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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. [...]

Deformation experiments involving hydrous magmas of different compositions (basalt and haplotonalite) have been performed in a Paterson press at 300 MPa, in the temperature range 600 °C–1020 °C, with water-saturated melts, during 2–4 h. Prior to deformation, the two end-member magmas were annealed at either 950 °C or 1000 °C, yielding magmas with crystal contents in the range 31–53 wt.% and 2 sets of viscosity contrasts. Under the experimental conditions investigated (i.e. moderate shear rates < 10 − 3 s − 1 ), mixing/mingling textures appear at temperatures > 950 °C. In the temperature range 950–985 °C, a few mixing and mingling textures occur, though both end-members essentially retain their physical integrity. It is only at, or above, 1000 °C that a dramatic jump in mingling efficiency happens, corresponding to a crystal fraction of 45 vol.%. Textures include entrainment of mafic crystals into the felsic magma, mafic–felsic banding, enclave formation, and diffusion-induced interface, the latter only over limited distances (< 300 μm) due to the short run durations. In the most strained parcels of interacting magmas, complex mixing/mingling textures were produced, similar to those observed in volcanic and plutonic rocks in arc settings. The experiments show that mixing between hydrous felsic and mafic magmas takes place at around 1000 °C, a temperature which is almost 200 °C lower than mixing under dry conditions. Magma mixing is commonly invoked as a trigger for volcanic eruptions; our experiments suggest that such eruptions can be driven by small (~ 15 °C) temperature fluctuation in the reservoir. Our results also suggest that slow replenishment of a felsic reservoir by mafic inputs will likely result in stratification between end-members rather than in a homogeneous mixture. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 SO 3 -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. [ABSTRACT FROM AUTHOR]

Santorini volcano in the Aegean region (Greece) is characterized by andesitic- to silicic-dominated explosive activity and caldera-forming eruptions, sourced from magmatic reservoirs located at various structural levels beneath the volcano. There is a good understanding of the silica-rich magmatism of the island whereas the andesite-dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood. To fill this gap we have performed crystallization experiments on a representative basalt from Santorini with the aim of determining the conditions of differentiation (pressure, temperature, volatile fugacities) and the parental magma relationship with the andesitic eruptive rocks. Experiments were carried out between 975 and 1040°C, in the pressure range 100-400 MPa, fO2 from QFM to NNO + 3·5 (where QFM is quartz-fayalite-magnetite and NNO is nickel-nickel oxide), with H2Omelt contents varying from saturation to nominally dry conditions. The results show that basalt phenocrysts within the basalt crystallized at around 1040°C in a magma storage reservoir located at a depth equivalent to 200-400 MPa pressure, with 3-5 wt % dissolved H2O, and fO2 around QFM. Comparison with the xenocryst and phenocryst assemblages of the Upper Scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000°C and 400 MPa, following 60-80 wt % crystallization of an ol + cpx + plag + Ti-mag + opx ± pig-ilm assemblage, with melt water contents around 4-6 wt %. At Santorini, the andesitic low-viscosity and water-rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs, eventually erupting upon mixing with resident magma. Changes in prevailing oxygen fugacity may control the tholeiitic-calc-alkaline character of Santorini magmas, explaining the compositional and mineralogical differences observed between the recent Thyra and old eruptive products from Akrotiri. [ABSTRACT FROM AUTHOR]

