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Explosive eruptions of trachytic to trachyandesitic composition produced thick and widespread pyroclastic deposits during the recent volcanic activity of Piton des Neiges (PdN), the largest volcano of La Réunion Island (France). However, without precise geochronology for these eruptions, the future behavior of the dormant volcano is difficult to assess. New high-resolution single crystal 40Ar/39Ar dates on alkali feldspar, together with petrochemical data on juvenile clasts, indicate that instead of multiple and small eruptions, PdN produced a single large-volume event (“Dalle Soudée” eruption) at 198.8 ± 2.5 kyrs in which its entire crustal plumbing system was reactivated by mafic injections in the silicic reservoir. After 60 kyrs of quiescence, PdN activity resumed with several small silicic eruptions. From a dome-forming trachytic event (Belouve eruption) represented by some of the most recent block-and-ash flow deposits near the present-day summit we report in-situ 238U/230Th disequilibrium dates on pristine zircons. Sanidine and zircon crystalized at ca. 875 °C in the trachytic magma. For this event, the corresponding 238U/230Th isochron age of the zircons overlaps with the youngest 40Ar/39Ar dates on single sanidine crystals and yields an age of 42.7 ± 3.8 kyr. The variability in trace elements of zircon argues for a prolonged crystallization during this phase prior to eruption. Similarly, the 40Ar/39Ar sanidine dates show a significant spread in both eruptions, covering ca. 80 kyrs, uncommon for a method with a predicted low closure temperature. Such a pattern of age distribution argues for the presence of inherited argon in sanidines and for long residence times in the magma reservoir. The recent eruptive history of Piton des Neiges, involving evolved magma compositions, long-living silicic reservoirs and millennia-long quiescence in volcanic activity, suggests that future potentially hazardous eruptions from this volcano cannot be excluded., Chemical Geology, 632, ISSN:0009-2541, ISSN:1872-6836

Lithium and hydrogen are volatile elements which diffuse rapidly in crystals and melt, making them powerful geochemical tools to reconstruct geological processes that take place on short time scales, such as syn- and post-eruptive degassing. Although the dynamics of hydrogen are fairly well understood to better constrain such processes, the assessment of Li behaviour within the magma reservoir relevant for ascent-related degassing still lacks detailed evaluation. Here, the first in situ Li concentrations and isotopic compositions (using SIMS analysis) of rhyolitic quartz-hosted, naturally glassy and crystallised melt inclusions (MIs) and groundmass glass (Mesa Falls Tuff, Yellowstone) are used to reconstruct Li elemental and isotopic evolution in the magma reservoir. Lithium concentrations in quartz-hosted glassy MIs (10–61 ppm) from a fallout deposit overlap with their groundmass glass (32–46 ppm) and their host quartz (8–15 ppm). Crystallised MIs from a later erupted flow pumice clast sample have higher Li concentrations (8–190 ppm) compared to the groundmass glass (32–51 ppm) and their host quartz (15–24 ppm). Lithium content in quartz from the early erupted sample is relatively homogenous, whereas it is up to a factor of two higher and heterogeneous in the later erupted sample, with a simultaneous increase in Li versus a decrease in H towards crystal rims. The δ7Li difference (expressed as Δ7LiMI–glass) between MIs (−8.0‰ to + 12.3‰) and groundmass glass (+9.0‰ to + 20.5‰) of two pyroclastic deposits reaches up to 29‰. Glassy MIs are internally heterogeneous in δ7Li and Li abundance. The cores of the glassy MIs record the δ7Li of the least modified melt during entrapment and the data distribution can be modelled by equilibrium fractionation between the melt and vapour phase during early open-system degassing in the magma reservoir. Late degassing during eruption triggers Li–H diffusional exchange between quartz and melt, as the degassing of H2O and the accompanying pressure change trigger H diffusion out of the host quartz and the MIs, which is charge balanced by inward Li diffusion. This results in the modification of Li contents in quartz and δ7Li values in the rims of the glassy MIs. Crystallised MIs reflect the loss of H2O from the MIs and the resulting enrichment of Li during the crystallisation. Additionally, the variations of δ7Li in the groundmass glass can be explained through modelling by kinetic fractionation between the melt and vapour during late stage open-system degassing linked with magma ascent. ISSN:0016-7037 ISSN:1872-9533

Understanding magmatic activity on the Early Earth remains a challenge for geoscientists, as most of its rock record has been destroyed or altered. The oldest exposed rocks belong to the Tonalite-Trondhjemite-Granodiorite (TTG) plutonic suite, only rarely associated with volcanic units of the same age. For this reason, TTGs are often interpreted as magmas that have not erupted, and their compositions thought to represent melts. However, if TTGs are the left-overs from shallow magma reservoirs that have lost some melt to the now-eroded volcanic record, their bulk composition would be at least partly biased towards crystal cumulates. As post-emplacement metamorphism typically overprints many of the chemical characteristics of the initial magmatic minerals, the more resistant magmatic minerals (quartz and zircons) within sedimentary successions derived from these systems provide the best chance of identifying volcanic lithologies that have been completely eroded. Here we use a novel approach to show that Ti-in-quartz and Ti-in-zircon thermometers can be used to recognise different magmatic sources in sedimentary rocks. In quartz, Ti thermometry calibrated against blue cathodoluminescence obtained from scanning electron microscopy allows for fast and statistically meaningful Ti quantification in hundreds of sedimentary quartz grains. This imaging-derived Ti distribution matches well with the distribution of Ti concentrations obtained by LA-ICP-MS spot measurements of individual crystals. We compare this quartz record to Ti distributions in zircons, which have the benefit of also providing a crystallisation age. We applied these techniques to the Pikes Peak Batholith (CO, USA), a 1.1 Ga A-type granite hosting several pegmatites, and the Tava sandstone, a series of Cryogenian intra-granite sedimentary dikes that represents the oldest terrestrial sediments in the Front Ranges of Colorado. Our data successfully separates plutonic from pegmatitic crystals and shows that quartz and zircon crystals in the Tava Sandstone crystallised at statistically higher temperatures than the ones observed in the Pikes Peak Batholith, implying potential contribution from a volcanic source that is no longer available on the surface. The proposed techniques can therefore be used to identify eroded magmatic lithologies and to estimate proportions of different magmatic components (volcanic, plutonic, pegmatitic) in sediments.

