absence of evidence for sudden, accelerated divergence of Nubia and Somalia. We focus on the Pliocene Gombe Stratoid Series and show how lithospheric thinning may result in pulsed magma generation from a plume‐influenced mantle. By solving the 1D advection‐diffusion equation using rates of plate thinning broadly equivalent to those measured geodetically today we show that despite elevated mantle potential temperature, melt generation may not occur and thereby result in extended intervals of quiescence. By contrast, an increase in the rate of plate thinning can generate magma volumes that are on the order of that estimated for the parental magma of the Gombe Stratoid Series. The coincidence of large‐volume stratiform basalt events within the East African Rift shortly before the development of axial zones of tectonic‐magmatic activity suggests that the plate thinning needed to form these stratiform basalts may herald the onset of the localization of strain. Plain Language Summary: The magmatic record in the Turkana Depression—part of the East African Rift System—is characterized by pulses of basaltic activity that are followed by long periods of relative magmatic quiescence. This is a puzzling observation assuming that these magmas are generated by decompression melting of the upper mantle; there is no obvious changes in the rate of plate motion between Nubia and Somalia. This study presents new geochemical data on the final pulse of basaltic volcanism (during the Pliocene) and interprets these data in the context of a mantle melting model. We find that pulses of basaltic volcanism and intervening periods of quiescence could be simulated using different rates of thinning of the plate. We examine the consequences of a period of enhanced plate thinning in context of melt generation both below and within the plate. Key Points: Pulses of basaltic volcanism and intervening periods of quiescence could be simulated using different rates of thinning of the plate [ABSTRACT FROM AUTHOR]

The timing and duration of volatile generation from crystallizing magma reservoirs and fluid release across the magmatic‐hydrothermal interface depend on complex coupled interactions controlled by non‐linear, dynamic properties of magmas, rocks and fluids. Understanding these mechanisms is essential to explain the rare formation of economic porphyry copper deposits. For this study, we further developed a coupled numerical model that can simultaneously resolve magma and hydrothermal flow by introducing a description of fluid transport within the magma reservoir and volatile release to the host rock. Our simulations use realistic magma properties derived from published experimental and modeling studies and cover different magma compositions and water contents. We show that magma convection at melt‐dominated states leads to homogenization, which delays fluid release and promotes a rapid evolution toward a mush state. The onset of magmatic volatile release can be near‐explosive with a tube‐flow outburst event lasting <100 years for high initial water contents of >3.5 wt% H2O that could result in the formation of hydrothermal breccias and vein stockworks or trigger eruptions. This event can be followed by sustained fluid release at moderate rates by volatile flushing caused by magma convection. Subsequent fluid release from concentric tube rings by radial cooling of non‐convecting magma mush with a volume of ∼100 km3 at ∼5 km depth is limited to remaining water contents of ∼3.1 wt% H2O and lasts 50–100 kyr. Ore formation from hydrous magmas may thus involve distinct phases of volatile release. Plain Language Summary: High‐temperature fluids are expelled from water‐bearing magma bodies as they cool in the subsurface. These fluids can lead to the formation of ore deposits and geothermal resources and even trigger volcanic eruptions. In this study, we investigate how fluids are transported within a coupled magmatic‐hydrothermal system and how the magma composition, including the initial amount of water, and magma convection can affect the process of subsurface fluid flow. Using insights from experimental and numerical studies, we designed computer simulations to investigate the formation of porphyry copper deposits. We find that magma convection plays a critical role in the initial stage of cooling, creating a uniform mixture of magma and volatiles that stays in suspension during an "incubation" period. During cooling, magmas with high water contents exhibit near‐explosive outgassing events that can be both precursors to volcanic eruptions or a pre‐ore stage. Subsequently, as convection ceases, the magma body undergoes a transition from homogeneous uniform to radial cooling with more gradual fluid release over 50–100 kyr, leading to metal enrichment in porphyry copper deposits. Key Points: Coupled simulations of magma convection, crystallization, degassing and hydrothermal fluid flow reveal distinct stages of volatile releaseAfter a magma incubation phase with bubble suspension, hydrous magmas can experience a near‐explosive tube‐flow outburst from magma mushAfter crystal lock‐up, shifting from uniform to radial cooling leads to continuous outgassing, potentially forming porphyry Cu deposits [ABSTRACT FROM AUTHOR]

The Gakkel Ridge in the Eurasian Basin has the slowest seafloor spreading worldwide. The western Gakkel Ridge (3°W–85°E; 14–11 mm/a) alternate between magmatic and sparsely magmatic zones, while the eastern Gakkel Ridge (85–126°E; 11–6 mm/a) appears to be dominated by magmatic zones despite ultraslow spreading. Little is known about the seafloor spreading conditions in the past along the entire ridge. Here, we exploit the residual bathymetry and basement roughness to assess the crustal accretion process of the Gakkel Ridge over time using 23 published regional multichannel seismic reflection profiles. Full seafloor spreading rates were faster (20–24 mm/a) up to ∼45 Ma, and residual bathymetry for the older crust is deeper than the world average in the entire Eurasian Basin. There is a sharp transition to 300–400 m shallower residual bathymetry for seafloor <45 Ma in the eastern Eurasian Basin. The crustal roughness versus spreading rate of the western Eurasian Basin is on the global trend, while that of the eastern is significantly below. Both low roughness and shallow residual bathymetry of the eastern Eurasian Basin is close to that of oceanic crust for spreading rates above 30 mm/a, demonstrating increased magmatic production of the eastern Gakkel Ridge since ∼45 Ma. A recent mantle tomography model predicts partial melting in the upper mantle based on the low Vs anomaly underneath. The sedimentary pattern toward the Lomonosov Ridge indicates that this hot mantle anomaly started to cause dynamic uplift of the area at ∼45 Ma. Plain Language Summary: The Gakkel Ridge has the slowest spreading ridge on earth, though rates were twice the present prior to ∼45 Ma. The sparsity of crustal‐scale seismic data leaves the spreading process uncertain. However, crustal roughness and residual bathymetry offer records of the plate‐spreading history. Lower roughness and shallower residual bathymetry are tied to increased magma production and thicker crust. We analyzed 23 published seismic profiles for these parameters. Seafloor older than ∼45 Ma has a residual bathymetry ∼550 m below the global average in both the eastern and western Eurasian Basin. For seafloor younger than 45 Ma basement depth is 300–400 m shallower in the east, while the west showed minimal change. The western Eurasian Basin's crustal roughness follows the global trend with spreading rate, whereas the eastern basin is much below. This indicates an unexpectedly robust magma production along the eastern Gakkel Ridge since ∼45 Ma. Recent upper mantle tomography models show a low shear‐wave velocity anomaly underneath, which is tied to increased temperature. Systematic shifts in the sedimentary depositional pattern in the east indicate that this anomaly started to cause dynamic uplift at ∼45 Ma, coeval with the increase in melting. Key Points: Residual bathymetry and crustal roughness used as proxy for spreading axis magma productivityThe eastern Eurasian Basin has low crustal roughness and near normal residual bathymetry since ∼45 Ma despite its ultraslow spreading rateThe eastern Gakkel Ridge spreading is magmatically robust since ∼45 Ma, the western variable [ABSTRACT FROM AUTHOR]

Axial Seamount is a submarine volcano on the Juan de Fuca Ridge with enhanced magma supply from the Cobb hotspot. We compare several deformation model configurations to explore how the spatial component of Axial's deformation time series relates to magma reservoir geometry imaged by multi‐channel seismic (MCS) surveys. To constrain the models, we use vertical displacements from seafloor pressure sensors and repeat autonomous underwater vehicle (AUV) bathymetric surveys between 2016 and 2020. We show that implementing the MCS‐derived 3D main magma reservoir (MMR) geometry with uniform pressure in a finite element model with uniform elastic host rock properties poorly fits the geodetic data. To test the hypothesis that there is compartmentalization within the MMR that results in heterogeneous pressure distribution, we compare analytical models using various horizontal sill configurations constrained by the MMR geometry. Using distributed pressure sources significantly improves the Root Mean Square Error (RMSE) between the inflation data and the models by an order of magnitude. The RMSE between the AUV data and the models is not improved as much, likely due to larger uncertainty of the AUV data. The models estimate the volume change for the 2016–2020 inter‐eruptive inflation period to be between 0.054 and 0.060 km3 and suggest that the MMR is compartmentalized, with most magma accumulating in sill‐like bodies embedded in crystal mush along the western‐central edge of the MMR. The results reveal the complexity of Axial's plumbing system and demonstrate the utility of integrating geodetic data and seismic imagery to gain insights into magma storage at active volcanoes. Plain Language Summary: Axial Seamount is a submarine volcano on the Juan de Fuca Ridge (NE Pacific Ocean) with enhanced magma supply from the Cobb hotspot. Its frequent activity and long‐term deformation time series covering eruptions in 1998, 2011, and 2015 make it an ideal place to study volcanic processes. Improved magma reservoir modeling at Axial will aid in understanding how magma transport and storage are related to surface deformation, seismicity, and eruption timing. Here we compare several models of Axial's magma reservoir to explore how the spatial component of the observed deformation at Axial compares to seismically imaged magma reservoir geometry. To constrain the models, we use vertical displacements covering an inflation period between 2016 and 2020, derived from pressure measurements collected at seafloor benchmarks and repeated bathymetric surveys. The models estimate the volume change for the 2016–2020 inflation period to be between 0.054 and 0.060 km3. Our results suggest that Axial's magma reservoir is compartmentalized, with most magma accumulating in sill‐like bodies embedded in crystal mush. The results reveal the spatial complexity of Axial's plumbing system and demonstrate how deformation data and seismic imagery can be used together to gain deeper insights into magma storage at active volcanoes. Key Points: Uniform pressurization of Axial Seamount's seismically imaged magma reservoir does not adequately fit the observed geodetic dataOur models estimate that Axial's magma reservoir inflated by 0.054–0.060 km3 during the inter‐eruptive recharge period between 2016 and 2020Axial's magma reservoir is likely compartmentalized, with magma accumulating in sills along the western‐central edge of the magma reservoir [ABSTRACT FROM AUTHOR]

Understanding the stress distribution around shallow magma chambers is vital for forecasting eruption sites and magma propagation directions. To achieve accurate forecasts, comprehensive insight into the stress field surrounding magma chambers and near the surface is essential. Existing stress models for pressurized magma chambers often assume a homogenous elastic half‐space or a heterogeneous crust with varying mechanical properties in horizontal layers. However, as many volcanoes have complex, non‐horizontal, and heterogeneous layers, we enhance these assumptions by considering mechanically stratified layers with varying dips. We employed the Finite Element Method (FEM) to create numerical models simulating three chamber geometries: circular, sill‐like and prolate. The primary condition was a 10 MPa excess pressure within the magma chamber, generating the stress field. Layers dips by 20‐degree increments, with differing elastic moduli, represented by stiffness ratios of the successive layers (EU/EL) ranging from 0.01 to 100. Our findings validate prior research on heterogeneous crustal modeling, showing that high stiffness ratios disrupt stress within layers and induce local stress rotations at mismatched interfaces. Layer dip further influences stress fields, shifting the location of maximum stress concentration over varying distances. This study underscores the significance of accurately understanding mechanical properties, layer dip in volcanoes, and magma chamber geometry. Improving forecasting of future eruption vents in active volcanoes, particularly in the Andes with its deformed, folded, and non‐horizontal stratified crust, hinges on this knowledge. By expanding stress models to incorporate complex geological structures, we enhance our ability to forecast eruption sites and magma propagation paths. Plain Language Summary: Understanding crustal stress distribution within active volcanoes is crucial to forecast how magma will propagate inside a volcano and where and when a volcanic eruption may occur. Currently, the models used to understand the stress field within volcanoes assume that the Earth's crust is made of the same materials or with horizontally layered different materials. Both cases are seldom correct. Volcanoes often have layers that are both mechanically diverse and dipping at different angles and directions. To improve forecast models, we considered a crust with layers of different mechanical properties and with variable dips. We used numerical models to simulate 3 pressurized magma chamber geometries. We changed the dip of the layers and investigated how the rocks respond to the applied pressure. We found that when the layers are inclined they respond differently to the applied pressure and the possible location of a new eruption is displaced by up to 2 km, compared to the expected location using common modeling protocols. This research improves volcano understanding, especially in regions like the Andes, where the crust is often made by geologically diverse dipping layers. By making our models more realistic we can better forecast where eruptions may occur. Key Points: FEM models used to analyze the influence of layer dip on crustal stresses resulting from magma chamber pressurizationPeak surface tensile stresses can shift by more than 2 km, deviating from homogeneous elastic half‐space assumptionsThe effect is more pronounced with an idealized circular chamber when compared to a sill‐like chamber [ABSTRACT FROM AUTHOR]

Crystal‐hosted melt embayments and melt inclusions partially record magmatic processes at depth, but it is not always obvious how to interpret this record. One impediment is our incomplete understanding of how embayments and melt inclusions form. In this study, we investigate the formation mechanism of embayments and melt inclusions during quartz growth to quantify the relationship between the compositions of the entrapped and average melt. We study the growth of embayments and inclusions through direct numerical simulations that couple the growth of a crystal surface with the evolution of the concentrations of incompatible components in the surrounding melt. We find that H2O is more enriched in the interior of defects on crystal surface compared to the exterior. The resultant lower disequilibrium in the defect interior causes lower growth rate than in the exterior, elongating the defect into an embayment. If crystal growth stops, the composition in the embayment equilibrates with the average melt within days to months. If crystal growth continues until the embayment neck closes, a melt inclusion forms. The melt entrapped by both embayments and melt inclusions is enriched in incompatible components, such as H2O and CO2. In addition to inclusion size, the enrichment of incompatible components in melt inclusions also depends on component diffusivity and the crystal growth regime. High‐diffusivity components like H2O have similar enrichment levels in all scenarios, while lower‐diffusivity components like CO2 are more enriched in melt inclusions with smaller sizes or formed in continuous crystal growth. Plain Language Summary: Embayments and melt inclusions are small droplets of melt entrapped by crystals. These droplets may contain valuable clues for understanding processes inside volcanoes inaccessible to direct observation, because they typically form at depth before the onset of eruptions. Embayments are partially connected to exterior melt and can record processes during eruption. Melt inclusions are fully isolated and can record magma compositions at depth. However, we do not yet understand how they form. Our lack of understanding hinders our ability to interpret this data. Here, we use numerical simulations to quantify how the enrichment of dissolved water in the melt near the crystal affects the formation of embayments and melt inclusions during quartz growth. There is a positive feedback between the water enrichment in the embayment interior and embayment elongation, because water is not integrated into the quartz as it grows. Our simulations put constraints on the application of embayments and melt inclusions to constrain volcanic processes: For embayments, component enrichment likely equilibrates with surrounding melt prior to eruption. For melt inclusions, high‐diffusivity components like water are similarly enriched in all scenarios, while lower‐diffusivity components like carbon dioxide are more enriched in smaller inclusions formed in continuous crystal growth. Key Points: Embayments can form from crystal defects because water is more enriched inside the defect than outsideEnrichment of incompatible components in embayments likely equilibrates in pre‐eruptive magma storageCarbon dioxide is more enriched in smaller melt inclusions formed in continuous crystal growth but water has similar enrichment in all cases [ABSTRACT FROM AUTHOR]

