Search

International audience; Nitrogen, carbon, hydrogen and sulfur are essential elements for life and comprise about 1 % of terrestrial planet masses. These elements dominate planetary surfaces due to their volatile nature, but the Earth's interior also constitutes a major C-H-N-S reservoir. Resolving the origin of the surficial versus deep volatile reservoirs requires the past 4.5 Giga-years of mantle outgassing and ingassing processes to be reconstructed, involving many unknowns. As an alternative, we propose to define the primordial distribution of volatiles resulting from degassing of the Earth's magma ocean (MO). The equilibrium partitioning of C-H-ON -S elements between the MO and its atmosphere is calculated by means of solubility laws, extrapolated to high temperatures and over a large range of redox conditions. Depending on the redox conditions, the amount of volatiles, and the size of the MO considered, we show that the last MO episode may have degassed 40-220 bar atmospheres, whereas hundreds to thousands of ppm of C-H-ON -S can be retained in the magma. Two contrasting scenarios are investigated: reduced vs. oxidized MO. For reduced cases (IW+2) would be dry and C-N-S-rich. An intermediate redox state produces a C-N atmosphere. In many cases, the present-day surficial abundances (atmosphere+ocean+crust) of C and N, the most volatile elements, are very close to the calculated primordial MO-atmosphere distribution. This probably means that lithospheric recycling and post-magma ocean degassing only moderately alter the surficial abundances of these elements. Sulfur, in contrast, must have been mostly outgassed by post-MO events. Changes in redox conditions during magma ocean degassing played a first order role in the composition of the primordial atmosphere of planets. We suggest that the more oxidized conditions on Venus due to H-loss may have played a role in the growth of a dry MO atmosphere on this planet compared to an H-bearing one on Earth. To verify these first order assertions, constraints on volatile behaviour under extreme magma ocean conditions and upon magma ocean solidification are urgently needed.

Anticipating volcanic eruptions at central volcanoes require knowing how magma chambers prepare for new eruptions. Pre-eruptive processes that occur in such magma chambers are recorded in the products of these eruptions, so their characterisation in terms of magma composition and physics offers the clues to understand past eruptions and predict future ones. Here, we study the very well preserved pyroclastic succession of El Palomar Member (712 ± 41 ka), in Las Cañadas caldera, Teide, Canary Islands. This deposit resulted from a single explosive eruption of a phonolitic magma that started with a sustained eruption column (sub-plinian or plinian) that formed a massive, 40 m thick non-welded fallout deposit, progressively changing into a lower intensity fire fountain that deposited a 25 m thick fallout succession of non-welded to strongly welded pumices. Stratigraphic and petrological data suggest that this eruption was related to a thermally-compositionally zoned and relatively shallow magma chamber in which the arrival of a hotter and more mafic magma rapidly triggered the eruption. The studied deposit shows how this zoned structure was maintained during the whole process, which allows one to reconstruct what happened during the eruption. Comparison of this eruption with the current situation at Teide volcano alerts on the potential rapid preparation for new eruptions in the case that sufficient phonolitic magma was available in the shallow plumbing system of this active volcano if new inputs of deeper magma take place.

International audience; Halogens are minor volatile species emitted during volcaniceruptions, whose injection into the atmosphere contributes to thediminution of the stratospheric ozone layer and causes majorclimatic impacts. However, the lack of detailed understandingconcerning halogens behavior in magmas – especially bromineand iodine - and their degassing during magma ascent, prevents afull assessment of halogen cycling in the Earth’s system.Previous studies on iron-free synthetic systems have shown thatthe partitioning of Cl, Br and I between fluids and silicate meltsincreasingly favors the former as the ionic radii of the halogenideions increases [1]. These results suggest that bromine and iodinefluxes to the atmosphere are probably underestimated. Toevaluate this phenomenon, additional experimental constraints onBr and I behavior in natural magmas are urgently needed. In thisstudy, the Br partitioning between fluids and natural silicatemelts has been investigated experimentally as a function of meltcomposition, pressure, temperature and oxygen fugacity.Composition investigated and corresponding geodynamicsettings are representative of alkaline/hyperalkaline magmaticsystems worldwide. Experiments were performed in internallyheated pressure vessels (IHPV) under controlled oxygen fugacityand allowing drop quench. Bromine abundances weredetermined by μ-XRF, LA-ICP-MS or nanoSIMS, while fluidcomposition was estimated by mass balance calculations. Ourresults confirm the preference of bromine for the fluid phase,whatever the experimental conditions investigated, being inagreement with previous studies on metaluminous compositions[2], DBrf/m increasing with melt silica content. Due to its very lowconcentration in magmas, iodine behavior remains particularlypoorly known. To address this issue, we have carried out HP-HTsolubility experiments to synthesize a series of reference glasses,in order to test several sensitive analytical techniques for iodinequantification, such as LA-ICP-MS, nanoSIMS or neutronirradiation noble gas mass spectrometric technique. Dataacquired in this study will enrich a still fragmentary knowledgeof deep halogens geochemistry and therefore constitute part ofthe essential basis to develop the first physically-basedquantitative framework for volcanic heavy halogen emissions.[1] Bureau et al., (2000), Earth And Planetary Science Letters183, 51-60.[2] Cadoux et al., (2018), Earth And Planetary Science Letters498, 450–463.

We performed phase equilibrium experiments on a natural Ca-poor pelite at 3 GPa, 750–1000°C, under moderately oxidizing conditions, simulating the partial melting of such lithologies in subduction zones. Experiments investigated the effect of sulphur addition on phase equilibria and compositions, with S contents of up to ∼ 2.2 wt. %. Run products were characterized for their major and trace element contents, in order to shed light on the role of sulphur on the trace element patterns of melts produced by partial melting of oceanic Ca-poor sediments. Results show that sulphur addition leads to the replacement of phengite by biotite along with the progressive consumption of garnet, which is replaced by an orthopyroxene-kyanite assemblage at the highest sulphur content investigated. All Fe-Mg silicate phases produced with sulphur, including melt, have higher MgO/(MgO+FeO) ratios (relative to S-free/poor conditions), owing to Fe being primarily locked up by sulphide in the investigated redox range. Secular infiltration of the mantle wedge by such MgO and K2O-rich melts may have contributed to the Mg and K-rich character of the modern continental crust. Addition of sulphur does not affect significantly the stability of the main accessory phases controlling the behaviour of trace elements (monazite, rutile and zircon), although our results suggest that monazite solubility is sensitive to S content at the conditions investigated. The low temperature (∼ 800 °C) S-bearing and Ca-poor sediment sourced slab melts show Th and La abundances, Th/La systematics and HFSE signatures in agreement with the characteristics of sediment-rich arc magmas. Because high S contents diminish phengite and garnet stabilities, S-rich and Ca-poor sediment sourced slab melts have higher contents of Rb, B, Li (to a lesser extent), and HREE. The highest ratios of La/Yb are observed in sulphur-poor runs (with a high proportion of garnet, which retains HREE) and beyond the monazite out curve (which retains LREE). Sulphides appear to be relatively Pb-poor and impart high Pb/Ce ratio to coexisting melts, even at high S content. Overall, our results show that Phanerozoic arc magmas from high sediment flux margins owe their geochemical signature to the subduction of terrigenous, sometimes S-rich, sediments. In contrast, subduction of such lithologies during Archean appears unlikely or unrecorded.

International audience; The rheology and thermodynamical evolution of magma, either in reservoirs, conduits or at the surface, are governed by temperature. To determine the field temperature, remote sensing methods based on measuring the infrared radiance are widely applied, but they are subject to assumptions and caveats that can propagate into large uncertainties. This is related to the poor knowledge of one of the most critical parameters, namely the spectral emissivity. In this work we aim at filling this gap through in situ spectral emissivity measurements performed over wide temperature (700-1600 K) and spectral ranges (1.25-25 µm) on two representative phonolitic compositions from Erebus (Antartica) and Teide (Spain) volcanoes. The laboratory spectra allow to determine precisely spectral emissivity in the thermal infrared (TIR), middle infrared (MIR), and shortwave infrared (SWIR) ranges. The results reveal the complexity and contrasted behavior of the radiative properties of the two rocks melts, despite their broadly similar composition. The spectral emissivity varies significantly as a function of temperature, composition, crystallinity, thickness, and thermal history. Altogether, the data reveal that emissivity cannot be considered as a constant value and question previous arguments that active lava always has lower emissivity than frozen lava. Finally, the laboratory-measured values of spectral emissivity were used to refine the temperature of Erebus lava lake gathered from previous remote sensing methods.

International audience; We established the phase equilibria of a representative tephriphonolitic sample of the 1631 eruption of Vesuvius (Italy). Experiments were conducted at 100 MPa, in the temperature range 950–1050 °C for melt water content ranging from 1.3 to 3.2 wt%, and at an oxygen fugacity (fO2) of NNO+1 to NNO+3 (one to three log unit above the fO2 of the Ni-NiO solid redox buffer). Results show that clinopyroxene, biotite and leucite dominate the crystallizing phase assemblage, with minor proportions of plagioclase and amphibole, in agreement with the petrological attributes of the tephra. Comparison between the phase proportions and compositions obtained in experiments and those observed in the rock indicates a pre-eruptive temperature of 950 ± 30° and a melt water content H2Omelt = 2.3 ± 0.3 wt%, for an oxygen fugacity around NNO+1. These T-H2O estimates are confirmed by empirical geothermometers based on experimental clinopyroxene and melt compositional trends. As for other Vesuvius eruptions, the most felsic part of the 1631 reservoir appears to have reached pre-eruptive leucite saturation, although a large amount of leucite microcrystals in the studied samples likely grew syn-eruptively. Our results confirm that magma storage conditions beneath Vesuvius became hotter, shallower, and more CO2-rich after the AD 79 Pompeii Plinian event.