The discovery of 18O-depleted igneous rocks at Krafla, Iceland, suggests that the system interacted with crustal rocks that experienced high-temperature hydrothermal alteration by a meteoric fluid to deviate from the expected mantle signature (δ18O = 5.5 ‰). Such assimilation is documented in low-δ18O settings worldwide, however, the mechanisms of this dynamic process remain poorly understood. Due to intense drilling activity and exploration at Krafla, both hydrothermally altered crustal rocks and parental magma are comparably well characterized, making Krafla a great case study for the application of a numerical model that can further advance the understanding of the formation process of low-δ18O magmas. In this study, we use a new three-phase two-component thermo-chemical-mechanical model to simulate the effect of variable crustal compositions on the assimilation process and the magma chamber dynamics. We define the simplified square-shaped magma chamber (10 x 10 m) of magma with initially basaltic composition (1250 °C) that assimilates the crustal rock (500 °C) at the top and bottom. Our results indicate that convective behaviour and the formation of cumulate layers can significantly hinder the assimilation process. While the crystal settling Stokes speed scale is the dominant driver for the formation of this boundary layer, depending on the assimilation timescales, the mushy chamber margins are able to grow to sufficient thickness to prohibit additional assimilation of low-δ18O crustal material. Density and buoyancy contrasts produce three types of convection: chamber convection, layered convection and plume driven convection. Final magma compositions in our preliminary model outputs range from mafic to intermediate but are not able to reach the felsic compositions encountered at Krafla. This suggests that evolution towards the erupted low-δ18O rhyolitic products involved multiple stages or included additional factors not yet accounted for in our model. Further refining of this and similar thermo-chemical-mechanical model setups may provide important new insights into the assimilation dynamics in the Krafla volcanic field and other low-δ18O settings worldwide.

As ascending magmas undergo cooling and crystallization, water and fluid-mobile elements (e.g., Li, B, C, F, S, Cl) become increasingly enriched in the residual melt until fluid saturation is reached. The consequential exsolution of a fluid phase dominated by H2O (magmatic volatile phase or MVP) is predicted to occur early in the evolution of long-lived crystal-rich “mushy” magma reservoirs and can be simulated by tracking the chemical and physical evolution of these reservoirs in thermomechanical numerical models. Pegmatites are commonly interpreted as the products of crystallization of late-stage volatile-rich liquids sourced from granitic igneous bodies. However, little is known about the timing and mechanism of extraction of pegmatitic liquids from their source. In this study, we review findings from thermomechanical models on the physical and chemical evolution of melt and MVP in near-solidus magma reservoirs and apply these to textural and chemical observations from pegmatites. As an example, we use a three-phase compaction model of a section of a mushy reservoir and couple this to fluid-melt and mineral-melt partition coefficients of volatile trace elements (Li, Cl, S, F, B). We track various physical parameters of melt, crystals, and MVP, such as volume fractions, densities, velocities, as well as the content in the volatile trace elements mentioned above. The results suggest that typical pegmatite-like compositions (i.e., enriched in incompatible elements) require high crystallinities (>70–75 vol% crystals) in the magma reservoir, at which MVP is efficiently trapped in the crystal network. Fluid-mobile trace elements can become enriched beyond contents expected from closed-system equilibrium crystallization by transport of MVP from more-evolved mush domains. From a thermomechanical perspective, these observations indicate that, rather than from melt, pegmatites may more likely be generated from pressurized, solute-rich MVP with high concentrations of dissolved silicate melt and fluid-mobile elements. Hydraulic fracturing provides a mechanism for the extraction and emplacement of such pegmatite-generating liquids in and around the main parental near-solidus mush as pockets, dikes, and small intrusive bodies. This thermomechanical framework for the extraction of MVP from mushes and associated formation of pegmatites integrates both igneous and hydrothermal realms into the concept of transcrustal magmatic distillation columns.

Lithium is an economically important element that is increasingly extracted from brines accumulated in continental basins. While a number of studies have identified silicic magmatic rocks as the ultimate source of dissolved brine lithium, the processes by which Li is mobilized remain poorly constrained. Here we focus on the potential of low-temperature, post-eruptive processes to remove Li from volcanic glass and generate Li-rich fluids. The rhyolitic glasses in this study (from the Yellowstone-Snake River Plain volcanic province in western North America) have interacted with meteoric water emplacement as revealed by textures and a variety of geochemical and isotopic signatures. Indices of glass hydration correlate with Li concentrations, suggesting Li is lost to the water during the water-rock interaction. We estimate the original Li content upon deposition and the magnitude of Li depletion both by direct in situ glass measurements and by applying a partition-coefficient approach to plagioclase Li contents. Across our whole sample set (19 eruptive units spanning ca. 10 m.y.), Li losses average 8.9 ppm, with a maximum loss of 37.5 ppm. This allows estimation of the dense rock equivalent of silicic volcanic lithologies required to potentially source a brine deposit. Our data indicate that surficial processes occurring post-eruption may provide sufficient Li to form economic deposits. We found no relationship between deposit age and Li loss, i.e., hydration does not appear to be an ongoing process. Rather, it occurs primarily while the deposit is cooling shortly after eruption, with δ18O and δD in our case study suggesting a temperature window of 40° to 70°C.

