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Calcareous lithics are commonly found within the products of some explosive eruptions of Somma-Vesuvius. The pumice fragments from the final phase of the Plinian fallout event of the Pomici di Avellino eruption contain abundant calcareous xenoliths. Previous work on that eruption, including numerical simulations, suggested that the release of CO2 from the entrapment of carbonates may have prolonged the magmatic phase of the eruption by maintaining sufficient driving pressure in the feeding dike. The texture and thermo-metamorphic reactions of carbonate xenolith-bearing pumice fragments of the Pomici di Avellino eruption are analyzed through petrography, scanning electron microscope images, energy dispersive spectrometer analyses, and micro-computed X-ray tomography to deduce the behavior of short-term carbonate-magma interaction and its contribution to the eruption dynamics. Results show that calcareous xenoliths experienced short-term magma-carbonate interaction, which took place in three steps: (i) entrainment, i.e., the mechanical process of carbonate xenoliths entrapment into a magma; (ii) decarbonation, related to high-temperature decomposition reaction of the xenoliths; and (iii) digestion or dissolution of the incorporated calcareous xenoliths into the melt with diffusion of Ca and Mg. The CO2 released during the syn-eruptive decarbonation process thus provided extra volatiles to the rising magma, which may have maintained magma buoyancy longer than expected if only magmatic volatiles were involved in the eruption. [ABSTRACT FROM AUTHOR]

The Southern Volcanic Zone (SVZ) in Chile is an active continental arc with a complex history of volcanism, where a range of magmatic compositions have been erupted in a variety of styles. In the Central SVZ, both monogenetic and polygenetic volcanoes exist, in close proximity to the Liquiñe-Ofqui Fault System (LOFS), but with variable local stress states. Previous studies have inferred varying crustal storage timescales, controlled by the orientation of volcanic centres relative to the N-S striking LOFS and σHMax in this region. To assess the relationship between volcanism and crustal stress states affected by large-scale tectonic structures and edifice controls, we present whole rock geochemical data, to ensure consistency in source dynamics and crustal processing, mineral-specific compositional data, thermobarometry, and Fe–Mg diffusion modelling in olivine crystals from mafic lavas, to assess ascent timescales, from the stratovolcanic edifice of Puyehue-Cordón Caulle and proximal small eruptive centres. Textural observations highlight differences in crystal maturation timescales between centres in inferred compression, transpression, and extension, yet source melting dynamics remain constant. Only samples from the stratovolcanic edifice (in regional compression) preserve extensive zonation in olivine macrocrysts; these textures are generally absent from proximal small eruptive centres in transtension or extension. The zonation in olivines from stratovolcanic lavas yields timescales on the order of a few days to a few weeks, suggesting that even in environments which inhibit ascent, timescales between unrest and eruption of mafic magmas may be short. Significantly, high-resolution compositional profiles from olivine grains in the studied samples record evidence for post-eruptive growth and diffusion, highlighting the importance of careful interpretation of diffusion timescales from zoned minerals in more slowly cooled lavas when compared with tephra samples. [ABSTRACT FROM AUTHOR]

Olivine‐hosted melt inclusions are an important archive of pre‐eruptive processes such as magma storage, mixing and subsequent ascent through the crust. However, this record can be modified by post‐entrapment diffusion of H+ through the olivine lattice. Existing studies often use spherical or 1D models to track melt inclusion dehydration that fail to account for complexities in geometry, diffusive anisotropy and sectioning effects. Here we develop a finite element 3D multiphase diffusion model for the dehydration of olivine‐hosted melt inclusions that includes natural crystal geometries and multiple melt inclusions. We use our 3D model to test the reliability of simplified analytical and numerical models (1D and 2D) using magma ascent conditions from the 1977 eruption of Seguam volcano, Alaska. We find that 1D models underestimate melt inclusion water loss, typically by ∼30%, and thus underestimate magma decompression rates, by up to a factor of 5, when compared to the 3D models. An anisotropic analytical solution that we present performs well and recovers decompression rates within a factor of 2, in the situations in which it is valid. 3D models that include multiple melt inclusions show that inclusions can shield each other and reduce the amount of water loss upon ascent. This shielding effect depends on decompression rate, melt inclusion size, and crystallographic direction. Our modeling approach shows that factors such as 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates and recovering water contents in natural magmatic systems. Plain Language Summary: The water content of olivine‐hosted melt inclusions can reveal important information about the generation and storage of magma beneath basaltic volcanoes. Diffusion of hydrogen (as H+) through the olivine host crystal, however, can modify the water content of melt inclusions over minutes to hours. Here we develop a new 3D diffusion model for water loss from olivine‐hosted melt inclusions which includes natural crystal shapes and multiple melt inclusions. We use our model to test the reliability of different types of analytical and numerical models using conditions of magma ascent from the 1977 eruption of Seguam volcano, Alaska. We find that 1D and 2D numerical models underestimate water loss and magma decompression rates because they do not account for additional water loss from all directions. An anisotropic analytical solution that we present compares well with the 3D model giving decompression rates within a factor of 2. Multiple melt inclusions can also shield each other and help to reduce water loss. Our modeling approach shows that factors such 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates, and recovering water contents in natural magmatic systems. Key Points: New 3D multiphase finite element diffusion model and anisotropic analytical solution for water loss from melt inclusions1D and 2D numerical models underestimate magma decompression rates compared to 3D models. The analytical solution performs wellShielding effect from multiple melt inclusions may limit water loss [ABSTRACT FROM AUTHOR]

The morphologies and size distributions of groundmass crystals record conditions of magma ascent through volcanic conduits. However, morphological information (such as crystal shapes) has not been incorporated into crystal size distributions (CSDs). Here, we focused on the crystal habit, especially the shape variation due to the combination of (hk0) faces (hereafter "tracht") of pyroxene microlites and nano-crystals, and measured CSDs for each crystal habit (tracht) to more comprehensively characterize the crystallization kinetics. We refer to the CSDs measured for each tracht as "tracht-specific CSDs." Pyroclasts from the 2011 eruption of Shinmoedake (Kirishima volcano group, Japan) were examined by field-emission scanning electron microscopy, electron backscatter diffraction analysis, synchrotron radiation X-ray computed nanotomography, and transmission electron microscopy. The samples contain groundmass pyroxenes of two main trachts: octagonal prisms consisting of {100}, {010}, and {110} faces and hexagonal prism lacking {100} faces. The pumice clasts formed by different eruption styles showed different trends of tracht-specific CSDs. Sub-Plinian pumice clasts were characterized by octagonal microlites (1–10 μm wide) and numerous hexagonal nano-crystals (0.2–2 μm wide), and a Vulcanian pumice clast with the same glass composition showed the same characteristics. In contrast, Vulcanian pumice clasts with more evolved glass compositions contained mostly octagonal pyroxenes. The tracht-specific CSDs and growth zonations indicate a change from octagon-dominant to hexagon-dominant growth conditions during syneruptive ascent. We infer that the hexagonal tracht resulted from a large degree of effective undercooling due to rapid decompression in the shallow conduit. Moreover, the texture of the less-evolved Vulcanian pumice indicates that a portion of the magma erupted on the Vulcanian eruption followed almost the same ascent paths just prior to the fragmentation as those during the sub-Plinian eruptions, and thus the Vulcanian eruption may have involved the rapid ascent of deeper magma. We propose that tracht analyses of groundmass pyroxenes provide detailed information about time-evolution of magma conditions during syneruptive ascent. [ABSTRACT FROM AUTHOR]

Abstract Olivine‐hosted melt inclusions are an important archive of pre‐eruptive processes such as magma storage, mixing and subsequent ascent through the crust. However, this record can be modified by post‐entrapment diffusion of H+ through the olivine lattice. Existing studies often use spherical or 1D models to track melt inclusion dehydration that fail to account for complexities in geometry, diffusive anisotropy and sectioning effects. Here we develop a finite element 3D multiphase diffusion model for the dehydration of olivine‐hosted melt inclusions that includes natural crystal geometries and multiple melt inclusions. We use our 3D model to test the reliability of simplified analytical and numerical models (1D and 2D) using magma ascent conditions from the 1977 eruption of Seguam volcano, Alaska. We find that 1D models underestimate melt inclusion water loss, typically by ∼30%, and thus underestimate magma decompression rates, by up to a factor of 5, when compared to the 3D models. An anisotropic analytical solution that we present performs well and recovers decompression rates within a factor of 2, in the situations in which it is valid. 3D models that include multiple melt inclusions show that inclusions can shield each other and reduce the amount of water loss upon ascent. This shielding effect depends on decompression rate, melt inclusion size, and crystallographic direction. Our modeling approach shows that factors such as 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates and recovering water contents in natural magmatic systems.