International audience; The 2015 reactivation of the Cotopaxi volcano urges us to understand the complex eruptive dynamics of Cotopaxi for better management of a potential major crisis in the near future. Cotopaxi has commonly transitioned from andesitic eruptions of strombolian style (lava flows and scoria ballistics) or nuées ardentes (pyroclastic flows and ash falls) to highly explosive rhyolitic ignimbrites (pumiceous pyroclastic flows), which entail drastically different risks. To better interpret geophysical and geochemical signals, Cotopaxi magma storage conditions were determined via existing phase-equilibrium experiments that used starting materials chemically close to the Cotopaxi andesites and rhyolites. The results suggest that Cotopaxi's most mafic andesites (last erupted products) can be stored over a large range of depth from ~7 km to ≥16 km below the summit (pressure from ~200 to ≥400 MPa), 1000 °C, NNO +2, and contain 4.5–6.0±0.7 wt% H2O dissolved in the melt in equilibrium with ~30–40% phenocrysts of plagioclase, two pyroxenes, and Fe-Ti oxides. These mafic andesites sometimes evolve towards more silicic andesites by cooling to 950 °C. Rhyolitic magmas are stored at 200–300 MPa (i.e. ~7–11 km below the summit), 750 °C, NNO +2, and contain ~6–8 wt% H2O dissolved in a nearly aphyric melt (

Pantelleria Island is the type locality of pantellerite, an iron and alkali-rich rhyolite (P.I=molar Na2O+K2O/Al2O3 >1.05). Peralkaline rhyolites (i.e pantellerite and comendite) and trachytes usually represent the felsic end-members in continental rift systems (e.g., Pantelleria, Tibesti, Ethiopia, Afar, Kenya, Bain and Range, South Greenland) and in oceanic sland settings (Socorro Is., Easter Is., Iceland and Azores). The origin of peralkaline rhyolites in the different tectonic settings is still a matter of debate and three hypotheses have been suggested: (a) crystal fractionation of alkali-basalt in a shallow reservoir to produce a trachyte which subsequently gives rise to a pantellerite (e.g., Barberi et al., 1975, Mungall & Martin 1995, Civetta et al., 1998,) (b) partial melting of cumulate gabbros to form a trachyte which then produces pantellerite (e.g., Lowestern & Mahood 1991; Bohrson & Reid 1997), (c) partial melting of different lithospheric sources fluxed by volatiles which add the excess alkalies to the melt (Bailey & Macdonald, 1975, 1987). Recent petrological work has helped to define the temperature range and redox conditions of pantellerite magmas (Scaillet & Macdonald 2001, 2003, 2006; White et al., 2005, 2009; Di Carlo et al., 2010,) as well as their pre-eruptive volatile contents (e.g., Gioncada & Landi 2010, Neave et al., 2012, Lanzo et al., 2013). In contrast little is known about the conditions of storage and evolution of the associated trachytes. At Pantelleria, trachytes and pantellerites constitute most of the outcropping rocks, the former being erupted dominantly as lava flows while pantellerites are erupted either explosively or effusively. We have experimentally investigated the phase relationship of two representative trachytes from Pantelleria island in order to shed light on their pre-eruptive conditions (pressure, temperature, H2Omelt, oxygen fugacity) and define their liquid lines of descent toward more evolved compositions. We have established the phase relationships over a P-T fO2-H2Omelt range of T=750-950°C, P=0.5-1.5 kbar, fO2=~FMQ and XH2Ofluid (H2O/H2O+CO2, in moles) between 1 and 0. By comparing the experimental phase assemblages, abundances and compositions with the natural products we set constraints on the storage conditions of trachytic magmas at Pantelleria and more generally, on the putative parent-daughter relationship between trachytic and pantelleritic magmas. Our results lay thebasis to understand the long debated petrological issue regarding the link 94 between silica oversaturated peralkaline and metaluminous magmas.

International audience; Volcanogenic halogens, in particular bromine, potentially play an important role in the ozone depletion of the atmosphere. Understanding bromine behaviour in magmas is therefore crucial to properly evaluate the contribution of volcanic eruptions to atmospheric chemistry and their environmental impact. To date, bromine partitioning between silicate melts and the gas phase is very poorly constrained, with the only relevant experimental studies limited to investigation of synthetic melt with silicic compositions. In this study, fluid/melt partitioning experiments were performed using natural silicate glasses with mafic, intermediate and silicic compositions. For each composition, experiments were run with various Br contents in the initial fluid (H2O–NaBr), at T–P conditions representative of shallow magmatic reservoirs in volcanic arc contexts (100–200 MPa, 900–1200 °C). The resulting fluid/melt partition coefficients (DBrf/m) are: 5.0 ± 0.3 at 1200 °C–100 MPa for the basalt, 9.1 ± 0.6 at 1060 °C–200 MPa for the andesite and 20.2 ± 1.2 at 900 °C–200 MPa for the rhyodacite. Our experiments show that DBrf/m increases with increasing SiO2 content of the melt (as for chlorine) and suggest that it is also sensitive to melt temperature (increase of DBrf/m with decreasing temperature). We develop a simple model to predict the S–Cl–Br degassing behaviour in mafic systems, which accounts for the variability of S–Cl–Br compositions of volcanic gases from Etna and other mafic systems, and shows that coexisting magmatic gas and melt evolve from S-rich to Cl–Br enriched (relative to S) upon increasing degree of degassing. We also report first Br contents for melt inclusions from Etna, Stromboli, Merapi and Santorini eruptions and calculate the mass of bromine available in the magma reservoir prior to the eruptions under consideration. The discrepancy that we highlight between the mass of Br in the co-existing melt and fluid prior to the Merapi 2010 eruption (433 and 73 tons, respectively) and the lack of observed BrO (from space) hints at the need to investigate further Br speciation in ‘ash-rich’ volcanic plumes. Overall, our results suggest that the Br yield into the atmosphere of cold and silicic magmas will be much larger than that from hotter and more mafic magmas.

International audience; Phase equilibrium experiments are essential for robustly and accurately constraining the intensive parameters of magma systems and their fractionation history, which is particularly true for A‐type granites crystallized from H2O‐rich melts and at reducing conditions. Here, we constrain the crystallization conditions of the ferroan (A‐type), Sn‐mineralized Qitianling granite of South China which formed during a major event of crustal formation and reorganization in the Mesozoic. To characterize the magma system conditions and fractionation, we have carried out a series of experiments on three representative, amphibole‐bearing samples. The experiments were performed at 100‐700 MPa (mainly at 200 MPa and 300 MPa), at an fO2 of ~NNO‐1.3 (1.3 log unit below the Ni‐NiO buffer) or ~NNO+2.4, at 660°C to 900°C, and at variable H2Omelt (~3‐9 wt%). They show that the Qitianling magmas last crystallized at ≥300‐350 MPa, at a H2Omelt ≥6.5‐8.0 wt%, and that magmatic fO2 was ~NNO‐1.3±0.5 at above‐solidus conditions. Amphibole texture in the rocks suggests an early crystallization of this mineral, hence that water‐rich (≥4 wt% H2O in melt) conditions prevailed early during the magmatic evolution, prior to amphibole crystallization. At all investigated conditions, amphibole crystallization requires at least 5‐6 wt% dissolved H2O, being even absent in the more potassic composition. We interpret this as resulting from the elevated K2O content of the investigated compositions that inhibits amphibole crystallization in metaluminous granitic systems. The experimental liquid line of descent obtained at 200‐300 MPa mimics the geochemical trend expressed by the pluton suggesting that fractionation occurred in the upper crustal reservoir.

International audience; The oxidation state of volcanic gas emissions influences the composition of the exosphere and planetary habitability. It is widely considered to be associated with the oxidation state of the melt from which volatiles exsolve. Here, we present a global synthesis of volcanic gas measurements. We define the mean oxidation state of volcanic gas emissions on Earth today and show that, globally, gas oxidation state, relative to rock buffers, is a strong function of emission temperature, increasing by several orders of magnitude as temperature decreases. The trend is independent of melt composition and geodynamic setting. This observation may explain why the mean oxidation state of volcanic gas emissions on Earth has apparently increased since the Archean, without a corresponding shift in melt oxidation state. We argue that progressive cooling of the mantle and the cessation of komatiite generation should have been accompanied by a substantial increase of the oxidation state of volcanic gases around the onset of the Great Oxidation Event. This may have accelerated or facilitated the transition to an oxygen-rich atmosphere. Overall, our data, along with previous work, show that there are no single nor simple relationships between mantle-, magma- and volcanic gas-redox states.

International audience; Volatile release during magma ascent in volcanic conduits (magma degassing) forms the basis for using volcanic gases as precursory signals. Recent high temperature high pressure experimental simulations have yielded results that challenge key assumptions related to magma degassing and are important for the interpretation of glass inclusion and gas data and for using volcanic gas as precursory signals. The experimental data show that, for ascent rates expected in natural systems, pure H2O basaltic melts will evolve mostly close to equilibrium when decompressed from 200 to 25 MPa. In the same way, degassing of H2O–S species evolves at near equilibrium, although this conclusion is limited by the number of S solubility data available for basaltic melts. However, degassing of CO2 is anomalous in all studies, whether performed on basaltic or rhyolitic melts. CO2 stays concentrated in the melt at levels far exceeding solubilities. The anomalous behaviour of CO2, when associated with near equilibrium H2O losses, yields post-decompression glasses with CO2 concentrations systematically higher than equilibrium degassing curves. Therefore, there is strong experimental support for disequilibrium degassing during ascent of CO2-bearing magmas. The existence of volatile concentration gradients around nucleated gas bubbles suggests that degassing is controlled by the respective mobilities (diffusivities) of volatiles within the melt. The recently formulated diffusive fractionation model reproduces the main characteristics, especially the volatile concentrations, of experimental glasses. The model also shows that the gas phase is more H2O-rich than expected at equilibrium because CO2 transfer toward the gas phase is hampered by its retention within the melt. However, only integrated gas compositions are calculated. Similarly, only bulk experimental fluid compositions are determined in recent experiments. Thus, constraints on the local gas phase are becoming necessary for the application to volcanoes. This stresses the need for the direct analysis of gas bubbles nucleated in decompression experiments. Pre-eruptive changes in volcanic CO2/SO2 and H2O/CO2 gas ratios are interpreted to reflect different pressures of gas-melt segregation in the conduit, an approach that assume gas-melt equilibrium. However, if disequilibrium magma degassing is accepted, the use of volatile saturation codes is no longer possible and caution must be exercised with the application of local equilibrium to volcanic gases. Future developments in the interpretation of gas data require progress from both sides, experimental and volcanological. One priority is to reduce the gap in scales between experiments and gas measurements.