The contemporary basaltic and rhyolitic volcanism with a dearth of intermediate compositions creates a compositional gap (‘Daly gap’), which is known to be pronounced in continental rifts/hotspots but absent in continental arcs. The intrinsic mechanism(s) producing the Daly gap remain poorly understood. Similar bimodal spectra recorded in melt inclusions from continental rifts/hotspots and arcs suggest that similar processes modulate the bulk composition of erupted magmas. A compiled data set of melt inclusion composition, phenocryst fraction, pre-eruptive temperature and melt H2O content enables precise calculations of melt viscosities and bulk magma viscosities for >100 eruptions of global volcanoes in various tectonic settings. Analyses of the data set suggest lower viscosities of rhyolitic melts in arcs than in rifts/hotspots (~104.5 ± 0.2 versus ~105.5 ± 0.2 Pa·s, respectively), at comparable temperatures, because of ~1.5 wt % higher melt H2O contents in arcs. Combining numerical constraints on ascent dynamics and data set observations, our study defines an eruption threshold of ~106.0 ± 0.3 Pa·s at the base of a volcanic conduit rooted in the subvolcanic reservoir. The lower viscosities of rhyolitic melts in arcs facilitate stirring/re-homogenization of upper crustal reservoirs upon recharge and accommodation of more phenocrysts in erupted products, leading to the common appearance of crystal-rich (30–50%) andesites and dacites (with rhyolitic interstitial melts) with mixing signatures in magmatic provinces of continental arcs. In contrast, the lower mixing efficiency and eruptibility of the more viscous magmas present in continental rifts/hotspots lead to preferential eruption of crystal-poor (

The chemical composition of gases emitted by active volcanoes reflects both magma degassing and shallower processes, such as fluid-rock hydrothermal interaction and mixing with atmospheric-derived fluids. Untangling the magmatic fluid endmember within surface gas emission is therefore challenging, even with the use of well-known magma degassing tracers such as noble gases. Here, we investigate the deep magmatic fluid composition at the Nisyros caldera (Aegean Arc, Greece) by measuring nitrogen and noble gas abundances and isotopes in naturally degassing fumaroles. Gas samples were collected from 32 fumarolic vents at water-boiling temperature between 2018 and 2021. These fumaroles are admixtures of magmatic fluids typical of subduction zones, groundwater (or air saturated water, ASW), and air. The N2, He, and Ar composition of the magmatic endmember is calculated by reverse mixing modeling and shows N2/He = 31.8 ± 4.5, N2/Ar = 281.6, δ15N = +7 ± 3 ‰, 3He/4He = 6.2 Ra (where Ra is air 3He/4He), and 40Ar/36Ar = 551.6 ± 19.8. Although N2/He is significantly low with respect to typical values for arc volcanoes (1,000–10,000), the contribution of subducted sediments to the Aegean Arc magma generation is reflected by the positive δ15N values of Nisyros fumaroles. The low N2/He ratio indicates N2-depletion due to solubility-controlled differential degassing of an upper-crustal silicic (dacitic/rhyodacitic) melt in a high-crystallinity reservoir. We compare our 2018–2021 data with N2, He, and Ar values collected from the same fumaroles during a hydrothermal unrest following the seismic crisis in 1996–1997. Results show additions of both magmatic fluid and ASW during this unrest. In the same period, fumarolic vents display an increase in magmatic species relative to hydrothermal gas, such as CO2/CH4 and He/CH4 ratios, an increase of ∼50 °C in the equilibrium temperature of the hydrothermal system (up to 325 °C), and greater amounts of vapor separation. These variations reflect an episode of magmatic fluid expulsion during the seismic crisis. The excess of heat and mass supplied by the magmatic fluid injection is then dissipated through boiling of deeper and peripheral parts of the hydrothermal system. Reverse mixing modeling of fumarolic N2-He-Ar has therefore important ramifications not only to disentangle the magmatic signature from gases emitted during periods of dormancy, but also to trace episodes of magmatic outgassing and better understand the state of the upper crustal reservoir., Geochimica et Cosmochimica Acta, 335, ISSN:0016-7037, ISSN:1872-9533

The origins of felsic low-δ18O melts (< +5.5‰) are usually attributed to assimilation of high-temperature hydrothermally altered (HTHA) rocks. Very few alkaline (silica-undersaturated and/or peralkaline) examples are known. Here, we classify the Miocene Fataga Group in Gran Canaria, a silica-undersaturated to mildly saturated alkaline volcanic sequence consisting of trachytic to phonolitic extra-caldera ignimbrites and lavas, as a new low-δ18O felsic locality. We provide new mineral, glass, and bulk geochemical data linked to a well-constrained stratigraphy to assess the processes involved in the magma reservoir that fed the Fataga eruptions. New high-precision single crystal feldspar 40Ar/39Ar ages of the study area span 13.931 ± 0.034 Ma to 10.288 ± 0.016 Ma. Fractional crystallization at shallow depths of sanidine/anorthoclase, biotite, augite/diopside, titanite, ilmenite, and titanomagnetite is the main driving process to produce phonolitic magmas from trachytic melts. Evidence of mafic hotter recharge is not found in the field, but some units exhibit trachytic compositions characterized by positive Eu/Eu* anomalies and high Ba contents, interpreted as melts of feldspar-dominated cumulates, the solid remnants of fractional crystallization. Hence, recharge magmas halted in the crystal mush and provided the heat needed to sustain cumulate melting and volcanic activity. This cumulate signature might be lost if fractional crystallization continues before the eruption. The interplay among meteoric water, the caldera-fault system, intra-caldera ignimbrites (Mogán Group), and the Fataga magma reservoir favoured assimilation of up to ca. 30% of HTHA rocks. Such assimilation is variable through time and recorded by δ18Omelt values down to +4.73‰. We did not find any direct relation between assimilation and silica saturation of the Fataga volcanic deposits.