Open conduit volcanoes erupt with the highest frequency on Earth. Their activity is characterized by an outgassing flux that largely exceeds the gas that could be released by the erupted magma; and by frequent small explosions intercalated by larger events that pose a significant risk to locals, tourists, and scientists. Thus, identifying the signs of an impending larger explosion is of utmost importance for the mitigation of volcanic hazard. Larger explosive events have been associated with the sudden ascent of volatile rich magmas, however, where and why magma accumulates within the plumbing system remains unclear. Here we show that the interaction between CO2‐rich fluids and magma spontaneously leads to the accumulation of volatile‐rich, low density and gravitationally unstable magma at depth, without the requirement of permeability barriers. CO2‐flushing forces the exsolution of water and the increase of magma viscosity, which proceeds from the bottom of the magma column upward. This rheological configuration unavoidably leads to the progressive thickening of a gas‐rich and low density (i.e., gravitationally unstable) layer at the bottom of the feeding system. Our calculations account for observations, gas monitoring and petrological data; moreover, they provide a basis to trace the approach to deeply triggered large or paroxysmal eruptions and estimate their size from monitoring data. Our model is finally applied to Stromboli volcano, an emblematic example of open conduit volcano, but can be applied to any other open conduit volcano globally and offers a framework to anticipate the occurrence of unexpectedly large eruptions. Plain Language Summary: Open conduit volcanoes erupt with the highest frequency on Earth and release significantly more gas than magma. Their activity is characterized by almost continuous and small explosions that are intercalated by significantly larger explosive events. The relatively continuous explosive activity makes these volcanoes attractive both for scientific research and tourism, however, unexpectedly large explosions pose a significant risk to scientists and tourists. The excess gas these volcanoes release is CO2‐rich and its interaction with magma leads to the accumulation of gas‐rich and low‐density magma at depth, below higher density magma. We suggest that the gravitational destabilization of this deep and gas rich magma is ultimately responsible for the large explosions that punctuate the activity of open conduit volcanoes. The approach to this larger events can be traced with continuous gas monitoring, which therefore provides a unique opportunity to anticipate this otherwise unexpected explosions. Key Points: Continuous CO2 flushing can prime basaltic volcanic systems toward more energetic eruptionsAccumulation of gas‐rich and low‐density magma at depth results from CO2 flushing alone and no ad‐hoc physical barrier is requiredBuild‐up to larger explosions can be monitored through continuous gas monitoring at the surface [ABSTRACT FROM AUTHOR]

Genuine non‐double‐couple (non‐DC) components of a seismic source, defined here as the non‐DC components that are not due to summation of pure double‐couple (DC) components, provide important insight into special physical processes in non‐earthquake sources such as explosion, volcano eruption and collapse etc. Yet they remain challenging to be resolved. To address the issue and explore the physical mechanism of those special events, we develop a waveform‐polarity‐based moment tensor (WPMT) inversion method and employ it to study physical process in the 2014–2015 Bárðarbunga volcano event sequence. The WPMT method incorporates P‐wave polarity data and seismic waveforms in the source inversion, designs a source simplicity test to check possible complex rupture in the seismic source, and employs a simulated annealing algorithm to search the best source solution. The simplicity test checks consistency of the source processes in the initiation stage of the event and the major energy release process of the event, thus ensuring that the inferred non‐DC components are genuine to the seismic source. Real event and synthetic tests indicate that the WPMT method can identify and resolve genuine non‐DC components in a seismic source. The WPMT inversions of the Bárðarbunga sequence yield many genuine non‐DC source components and reveal that the eruptions are accompanied by seismic activities in depths of 1–5 km with magma migrations out of chambers, collapses of conduits, failures of normal faults, and a magma recharge at a depth of 9 km accompanied by a failure on a nearby normal fault. Plain Language Summary: Special seismic events such as explosion, collapse and volcanic eruption possess source components that are different from fault solutions used to represent a typical tectonic earthquake of simple shear. The identification and quantification of the source components of those special seismic sources thus play an important role in deciphering the physical process during those events. Yet, it remains challenging to resolve the source components of special seismic events due to the ambiguity of distinguishing them from an apparent one due to complex earthquake rupture. We develop a waveform‐polarity‐based moment‐tensor inversion to resolve the source components of a special seismic source and employ the method to study the physical process in the 2014–2015 Bárðarbunga volcano event sequence in Iceland. Real event and synthetic tests confirm method's capability of identifying and constraining the source components of special non‐earthquake events. The study of the Bárðarbunga sequence reveals that the eruptions are accompanied by seismic activities in depths of 1–5 km with magma migrations out of chambers, collapses of conduits, failures of normal faults, and a magma recharge at a depth of 9 km accompanied by a failure on a nearby normal fault. Key Points: We propose a waveform‐polarity‐based moment tensor inversion method to extract non‐double‐couple components of moderate seismic eventsReal event and synthetic tests show the method is effective in distinguishing and resolving genuine non‐double‐couple componentsMethod application reveals magma activities, conduit collapses and induced earthquakes during the 2014–2015 Bárðarbunga eruptions [ABSTRACT FROM AUTHOR]

Basaltic fissure eruptions, which are the most common type of eruption on Earth, are fed by dykes which mediate magma transport through the crust. Dyke propagation processes are important because they determine the geometry of the transport pathway and the nature of any geophysical signals associated with magma ascent. Here, we investigate small‐scale (mm–cm wide) banding features at the margins of dykes in the Teno Massif (Tenerife, Spain) and the Columbia River Basalt Province (CRBP) (USA). Similar marginal bands have been reported for dykes in numerous localities around the world. Dyke margins record valuable information about propagation because they are the first material to solidify against the host rock at the propagating dyke tip. We find that the marginal bands are defined by cyclic variations in phenocryst concentration and vesicularity, and we infer that these cyclic variations in texture are a product of cyclic variations in magma flow rates and pressures within the dyke tip. This indicates that dyke emplacement occurs in pulses, with propagation repeatedly hindered by the rapid cooling and solidification of magma in the narrow dyke tip. Using a 1D conduction model, we estimate the time taken for each band to cool and solidify, which provides a timescale of several minutes to tens of minutes for the pulses. The occurrence of similar bands in various volcanic settings suggests that pulsatory propagation is a common, if not ubiquitous, process associated with dyke emplacement. Plain Language Summary: Dykes are cracks in the Earth's crust through which magma rises, sometimes feeding volcanic eruptions. Here, we investigate small‐scale banding features that are commonly found in solidified magma in dykes, using examples from the Teno Massif (Tenerife, Spain) and the Columbia River Basalt Province (USA). The bands form at the margin of the dyke, against the crack wall, which means that they record what happened when the dyke first formed; that is, when magma first flowed into the crack. We find that the bands are defined by repeated variations in the amount of crystals and bubbles, which we infer to have resulted from variations in magma flow rates and pressures in the dyke tip. This shows that the dykes formed in steps, rather than all in one go. By calculating the time taken for the bands to cool and solidify, we find that each propagation step lasts several minutes to tens of minutes. The occurrence of similar bands in various volcanic settings around the world suggests that most or all dykes form this way. This is important because the stepwise formation of the cracks might create small earthquakes that can be detected as the magma rises toward the Earth's surface. Key Points: Banded textures at dyke margins are the result of pulsatory magma flow in the tip of the propagating dykeDyke propagation is not continuous, but occurs in steps via a cycle of cooling, stalling, inflating, rupturing and propagatingCooling and solidification of magma in the narrow dyke tip has a strong influence on dyke propagation [ABSTRACT FROM AUTHOR]

All instrumented basaltic caldera collapses have generated Mw > 5 very long period earthquakes. However, previous studies of source dynamics have been limited to lumped models treating the caldera block as rigid, leaving open questions related to how ruptures initiate and propagate around the ring fault, and the seismic expressions of those dynamics. We present the first 3D numerical model capturing the nucleation and propagation of ring fault rupture, the mechanical coupling to the underlying viscoelastic magma, and the associated seismic wavefield. We demonstrate that seismic radiation, neglected in previous models, acts as a damping mechanism reducing coseismic slip by up to half, with effects most pronounced for large magma chamber volume/ring fault radius or highly compliant crust/compressible magma. Viscosity of basaltic magma has negligible effect on collapse dynamics. In contrast, viscosity of silicic magma significantly reduces ring fault slip. We use the model to simulate the 2018 Kı̄lauea caldera collapse. Three stages of collapse, characterized by ring fault rupture initiation and propagation, deceleration of the downward‐moving caldera block and magma column, and post‐collapse resonant oscillations, in addition to chamber pressurization, are identified in simulated and observed (unfiltered) near‐field seismograms. A detailed comparison of simulated and observed displacement waveforms corresponding to collapse earthquakes with hypocenters at various azimuths of the ring fault reveals a complex nucleation phase for earthquakes initiated on the northwest. Our numerical simulation framework will enhance future efforts to reconcile seismic and geodetic observations of caldera collapse with conceptual models of ring fault and magma chamber dynamics. Plain Language Summary: Caldera collapse manifests as the rapid subsidence of a kilometer‐scale block of crust circumscribed by a near‐circular fault on top of a volcano. The subsidence of the caldera block is caused by the eruption‐induced withdrawal of magma and reduction in pressure in the underlying magma chamber. All scientifically instrumented caldera collapses at volcanoes with low‐viscosity magma are accompanied by earthquakes of magnitude 5 and above. How do magma viscosity and the seismic wave radiation influence the amount of slip per earthquake on the fault? What can we learn about the dynamics of these earthquakes from seismic records? We address these questions by performing computer simulations of caldera collapse earthquakes and compare the results to the seismic records from the Kı̄lauea caldera collapse of 2018. Key Points: Seismic wave radiation through ring fault and magma, as well as high magma viscosity, reduce fault slip by up to half during collapseRupture propagation, downward momentum transfer via magma pressure waves, and chamber pressurization are identified in unfiltered seismogramsComparison between simulated and observed near‐field seismograms from Kı̄lauea 2018 reveals complex nucleation phase on the ring fault [ABSTRACT FROM AUTHOR]

Altered oceanic crust (AOC) plays a critical role in geochemical recycling in subduction zones. However, identifying contributions of subducted AOC to arc magmas remains a conundrum due to the lack of effective tracers. Here, we investigate the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc. Based on a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes, we confirm that AOC‐derived fluid (or hydrous melt), rather than sediment‐derived melt or fluid, is responsible for the Sr‐Nd isotope decoupling (i.e., 87Sr/86Sr is "excessively" enriched relative to 143Nd/144Nd when compared to the "normal" mantle derivates) observed in island arc lavas. We show that the arc lavas with increasingly decoupled Sr‐Nd isotopes generally have heavier Ba isotope ratios, which is also a characteristic feature of AOC‐derived fluids. Thus, these results establish an intimate link between subducted AOC, heavy Ba isotope compositions, and Sr‐Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones. Furthermore, a similar correlation is observed between Sr‐Nd isotope decoupling and heavy B isotope ratios for global arc lavas, implying that the recycling of AOC component is generally linked to serpentinite dehydration in subduction zones. Plain Language Summary: Altered oceanic crust (AOC) is an important subduction component that contributes to the generation of arc magmas. However, identifying this contribution to arc magmas is challenging for the lack of effective tracers. Nonetheless, AOC has two characteristic features: (a) Sr‐Nd isotope decoupling (i.e., enriched in 87Sr/86Sr relative to 143Nd/144Nd in contrast to the "normal" mantle array); and (b) relatively heavy Ba isotope compositions. Here, we analyze the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc, which are combined with a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes. Our results confirm that fluid (or hydrous melt) derived from subducted AOC, rather than sediment, is responsible for the Sr‐Nd isotope decoupling observed in island arc lavas. Strikingly, the magnitude of the Sr‐Nd isotope decoupling correlates well with the Ba isotope compositions of the arc lavas. Thus, our results establish an intimate link between subducted AOC, heavy Ba isotope ratios, and Sr‐Nd isotope decoupling signature in island arcs. This combination of Ba‐Sr‐Nd isotopes can therefore now be used as a powerful tracer to identify the contributions of subducted AOC to arc magmas. Our results also suggest that AOC and serpentinite show synergistic behavior during subduction recycling. Key Points: Global arc data confirm that the Sr‐Nd isotope decoupling of arc lavas is caused by AOC‐derived fluidDecoupled Sr‐Nd isotopes plus heavy Ba isotope is a good tracer for recycled altered oceanic crust (AOC) in arc magmasAOC and serpentinite show synergistic behaviors during subduction recycling [ABSTRACT FROM AUTHOR]

The thermal properties of the Earth's primordial magma are the key factors that constrained crystallization and other thermo‐chemical processes in Earth's primitive magma ocean and therefore controlled the Earth's long‐term evolution. Thermal conductivity of the primordial magma is conventionally assumed to be a constant of about 4 W m−1 K−1 under the high pressure‐temperature conditions of the primitive magma ocean. Here we measured the lattice thermal conductivity of a variety of basaltic and silicate glasses at high pressures and a wide range of temperatures. Our results suggest that the primordial magma, if it is indeed represented by basaltic melts, had a thermal conductivity of ∼1.0–1.9 W m−1 K−1, much lower than previously thought. Such low thermal conduction reduced heat loss and thus prolonged the cooling time of the early magma ocean, promoting convection in the solidifying mantle and preventing a global overturn. Moreover, if the seismic ultralow velocity zones presently observed in the lowermost mantle are made of basaltic melts, originating either from remnants of the primitive magma ocean or pieces of subducted crust, the material in these zones must have an ultralow thermal conductivity, which would reduce cooling and thus influence the thermo‐chemical evolution of the present day core‐mantle boundary. Plain Language Summary: It is believed that a global magma ocean existed in the early Earth's interior. The thermo‐chemical evolution of the primitive magma ocean holds the key to understand the Earth's subsequent evolution and even its present day structure. Thus, knowledge of the fundamental physical properties of the primitive magma ocean, in particular, the thermal properties, would significantly advance our comprehension of our planet's history. Here, we precisely determined the thermal conductivity of a series of basaltic and silicate glasses at the high pressure‐temperature conditions assumed to be characteristic of the primitive magma ocean. Our results suggest that the primitive magma ocean, if it indeed consisted of basaltic melts, had a thermal conductivity much lower than previously assumed, which substantially slowed down cooling and allowed convection of the magma ocean before it entirely solidified. Furthermore, if the ultralow seismic velocity zones observed at present are remnants of the primitive magma ocean, their exceptionally low thermal conductivity should hinder heat transfer and thus strongly affect the thermo‐chemical state of the core‐mantle boundary. Key Points: We measured lattice thermal conductivity of basaltic and silicate glasses at high pressures and wide range of temperaturesIf primordial magma can be modeled as basaltic melts, its thermal conductivity in the mantle is much lower than previously thoughtSuch poorly conductive primitive magma ocean has a longer lifetime, facilitating solid‐state convection in the solidified mantle [ABSTRACT FROM AUTHOR]

Processes of magma generation and transportation in global mid‐ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end‐member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow‐spreading ridges. Here we present detailed mineralogical studies of high‐Mg and low‐Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High‐Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high‐An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low‐Mg basalts contain low‐Mg# Ol and low‐An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low‐Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high‐Mg and low‐Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high‐Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low‐Mg melts may have evolved from the high‐Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow‐spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum. Plain Language Summary: Mid‐ocean ridges are divergent plate boundaries where asthenospheric mantle undergoes decompressional melting to form basaltic melts and lithospheric mantle. The southwest Indian ridge (SWIR) represents the slowest spreading class of ocean ridges. It is characterized by intermittent volcanism resulting in thin or missing oceanic crust. To better understand the magma dynamics of such specific ridges and their correlation with the spreading rates, we calculated the depth of mineral crystallization in high‐Mg and low‐Mg basalts collected together and constrained the residence time of individual crystals in the melt through Fe‐Mg diffusion profiles. We show that the high‐Mg melts started crystallization close to the base of the tectonic plate, and likely partially crystallized in a deep magma chamber in the mantle to form low‐Mg magmas prior to eruption to the seafloor. This study determined that the rate at which the melts flowed up through the mantle to the crust was the slowest yet found for any ocean ridges. The mineral dynamics of the ultraslow‐spreading SWIR therefore illustrate the critical role of the velocity at which the tectonic plates separate on the formation of magmas at mid‐ocean ridges. Key Points: Deep crystallization depths (∼32 ± 7.8 km) and a deep axial magma chamber (AMC) (∼13 ± 7.8 km) may exist under the ultraslow‐spreading Southwest Indian Ridge (SWIR)The ascent rate from the AMC to seafloor of the SWIR is ∼0.002–0.01 m/s, which is the slowest in global mid‐ocean ridge systemsMagma ascent rates correlate positively with the spreading rates in the global mid‐ocean ridge systems [ABSTRACT FROM AUTHOR]