Abstract The eruption activity of Shinmoedake in the Kirishima volcanic group of Japan resumed in 2017–18, following a quiet period during 2011–17. Subplinian eruptions preceded lava effusion in 2011; however, no subplinian eruption occurred during 2017–18. Petrological studies and melt inclusion analyses were conducted to investigate the ascent and degassing of the magma to understand the cause of the different eruption styles. Chemical analysis of the melt inclusions from the 2011 eruption indicates that mafic magma with high volatile content (6.2 wt% H2O, 0.25–1.4 wt% CO2) ascended into the shallow felsic magma (1.9–3.7 wt% H2O, 0.025–0.048 wt% CO2) at depths of 5–6 km. Calculations indicate that the mafic magmas were of lower density (1717–1835 kg m−3) than the felsic magma (2264–2496 kg m−3) at 125 MPa and that the two magmas were mixed. The 2011 mixed magma with high volatile content (4.0 wt% H2O, 0.14–0.70 wt% CO2) had a bubble volume of approximately 50 vol% at 50 MPa, which is likely to have caused the subplinian eruption. The whole-rock and chemical compositions of the plagioclase, clinopyroxene, and orthopyroxene phenocryst cores from 2018 and 2011 were similar, suggesting that the 2018 magma was a remnant of the 2011 magma. Chemical analyses of the groundmass from 2018 and the MELTS calculation indicate that the magma approached chemical equilibrium during 2011–18. Melt inclusion analyses and volcanic gas observation noted a lower bulk volatile content in the 2018 magma (2.1–3.0 wt% H2O, 0.087–0.10 wt% CO2) than that in the 2011 magma. Comparison of the degassed-magma volumes estimated from the S and Cl contents of the melt inclusions, SO2 flux and volcanic gas composition, and erupted-magma volume indicates that excess degassing has been occurring in the magma due to convection since February 2011, which may have decreased the volatile content of the magma. The relatively low volatile content meant that the 2018 magma could not erupt explosively and lava was instead erupted via effusion. Graphical Abstract

The eruption activity of Shinmoedake in the Kirishima volcanic group of Japan resumed in 2017–18, following a quiet period during 2011–17. Subplinian eruptions preceded lava effusion in 2011; however, no subplinian eruption occurred during 2017–18. Petrological studies and melt inclusion analyses were conducted to investigate the ascent and degassing of the magma to understand the cause of the different eruption styles. Chemical analysis of the melt inclusions from the 2011 eruption indicates that mafic magma with high volatile content (6.2 wt% H2O, 0.25–1.4 wt% CO2) ascended into the shallow felsic magma (1.9–3.7 wt% H2O, 0.025–0.048 wt% CO2) at depths of 5–6 km. Calculations indicate that the mafic magmas were of lower density (1717–1835 kg m−3) than the felsic magma (2264–2496 kg m−3) at 125 MPa and that the two magmas were mixed. The 2011 mixed magma with high volatile content (4.0 wt% H2O, 0.14–0.70 wt% CO2) had a bubble volume of approximately 50 vol% at 50 MPa, which is likely to have caused the subplinian eruption. The whole-rock and chemical compositions of the plagioclase, clinopyroxene, and orthopyroxene phenocryst cores from 2018 and 2011 were similar, suggesting that the 2018 magma was a remnant of the 2011 magma. Chemical analyses of the groundmass from 2018 and the MELTS calculation indicate that the magma approached chemical equilibrium during 2011–18. Melt inclusion analyses and volcanic gas observation noted a lower bulk volatile content in the 2018 magma (2.1–3.0 wt% H2O, 0.087–0.10 wt% CO2) than that in the 2011 magma. Comparison of the degassed-magma volumes estimated from the S and Cl contents of the melt inclusions, SO2 flux and volcanic gas composition, and erupted-magma volume indicates that excess degassing has been occurring in the magma due to convection since February 2011, which may have decreased the volatile content of the magma. The relatively low volatile content meant that the 2018 magma could not erupt explosively and lava was instead erupted via effusion. [ABSTRACT FROM AUTHOR]

The birth of a new monogenetic volcano is difficult to forecast with precision, both in space and time. Nevertheless, seismic activity can alert of the imminence of such an eruption because it usually occurs as small-magnitude earthquake swarms that can last for a few weeks to months prior to an eruption. These swarms are usually related to magma that becomes stalled in the Earth's crust for variable periods of time before its eventual eruption at the surface. For several reasons, volcanic seismic swarms have rarely been recorded with seismometers before the birth of a new monogenetic volcano. Over the past 25 years, six distinct seismic swarms (in 1997, 1999, 2000, 2006, 2020, and 2021) were detected between Tancítaro and Paricutin volcanoes, in the southwestern part of México's Michoacán-Guanajuato volcanic field. They are believed to represent repeated attempts of magma to reach the surface hinting that in this region magma might become stalled for some time, so as to not reach the surface in a single ascent event from its source in the mantle. To better understand the magma's migration path through the crust, we re-located with greater precision some of these seismic swarms by using the same methodology and velocity model to the entire data set. Our results show that these swarms originated within a small area beneath the NE flank of Tancítaro at depths of between 15 and 8 km below sea level (bsl). Apparently, magma is trying to reach the surface within the same conduit network at these crustal depths, but stalls when reaching a depth of ~ 8 km bsl. It is crucial to study these swarms because they might be precursors to a new eruption in this part of the Michoacán-Guanajuato volcanic field. This monogenetic field has been very active, producing several dozen eruptions during the Holocene, the last two Jorullo (1759–1774) and Paricutin (1943–1952). Furthermore, the Tancítaro area displays one of the highest densities of Holocene volcanoes within the entire field, making it a probable candidate location for the birth of a future monogenetic volcano. For these reasons, a permanent seismic network should be installed as soon as possible. [ABSTRACT FROM AUTHOR]

The recent eruption of the Cumbre Vieja Volcanic Ridge in La Palma started on September 19, 2021, after a series of seismic swarms and lasted 85 days. Seismic swarms showed a large variability but could be grouped in two categories: long-term swarms located at subcrustal depth, showing b-values close to 2.2 and low rates (from 4.1 to 56 events/hour) and short-term swarms located at very shallow depths, showing moderate b-values [0.9, 1.6] and high rates (from 58 to 460 events/hour). Magma ascent velocity within the 0–10 km depth range varied between 0.03 and 0.21 km/h. Spatial and temporal characterization of the b-values for the entire magmatic process during the selected study period also indicated three distinct dynamics during a preparation phase starting from at least October 2017–August 2021, the eruptive phase from September 2021–December 2021 and a post-eruption process from the end of the eruption in December 2021–May 2022. These stages with clearly different values reflect two scenarios: one corresponding to the time before the eruption with magma ascending from a 39-km depth to the surface, and a second stage corresponding to the eruption itself which includes two seismicity concentrations within the 30–40 km depth interval and a shallow concentration from 0–20 km, corresponding to the crustal reservoir and movement towards the surface. [ABSTRACT FROM AUTHOR]