International audience; Volcano sector collapses are catastrophic events that can mobilize huge volumes of material and cause changes in the magmatic plumbing system, leading to variations in growth rate and/or composition. Dating pre-historic volcanic debris avalanche deposits is challenging. Geological materials directly recording avalanche formation and amenable to radiometric dating are rare, and, in the case of Holocene events, the applicable radiometric dating techniques are scarce. Socompa volcano suffered the largest collapse event in the Central Volcanic Zone of the Andes, producing one of Earth's most spectacular and best-preserved volcano avalanche deposits. We apply multiple dating techniques to unravel the timing of Socompa's collapse and gain insights into its pre- and post-collapse eruptive history. The age of the collapse event is constrained by a 14C age of 6,200-6,400 BP of a paleosol buried by the avalanche, and by a post-collapse lava flow dated at 5.91 ± 0.43 ka by zircon double-dating. Bayesian age sequence modeling integrating these ages determines that the collapse occurred at 6.18-0.64+0.28 ka. Four zircon eruption dates and one unspiked K-Ar age between 69.2 ± 6.0 and 22.1 ± 1.9 ka constrain the age of the youngest stage of activity before the collapse. The ages, together with paleosurface modeling and volume calculations, allow estimating growth rates for the young pre-collapse and post-collapse stages of ~ 0.2-0.3 km3/kyr and ~ 0.5-2 km3/kyr, respectively, indicating a significant increase in activity after the collapse event. The collapse may have triggered a new growth phase or accelerated an ongoing one and was likely associated with a Plinian eruption that produced widespread pumice fallout. The pre- and post-collapse lavas have similar compositions and zircon crystallization age distributions, suggesting that the same or a similar magma reservoir was tapped before and after the collapse. Thus, huge collapses such as Socompa's event can promote increased volcanic activity as a consequence of the unloading effect, but the overall plumbing system may not be affected enough to show significant variations in erupted compositions, at least transiently. Our results highlight the efficacy of the zircon double-dating method for dating very young felsic lavas and for constraining the age of debris avalanche deposits. This is particularly relevant in the Andean Central Volcanic Zone and other regions with arid climates, where organic material is rare and hence 14C dating is often unfeasible. Furthermore, the post-collapse zircon eruption age of 5.91 ± 0.43 ka is the youngest radiometric age yet obtained for a lava flow in the southern Central Volcanic Zone, highlighting the youth of volcanic activity at Socompa, and confirming its status as a Holocene active volcano.

Inter-session excess variance of U/Pb and Pb/Pb ratios in LA-ICP-MS zircon dating is the largest contributor to systematic errors, which in turn limit the accuracy of age determinations. Quantifying long-term excess variance of reference materials allows for the estimation of excess variance in samples, but such compilations are not available in the literature. Here, we present the results of over 100 measurement sessions over 6 years for seven common zircon reference materials and calculate a characteristic excess variance for each. For Pb-206/U-238 ages, these values (2s) are as follows: AusZ7-1 = 1.7% (115 sessions), AusZ7-5 = 0% (74), OD-3 = 0.6% (19), Temora2 = 1.2% (86), Plesovice = 1.1% (100), 91500 = 1.0% (146) and Mud Tank = 3.0% (12). For Pb-207/Pb-206 ages, smaller excess variances are observed: Temora2 = 0.3%, Plesovice = 0.4%, Mud Tank = 1.7% and 91500 = 0.4%. These values are well-constrained estimates of inter-session excess variance for the ETH Zurich LA-ICP-MS laboratory using a sector-field ICP-MS, and may provide either a first-order estimate for other laboratories or a value to compare against their own reference material compilations., Geostandards and Geoanalytical Research, 46 (3), ISSN:1639-4488, ISSN:1751-908X

Caldera-forming eruptions are among the most hazardous natural events on Earth and pose a significant risk for global consequences in the future. Recent petrological re-evaluations of caldera and intercaldera deposits in several volcanic systems worldwide suggest a cyclic behavior in the evolution of these subvolcanic reservoirs, comprising distinct maturation, fermentation and recovery phases. Here, we test the application of this caldera cycle framework on the Aso system in Central Kyushu (Japan) by evaluating pre-caldera activity of the Aso-4 caldera-forming event.The Aso system is an archetypical example of a multi-cyclic caldera-forming volcanic edifice, which was built by four catastrophic caldera-forming events between 266 ka (Aso-1) and 86.4 ka (Aso-4). These caldera-forming eruptions are separated by extensive post- and pre-caldera activity from numerous vents inside and outside the caldera areas. The deposits produced from these vents range from basalts to rhyolites in bulk-rock compositions and are dominated by a mineral assemblage consisting of plagioclase, ortho- and clinopyroxene, Fe-Ti oxides and apatite.Plagioclase and orthopyroxene are the most abundant minerals in the Aso system. However, their often wide compositional ranges, recording multiple stages of magma evolution, make it difficult to pinpoint chemical differences between pre- or post-caldera eruptions; geochemical analyses of orthopyroxene and plagioclase give overlapping ranges for pre-Aso-4 and Aso-4 caldera-forming events with Mg# between 71 to 75 and An% between 35 to 80, respectively. On the other hand, accessory mineral phases, such as titanomagnetite and apatite, chemically re-equilibrate fast with the melt and hence predominantly inherit conditions prevailing just prior to eruption. Average MnO contents in titanomagnetite record a distinct increase from ~0.8 wt% to ~0.9 wt% in pre-Aso-4 crystals to 1.2 wt% in the silicic cap of the Aso-4 system, indicating the progressive evolution of the system towards more differentiated compositions. Similarly, the F-Cl-OH record of apatite in the pre-Aso-4 and the Aso-4 system indicate the transition from water-undersaturated conditions during magmatic evolution in the pre-Aso-4 system to water-saturated conditions in the silicic cap of the Aso-4 system. Concomitant with these variations, changes in dissolved water contents in melts and storage temperatures are observed, with temperatures progressively decreasing from the pre-Aso-4 units (> 900 °C) to the silicic portion of the Aso-4 deposits (~860-880 °C). Dissolved water contents in the melt, in turn, increase from ~3-4 wt% in the pre-caldera system to ≥ 4.6 wt% in the Aso-4 silicic cap, close to or even at volatile saturation considering ~100-400 ppm CO2 in the melt at storage depths (~1.5 - 2 kbar). These findings are in good agreement with the caldera cycle framework and suggest the progressive differentiation of the Aso system during a maturation phase through volatile exsolution in the fermentation stage to the catastrophic Aso-4 caldera-forming eruption.