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita‐Ioto Island, south of Japan, unusual M ∼ 5 non‐double‐couple volcanic earthquakes exhibited quasi‐regular recurrence near a submarine caldera. Following the earthquakes in 2008 and 2015, a distant ocean bottom pressure sensor recorded distinct tsunami signals. In this study, we aim to find a source model of the tsunami‐generating earthquake and quantify the pre‐seismic magma overpressure within the caldera's magma reservoir. Based on the earthquake's characteristic focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish mechanical earthquake models that link pre‐seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring‐fault system and a reservoir of the caldera. The trapdoor faulting with large fault slip due to magma‐induced shear stress in the submarine caldera reproduces well the observed tsunami waveform. Due to limited data, uncertainties in the fault geometry persist, leading to variations of magma overpressure estimation: the pre‐seismic magma overpressure ranging approximately from 5 to 20 MPa, and the co‐seismic pressure drop ratio from 10% to 40%. Although better constraints on the fault geometry are required for robust magma pressure quantification, this study shows that magmatic systems beneath calderas are influenced significantly by intra‐caldera fault systems and that tsunamigenic trapdoor faulting provides rare opportunities to obtain quantitative insights into remote submarine volcanism hidden under the ocean. Plain Language Summary: Monitoring submarine volcanoes is essential to understand and prepare for potential volcanic hazards in/around oceans, but it's challenging because these volcanoes are located in inaccessible environments. In a submarine volcano with a caldera structure in the south of Japan, unusual volcanic earthquakes took place every several years. After one of these earthquakes in 2008, a pressure sensor deployed on the sea bottom recorded a clear signal of tsunami waves. By utilizing the tsunami signal from the earthquake, we attempt to measure how much magma pressure was building up beneath the volcano before the earthquake. Assuming that the earthquake happened with sudden rupture on an intra‐caldera fault system due to highly pressurized magma beneath the volcano, we develop a method to assess the built‐up magma pressure through quantification of the earthquake and tsunami sizes. By applying the method, we estimate that the volcanic edifice was under a highly stressed condition before the earthquake, suggesting the active magma accumulation process that has continued beneath the volcano. Signals emitted from volcanic earthquakes under oceans shed light on the activity of poorly monitored submarine volcanoes. Key Points: Non‐double‐couple earthquakes with seismic magnitudes of 5.2–5.3 recurred in the vicinity of a submarine caldera near Kita‐Ioto IslandA mechanical model of trapdoor faulting based on tsunami data of the 2008 earthquake infers pre‐seismic overpressure in a magma reservoirUncertainty in fault geometry varies our estimate of pre‐seismic overpressure (5–20 MPa) and co‐seismic pressure drop ratio (10%–40%) [ABSTRACT FROM AUTHOR]

The transition between non‐rifted continental lithosphere and oceanic lithosphere in rifted margins can display a wide range of characteristics, depending on the regional tectonic evolution. The velocity and duration of the rifting process as well as the geodynamic setting influence the properties and geometry of the margins, which are often grouped into two main categories: magma‐poor and magma‐rich. We show how different types of rifted margins can influence the dynamics of continental collision, focusing on the time and depth of slab break‐off after collision and the fate of margin material. We find that rifted margins have a noticeable impact on subduction dynamics, as we observe large variability in slab break‐off times and depths. In particular, the presence of a rifted margin can delay slab break‐off to up to 60 Myr after the onset of collision. Our results show that a large portion of the weak crust of magma‐poor margins is likely to detach from the subducting plate and accrete to the upper plate, while the dense and strong mafic and ultramafic component of magma‐rich margins causes most of the margin to subduct and be lost into the mantle, leaving only a small fraction of transitional and oceanic crust at the surface. Therefore, the volume of accreted material is much larger when the margin is magma‐poor than magma‐rich, which is consistent with geological observations that fossil magma‐poor rifted margins are preserved in many mountain ranges, whereas remnants of magma‐rich rifted margins are scarce. Plain Language Summary: The transition between continental and oceanic lithosphere at rifted margins in nature is not a sharp boundary, but it is gradual and often characterized by the presence of altered crust. Such margins can be either magma‐poor or magma‐rich, depending on the volume and timing of magmatism during extension; in nature, we can find traces of rifted margins in collisional mountain belts, with magma‐poor margins being more easily preserved than magma‐rich ones. The available models for continental collision, however, do not often consider the presence of such margins. We model continental collision with the presence of a magma‐poor or magma‐rich rifted margin, aiming to provide an explanation for these observations and to see how the margins can affect the subduction process. We find that the presence of a rifted margin can have a significant impact on the evolution of the subduction system, with slab break‐off being significantly delayed in all cases. In the models, we find that part of the margin is always transferred to the overriding plate, and our results show that magma‐poor margins are more likely to be recovered, matching the observations that magma‐poor margins are more easily preserved than magma‐rich. Key Points: The presence of rifted margins in numerical models of continental collision can significantly delay slab break‐offThe type of margin (magma‐poor vs. magma‐rich) controls the dynamic of collision and the likelihood for the margin to be preservedThe volume of preserved margin material is much higher when to subduct is a magma‐poor margin than a magma‐rich one [ABSTRACT FROM AUTHOR]

Magma supply likely exerts primary control on seafloor morphology of oceanic crust, but most studies have related morphology to spreading rate. Here we examine global patterns of morphology on mid‐ocean ridge (MOR) flanks in relation to magma supply derived from residual mantle Bouguer gravity anomaly (proxy for relative crustal thickness) and spreading rate. We use multibeam bathymetry to characterize morphology using both qualitative (descriptive) and quantitative approaches, and we compare results to both magma supply and spreading rate. Morphology becomes more isotropic and abyssal hills are more irregular and discontinuous as magma supply decreases, while roughness, area of steeper slopes, and anomalous fabric orientation increase. We interpret these changes to reflect changing magma distribution along‐axis, from large‐volume and spatially extensive to progressively reduced, increasingly localized, and more irregularly emplaced. Observed relations between crustal thickness and morphology imply that average thickness of purely magmatic crust in the Atlantic and parts of the Indian ridge system is significantly less than average seismically determined crust. Thus seismically defined crustal thickness in those regions likely includes significant non‐magmatic components such as serpentinized mantle. Excepting regions of extensive mantle exposure, most morphologic parameters that we examined are sensitive to estimated magma supply but not necessarily to spreading rate alone. We summarize our results in schematic models that relate morphologic variations to changes in magma supply and mantle serpentinization throughout the global MOR system. Finally, we note that combined qualitative and quantitative results of our study may be useful for developing automated morphologic classification schemes. Plain Language Summary: Mid‐ocean ridges encircle the globe and spread apart along their axes to form ocean crust. Seafloor morphology of crust on the flanks of different ridges can be highly variable. The variations classically have been attributed to differences in spreading rate but there are notable exceptions to this correlation. Here we use changes in gravity over different mid‐ocean ridges (MORs) together with spreading rate to infer magma supply to the ridges, and we examine how morphology varies with changes in the magma supply. We find that features such as long, linear, and structurally continuous abyssal hills correlate well with high magma supply while progressively rougher, steeper, and more irregular morphology forms as magma supply is reduced; this is observed irrespective of spreading rate, indicating that magma supply better predicts morphologic style. Observed relations between crustal thickness and morphology imply that seismically determined crustal thickness in the Atlantic and parts of the Indian Ocean MOR system includes significant components of non‐magmatic crust (e.g., altered mantle). Our study characterizes crustal morphology both qualitatively (visually) and quantitatively; correlations between these characterizations may be useful for developing automated morphologic classification schemes. Key Points: Variable morphology of global mid‐ocean ridge flanks is examined qualitatively and quantitatively using multibeam bathymetric dataChange in magma supply as derived from gravity and spreading rate, plus mantle serpentinization, explains observed morphologic variationsMorphologic changes indicate thinner magmatic crust than seismically determined crust in the Atlantic and parts of the Indian Ocean [ABSTRACT FROM AUTHOR]

Oceanic detachment faulting, a major mode of seafloor accretion at slow and ultraslow spreading ridges, is thought to occur during magma‐poor phases and be abandoned when magmatism increases. In this framework, detachment faulting is the result of temporal variations in magma flux, which is inconsistent with recent geophysical observations at the Longqi segment on the Southwest Indian Ridge (49°42′E). In this paper, we focus on this sequentially active detachment faulting system that includes an old, inactive detachment fault and a younger, active detachment fault. We investigate the mechanisms controlling the temporal evolution of this tectonomagmatic system by using 2D mid‐ocean ridge spreading models that simulate faulting and magma intrusion into a visco‐elasto‐plastic continuum. Our models show that temporal variations in magma flux alone are insufficient to match the inferred temporal evolution of the sequentially active detachment system. Rather we find that sequentially active detachment faulting spontaneously occurs at the Longqi segment as a function of lithospheric thickness. This finding is in agreement with an analytical model, which shows that a thicker axial lithosphere results in a smaller fault heave and that a flatter angle in lithosphere thickening away from the accretion axis stabilizes the active fault. A thicker axial lithosphere and its flatter off‐axis angle combined have the potential to modulate sequentially active detachment faulting at the Longqi segment. Our results thus suggest that temporal changes of magmatism are not necessary for the development and abandonment of detachment faults at ultraslow spreading ridges. Plain Language Summary: The oceanic lithosphere is frequently generated through slip on long‐lived faults called oceanic detachment faults, which exhume lower crustal and upper mantle materials on the seabed and form dome‐shaped features. Yet, oceanic detachment faulting is a sequentially active process and previous studies have focused on temporal variations in magma flux as the primary cause. Magma flux refers to the rate at which the continuous flow of magma moves from melted mantle at depth toward the ridge axis through volcanic vents or fissures. In the presented manuscript we show that this standard model does not work for the ultraslow spreading Southwest Indian Ridge (49°42′E). Here, we present geological and geophysical evidence that the magma flux had remained stable during the two most recent phases of detachment faulting. We investigate the mechanisms controlling the life cycle of the detachment fault by using a 2D geodynamics model. We compare model results with observations and propose that sequentially active detachment faulting at the Longqi segment is a function of lithospheric structure rather than variations in magma flux. These findings show that temporal changes of magmatism are not required for sequentially active detachment faulting to occur. Key Points: Our tectonomagmatic models provide a mechanical explanation for the sequentially active detachment fault system at the Longqi segment on the ultraslow spreading ridgesLithospheric structure at spreading centers plays a key role in the lifecycle of oceanic detachment faults [ABSTRACT FROM AUTHOR]

Melt percolation through partially molten "mushy" regions of the crust underpins models for magma migration, accumulation, and processes that prime systems for eruption. Knowledge of the hydraulic properties of magma mush, specifically permeability, is required for accurate predictions of melt migration rates and accumulation timescales. Previous studies, validated for cuboidal crystal analogs, show that crystal shape exerts a first‐order control on the permeability, and is tested here for anisometric natural crystal shapes using X‐ray CT 3D data sets of magma mush analogs made from packed confectionary particles arranged randomly. We use a lattice‐Boltzmann fluid flow simulation tool to determine the permeability of the analogue melt phase network between the packed particles. We find excellent agreement with our data sets to within ∼0.1 log units, when the specific surface area is measured. To extend this to more typical cases where the specific surface area is unknown, we use the shape and size of the objects determined in both 3D and 2D to estimate the specific surface area assuming a cuboid approximation. These approximate solutions give good results to within ∼0.5 log units of the measured permeability and offer a method by which permeability could be estimated from a thin section of a cumulate or pluton sample. Our shape‐sensitive approach is more accurate than existing models for permeability of magma mush, most approximating natural crystal shapes to spheres. We therefore propose that these could be implemented in dynamic magma mush models for melt movement in the crust to produce more accurate flux predictions. Plain Language Summary: Magma chambers in the Earth are "mushy," meaning that liquid magma is trapped in between solid crystals. In many cases, the liquid magma must escape before an eruption occurs. For the liquid magma to escape, it must move through the tight spaces between the solid crystals, which occurs at a speed dictated by the "permeability" of the crystal framework. Here, we use fudge and sugar crystals as a proxy for the solid crystals in these magma chambers, and we use simulations to observe how the fluid between the fudge and sugar moves. The key advance here is that we show how the complicated shape of fudge and sugar crystals changes the speed at which the fluid is able to move. Key Points: Crystal shape exerts a first order control on magma mush permeabilityThe specific surface area of magmatic crystals is the relevant metric to predict the permeability, across all crystal shapesMagma mush permeability can be calculated from crystal size measurements made on a 2D thin section, via a 2D‐to‐3D conversion [ABSTRACT FROM AUTHOR]

The processes that control the crystallization of magmas, namely nucleation and growth, remain poorly constrained. Classical nucleation theory (CNT) has so far not provided a unified framework to explain crystal number density in laboratory experiments and the field. We use numerical experiments to study the influence of different cooling rates and CNT parameters on the crystal number density measured under constrained conditions in laboratory experiments. With varying the magnitude of pre‐exponential factor (J0) of nucleation rate, which represents the number of cluster as heterogeneous nucleation sites, we find that the cooling rate exponent (ξ) of the crystal number density varies from 3/2 to −1, depending on the magnitude of J0, cooling rate and surface tension. Using the regime maps of ξ as functions of J0 and surface tension, we can successfully interpret the diversity of cooling rate exponents reported by laboratory experiments in terms of J0 and surface tension. For extreme case of a negative cooling rate exponent ξ = −1, we propose a new crystallization model with constant rates of nucleation and crystallization, which reproduces a log‐linear crystal size distribution. In this condition, it is interesting that the crystal growth rate is inversely proportional to time, even if the diffusion‐limited growth is inversely proportional to the square root of time. For the case study of zoning profiles in microlites formed by decompression‐induced crystallization during the Shinmoedake 2011 subplinian eruptions, the comparison between theoretical predictions and natural volcanic products supports that our model is well applicable to magma ascent processes. Plain Language Summary: Crystallization of magmas denotes the formation of crystals in cooling magmas owing to heat loss during their ascension to the Earth surface or to the decompression and vesiculation occurring during an eruption. Since it strongly depends on cooling rate, the crystal number density is the most relevant textural index of the final products of the ascending magmas, the erupted rocks. Continuous efforts have been made in theory, laboratory experiments and observation of natural rock samples to understand how the cooling rate of magmas controls the texture of the erupted materials. Yet the role that different factors play on the texture of rocks during crystallization remains poorly constrained. For instance, the crystal number density has been observed to increase with cooling rate in some laboratory experiments and show no dependence or even decrease in others. The model proposed here identifies the nucleation rate as the main parameter controlling the formation of crystals. This study provides a consistent framework to relate the crystal number densities measured in laboratory experiments and observed in natural samples to their cooling conditions. Key Points: Pre‐exponential factor of nucleation rate in classical theory has the greatest effect on number density of crystals formed by coolingThe model can explain the diversity of the experimental and natural dataConstant rates of nucleation and crystallization yield log‐linear CSDs [ABSTRACT FROM AUTHOR]

A high‐precision earthquake catalog is made from continuous waveform data recorded at 360 portable seismographs deployed for one month in the Changbaishan‐Tianchi volcanic area based on an automatic earthquake detection and location workflow. Then we investigate detailed 3‐D structures of P and S wave velocities (Vp and Vs) and Vp/Vs ratio of the crust beneath the study region. Our results reveal a high‐Vp/Vs zone accompanied with active volcanic earthquakes beneath the Tianchi caldera at ∼5 km depth, which may reflect a magma chamber. The intense seismic activities within 2 km depth beneath the Tianchi caldera from 2002 to 2005 occurred directly above the magma chamber revealed by this study, which could be related to upward migration of high‐pressure volatiles originating from the magma chamber. A potential middle‐crustal magma chamber might provide continuous magmatic materials feeding the upper‐crust magma chamber and lead to ruptures of surrounding rocks, resulting in two swarms of volcanic seismicity during the observation period. We deem that the diverse eruption styles of the Changbaishan volcano are closely associated with the multilevel structure of magma chambers in the upper and middle crust. Plain Language Summary: It is difficult for the earthquake monitoring system to identify and locate microearthquakes by using traditional seismic detection methods due to their weak signals, leading to a relatively low quantity and quality of seismic data in a study region, which cannot meet the demands of high‐resolution seismic tomography. In this study, we detect and locate microseismic events beneath the Changbaishan‐Tianchi volcanic area using 1‐month waveform data recorded at our 360 portable seismographs deployed in the volcanic area. Then we investigate high‐resolution 3‐D crustal structures of P and S wave velocities (Vp, Vs) and Vp/Vs ratio beneath the study area. Our results reveal a possible crustal magma reservoir at ∼5 km depth beneath the Tianchi caldera, which is accompanied with volcanic earthquakes during our observation period. In addition, small earthquakes occurred actively at a depth of ∼2 km during 2002–2005, which are located right above the magma reservoir. We deem that the joint effect of magma reservoirs in the upper and middle crust could explain the active volcanic seismicity and distinctive eruption behaviors of the Changbaishan volcano. Key Points: A magma chamber at ∼5 km depth beneath the Tianchi caldera of the Changbaishan volcano (CBV) is revealedThe CBV unrest during 2002–2005 was related to reactivation of the crustal magma chamberDistinctive eruption behaviors of the volcano are related to multilevel magma chambers in the crust [ABSTRACT FROM AUTHOR]