Magmatic degassing is ubiquitous and enduring, yet its impact on both planetary surficial chemistry and how it may have varied among planetary systems remains imprecise. A large number of factors are likely to be involved in the control of magmatic gas compositions, leading roles being given to the redox state and volatile abundances in planetary interiors, and the fate of the latter during mantle melting. We however show that the pressure at which degassing occurs, that is the atmospheric pressure in most sensible cases, has a prime influence on the composition of subaerial volcanic gases on planets: high surface pressure produces N 2 - and CO 2 -rich and dry volcanic gases, while low pressure promotes sulfur-rich gases. In-between, atmospheric pressures close to 1 bar trigger volcanic gases dominated by H 2 O. This simple pattern broadly mirrors the atmospheres of Venus–Earth–Mars–Io planetary suite and constitutes benchmarks for the prediction and interpretation of atmospheric features of extra-solar planets. Volatile abundances within the planetary body interiors also matter but they play a secondary role. Furthermore, our analysis shows that any difference in redox conditions prevailing during partial melting tends to disappear with the degassing process itself, converging toward a unique – planetary oxygen fugacity – at the venting pressure. A feedback relationship between volcanic gas compositions and atmospheric pressure implies a runaway drying during atmospheric growth; that is volcanic gases must become CO 2 richer as the atmospheric mass increases. This may explain some features of the Venusian atmosphere. On Earth, impact ejection of the atmosphere and CO 2 -sink mechanisms, such as carbonate precipitation and plate tectonics, must have decreased atmospheric pressure allowing the reestablishment of water-rich volcanic gases. [ABSTRACT FROM AUTHOR]

Sulfate-dominated sedimentary deposits are widespread on the surface of Mars, which contrasts with the rarity of carbonate deposits, and indicates surface waters with chemical features drastically different from those on Earth. While the Earth's surface chemistry and climate are intimately tied to the carbon cycle, it is the sulfur cycle that most strongly influences the Martian geosystems. The presence of sulfate minerals observed from orbit and in-situ via surface exploration within sedimentary rocks and unconsolidated regolith traces a history of post-Noachian aqueous processes mediated by sulfur. These materials likely formed in water-limited aqueous conditions compared to environments indicated by clay minerals and localized carbonates that formed in surface and subsurface settings on early Mars. Constraining the timing of sulfur delivery to the Martian exosphere, as well as volcanogenic HO is therefore central, as it combines with volcanogenic sulfur to produce acidic fluids and ice. Here, we reassess and review the Martian geochemical reservoirs of sulfur from the innermost core, to the mantle, crust, and surficial sediments. The recognized occurrences and the mineralogical features of sedimentary sulfate deposits are synthesized and summarized. Existing models of formation of sedimentary sulfate are discussed and related to weathering processes and chemical conditions of surface waters. We also review existing models of sulfur content in the Martian mantle and analyze how volcanic activities may have transferred igneous sulfur into the exosphere and evaluate the mass transfers and speciation relationships between volcanic sulfur and sedimentary sulfates. The sedimentary clay-sulfate succession can be reconciled with a continuous volcanic eruption rate throughout the Noachian-Hesperian, but a process occurring around the mid-Noachian must have profoundly changed the composition of volcanic degassing. A hypothetical increase in the oxidation state or in water content of Martian lavas or a decrease in atmospheric pressure is necessary to account for such a change in composition of volcanic gases. This would allow the pre mid-Noachian volcanic gases to be dominated by water and carbon-species but late Noachian and Hesperian volcanic gases to be sulfur-rich and characterized by high SO content. Interruption of early dynamo and impact ejection of the atmosphere may have decreased the atmospheric pressure during the early Noachian whereas it remains unclear how the redox state or water content of lavas could have changed. Nevertheless, volcanic emission of SO rich gases since the late Noachian can explain many features of Martian sulfate-rich regolith, including the mass of sulfate and the particular chemical features (i.e. acidity) of surface waters accompanying these deposits. How SO impacted on Mars's climate, with possible short time scale global warming and long time scale cooling effects, remains controversial. However, the ancient wet and warm era on Mars seems incompatible with elevated atmospheric sulfur dioxide because conditions favorable to volcanic SO degassing were most likely not in place at this time. [ABSTRACT FROM AUTHOR]

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 and to changes in the compositions of volcanic gases, 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. 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 SO2/H2S?