The petrogenesis of Palaeoarchaean mafic–ultramafic rock suites from the western Iron Ore Group, Singhbhum Craton, eastern India, has been evaluated based on the chemistry of constituent mineral phases. The rock suites include basaltic rocks and mafic (gabbro) to ultramafic (serpentinized peridotite) intrusive rocks, which occur in host rocks covering phyllite, ferruginous shale, banded haematite quartzite and jasper. The constituent clinopyroxene shows dominant uralitization, whereas plagioclase grains are generally saussuritized, being marked by relatively tiny granular aggregates of albite, chlorite, epidote and K-feldspar. The ultramafic intrusive rocks are overwhelmingly serpentinized. Clinopyroxene compositions are augitic, whereas relict plagioclase is typically bytownite. Amphiboles of the investigated rock suites are divisible into the 'uralite' type (occurring peripherally to clinopyroxene) and the 'completely changed-over amphibole' type (with no traces of initial clinopyroxene). Both the amphibole types belong to the 'calcic group', showing a compositional spectrum from actinolite–magnesiohornblende–ferrohornblende–ferroactinolite. Opaque minerals include magnetite (both Cr-magnetite and Al-magnetite), chromite (Al-chromite) and ilmenite, whereas serpentine (belonging to the ultramafic intrusive rocks) corresponds to lizardite. Looking at the mineral compositions of the pyroxene, glass and amphibole, the studied rock suites show a wide equilibration temperature–pressure domain range (∼750 °C to ∼1400 °C at ∼0.26 kbar to ∼21 kbar), which corresponds to an ascending magma that underwent a 'hydration event' at a shallow level. The assessment of the clinopyroxene and spinel chemistry characteristically suggests an arc setting for the parent magma that has undergone both equilibrium and fractional crystallization in the course of magmatic evolution. During differentiation, the magmatic density remains almost constant, with variable oxygen fugacity. [ABSTRACT FROM AUTHOR]

Villarrica or Rukapillan (35.9°S; 2,847 m a.s.l.) is one of the most active volcanoes in South America and is the highest-risk volcano in Chile. It has an open conduit with a persistent lava lake. On the 3 March 2015, Strombolian activity rapidly progressed into a 1.5-km-high lava fountain, erupting at least ∼ 2.4 × 106 m3 of tephra. Soon after, the activity returned to mild Strombolian "background" explosions, which lasted until early 2017. Understanding the pre-eruptive conditions of such paroxysmal events is fundamental for volcanic hazard assessment. We present major and trace element geochemistry for glass and crystalline phases of basaltic andesite paroxysm pyroclasts (52–56 wt.% SiO2), and for the subsequent Strombolian "background" activity through February 2017 (54–56 wt.% SiO2). The lava fountain source magma was initially stored in a deeper and hotter region (9.4–16.3 km; ca. 1140 °C) and was then resident in a shallow (≤ 0.8 km) storage zone pre-eruption. During storage, crystallising phases comprised plagioclase (An66–86), olivine (Fo75–78) and augite (En46–47). Equilibrium crystallisation occurred during upper-crustal magmatic ascent. During storage in the shallower region, magma reached H2O saturation, promoting volatile exsolution and over-pressurization, which triggered the eruption. In contrast, subsequent "background" explosions involving basaltic-andesite were sourced from a depth of ≤ 5.3 km (ca. 1110 °C). Pre-eruptive conditions for the 2015 lava fountain contrast with historical twentieth-century eruptions at Villarrica, which were likely driven by magma that underwent a longer period of mixing to feed both effusive and explosive activity. The rapid transition to lava-fountaining activity in 2015 represents a challenging condition in terms of volcano monitoring and eruption forecasting. However, our petrological study of the pyroclastic materials that erupted in 2015 offers significant insights into eruptive processes involving this type of eruption. This aids in deciphering the mechanisms behind sudden eruptions at open conduit systems. [ABSTRACT FROM AUTHOR]

Large silicic eruptions can be preceded by small eruptions of different styles and volumes. The Holocene Llao Rock, Cleetwood, and climactic eruptions of Mt. Mazama, OR, USA, were sourced from the same magma and followed this pattern. The Llao Rock and Cleetwood eruptions are both relatively small, have pyroclasts with microlites and a wide range of vesicularities, and each consisted of an explosive phase followed by an effusive phase. The climactic eruption had no effusive phase and created highly vesicular pyroclasts with no microlites. We analyzed microlite crystallization using phase equilibria and decompression experiments. Comparing the results to the pyroclasts from the Llao Rock and Cleetwood eruptions, we find that the differences between the small and climactic eruptions are likely caused by different magma ascent dynamics. Our experiments show that plagioclase and pyroxene microlites crystallize only during decompressions that are most likely too slow to result in explosive eruptions. The Llao Rock magma likely stalled at shallow depths before continuing fast ascent, which allowed for microlite crystallization that might have also caused explosive eruptions. The Cleetwood magma likely took two separate ascent paths, a majority fraction that ascended quickly from high pressure without stalling and a minority fraction that stalled at shallow depths before continuing ascent along with the majority fraction. These ascent dynamics of the Llao Rock and Cleetwood magmas led to the creation of obsidian pyroclasts from sidewall sintering of fragmented majority-fraction ash. The climactic magma did not stall at shallow depths and instead ascended from depth quickly to the surface, creating the conditions necessary for caldera collapse. [ABSTRACT FROM AUTHOR]

Groundmass crystals indicate syneruptive magmatic conditions, and thus their crystal size distributions (CSDs) are used to infer magma ascent histories. Three-dimensional (3D) CSDs are most commonly estimated from two-dimensional (2D) observations and plotted against long-axis length, L (hereafter referred to as "L-plot CSDs"). However, L-plot CSDs have two significant problems: the error because of the conversion from 2D to 3D and a lowered sensitivity to changes in the degree of effective undercooling (ΔTeff), which arises because a crystal's growth rate varies with ΔTeff most strongly along its long dimension. Although these problems can result in false interpretations of magma ascent dynamics, there has been little discussion of the influence of the size criteria on CSDs. In this study, we investigated which 3D size criterion [i.e., long (L), intermediate (I), or short (S) axis length] is optimum for 2D-estimated CSDs of groundmass crystals from two perspectives: (1) conformity with the actual distributions, and (2) the sensitivity of CSD slopes to the magma ascent conditions in the conduit. We observed groundmass pyroxene crystals in pumice clasts from sub-Plinian and Vulcanian eruptive phases during the 2011 eruption of Shinmoedake (andesitic volcano, Japan) by using synchrotron radiation-based X‑ray computed nanotomography (SR-XCT) and field-emission scanning electron microscopy (FE-SEM) and reinvestigated the crystallization kinetics of pyroxene nanolites ranging in width from a few hundred nanometers to 1 μm. The SR-XCT observations provided the detailed 3D shapes and 3D CSDs (CT-CSDs) of these nanolites directly. The FE-SEM observations allowed us to estimate 3D aspect ratios (S:I:L) and CSDs (SEM-CSDs). L-plot SEM-CSDs, acquired using the program CSDCorrections, were used to calculate S-plot SEM-CSDs and I-plot SEM-CSDs. We compared the data from FE-SEM with those from SR-XCT to evaluate the accuracy of 3D aspect ratios and CSDs estimated from 2D data. The L-plot SEM-CSDs from the sub-Plinian pumice sample showed significant inconsistencies with the CT-CSD, a result of the difficulty in estimating representative 3D aspect ratios from 2D observations for elongated groundmass crystals. In contrast, the S- and I-plot SEM-CSDs kept the effect of aspect ratio to a minimum and preserved their actual slopes, except for a vertical discrepancy between the CSDs. Moreover, the slopes of S- and I-plot CSDs of the nanolites differed more markedly between the two eruptive styles (by ~20% more) than those of L-plot CSDs. For estimating magma ascent dynamics, we propose that the optimum method for acquiring SEM-CSDs is to measure the cross-sectional widths of crystals and convert the resulting 2D data set into S-plot CSDs. Our new finding that the 3D shapes and CSDs of pyroxene nanolites differ according to eruptive style means that nanolites indicate distinct differences in ascent histories at the shallow conduit: increasing ΔTeff just before sub-Plinian eruptions and decreasing ΔTeff before Vulcanian eruptions. Given the similarity in CSDs of microlites, our results suggest that eruptive style was determined in the shallow conduit. Monitoring the condition of the shallow conduit may contribute to predicting the time evolution of eruptive activity. [ABSTRACT FROM AUTHOR]

