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Abstract: We report lithium (Li) isotopic measurements in seawater-derived waters that were discharged from geothermal wells, thermal springs, and sub-marine 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. [Copyright &y& Elsevier]

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. • The oxidation state of volcanic gases is correlated with emission temperature. • It is independent of geodynamic setting and melt composition. • Closed-system re-equilibration of volcanic gases dictates their oxidation state. • Dichotomy between melt and volcanic gases f O 2 in the early earth is reconciled. • Shift in volcanic gases oxidation state must have occurred near the GOE time. [ABSTRACT FROM AUTHOR]

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 (H 2 O–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 (D Br f/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 D Br f/m increases with increasing SiO 2 content of the melt (as for chlorine) and suggest that it is also sensitive to melt temperature (increase of D Br f/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. [ABSTRACT FROM AUTHOR]

Modeling magmatic degassing, or how the volatile distribution between gas and melt changes at pressure varies, is a complex task that involves a large number of thermodynamical relationships and that requires dedicated software. This article presents the software D-Compress, which computes the gas and melt volatile composition of five element sets in magmatic systems (O–H, S–O–H, C–S–O–H, C–S–O–H–Fe, and C–O–H). It has been calibrated so as to simulate the volatiles coexisting with three common types of silicate melts (basalt, phonolite, and rhyolite). Operational temperatures depend on melt composition and range from 790 to 1400 °C. A specificity of D-Compress is the calculation of volatile composition as pressure varies along a (de)compression path between atmospheric and 3000 bars. This software was prepared so as to maximize versatility by proposing different sets of input parameters. In particular, whenever new solubility laws on specific melt compositions are available, the model parameters can be easily tuned to run the code on that composition. Parameter gaps were minimized by including sets of chemical species for which calibration data were available over a wide range of pressure, temperature, and melt composition. A brief description of the model rationale is followed by the presentation of the software capabilities. Examples of use are then presented with outputs comparisons between D-Compress and other currently available thermodynamical models. The compiled software and the source code are available as electronic supplementary materials. [ABSTRACT FROM AUTHOR]

In the Rio Maria and Sapucaia Domains of the Carajás Province, during the Mesoarchean, tonalite-trondhjemite-granodiorite (TTG) series (2.98–2.92 Ga; Colorado Trondhjemite) and sanukitoid rocks (∼2.87 Ga; Rio Maria Suite of Ourilândia do Norte) were formed. They were followed, in the Neoarchean (2.75–2.73 Ga), by numerous stocks of granitoids similar to A-type granites (Vila Jussara Suite). Despite the compositional differences between these granitoids, epidote is a mineral phase common to all of them, with pistacite contents of 26–29 mol% in TTGs, 22–33 mol% in sanukitoids and 25–30 mol% in Neoarchean granitoids. The study of the dissolution kinetics of Archean epidotes of the Carajás Province reveals that the partial dissolution time of their crystals was ∼4–10 years, with corresponding magma ascent rates of 4–8 km/year. Magma viscosities at liquidus temperature were estimated at 105.3 Pa s for TTG magma and 102.5 Pa s for sanukitoid magma, whereas monzogranitic magmas of the Vila Jussara Suite exhibited a viscosity of 104.4 Pa s. In contrast, the viscosity of tonalitic magma of the Vila Jussara Suite was 103.5 Pa s. The preservation of magmatic epidote in Archean granitoids requires that the plutons grew in an incremental way, similar to their Phanerozoic counterparts, with the stacking of small sill-dykes of about 100 m thickness each intruded in a rather cold crust, allowing fast cooling rates so as to prevent complete dissolution of epidote at the final emplacement level. • Epidote is relatively common in the Archean granitoids of the Carajás Province. • The epidote crystal dissolution time suggests that Archean magmas were emplaced through a mechanism involving incremental growth. • Epidote dissolution kinetics and viscosity estimates, help to understand the magma rise, emplacement, and crystallization processes of the Archean plutons. [ABSTRACT FROM AUTHOR]