Experiments were conducted to determine the water solubility of alkali basalts from Etna, Stromboli and Vesuvius volcanoes, Italy. The basaltic melts were equilibrated at 1,200°C with pure water, under oxidized conditions, and at pressures ranging from 163 to 3,842 bars. Our results show that at pressures above 1 kbar, alkali basalts dissolve more water than typical mid-ocean ridge basalts (MORB). Combination of our data with those from previous studies allows the following simple empirical model for the water solubility of basalts of varying alkalinity and fO to be derived: $$ {\text{H}}_{ 2} {\text{O}}\left( {{\text{wt}}\% } \right) = {\text{ H}}_{ 2} {\text{O}}_{\text{MORB}} \left( {{\text{wt}}\% } \right) + \left( {5.84 \times 10^{ - 5} *{\text{P}} - 2.29 \times 10^{ - 2} } \right) \times \left( {{\text{Na}}_{2} {\text{O}} + {\text{K}}_{2} {\text{O}}} \right)\left( {{\text{wt}}\% } \right) + 4.67 \times 10^{ - 2} \times \Updelta {\text{NNO}} - 2.29 \times 10^{ - 1} $$ where HO is the water solubility at the calculated P, using the model of Dixon et al. (). This equation reproduces the existing database on water solubilities in basaltic melts to within 5%. Interpretation of the speciation data in the context of the glass transition theory shows that water speciation in basalt melts is severely modified during quench. At magmatic temperatures, more than 90% of dissolved water forms hydroxyl groups at all water contents, whilst in natural or synthetic glasses, the amount of molecular water is much larger. A regular solution model with an explicit temperature dependence reproduces well-observed water species. Derivation of the partial molar volume of molecular water using standard thermodynamic considerations yields values close to previous findings if room temperature water species are used. When high temperature species proportions are used, a negative partial molar volume is obtained for molecular water. Calculation of the partial molar volume of total water using HO solubility data on basaltic melts at pressures above 1 kbar yields a value of 19 cm/mol in reasonable agreement with estimates obtained from density measurements. [ABSTRACT FROM AUTHOR]

Experiments were conducted to determine CO solubilities in alkali basalts from Vesuvius, Etna and Stromboli volcanoes. The basaltic melts were equilibrated with nearly pure CO at 1,200°C under oxidizing conditions and at pressures ranging from 269 to 2,060 bars. CO solubility was determined by FTIR measurements. The results show that alkalis have a strong effect on the CO solubility and confirm and refine the relationship between the compositional parameter Π devised by Dixon (Am Mineral 82:368-378, ) and the CO solubility. A general thermodynamic model for CO solubility in basaltic melts is defined for pressures up to 2 kbars. Based on the assumption that O and CO mix ideally, we have:Then, from the thermodynamic model, we obtain ln K = 0.893 Π − 15.247. The new CO solubility model yields saturation pressures lower by as much as 50% relative to some existing models when applied to volatile-rich alkali basalts. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Many basaltic flood provinces are characterized by the existence of voluminous amounts of silicic magmas, yet the role of the silicic component in sulphur emissions associated with trap activity remains poorly known. We have performed experiments and theoretical calculations to address this issue. The melt sulphur content and fluid/melt partitioning at saturation with either sulphide or sulphate or both have been experimentally determined in three peralkaline rhyolites, which are a major component of some flood provinces. Experiments were performed at 150 MPa, 800–900°C, fO2 in the range NNO – 2 to NNO + 3 and under water-rich conditions. The sulphur content is strongly dependent on the peralkalinity of the melt, in addition to fO2, and reaches 1000 ppm at NNO + 1 in the most strongly peralkaline composition at 800°C. At all values of fO2, peralkaline melts can carry 5–20 times more sulphur than their metaluminous equivalents. Mildly peralkaline compositions show little variation in fluid/melt sulphur partitioning with changing fO2 (DS ≈270). In the most peralkaline melt, DS rises sharply at fO2 > NNO + 1 to values of >500. The partition coefficient increases steadily for Sbulk between 1 and 6 wt % but remains about constant for Sbulk between 0·5 and 1 wt %. At bulk sulphur contents lower than 4 wt %, a temperature increase from 800 to 900°C decreases DS by ∼10%. These results, along with (1) thermodynamic calculations on the behaviour of sulphur during the crystallization of basalt and partial melting of the crust and (2) recent experimental constraints on sulphur solubility in metaluminous rhyolites, show that basalt fractionation can produce rhyolitic magmas having much more sulphur than rhyolites derived from crustal anatexis. In particular, hot and dry metaluminous silicic magmas produced by melting of dehydrated lower crust are virtually devoid of sulphur. In contrast, peralkaline rhyolites formed by crystal fractionation of alkali basalt can concentrate up to 90% of the original sulphur content of the parental magmas, especially when the basalt is CO2-rich. On this basis, we estimate the amounts of sulphur potentially released to the atmosphere by the silicic component of flood eruptive sequences. The peralkaline Ethiopian and Deccan rhyolites could have produced ∼1017 and ∼1018 g of S, respectively, which are comparable amounts to published estimates for the basaltic activity of each province. In contrast, despite similar erupted volumes, the metaluminous Paraná–Etendeka silicic eruptives could have injected only 4·6 × 1015 g of S in the atmosphere. Peralkaline flood sequences may thus have greater environmental effects than those of metaluminous affinity, in agreement with evidence available from mass extinctions and oceanic anoxic events. [ABSTRACT FROM AUTHOR]