• We integrate conduit models of magma ascent with volatile diffusion chronometry. • Assuming constant magma decompression overestimates the actual magma ascent speed. • In this case, magma decompression rate can be overestimated by a factor of 7. • This method anchors diffusion chronometry with physical theories of magma ascent. The pre- and syn-eruptive magma decompression rate is recognized as a key parameter modulating eruption dynamics, with explosive eruptions being generally associated with much larger decompression rates than effusive ones. Magma decompression rates cannot be directly measured and thus are typically inferred from petrological, geochemical, numerical modelling, and seismic data. Most studies use petrological information of volatile element diffusive equilibration in glass and crystals to infer a single value for the magma ascent rate for a given eruption, even though numerical volcano conduit simulations show that changes of velocity are expected during magma ascent. Here we integrate magma ascent conduit models with diffusion chronometry of volatiles in melt embayments and phenocrysts to obtain a more comprehensive understanding of magma ascent rates. We find that incorporating a more realistic boundary condition that depends on the magma ascent path with variable velocities gives time estimates that can be up to a factor of 7 longer than from the standard assumption of constant magma ascent rate. Therefore, previous magma ascent rates from diffusion chronometry of volatiles in crystals and melts with a fixed boundary condition may be significantly overestimated. Overall, we show that coupling of magma ascent models with diffusion chronometry can provide more robust inferences of magma ascent and thus improve the understanding of the role of this parameter into the explosive and effusive eruption controls. [ABSTRACT FROM AUTHOR]

The Bandelier Tuff is the result of two subsequent caldera-forming eruptions of similar volume and composition which produced the Otowi (1.61 Ma) and Tshirege (1.26 Ma) members. Their remarkably similar characteristics and shared caldera boundaries provides a unique platform to investigate whether magma ascent is affected by the presence of a preexisting caldera boundary. Here, we present decompression rates for discrete layers within the initial plinian phase of each member by modeling volatile gradients (H2O, CO2 absent) in quartz-hosted reentrants (unsealed melt inclusions). Successful best-fit 1D diffusion models for the lower (n = 4/9) and upper (n = 11/13) units resulted in average decompression rates of 0.041 MPa/s and 0.026 MPa/s, respectively. Strong overlap between rates extracted from the two eruptions suggests there was no significant change in ascent dynamics. However, the older Otowi member contains a larger number of reentrants that cannot be modeled adequately, suggesting a more complicated path than can be reconstructed with our constant decompression approach. In contrast, reentrants from the Tshirege can be readily modeled from storage depths, an observation that suggests conduit formation was more efficient in the second eruption. To further evaluate the robustness of these extracted rates, we then applied a 2D diffusion model, which considers various reentrant geometries; surprisingly, we find little alteration to 1D-derived rates. By contrast, incorporating the uncertainty in Bandelier temperature (~ 130 °C) shifts rates by 340–440%. However, we argue that the largest source of variation from decompression rates extracted from reentrants lies in the extreme range preserved within each fall deposit, each spanning three orders of magnitude, suggesting extreme conduit dynamic shifts, and emphasizing that petrologic-based ascent rates may vary widely, even within a single-sampled layer. Finally, the lack of detectable CO2 concentrations in measured profiles is at odds with the amounts detected in sealed melt inclusions (< 200 ppm), an observation that has been made in other silicic systems (e.g., Bishop, USA; Oruanui, NZ; Santorini, GR). We propose two mechanisms to remove CO2 from the system prior to eruption: (1) additional crystallization drove CO2 into the fluid phase prior to ascent or (2) reentrants reset to a CO2 free environment due to a small, initial pre-eruptive pressure decrease. Both scenarios have important implications for the pre-eruptive state of the magma body. [ABSTRACT FROM AUTHOR]

Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microlites highlight the same level of intracrystalline variations but require nanoscale resolution which is globally less available. The southern Taupo Volcanic Zone (TVZ), New Zealand, has produced a wide range of explosive eruptions yielding glassy microlite-bearing tephras. Major oxide analyses and textural information reveal that microlite rims are commonly out of equilibrium with the surrounding glass. We mapped microlites and microcrysts at submicron resolution for major and trace element distributions and observed three plagioclase textural patterns: (1) resorption and overgrowth, (2) oscillatory zoning, and (3) normal (sharp) zoning. Pyroxene textures are diverse: (1) resorption and overgrowth, (2) calcium-rich bands, (3) hollow textures, (4) oscillatory zoning, (5) sector zoning, (6) normal zoning and (7) reverse zoning. Microlite chemistry and textures inform processes operating during pre-eruptive magma ascent. They indicate a plumbing system periodically intruded by short-lived sub-aphyric dykes that entrain microantecrysts grown under diverse physico-chemical conditions and stored in rapidly cooled, previously intruded dykes. Changes in temperature gradients between the intrusion and the host rock throughout ascent and repeated magma injections lead to fluctuations in cooling rates and generate local heterogeneities illustrated by the microlite textures and rim compositions. Late-stage degassing occurs at water saturation, forming thin calcic microcryst rims through local partitioning effects. This detailed investigation of textures cryptic to conventional imaging shows that a significant proportion of the micrometre-sized crystal cargo of the TVZ is of antecrystic origin and may not be attributed to late-stage nucleation and growth at the onset of volcanic eruptions, as typically presumed. [ABSTRACT FROM AUTHOR]

The Moon is not volcanically active at present, therefore, we rely on data from lunar samples, remote sensing, and numerical modeling to understand past lunar volcanism. The role of different volatile species in propelling lunar magma ascent and eruption remains unclear. We adapt a terrestrial magma ascent model for lunar magma ascent, considering different compositions of picritic magmas and various abundances of H2, H2O, and CO (measured and estimated) for these magmas. We also conduct a sensitivity analysis to investigate the relationship between selected input parameters (pre‐eruptive pressure, temperature, conduit radius, and volatile content) and given outputs (exit gas volume fraction, velocity, pressure, and mass eruption rate). We find that, for the model simulations containing H2O and CO, CO was more significant than H2O in driving lunar magma ascent, for the range of volatile contents considered here. For the simulations containing H2 and CO, H2 had a similar or slightly greater control than CO on magma ascent dynamics. Our results showed that initial H2 and CO content has a strong control on exit velocity and pressure, two factors that strongly influence the formation of an eruption plume, pyroclast ejection, and overall deposit morphology. Our results highlight the importance of (a) quantifying and determining the origin of CO, and (b) understanding the abundance of different H‐species present within the lunar mantle. Quantifying the role of volatiles in driving lunar volcanism provides an important link between the interior volatile content of the Moon and the formation of volcanic deposits on the lunar surface. Plain Language Summary: Unlike the Earth, the Moon does not have any active volcanoes, and has not had any active volcanoes for at least the last 100 million to 1 billion years. Therefore, we rely on studying samples collected by astronauts and robotic landers, satellite data, and computer models to understand what volcanic activity on the Moon was like. Volcanic eruptions are mostly driven by gas: as magma rises in the Moon's crust, gases will separate out, allowing magma to rise more quickly. As a result of this, we can use volcanic eruptions to understand how much gas exists within the Moon, which is important for understanding how the Moon formed. We used a computer model to simulate magma ascent on the Moon in order to understand what gases were more significant in driving magma ascent. We found that molecular hydrogen and carbon monoxide had a bigger effect on magma ascent than water, so would have a greater effect on the style of volcanic eruptions on the Moon. The source of carbon monoxide and the relative amounts of molecular hydrogen and water in the Moon's mantle are currently unknown and our results highlight the importance of understanding this information for understanding lunar volcanic activity. Key Points: We use a magma ascent model and sensitivity analysis to understand the relative significance of different volatiles on lunar magma ascentFor the range of initial volatile abundances considered, CO and H2 were more significant than H2O in driving lunar magma ascentResults highlight the importance of quantifying and determining the origin of CO, and understanding H‐speciation within the lunar mantle [ABSTRACT FROM AUTHOR]