The critical role of iron on crystal-silicate liquid relationships and melt differentiation is mainly controlled by the redox conditions prevailing in magmas, but the presently available database merely constrains the thermodynamic properties of iron-bearing components in strongly reduced and anhydrous molten silicate where iron is in the ferrous form. This paper provides new standard states for pure ferrous (FeOliq) and ferric (Fe2O3liq) molten iron oxides and extends the experimental database towards oxidizing and water-bearing domains. Iron-iridium, iron-platinum alloys, magnetite or hematite were equilibrated with synthetic silicic liquids at high temperature and high pressure under controlled oxygen fugacity (fO2) to determine activity-composition relationships for FeOliq and Fe2O3liq. Between 1000 and 1300°C, the fO2 ranges from that in air to 3-log units below that of the nickel-nickel oxide buffer (NNO). Experiments were performed on both anhydrous and hydrous melts containing up to 6-wt.% water. Incorporation of water under reducing conditions increases the activity coefficient of FeOliq but has an opposite effect on Fe2O3liq. As calcium is added to system, the effect of water becomes weaker and is inverted for Fe2O3liq. Under oxidizing conditions, water has a negligible effect on both activities of FeOliq and Fe2O3liq. In contrast, changes in redox conditions dominate the activity coefficients of both FeOliq and Fe2O3liq, which increase significantly with increasing fO2. The present results combined with the previous work provide a specific database on the energetics of iron in silicate melts that cover most of the condition prevailing in natural magmas. [Copyright &y& Elsevier]

Despite CO 2 is the second most abundant volatile species implied in magmatic systems, its impact on the molecular structure of aluminosilicate glasses in complex systems is currently not well-constrained. Inasmuch, whether CO 2 induces an increase in glass polymerization or not is yet not clear for complex glass compositions. Using the set of nephelinite glass samples investigated in Part I (Morizet et al., 2014), in which the CO 2 solubility and speciation have been constrained as a function of the #K (molar K 2 O/K 2 O + Na 2 O), we have conducted a thorough investigation of the change in the silicate network structure associated to CO 2 dissolution. The change in silicate structure has been studied as a function of volatiles solubility (CO 2 up to 4.5 wt.%, H 2 O up to 3 wt.%) as well as a function of the #K (between 0 and 0.75) of the glasses. We used 29 Si Solid-State NMR for probing the silicon local environment in the quenched glasses. We observe that in such depolymerized nephelinite compositions the exchange between Na and K in volatile-free glasses induces a change in the glass structure attributed to the Mixed Alkali Effect (MAE) invoked in previous works. The observed changes might be related to geometric changes in the glass structure (change in network species bond lengths or angles). The addition of H 2 O only produces a negligible change in the degree of polymerization being probed by the constant NBO/T (Non-Bridging Oxygen per Tetrahedron) as H 2 O is added to volatile-free glasses. In contrast, we observed that the addition of CO 2 induces a strong change in the glass structure which might be interpreted as an increase in polymerization. Alternatively, considering the identified CO 2 dissolution mechanism (mostly as non-bridging carbonates units), the glass structure changes might reflect an apparent glass polymerization through the creation of Si–O–C bonds rather a true polymerization through the creation of Si–O–Si bonds. [ABSTRACT FROM AUTHOR]

Silica undersaturated melts such as nephelinites are very peculiar magmatic materials. Their occurrence on Earth is often associated with carbonatite melts. These low-silica melts can dissolve a large quantity of CO 2 and are rich in alkalis. However, the way CO 2 dissolves into these melts and the effect of different alkali elements are poorly constrained. We present experimental results on the CO 2 solubility and speciation in synthetic nephelinite in the NKCMAS system, equilibrated at high-pressure (50–300 MPa), high-temperature (1250 °C) with an excess C–O–H fluid phase. The nephelinitic glasses with 20 mol.% total alkali oxides were synthesized with varying K 2 O/(K 2 O + Na 2 O) ratio (denoted as #K) in order to investigate the differential effect of those two alkali cations on CO 2 solubility and speciation. All experiments were conducted under oxidizing conditions (>NNO+3) resulting in binary fluid phase compositions with CO 2 and H 2 O species. The CO 2 content and speciation were investigated using micro-Raman and Solid State NMR spectroscopies for 13 C nucleus. We observe an increase in CO 2 content as a function of pressure, consistent with previous studies but CO 2 solubility is much higher than in alkali-poorer melts. The CO 2 content is above 1 wt.% at 50 MPa and increases up to 4.5 wt.% at 300 MPa. The progressive replacement of Na by K (#K between 0 and 1) induces an increase in CO 2 content. At 50 MPa, the CO 2 solubility is ∼1.75 wt.% in the K-free glass (#K = 0) and increases up to ∼3.0 wt.% CO 2 in Na-free glass (#K = 1). The change in CO 2 solubility as a function of #K is discussed in terms of carbonatite genesis. The 13 C NMR spectra show that carbonate (CO 3 2− ) environments can be attributed to carbonate species associated to non-bridging oxygen in agreement with the depolymerized nature of the investigated compositions. Two singular additional carbonated species were also identified with 13 C NMR signatures at 161 and 165 ppm. Those species are assigned to isolated K + …CO 3 2− …H + and Na + …CO 3 2− …H + carbonate species. The presence of such isolated carbonate species is interpreted as possible precursors to carbonatitic melt genesis. [ABSTRACT FROM AUTHOR]