The near-liquidus crystallization of a high-K basalt (PST-9 golden pumice, 49·4 wt % SiO2, 1·85 wt % K2O, 7·96 wt % MgO) from the present-day activity of Stromboli (Aeolian Islands, Italy) has been experimentally investigated between 1050 and 1175°C, at pressures from 50 to 400 MPa, for melt H2O concentrations between 1·2 and 5·5 wt % and ΔNNO ranging from −0·07 to +2·32. A drop-quench device was systematically used. AuPd alloys were used as containers in most cases, resulting in an average Fe loss of 13% for the 34 charges studied. Major crystallizing phases include clinopyroxene, olivine and plagioclase. Fe–Ti oxide was encountered in a few charges. Clinopyroxene is the liquidus phase at 400 MPa down to at least 200 MPa, followed by olivine and plagioclase. The compositions of all major phases and glass vary systematically with the proportion of crystals. Ca in clinopyroxene sensitively depends on the H2O concentration of the coexisting melt, and clinopyroxene Mg-number shows a weak negative correlation with ΔNNO. The experimental data allow the liquidus surface of PST-9 to be defined. When used in combination with melt inclusion data, a consistent set of pre-eruptive pressures (100–270 MPa), temperatures (1140–1160°C) and melt H2O concentrations is obtained. Near-liquidus phase equilibria and clinopyroxene Ca contents require melt H2O concentrations <2·7–3·6 and 3 ± 1 wt %, respectively, overlapping with the maximum frequency of glass inclusion data (2·5–2·7 wt % H2O). For olivine to crystallize close to the liquidus, pressures close to 200 MPa are needed. Redox conditions around ΔNNO = +0·5 are inferred from clinopyroxene compositions. The determined pre-eruptive parameters refer to the storage region of golden pumice melts, which is located at a depth of around 7·5 km, within the metamorphic arc crust. Golden pumice melts ascending from their storage zone along an adiabat will not experience crystallization on their way to the surface. [ABSTRACT FROM AUTHOR]

Shows that the melting of basalt under both water-added and low-temperature conditions can yield extremely alkali-rich silicic liquids. Properties of the liquids; Similarity to glasses preserved in mantle xenoliths found in subduction zones and to veins in exhumed metamorphic terranes of fossil convergent zones; Interaction with mantle olivine, which produces modal mineralogies identical to those observed in metasomatized Alpine-type peridotites; Suggestion that mantle metasomatism by slab-derived melt is more common than believed.

Saal et al. reply - Scaillet and Pichavant raise an important point about the role that fO2 plays in determining C-H-O fluid speciation and in estimating the degree of vapour saturation in oceanic basalts. However, this does not seem to be relevant to the volatile geochemistry of mid-ocean-ridge basalt (MORB) magmas in general and of Siqueiros MORB in particular. [ABSTRACT FROM AUTHOR]