©2015. The Authors. Volcanic edifice collapses generate some of Earth's largest landslides. How such unloading affects the magma storage systems is important for both hazard assessment and for determining long-term controls on volcano growth and decay. Here we present a detailed stratigraphic and petrological analyses of volcanic landslide and eruption deposits offshore Montserrat, in a subduction zone setting, sampled during Integrated Ocean Drilling Program Expedition 340. A large (6-10km3) collapse of the Soufrière Hills Volcano at ~130ka was followed by explosive basaltic volcanism and the formation of a new basaltic volcanic center, the South Soufrière Hills, estimated to have initiated

Abstract Petrological observations and chemical analyses of melt inclusions in scoria were used to investigate the magma ascent and eruption processes of the 1979, 1989, and 2014 eruptions of Nakadake, Aso volcano, Japan. Major elements and sulfur contents of the melt inclusions were determined using an electron probe microanalyzer, and their water and CO2 contents were determined using secondary ion mass spectrometry. Five scoria specimens from the 2014 eruptions had an andesite composition identical to the scoria from the 1979 and 1989 eruptions. Thermometry using the chemical composition of the groundmass and the rims of the phenocrysts indicated that the temperature of the 2014 magma was 1042–1092 °C. Melt inclusions in plagioclases, clinopyroxenes, and olivines in the 2014 scoria had an andesite composition similar to that of the groundmass. The volatile content of the melt inclusions was 0.6–0.8 wt% H2O, 0.003–0.017 wt% CO2, and 0.008–0.036 wt% S. The variation in CO2 and S content of the melt inclusions was not correlated with the K2O content, suggesting that the magma degassed as pressure decreased. Melt inclusions in plagioclases, clinopyroxenes, and olivines from the 1979 and 1989 scoria had similar major elements and volatile content to the 2014 eruption specimens. The similarity in chemical composition of both the whole-rock and melt inclusions among all samples suggests that the magmas of these eruptions were derived from the same magma chamber. The gas saturation pressure estimated from the H2O and CO2 contents of the 1979, 1989, and 2014 scoria ranged from 18 to 118 MPa, corresponding to depths of 1–4 km. Comparison of this depth with geophysical observations suggests that the inclusion entrapments occurred in the upper part of the magma chamber and/or a conduit. By combining the melt inclusion analysis with volcanic gas observations, we estimated the bulk volatile content of the magma. Based on the bulk sulfur content of the magma and the SO2 flux between January 2014 and December 2017, the amount of degassed magma over that period was estimated to be the equivalent of 1–3 km3 of dense rock. The estimated volume was more than 600 times larger than that of products erupted during the same period. This suggests that magma degassing occurred at several depths in the magma chamber due to magma convection in a conduit.

Boninites are rare high magnesium andesites that often contain trace chromites. These chromites precipitate from primitive boninitic melts and are thought to be carried to the surface in melts that continue to crystallize a significant volume of silicate minerals. Such magmatic differentiation drives primitive chromite out of equilibrium with the residual melt with respect to both divalent and trivalent cation proportions. Diffusion then operates to alter primitive chromite toward a composition in equilibrium with residual melt. To simulate this process, we have performed internally heated pressure vessel experiments, providing insights into the processes of chromite-melt re-equilibration through time. While Fe-Mg exchange at magmatic temperatures equilibrates the tetrahedrally coordinated divalent cations in chromite in less than 6 hours, equilibration of trivalent cations in octahedral coordination with residual melts is slower. Our experimental results show that chromite Al concentrations are ubiquitously lower, and Fe2+/Fe3+ values are ubiquitously higher than modelled equilibrium values, indicating that there was insufficient time for significant Al and Fe3+ replacement of Cr. Similar observations can be made for natural chromite compositions in the extrusive sequence of the Troodos ophiolite (Cyprus). Based on a simple model of diffusive equilibration, we estimate that microphenocrystic chromites up to 60 μm in diameter take between 60 days and c. 170 years to equilibrate under conditions analogous to the physiochemical state of melt immediately prior to eruption. For the Troodos ophiolite extrusive sequence, this implies that mafic magmas are erupted less than c. 170 years after extraction from the mantle for disequilibrium textures and compositions to be preserved. [ABSTRACT FROM AUTHOR]

Phenocrysts in volcanic rocks are recorders of magmatic processes that have occurred at depth before and during a volcanic eruption. Our petrological investigations of stratigraphically controlled tephrite and phonotephrite samples from the latest eruption of Fogo (Cape Verde Islands) aimed to reconstructing magma storage and transport. The dates of sample emplacement have been determined using satellite instrument – derived high resolution thermal infrared maps. All samples are strongly phyric and commonly contain complexly zoned clinopyroxene crystals and cumulate fragments. Clinopyroxenes from all samples exhibit 10–50 μm wide rim zones, inferred to have grown in a few days to weeks during the ongoing eruption as a consequence of H2O loss from the melt. Clinopyroxene-melt thermobarometry using tephrite groundmass compositions suggests that the rims formed at upper mantle pressures of around 600 MPa (21 km depth). This level is interpreted to reflect temporary reduction in magma ascent velocity by near-isobaric movement through a complex storage system. Previously, the tephrite magma had accumulated at a deeper level, possibly between 700 and 900 MPa as indicated by clinopyroxene cores (Mata et al., 2017). The cause for H2O loss initiating rim growth could be degassing after rise of the magma from the deeper level, or CO2 flushing by a carbonic fluid phase released at depth. Corresponding data from phonotephrites indicate last equilibration at around 440 MPa (16 km); the phonotephrite magma is inferred to be a residuum from an earlier magmatic event that was entrained by advancing tephrite. Microthermometry of CO2-dominated fluid inclusions in tephrite clinopyroxenes results in pressures of around 330 MPa (12 km), indicating another short pause in magma ascent in the lowermost crust. Rim zonations of olivine phenocrysts indicate that after leaving this final stalling zone, the magma ascended to the surface in less than half a day. In strong contrast to these petrological equilibration depths, seismic events precursory to the eruption were located at < 5 km below sea level, with only two exceptions at 17 and 21 km depth consistent with our barometry. Our results enhance the understanding of this potentially dangerous volcano, which helps to interpret future pre-eruptive unrest.