International audience; Constraining crystallization pressure and thus intrusion depth of granites in various geodynamic settings remains challenging, yet important to further our understanding of magma system and crustal evolution. We propose that titanite, which is a common accessory in metaluminous and weakly peraluminous granites, can be used as a barometer if it crystallized in magmatic, near-solidus conditions and in equilibrium with amphibole, plagioclase, K-feldspar, quartz, biotite, and magnetite ± ilmenite. Titanite Al2O3 increases with pressure (P) according to: P (in MPa) = 101.66 × Al2O3 in titanite (in wt%) + 59.013 (R2 = 0.83) with estimated uncertainties of ~±60 to ~±100 MPa for crystallization between ~150 and 400 MPa. We highlight that the current calibration dataset is limited, and that systematic experimental studies are needed to rigorously quantify the relation. The most important use of this empirical barometer will be for rocks in which amphibole is present but significantly altered, or in combination with amphibole barometry, as titanite can be easily dated by LA-ICP-MS.

International audience; From oxic atmosphere to metallic core, the Earth's components are broadly stratified with respect to oxygen fugacity. A simple picture of reducing oxygen fugacity with depth may be disrupted by the accumulation of oxidised crustal material in the deep lower mantle, entrained there as a result of subduction. While hotspot volcanoes are fed by regions of the mantle likely to have incorporated such recycled material, the oxygen fugacity of erupted hotspot basalts had long been considered comparable to or slightly more oxidised than that of mid-ocean ridge basalt (MORB) and more reduced than subduction zone basalts. Here we report measurements of the redox state of glassy crystal-hosted melt inclusions from tephra and quenched lava samples from the Canary and Cape Verde Islands, that we can independently show were entrapped prior to extensive sulphur degassing. We find high ferric iron to total iron ratios (Fe3+/∑Fe) of up to 0.27–0.30, indicating that mantle plume primary melts are significantly more oxidised than those associated with mid-ocean ridges and even subduction zone. These results, together with previous investigations from the Erebus, Hawaiian and Icelandic hotspots, confirm that mantle upwelling provides a return flow from the deep Earth for components of oxidised subducted lithosphere and suggest that highly oxidised material accumulates or is generated in the lower mantle. The oxidation state of the Earth's interior must therefore be highly heterogeneous and potentially locally inversely stratified.

Erratum to: Bull Volcanol (2017) 79: 36DOI 10.1007/s00445-017-1114-zDuring the steps of corrections, the publisher inadvertently changed the author affiliations so that they were no longer correct. The correct information is given below. The publisher regrets this mistake.; International audience; Volcanic gas emission measurements inform predictions of hazard and atmospheric impacts. For these measurements, Multi-Gas sensors provide low-cost in situ monitoring of gas composition but to date have lacked the ability to detect halogens. Here, two Multi-Gas instruments characterized passive outgassing emissions from Mt. Etna’s (Italy) three summit craters, Voragine (VOR), North-east Crater (NEC) and Bocca Nuova (BN) on 2 October 2013. Signal processing (Sensor Response Model, SRM) approaches are used to analyse H2S/SO2 and HCl/SO2 ratios. A new ability to monitor volcanic HCl using miniature electrochemical sensors is here demonstrated. A “direct-exposure” Multi-Gas instrument contained SO2, H2S and HCl sensors, whose sensitivities, cross-sensitivities and response times were characterized by laboratory calibration. SRM analysis of the field data yields H2S/SO2 and HCl/SO2 molar ratios, finding H2S/SO2 = 0.02 (0.01–0.03), with distinct HCl/SO2 for the VOR, NEC and BN crater emissions of 0.41 (0.38–0.43), 0.58 (0.54–0.60) and 0.20 (0.17–0.33). A second Multi-Gas instrument provided CO2/SO2 and H2O/SO2 and enabled cross-comparison of SO2. The Multi-Gas-measured SO2-HCl-H2S-CO2-H2O compositions provide insights into volcanic outgassing. H2S/SO2 ratios indicate gas equilibration at slightly below magmatic temperatures, assuming that the magmatic redox state is preserved. Low SO2/HCl alongside low CO2/SO2 indicates a partially outgassed magma source. We highlight the potential for low-cost HCl sensing of H2S-poor HCl-rich volcanic emissions elsewhere. Further tests are needed for H2S-rich plumes and for long-term monitoring. Our study brings two new advances to volcano hazard monitoring: real-time in situ measurement of HCl and improved Multi-Gas SRM measurements of gas ratios.

International audience; Andesitic arc volcanism is the most common type of subduction-related magmatism on Earth. How these melts are generated and under which conditions they evolve towards silica-rich liquids is still a matter of discussion. We have performed crystallization experiments on a representative andesite sample from the Upper Scoriae 1 (USC-1) eruption of Santorini (Greece) with the aim of understanding such processes. Experiments were performed between 1000 and 900 °C, in the pressure range 100–400 MPa, at fO2 from QFM (quartz–fayalite–magnetite) to NNO (nickel–nickel oxide) + 1·5, with H2Omelt contents varying from saturation to nominally dry conditions. The results show that the USC-1 andesitic magma was generated at 1000 °C and 12–15 km depth (400 MPa), migrated to shallower levels (8 km; 200 MPa) and intruded into a partially crystallized dacitic magma body. The magma cooled to 975 °C and generated the phenocryst assemblage and compositional zonations that characterize the products of this eruption. An injection of basaltic magma problably subsequently triggered the eruption. In addition to providing the pre-eruptive conditions of the USC-1 magma, our experiments also shed light on the generation conditions of silica-rich magmas at Santorini. Experimental of runs performed at fO2 ∼ NNO + 1 (± 0·5) closely mimic the compositional evolution of magmas at Santorini whereas those at reduced conditions (QFM) do not. Glasses from runs at 1000–975 °C encompass the magma compositions of intermediate-dominated eruptions, whereas those at 950–900 °C reproduce the silicic-dominated eruptions. Altogether, the comparison between our experimental results and natural data for major recent eruptions from Santorini shows that different magma reservoirs, located at different levels, were involved during highly energetic events. Our results suggest that fractionation in deep reservoirs may give rise to magma series with a tholeiitic signature whereas at shallow levels calc-alkaline trends are produced.

International audience; Experiments have been performed both on a peraluminous leucogranitic (DK89) and a F-, Li-, P-rich pegmatitic (B2) melt to constrain the stability of micas in evolved crustal silicic magmas and refine mica-melt partition coefficients for F, Li, and Be. The experiments were conducted in parallel in two fO2 ranges, “oxidizing” (NNO +1 to +3) and “reducing” (NNO –1.6 to –1.4). One two-stage reducing-oxidizing experiment was conducted in a vessel fitted with a H2-permeable Shaw-type membrane. The approach toward equilibrium was tested by imposing long experimental durations and combining mica crystallization experiments with mica dissolution experiments using mica seeds. Experimental micas and melts were analyzed for major elements by electron microprobe and for light elements by nuclear microprobe. At 3 kbar, 620 °C, and under oxidizing conditions, B2 crystallized only muscovite, the biotite seeds reacting to form a new mica intermediate between phengite and Li-rich phengite. Under reducing conditions, biotite (siderophyllite composition) appeared as the stable mica. The two-stage experiment yielded a composite mica assemblage with siderophyllite cores mantled by muscovite rims. At 3.5–3.8 kbar, 720 °C, and under oxidizing conditions, DK89 crystallized only aluminous biotite and muscovite seeds reacted to form biotite-bearing assemblages; muscovite appeared together with biotite at 700 °C. Under reducing conditions, Al-rich biotite is also the stable mica at 720 °C. Partition coefficients show that F and Li are preferentially incorporated in biotite rather than in muscovite, the opposite as for Be. Biotite fractionation buffers the F and increases the Li and Be contents of the residual melt. Muscovite increases the Li content of the melt and has little influence on F and Be concentrations. Our experiments reproduce mica assemblages and compositions typical of Variscan pegmatites and leucogranites, yet very Li-rich micas (e.g., lepidolites) were not obtained. The results stress the differential influence of fO2 on mica stability in moderately and highly fractionated crustal melts. Mica crystallization in leucogranites does not appear to be strongly dependent on fO2. In contrast, a very strong influence of fO2 on stable mica assemblages is demonstrated for the pegmatitic melt. The reducing experiments emphasize the existence of a stability field for biotite in melts poor in Fe, Mg, and Ti. If fO2 is reducing, biotite must crystallize in moderately to highly evolved peraluminous crustal melts. In contrast, the crystallization of muscovite as the sole mica in evolved crustal melts constitutes an indicator of oxidizing fO2. Such an oxidizing evolution that deviates from classical buffered T-logfO2 trajectories is the consequence of a mechanism of magma “self-oxidation” that is proposed to result from dissociation of H2O in the melt.

Abundant granitic rocks exposed in ancient mountain belts suggest that crustal melting plays a major role in orogenic processes. However, complex field relations and superposition of multiple tectonic events make it difficult to determine the role of melting in orogenesis. In contrast, geophysical measurements image present-day crustal conditions but cannot discriminate between partial melt and aqueous fluids. Here we connect pressure–temperature paths of Himalayan Miocene crustal rocks to the present-day conditions beneath the Tibetan plateau imaged with geophysical data. We use measurements of electrical conductivity to show that 4–16% water-rich melt is required to explain the crustal conductivity in the north-western Himalaya. In southern Tibet, higher melt fractions >30% reflect a crust that is either fluid-enriched (+1% H2O) or hotter (+100 °C) compared to the Miocene crust. These melt fractions are high enough for the partially molten rocks to be significantly weaker than the solid crust., Crustal melting may play a fundamental role in orogenic processes, but quantifying crustal melt remains difficult. Here, the authors combine pressure-temperature paths, electrical conductivity and geophysical data to elucidate the melting conditions in Tibet since the Miocene.

