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The extent to which sulfur dissolves in silicate melts saturated in an immiscible sulfide phase is a fundamental question in igneous petrology and plays a primary role in the generation of magmatic ore deposits, volcanic degassing, and planetary differentiation. In igneous systems, sulfide melts can be described as FeS-NiS-CuS0.5 solutions with Fe/(Fe+Ni+Cu) significantly less than 1. Despite the presence of Ni and Cu in the sulfide, however, most experimental studies to date have concentrated on the effects of silicate melt composition on sulfur solubility and have used essentially pure FeS as the sulfide liquid. We have carried out 49 new experiments at pressures of 1.5-24 GPa and temperatures of 1400 to 2160 °C to investigate the effects of sulfide composition on sulfur solubility as well as extending the pressure and temperature ranges of the available data on sulfide saturation. We find that in the compositional range of most igneous sulfide melts [Fe/(Fe+Ni+Cu) > 0.6] sulfur solubility decreases linearly with Fe content such that at Fe/(Fe+Ni+Cu) of 0.6 the sulfur content at saturation is 0.6 times the value at pure FeS saturation. At lower values of Fe/(Fe+Ni+Cu), however, deviations from this ideal solution relationship need to be taken into consideration. We have treated these non-idealities by assuming that FeS-NiS-CuS0.5 liquids approximate ternary regular solutions.We have fitted our data, together with data from the literature (392 in total), to equations incorporating the effects of silicate melt composition, sulfide liquid composition, and pressure on the solubility of sulfur at sulfide saturation ([S]SCSS). The temperature dependence of [S]SCSS was assumed either to be an unknown or was taken from 1 bar thermodynamic data. The most important best-fit silicate melt compositional term reflects the strongly positive dependence of [S]SCSS on the FeO content of the silicate melt. The best-fit value of this parameter is essentially independent of our assumptions about temperature dependence of [S]SCSS or the solution properties of the sulfide. All natural compositions considered here exhibit positive dependences of [S]SCSS on temperature and negative dependences on pressure, in accord with previous studies using smaller data sets.

The thermodynamic and transport properties of the aluminosilicate melts at the heart of most magmatic processes vary in complex ways with composition, temperature, and pressure. Insights into these properties can come from information on the structure of the melts themselves, and more commonly from their glassy, quenched equivalents. Although most such connections remain qualitative or semi-quantitative, they are fundamentally important in interpretation of observations on igneous systems in nature and the laboratory, and in the formulation of physically accurate models. This review presents some of the important concepts of aluminosilicate glass and melt structure and dynamics that are most relevant to furthering our understanding of the igneous processes so central to how our planet has formed and evolved. The relationships among glasses, melts, and crystals are introduced. The structural underpinnings of temperature and pressure effects on melt free energies, densities, and viscosities, constraints on the extent of order/disorder among cations and anions in melts, why silica activity varies so strongly with composition, and how liquid-liquid phase separation can be understood, are discussed. Some simple, but useful, general views are presented on melt disorder and the shapes of liquidus surfaces (key to magmatic phase equilibria), as are links between atomic-scale dynamics and viscous flow and diffusion.

The partition coefficients of V and Sc between clinopyroxene and silicate melt (DVCpx/SM and DScCpx/SM) have been determined experimentally at 1200-1400 °C and 0.8-2.3 GPa, using a hornblende- and clinopyroxene-rich mantle rock in graphite-lined Pt95Rh05 capsules. The results show that the DVCpx/SM and DScCpx/SM values decrease from 3.8 to 2.3 and from 2.6 to 1.1, respectively, as the experimental temperature and pressure vary from 1200 °C and 0.8 GPa to 1400 °C and 2.3 GPa. The presence of water in silicate melts may also reduce DVCpx/SM and DScCpx/SM. These results imply that the effects of temperature, pressure, and melt water content on DVCpx/SM should be considered when using V systematics in cratonic mantle peridotites to constrain cratonic mantle oxygen fugacity (fO2). However, although the dominant V in the present silicate melt is mixed V3+ and V4+, the DVCpx/SM/DScCpx/SM together with literature data obtained at similar fO2 shows a nearly constant value of 1.68 ± 0.26, regardless of temperature, pressure, melt composition, and melt water content, indicating that these factors cannot cause fractionation of Sc3+ from mixed V3+ and V4+ in mantle melts through clinopyroxene/silicate melt partitioning. Therefore, in combination with V/Sc systematics in primitive MORBs and arc basalts, using DVCpx/SM and DScCpx/SM obtained at 1 bar and dry conditions should be valid to constrain mantle fO2, except for the case that the DCpx/SM for Sc3+ can be demonstrated to be fractionated from the DCpx/SM for mixed V4+ and V5+, which are present in oxidized basalts.