Igneous dike intrusion is a primary crust‐forming process at the Earth's plate boundaries. Understanding its mechanism is thus crucially important in lithospheric studies. Our present article combines experimental and field observations to investigate the problem of dike emplacement from a mechanical perspective. Scaled laboratory experiments were conducted by injecting immiscible liquids into visco‐elastic and visco‐elasto‐plastic host materials at varying volumetric flow rates (VFR = 0.100 to 1.670 ml s−1). Another set of experiments used different injecting liquid–host combinations to set their viscosity ratios (η*) at low (105), moderate (106), and high (109) values. These two lines of experiments allow us to recognize three principal mechanisms of liquid pathways: tensile fracturing of the host, wave instability at the liquid‐host interface, and coupled fracturing‐wave instability process. The three mechanisms give rise to a wide variation in intrusion geometry, ranging from planar structures with elliptical outlines to typical bulbous geometry, with intermediate patterns characterized by in‐plane and off‐plane wavy interfaces with the host. This study uses the experimental data to constrain the VFR conditions that determine the fracturing versus wave instability‐controlled mechanisms. It is also shown from the experiments that η* can significantly influence the evolution of three‐dimensional intrusive geometry. The Chotonagpur Granite Gneiss Complex in eastern India is chosen as a field study area to validate our laboratory findings using a 2D shape analysis of the intrusive boundaries in terms of their fractal dimensions (D) and skewness‐kurtosis estimates. Plain Language Summary: Magmas generated deep inside the Earth flow through solid rock strata both in horizontal and vertical directions. However, how they create passages for their flow is not yet fully understood, and it needs a systematic study of this problem, which is the primary goal of our article. We mechanically reproduced magma flows in rock analog materials under laboratory conditions and studied the mechanisms of passage generation by magma to intrude into the host material. The experimental results show three primary mechanisms: (a) opening mode of host fracturing, (b) wave formation at the liquid–host interface, and (c) fracturing, coupled wave formation at the fracture walls. It is shown that these mechanisms are related to the volumetric magma influx rate and the viscosity ratio of host materials to magma. Each mechanism gives rise to characteristic geometry of magma intrusions, such as straight tabular, lobate geometry, or complex network patterns. To validate the laboratory findings, this study provides field examples from the Chotonagpur Granite Gneiss Complex of Shingbhum Craton, Eastern India. A two‐dimensional shape analysis of intrusion boundaries presented in this article uses a set of geometrical parameters to show their geometry quantitatively as an indicator of the emplacement mechanisms. Key Points: Magmatic intrusions into host rocks can occur by host fracturing, interface instability, or a combination of fracturing and instabilityEach intrusion mechanism gives rise to characteristic geometry of the dike‐host boundaries and the spatial patterns of dikesThe magma influx rate and the host‐to‐magma viscosity ratio are the two crucial factors in controlling the intrusion processes [ABSTRACT FROM AUTHOR]

Recent geodetic data has clearly imaged significant contraction of rift zones during effusive eruptions, which is attributed to deformation of active, elongated feeders, challenging the rigid conduit paradigm. In this work we develop a physical model to understand the impact of conduit deformation on the eruptive dynamics. Numerical calculations show that magma overpressure increases the width of the conduit, resulting in higher discharge rates than would be expected from the rigid case. At the same time, conduits with high aspect ratio, have larger compressibility and can store significant amount of magma, thereby acting as secondary reservoirs. The net result is that rift zones can maintain high fluxes over prolonged periods of time, leading to large volume eruptions and biasing magma compressibility estimates. We apply our findings to the 2018 Kīlauea eruption where episodic collapse of the summit led to pressure pulses that propagated down‐rift and that were recorded by both tiltmeters and peaks in the effusion rates. Inversion of the data indicates a conduit with a height of 700–800 m and a maximum opening of 4 m, located at a depth of 2.5 km in the Upper East Rift Zone, becoming shallower in the Puʻu ʻŌʻō region and propagating sub‐horizontally in the Middle and Lower East Rift Zone. Based on these properties we infer that up to 30% of the erupted volume can be attributed to magma stored in the rift zone. In agreement with recent studies, we find that magma over‐pressure was low at the end of the 2018 eruption. Plain Language Summary: Deformation of the volcanic edifice shows a contraction of the conduits feeding eruptions. We have developed a physical model to understand how such deformation affects the amount of magma extruded and for how long the eruption will last. The results of the model indicate that elastic conduit deformation allows for larger volumes of magma to surface more quickly, compared to cases where the conduit behaves rigidly. One important finding was that elastic conduits act as secondary magma reservoirs. We applied the model to estimate the properties of the storage and transport system of the 2018 Kīlauea eruption. We found that the conduit had a height of about 750 m and a maximum opening of 4 m. The conduit is located at a depth of about 2.5 km in the Upper East Rift Zone, becoming shallower in the Pu'u 'Ō'ō region and propagating sub‐horizontally in the Middle and Lower East Rift Zone. As much as 30% of the total erupted volume could have originated from the conduit itself. Indeed, the pressure that drove the flow of the magma was low at the end of the eruption. Key Points: Geodetic data show significant deformation of rift zones during eruptionsA physical model of elastic conduit is derived to understand deformation effects on eruption dynamicsThe model is applied to 2018 Kīlauea eruption to invert the parameters of its plumbing system [ABSTRACT FROM AUTHOR]

This study describes an ongoing intense earthquake swarm in the crust of the northeastern Noto Peninsula, Japan, that began around the end of 2019. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust. However, no volcanic activity has occurred in this region since the Middle Miocene (15.6 Ma). This study investigates the cause of this earthquake swarm based on the spatiotemporal evolution of earthquake hypocenters and seismic reflectors. The hypocenter relocation of 10,940 earthquakes (M > 1) reveals that they are all crustal and migrated upward, activating a complex network of faults at depths shallower than 20 km. The initiation of this earthquake swarm occurred at a locally deep depth (z = 17 km), and the local hypocenter distribution shows a characteristic circular pattern. We find a distinctive S‐wave reflector in the immediate vicinity. A low‐velocity anomaly exists below the reflector, and a high helium isotope ratio and a low‐gravity anomaly were observed at the surface. These observations suggest that the current seismicity is associated with fluids released by ancient or possibly unrecognized modern magmatic activity, although no volcanic activity has been documented in this area for 15 million years. Significant crustal deformation was observed during this swarm and is probably related to the fluid movement and aseismic deformation that contributed to this earthquake swarm. The strongest M5.4 earthquake (as of October 2022) occurred near the migration front on the largest planar structure, leaving the shallow extension unruptured. Plain Language Summary: An intense earthquake swarm is currently occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but no volcanic activity has occurred in this region since the middle Miocene (15.6 Ma). We investigate the cause of this earthquake swarm using precisely determined earthquake locations and seismic reflectors. We find that the earthquakes moved from deep to shallow depths via many planes, similar to earthquake swarms near volcanoes. The strongest M5.4 earthquake occurred near the migration front on the largest planar structure. Further earthquakes may be possible in the shallow part of this fault. We find a distinctive S‐wave reflector, suggesting a fluid source, in the immediate vicinity of the initiation point of this swarm. The local hypocenters show a circular pattern similar to the ring fault that forms above the magma reservoir. These observations may indicate that the current seismic activity is being impacted by fluids related to ancient or new hidden magmatic activity. The present results suggest that hidden magma‐induced structures and fluids can generate earthquakes even in areas where no volcanic activity has been observed for over 10 million years. Key Points: An intense seismic swarm continues in a non‐volcanic region, with upward earthquake migration through a complex network of faultsA deep S‐wave reflector located near the seismicity initiation point and above the low‐velocity anomaly may represent a fluid sourceThe significant surface deformation observed during this swarm was caused by aseismic deformation activated by fluid migration [ABSTRACT FROM AUTHOR]

We integrate textural and in situ compositional information from plagioclase and clinopyroxene (Cpx) phenocrysts together with groundmass compositions in early Cretaceous andesite dykes within the Sulu belt of China to propose a new petrogenetic model for andesite. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strength elements (HFSE). However, Cpx phenocrysts are either reversely zoned (type I) or homogeneous (type II), with the zoned Cpx divided into subtypes IA and IB. All Cpx has high Mg#, low Na2O and generally low Al2O3, with depletions in HFSE and variably high 87Sr/86Sr ratios, suggesting crystallization above the Moho from magmas derived from enriched lithospheric mantle. The cores of type IA/IB and type II Cpx have normal major‐ and trace‐element compositional variations and similar 87Sr/86Sr ratios to each other and to plagioclase, consistent with fractional crystallization from a common magma (magma 1). The rims of type IA and IB Cpx also have normal major‐ and trace‐element compositional variations, but these are not as evolved as the cores, and the rims have lower 87Sr/86Sr ratios, demonstrating crystallization from an isotopically distinct magma (magma 2). Based on modeled major and rare earth element compositions of magmas inferred to have been in equilibrium with different Cpx (±plagioclase) domains, the measured groundmass compositions can be reproduced by variable mixing between the two magmas. Our study demonstrates for the first time that andesite magma can be made through fractionation and shallow hybridization of magmas derived from variably enriched lithospheric mantle. Plain Language Summary: Andesite is the characteristic volcanic rock that is formed where Earth's tectonic plates are thrust one under another, as occurs along the Andes, the Cordillera of Central and North America, and the western margin of the Pacific Ocean, but how the magmas form remains controversial. In the mantle model, an enriched source is thought to form by reaction between mantle peridotite and metamorphic fluids derived from seafloor sediments and/or altered oceanic lithosphere during under‐thrusting of an ocean plate beneath a continental margin. By contrast, the shallow model proposes that andesite magmas are formed by differentiation of basaltic magmas or mixing of magmas from mantle and crustal sources in the lithosphere of an overriding continental plate. In either case, the composition of the magma may evolve due to crystal fractionation or be modified by crustal assimilation. Here, we use petrographic observations and in situ major‐ and trace‐element, and Sr isotope compositions of plagioclase and clinopyroxene phenocrysts and groundmass in early Cretaceous andesite dykes from the Sulu belt of China, to show for the first time that andesite can be made through fractionation and shallow hybridization of two different magmas derived from variably enriched lithospheric mantle of the underlying continental plate. Key Points: Using compositions of plagioclase/clinopyroxene phenocrysts and the groundmass in andesite we develop a new model of andesite petrogenesisThis model requires polybaric crystallization and hybridization between 2 enriched lithospheric mantle‐derived magmas at crustal depthWe demonstrate that andesite of similar composition to continental crust can be made through mixing magmas derived from the enriched subcontinental lithospheric mantle [ABSTRACT FROM AUTHOR]

Previous work at Fernandina, the most active volcano of the Western Galápagos (Ecuador), revealed evidence for both a shallow and a deep magma reservoir, but the relative contribution of the two reservoirs to eruptions remains unclear. Here we investigate the September 2017 circumferential eruption and the June 2018 radial eruption using interferometric synthetic aperture radar data and geodetic modeling. Our results show that during the 2017 eruption magma was simultaneously withdrawn from the deep reservoir, injected upwards through the shallow reservoir, and then fed the circumferential feeder dike to the SW of the caldera. Two episodes of inflow of new magma occurred in both the deep and shallow magma reservoirs in the inter‐eruptive period from December 2017 to May 2018. During the 2018 eruption, both reservoirs fed two radial feeder dikes below the north flank, probably interacting with an underlying peripheral melt pocket, and an inclined sheet below the NW sector of the caldera. Our results highlight the primary role of the deeper reservoir which accumulates most of the magma before eruptions. Both eruptions were characterized by rapid magma transfer from the deeper to the shallower reservoir. This is similar to what is observed at the nearby Wolf volcano, but unlike nearby Sierra Negra, where a shallower reservoir accumulates higher volumes of magma before eruptions. These differences in the pre‐eruptive role of the deeper and shallower reservoirs might be related to the different evolutionary stages of Fernandina and Wolf with regard to the more mature Sierra Negra. Plain Language Summary: Fernandina is the most active volcano of the Western Galápagos (Ecuador). Two magma reservoirs at different depths below the volcano trigger eruptions both near the caldera rim (circumferential eruptions) and lower on the volcano flanks (radial eruptions). The role played by the two reservoirs in the eruptions is poorly understood. Here we studied the 2017 and the 2018 eruptions that occurred at Fernandina by using deformation data derived from radar satellites to investigate the inputs and outputs of new magma into the two reservoirs when eruptions occurred. The 2017 eruption had similar characteristics to the 2005 eruption, while the 2018 eruption had more complex mechanisms of shallow magma transfer, with magma transported in multiple directions below the north flank of Fernandina. We found that rapid magma transfer from the deep to the shallow reservoir provided most of the magma for both eruptions, similar to the nearby Wolf volcano, and unlike nearby Sierra Negra, a feature that may be due to the evolutionary stage of the volcanoes. Key Points: The 2017 circumferential eruption at Fernandina shows similar characteristics to the 2005 eruption, occurred in the same areaThe 2018 eruption is connected to the propagation of multiple dikes and sills, with most of the magma remaining intrudedBoth eruptions were characterized by rapid magma transfer from the deeper to the shallower reservoir, similar to what is observed at Wolf [ABSTRACT FROM AUTHOR]

Dehydration of subducting oceanic lithosphere (slab) induces Li‐isotope fractionation between the fluid and the slab, suggested by the δ7Li variation (∼10‰) in exhumed subduction complexes. Given that arc magmas represent melt of the supraslab mantle, a large δ7Li variation is anticipated for arc volcanic rocks. However, the δ7Li values in these rocks are mostly homogeneous within the range of mid‐ocean ridge basalts (+1.6 to +5.6‰). The lack of a subduction‐related δ7Li signature has been explained by (a) homogenization by mixing of different magma sources, (b) loss of Li from the slab via dehydration, or (c) homogenization by diffusive exchange of slab‐derived Li and the mantle. The Chugoku district in SW Japan is an ideal place to study the process responsible for Li‐isotope variation in arc magmas, since the Chugoku volcanic rocks show large δ7Li variation (−1.9 to +7.4‰). High δ7Li values (+6.3 to +7.4‰) are found in some high‐Sr andesites and dacites (adakites) whereas low δ7Li values (−1.0 to −0.1‰) are found in high‐Mg andesites. The parental magmas of these rocks have been sourced from subducted oceanic crust and sediments, respectively, with various extents of the interaction with wedge mantle. The limited extents of Li isotope modification are indicated by the similarity of the δ7Li values of these rocks and their supposed sources. The models for a slab dehydration and a diffusive exchange between slab‐derived melt and mantle demonstrate that the δ7Li signatures of the sources can be preserved in the adakites if they ascent rapidly in mantle. Plain Language Summary: Many of Earth's volcanoes occur near the trenches, where one plate subducts beneath another. These volcanoes are formed as a result of the melting of mantle that contains materials of subducted plates and the transportation of melts through the mantle. A quantitative understanding of these important processes has been hampered by the lack of geochemical tools to examine consecutive chemical reactions beneath the volcanoes. Here we conducted a lithium isotope investigation of the volcanic rocks from the Chugoku district in Southwest Japan. This district hosts the volcanoes of andesites and dacites which have been derived from the melting of materials in the subducted plate. We found that these volcanic rocks preserve Li‐isotope compositions of the original materials when they melted. This in turn suggests that these magmas ascent rapidly (<300,000 years) in the mantle. Key Points: Volcanic rocks in the Chugoku district in Southwest Japan show large δ7Li variation (−1.9‰ to +7.4‰)Li isotopes fractionate to the limited extent during arc magma productionSlab‐derived δ7Li features can be preserved if slab‐derived fluids/melts have distinct δ7Li values from mantle and ascend rapidly [ABSTRACT FROM AUTHOR]