A new, pre-Green Tuff (46 ka) volcanic stratigraphy is presented for the peralkaline Pantelleria Volcano, Italy. New 40Ar/39Ar and paleomagnetic data are combined with detailed field studies to develop a comprehensive stratigraphic reconstruction of the island.We find that the pre-46 ka succession is characterised by eight silicarich peralkaline (trachyte to pantellerite) ignimbrites,many ofwhich blanketed the entire island. The ignimbrites are typically welded to rheomorphic, and are commonly associated with lithic breccias and/or pumice deposits. They record sustained radial pyroclastic density currents fed by lowpyroclastic fountains. The onset of ignimbrite emplacement is typically preceded (more rarely followed) by pumice fallout with limited dispersal, and some eruptions lack any associated pumice fall deposit, suggesting the absence of tall eruption columns. Particular attention is given to the correlation of well-developed lithic breccias in the ignimbrites, interpreted as probable tracers of caldera collapses. They record as many as five caldera collapse events, in contrast to the two events reported to date. Inter-ignimbrite periods are characterised by explosive and effusive eruptions with limited dispersal, such as small pumice cones, as well as pedogenesis. These periods have similar characteristics as the current post-Green Tuff activity on the island, and, while not imminent, it is reasonable to postulate the occurrence of another ignimbrite-forming eruption sometime in the future. © 2017 Elsevier B.V. All rights reserved.

Experiments were performed to determine the sulfur solubilities of hydrous basalts from Vesuvius, Etna and Stromboli (Italy). The melts were equilibrated at 1050 and 1200 °C with H 2 O and sulfur (added as pyrrhotite), and at pressures ranging from 250 to 2000 bar. Most experiments were performed under oxidising conditions (NNO + 2), and a few under reducing conditions (NNO − 1), with melt water contents of 0.5–3.5 wt.%. Sulfur contents in glasses were determined by electron microprobe and range from 860 up to 6700 ppm. No compositional effect is found between the three alkali basaltic melts. The fugacities of S-bearing species were derived using an MRK equation of state applied to an O–H–S fluid, knowing H 2 and H 2 O fugacities, and range from 50 up to 3000 bar. A thermodynamic species-based model is derived from our results along with available data in the literature, assuming that sulfur dissolution results from the additive contributions of both H 2 S and SO 2 dissolution reactions. Compared to similar models developed for silicic melts, basalt compositions requires the incorporation of an Fe term, which accounts for the strong association between Fe and S in silicate melts, and considers the elevated Fe content of mafic melts. The model shows that, at any fixed f S 2 , the sulfur solubility in hydrous basalt displays a pronounced minimum around NNO, the position of which depends on temperature. The minimum in sulfur solubility coincides with the redox range were the abundance of S 2 in the fluid reaches its maximum compared to either H 2 S or SO 2 species. Such a minimum in solubility is in agreement with experimental constraints at 1 bar under carefully controlled f O 2 and f S 2 . Calculated proportions of dissolved species in the melt depend on the prevailing f S 2 and f O 2 , being in general agreement with available spectroscopic models. Calculations of gas saturation pressures, which classically consider only H 2 O and CO 2 dissolved volatiles, are strongly affected by S-bearing species. At f O 2 close to, or higher than, NNO + 1, omission of sulfur species may result in underestimates of gas saturation pressures of 1 kbar or more. The same happens at f O 2 below NNO − 1.

International audience; Deformation experiments involving hydrous magmas of different compositions (basalt andhaplotonalite) have been performed in a Paterson press at 300 MPa, in the temperature range600°C-1020°C, with water-saturated melts, during 2-4 hours. Prior to deformation the twoend-member magmas were annealed at either 950°C or 1000°C, yielding magmas with crystalcontents in the range 31-53 wt% and 2 sets of viscosity contrasts. Under the experimentalconditions investigated (i.e. moderate shear rates 950°C. In the temperature range 950-985°C a few mixing and minglingtextures occur, though both end-members essentially retain their physical integrity. It is onlyat, or above, 1000°C that a dramatic jump in mingling efficiency happens, corresponding to acrystal fraction of 45 vol%. Textures include entrainment of mafic crystals into the felsicmagma, mafic-felsic banding, enclave formation, diffusion-induced interface, the latter only over limited distances (< 300 microns) due to the short run durations. In the most strainedparcels of interacting magmas, complex mixing/mingling textures were produced, similar tothose observed in volcanic and plutonic rocks in arc settings. The experiments show thatmixing between hydrous felsic and mafic magmas takes place at around 1000°C, atemperature which is almost 200°C lower than mixing under dry conditions. Magma mixingis commonly invoked as a trigger for volcanic eruptions; our experiments suggest that sucheruptions can be driven by small (~15°C) temperature fluctuation in the reservoir. Our resultsalso suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result instratification between end-members rather than in a homogeneous mixture.

International audience; To interpret primitive magma compositions in the Aeolian arc and contribute to a better experimental characterization of ultra-calcic arc melts, equilibrium phase relations have been determined experimentally for the La Sommata basalt (Som-1, Vulcano, Aeolian arc). Som-1 (Na2O + K2O = 4.46 wt.%, CaO = 12.97 wt.%, MgO = 8.78 wt.%, CaO/Al2O3 = 1.03) is a reference primitive ne-normative arc basalt with a strong ultra-calcic affinity. The experiments have been performed between 44 and 154 MPa, 1050 and 1150 °C and from NNO + 0.2 to NNO + 1.9. Fluid-present conditions were imposed with H2O–CO2 mixtures yielding melt H2O concentrations from 0.7 to 3.5 wt.%. Phases encountered include clinopyroxene, olivine, plagioclase and Fe-oxide. Clinopyroxene is slightly earlier than olivine in the crystallization sequence. It is the liquidus phase at 150 MPa, being joined by olivine on the liquidus between 44 and 88 MPa. Plagioclase is the third phase to appear in the crystallization sequence and orthopyroxene was not found. Experimental clinopyroxenes (Fs7–16) and olivines (Fo78–92) partially reproduce the natural phenocryst compositions (respectively Fs5–7 and Fo87–91). Upon progressive crystallization, experimental liquids shift towards higher SiO2 (up to ~ 55 wt.%), Al2O3 (up to ~ 18 wt.%) and K2O (up to ~ 5.5 wt.%) and lower CaO, MgO and CaO/Al2O3. Experimental glasses and natural whole-rock compositions overlap, indicating that progressive crystallization of Som-1 type melts can generate differentiated compositions such as those encountered at Vulcano. The low pressure cotectic experimental glasses reproduce glass inclusions in La Sommata clinopyroxene but contrast with glass inclusions in olivine which preserve basaltic melts more primitive than Som-1. Phase relations for the La Sommata basalt are identical in all critical aspects to those obtained previously on a synthetic ultra-calcic arc composition. In particular, clinopyroxene + olivine co-saturation occurs at very low pressures (≤ 100 MPa). Ultra-calcic arc compositions do not represent primary mantle melts but result from the interaction between a primary mantle melt and clinopyroxene-bearing rocks in the arc crust. At Vulcano, primitive ultra-calcic end-member melts were generated between 250 and 350 MPa in the lower magma accumulation zone by reaction between hot primitive melts and wehrlitic or gabbroic lithologies. At Stromboli, golden pumices and glass inclusions with an ultra-calcic affinity were also generated at shallow pressures, between 150 and 250 MPa, suggesting that the interaction model is of general significance in the Aeolian arc.

International audience; One of the key issues in utilizing precursor signals of volcanic eruption is to reliably interpret geophysical and geochemical data in terms of magma movement towards the surface. An important first step is to identify where the magma is stored prior to ascent. This can be studied through phase-equilibrium experiments designed to replicate the phase assemblage and compositions of natural pyroclasts or by measuring volatiles in melt inclusions from previous eruptions. The second crucial step is to characterize the magmatic conditions and processes that will guide the eruption style. This may be addressed through controlled dynamic decompression or deformation experiments to examine the different rates that govern the kinetics of syn-eruptive degassing, crystallization, and strain. Comparing the compositional and textural characteristics of these experimental products with the natural samples can be used to retrieve magma ascent conditions. These experimental simulations allow interpretation of direct observations and in situ measurements of syn-eruptive processes leading to more accurate forecasting of future eruptive scenarios.

Understanding the plumbing system structure below volcanoes and the storage conditions (temperature, pressure, volatile content and oxygen fugacity) of erupted magmas is of paramount importance for eruption forecasting and understanding of the factors controlling eruptive dynamics. Phase equilibria experiments have been performed on a Tungurahua andesite (Ecuador) to shed light on the magmatic conditions that led to the July-August 2006 eruptions and the parameters that controlled the eruptive dynamics. Crystallization experiments were performed on a representative August 2006 mafic andesite product between 950 and 1025A degrees C, at 100, 200 and 400 MPa and NNO + 1 and NNO + 2 (where NNO is nickel-nickel oxide buffer), and water mole fractions in the fluid (XH2O) from 0 center dot 3 to 1 (water-saturation). Comparison of the natural phenocryst assemblage, proportions and phenocryst compositions with our experimental data indicates that the natural andesite experienced two levels of ponding prior to the eruption. During the first step, the magma was stored at 400 MPa (15-16 km), 1000A degrees C, and contained c. 6 wt % dissolved H2O. In the second step, the magma rose to a confining pressure of 200 MPa (8-10 km), where subsequent cooling (to 975A degrees C) and water-degassing of the magma led to the crystallization of reversely zoned rims on pre-existing phenocrysts. The combination of these processes induced oxidation of the system and overpressure of the reservoir, triggering the July 2006 eruption. The injection of a new, hot, volatile-rich andesitic magma from 15-16 km into the 200 MPa reservoir shortly before the eruption was responsible for the August 2006 explosive event. Our results highlight the complexity of the Tungurahua plumbing system in which different magmatic reservoirs can coexist and interact in time and are the main controlling factors of the eruptive dynamics.