The understanding of magma ascent dynamics is essential in forecasting the scale, style and timing of volcanic eruptions. The monitoring of near-field deformation is widely used to gain insight into these dynamics, and has been linked to stress changes in the upper conduit. The ascent of magma through the conduit exerts shear stress on the conduit wall, pulling up the surrounding edifice, whilst overpressure in the upper conduit pushes the surrounding edifice outwards. How much shear stress and pressure is produced during magma ascent, and the relative contribution of each to the deformation, has until now only been explored conceptually. By combining flow and deformation modeling using COMSOL Multiphysics, we for the first time present a quantitative model that links magma ascent to deformation. We quantify how both shear stress and pressure vary spatially within a cylindrical conduit, and show that shear stress generally dominates observed changes in tilt close to the conduit. However, the relative contribution of pressure is not insignificant, and both pressure and shear stress must be considered when interpreting deformation data. We demonstrate that significant changes in tilt can be driven by changes in the driving pressure gradient or volatile content of the magma. The relative contribution of shear stress and pressure to the tilt varies considerably depending on these parameters. Our work provides insight into the range of elastic moduli that should be considered when modeling edifice-scale rock masses, and we show that even where the edifice is modeled as weak, shear stress generally dominates the near field deformation over pressurization of the conduit. While our model addresses cyclic tilt changes observed during activity at Tungurahua volcano, Ecuador, between 2013 and 2014, it is also applicable to silicic volcanoes in general.

Microlite textural characteristics are analyzed in the products of Vulcanian explosions, ash venting occurring synchronously with lava effusion (syn -extrusive ash venting), a precursory explosion, and lava dome effusion associated with Phases 3, and 5 of the current Soufrière Hills Volcano (SHV) eruption. 2D microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism. In addition, magma ascent rates for each eruptive style were calculated using microlite number density (Toramaru et al. 2008). Strong differences between pyroclastic and effusive microlites are observed in 2D population statistics and crystal size distributions, indicating higher amounts of smaller microlites (<0.0054 mm) in the lava dome compared with pyroclastic products. Crystal size distributions show that the lava dome and Vulcanian explosion samples have similar amounts of large microlites (>0.0084 to 0.012 mm). However, syn -extrusive ash venting samples have comparatively greater amounts of large microlites. The similarity between lava dome and Vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system (>2 km depth). The contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at SHV. The greater amounts of large microlites in syn -extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit, inferred to result from differences in ascent rate. Sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn -extrusive ash venting compared with the lava dome. Calculated magma ascent rates for Vulcanian explosions and syn -extrusive ash venting have similar ranges, while lava effusion is up to a factor of 10 faster. Differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style. Ascent rate is calculated at microlite nucleation depth using the method of Toramaru et al. (2008). However, the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent. More small microlites in the lava dome suggest greater nucleation rates in the shallow conduit, recording an on average shallower magma ascent rate (<1 km depth). Rapid decompression during pyroclastic eruptions restricts shallow crystallization, suggesting the microlite population is on average recording the deeper conduit system (>2 km depth). • Examined microlite textures from a range of juvenile pyroclastic and effusive samples • Synchronous dome effusion and ash venting display contrasting microlite textures • Differences in microlite crystallization depth and rate record magma conditions at different depths depending on eruption style • Differences in ascent rate across the magma column produce lower decompression rates in conduit margin magma observed as ash venting • Ascent rates calculated from microlites record deeper values for pyroclastic activity due to rapid decompression restricting crystallization [ABSTRACT FROM AUTHOR]

While long-term interactions of magma with carbonate wall-rock (a.k.a. carbonate assimilation) are well-studied, only recently some experimental studies focused on short-term interactions (seconds to minutes) at magma chamber conditions (0.5 GPa and 1200 °C). They have shown that carbonate assimilation can effectively release CO 2 and dissolve the ingested clast in syn -eruptive timescales. Carbonate wall-rock xenoliths in eruptive products can hence be seen as proof of even shallower ingestion (i.e., within the feeding dyke). To study these shallower interactions, we performed 66 experiments at atmospheric pressure (i.e., at the second endmember of the volcanic feeding system) and at 950–1230 °C with varying melt compositions and limestone compositions. Decarbonation was found to be mainly dependent on temperature and limestone composition while clast dissolution is largely dependent on magma composition, temperature, pressure and interaction time. In natural systems during magma ascent and with increasing quantities of assimilated wall-rock, the magma temperature would steadily decrease, limiting its own decarbonation and assimilation ability. But even in the 950 °C-experiments decarbonation (i.e., CO 2 release) remained a syn -eruptive process. We subsequently discussed the limits of carbonate assimilation as well as the potential effect of syn -eruptive addition of CO 2 to the magmatic mixture on magma ascent and eruption dynamics. • Carbonate assimilation within the whole volcanic plumbing system. • Controlling parameters and relative timescales of decarbonation and clast dissolution. • Decarbonation occurs in syn -eruptive timescales, independent of depth of clast ingestion. • Clast dissolution dominant at magma chamber, but only of very localized importance in shallow depths. • Additional CO 2 will form bubbles and increase eruption explosivity. [ABSTRACT FROM AUTHOR]

Late-orogenic granitoids carry unique geochemical and structural evidence of post-collisional exhumation, commonly associated with decompression melting, mixing and mingling of individual magma pulses and differentiation. The Weinsberg Type Granitoids (WTG) forming the 313.6 Ma Kaplice Sheet and 322 Ma Hojná Voda Sheet (northwestern Weinsberg Composite Pluton) have geochemical signature that demonstrates the substantial role played by the partial melting of metasedimentary sequences and the magmatic fractionation of K-feldspar, plagioclase and biotite during magma ascent and emplacement. The composition of highly peraluminous WTG were partly modified by the assimilation of host metasedimentary xenoliths. The textures of the WTG are characterised by straight crystal size distribution (CSD) consistent with in-situ crystallization and crystal/melt separation, forming abundant K-feldspar phenocryst-rich domains (KPD). The KPD were formed by redistribution of K-feldspar phenocrysts (crystal accumulation) in a suspension during magma movement followed by interstitial melt extraction and escape from the highly crystallised mush. The multiple fabric pattern within the Kaplice Sheet is the result of changeover of local stress field by varying increment of regional stress conditions and buoyancy forces at the time of magma ascent and emplacement. The steeply dipping foliation (M 1) reflects the magma flow with the increment of regional stress-field (ENE–WSW oriented extension). The superimposition of flat-lying foliation (M 2) with abundant particle accumulations (i.e. K-feldspar phenocrysts, enclaves and host-rock xenoliths) were associated with (a) increasing buoyancy force in ascending magma above the rate of the regional stress field or (b) collapse of ascending magma after the decreasing the buoyancy force resulting in the mush moving down. • Late-orogenic granitoids bear the evidence of post-collisional exhumation. • Geochemical signature of metasedimentary sources and magmatic fractionation. • Phenocryst-rich domains reflect redistribution of Kfs crystals and melt extraction. • The fabric pattern reflects stress-field changeover. [ABSTRACT FROM AUTHOR]

Volcanic eruptions occur when a conduit forms to connect a crustal magma reservoir to Earth’s surface. Conduit formation is generally assumed to be a ‘bottom–up’ process and a major driver of precursory volcanic seismicity, which is the most commonly monitored parameter at volcanoes worldwide. If both assumptions are true, initial precursory seismicity should coincide spatially with petrologically-estimated magma reservoir depths. A review of six well-constrained case studies of arc volcanoes that erupt after repose intervals of decades indicates that, to the contrary, initial precursory seismicity is consistently several kilometers shallower than the magma reservoir. We propose a model involving a three-phase process of unrest and eruption: initial (partial) conduit formation occurs during a ‘staging’ phase, either aseismically or long before the onset of the immediate precursory run-up to eruption. Staging may involve slow ascent rates and/or small volumes. A destabilization phase then coincides with the onset of precursory seismicity, leading to a ‘tapping’ phase that involves additional magma ascent from the magma reservoir. This model implies that, most critically, it may be possible to detect precursory magma ascent well before the onset of seismic activity by continuous monitoring of the state of stress in the mid to shallow crust.