The fate of many economically important elements is controlled at the magmatic-hydrothermal transition, where the co-occurrence of melt and magmatic fluid may significantly change partitioning. With increasing usage of batteries across a range of appliances the requirement for lithium (Li) is growing. Despite this, the behaviour of Li in silicic magmatic systems remains poorly known. Here, we illustrate how compositionally unusual biotites from the Bishop and Kos Plateau tuffs may contain a magmatic volatile phase trapped between layers of biotite crystals. These biotite crystals come from pristine volcanic deposits, appear fresh under the binocular microscope and return oxygen isotopic compositions that, taken together with other phases in the same sample, indicate high-temperature equilibrium. Biotite separates show expected XRD specta indicating the absence of other crystalline phases and yet these biotites return low (< 95 wt.%) analytical totals via electron microprobe (EMP) consistent with the presence of considerable amounts of light elements (non-measurable by EMP). Lithium abundances of these biotites are remarkable with values reaching >2,300 ppm with similar results from both spots and line analyses. Lithium isotopic compositions in these biotites are exceptionally light (δ7Li as low as -27.6‰) and large isotopic fractionation between biotite and corresponding bulk samples (Δ7Libt–bulk as low as −37.3‰). Groundmass glasses, melt inclusions and other mineral phases from the Kos and Bishop systems do not support an extremely Li-rich melt prior to eruption. In contrast, biotites from the phonolitic systems of Campi Flegrei and Tenerife do not exhibit such extreme compositions with bioites and melts having approximately equivalent Li contents with Δ7Libt–bulk to a maximum of −10.9‰. We infer this difference in behaviour to the appearance of biotite in alkaline systems occurring prior to the widespread exsolution of a magmatic volatile phase in the magma reservoir. In the rhyolitic suites, biotite crystallises at lower temperatures and so most biotite growth occurs in the presence of an exsolved fluid phase allowing such a fluid to be trapped within the biotites.

Most volcanoes, across tectonic settings, can show both explosive and effusive eruptions, either as separated eruptive events or within the same eruptive episode. Such differences in eruptive style have significant implications for depositional morphology and hazards associated with the eruptions. In many cases, chemically nearly identical magmas may produce either explosive or effusive events. In contrast, we find the intercalated ignimbrites and lavas of trachytic to phonolitic compositions of the Fataga Group, Gran Canaria, differ markedly in several aspects. First, field observations revealed that the lavas pinch out toward the caldera rim and are therefore likely originated from extra-caldera sources. Second, while the explosive deposits vary in crystallinity (here referring to crystals with a long axis > 250 µm) from a few percent in crystal-poor portions to 10 – 20% in crystal-rich clasts, the lavas are almost aphyric with crystallinities < 1%. Further, while the ignimbrites have a mineral assemblage containing alkali-feldspar, biotite, pyroxene, amphibole, titanite, and Fe-Ti oxides, the lavas mostly contain alkali feldspar as a nearly unique mineral phase. Third, major elemental compositions show that while ignimbrites and lavas may overlap within the trachyte field on a TAS plot, only the lavas have compositions that extend to phonolite. Trace elemental compositions of lavas suggest extensive fractionation with compatible elements (e.g. Ba, max 227 ppm, avg. 30 ppm) depleted and incompatibles (e.g. Zr, Hf, Ce, Rb) enriched. While crystal-poor juveniles from the ignimbrites may have compositions approaching those of the lavas, crystal-rich juvenile clasts are markedly enriched in feldspar-phyric elements (e.g. Ba, max 1892 ppm, avg. 1496 ppm) suggesting involvement with a feldspar-dominated cumulate pile. Furthermore, CIPW norm calculations show that the lavas trend towards nepheline-normative compositions. The normative prediction of nepheline occurrence in the lavas is confirmed by petrographic observation of nepheline both as groundmass constituent and rarely as phenocrysts and thus suggests a somewhat different petrogenetic history for the lavas. Fourth, on a crystal scale, the feldspar phenocrysts in the lavas have relatively restricted, and low Ba contents (20 - 500 ppm), while ignimbrites have extremely Ba-enriched feldspars (up to 18,000 ppm Ba) in their crystal-rich portions.Deposit geometry, petrography, and geochemical data lead us to the conclusion that the Fataga lavas must have followed a different petrogenetic path than their explosive counterparts. We, therefore, suggest the magmas feeding the effusive eruptions may have bypassed the main caldera system and thus provide a different window into the Miocene magmatism of Gran Canaria.