International audience; Two mafic eruptive products from Vesuvius, a tephrite and a trachy-basalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to !12008C) and both at 0Á1 MPa and at high pressures (from 50 to 200 MPa) under H 2 O-bearing fluid-absent and H 2 O-and CO 2 -bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crys-tals to force olivine saturation. Melt H 2 O concentrations reached a maximum of 6Á0 wt % and experimental fO 2 ranged from NNO ^ 0Á1 to NNO þ 3Á4 (where NNO is nickel^nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H 2 O in melt 53 wt %, and by phlogopite (Mg# up to 81) for H 2 O in melt 43 wt %. Olivine (Fo 85) crystallized spontaneously in only one ex-perimental charge. Plagioclase was not found. Upon progressive crys-tallization of clinopyroxene, glass K 2 O and Al 2 O 3 contents strongly increase whereas MgO, CaO and CaO/Al 2 O 3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H 2 O and CO 2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was re-constructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K 2 O $ 4Á5^5Á5 wt %, MgO ¼ 8^9 wt %, Mg# ¼ 75^80, CaO/Al 2 O 3 ¼ 0Á9^0Á95) that crystallize Fo-rich olivine (90^91) as the liquidus phase between 1150 and 12008C and from 300 to 5200 MPa. Primitive Vesuvius melts are volatile-rich (1Á5^4Á5 wt % H 2 O and 600^ 4500 ppm CO 2 in primitive melt inclusions) and oxidized (from NNO þ 0Á4 to NNO þ1Á2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is pro-posed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma^carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.

International audience; The intensive variables of dacitic-rhyodacitic magmas prior to four large Plinian eruptions of Santorini Volcano over the last 200 kyr (Minoan, Cape Riva, Lower Pumice 2 and Lower Pumice 1) were determined by combining crystallization experiments with study of the natural products, including the volatile contents of melt inclusions trapped in phenocrysts. Phase equilibria of the silicic magmas were determined at pressures of 1, 2 and 4 kbar, temperatures of 850-900°C, fluid (H2O + CO2)-saturation, XH2O [= molar H2O/(H2O + CO2)] between 0*6 and 1 (melt H2O contents of 2-10 wt %), and redox conditions of FMQ (fayalite-magnetite-quartz buffer) or NNO + 1 (where NNO is Ni-NiO buffer). Experiments were generally successful in reproducing the phenocryst assemblage of the natural products. The phase relationships vary significantly among the investigated compositions, revealing a sensitivity to small variations in whole-rock compositions. Our results show that the pre-eruptive storage conditions of the four silicic magmas were all very similar. The magmas were stored at T = 850-900°C and P ≥ 2 kbar, under moderately reduced conditions (ΔNNO = −0*9 to −0*1), and were poor in fluorine (∼500-800 ppm) and sulphur (≤100 ppm), but rich in water and chlorine (5-6 wt % and 2500-3500 ppm, respectively). In all cases, the melts were slightly undersaturated with respect to H2O, but most probably saturated with respect to H2O + Cl ± CO2 and a brine. The Santorini magma plumbing system appears to be dominated by a large, long-lived (≥200 kyr) predominantly silicic magma storage region situated at ≥8 km depth, from which crystal-poor melt batches were extracted during the largest caldera-forming eruptions of the volcanic system.

International audience; It is widely believed that the Earth was strongly reduced during its early accretion, however, the transition from the reduced state that prevailed during Earth's early period to the modern oxidized crust and mantle has never been captured by geochemical surveys on Earth materials as old as ∼4.0 billion years ago. By combining available trace-elements data of igneous zircons of crustal origin, we show that the Hadean continental crust was significantly more reduced than its modern counterpart and experienced progressive oxidation till ∼3.6 billions years ago. We suggest that the increase in the oxidation state of the Hadean continental crust is related to the progressive decline in the intensity of chondritic addition during the late veneer. Inputs of carbon- and hydrogen-rich chondritic materials during the formation of Hadean granitic crust must have favored strongly reduced magmatism. The conjunction of cold, wet and reduced granitic magmatism during the Hadean implies the production of methane-rich fluids, in addition to the CO- and H2-rich volcanic species produced by degassing of hot reduced basaltic melts as modified by delivered materials during the late veneer. When the late veneer events ended, magma produced by normal decompression melting of the mantle imparted more oxidizing conditions to erupted lavas and the related crust, emitting CO2- and H2O-rich gases. Our model suggests that the Hadean continental crust was possibly much weaker than present-day, facilitating intrusion of underplating magma and thus allowing faster crustal growth in the early Earth.

Seismic and magnetotelluric field campaigns carried out across the Himalaya and the Tibetan Plateau show mid-crustal low resistivity and low velocity zones. Interpretation of these anomalous observations, either saline fluids or partial melts, is still largely debated partly because experimental data simulating crustal melting under relevant pressure, temperature and water content conditions have not been provided. We report laboratory measurements constraining the resistivity as a function of temperature and the viscosity at 800 °C of natural metapelite during partial melting. Dehydration-melting of muscovite is simulated using a Paterson press at 300 MPa in the temperature range 600–850 °C. Electrical resistivity has been measured from the solid state ( 800 °C) and viscosity has been determined at 800 °C. Our results together with recent experimental constraints on seismic properties of partially molten rocks strongly suggest that the electrical and seismic anomalies measured underneath the Himalayan–Tibetan collisional orogen are best explained by partially molten rocks or local accumulation of pure melt bodies. This is also remarkably corroborated by the temperature–depth conditions of crustal partial melting and melt ponding expected from petrological surveys in the Himalayan range. However, our data suggest much higher melt fraction than previously thought and this implies regions in the middle crust having viscosities several orders of magnitude lower than previously assumed. High degree partial melting in the middle crust of the Himalayan–Tibetan orogenic system suggests revision of conceptual models on the development of mountain belts or that geophysical models addressing electrical resistivity at depth must be re-evaluated.

The island of Pantelleria (Sicily Strait), the type locality for pantellerite, has been the locus of major caldera-forming eruptions that culminated, ca. 50 ka ago, in the formation of the Cinque Denti caldera produced by the Green Tuff eruption. The post-caldera silicic activity since that time has been mostly confined inside the caldera and consists of smaller-energy eruptions represented by more than twenty coalescing pantelleritic centers structurally controlled by resurgence and trapdoor faulting of the caldera floor. A high-resolution 40Ar/39Ar study was conducted on key units spanning the recent (post-20 ka) intracaldera activity to better characterize the present-day status (and forecast the short-term behavior of) the system based on the temporal evolution of the latest eruptions. The new 40Ar/39Ar data capture a long-term (> 15 ka) decline in eruption frequency with a shift in eruptive pace from 3.5 ka−1 to 0.8 ka−1 associated with a prominent paleosol horizon marking the only recognizable volcanic stasis around 12–14 ka. This shift in extraction frequency occurs without major changes in eruptive style, and is paralleled by a subtle trend of decreasing melt differentiation index. We speculate that this decline probably occurred (i) without short-term variations in melt production/differentiation rate in a steady-state compositionally-zoned silicic reservoir progressively tapped deeper through the sequence, and (ii) that it was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka. The intracaldera system is experiencing a protracted stasis since 7 ka. Coupled with recent geodetic evidence of deflation and subsidence of the caldera floor, the system appears today to be on a wane with no temporal evidence for a short-term silicic eruption.

International audience; We present the results of phase equilibrium experiments carried out on basanite and phonotephrite lavas from Ross Island, Antarctica. Experiments were designed to reproduce the P–T–X–f O 2 conditions of deep and intermediate magma storage and to place constraints on the differentiation of each of the two predominant lava suites on the island, which are thought to be derived from a common parent melt. The Erebus Lineage (EL) consists of lava erupted from the Erebus summit and the Dry Valley Drilling Project (DVDP) lineage is represented by lavas sampled by drill core on Hut Point Peninsula. Experiments were performed in internally heated pressure vessels over a range of temperatures (1000–1150°C) and pressures (200–400 MPa), under oxidized conditions (NNO to NNO + 3, where NNO is the nickel–nickel oxide buffer), with XH2O of the H 2 O–CO 2 mixture added to the experimental capsule varying between zero and unity. The overall mineralogy and mineral compositions of the natural lavas were reproduced, suggesting oxidizing conditions for the deep magma plumbing system, in marked contrast to the reducing conditions (QFM to QFM – 1, where QFM is the quartz–fayalite–magnetite buffer) in the Erebus lava lake. In basanite, crystallization of spinel is followed by olivine and clinopyroxene; olivine is replaced by kaersutitic amphibole below ∼1050°C at intermediate water contents. In phonotephrite, the liquidus phase is kaersutite except in runs with low water content ( X H 2 O fluid

International audience; Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of T and fO2 in the magma reservoir. In relatively reduced (ΔNNO < 1) siliceous magmas (e.g., Bishop Tuff, Taupo units), Ti behaves compatibly (DTi ≈ 2–3.5), leading to a decrease in TiO2 activity in the melt with cooling and fractionation. In contrast, FeTi-oxides are poorer in TiO2 in more oxidized magmas (ΔNNO > 1, e.g., Fish Canyon Tuff, Pinatubo), and the d(aTiO2)/dT slope can be negative. Biotite, FeTi-oxides, liquid, and possibly plagioclase largely maintained equilibrium in the Bishop Tuff magma (unlike the pyroxenes, and cores of quartz, sanidine, and zircon) prior to and during a mixing event triggered by a deeper recharge, which, based on elemental diffusion profiles in minerals, took place at least several decades before eruption. Equilibrating phases and pumice compositions show evolving chemical variations that correlate well with mutually consistent temperatures based on the FeTi-oxides, sanidine-plagioclase, and Δ18O quartz-magnetite pairs. Early Bishop Tuff (EBT) temperatures are lower (700 to ~780 °C) than temperatures (780 to >820 °C) registered in Late Bishop Tuff (LBT), the latter defined here not strictly stratigraphically, but by the presence of orthopyroxene and reverse-zoned rims on quartz and sanidine. The claimed similarity in compositions, Zr-saturation temperatures and thermodynamically calculated temperatures (730–740 °C) between EBT and less evolved LBT reflect the use of glass inclusions in quartz cores in LBT that were inherited from the low-temperature rhyolitic part of the reservoir characteristic of the EBT. LBT temperatures as high as 820 °C, the preservation of orthopyroxene, and the presence of reverse-zoned minerals (quartz, sanidine, zircons) are consistent with magma recharge at the base of the zoned reservoir, heating the cooler rhyolitic melt, partly remelting cumulate mush, and introducing enough CO2 (0.4–1.4 wt%, mostly contained in the exsolved fluid phase) to significantly lower H2O-activity in the system.