We conducted analogue experiments to examine flux‐driven and buoyancy‐driven magma ascent, which included a series of isothermal experiments and thermal, solidification‐prone experiments. We measured the internal flow using 2D particle image velocimetry, which indicates that buoyancy has a strong control on the flow pattern of isothermal dikes. Dikes that are not buoyant (likely driven by source pressure) take on a circulating pattern, while buoyant dikes assume an ascending flow pattern. Solidification modifies the flow field so that flow is confined to the dike's upper head region. The lower tail becomes mostly solidified, with a narrow conduit connecting the source to the head. We interpret that this conduit acts as a high velocity point source to the head, promoting a circulating flow pattern, even as the dike becomes buoyant. We then perform particle tracking velocimetry on several particles to illustrate the complexity of their paths. In a circulating flow pattern, particles rise to the top of the dike, descend near the lateral edge, and then are drawn back into the upward flow. In an ascending pattern, particles ascend slightly faster than the propagation velocity, and therefore are pushed to the side as they approach the upper tip. In erupting dikes, particles simply flow to the vent. In the context of crystal growth in magmatic dikes, these results suggest that crystal growth patterns (e.g., normal or oscillatory zoning) can reflect the magma flow pattern, and potentially the driving forces. Plain Language Summary: Dikes are magma‐filled cracks, which grow through the earth and can feed volcanic eruptions. Depending on the forces acting on the magma, including buoyancy, the magma inside can flow in different ways. We studied the flow inside of dikes using small, laboratory experiments. We injected oil, water, or liquid gelatin, mimicking the magma, into a block of transparent gelatin, mimicking the Earth's crust. We added small tracer particles to the liquids, which could be tracked using cameras, to understand the flow inside of our experiments. We found that when buoyancy is low, for example, when dikes are small, the internal flow circulates up and down. However, as the buoyancy increases, they begin to flow upward. These results can be related to small crystals that form in magma in nature and record chemical information in layers (known as zoning), like rings in a tree. If a crystal circulates up and down, it can look different from one that just rises up. Such crystals may give us information about the forces that control dikes at volcanoes. Key Points: In isothermal, buoyantly propagating dikes, a dike's internal flow ascends, whereas in source‐pressure controlled dikes, it circulatesSolidification generates a conduit in the dike tail, which acts as a high velocity point source to the head, promoting circulationErupting dikes assume a simple ascending flow pattern, as liquid flow directly from the source to surface [ABSTRACT FROM AUTHOR]

Simulating magma propagation pathways requires both a well‐calibrated model for the stress state of the volcano and models for dike advance within such a stress field. Here, we establish a framework for calculating computationally efficient and flexible magma propagation scenarios in the presence of caldera structures. We first develop a three‐dimensional (3D) numerical model for the stress state at volcanoes with mild topography, including the stress induced by surface loads and unloading due to the formation of caldera depressions. Then, we introduce a new, simplified 3D model of dike propagation. Such a model captures the complexity of 3D magma trajectories with low running time, and can backtrack dikes from a vent to the magma storage region. We compare the new dike propagation model to a previously published 3D model. Finally, we employ the simplified model to produce shallow dike propagation scenarios for a set of synthetic caldera settings with increasingly complex topographies. The resulting synthetic magma pathways and eruptive vent locations broadly reproduce the variability observed in natural calderas. Plain Language Summary: Understanding the pathways that bring magma from an underground chamber to the surface helps to prepare for future eruptions in volcanic areas. Dikes are fractures filled with magma and represent the most common mechanism of magma transport in the Earth's crust. Their trajectories may be curved if the Earth's crust is deformed by the load of topography or by tectonic forces. Here we first discuss a model of such deformation processes in volcanic regions with complex but mild topography. Then, we develop a simplified dike propagation model that we compare to a more sophisticated one. Next, we combine our models and simulate magma pathways in artificially‐generated scenarios. Key Points: We present numerical models of crustal stress state in the presence of caldera structuresWe develop a fast dike propagation model and validate it on a previous numerical modelWe combine our stress and dike models to simulate magma pathways at synthetic calderas [ABSTRACT FROM AUTHOR]

The high flux of subaerial volcanic CO2 emissions around the circum‐Mediterranean region requires the involvement of an unusually carbon‐rich reservoir, but the origin of which is still unclear. Here, we aim to resolve this problem by analyzing Mg and Zn isotopes for the widely distributed mafic potassic to ultrapotassic lavas in this region. These K‐rich lavas have lower δ26Mg but similar δ66Zn compared to mid‐ocean ridge basalts (MORB). No known magmatic processes can explain the isotopic data, which must therefore be characteristics of the mantle sources. Recycled carbonate sediments are capable of explaining the low δ26Mg, but they typically also have high δ66Zn. Thus, the low δ26Mg but unfractionated δ66Zn of these K‐rich lavas define "Mg‐Zn isotopic decoupling" which has not yet been observed for other types of mantle‐derived lavas. The carbonate‐bearing silicate sediments analyzed here possess low δ26Mg and MORB‐like δ66Zn, which can account for the Mg‐Zn isotopic decoupling. Therefore, the nature of recycled materials (carbonates vs. carbonate‐bearing silicate sediments) in the mantle can be distinguished by the coupling or decoupling of Mg and Zn isotopes of mantle‐derived magmas. The input flux of carbon from the sediments to the lithospheric mantle is estimated to be ∼8.1 Mt/yr, and ∼22.4 Mt/yr of CO2 emissions are predicted, which fit well with the observed output flux of 20.1 ± 13.4 Mt/yr. Our results demonstrate that recycled crustal carbon stored in the lithospheric mantle is an important source for the extensive subaerial volcanic CO2 emissions in the circum‐Mediterranean region. Plain Language Summary: Extensive volcanic CO2 emissions around the circum‐Mediterranean region constitute a significant proportion of the global total, but the source of this carbon‐rich reservoir is unclear. We analyze Mg and Zn isotopes for the widespread, lithospheric mantle‐derived K‐rich lavas in this region and find that they have lower Mg but similar Zn isotopic data compared to mid‐ocean ridge basalts. This Mg‐Zn isotopic feature is defined as "Mg‐Zn isotopic decoupling" and explained as resulting from recycled carbonate‐bearing sediments in their mantle sources. Thus, the lithospheric mantle beneath the circum‐Mediterranean region is carbon‐rich, providing an important source for the extensive volcanic CO2 emissions. Key Points: Mg and Zn isotopic decoupling is reported for the first time in circum‐Mediterranean K‐rich lavasCarbonate‐bearing silicate sediments have been recycled into the lithospheric mantle beneath the circum‐Mediterranean regionCarbon‐rich lithospheric mantle is an important source for extensive volcanic CO2 emissions [ABSTRACT FROM AUTHOR]

Active magma chambers are periodically replenished upon a combination of buoyancy and pressure forces driving upward motion of initially deep magma. Such periodic replenishments concur to determine the chemical evolution of shallow magmas, they are often associated to volcanic unrests, and they are nearly ubiquitously found to shortly precede a volcanic eruption. Here, we numerically simulate the dynamics of shallow magma chamber replenishment by systematically investigating the roles of buoyancy and pressure forces, from pure buoyancy to pure pressure conditions and across combinations of them. Our numerical results refer to volcanic systems that are not frequently erupting, for which magma at shallow level is isolated from the surface ("closed conduit" volcanoes). The results depict a variety of dynamic evolutions, with the pure buoyant end‐member associated with effective convection and mixing and generation of no or negative overpressure in the shallow chamber, and the pure pressure end‐member translating into effective shallow pressure increase without any dynamics of magma convection associated. Mixed conditions with variable extents of buoyancy and pressure forces illustrate dynamics initially dominated by overpressure, then, over the longer term, by buoyancy forces. Results globally suggest that many shallow magmatic systems may evolve during their lifetime under the control of buoyancy forces, likely triggered by shallow magma degassing. That naturally leads to long‐term stable dynamic conditions characterized by periodic replenishments of shallow partially degassed, heavier magma by volatile‐rich fresh deep magma, similar to those reconstructed from petrology of many shallow‐emplaced magmatic bodies. Plain Language Summary: Periodic injections of magma into shallow magma chambers are a fundamental process known to govern the evolution of magmatic systems. According to our overall understanding of magmatic systems, we simulate the time‐space dynamics in a composite magmatic system under the action of buoyancy and pressure forces. The formers are due to different chemical compositions of the shallow and deep magmas. The latter may result from chemical reactions, tectonic stress accumulation, or other deep processes. The numerical results illustrate an ample variety of situations mostly controlled by buoyancy or pressure forces. It is found that the "normal" condition for many volcanic systems is likely characterized by periodic arrivals of light batches of magma giving rise to efficient convection and mixing at shallow level, and parallel sinking of partially degassed, dense magma. This ensures essentially stable conditions unlikely to give origin to any pressure buildup leading to an eruption; it provides a mechanism for long life times of relatively small, shallow magmatic systems, and a framework for interpreting petrologic observations suggesting long sequences of mixing events at magmatic systems worldwide. Key Points: A numerical model for time‐space dynamics in shallow magma chambers replenished by deep magma is presentedEvolution under pressure and buoyancy forces is initially dominated by overpressure, and over the long term by buoyant convection and mixingLifetime evolution of volcanic systems is likely toward stable conditions with no pressure buildup leading to an eruption [ABSTRACT FROM AUTHOR]

Recharges of magma underneath basaltic volcanoes can occur as precursory events prior to an eruption but are not always revealed in geophysical data streams or erupted lavas compositions. In contrast, phosphorus within primitive, Mg‐rich (Fo89‐90), olivine can preserve recharge information lost by the mixed melt. Evidence of rapid growth and dissolution are preserved only in phosphorus X‐ray intensity maps, which reveal that Mg‐rich olivine from eruptions occurring between 2008 and 2020 at Kīlauea Volcano (Hawaiʻi) experienced at least two episodes of magma intrusion. We develop numerical diffusion models that evaluate the fidelity of the Fe‐Mg compositional archive by quantifying three factors that influence Fo population distributions: (a) the frequency at which an Mg‐rich basaltic liquid (in equilibrium with Fo90 olivine) intrudes the reservoir, (b) the pre‐existence of a polymodal distribution of olivine crystal sizes and their shapes (c) the effects of sectioning on apparent olivine core compositions. We find that most crystals lose their initial Mg‐rich composition if they are held at temperatures relevant to summit magma storage conditions (1,160–1,190°C) for more than 10 years. Thus, previous assertions that Mg‐rich olivine crystals at Kīlauea are scavenged from centuries‐old stored magmas are unrealistic. Our method permits critical evaluation of contrasting explanations of heterogeneous Fe‐Mg contents of olivine cargo: (a) different total durations of mush storage with partial diffusive erasure of compositional traits, or (b) coexistence of multiple chemically distinct magmas. Our approach provides general guidance for the conservative interpretation of temporal information preserved within olivine Fe‐Mg compositional archives. Plain Language Summary: Geochemical characterization of olivine crystals is a valuable petrological tool that provides clues into the composition of primitive (Mg‐rich) basaltic melts. Olivine from three recent Kīlauea eruptions (2008 and 2020 Halemaʻumaʻu lava lake at the Kīlauea summit, 2018 lower East Rift Zone) are analyzed for their iron (Fe), magnesium (Mg) and phosphorus (P) compositions. Analyzing concentration changes in slow diffusing elements like P in olivine and in fast diffusing elements like Fe‐Mg helps to understand growth, dissolution, and diffusive re‐equilibration cycles in the crystals. These cycles are inferred to have been caused by magma intrusions for all three eruptions. Then, we develop an interpretative framework that incorporates the disparate patterns of chemical heterogeneity revealed in the chemical maps of sectioned olivine crystals. Using numerical diffusion models, we evaluate how the frequency of Mg‐rich primitive intrusions, the size of the olivine crystals, and crystal sectioning affect the Fe‐Mg content within a given olivine population. We show that the variable compositions of olivine crystals erupted in Kīlauea lavas cannot result from remobilization of century‐old crystal mush. Key Points: Olivine crystals from three recent Kīlauea eruptions have sharp truncations in phosphorus zoning that mark olivine dissolution event(s)Dissolution‐recrystallization of high‐Fo olivine at Kīlauea can occur in less than a yearFo contents in Kīlauea olivine crystals reflects either a high frequency of magma recharges (>10 events/y) or short diffusion times (<5 years) [ABSTRACT FROM AUTHOR]

The intrusion of magma into Kīlauea's lower East Rift Zone in May 2018 led to the largest eruption along this segment of the volcano in over 200 years. As magma drained from the rift zone, leading to the collapse of Pu'u 'Ō'ō, pressure at the summit initially remained elevated and dropped at a slower rate compared to historical intrusion events. The anomalously long timescale of summit deflation suggests that the dike was fed from multiple sources. Here we show that dikes can serve as "dipsticks" of magma reservoirs and that the co‐evolution of dike growth and reservoir deflation constrains key magma transport parameters. Using coupled dike‐chamber models constrained by ground deformation and seismicity, we test four configurations of magma plumbing in order to illuminate which reservoirs and transport pathways were activated during the intrusion phase (30 April to 3 May) of the 2018 event. Slow summit deflation relative to the rate of dike propagation is best explained by a model in which the dike initiates from a compressible magma reservoir in the East Rift Zone, which then drains magma upstream from the Halema'uma'u reservoir through a shallow transport system. We use a Bayesian Markov chain Monte Carlo (MCMC) approach to estimate storage parameters for both reservoirs as well as the effective conductivity of the shallow magma transport system in the East Rift Zone, finding good agreement with independent estimates. Our results suggest that the rupture of reservoirs from within the East Rift Zone presents a unique hazard at Kīlauea. Plain Language Summary: We investigate early phases of the 2018 unrest at Kīlauea Volcano, Hawai'i. Between 30 April and 3 May, small earthquakes accompanied magma movement through the lower East Rift Zone (LERZ). At the same time, the ground surface at the summit and the lava lake within the Halema'uma'u crater began to drop, indicating that magma was being withdrawn from the shallow summit reservoir. If the shallow summit reservoir were the only source of magma for the LERZ intrusion, we would expect the timescales of magma intrusion into the LERZ and out of the summit region to be similar. However, the rate of ground subsidence at the summit is anomalously slow given the rate of magma movement through the rift zone. We propose that an additional source of magma fed the intrusion, and we test physics‐based models for the growth of a dike fed by multiple magma chambers. By comparing the model output to data, we constrain the additional source of magma to be a large and/or compressible reservoir within the East Rift Zone. We estimate how easily magma can flow through the shallow East Rift Zone. Our work highlights the hazard posed by magma stored within the flanks of the volcano. Key Points: Deflation of Kīlauea's summit was anomalously slow during the intrusion of magma into the lower East Rift Zone in 2018Dynamic models suggest the dike broke out from a shallow East Rift Zone reservoir, buffering pressure upstream in the Halema'uma'u reservoirTemporal evolution of dike growth and summit deflation constrain magma conductivity through a shallow East Rift Zone transport pathway [ABSTRACT FROM AUTHOR]

