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To test mechanisms of basaltic magma degassing, continuous decompressions of volatile-bearing (2.7-3.8 wt% [H.sub.2]O, 600-1,300 ppm C[O.sub.2]) Stromboli melts were performed from 250-200 to 50-25 MPa at 1,180-1,140 °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 [10.sup.4] to [10.sup.6] [cm.sup.-3] , similar to Stromboli pumices. Vesicularities range from 0 to ~20 vol%. Final melt [H.sub.2]O concentrations are homogeneous and always close to solubilities. In contrast, the rate of vesiculation controls the final melt C[O.sub.2] concentration. High vesicularity charges have glass C[O.sub.2] concentrations that follow theoretical equilibrium degassing paths, whereas glasses from low vesicularity charges show marked deviations from equilibrium, with C[O.sub.2] concentrations up to one order of magnitude higher than solubilities. FTIR profiles and maps reveal glass C[O.sub.2] 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 C[O.sub.2] and [H.sub.2]O 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 gasmelt interface and is kinetically limited by the diffusivities of volatiles in the melt. Our experiments show that C[O.sub.2]-oversaturated melts can be generated as a result of magma decompression. They provide a new explanation for the occurrence of C[O.sub.2]-rich natural basaltic glasses and open new perspectives for understanding explosive basaltic volcanism. Keywords Basaltic melts * Volatiles * Decompression experiments * Magma degassing * C[O.sub.2]-oversaturation * Explosive volcanism, Introduction During magma ascent, decompression leads to the exsolution of volatiles (mainly [H.sub.2]O and C[O.sub.2]) from the melt, and to their progressive transfer from the deep Earth to the atmosphere. [...]

The Precambrian history of our planet is marked by two major events: a pulse of continental crust formation at the end of the Archaean eon and a weak oxygenation of the atmosphere (the Great Oxidation Event) that followed, at 2.45 billion years ago. This oxygenation has been linked to the emergence of oxygenic cyanobacteria (1,2) and to changes in the compositions of volcanic gases (3,4), but not to the composition of erupting lavas--geochemical constraints indicate that the oxidation state of basalts and their mantle sources has remained constant since 3.5 billion years ago (5,6). Here we propose that a decrease in the average pressure of volcanic degassing changed the oxidation state of sulphur in volcanic gases, initiating the modern biogeochemical sulphur cycle and triggering atmospheric oxygenation. Using thermodynamic calculations simulating gas-melt equilibria in erupting magmas, we suggest that mostly submarine Archaean volcanoes produced gases with S[O.sub.2]/[H.sub.2]S < 1 and low sulphur content. Emergence of the continents due to a global decrease in sea level and growth of the continental crust in the late Archaean then led to widespread subaerial volcanism, which in turn yielded gases much richer in sulphur and dominated by S[O.sub.2]. Dissolution of sulphur in sea water and the onset of sulphate reduction processes could then oxidize the atmosphere., Mass-independent fractionation of sulphur (MIF), quantified as [Δ.sup.33]S, is commonly recorded in sediments older than 2.45 billion years (Gyr) (7). A proliferation of MIF has been clearly identified between 2.7 [...]

Mount Vesuvius, Italy, quiescent since A.D. 1944, is a dangerous volcano currently characterized by elevated C[O.sub.2] emissions of debated origin. We show that such emissions are most likely the surface manifestation of the deep intrusion of alkalic-basaltic magma into the sedimentary carbonate basement, accompanied by sidewall assimilation and C[O.sub.2] volatilization. During the last eruptive period (1631-1944), the carbonate-sourced C[O.sub.2] made up 4.7-5.3 wt% of the vented magma. On a yearly basis, the resulting C[O.sub.2] production rate is comparable to C[O.sub.2] emissions currently measured in the volcanic area. The chemical and isotopic composition of the fumaroles supports the predominance of this crust-derived C[O.sub.2] in volatile emissions at Mount Vesuvius.

Indonesia hosts the largest number of active volcanoes, several of which are renowned for climate-changing historical eruptions. This pedigree might suggest a substantial fraction of global volcanic sulfur emissions from Indonesia and are intrinsically driven by sulfur-rich magmas. However, a paucity of observations has hampered evaluation of these points—many volcanoes have hitherto not been subject to emissions measurements. Here we report new gas measurements from Indonesian volcanoes. The combined SO2 output amounts to 1.15 ± 0.48 Tg/yr. We estimate an additional time-averaged SO2 yield of 0.12-0.54 Tg/yr for explosive eruptions, indicating a total SO2 inventory of 1.27-1.69 Tg/yr for Indonesian. This is comparatively modest—individual volcanoes such as Etna have sustained higher fluxes. To understand this paradox, we compare the geodynamic, petrologic, magma dynamical and shallow magmatic-hydrothermal processes that influence the sulfur transfer to the atmosphere. Results reinforce the idea that sulfur-rich eruptions reflect long-term accumulation of volatiles in the reservoirs. Indonesia is the country with the most active volcanoes, several of which are renowned for climate-changing eruptions. Here the authors show a rather moderate sulfur emissions budget and reinforce the idea that sulfur-rich eruptions reflect long-term accumulation of volatiles in the reservoirs. [ABSTRACT FROM AUTHOR]

Author(s): Alain Burgisser (corresponding author) [1]; Bruno Scaillet [1] Volatiles carried by magmas, either dissolved or exsolved, have a fundamental effect on a variety of geological phenomena, such as magma [...]