International audience; Peritectic crystals in igneous rocks may be derived from either the source or country rocks, or may have formed by reactive assimilation of source-inherited solids, primary magmatic minerals during self- or magma mixing, or country-rock xenoliths or xenocrysts. Identifying such peritectic crystals is important for constraining the components and textures of igneous rocks and the underlying processes of magmatic evolution. In this study we demonstrate that peritectic olivine formed in melting experiments crystallizes as clusters of euhedral to subhedral crystals. Olivine replacing orthopyroxene, amphibole, and phlogopite forms crystal clusters with distinct crystal to melt ratios, 2D surface area, grain boundary segmentation, and inclusion relations. In our experiments the textures of peritectic crystals are primarily controlled by the stability temperature and decomposition rate of reactive minerals. High-temperature minerals such as orthopyroxene slowly decompose to form high-density clusters of large crystals with long grain boundary segments. The SiO2-rich peritectic melt produced favours formation of melt inclusions. Low-temperature minerals such as amphibole and phlogopite rapidly decompose to form low-density clusters of small crystals with short grain boundary segments. The relatively SiO2-poor peritectic melt produced results in the formation of fewer melt inclusions. Host melt composition has a minor effect on the textures of peritectic olivine formed in the melting experiments of this study and previous contamination experiments, but affects the assemblages of the peritectic crystal clusters. Cluster density and 2D surface area of peritectic olivine tend to decrease, whereas grain boundary segment length increases with increasing experimental temperature and H2O content. Using textural criteria that distinguish olivine formed after different minerals in our melting experiments, we hypothesize that two olivine populations from a basaltic-andesitic lava flow of the Tatara-San Pedro volcanic complex, Chile, may be peritectic crystals formed after amphibole and orthopyroxene. Both amphibole and orthopyroxene are common in xenoliths preserved in some Tatara-San Pedro lava flows. One notable difference between the experimental and natural olivine crystals is that the natural olivine crystals have 2D surface areas and 2D grain boundary segments up to ∼1000 and ∼100 times larger, respectively, than those produced in our experiments. We propose that this size difference is primarily controlled by comparatively slow heating and decomposition of reactive crystals and textural coarsening of peritectic crystals during prolonged magma residence in the natural system.

International audience; We report petrological and geochemical data for the 2*87 Ga Rio Maria sanukitoid granodiorite and associated rocks from Mesoarchean granite-greenstone terranes of the eastern Amazonian craton, Brazil. The dominant rocks have granodiorite to subordinate monzogranitic compositions, with minor proportions of intermediate quartz diorites or quartz monzodiorites, in addition to mafic endmembers occurring as layered rocks or as enclaves. The mineral assemblage is dominated by amphibole-plagioclase-biotite and epidote minerals, all of inferred magmatic origin, pyroxenes being notably absent. Textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the Rio Maria rocks. Crystallization conditions have been derived from a comparison between natural phase assemblages, proportions and compositions and experimental studies carried out on similar magma compositions. The comparison shows that the parental magmas were water-rich, with more than 7 wt % dissolved H2O, with crystallization temperatures in the range 950-680°C. The Mg/(Mg + Fe) ratios of both amphibole and biotite indicate fO2 conditions in the range NNO + 0*5 to NNO + 2*5 (where NNO is nickel-nickel oxide buffer), therefore pointing to both water-rich and oxidizing conditions for sanukitoid magmas. Amphibole compositions indicate emplacement at around 200 MPa, and record a high-pressure stage of magma crystallization around 600-900 MPa. Sanukitoid magmas share two of the principal characteristics of modern arc magmas, elevated redox state and volatile contents, which suggest that they may have formed in a geodynamic environment broadly similar to present-day subduction zones.

International audience; We report lithium (Li) isotopic measurements in seawater-derived waters that were discharged from geothermal wells, thermal springs, and submarine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). While Li isotopic signatures of the geothermal fluids collected from deep reservoirs were found to be homogeneous for a given site, the δ7Li signatures for each of these reservoirs were significantly different. The first low temperature (25-250°C) experiments of Li isotope exchange during seawater/basalt interaction confirmed that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranged from +19.4‰ (Δ solution - solid) at 25°C to +6.7‰ at 250°C. These experiments demonstrated the importance of Li isotopic fractionation during the formation of Li-bearing secondary minerals and allowed us to determine the following empirical relationship between isotopic fractionation and temperature: Δ solution – solid = 7847 / T – 8.093. Application of experimental results and literature data to the Bouillante area suggested that geothermal water was in equilibrium at 250-260°C. It likely has a deep and large reservoir located in the upper sheeted dike complex of the oceanic crust, just below the transition zone between andesite volcanic flows and the basaltic dikes. The upper dike section, from which Li is extracted by hydrothermal fluids, was characterized by light Li isotopic values in the rocks, indicating retention of 6Li by the altered rocks. For the Lamentin and Diamant areas, the geothermal fluids appeared to be in equilibrium with reservoir volcano-sedimentary rocks at 90-120°C and 180°C, respectively. Further evidence for this argument is provided by the fact that only the Na/Li thermometric relationship determined for sedimentary basins yielded temperature values in agreement with those measured or estimated for the reservoir fluids. This suggests the importance of a sedimentary signature in these reservoir rocks. Altogether, this study highlights that the use of Li isotopic systematics is a powerful tool for characterizing the origin of geothermal waters as well as the nature of their reservoir rocks.

International audience; Both high sulfur contents of the martian regolith and lack of detection of extensive carbonate deposits suggest that the latest geological events that shaped the landscapes of Mars were dominated by acidic waters possibly related to appreciable SO2 concentrations in the atmosphere. On the basis of fundamental thermochemical principles, we model here the likely sulfur contents of (1) the martian and terrestrial mantles and (2) the volcanic gases delivered by the corresponding basaltic magmas. We find that the martian mantle contains at least 3-4 times as much sulfur as its terrestrial counterpart, yielding basaltic melts richer in sulfur than those on Earth. Such an S-enrichment is explained by contrasted redox conditions prevailing during magma ocean equilibration, which lead to distinct iron contents of the martian and terrestrial mantles and of their basaltic derivatives. Calculated volcanic gas compositions in equilibrium with a magma ocean sustaining a denser atmosphere are shown to be dominated by CO ± CO2 and H2 ± H2O species, depending on fO2, sulfur species amounting to only ~ 1%. In contrast, volcanic gases supplied at later stages of Mars evolution, such as during the building of the Tharsis province, are shown to be significantly richer in sulfur, with S contents on average 10-100 times that of gases emitted by magmas on Earth. If degassing during such a period occurred in a tenuous atmosphere (1 bar or less), volcanic gases were dominated by SO2 rather than by H2S, which should have favored the acidification of any persistent water layer. The calculated amounts of S emitted by the Tharsis volcanic region turn out to be equivalent to a 20-60 m thick layer of sulfate minerals if uniformly covering the martian surface, in qualitative agreement with remote sensing of the martian regolith.

Forecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano1,2. Critical to the evaluation of hazards associated with the next eruption is the estimation of the depth of the magma reservoir, one of the main parameters controlling magma properties and eruptive style. Petrological studies have indicated that during past activity, magma chambers were at depths between 3 and 16km (refs 3– 7). Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8–9km depth8, and these have been tentatively interpreted as levels of preferential magma accumulation. By using experimental phase equilibria, carried out on material from four main explosive events at Vesuvius, we show here that the reservoirs that fed the eruptive activity migrated from 7–8km to 3–4km depth between the AD 79 (Pompeii) and AD 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago9 and the 1944 Vesuvius eruption7 are included, the total upward migration of the reservoir amounts to 9–11 km. The change of preferential magma ponding levels in the upper crust can be attributed to differences in the volatile content and buoyancy of ascending magmas, as well as to changes in local stress field following either caldera formation10 or volcano spreading11. Reservoir migration, and the possible influence on feeding rates12, should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius.

The compositions of volcanic gases can be used by geologists to monitor volcanic activity or as a pointer to how the composition of the atmosphere has evolved with time. It is implicitly assumed that these gases faithfully reflect the conditions, in particular the redox state, of their source magma. But detailed physico-chemical modelling of the degassing processes reveals that this assumption is not generally valid and that the redox state of erupted magmas does not necessarily reflect the redox state of the gases emitted into the atmosphere. A coupled chemical–physical model of conduit flow shows that the oxidation state of an ascending magma is strongly dependent on both the composition and amount of gas in the reservoir. Volatiles carried by magmas, either dissolved or exsolved, have a fundamental effect on a variety of geological phenomena, such as magma dynamics1,2,3,4,5 and the composition of the Earth’s atmosphere6. In particular, the redox state of volcanic gases emanating at the Earth’s surface is widely believed to mirror that of the magma source, and is thought to have exerted a first-order control on the secular evolution of atmospheric oxygen6,7. Oxygen fugacity ( ) estimated from lava or related gas chemistry, however, may vary by as much as one log unit8,9,10, and the reason for such differences remains obscure. Here we use a coupled chemical–physical model of conduit flow to show that the redox state evolution of an ascending magma, and thus of its coexisting gas phase, is strongly dependent on both the composition and the amount of gas in the reservoir. Magmas with no sulphur show a systematic increase during ascent, by as much as 2 log units. Magmas with sulphur show also a change of redox state during ascent, but the direction of change depends on the initial in the reservoir. Our calculations closely reproduce the H2S/SO2 ratios of volcanic gases observed at convergent settings, yet the difference between in the reservoir and that at the exit of the volcanic conduit may be as much as 1.5 log units. Thus, the redox state of erupted magmas is not necessarily a good proxy of the redox state of the gases they emit. Our findings may require re-evaluation of models aimed at quantifying the role of magmatic volatiles in geological processes.