We investigated essential factors controlling magma fragmentation during the 2011 sub‐Plinian eruptions of Kirishima Shinmoe‐dake volcano, Japan, based on observational data and the analysis of the conduit flow system. Key observational data of the Shinmoe‐dake eruptions are geophysically estimated magma discharge rate and petrologically measured crystallinity and porosity at the time of fragmentation. Using formulations of fragmentation criterion and conduit flow model, we derived analytical expressions describing the relationship among magmatic and geological parameters at the time of strain rate‐induced or stress fragmentation. The combination of the observational data and the analytical expressions revealed the relationship among weakly constrained parameters at the fragmentation, such as pressure, gas permeability in the magma, conduit radius, and maximum packing fraction of crystals, β*. From this relationship, we found that a smaller β*, less than approximately 0.5, is necessary for the fragmentation to occur for a wide range of the conduit radii under realistic pressure and gas permeability values. This reflects that in the case of the sub‐Plinian eruption with a moderate magma discharge rate, a drastic increase in magma viscosity induced by the smaller β* plays a crucial role in the occurrence of fragmentation. The strong dependence of the fragmentation process on β* indicates that the suspension rheology of crystal‐bearing magma is an essential factor controlling the magma ascent process during the moderate explosive eruption. Plain Language Summary: Magma ascent from the magma chamber to the surface through a volcanic conduit is an important process that controls the style and intensity of volcanic eruptions. In explosive eruptions, magma fragmentation during the magma ascent is a crucial process causing high‐speed ejection of gas and pyroclasts, leading to hazardous surface phenomena such as eruption clouds and pyroclastic flows. Since direct observation of the magma ascent is impossible, it is necessary to integrate indirect observations, models, and experiments for detailed understanding. We investigated essential factors controlling magma fragmentation during moderate‐intensity explosive eruptions by combining observational data and newly developed analytical expressions describing the parameter relationship at fragmentation. The target of our study is the 2011 explosive eruption of Shinmoe‐dake volcano, in which a typical moderate‐intensity eruption was densely observed by various methods. By utilizing these observations, we revealed that during moderate‐intensity explosive eruptions, a drastic increase in magma viscosity induced by magma crystallization plays a crucial role in the occurrence of fragmentation. The viscosity increase with crystallization is closely related to the microscopic structure of crystal‐bearing magma. Therefore, combining petrological information and conduit flow modeling is essential for a detailed analysis of the magma ascent process during moderate‐intensity explosive eruptions. Key Points: Observations of the Shinmoe‐dake eruptions, which provide constraints on magma fragmentationDerivation of analytical expressions describing the relationship among parameters at fragmentationStrong effects of magma rheology on fragmentation during moderate‐intensity explosive eruptions [ABSTRACT FROM AUTHOR]

Okmok volcano located on the northeastern part of the Umnak Island is one of the most active volcanoes in the Aleutian Arc. It was initially built as a large shield volcano, but 12,000 and 2,050 years ago, it experienced two caldera‐forming eruptions that destroyed the central part of the volcano. The post‐caldera eruptions have occurred mostly along the inner perimeter of the caldera from cinder and tuff cones. Here, we use the local earthquake data of the Alaska Volcano Observatory (AVO) in the time period from 2003 to 2017 to build a model with the 3D distributions of the P and S wave velocities and Vp/Vs ratio. At depths of more than 10 km, we observe a vertically aligned anomaly of high Vp/Vs ratio interpreted as a long‐lived conduit likely responsible for the volcano evolution since its origin. Above this conduit, we reveal a large anomaly of high Vp/Vs ratio representing the main magma reservoir that is connected with several shallow anomalies located below the centers of recent eruptions in the caldera. One of such anomalies represents a large shallow reservoir below the Cone A, which was the source of most of Okmok's historical eruptions. The most recent eruption occurred in 2008 and was fed by a magma diapir that was initially formed in the central magma reservoir and then slowly ascended through a ductile silica rich layer at depths from 6 to 2 km. This interpretation is consistent with the petrology studies and modeling of ground deformations. Plain Language Summary: Okmok is one of the most active volcanoes of the Aleutian Arc. Initially it was formed as an isometrical shield volcano, which was later destroyed by two caldera‐forming eruptions. The post‐caldera activity mostly occurred at several cones distributed along the inner perimeter of the caldera. The latest eruption with the explosivity index of VEI 4 took place in 2008. This eruption, which produced a new large cone Ahmanilix in the northeastern part of the caldera, was significantly different in composition and eruption style compared to other intra‐caldera eruptions. We present a new seismic tomography model, which was constructed based on the arrival times of the P and S waves from local seismicity. Below 10 km depth, we observe an anomaly of high Vp/Vs ratio, which may represent a steady magma conduit that is responsible for the long‐term formation of the entire Okmok volcanic complex. In the upper crust, the model reveals a series of separate local magma sources beneath volcanic centers where historical eruptions took place. The 2008 eruption was fed by a magma diapir that was initially formed in the deep conduit and then slowly ascended through a ductile layer at depths from 6 to 2 km. Key Points: The new crustal model beneath the Okmok Caldera with the 3D distributions of Vp, Vs and Vp/Vs ratio reveals the geometry of magma sourcesBelow 10 km depth, an anomaly of high Vp/Vs ratio indicates the location of the main magma conduit that fed the long‐term formation of OkmokAt shallow depths, seismic anomalies reveal several magma sources that fed eruptions along the inner perimeter of the caldera [ABSTRACT FROM AUTHOR]

Results from nine microgravity campaigns from Kı̄lauea, Hawaiʻi, spanning most of the volcano's 2008–2018 summit eruption, indicate persistent mass accumulation at shallow levels. A weighted least squares approach is used to recover microgravity results from a network of benchmarks around Kı̄lauea's summit, eliminate instrumental drift, and restore suspected data tares. A total mass of 1.9 × 1011 kg was determined from these microgravity campaigns to have accumulated below Kı̄lauea Caldera during 2009–2015 at an estimated depth of 1.3 km below sea level. Only a fraction of this mass is reflected in surface deformation, and this is consistent with previously reported discrepancies between subsurface mass accumulation and observed surface deformation. The discrepancy, amongst other independent evidence from gas emissions, seismicity, and continuous gravimetry, indicate densification of magma in the reservoirs below the volcano summit. This densification may have been driven by degassing through the summit vent. It is hypothesized that during the final years of the summit eruption, magma densification resulted in a buildup of pressure in the reservoirs that may have contributed to the lower East Rift Zone outbreak of 2018. The observed mass accumulation beneath Kı̄lauea could not have been detected through other techniques and illustrates the importance of microgravity measurements in volcano monitoring. Plain Language Summary: Kı̄lauea, Hawaiʻi, is one of the best known shield volcanoes in the world. In 2008, a summit eruption began and lasted for over a decade, ending with an outbreak in 2018 from the volcano's lower East Rift Zone (ERZ) that destroyed hundreds of homes. Over the course of the 2008–2018 summit eruption, nine microgravity campaigns were completed around the volcano's caldera. These campaign measurements provide unique insight into mass changes below the surface of the volcano that would not have been detected otherwise. The results show that during 2009–2015, mass was steadily accumulating in the magma reservoirs below Kı̄lauea's summit. Usually, the accumulation of mass below volcanoes is accompanied by surface inflation, but for Kı̄lauea it was found that the mass increase was much larger than expected for the observed increase in volume, indicating densification of gas‐rich magma in the reservoir. Magma densification may lead to a mass increase without significantly increasing the source volume. The subsequent pressure increase may have contributed to the 2015 summit intrusion and the 2018 lower ERZ eruption. Key Points: Nearly a decade of microgravity campaign data from Kı̄lauea during its 2008–2018 eruption are analyzed using a weighted least squares approachSubsurface mass accumulation between 2009—2015 of order 1.9 × 1011 kg at 1.3 km depth b.s.l. without commensurate surface deformationCompression and densification of gas‐rich magma may have accommodated increased pressurization in the magmatic system [ABSTRACT FROM AUTHOR]

The Longgang volcanic field (LVF) is one of the largest intraplate monogenetic volcanic regions in China, and eruption lasted from the early Pleistocene to the Holocene. Previous research shows that the LVF is not only experiencing rapid surface uplift but also significant seismic activity. The magmatic system under the LVF poses a hazard of potential eruption. The three‐dimensional electrical resistivity model developed by magnetotelluric data shows that the upper crust of the LVF and the surrounding areas hosts a high‐resistivity body composed of volcanic rocks produced by massive eruptions during past volcanic episodes. There are two main high‐conductivity structures below the depth of 10 km in the southwest and northeast of the LVF. The two structures merge in the lower crust and extend to the mantle. We interpret these high‐conductivity structures to be magmatic channels (the resistivity of C1 is 20–50 and C2 is 5–20 Ωm). Magmatic channel upwelling along the C1 may have caused the eruption of the Jinlongdingzi volcano 1,700 years ago. The C2 magmatic channel in the northeast of the LVF is larger and lower resistivity than C1. The modern surface uplift and seismicity in the LVF is a result of upwelling and intrusion along C2 magmatic channel. The Hunjiang fault passing through the LVF may also influence the upwelling of material along the C2 magmatic channel, posing a hazard of future eruption in the LVF. The high‐conductivity structures in the LVF pass through the Moho and extend to the upper mantle and are connected from the top to bottom. Combined with geochemical data, we speculate that the magmatic system in the LVF may directly come from the upper mantle and the magma has experienced rapid ascent with limited crustal internal evolution. Plain Language Summary: The Longgang volcanic field (LVF) is one of the main active volcanic regions in Jilin Province, northeast China. It is densely populated and economically developed, and it is also the most important food production place and tourist region in China. Therefore, the risk of volcanic and seismic activity in this area has always been the focus of attention and research. Through a geophysical method called Magnetotellurics, we develop a three‐dimensional electrical resistivity model of the LVF. The model reveals two crustal and mantle scale magmatic channels C1 and C2. The modern crustal uplift and seismicity are result of upwelling and intrusion of the C2 magmatic channel, posing a hazard of future eruption in the LVF. The results of this paper can provide deep geophysical basis for the prediction of volcanic magma system and eruption risk. Key Points: Broadband magnetotelluric data were used to generate a three‐dimensional electrical structure of the Longgang volcanic field (LVF) and adjacent areasThe modern crustal uplift and seismicity in the LVF is a result of upwelling and intrusion of the electrically conductive zone interpreted as a magmatic channelThe magmatic system in the LVF is sourced from the deep mantle and has experienced rapid uplift and limited crustal internal evolution [ABSTRACT FROM AUTHOR]

Biotite (sensu lato) is a widespread rock‐forming mineral in magmatic rocks that can be stable in a broad range of pressure and temperature, but appropriate biotite thermometers or barometers are lacking. Based on a collected experimental dataset (n = 839, T = 625–1,325°C, P = 1–48 kbar) containing biotites that span a wide compositional range [e.g., Mg/(Mg + Fe) = 0–1, TiO2 = 0–9 wt%], we have trained several machine learning algorithms for calibrating a biotite thermobarometer. Our evaluation on model performance reveals that the thermobarometry derived from extremely randomized trees is the best option, which returns coefficients of determination (R2) ≥0.97 for estimating both temperature and pressure using either biotite‐only or biotite + melt model. The model reliability were evaluated using three different approaches for the biotite‐only and biotite + melt thermobarometers respectively, including Monte Carlo cross‐validation (RMSEs are 65°C and 4.7 kbar, 38°C and 3.2 kbar, respectively), testing with independent test set (RMSEs are 54°C and 4.4 kbar, 35°C and 2.4 kbar, respectively), and error propagation from assumed analytical uncertainty (2*MAD are 54°C and 1.27 kbar, 10°C and 1.26 kbar, respectively). Quantified relative importance of involved components in the thermobarometers supports an intrinsic control of thermodynamics in the stability of biotite as a function of pressure and temperature. We applied the new biotite thermobarometer for biotite‐bearing andesitic, phonolitic and rhyolitic volcanic systems provide reliable constraints of temperature and pressure for magma storage, ascent, and evolution. We also offer a user‐friendly webpage for online performance of the thermobarometers (https://lixiaoyan.shinyapps.io/Biotite_thermobarometer/). Plain Language Summary: Biotite is a common rock‐forming mineral in a variety of igneous rocks, and its chemical composition has been widely used to retrieve associated magma properties and magmatic processes. Temperature and pressure are the two most important intensive parameters of magmatic processes, but a reliable biotite‐based thermobarometry is lacking in literature, in contrast to the considerable attentions that have been paid to other rock‐forming rocks such as clinopyroxene and amphibole. In this study, we provide a robust biotite thermobarometer based on machine learning methods. Several potentially useful algorithms have been tested on a carefully collected experimental data that include full information of temperature, pressure, and biotite and coexisting melt compositions. Our evaluations demonstrate that an optimized thermometry model comes from the method called extremely randomized trees. We also applied the proposed biotite thermobarometer to natural biotites from volcanic eruptions, including andesite (Central Volcanic Zone of Andes), phonolite (Canary Islands), and rhyolite (Bishop Tuff), and the estimated temperature and pressure are informative and complementary to the pre‐eruption conditions estimated from other thermobarometers. In addition, we also provide a webpage that can be easily used for applying our biotite thermobarometry. Key Points: Experimental data of biotite‐bearing magmas are used to train machine learning algorithms for establishing thermobarometerThe method of Extremely Randomized Trees is demonstrated to provide an optimal biotite‐based thermobarometerApplication of the thermobarometer to biotite‐bearing volcanic systems provides reliable constraints on magmatic processes [ABSTRACT FROM AUTHOR]

Early arrival traveltime tomography and full waveform inversion were conducted on downward continued streamer seismic data at Dante's Domes oceanic core complex (OCC), providing unprecedented details of shallow P wave velocity structure. Together with reverse time migration images, seafloor morphology, in situ geological samples, magnetic and gravity data, the seismic constraints are used to infer the lithological distribution along the seismic profiles. Based on the striking similarity in velocity structure beneath the corrugated domes with other OCCs and drilling results from Atlantis Massif, we confidently reconfirmed the Southern Dome as dominantly gabbroic rocks, and the Northern Dome as serpentinized peridotites. A series of isolated gabbroic bodies embedded in the diabase and basaltic layers is observed in the breakaway zone, suggesting that the initiation of Dante's Domes OCC occurred over a long period during which there were several failed attempts to form a long‐lived detachment fault. This early development of the OCC probably occurred under a regime of alternating magma starvation and magma replenishment. The predominantly gabbroic section, beneath the Southern Dome and extending to termination, indicates the OCC has been created with relatively high magma flux. We also imaged distinct shallow subseafloor reflections which are also termed as D reflectors underneath the corrugated domes. The location of the D reflectors is similar to those in the Atlantis Massif, with depths well correlated with the top of exhumed gabbroic bodies and the discontinuities in the D reflectors between gabbroic bodies. Our findings contribute to the understanding of processes controlling the OCCs initiation and evolution at slow spreading ridges. Plain Language Summary: The lithospheric structure generated along the mid‐ocean ridges is determined by the magmatic accretion and tectonic extension, the spatiotemporal variation of which generally depends on the spreading rates. Along slow spreading ridges, tectonic extension dominates and sometimes forms oceanic core complexes (OCCs) generated by heterogeneous exhumed lower crustal and upper mantle section caused by long‐lived, large‐offset detachment faults. The Dante's Domes OCC, located between 26°32′N and 26°40′N on the African plate along the Mid‐Atlantic Ridge, was created by a ∼18‐km‐long detachment fault. Here detailed structural variability within the Dante's Domes OCC was obtained from advanced seismic imaging techniques. The lithological distribution interpreted from seismic imaging, which could be employed to infer magmatic accretion and tectonic extension behaviors, improved our understanding of the initiation and evolution of Dante's Domes OCC. A series of linear ridges are interpreted as breakaways where the OCC initiated, suggesting that the Dante's Domes OCC experienced several failed attempts to form long‐lived detachment fault, and was not exhumed quickly as previously postulated. Key Points: Detailed structure of Dante's Domes oceanic core complex (OCC) was obtained from downward continued streamer waveform inversion and reverse time migrationA series of narrow ridges interpreted as breakaways indicates that the initiation of Dante's Domes OCC was not fast as previously postulatedVelocity contrasts of the shallow low‐velocity and fractured layer and deeper high‐velocity gabbros contributes to the shallow reflectivity [ABSTRACT FROM AUTHOR]