Many volcanic eruptions produce pyroclastic deposits that display pronounced internal gradients in composition, mineralogy, and phenocryst abundance. The volcanic succession of the Mogán Group (14.2–13.6 Ma) of Gran Canaria, Spain, provides an exceptional opportunity to study the temporal evolution of a long-lived magmatic system and the associated diverse styles of zoning preserved in the volcanic record.The Upper Mogán Formation (13.9–13.6 Ma) contains strongly to moderately welded, peralkaline trachytic to rhyolitic ignimbrites erupted from the multicyclic Tejeda Caldera. Many ignimbrites exhibit upward increases in crystallinity and pumice size and abundance associated with decreases in welding intensity and abundance of lithic clasts. A typical characteristic of the deposits is the coexistence of crystal-poor and crystal-rich pumices along with mingled varieties within a given cooling unit. Major element variations between pumice types are commonly small yet some trace elements (Rb, Zr, Sr, Ba) may differ substantially. We focus here on ignimbrite ‘D’ (13.7 Ma, ~15 km3), which contains dominantly crystal-poor trachydacite fiamme (SiO2 68–69 wt%, Ba 80–180 ppm, Zr 2250–2460 ppm) in the lower parts that show increased mingling with a crystal-rich comenditic trachyte (SiO2 64–66 wt%, Ba 540–2090 ppm, Zr 420–930 ppm) in the upper parts of the deposit. The mineral assemblage of ‘D’ is dominated by anorthoclase and amphibole with minor clinopyroxene, plagioclase, FeTi-oxides and apatite. Phenocryst abundance increases from nearly aphyric at the base to ca. 15 vol% towards the top. The crystal-poor trachydacite is enriched in components that are incompatible with the observed phenocryst assemblage (FeO, Zr, Rb, Nb, REE). Conversely, the crystal-rich trachyte is enriched in typically compatible major and trace elements (Al2O3, Na2O, P2O5, Ba, Sr).We suggest that crystal-poor pumices in ignimbrite D are derived from evolved aphyric melts extracted from highly crystalline portions of an upper crustal magma reservoir and that crystal-rich pumices contain a considerable component of remobilised shallow cumulate. This interpretation is supported by strong enrichments in Ba in the crystal-rich pumices, significantly larger than expected in melts evolving along liquid lines of descent from a trachybasaltic parent. We show that extensive fractionation of a mineral assemblage dominated by anorthoclase is an important petrogenetic mechanism in the Upper Mogán Formation and not only produces the observed depletions in Ba, Sr and negative Eu anomalies in the crystal-poor pumices but also lays the foundation for remelting of a feldspar-dominated cumulate.This type of zoning strongly contrasts with that of cooling unit ‘P1’ (14.2 Ma, ~45 km3), which is compositionally zoned from a silicic lower part to a basaltic top and the result of mixing between rhyolitic, trachytic and basaltic magma components. We propose that the observed temporal changes in zoning style and decrease in frequency of basaltic eruptions are related to progressive thermal conditioning and increasing ‘mushification’ of the upper crust, which creates a thermal and viscosity barrier suppressing basaltic eruptions.

The granite solidus curve is generally placed around 660-700°C, depending on the pressure conditions and water content of the melt. However, recent studies have documented evidence of subsolidus crystallisation in granitic rocks, with minerals recording temperatures

The location, volume and physical states of magma reservoirs are primary controls on the eruptive behavior of volcanic systems. Fundamental to understanding and monitoring these systems is the ability to identify reservoir size and physical properties, in particular melt fraction which plays an important role in the rheology and stability of a magmatic system. Large silicic volcanic eruptions in the geological record suggest that extensive pockets of melt-rich silicic magma must exist in the subsurface but such melt pockets have not been detected by geophysics. This has led to the question of whether the reservoirs that feed large volcanic eruptions are only melt-rich for a short time and thus would only be detected by geophysics shortly prior to an eruption. Magnetotelluric data measure the electrical resistivity of the subsurface and are sensitive to subsurface fluids and partial melts making it a powerful tool for imaging subvolcanic magma reservoirs. This study examines the ability for magnetotelluric data to accurately estimate melt fraction using both stochastic Bayesian inversion and deterministic regularized inversion. Results from synthetic modelling indicate that magnetotelluric data are best able to predict the melt fraction for the thick melt-rich layer using both inversion methods, though both methods under-estimate the true amount of melt. In addition, magnetotelluric data can accurately detect changes in melt fraction from crystal- rich mush (0.1 melt fraction) to melt-rich magma (0.9 melt fraction) for thick layers. Thickness is a key parameter which provides a method to assess the total volume of melt present, but it is difficult to estimate using smooth regularized inversions. Fixed-dimension Bayesian inversions provide estimates of layer thickness and their un- certainties and it is shown that estimates of total volume of melt are more accurate for both thin (0.2 km) and thick (1 km) layers than individual estimates of melt fraction or thickness alone. The melt fraction of the thin melt-rich layer is under-estimated using both the deterministic inversion method and the most probable solution of the Bayesian inversion. The Bayesian distribution of solutions has long tails which includes the true solution and better represents the uncertainty of the modelled parameters. As we consider past large melt-rich caldera eruptions at Taupo (New Zealand), Toba (Indonesia), Yellowstone and Long Valley (USA), our analysis suggests that thin melt-rich, potentially eruptible zones involved in large (e.g. > 500 km3) eruptions could be systematically misinterpreted as thicker non-eruptible crystal-rich mush zones. Testing the sensitivity of MT to detect melt accumulation in active magmatic systems has important implications for volcanic hazard assessment and the use of Bayesian inversions allows for a better understanding of the range of possible interpretations., Journal of Volcanology and Geothermal Research, 423, ISSN:0377-0273