The sedimentary record of the Halslon dam, 20 km north of the Vatnajokull, allows a better understanding of the glaciation–deglaciation interface for MIS 6/5e and MIS2/1 in Northern Iceland. This sequence evidences the full Eemian deglaciation, the record of a Mid-Eemian cooling glacial complex and an abrupt deglaciation from the Bolling onward. Northern Iceland also experienced limited deglaciation and polar desert during the LGM, in connection with precipitation starvation due to the deflection towards the south of the Irminger current and of the Gulf Stream and the development of sea ice along the northern coastline.

International audience; We present a comprehensive overview of the magnetic properties of the four known tektite fields and related fully melted impact glasses (Aouelloul, Belize, Darwin, Libyan desert and Wabar glasses, irghizites, and atacamaites), namely magnetic susceptibility and hysteresis properties as well as properties dependent on magnetic grain-size. Tektites appear to be characterized by pure Fe2+ paramagnetism, with ferromagnetic traces below 1 ppm. The different tektite fields yield mostly non-overlapping narrow susceptibility ranges. Belize and Darwin glasses share similar characteristics. On the other hand the other studied glasses have wider susceptibility ranges, with median close to paramagnetism (Fe2+ and Fe3+) but with a high-susceptibility population bearing variable amounts of magnetite. This signs a fundamental difference between tektites (plus Belize and Darwin glasses) and other studied glasses in terms of oxygen fugacity and heterogeneity during formation, thus bringing new light to the formation processes of these materials. It also appears that selecting the most magnetic glass samples allows to find impactor-rich material, opening new perspectives to identify the type of impactor responsible for the glass generation.

International audience; The Earth's mantle redox state regulates the fate and transfer of metals by magmatism, buffers the igneous inputs of volcanic gases in the atmosphere and controls the depth of mantle melting. It therefore strongly affects ore forming processes, biogeochemical cycles and deep geodynamic processes. This paper reviews the current knowledge on the redox state of the upper mantle and of magmas produced by mantle melting. The geochemical processes likely to control and modify it through space and time are discussed.We analyze the link between the redox state of magma and that of their mantle source and we conclude that melts produced in the mantle may well all equilibrate in a narrow range of oxidation state, where the speciation of sulfur in basalts shifts from sulfide to sulfate, that is, FMQ+1 ± 1 (1 log unit below and above the oxygen fugacity buffered by the assemblage fayalite–magnetite–quartz). Subsequently, degassing and partial crystallization of melts can affect their redox states, producing most of the range of redox states observed on magmas reaching Earth's surface. The asthenosphere sourcing basaltic magmas may therefore be more oxidized than the FMQ−1 value generally assumed.We also discuss redox transfers from the mantle to the atmosphere via volcanic degassing and the backward fluxes via subduction processes of the hydrothermalized oceanic lithosphere. Arc-magmas are oxidized (up to FMQ+4) but it is unclear when this feature is acquired since strongly oxidized primary arc-basalts have yet to be found. The oxidizing event may be the assimilation of slab-derived SO3-rich fluids by primary basalt generated by decompression melting in the mantle-wedge.Overall, subduction must result in a transfer of oxygen from the Earth's surface down to the mantle. This must imply that subduction and its initiation can hardly be the trigger of the great oxidation event at the end of the Archaean. In contrast, the cooling of the Earth's interior through time must have impacted on the redox state of basalts, by decreasing the depth of mantle melting. According to the long-established vertical stratification of the Earth's mantle, ancient primary magmas are therefore likely to have been more reduced (i.e. < FMQ−3) than present-day ones. However, geochemical observations on ancient basalts suggest a constant oxidation state since the early Archaean.We conclude that large uncertainties in the calibration of mineralogical oxygen barometer probably explains why we have difficulties in identifying (i) ancient primary basalts being more reduced than recent ones and (ii) primary basalts from subduction zones being as oxidized as arc-lavas reaching the surface. Finally, the degree of mantle melting is certainly a key issue for the interpretation of the mantle oxidation state. Extremely oxidized melts, enriched in C–H–S volatile species, produced by very low degrees of mantle melting may be indicative of an Earth's mantle more oxidized than usually considered.

International audience; To test mechanisms of basaltic ma gma degassing, continuous decompressions of volatile-bearing (2.7-3.8 wt% H2O, 600-1300 ppm CO2) Stromboli melts were performed from 250-200 to 50-25 MPa at 1180-1140°C. Ascent rates were varied from 0.25 to ~ 1.5 m/s. Glasses after decompression show a wide range of textures, from totally bubble-free to bubble-rich, the latter with bubble number densities from 104 to 106/cm3, similar to Stromboli pumices. Vesicularities range from 0 to ~ 20 vol%. Final melt H2O concentrations are homogeneous and always close to solubilities. In contrast, the rate of vesiculation controls the final melt CO2 concentration. High vesicularity charges have glass CO2 concentrations that follow theoretical equilibrium degassing paths whereas glasses from low vesicularity charges show marked deviations from equilibrium, with CO2 concentrations up to one order of magnitude higher than solubilities. FTIR profiles and maps reveal glass CO2 concentration gradients near the gas-melt interface. Our results stress the importance of bubble nucleation and growth, and of volatile diffusivities, for basaltic melt degassing. Two characteristic distances, the gas interface distance (distance either between bubbles or to gas-melt interfaces) and the volatile diffusion distance control the degassing process. Melts containing numerous and large bubbles have gas interface distances shorter than volatile diffusion distances, and degassing proceeds by equilibrium partitioning of CO2 and H2O between melt and gas bubbles. For melts where either bubble nucleation is inhibited or bubble growth is limited, gas interface distances are longer than volatile diffusion distances. Degassing proceeds by diffusive volatile transfer at the gas-melt interface and is kinetically limited by the diffusivities of volatiles in the melt. Our experiments show that CO2-oversaturated melts can be generated as a result of magma decompression. They provide a new explanation for the occurrence of CO2-rich natural basaltic glasses and open new perspectives for understanding explosive basaltic volcanism.

International audience; The Moon was once thought to be depleted in volatile elements. Analyses of the carbon contents of lunar volcanic glasses reveal that carbon monoxide degassing could have produced the fire-fountain eruptions from which these glasses were formed.

The effect of sulfur dissolved in anorthite-diopside eutectic (AD) glasses on the glass transition temperature (Tg) has been investigated via Differential Scanning Calorimetric measurements (DSC) and Thermogravimetric Analysis (TGA) under moderately reducing to oxidizing conditions. In a series of AD glasses, we have measured the change in Tg as a function of S content present as SO4 2- (HS- is also identified to a lesser extent) and H2O content. The AD glasses investigated have S contents ranging from 0 to 7519 ppm and H2O contents ranging from 0 to 5.3 wt.%. In agreement with previous studies, increasing H2O content induces a strong exponential decrease in Tg: volatile free AD glass has a Tg at 758±13C and AD glass with 5.18±0.48 wt.% H2O has a Tg at 450±11C. The change in Tg as a function of H2O is wellreproduced with a third-order polynomial function and has been used to constrain Tg at any H2O content. The effect of S on Tg is almost inexistent or towards a decrease in Tg with increasing S content. For instance, at ~2.4 wt.% H2O, the addition of S induces a change in Tg from 585±10°C with 0 ppm S to 523±3C with 2365±138 ppm S; a further increase in S up to 7239±90 ppm S does not induce a dramatic change in Tg measured at 529±2C. The limited effect of S on the glass transition temperature contrasts with recent spectroscopic measurements suggesting that S dissolution as SO4 2- groups provokes an increase in the polymerization degree. We propose an alternative view which reconciles the spectroscopic evidence with the Tg measurements. The dissolution of S as SO4 2- does not induce the formation of Si-O-Si molecular bonding through consumption of available non-bridging oxygens (NBO) but instead we suggest that Si-O-S molecular bonds are formed which are not detectable by DSC measurements but mimic the increase in glass polymerization. Therefore, spectroscopic measurements must be used with caution in order to extract melt physical properties.