Abstract: On the Moon, the low‐density crust exerts a strong filter to magma ascent. Many lunar craters are filled with mare, and evidence of pyroclastic activity and shallow magmatism is often located within craters called floor‐fractured craters (FFCs). Interpreting quantitative observations on mare‐filled craters and FFCs based on mechanical models, we show that a surface unloading caused by an impact crater provides a driving overpressure to the magma stalling at depth. This overpressure counterbalances the melt negative buoyancy, favoring its ascent through the crust. Providing a large unloading and a thin crust, magma can ascend up to the crater floor. FFC characteristics are consistent with a magma denser than the crust by 200–300 kg/m3 and an elastic lithosphere thickness larger than 70 km. Our study suggests that small impact cratering likely induced magmatism and thereby crustal evolution in the early times of terrestrial planets. [ABSTRACT FROM AUTHOR]

The cellular automaton model of Piegari, Di Maio, Scandone and Milano, J. Volc. Geoth. Res. , 202 , 22–28 (2011) is extended to include magma-induced stress (i.e. a local magma-related augmentation of the stress field). This constitutes a nonlinear coupling between the magma and stress fields considered by this model, which affects the statistical distributions of eruptions obtained. The extended model retains a power law relation between eruption size and frequency for most events, as expected from the self-organised criticality inspiring this model; but the power law now applies for a reduced range of size, and there are new peaks of relatively more frequent eruptions of intermediate and large size. The cumulative frequency of repose time between events remains well modelled by a stretched exponential function of repose time (approaching a pure exponential distribution only for the longest repose times), but the time scales of this behaviour are slightly longer, reflecting the increased preference for larger events. The eruptions are relatively more likely to have high volatile (water) content, so would generally be more explosive. The new model also naturally favours a central ‘axial’ transport conduit, as found in many volcano systems, but which otherwise must be artificially imposed within such models. [ABSTRACT FROM AUTHOR]

Volatiles play an important role in magmatic and volcanic processes. Melt inclusions are a powerful tool to study pre-eruptive volatiles, but interpretation of their H2O and CO2 variations can be difficult. The H2 O and CO2 contents of melt inclusions from nine basanites from Hut Point Peninsula, Mt Terror and Mt Bird on Ross Island, Antarctica, were studied to understand better the behavior of volatiles in the magmas and to provide insight into magma transport and storage processes. Ninety olivine-hosted (Fo78-88) melt inclusions were examined along with the composition of the associated bulk-rock samples. The H2 O (0.4-2.0wt %) and CO2 (0.2-9wt %, or 0.2-1-8wt % after correction for vapor bubbles) variations in the melt inclusions cannot be explained by equilibrium degassing. A strong correlation between melt inclusion radius and H2 O content for Hut Point samples indicates that diffusive loss of Hþ has occurred. Based on vapor saturation pressure trends, it is inferred that a magma reservoir existed below Hut Point at a depth of ∼18 km and by modeling diffusive loss of H+ for melt inclusions, it is shown that the magmas ascended from this depth in less than a year. Melt inclusions from Terror and Bird lack evidence of diffusive loss of H+ and there are no strong chemical indicators of CO2 fluxing. Compositional heterogeneities in melt inclusions indicate that magma mixing occurred, making it difficult to interpret H2 O and CO2 trends. Melt inclusions from these volcanoes were entrapped polybarically, inconsistent with entrapment in a single storage region. Published analyses of 54 olivine-hosted (Fo53-83) melt inclusions in seven samples from Erebus volcano on Ross Island were re-examined for comparative purposes. Low H2 O (∼0.1wt %) and CO2 (0-0.2wt %) contents and the evolved compositions of these indicate that Erebus magmas undergo shallow (<6 km) crystallization before eruption, probably in a shallow storage region. Magmas from the surrounding volcanoes show no sign of shallow storage. [ABSTRACT FROM AUTHOR]

Although kimberlite magma carries large amounts of mantle-derived xenocrysts and xenoliths (with sizes up to meters), this magma ascends from the Earth's mantle (> 150–250 km) to the surface in a matter of hours or days, which enables diamonds to survive. The recently proposed assimilation-fuelled buoyancy model for kimberlite magma ascent emphasizes the importance of fluid CO 2 that is produced via the reactive dissolution of mantle-derived orthopyroxene xenocrysts into kimberlite melt, which initially has carbonatitic composition. Here, we use a series of high-pressure experiments to test this model by studying the interaction of orthopyroxene (Opx) with an alkali-dolomitic melt (simplified to 0.7Na 2 CO 3 + 0.3K 2 CO 3 + 2CaMg(CO 3 ) 2 ), which is close to the melt that is produced by the partial melting of a kimberlite source, at P = 3.1–6.5 GPa and T = 1200–1600 °C, i.e., up to pressures that correspond to depths (~ 200 km) from where the ascent of kimberlite magma would start. During the first set of experiments, we study the reaction between powdered Opx and model carbonate melt in a homogeneous mixture. During the second set of experiments, we investigate the mechanism and kinetics of the dissolution of Opx crystals in alkali-dolomitic melt. Depending on the P-T conditions, Opx dissolves in the alkali-dolomitic melt (CL) either congruently or incongruently via the following reactions: Mg 2 Si 2 O 6 (Opx) + CaMg(CO 3 ) 2 (CL) = CaMgSi 2 O 6 (clinopyroxene) + 2MgCO 3 (CL) and Mg 2 Si 2 O 6 (Opx) = Mg 2 SiO 4 (olivine) + SiO 2 (CL). The experiments confirm that the dissolution of Opx causes gradual SiO 2 enrichment in the initial carbonate melt, as previously suggested. However, the assimilation of Opx by carbonate melt does not produce fluid CO 2 in the experiments because the CO 2 is totally dissolved in the evolved melt. Thus, our results clearly demonstrate the absence of exsolved CO 2 fluid at 3.1–6.5 GPa in ascending kimberlite magma and disprove the assimilation-fuelled buoyancy model for kimberlite magma ascent in the lithospheric mantle. We alternatively suggest that the extreme buoyancy of kimberlite magma at depths of 100–250 km is an exclusive consequence of the unique physical properties (i.e., low density, ultra-low viscosity and, thus, high mobility) of the kimberlite melt, which are dictated by its carbonatitic composition. [ABSTRACT FROM AUTHOR]

Volcanic activity exhibits a wide range of eruption styles, from relatively slow effusive eruptions that produce lava flows and lava domes, to explosive eruptions that can inject large volumes of fragmented magma and volcanic gases high into the atmosphere. Although controls on eruption style and scale are not fully understood, previous research suggests that the dynamics of magma ascent in the shallow subsurface (< 10 km depth) may in part control the transition from effusive to explosive eruption and variations in eruption style and scale. Here we investigate the initial stages of explosive eruptions using a 1D transient model for magma ascent through a conduit based on the theory of the thermodynamically compatible systems. The model is novel in that it implements finite rates of volatile exsolution and velocity and pressure relaxation between the phases. We validate the model against a simple two-phase Riemann problem, the Air-Water Shock Tube problem, which contains strong shock and rarefaction waves. We then use the model to explore the role of the aforementioned finite rates in controlling eruption style and duration, within the context of two types of eruptions at the Soufrière Hills Volcano, Montserrat: Vulcanian and sub-Plinian eruptions. Exsolution, pressure, and velocity relaxation rates all appear to exert important controls on eruption duration. More significantly, however, a single finite exsolution rate characteristic of the Soufrière Hills magma composition is able to produce both end-member eruption durations observed in nature. The duration therefore appears to be largely controlled by the timescales available for exsolution, which depend on dynamic processes such as ascent rate and fragmentation wave speed. [ABSTRACT FROM AUTHOR]