Chemical diffusion of F and Cl has been experimentally determined in a rhyodacitic melt obtained from remelting a sample of Hekla pumice (Iceland). Diffusion couple experiments were conducted in a vertical tube furnace over a temperature range of 750-950°C and in air for durations of 1 to 35 days. Concentration profiles of F and Cl were obtained for the quenched samples using an electron microprobe. Fluorine and chlorine exhibit Arrhenian behavior over the range of temperature investigated here. The pre-exponential factors of F and Cl are D0(F)=4.3×10-4 and D0(Cl)=1.6×10-5 m2/s. Fluorine diffusion coefficients vary in the order of 1×0-15 to 1×10-13 m2/s, whereas Cl diffusivity is up to two orders of magnitude slower. The activation energies for F and Cl diffusivities are equal within error at 223±31 and 229±52 kJ/mol, respectively. The difference in diffusivity between F and Cl is particularly pronounced in the melt of our study, compared to results obtained for other magmatic melt compositions. This means that the potential for diffusive fractionation exists and may occur especially under conditions of magma ascent and bubble growth, as this would favor partitioning of the relatively fast-diffusing halogens into growing bubbles, due to H2O exsolution. A dependence of diffusivity on atomic radius observed here is enhanced over that observed in more basic, less viscous melts, indicating that diffusive fractionation is more likely to be pronounced in more silicic, more viscous systems. A proper parameterization and modeling of diffusive fractionation of halogens in actively degassing volcanic systems thus holds the potential of serving as a tool for quantifying the processes responsible for volcanic unrest.

Carbonatites are rare igneous carbonate-rich rocks. Most carbonatites contain a large number of accessory oxide, sulfide, and silicate minerals. Baddeleyite (ZrO2) and zircon (ZrSiO4) are common accessory minerals in carbonatites and because these minerals host high concentrations of U and Th, they are often used to determine the ages of formation of the carbonatite. In an experimental study, we constrain the stability fields of baddeleyite and zircon in Ca-rich carbonate melts with different silica concentrations. Our results show that SiO2-free and low silica carbonate melts crystallize baddeleyite, whereas zircon only crystallizes in melts with higher concentration of SiO2 We also find that the zirconsilicate baghdadite (Ca3ZrSi2O9) crystallizes in intermediate compositions. Our experiments indicate that zircon may not be a primary mineral in a low-silica carbonatite melt and care must be taken when interpreting zircon ages from low-silica carbonatite rocks.