This paper reports the results of hydrothermal synthesis in the system [Na.sub.2]O-MgO-FeO-[Fe.sub.2][O.sub.3]-Si[O.sub.2][H.sub.2]O. Four samples of stoichiometric magnesioriebeckite composition, ideally [[][Na.sub.2][Mg.sub.3][Fe.sup.3+.sup.2][Si.sub.8] [O.sub.22](OH).sub.2], were run at 700-800 [degrees]C, 0.4 GPa, and redox conditions varying from NNO (Nickel-Nickel Oxide) to NNO + 2.3 log f[o.sub.2]. Powder XRD and SEM-EDX show a high (>85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [[Na[Na.sub.2][Mg.sub.4][Fe.sup.3+][Si.sub.8][O.sub.22](OH).sub.2]]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mossbauer spectra show that in all samples, [Fe.sup.3+] is completely ordered at M2, whereas [Fe.sup.2+] occurs at the M1, M3, and M4 sites. The [Fe.sup.3+]/[Fe.sup.2+] ratio is a function of f[o.sub.2]: for increasing oxidation conditions, there is significant increase in [sup.M2][Fe.sup.3+] and decrease in [Fe.sup.2+], notably in [sup.M4][Fe.sup.2+]. Mossbauer spectra also show significant variation in [sup.M1][Fe.sup.2+] and [sup.M3][Fe.sup.2+] quadrupole splitting as a function of the [Fe.sup.3+] content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies

Several geophysical surveys carried out in southern Tibet have revealed the occurrences of bright spots of high electrical conductivity at 15-20 km depth, corresponding to zones of seismic attenuation and correlated to high heat flow at the surface. Such zones have been variably interpreted as intruding magmas, partially melted middle crust, or concentrations of free fluids. Here we show that experimentally derived electrical conductivities of hydrous granite melts under pressure-temperature-H20 conditions appropriate for crust-derived magmas are in perfect agreement with those inferred from magnetotelluric data, strongly suggesting that the Tibetan bright spots image present-day pluton assembly in the middle crust, in a manner analogous to what happened during the Miocene to form the leucogranite plutons now exposed southward in the High Himalaya range. Keywords: leucogranite, Himalayas, Tibet, conductivity, crustal melting.

Kinetics and reaction paths of [Fe.sup.3+] reduction by [H.sub.2] in high-Fe and low-[H.sup.2]O silicate melts have been investigated at 800[degrees]C. Time-series experiments were performed in cold-seal pressure vessels at 50 bars of pure [H.sub.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.sup.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.sub.2]O, a different and mobile water-derived species seems likely. One possible mechanism is the reduction of [Fe.sup.3+] by the transport of molecular [H.sub.2]. As this process is limited by the flux of [H.sub.2], it will depend on both diffusivity and solubility of [H.sub.2] in the melt. Alternatively, migration of protons ([H.sup.+]) 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.

The kinetics of Fe oxidation-reduction in two hydrous rhyolitic melts, one metaluminous and the other peralkaline, have been studied at 800 [degrees]C, 2kb, for melt water contents from ~5 wt% to saturation and [f.sub.O.sub.2] between NNO-2 and NNO + 3 (NNO = nickel-nickel oxide redox buffer). The metaluminous melt (~1 wt% Fe[O.sub.T]) reached redox equilibrium after 10 hours and the peralkaline one (~3 wt% Fe[O.sub.T]) after 3 hours. The kinetics of Fe oxidation and reduction are similar and unaffected by the presence or absence of a hydrous fluid phase. No redox front is observable in the glass as the [Fe.sup.3+]/[Fe.sup.2+] ratio evolves, implying that the Fe redox kinetics in hydrous silicic melts is rate-limited neither by the diffusion of [H.sub.2] nor by the mobilities of divalent cations, as observed for anhydrous basaltic melts. We propose a two-step reaction mechanism that involves: (1) virtually instantaneous diffusion of [H.sub.2] in the sample, followed by (2) slower structural/chemical reorganizations around Fe atoms. The overall redox process involving iron and hydrogen in Fe-poor, [H.sub.2]O-rich melts is thus reaction-limited and obeys a first-order logarithmic rate law. The relatively slow kinetics of oxidation/reduction explains why melt [Fe.sup.3+]/[Fe.sup.2+] can be readily quenched in laboratory experiments. Simulation of oxidation of magmas due to [H.sub.2] exchange with wall rocks is performed using these new kinetics laws and two [D.sub.H.sub.2] values extracted from the literature. We demonstrate that the metaluminous composition is not significantly modified whereas the peralkaline composition undergoes important and fast changes of [Fe.sup.3+]/[Fe.sup.2+] during short processes such as ascent prior to Plinian-style eruptions.

Author(s): Gaelle Prouteau [1, 2]; Bruno Scaillet (corresponding author) [3]; Michel Pichavant [3]; René Maury [1] The low concentrations of niobium, tantalum and titanium observed in island-arc basalts are thought [...]

Since May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km³ of basanitic magma. The eruption occurred along a WNW-ESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels (≤12-15 km) yields a phonolitic residual liquid containing up to 3-4 wt% H2O after ≥65 wt% of an assemblage of olivine+plagioclase+amphibole+clinopyroxene+biotite+magnetite+ilmenite+apatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6-8 km depth, while a shallower differentiation (≤4-5 km) results in the production of liquids with trachyte-benmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in SiO2-Al2O3 but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage atMayotte is a rather shallow process. [ABSTRACT FROM AUTHOR]