Crystal-bearing magmas are often assumed to be intrinsically non-Newtonian, but the mechanism underlying suspension shear thinning remains unconstrained. Here, we test the hypothesis that shear thinning in these suspensions arises solely from unrelaxed shear thinning in the melt phase or from viscous heating or both. We compile existing data for the rheology of high-temperature crystal-bearing silicate magmas across a wide range of conditions. In order to test this, we define a 'lever' function L (ϕ) which scales the strain rate in the melt phase between crystals relative to the bulk strain rate of the suspension as a function of crystal volume fraction ϕ. We show that, in general, the parameterisation of the amplification of strain rates via the L (ϕ) factor coupled with a shear-thinning law for the melt phase fully accounts for the non-Newtonian behaviour of the suspension. Next, we use the Brinkman number Br to demonstrate that some existing data derive from experiments that were subject to viscous heating, resulting in a further manifestation of apparent shear thinning. Taken together, our results provide a theoretical framework that predicts microphysical regimes for shear thinning. This implies that crystal-bearing magmas are Newtonian except where the scaled conditions for viscous heating are met, or when the strain rates in the melt phase approach the inverse of the structural relaxation timescale. This shear thinning due to non-Newtonian behaviour of the melt between crystals occurs very close to the brittle threshold, yielding an extension of the brittle failure criterion to crystal-bearing magmas, with direct implications for volcanic eruption dynamics. • Magma shear thinning can have a microphysical origin in the gaps between crystals. • A lever function approach shows that shear thinning onsets scale with viscosity. • Shear thinning is closely associated with the brittle threshold via melt relaxation. • Some existing experimental data are in the viscous heating regime. [ABSTRACT FROM AUTHOR]

A shallow water eruption started on October 10, 2011, ~2 km south off the coast of El Hierro (Canary Islands, Spain). The eruption lasted about five months and ended by early March 2012. Three months of unrest preceded this event with more than 10,000 localized earthquakes and up to 6 cm of vertical ground deformation. In the Canary Islands, this is the first eruption to be monitored by the network of Instituto Geográfico National (IGN) since the very beginning of the seismic unrest. This provided unprecedented time series that include geophysical (seismic and gravimetric), geodetic, geochemical and petrological data. In this work we discuss and interpret these data in order to describe the mechanisms of 2011-2012 El Hierro eruption, including ascent from magmatic source, a crustal storage, and the final intrusion in the South Rift before the eruption. Our research approach provides a multidisciplinary view of the dynamics of magma ascent and improves previous interpretations formulated during or shortly after the end of the eruption. According to our results, a major intrusion occurred beneath and around preexisting high-density magmatic bodies, localized at depth below the central part of the island. After a failed attempt to reach the surface through a low fractured zone located below the central-northern part of the island, the ascending magma finally found its way nearby the El Hierro South Rift Zone and erupted off the coast of La Restinga village, 350 m below sea level. The eruption was fed by the ascent of an important volume of material from the upper mantle that was emplaced near the crust-mantle boundary and progressively tapped during the eruption.

Clinopyroxene (Cpx) phenocrysts have the potential to record magmatic water contents and magma ascent rates through their concentration of H2O (incorporated as H+ and commonly referred to as water). Here we investigate three issues related to the fidelity and utility of the Cpx water record: the partitioning of water between Cpx and melt, the diffusivity of water in natural Cpx phenocrysts, and the possibility for water loss in Cpx erupted in lava flows. Samples studied are from volcanic ash of the 1974 eruption of Volcán de Fuego (Guatemala) and scoria and lava from the 1977 eruption on Seguam Island (Alaska). The partitioning of water was determined by analyzing melt inclusions (MIs) and the adjacent clinopyroxene host by ion microprobe. For seven Cpx-hosted MIs from Seguam, the partition coefficient is well predicted (within 0·1 wt % H2O on average) using the temperature-dependent parameterization of O'Leary et al. (2010; Earth and Planetary Science Letters 297, 111-120). For the determination of diffusivity, H2O concentration profiles were measured in oriented Cpx from Fuego by ion microprobe. Water decreases toward the rim, consistent with diffusive re-equilibration during ascent-driven degassing. Using previously estimated durations of ascent (7-12 min), we determined the H+ diffusivity (10-9·7-10-10·3 m² s-1 at a temperature of 1030 °C) that would satisfy these timescales. These calculated DH+Cpx values are comparable with the medial values determined for natural Cpx (Mg# < 92·5) in laboratory diffusion studies. Tephra and lava co-erupted on Seguam bear similar Cpx populations in their major and trace element compositions, but the lava Cpx contain 80% lower H2O concentrations on average than the tephra Cpx. Using the DH+Cpx values obtained from the Fuego Cpx, the difference in H2O between the lava and tephra Cpx can be attributed to post-eruption H2O loss during the estimated ~20 min the sample remained above the H+ closure temperature. The results from this study indicate that clinopyroxene from slowly cooled basaltic lavas should not be used to reconstruct initial magmatic water contents. High ascent rates, rapid post-eruptive cooling, large phenocrysts, and cooler magma (e.g. andesites and rhyolites) favor better preservation of water in Cpx. In cases of H+ loss, Cpx zonation can be exploited as a chronometer for syn- and post-eruptive volcanic processes. [ABSTRACT FROM AUTHOR]

The ascent of H2O- and H2O-CO2-bearing basaltic melts from the deeper to the shallower part of the Stromboli magmatic system and their vesiculation were simulated from decompression experiments. A well-studied "golden" pumice produced during an intermediate- to a large-scale paroxysm was used as starting material. Volatile-bearing glasses were synthesized at an oxygen fugacity (...) ranging from NNO-1.4 to +0.9, 1200 °C and 200 MPa. The resulting crystal- and bubble-free glasses were then isothermally (1200 °C) decompressed to final pressures Pf ranging between 200 and 25 MPa, at a linear ascent rate of 1.5 m/s (or 39 kPa/s) prior to be rapidly quenched. Textures of postdecompression glasses that were characterized by X-ray computed tomography result from different mechanisms of degassing that include bubble nucleation, growth, coalescence, and outgassing, as well as fragmentation. Homogeneous bubble nucleation occurs for supersaturation pressures (difference between saturation pressure and pressure at which bubbles start to form homogeneously, ΔPHoN) ≤ 50 MPa. In the CO2-free melts, homogeneous nucleation occurs as two distinct events, the first at high Pf (200-150 MPa) and the second at low Pf (50-25 MPa) near the fragmentation level. In contrast, in the CO2-bearing melts, multiple events of homogeneous bubble nucleation occur over a substantial Pf interval along the decompression path. Bubble coalescence occurs in both H2O- and H2O-CO2-bearing melts and is the more strongly marked between 100 and 50 MPa Pf. The CO2-free melts follow equilibrium degassing until 100 MPa Pf and are slightly supersaturated at 60 and 50 MPa Pf, thus providing the driving force for the second bubble nucleation event. In comparison, disequilibrium degassing occurs systematically in the CO2-bearing melts that retain high CO2 concentrations. Fragmentation was observed in some CO2-free charges decompressed to 25 MPa Pf and is intimately associated with the occurrence of the second bubble nucleation event. Textures of H2O-CO2-bearing glasses reproduce certain critical aspects of the Stromboli natural textures (bubble number densities, shapes, sizes, and distributions) and chemistries (residual volatile concentrations). Average bubble sizes, bubble size distribution (BSD), and bubble number density (BND) data are used together to estimate that the "golden" pumice magmas ascend from their source region in 43 to 128 min. [ABSTRACT FROM AUTHOR]