A thermodynamic model for estimating the saturation conditions of H 2 O–CO 2 mixed fluids in multicomponent silicate liquids is described. The model extends the capabilities of rhyolite-MELTS (Gualda et al. in J Petrol 53:875–890, 2012a ) and augments the water saturation model in MELTS (Ghiorso and Sack in Contrib Mineral Petrol 119:197–212, 1995 ). The model is internally consistent with the fluid-phase thermodynamic model of Duan and Zhang (Geochim Cosmochim Acta 70:2311–2324, 2006 ). It may be used independently of rhyolite-MELTS to estimate intensive variables and fluid saturation conditions from glass inclusions trapped in phenocrysts. The model is calibrated from published experimental data on water and carbon dioxide solubility, and mixed fluid saturation in silicate liquids. The model is constructed on the assumption that water dissolves to form a hydroxyl melt species, and that carbon dioxide both a molecular species and a carbonate ion, the latter complexed with calcium. Excess enthalpy interaction terms in part compensate for these simplistic assumptions regarding speciation. The model is restricted to natural composition liquids over the pressure range 0–3 GPa. One characteristic of the model is that fluid saturation isobars at pressures greater than ~100 MPa always display a maximum in melt CO 2 at nonzero H 2 O melt concentrations, regardless of bulk composition. This feature is universal and can be attributed to the dominance of hydroxyl speciation at low water concentrations. The model is applied to four examples. The first involves estimation of pressures from H 2 O–CO 2 -bearing glass inclusions found in quartz phenocrysts of the Bishop Tuff. The second illustrates H 2 O and CO 2 partitioning between melt and fluid during fluid-saturated equilibrium and fractional crystallization of MORB. The third example demonstrates that the position of the quartz–feldspar cotectic surface is insensitive to melt CO 2 contents, which facilitates geobarometry using phase equilibria. The final example shows the effect of H 2 O and CO 2 on the crystallization paths of a high-silica rhyolite composition representative of the late-erupted Bishop Tuff. Software that implements the model is available at ofm-research.org, and the model is incorporated into the latest version (1.1+) of rhyolite-MELTS.

Reactions involving carbon in the deep Earth have limited manifestations on Earth's surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth's accretion and may have sequestered substantial carbon in Earth's core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth's inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The 10-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and has helped to identify gaps in our understanding that motivate and give direction to future studies.

Magma reservoirs play a key role in controlling numerous processes in planetary evolution, including igneous differentiation and degassing, crustal construction, and volcanism. For decades, scientists have tried to understand what happens in these reservoirs, using an array of techniques such as field mapping/petrology/geochemistry/geochronology on plutonic and volcanic lithologies, geophysical imaging of active magmatic provinces, and numerical/experimental modeling. This review paper tries to follow this multi-disciplinary framework while discussing past and present ideas. We specifically focus on recent claims that magma columns within the Earth's crust are mostly kept at high crystallinity (\"mush zones\"), and that the dynamics within those mush columns, albeit modulated by external factors (e.g., regional stress field, rheology of the crust, pre-existing tectonic structure), play an important role in controlling how magmas evolve, degas, and ultimately erupt. More specifically, we consider how the chemical and dynamical evolution of magma in dominantly mushy reservoirs provides a framework to understand: (1) the origin of petrological gradients within deposits from large volcanic eruptions (\"ignimbrites\"); (2) the link between volcanic and plutonic lithologies; (3) chemical fractionation of magmas within the upper layers of our planet, including compositional gaps noticed a century ago in volcanic series (4) volatile migration and storage within mush columns; and (5) the occurrence of petrological cycles associated with caldera-forming events in long-lived magmatic provinces. The recent advances in understanding the inner workings of silicic magmatism are paving the way to exciting future discoveries, which, we argue, will come from interdisciplinary studies involving more quantitative approaches to study the crust-reservoir thermo-mechanical coupling as well as the kinetics that govern these open systems.