Sedimentological and zircon U–Pb–Hf isotopic analyses of the Carboniferous strata in the Bainaimiao arc belt (BAB) provide provenance information for the northern margin of the North China Craton (NCC). The late Carboniferous strata in the eastern BAB were dominantly shallow marine carbonate-clastic deposits. Detrital zircon data show that: (1) sandstone from lower Sidaozhangpeng Formation displays two major age peaks of 369 Ma and 2522 Ma, with eHf (t) values of − 25.9 to − 20.4 and 2.2–6.4, respectively; (2) sandstone from upper Sidaozhangpeng Formation shows a significant age peak at 384 Ma, with eHf (t) values of − 25.3 to − 14.5. Determination of detrital zircon parent rock types based on trace element proxies suggests that the protolith rock types consist of felsic (73%) and mafic rocks (22%), with minor alkaline rocks (5%), implying a continental arc setting. The U–Pb–Hf dating of the Carboniferous strata in the western and eastern BAB shows different provenances: the Carboniferous sediments in the western BAB were derived from early Paleozoic magmatic rocks, micro-block (Hunshandake Block) and the NCC basement; the provenances of the Carboniferous strata from the eastern BAB were mainly local Devonian magmatic rocks, with subordinate input of the Neoarchean NCC basement rocks. The early Paleozoic zircon grains recorded by the early Devonian rhyolitic tuff and late Carboniferous sandstone indicate that the early Paleozoic arc magmatic activities also occurred in the eastern BAB.

Orogenic crustal thickening leads to increased continental elevation and runoff into the oceans, but there are fundamental uncertainties in the temporal patterns of thickening through Earth history. U‐Pb age and trace element data in detrital zircons from Antarctica are consistent with recent global analyses suggesting two dominant peaks in average crustal thickness from ~2.6 to 2.0 Ga and ~0.8 to 0.5 Ga. Shifts in marine carbonate 87Sr/86Sr ratios show two primary peaks that post‐date these crustal thickness peaks, suggesting significant weathering and erosion of global continental relief. Both episodes correlate well with zircon trace element and isotope proxies indicating enhanced crustal and fluid input into subduction zone magmas. Increased crustal thickness correlates with increased passive margin abundance and overlaps with snowball Earth glaciations and atmospheric oxygenation, suggesting a causal link between continental rift‐drift phases and major transitions in Earth's atmospheric and oceanic evolution.

Pre- to syn-eruptive fluids released by the magmatic system forming the non-welded rhyolitic Kos Plateau Tuff ignimbrite, Greece, were characterized using fluid inclusions entrapped in quartz from granitic clasts entrained from the roof or walls of the feeding magma reservoir. This fluid was initially of intermediate density and relatively low salinity (3–11 wt% NaClequivalent). Following significant pressure drop(s), it separated into immiscible brine and CO2-bearing vapor occurring as texturally less mature inclusions. The transition from single- to two-phase-fluid occurred at magmatic conditions, as supported by high fluid inclusion homogenization temperatures (ca. 700 °C) and simultaneous trapping of fluid inclusions with silicate melt inclusions. The large recorded pressure drop together with texturally immature fluid inclusion shapes suggests a possible direct relation between the fluid phase change and the incipient 161 ka caldera-forming eruption. All inclusion types occur predominantly along (pseudo-) secondary trails in the granitic clasts, indicating efficient entrapment in microfractures formed at high crystallinity, as attested to by elevated concentrations of incompatible elements in the silicate melt inclusions. Fast quenching of the inclusions due to eruption prevented significant post-entrapment chemical modification, providing pristine samples of magmatic fluid compositions. Element concentrations in co-existing fluids and silicate melt were analyzed using laser ablation inductively-coupled plasma mass spectrometry and compared in view of element transporting capabilities and magmatic-hydrothermal ore formation. Economically important metals such as Cu, Zn, Mo, and W were extracted from the silicate melt by the fluid. In particular, Cu concentrations in the single-phase fluid in the range of 100–500 ppm agree well with previous experimental and modeling studies and, therefore, represent values expected for primary magmatic fluids. Most elements got further enriched in the brine upon phase separation, except Li, B, and As, which may have contributed significantly also to the vapor phase. Considering the similarity between vapor and intermediate-density fluid in terms of salinity, however, the metal-enriched brine phase only comprised a minor fraction of the bulk fluid.

Somma-Vesuvius is one of the most iconic active volcanoes with historic and archeological records of numerous hazardous eruptions. Petrologic studies of eruptive products provide insights into the evolution of the magma reservoir before eruption. Here, we quantify the duration of shallow crustal storage and document the evolution of phonolitic magmas before major eruptions of Somma-Vesuvius. Garnet uranium-thorium petrochronology suggests progressively shorter pre-eruption residence times throughout the lifetime of the volcano. Residence times mirror the repose intervals between eruptions, implying that distinct phonolite magma batches were present throughout most of the volcano’s evolution, thereby controlling the eruption dynamics by preventing the ascent of mafic magmas from longer-lived and deeper reservoirs. Frequent lower-energy eruptions during the recent history sample this deeper reservoir and suggest that future Plinian eruptions are unlikely without centuries of volcanic quiescence. Crystal residence times from other volcanoes reveal that long-lived deep-seated reservoirs and transient upper crustal magma chambers are common features of subvolcanic plumbing systems., Science Advances, 8 (2), ISSN:2375-2548