• A magma ocean degassing code to calculate the composition of the outgassed atmosphere. • Oxygen fugacity, volatile contents and size of the magma ocean are variable. • Oxygen fugacity has a pivotal role of the composition and size of the atmosphere. • The masses of C and N outgassed by the magma are very close to the present-day surficial reservoirs. 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-O-N-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-O-N-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 behavior under extreme magma ocean conditions and upon magma ocean solidification are urgently needed. [ABSTRACT FROM AUTHOR]

Abstract: The development of an accurate analytical procedure for determination of dissolved water in complex alumino-silicate glasses via micro-Raman analysis requires the assessment of the spectra topology dependence on glass composition. We report here a detailed study of the respective influence of bulk composition, iron oxidation state and total water content on the absolute and relative intensities of the main Raman bands related to glass network vibrations (LF: ∼490cm−1; HF: ∼960cm−1) and total water stretching (H2OT: ∼3550cm−1) in natural glasses. The evolution of spectra topology was examined in (i) 33 anhydrous glasses produced by the re-melting of natural rock samples, which span a very large range of polymerisation degree (NBO/T from 0.00 to 1.16), (ii) 2 sets of synthetic anhydrous basaltic glasses with variable iron oxidation state (Fe3+/FeT from 0.05 to 0.87), and (iii) 6 sets of natural hydrous glasses ( from 0.4 to 7.0wt%) with NBO/T varying from 0.01 to 0.76. In the explored domain of water concentration, external calibration procedure based on the H2OT band height is matrix-independent but its accuracy relies on precise control of the focusing depth and beam energy on the sample. Matrix-dependence strongly affects the internal calibrations based on H2OT height scaled to that of LF or HF bands but its effect decreases from acid (low NBO/T, SM) to basic (high NBO/T, SM) glasses. Structural parameters such as NBO/T (non-bridging oxygen per tetrahedron) and SM (sum of structural modifiers) describe the matrix-dependence better than simple compositional parameters (e.g. SiO2, Na2O+K2O). Iron oxidation state has only a minor influence on band topology in basalts and is thus not expected to significantly affect the Raman determinations of water in mafic (e.g. low SiO2, iron-rich) glasses. Modelling the evolution of the relative band height with polymerisation degree allows us to propose a general equation to predict the dissolved water content in natural glasses:where is the total water content (in wt%) dissolved in glass; TOT N represents the computed I LF/I HF variation as a function of the calculated NBO/T and SM parameters; is the H2O band height scaled to ratio of the reference bands; k is the linearity spectrometer response on the H2OT band in function of water content. The water concentrations of the reference glasses are reproduced using this equation with a standard deviation of 0.06wt%. The adopted parameterisation provides a useful tool towards the characterisation of composition dependence of micro-Raman procedures for silicate glasses. We show, based on the widest range of glass compositions so far investigated, that accurate evaluation of dissolved water content is achieved by micro-Raman spectroscopy. [Copyright &y& Elsevier]

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. • Mantle plumes are two orders of magnitude more oxidised than MOR mantle. • Melt inclusions from five hotspots all show a reduction during S degassing. • Mantle plumes transport oxidized material out of the deep earth. [ABSTRACT FROM AUTHOR]