A new model describing zircon saturation in silicate melts is presented that combines the results of 196 data from new experiments with data from previous experimental studies. In the new experiments, the concentration of Zr in melts coexisting with zircon was determined at temperatures between 800 and 1500 °C for 21 compositions (with alumina saturation index, ASI, from 0.20 to 1.15), containing ~ 1 to 16 wt % FeO T and, for a subset of these conditions, at variable pressure (0.0001 to 4.0 GPa) and water content (0 to 15 wt %). The collated dataset contains 626 data, with 430 from 26 literature studies, and covers conditions from 750 to 1620 °C, (including 45 new data and 106 literature data for temperatures < 1000 °C), ASI 0.20 to 2.00, 0.0001 to 4.0 GPa and 0 to 17 wt % H 2 O. A limitation of previous models of zircon saturation is the choice of parameter used to describe the silicate melt, which may not be appropriate for all compositions and can result in differences in predicted temperatures of over 200 °C for granitic systems. Here we use optical basicity ( Λ ), which can be easily calculated from the major oxide components of a melt, to parameterise the composition. Using a non-linear least-squares multiple regression, the new zircon saturation model is: log Zr = 0.96 ( 5 ) - 5790 ( 95 ) / T - 1.28 ( 8 ) P + 12.39 ( 35 ) Λ + 0.83 ( 9 ) x . H 2 O + 2.06 ( 16 ) P Λ where Zr is in ppm, T is temperature in K, P is pressure in GPa, Λ is the optical basicity of the melt, x .H 2 O is the mole fraction of water in the melt, and the errors are 1σ. This model confirms that temperature and melt composition are the dominant controls on zircon solubility. In addition, pressure and melt water content exert small but resolvable effects on the solubility and are included, for the first time, in a model. Using this new calibration, 92% of the predicted temperatures are within 10% of the experimental temperatures for the collated dataset (with an average temperature difference of 57 °C), while predicted temperatures for only 78 and 62% of the collated dataset are within 10% of the experimental temperature (with average temperature differences > 80 °C) using the widely cited Watson and Harrison (Earth Planet Sci Lett 64:295–304, 1983) and Boehnke et al. (Chem Geol 351:324–334, 2013) models, respectively. This new model can be extrapolated to temperatures below those included in the calibration with greater accuracy and when applied to melt inclusions from the Bishop Tuff, gives temperatures that are in excellent agreement with independent estimates.

Carbon‐bearing dense silicate melts may account for the seismically weak layers on top of the mantle transition zone (MTZ) at 13–14 GPa. How carbon is incorporated into silicate melts at such high pressures has not been experimentally probed, making the nature of carbon in dense melts elusive. Here, our nuclear magnetic resonance experiments reveal how carbon and pressure influence the structure of mantle melts. The pressure‐driven changes in the Si coordination environments and network connectivity in dense melts are accompanied by a dramatic increase in the fraction of structurally bound carbonates above 8 GPa up to 14 GPa, further polymerizing melt networks. The pressure‐driven melt polymerization and increase in diversity in carbon configurations reveal the mechanisms behind the pressure‐induced propensity to store carbon in silicate melts under compression. The substantial carbon incorporation in silicate melts toward the MTZ may potentially filter out the carbon from subducting slabs, controlling the deep carbon cycle. Plain Language Summary The dissolution of carbon into mantle rocks promotes the formation of partial melts that may comprise the thin low‐velocity layer on top of the MTZ at a depth of ∼380–410 km (13–14 GPa). While the investigation of the carbon speciation in the dense melts is essential for elucidating the nature of the melts and their impact on deep carbon cycles, probing of the structures of carbon‐bearing mantle melts toward the MTZ has been experimentally challenging. Here, we explored the carbon, silicon and oxygen environments in the dense melts near MTZ, revealing that the structurally bound carbonates species in the silicate melts are dominant toward the top of MTZ. Diverse carbon species in the melts under pressure play distinct structural roles, accounting for the pressure‐driven variations in melt properties. The formation of carbon‐bearing silicate melts highlights the prominent carbon storing capacity of silicate melts on the lowest upper‐mantle. Our findings indicate that carbon‐bearing mantle melts formed below a depth of ∼410 km may further filter out carbon from subducting slabs and may inhibit the efficient distribution of carbon into lower mantle, thus controlling deep carbon cycle. Key Points The speciation of carbon in mantle melts under compression toward the mantle transition zone (MTZ) was experimentally revealed The structurally bound carbonate species could prevail in dense silicate melts on top of the MTZ The carbonated silicate melts have potential to store carbon on the lowest upper‐mantle, inhibiting its infiltration to lower mantle