International audience; The role of volcanogenic halogen-bearing (i.e. chlorine and bromine) compounds in stratospheric ozone chemistry and climate forcing is poorly constrained. While the 1991 eruption of Pinatubo resulted in stratospheric ozone loss, it was due to heterogeneous chemistry on volcanic sulfate aerosols involving chlorine of anthropogenic rather than volcanogenic origin, since co-erupted chlorine was scavenged within the plume. Therefore, it is not known what effect volcanism had on ozone in pre-industrial times, nor what will be its role on future atmospheres with reduced anthropogenic halogens present. By combining petrologic constraints on eruption volatile yields with a global atmospheric chemistry-transport model, we show here that the Bronze-Age 'Minoan' eruption of Santorini Volcano released far more halogens than sulfur and that, even if only 2% of these halogens reached the stratosphere, it would have resulted in strong global ozone depletion. The model predicts reductions in ozone columns of 20 to >90% at Northern high latitudes and an ozone recovery taking up to a decade. Our findings emphasise the significance of volcanic halogens for stratosphere chemistry and suggest that modelling of past and future volcanic impacts on Earth's ozone, climate and ecosystems should systematically consider volcanic halogen emissions in addition to sulfur emissions. Halogens (especially chlorine and bromine) play important roles in the catalytic destruction of atmospheric ozone 1–3. However, the role of 'volcanogenic' halogens in stratospheric ozone chemistry and climate forcing remains poorly constrained. It is commonly believed that most of the halogens in explosive volcanic plumes do not enter the stratosphere, because they are removed by hydrometeors in the trop-osphere 4,5. In contrast, volcanic sulfur emissions are known to play a key role in stratospheric ozone change and climate forcing on annual to decadal timescales 6. As stratospheric sulfate aerosols backscatter solar radiation, they act to cool the Earth's troposphere and surface 7. In addition, the surface of sulfate aerosols provides sites for heterogeneous chemical reactions that activate halogen species which destroy ozone 8,9. In the case of recent large volcanic eruptions (1982 El Chichón eruption, 1991 Pinatubo eruption), the halogen compounds involved in such reactions were sourced from anthropogenic emissions 1,10 (e.g., chlorofluorocarbons). The 1991 eruption of Mount Pinatubo (Philippines) furnished particularly significant insights into the mechanisms, feedbacks and timescales of such processes 1. However, the Pinatubo magma was relatively poor in halogens compared to other volcanic eruptions. In addition, it occurred while the global atmosphere was still loaded with anthropogenic emissions of organic halogens. This calls into question the general applicability of the conclusions derived from Pinatubo observations and ensuing modelling to past or forthcoming eruptions. In particular, it is not known what would happen with a volcanic event with a different halogen yield occurring under pre-industrial atmospheric conditions. In addition to these uncertainties, recent models have re-evaluated the fraction of explosively-emitted halogens crossing the tropopause, suggesting significantly higher values, up to 25%

The phase relationships and compositions of a pantellerite from the Eburru complex in the Kenya Rift Valley have been determined at 150 MPa and under reducing conditions, 2 log units below the Ni–NiO solid buffer. The effects of temperature and melt water content on phase relationships have been explored. Alkali feldspar and quartz crystallise alone at temperatures above 700 °C, irrespective of melt water content. Below 700 °C, sodic amphibole and clinopyroxene also crystallise; the amphibole being the liquidus phase under water-rich conditions. The coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal-poor pantellerite implies temperatures below 700 °C and melt water contents higher than 4 wt.%, possibly up to 5–6 wt.%. Pantellerites have lower liquidus temperatures than associated comendites, which supports a parent–daughter relationship between the two magma types. The melts produced in the experiments extend the compositional trend displayed by the natural rock series, and reproduce some extreme compositions occasionally observed in alkaline volcanic series, with FeO⁎ contents above 12 wt.% and Na2O contents approaching 10 wt.%. Pantellerites are therefore the true near-minimum melt compositions of alkaline oversaturated magma series.

Experiments performed at 2 kbar, in the temperature range 800–1000°C, with fO2 between NNO–2·3 and NNO+2·9 (where NNO is the nickel–nickel oxide buffer), and varying amounts of sulphur added to hydrous metaluminous rhyolite bulk compositions, were used to constrain the solubility of sulphur in rhyolite melts. The results show that fS2 exerts a dominant control on the sulphur solubility in hydrous silicate melts and that, depending on fO2, a rhyolitic melt can reach sulphur contents close to 1000 ppm at high fS2. At fO2 below NNO+1, the addition of iron to a sulphur-bearing rhyolite magma produces massive crystallization of pyrrhotite and does not enhance the sulphur solubility of the melt. For a given fO2, the melt-sulphur-content increases with fS2. For fixed fO2 and fS2, temperature exerts a positive control on sulphur solubilities, at least for fO2 below NNO+1. The mole fraction of dissolved sulphur exhibits essentially linear dependence on fH2S at low fO2 and, although the experimental evidence is less clear, on fSO2 at high fO2. The minimum in sulphur solubility corresponds to the redox range where both fH2S and fSO2 are approximately equal. A thermodynamic model of sulphur solubility in hydrous rhyolite melts is derived assuming that total dissolved sulphur results from the additive effects of H2S and SO2 dissolution reactions. The model reproduces well the minimum of sulphur solubility at around NNO+1, in addition to the variation of the sulphide to sulphate ratio with fO2. A simple empirical model of sulphur solubility in rhyolitic melts is derived, and shows good correspondence between model and observations for high-silica rhyolites.

Kinetics and reaction paths of Fe 3+ reduction by H 2 in high-Fe and low-H 2 O silicate melts have been investigated at 800 °C. Time-series experiments were performed in cold-seal pressure vessels at 50 bars of pure H 2 using rapid-heating and rapid-quench strategies. Within the first minutes of the experiments, a fast partitioning of Na occurred between the gas and the melt due to the reducing conditions. Kinetically decoupled from the Na partitioning, the progression of a front of Fe 3+ reduction within the quenched melt was observed and was identified as a diffusion-limited process. The growth of the reduced layer is accompanied by an increase in concentration of OH-groups suggesting that reduction operates through proton incorporation within the melt. As this growth rate is slightly faster than predicted from the diffusion of molecular H 2 O, a different and mobile water-derived species seems likely. One possible mechanism is the reduction of Fe 3+ by the transport of molecular H 2 . As this process is limited by the flux of H 2 , it will depend on both diffusivity and solubility of H 2 in the melt. Alternatively, migration of protons (H + ) and electronic species within the melt could control the velocity of the reduction front. The increase in concentration of the reaction-derived OH groups produces a water over saturation followed by partial dehydration of the melt. This dehydration leads to a change in the redox conditions within the gas that influences the Na partitioning between gas and melt.

International audience; New experimental data of Ar and Ne solubility at pressures up to 360 MPa in alkali-basaltic (Mt. Etna, Italy) and rhyolitic (Vulcano Island, Italy) melts are presented. Solubility experiments have been conducted in internally heated pressure vessels at 1200 °C under nominally anhydrous conditions. Ar and Ne contents dissolved in the experimental glasses were then measured by quadrupole mass spectrometry. Over the pressure range investigated, Ar and Ne solubilities vary linearly with Ar and Ne pressures and can be described by Henry's constant (kAr,Ne = PAr, Ne / xAr, Ne, where PAr, Ne is the partial pressure of Ar or Ne and xAr, Ne is the molar fraction of Ar or Ne in the melt) of 7.6 ± 0.8 × 105 and 1.9 ± 0.4 × 105 MPa, respectively for Ar and Ne in the basaltic melt and 1.5 ± 0.2 × 105 and 3.8 ± 0.2 × 104 MPa, respectively for Ar and Ne in the rhyolitic melt. In accordance with existing models, rhyolitic melts show higher noble gas solubilities than basaltic melts, Ne solubility being higher than that of Ar in a given composition. We propose a semi-empirical model of noble gas (Ar, Ne and He) solubility calibrated on a very large set of measurements in natural and synthetic silicate melts. The model expands the concept of ionic porosity in terms of porosity accessible for noble gas dissolution in melt, taking into account the large-scale structural effects of cations, as well as temperature and pressure. The model is valid over a wide range of temperatures (800–1600 °C), pressures (up to 3 GPa) and compositions, being useful for both geological and physico-chemical studies.

International audience; New experiments have been performed on a high-K basalt (PST-9) from Stromboli volcano, Italy, to constrain the physical conditions of golden pumice magmas at their storage level and discuss their petrogenesis. Fluid-present, H2O- and CO2-bearing, near-liquidus experiments were performed at 11508C between 100 and 400MPa and under oxidizing conditions. Glasses were analyzed by Fourier transform IR spectroscopy and their H2O and CO2 concentrations compared with those in glass inclusions.Most glass inclusions cluster near the 200MPa isobar, suggesting entrapment at a depth of ~8 km. Golden pumice magmas have viscosities of 7.9 Pa s and densities of 2.48-2.57 g/cm3. Compositions of experimental fluids coexisting with melts along the liquidus have been estimated by mass balance. They range from CO2-rich (XH2O~0.2) at 400MPa to H2O-rich (XH2O~0.8) at 100MPa.The free fluid phase present at the reservoir level has an XH2O of ~0.6, consistent with equilibration with a near-liquidus golden pumice magma below 300MPa. Clinopyroxene is the liquidus phase in all highpressure experiments, either fluid-absent (H2O) or fluid-present (H2O-CO2). In contrast, at 0.1MPa, cpx and ol appear together on the liquidus and olivines are more Fo-rich (up to 89.1) than those crystallizing at high pressures (up to 87.3).The composition of cotectic liquids multiply saturated in cpx and ol has been experimentally determined. Most pumices and a majority of melt inclusions have compositions of cotectic melts. In contrast, PST-9 plots in the cpx field and is representative of less evolved ankaramitic magmas parental to golden pumices. Melt inclusions trapped in Fo 487 olivines form a group of ultra-calcic compositions plotting in the cpx field, interpreted as boundary layer melts locally generated by cpx dissolution in the deep reservoir. Ankaramitic melts in the Stromboli feeding system are proposed to derive from primitive mantle melts by combined crystallization, mixing, wall-rock interaction and assimilation.

Partitioning experiments between a basaltic melt from Mt. Etna and a low-density hydrous fluid or vapor containing H(2)O or H(2)O-CO(2) were performed at 1200-1260 degrees C, at pressures between 1 and 200 MPa, either near the nickel-nickel oxide (NNO) buffer or at two log units above it (NNO + 2), and with different chloride concentrations. Most of the experiments were done at chloride-brine-undersaturated conditions, although at the highest Cl concentrations explored brine saturation might have been reached. The average partition coefficients (D(Cl)(fluid/melt)) over the range of Cl concentrations were derived on a weight basis by plotting the calculated concentrations of Cl in the fluid phase versus the measured ones in the melt. For H(2)O-Cl experiments in which the Cl concentration in the melt was = 2. Addition of CO(2) at NNO yields lower partition coefficients than in CO(2)-free conditions over the pressure range investigated. The negative pressure dependence observed for H(2)O-Cl experiments disappears when CO(2) is present in the system. Overall, once CO(2) is introduced in the system, Cl fugacity in the silicate melt tends to increase, thus resulting in a decrease of D(Cl)(f/m). Application to Mt. Etna shows that the composition of the volcanic plume in terms of Cl records very shallow pressures of magma equilibration with its exsolved fluid.