Large and easily accessible porphyry Cu deposits have already been identified, exploited, and gradually exhausted. Novel strategies, therefore, are required to identify new, deeply buried deposits. Previous studies have proposed several lithogeochemical and mineralogical approaches for identifying porphyry Cu systems. Most of these methods, however, require significant a priori knowledge of the exploration region and are, generally, of low effectiveness. In this study, machine learning models using Random Forest and Deep Neural Network algorithms are utilized to characterize magma fertility. The two models have first been trained using a large trace‐element data set of magmatic zircon and then validated on unseen data set during the training process. The performance of both models was evaluated using a fivefold cross‐validation technique, which demonstrates that the models provide consistent results and yield good classification accuracy (∼90% on average) with low false positive rates. Feature importance analysis of the models suggests that Eu/Eu*, Eu/Eu*/Y, Ce/Nd, Ce/Ce*, Dy, Hf, and Ti are the important parameters that distinguish fertile and barren zircons. The real‐world applicability of the validated models was evaluated using two well‐characterized porphyry Cu deposits in subduction and postcollisional settings—the Highland Valley porphyry Cu district (south‐central British Columbia, Canada) and the southern Gangdese belt (Tibet, China), respectively. The results demonstrate that our generalized models can discriminate zircon from igneous rocks associated with porphyry Cu deposits from those in nonmineralized systems with high accuracy and independent of geological setting, suggesting that this new approach can be used effectively in greenfield and brownfield exploration. Plain Language Summary: Mineral resources, which are important to the development of human society, are increasingly being consumed as our need for technological advancement increases. With this consumption comes the need to identify additional resources, many of which may be invisible at the Earth's surface. This challenges current exploration methods, requiring novel techniques to be developed to identify variably mineralized rock below Earth's surface. In this study, machine learning methods are trained using a global data set of zircon chemistry to evaluate the prospectivity of porphyry Cu mineralization in magmatic districts. This work demonstrates that machine learning methods provide a robust, accurate, and effective approach to identifying porphyry Cu mineral resources. Key Points: A global data set of magmatic zircon trace‐element chemistry is used to train two separate machine learning algorithmsThe Eu/Eu*, Eu/Eu*/Y, Ce/Nd, and Ce/Ce* ratios, and Dy, Hf, and Ti contents are important for distinguishing fertile and barren zirconsThe performance of the machine learning models is not affected by regional differences and geological setting [ABSTRACT FROM AUTHOR]

Crustal magmatic systems likely consist of magmatic reservoirs dominated by crystal mush. Recent studies suggest that the physical processes occurring in crystal mush could alter the response of magmatic reservoirs during volcanic unrest. Here, we present a magma chamber deformation model that incorporates two new aspects in crystal mush: heat and exsolved gas. The model is based on earlier studies by Liao et al. (2021, https://doi.org/10.1029/2020JB019395) with additional processes including thermal‐mechanical coupling, dependence of material properties on gas content, and temperature evolution following an injection of hotter magma. The post‐injection time‐dependent evolution of the system can be grouped into three periods, which are dominated by poroelastic diffusion (short term), viscoelastic relaxation (mid term), and thermal equilibration (long term). All three time‐regimes are strongly affected by gas distribution, which alters the relative compressibility of the crystal‐rich and crystal‐poor regions in the chamber. The contribution of thermal evolution emerges during the mid‐term evolution. The time‐dependent evolution of the system highlights the intrinsic ability of a gas‐bearing mushy magma chamber to generate non‐monotonic time series of stresses, deformation, and magma transport. Plain Language Summary: The processes that occur in magmatic reservoirs and their surrounding crustal rocks can modulate the triggering, duration, and style of volcanic eruptions. Although magmatic reservoirs are typically modeled as cavities filled with fluid magma, geophysical and petrological observations have long suggested that they likely contain crystal mush, an ensemble of solid crystals and fluid (gas/melt) that reside in interstitial pore spaces. Some recent studies show that a mushy magma chamber could behave differently than a fluid‐filled chamber, leading to different interpretation of observations such as ground deformation. Here, we extend these studies to incorporate two new aspects that are typical for crustal magmatic systems: gas phase (in the form of disconnected gas bubbles) and nonuniform temperature. The incorporation of these two new aspects allows for more processes that can be examined quantitatively, and a more realistic depiction of crustal magmatic reservoirs. We find that when gas and heat distribute unevenly, a simple magma injection event could result in complex time‐dependent changes in pressure, stress, and magma flows in the chamber. Key Points: A physics‐based model explores the influence of gas and heat in the a magma chamber containing crystal mush upon magma injectionThe non‐uniform distributions of gas and heat cause non‐monotonic time evolution of deformation, crustal stresses, and magma transportPoroelastic diffusion, viscous relaxation, and thermal equilibration lead to three time regimes in the post‐injection evolution [ABSTRACT FROM AUTHOR]

As volcanoes undergo unrest, understanding the conditions and timescales required for magma reservoir failure, and the links to geodetic observations, are critical when evaluating the potential for magma migration to the surface and eruption. Inferring the dynamics of a pressurized magmatic system from episodes of surface deformation is heavily reliant on the assumed crustal rheology, typically represented by an elastic medium. Here, we use Finite Element models to identify the rheological response to reservoir pressurization within a temperature‐dependent Standard Linear Solid viscoelastic ("thermo‐viscoelastic") domain. We assess the mechanical stability of a deforming reservoir by evaluating the overpressures required to initiate brittle failure along the reservoir wall, and the sensitivity to key parameters. Reservoir inflation facilitates compression of the ductile wall rock, due to the non‐uniform crustal viscosity, impacting the temporal evolution of the induced tensile stress. Thermo‐viscoelasticity enables a deforming reservoir to sustain greater overpressures prior to failure, compared to elastic analyses. High‐temperature (e.g., mafic) reservoirs fail at lower overpressures compared to low‐temperature (e.g., felsic) reservoirs, producing smaller coincident displacements at the ground surface. The impact of thermo‐viscoelasticity on reservoir failure is significant across a wide range of overpressure loading rates. By resisting mechanical failure on the reservoir wall, thermo‐viscoelasticity impacts dyke nucleation and formation of shear fractures. Numerical models may need to incorporate additional processes that act to promote failure, such as regional stresses (e.g., topographic and tectonic), external triggers (e.g., earthquake stress drops), or pre‐existing weaknesses along the reservoir wall. Plain Language Summary: Understanding the conditions required for magma reservoir failure, which can result in the transport of magma to the ground surface, and its links to ground deformation is a persistent challenge when assessing the eruption potential of a volcano. Numerical models are commonly used to account for different complexities observed in nature, which can significantly affect the ambient subsurface stress field or the threshold for reservoir failure. Most numerical models do not consider the impact of crustal behavior; whilst the results can be instructive, an accurate model is of paramount importance when modeling reservoir failure. Here we investigate the conditions for failure within a thermo‐viscoelastic crust, accounting for the increased temperatures and ductility surrounding the reservoir. Our results highlight that an inflating reservoir results in the compression of the surrounding crustal rock due to the non‐uniform viscosity, which is not observed in simpler elastic models. This hinders the development of the tensile stress, and may require overpressures greater than usually considered to initiate reservoir failure. As the behavior of the crust is controlled by its temperature, and can strongly affect the conditions for reservoir failure, we suggest that thermo‐viscoelasticity should be incorporated in deformation models and failure analyses. Key Points: Reservoir failure within a thermo‐viscoelastic crust is controlled by thermal heterogeneity and the associated viscous timescalesNon‐uniform crustal viscosity enables compression of the surrounding host rock, altering the tensile stress evolution around the reservoirLow‐temperature reservoirs sustain elevated overpressures prior to failure, due to long viscous timescales, increasing surface deformation [ABSTRACT FROM AUTHOR]

The magma plumbing in the lower crust beneath the Coso volcanic field (CVF) remains controversial, largely because of the absence of high‐resolution lower crustal velocity models. For the first time, we develop a high‐resolution crustal P‐wave velocity model for the Coso‐Ridgecrest region by jointly inverting 137,992 first P and 8,636 PmP travel‐time data using an eikonal equation‐based seismic reflection tomography method. More than half of the PmP travel times are picked from earthquakes after the 2019 Mw 7.1 Ridgecrest earthquake. Such abundant PmP travel times significantly improve the resolution of the lower crust. Our final velocity model reveals a prominent low‐velocity body sitting right beneath the CVF at 5–20 km depths, which we interpret as a rhyolite magma reservoir that supplies heat flux to the hot springs and also feeds the volcanic activities at Coso. We find that the upper‐middle crustal low‐velocity body dips southwards into the lower crust, extending to regions beneath the Indian Wells Valley and the Garlock Fault at depth greater than 20 km. We ascribe the lower crustal low‐velocity body (more than 4% Vp reduction) to a basaltic magma reservoir that connects the melts in the uppermost mantle with the eruptible rhyolitic reservoir at shallower depths. The basaltic magma reservoir constitutes an important part of a continuous N‐S elongated crustal magma plumbing system beneath the CVF, formed as a combined result of local extension, faulting, and stress distribution. Plain Language Summary: The deep origin of the Coso volcanic field (CVF) in the lower crust is not fully understood. To well constrain the deep origin of magmas, we use a newly developed seismic reflection tomography method to image the entire crustal P‐wave velocity structure beneath the CVF by incorporating the direct P and the Moho reflected waves. Tomographic results reveal a significant low‐velocity anomaly right beneath the CVF in the upper‐middle crust, which dips southwards into the lower crust and extends to the Indian Wells Valley and the Garlock Fault. The low‐velocity anomaly in the upper‐middle crust is interpreted as a rhyolite magma reservoir (contains mostly crystallized silicic magma) directly supplying hot springs at or near the surface, whereas the low‐velocity anomaly in the lower crust is interpreted as a basaltic magma reservoir (contains fewer silica contents) that constitutes a continuous magma plumbing system mobilizing melts from the upper mantle along N‐S direction into the more eruptible shallower Coso reservoir. Key Points: We obtain a new Vp model of the whole crust beneath the Coso volcanic field by inverting the first and Moho reflected P‐wave arrivalsOur model shows a pervasive low‐velocity anomaly in the upper‐middle crust, which dips southwards beneath the Coso volcanic fieldLower crust low Vp may represent a basaltic magma reservoir that ascends northwards and differentiates into a shallower rhyolite reservoir [ABSTRACT FROM AUTHOR]

Towada volcano is an active volcano in northeast Japan, that caused two large caldera‐forming eruptions at 36 ka (episode N) and 15.5 ka (episode L). Petrological analyses and phase equilibrium experiments were performed on silicic endmember rhyolitic pumices from these two eruptions to constrain the pre‐eruptive magma storage conditions. A common mineral assemblage of plagioclase + orthopyroxene + clinopyroxene + magnetite + ilmenite was observed in both eruptions, whereas amphibole (hornblende) was observed only in the episode L pumice. Mineral thermometers presented temperatures of 832–883°C and 802–870°C for the episode N and L pumices, respectively, with an oxygen fugacity of ∼nickel–nickel oxide (NNO) + 1 in log units. The water‐saturation pressures evaluated using the plagioclase‐melt hygrometer were in the range of ∼100–200 MPa. Experiments at 100–350 MPa and 825–900°C under water‐saturated and NNO‐buffered conditions successfully reproduced the mineral assemblages in both pumices, except for magnetite. Further constraints by phase compositions and phase proportions resulted in the preferred storage conditions of 840–850°C and 150–170 MPa for both eruptions. The emergence of hornblende in the episode L magma was likely caused by the higher CaO content compared to the episode N magma and not by the difference in the storage conditions. The storage depths at ∼5–7 km coincide with the depth of the low seismic velocity beneath the present Towada volcano, suggesting possible magma accumulation at the same depths as in the past caldera‐forming eruptions. Plain Language Summary: Experimental constraints on the pre‐eruptive storage conditions of voluminous SiO2‐rich magma are fundamentally important to mitigate the risks of serious future volcanic hazards. Towada volcano is an active volcano located in northeast Japan, that caused two large caldera‐forming eruptions 36 and 15.5 kyr ago. Hornblende is present only in the later eruption, potentially reflecting differences in magma storage conditions and/or bulk compositions. We conducted water‐saturated phase equilibrium experiments on the most SiO2‐rich pumices derived from two caldera‐forming eruptions at Towada volcano to find the pressure and temperature conditions that best explain the mineral assemblage, phase compositions, and phase proportions recorded in the natural pumice. Our preferred pre‐eruptive conditions were 840–850°C and 150–170 MPa for both eruptions under water‐saturated conditions at oxygen fugacity buffered by nickel–nickel oxide. We emphasize the importance of subtle differences in the whole rock compositions to crystallize hornblende in the later eruption. The pre‐eruptive storage depths of 5–7 km coincide with the depth of the low seismic velocity signal beneath the present Towada volcano. This suggests that magma, if present, is stored at the same depth as in the caldera‐forming stage. Key Points: Water‐saturated phase equilibrium experiments successfully reproduced the mineral assemblages in the two magmas, except for magnetitePre‐eruptive depths at 5–7 km for the two magmas coincide with the depth of the low seismic velocity signal beneath the volcanoHornblende crystallization in the later eruption was likely due to the difference in bulk compositions, but not in storage conditions [ABSTRACT FROM AUTHOR]

The transcrustal, mush‐dominated magma storage paradigm, which posits liquid melt is heterogeneously distributed within a vertically extensive magma mush, differs significantly from classical geodetic models, where melt is stored within liquid‐dominant chambers within an elastic crust. Here, we present mechanical models consistent with transcrustal melt storage by separating the magmatic system into three domains: liquid melt lenses, surrounding crystal‐dominated poroelastic magma mush, and elastic crust. Our results indicate that pressure changes within the melt lens may induce surface displacements that approximate the displacements predicted by spheroidal pressure sources that mimic the geometry of the mush zone. Adopting constitutive parameters of the mush dependent on mush melt fraction, we show that a magma storage system will have an effective geometry inferred from surface displacements that smoothly transitions from the geometry of the melt lens to the geometry of the mush as mush melt fraction increases. This holds true across multiple storage zone geometries, including a "transcrustal" storage zone with a magma mush that extends deep in the crust. Accounting for the presence of a magma mush can lead to an increase in the estimated volume of injected or withdrawn magma (by several multiples) compared to values obtained using fully elastic models. Comparing erupted magma volumes to source volume changes allows for an estimation of magma compressibility; we show the presence of a mush can increase this estimated magma compressibility by up to approximately 50%, suggesting magmas may have higher bubble fraction than previous geodetically derived estimates. Plain Language Summary: Different kinds of geophysical observations provide differing views into the nature of magma storage regions within the crust. Many geophysical observations, such as seismic data, indicate that melt is stored within discrete, horizontally elongate pockets that are contained within a "mush" of solid crystals and liquid melt. However, surface deformations (geodetic data) are typically explained with models that ignore the presence of a magma mush. In this paper, we attempt to reconcile these views of magma storage regions, by incorporating the effects of a magma mush into models of ground deformation. Our results show that ground deformation measurements are potentially more sensitive to the magma mush than the discrete pockets of melt within the mush, and that appropriately accounting for the presence of a magma mush can change estimates of important quantities, such as the volume of injected magma and magma compressibility. Key Points: We develop a numerical poroelastic model for surface displacements induced by melt migration within a mush‐dominated melt storage regionSurface displacements may be more sensitive to the geometry of a low melt content mush than a melt‐rich lens within the mushAccounting for magma mush zones leads to important differences in estimation of quantities of interest, such as inferred volume change [ABSTRACT FROM AUTHOR]