Granitic melts may form either directly, by melting of pre-existing crustal rocks, or by fractionation of mafic to intermediate magmas, typically mantle-derived. Each model is applicable to distinct portions of the Earth at different times. Whenever there is an important flux of mafic magmas from the mantle, differentiation of basaltic magmas dominates. In contrast, locations with a lower mafic magma flux, elevated thermal gradients and/or a fertile, thick crust are dominated by crustal melting and reworking. This only partly overlaps with the dichotomy between magmatic arcs and collision zones, as places like e.g. inverted back-arcs also feature large-scale crustal melting, whereas post-collisional domains include a sizeable mantle-derived component. Petrogenesis of individual granitic suites probably cannot be accounted for by pure, end-member models, as these suites typically feature a fair proportion of hybrid or ambiguous granitoids. Thus, we explore various petrogenetic and geological scenarios leading to the formation of hybrid granitoids at various scales. Finally, we outline possibilities to quantify the respective contributions of crust and mantle involved in their formation – hopefully, paving the way for a better understanding and more rigorous discussion of the mechanisms of crustal growth and recycling.

The growth and evolution of crustal-scale magmatic systems play a key role in the generation of the continental crust, the largest eruptions on Earth, and the formation of metal resources vital to our society. However, such systems are rarely exposed on the Earth’s surface, limiting our knowledge about the magmatic processes occurring throughout the crust to indirect geochemical and petrographic data obtained from the shallowest part of the system. The Hf isotopic composition of accessory zircon is widely used to quantify crust-mantle evolution and mass transfers to and within the crust. Here we combine single-grain zircon Hf isotopic analysis by LA-MC-ICP-MS with thermal modelling to one of the best-studied crustal-scale igneous systems (Sesia Magmatic System, northern Italy), to quantify the relative contribution of crustal- and mantle-derived magmas in the entire system. Zircons from the deep gabbroic units define a tight range of εHf (−2.5 ± 1.5). Granites and rhyolites overlap with this range but tail towards significantly more negative values (down to −9.5). This confirms that the entire system consists of hybrid magmas that stem from both differentiation of mantle-derived magmas and melting of the crust. Thermal modelling suggests that crustal melting and assimilation predominantly occurs during emplacement and evolution of magmas in the lower crust, although melt production is heterogeneous within the bodies both spatially and temporally. The spatial and temporal heterogeneity resolved by the thermal model is consistent with the observed Hf isotope variations within and between samples, and in agreement with published bulk-rock Sr–Nd isotopic data. On average, the crustal contribution to the entire system determined by mixing calculations based on Hf isotopic data range between 10 and 40%, even with conservative assumptions, whereas the thermal model suggests that this space- and time-averaged contribution does not exceed 20%. However, spatial and temporal variations in the crustal melt proportion (from 0 up to 80% as observed in the thermal model) may impart significant isotopic variability to different batches of magma observed on the outcrop scale, emphasizing the need to consider a magmatic system as a whole, i.e., by integrating all spatial and temporal scales, to more precisely quantify crustal growth vs. reworking., Contributions to Mineralogy and Petrology, 176 (11), ISSN:0010-7999, ISSN:1432-0967

International audience; Recent advances in zircon geochronology by isotope dilution thermal-ionization mass spectrometry (ID-TIMS) techniques allow for age precision beyond the 0.02 % level for single zircon 206Pb/238U dates [1] and can resolve zircon crystallization heterogeneities in a dispersed age population. As zircon saturation is dependent on melt temperature and composition, quantitative analysis of dispersed zircon age populations can provide insight into the timing and magnitude of intensive parameter variations during the lifetime of magma reservoirs.

Most volcanoes erupt both effusively and explosively, with explosive behaviour being responsible for most human fatalities. Eruption style is thought to be strongly controlled by fast conduit processes, limiting our ability for prediction. Here we address a critical question in the quest to develop timely forecasting of eruptive behaviour: are there conditions in which the outcome of an eruption is predetermined by the state of the magma in the subvolcanic reservoir? We analyse the pre-eruptive storage conditions of 245 units from volcanoes around the world. We show that pre-eruptive crystallinity, dissolved water content and the presence of exsolved volatiles in the chamber exert a primary control on eruptive styles. Magmas erupt explosively over a well-defined range in dissolved water content (~4–5.5 wt%) and crystallinity (less than 30 vol%). All other conditions, namely higher crystallinity, dissolved water contents below 3.5 wt% and, counterintuitively, in excess of 5.5 wt%, favour effusive activity. Between these ranges, there is a narrow field of transitional storage properties that do not discriminate between eruptive styles, and where the conduit exerts the main control on eruptive behaviour. Our findings suggest that better estimates of crystallinity and water content in subvolcanic chambers are key to forecasting eruptive style. The effusive or explosive nature of a volcanic eruption may be determined by the crystallinity, water content and presence of exsolved volatiles in subvolcanic chambers, according to analysis of the pre-eruptive storage conditions of global volcanoes.

The Ordovician rocks of Sierra Las Planchadas are an exceptionally preserved natural example of a plutonic-volcanic connection. Located in northwestern Argentina, the Las Planchadas Formation consists of a monzogranitic batholith that displays a window into the source of large subvolcanic rhyolitic rocks, cryptodomes and volcanic feeder-dykes. Mafic intrusions and felsic dykes occur cutting the plutonic rocks. Grain-scale relationships, whole-rock compositions and mineral chemistry indicate that monzogranites and rhyolites are two comagmatic members which differentiated within upper crust magma reservoirs. Monzogranites accumulated some plagioclase and trapped a large volume of melt, denoted by a high modal percentage (~64–71%) of near-solidus minerals, comprised of alkali feldspar, albite-rich plagioclase (An