Crystals retain an imprint of the dynamic changes within a magma reservoir and hence contain invaluable information about the evolving conditions inside volcanic plumbing systems. However, instead of telling a single, simple story, they comprise overprinted evidence of numerous processes relating to temperature, pressure and composition that drive crystal precipitation and dissolution in magmatic systems. To decipher these different elements in the story that crystals tell, we attempt to identify the observational signatures of a simple, yet ubiquitous process: crystal precipitation and dissolution during magma cooling. To isolate this process in a complex magmatic system with intricate dynamic feedbacks, we assume that synthetic crystals precipitate and dissolve rapidly in response to deviations from thermodynamic equilibrium. In our crystalline‐scale simulations, synthetic crystals drag along the cooler‐than‐ambient melt in which they precipitated and can drive a temperature‐dependent, crystal‐driven convection. We analyze the non‐dimensional conditions for this coupled convection and record the heterogeneous thermal histories that synthetic crystals in this flow regime experience. We show that many synthetic crystals dissolve, loosing their thermal record of the convection. Based on our findings, we suggest that heterogeneity in the thermal history of crystals is more indicative of local, crystal‐scale processes than the overall, system‐wide cooling trend. Plain Language Summary: Similar to tree rings, crystals tell a story of their past through their sequential growth records. However, unlike trees, crystals are dynamic and can travel throughout the magmatic domain. We hypothesize that this mobility may result in a bias in the crystal record and prevent it from accurately recording the magma reservoir history. In order to understand what patterns natural sample crystals would record during cooling, we built a simulator that tracks individual crystals and the thermal environment they experience. We simulate how crystals drive flow, but also precipitate and dissolve in response to temperature change. Our results show that each synthetic crystal has a unique record of its past that is dependent on the temperature distribution within its vicinity. Additionally, most of the synthetic crystals dissolve in the convection, so we lose their record. Overall, the synthetic crystals in our simulation can reproduce some of the variable patterns that are often found in natural samples, suggesting that real crystals are imperfect witnesses of the magmatic dynamics they are exposed to. Key Points: Crystals in convection preserve the least information about the most dynamic flow domainCrystals in convection show signs of dissolution, suggesting a slower than actual cooling rateCrystals can record cooling, heating, and complex thermal histories even during steady cooling [ABSTRACT FROM AUTHOR]

The Campi Flegrei caldera is considered the most dangerous volcano in Europe and is currently in a new phase of unrest (started in 2000 and still ongoing) that has persisted intermittently for several decades (main crisis occurred from 1950 to 1952, 1970–1972, and 1982–1984). Here, by combining the petrological and geochemical data collected in recent decades with numerical simulations, we place new constraints on the source(s) of the current dynamics of the volcano. In particular, we show that the measured (N2‐He‐CO2) geochemical changes at the fumaroles of Solfatara hydrothermal site are the result of massive (about 3 km3) magma degassing in the deep portion (≥200 MPa, 8 km of depth) of the plumbing system. This degassing mechanism would be able to flood the overlying hydrothermal system with hot gas, thus heating and fracturing the upper crust inducing shallow seismicity and deformation. This implies that the deep magma transfer process (≥8 km) has been decoupled from the source of deformation and seismicity, localized in the first kilometers (0–4 km) of caldera‐filling rocks. This information on magma transfer depth can have important implications for defining the best monitoring strategies and for forecasting a future eruption. Finally, this study highlights how petrological and geochemical data allow us to explore the dynamics of the deep portion of the plumbing system and thus trace the occurrence of recharge episodes, in a portion of the ductile lower crust where magma transfer occurs in the absence of earthquakes. Plain Language Summary: Calderas are volcanic depressions formed as the ground collapses during huge volcanic eruptions. They often exhibit pronounced unrest, with frequent earthquakes, ground uplift, and considerable heat and mass flux that are monitored by volcanologists for eruption forecasting. However, as this activity is due to the complex interactions among magma and hydrothermal system stored beneath the volcano, it is always difficult to predict the evolution of the unrest toward critical conditions until to eruption. The Campi Flegrei caldera is among the most dangerous volcanos in Europe and is currently in a new phase of unrest that has lasted for several decades, whose nature (magmatic or not magmatic) has remained unclear. Here, we combine petrological and geochemical observations collected in recent decades with numerical simulations to place new constraints on the source of the recent dynamics of the volcano. In particular, we show that new deep magma has recharged the shallow reservoir beneath the volcano and flooded the overlying hydrothermal system with hot gas; thereby weakening the upper rocks allowing deformation (ground uplift) and fracturing (seismicity). This information is particularly important in the case of high‐risk Campi Flegrei caldera, because it can help to improve defense strategies in case of future eruption. Key Points: We explore recent magma dynamics under Campi Flegrei restless caldera (Italy)New petrological and geochemical data trace mafic magma recharges in the lower crustDeep‐gas flooding of hydrothermal system justifies shallow seismicity and deformation [ABSTRACT FROM AUTHOR]

Melt Fe3+/FeT ratios are widely used as a proxy for the oxygen fugacity (fO2) in magma systems. A particular challenge is a difficulty in quantifying the Fe2+ and Fe3+ contents, which are not easily measurable by conventional techniques. Using the Mn–Fe2+ exchange coefficient (KD) between olivine (ol) and melt (m), this study develops a novel approach to this problem using the relationship of Fem3+ ${\mathrm{Fe}}_{\mathrm{m}}^{3+}$/FemT ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$ = 1 − KDol/mMn−Fe2+ ${\mathrm{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ * (FeolT ${\mathrm{Fe}}_{\mathrm{ol}}^{\mathrm{T}}$/Mnol)/(FemT ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$/Mnm). The key for applying this equation is to combine well‐defined KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ with accurate Mn and FeT analyses of olivine and coexisting melt. The Mn and FeT analyses are readily measured by electron microprobe. To define KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$, we performed experiments at 1.0–1.75 GPa and 1250–1450°C under graphite‐buffered conditions, where FeT can be used as Fe2+ to calculate KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$. The results show that KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ = 0.730 ± 0.025 is nearly constant over a realistic range of experimental conditions (temperature, pressure, and melt composition) relevant to basaltic magmas, allowing calculation of melt Fe3+/FeT ratios without the need for any further knowledge of these parameters. A comparison with published data, where melt Fe3+/FeT ratios have been directly measured, demonstrates that our calculation method is as accurate as Mossbauer spectroscopy and XANES techniques, thus providing a very simple and accessible approach to melt Fe3+/FeT ratio estimation. However, it should be noted that the accuracy of this method is critically reliant on: (a) accurate Mn and Fe analyses and (b) olivine‐melt chemical equilibrium. Plain Language Summary: The melt Fe3+/FeT ratios are well constrained as a function of oxygen fugacity (fO2) but difficult to measure. As nearly all Fe in olivine is Fe2+ and all Mn in olivine and basaltic melt are Mn2+, the Mn–Fe2+ exchange coefficient (KD) between olivine (ol) and melt (m) can be developed into a simple back calculation method for melt Fe3+/FeT using the formulation Fem3+ ${\mathrm{Fe}}_{\mathrm{m}}^{3+}$/FemT ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$ = 1 − KDol/mMn−Fe2+ ${\mathrm{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$* (FeolT ${\mathrm{Fe}}_{\mathrm{ol}}^{\mathrm{T}}$/Mnol)/(FemT ${\mathrm{Fe}}_{\mathrm{m}}^{\mathrm{T}}$/Mnm). This equation only needs to know Mn and FeT contents in olivine and a coexisting melt and KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$. The former is measurable accurately by careful electron microprobe analysis. The latter is obtained here by the graphite‐buffered experiments, where FeT is used as Fe2+ to calculate KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$. We find that KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ (0.730 ± 0.025) is close to constant over the temperature, pressure, and melt composition range relative to most basalts on Earth, which allows us to calculate the melt Fe3+/FeT ratio with the above equation, providing a simple determination of the magma fO2. This method has an accuracy comparable to more complicated and less accessible Mossbauer spectroscopy and XANES techniques. Key Points: A new oxybarometer is developed based on olivine‐melt Mn‐Fe2+ exchange coefficient (KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$)KDol/mMn−Fe2+ ${\text{KD}}_{\mathrm{ol}/\mathrm{m}}^{\mathrm{Mn}-{\mathrm{Fe}}^{2+}}$ is nearly constant with the variation of experimental conditionsThe new oxybarometer provides a simple and efficient method for melt Fe3+/FeT calculation [ABSTRACT FROM AUTHOR]

Persistent volcanic activity is thought to be linked to degassing, but volatile transport at depth cannot be observed directly. Instead, we rely on indirect constraints, such as CO2‐H2O concentrations in melt inclusions trapped at different depth, but this data is rarely straight‐forward to interpret. In this study, we integrate a multiscale conduit‐flow model for non‐eruptive conditions and a volatile‐concentration model to compute synthetic profiles of volatile concentrations for different flow conditions and CO2 fluxing. We find that actively segregating bubbles in the flow enhance the mixing of volatile‐poor and volatile‐rich magma in vertical conduit segments, even if the radius of these bubbles is several orders of magnitude smaller than the width of the conduit. This finding suggests that magma mixing is common in volcanic systems when magma viscosities are low enough to allow for bubble segregation as born out by our comparison with melt‐inclusion data: Our simulations show that even a small degree of mixing leads to volatile concentration profiles that are much more comparable to observations than either open‐ or closed‐system degassing trends for both Stromboli and Mount Erebus. Our results also show that two of the main processes affecting observed volatile concentrations, magma mixing and CO2 fluxing, leave distinct observational signatures, suggesting that tracking them jointly could help better constrain changes in conduit flow. We argue that disaggregating melt‐inclusion data based on the eruptive behavior at the time could advance our understanding of how conduit flow changes with eruptive regimes. Plain Language Summary: Persistently degassing volcanoes typically erupt multiple times a day, emitting copious gas and thermal energy but relatively little magma. The ascent of the erupted magma is likely a byproduct of degassing, but it is unclear how exactly eruptive behavior and degassing are related. Without the ability to observe degassing processes at depth, we rely on erupted samples to derive constraints on pre‐eruptive flow conditions. Some of these samples seal in magma droplets named melt inclusions, which represent valuable snapshots of volatile concentrations at depth. Here, we use numerical simulations to study how magma flow in vertical conduit‐like structures at depth alters the volatile concentrations in melt inclusions. We demonstrate that volatile‐rich, ascending magma mixes with volatile‐poor, descending magma that has lost volatiles at the surface. The degree of mixing depends on the physical properties of magma and bubbles. This magma mixing, together with the carbon dioxide fluxing, can significantly shift the water and carbon dioxide concentrations in melt inclusions. Our study suggests that some degree of magma mixing is almost inevitable in persistently degassing volcanoes, but that mixing may vary considerably with depth. We suggest that melt inclusion data could potentially help tracking the evolving flow conditions in volcanic conduits. Key Points: Magma mixing occurs commonly at the interface between volatile‐rich and volatile‐poor magma in persistently degassing volcanoesBubble speed, magma viscosities, and bubble volume fraction control magma mixingMagma mixing and carbon dioxide fluxing leave distinct signatures in melt‐inclusion data [ABSTRACT FROM AUTHOR]

We use earthquake‐based adjoint tomography to invert for three‐dimensional structure of the North Island, New Zealand, and the adjacent Hikurangi subduction zone. The study area, having a shallow depth to the plate interface below the North Island, offers a rare opportunity for imaging material properties at an active subduction zone using land‐based measurements. Starting from an initial model derived using ray tomography, we perform iterative model updates using spectral element and adjoint simulations to fit waveforms with periods ranging from 4–30 s. We perform 28 model updates using an L‐BFGS optimization algorithm, improving data fit and introducing P‐ and S‐wave velocity changes of up to ±30%. Resolution analysis using point spread functions show that our measurements are most sensitive to heterogeneities in the upper 30 km. The most striking velocity changes coincide with areas related to the active Hikurangi subduction zone. Lateral velocity structures in the upper 5 km correlate well with New Zealand geology. The inversion reveals increased along‐strike heterogeneity on the margin. In Cook Strait we observe a low‐velocity zone interpreted as deep sedimentary basins. In the central North Island, low‐velocity anomalies are linked to surface geology, and we relate velocity structures at depth to crustal magmatic activity below the Taupō Volcanic Zone. Our velocity model provides more accurate synthetic seismograms with respect to the initial model, better constrains small (< $< $50 km), shallow (< $< $15 km) and near‐offshore velocity structures, and improves our understanding of volcanic and tectonic structures related to the active Hikurangi subduction zone. Plain Language Summary: We perform seismic imaging of the Earth's crust below the North Island of New Zealand, which sits above an active plate boundary known as the Hikurangi subduction zone. By comparing computer simulations of earthquake ground motion with recordings of ground motion, our imaging method iteratively improves models of Earth's subsurface structure. Our data set consists of earthquake waveforms from 1,800 unique source–receiver pairs. We incrementally update the seismic velocities of the initial model 28 times, resulting in velocity changes of up to ±30%. Variations in the subsurface structure are most strongly resolved in the upper 30 km. Seismic velocity structures in the upper 5 km correspond well with known surface geology. The strongest velocity changes correspond to regions related to the Hikurangi subduction zone, such as a deep sedimentary basin in Cook Strait, and anomalous velocity structures related to the Taupō Volcanic Zone. The newly derived velocity model improves predictions of earthquake ground motion and improves our understanding of volcanic and tectonic structures associated with the active Hikurangi subduction zone. Key Points: We develop a high‐resolution (4–30 s) 3D velocity model of the North Island of New Zealand with 28 adjoint tomography iterationsDistinct P‐ and S‐wave velocity changes of up to ±30% are made to the existing model in the upper 30 kmThe tomographic results provide improved images of tectonic and magmatic structures throughout the Hikurangi subduction margin [ABSTRACT FROM AUTHOR]

The effects of buoyant ridge subduction have been researched for decades. However, it remains unclear how this process influences magma chemistry. Here we use a compilation of geochemical data, well‐established geochemical proxies (i.e., Ba/Nb, Th/Nb) and mantle redox modeling (i.e., V/Sc, Cu) to propose that the subduction of a ridge underneath the central portion of the Vanuatu arc causes shallow‐angle subduction and the development of a slab tear. We suggest that the shallow slab pinches out the asthenospheric mantle and bulldozes ancient metasomatized lithospheric mantle from the forearc toward the main‐arc. Slab‐fluxed melting of this bulldozed material could account for the along‐arc 87Sr/86Sr‐143Nd/144Nd variations of the Vanuatu magmas. The influx of hot sub‐slab material into the Vanuatu arc mantle wedge through a slab tear produces magmatism within the forearc. Modeling of V/Sc and Cu systematics suggest that the mantle source of the forearc magmas has a higher fO2 and Cu content than the source of the main‐arc and rear‐arc samples. The main‐arc and rear‐arc mantles were metasomatized by both slab‐derived fluids and melts. Whereas release of high Cu‐SO2 slab‐derived fluids caused oxidation and Cu enrichment of the forearc mantle. These systematics indicate a decrease in the fO2 of slab fluxes with increasing depth‐to‐slab and distance‐from‐trench. Our findings highlight the role of ridge subduction in controlling the along‐arc and across‐arc variations in the chemistry of Vanuatu arc magmas. This updated geodynamic model, based on geochemistry, is consistent with recent geophysical constraints and 3D numerical modeling. Plain Language Summary: Subduction of oceanic plates (slabs) at convergent plate margins (trenches) causes the slab to heat up and release fluids, which causes the overlying mantle wedge to melt and produce magmatism. Some parts of the slab (e.g., ridges) are less dense than others and therefore subduct at shallower angles. This can cause (a) the subducting slab to tear and (b) the material above the subducting slab to be pushed away from the trench toward the main‐arc (a process termed "bulldozing"). It remains unclear whether the effects of ridge subduction can be detected using the geochemistry of subduction‐related arc magmas. We use a combination of geochemical data and modeling to suggest that the latitudinal variations in the chemistry of Vanuatu arc samples results from "bulldozing" effects associated with shallow angle ridge subduction. We suggest that the mantle near the Vanuatu trench might be a copper‐rich reservoir, which would not normally melt because it is too cold. However, the inflow of hot sub‐slab mantle through a slab tear appears to provide the heat necessary to melt this source and generate Cu‐rich magmas anomalously close to the trench. Hence, ridge subduction appears to have a profound influence on arc magma chemistry. Key Points: Buoyant ridge subduction causes shallow‐angle subduction and a slab tear to develop beneath the Vanuatu arcAlong‐arc changes in Sr‐Nd isotopes of Vanuatu lavas are linked to slab‐fluxed melting of bulldozed keel as a result of shallow subductionInflux of hot sub‐slab material through a slab tear into the cold and oxidized forearc mantle produces forearc magmas with high fO2 and Cu [ABSTRACT FROM AUTHOR]