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1. Behavior and properties of water in silicate melts under deep mantle conditions

   https://doi.org/https://doi.org/10.1038/s41598-021-90124-7  

   Karki, Bijaya B ; Ghosh, Dipta B ; Karato, Shun-ichiro ( May 2021 , Scientific reports)

   null (Ed.)

   Water (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we investigate the equation of state, speciation, and transport properties of water dissolved in Mg1-xFexSiO3 and Mg2(1-x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000 to 4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt+water solution is negative at low pressures and becomes zero above 15 GPa. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as -O-H-O-, -O-H-O-H- and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations. 

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2. Effects of H2O–CO2 Fluids, Temperature, and Peridotite Fertility on Partial Melting in Mantle Wedges and Generation of Primary Arc Basalts

   https://doi.org/10.1093/petrology/egad047  

   Lara, Michael ; Dasgupta, Rajdeep ( July 2023 , Journal of Petrology)

   Many lines of evidence from high P–T experiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon. However, constraints on the effects of H2O–CO2 fluids on mantle melting, particularly at mantle wedge P–T conditions, are limited. Here, we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2 [= molar CO2 / (CO2 + H2O)] of 0.04–0.17. Experiments were performed at 2–3 GPa and 1350°C to assess how temperature, peridotite fertility, and XCO2 of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene +7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO*, and MgO and poorer CaO, Al2O3, and alkalis when influenced by H2O–CO2 fluids. At constant P–T and bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2 of slab-fluid. High XCO2 depleted compositions generate ~7 wt.% melt, whereas, at identical P–T, low XCO2 fertile compositions generate ~30 to 40 wt.% melt. Additionally, peridotite fertility and XCO2 have significant effects on peridotite partial melt compositions. At a constant P–T–XCO2, fertile peridotites generate melts richer in CaO and Al2O3 and poorer in SiO2, MgO + FeO, and alkalis. Similar to previous experimental studies, at a constant P–T fertility condition, as XCO2 increases, SiO2 and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2 on melt SiO2 and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2 studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across most P–T–fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2 ≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2 = 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2 limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2 < 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3 signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites with 0 < XCO2 < 0.11 at 2–3 GPa, or from nominally CO2-free hydrous fertile peridotites at P > 3 GPa.

    

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3. Mixed incorporation of carbon and hydrogen in silicate melts under varying pressure and redox conditions

   https://doi.org/10.1016/j.epsl.2020.116520  

   Karki, Bijaya B ; Ghosh, Dipta B ; Banjara, Dipendra ( November 2020 , Earth and planetary science letters)

   null (Ed.)

   Volatiles including carbon and hydrogen are generally considered to be more soluble in silicate melts than in mantle rocks. How these melts contribute to the storage and distribution of key volatiles in Earth's interior today and during its early evolution, however, remains largely unknown. It is essential to improve our knowledge about volatiles-bearing silicate magmas over the entire mantle pressure regime. Here we investigate molten Mg FexSiO3 ( , 0.25) containing both carbon and hydrogen using first-principles molecular dynamics simulations. Our results show that the dissolution mechanism of the binary volatiles in melts varies considerably under different conditions of pressure and redox. When incorporated as CO2 and H2O components (corresponding to oxidizing conditions) almost all carbon and hydrogen form bonds with oxygen. Their speciation at low pressure consists of predominantly isolated molecular CO2, carbonates, and hydroxyls. More oxygenated species, including tetrahedrally coordinated carbons, hydrogen (O-H-O) bridges, various oxygen-joined complexes appear as melt is further compressed. When two volatiles are incorporated as hydrocarbons CH4 and C2H6 (corresponding to reducing conditions), hydroxyls are prevalent with notable presence of molecular hydrogen. Carbon-oxygen bonding is almost completely suppressed. Instead carbon is directly correlated with itself, hydrogen, and silicon. Both volatiles also show strong affinity to iron. Reduced volatile speciation thus involves polymerized complexes comprising of carbon, hydrogen, silicon, and iron, which can be mostly represented by two forms: C1−4H1−5Si0−5O0−2 (iron-free) and C5−8H1−8Si0−6Fe5−8O0−2. The calculated partial molar volumes of binary volatiles in their oxidized and reduced incorporation decrease rapidly initially with pressure and then gradually at higher pressures, thereby systematically lowering silicate melt density. Our assessment of the calculated opposite effects of the volatile components and iron on melt density indicates that melt-crystal density crossovers are possible in the present-day mantle and also could have occurred in early magma ocean environments. Melts at upper mantle and transition zone conditions likely dissolve carbon and hydrogen in a wide variety of oxidized and non-oxygenated forms. Deep-seated partial melts and magma ocean remnants at lower mantle conditions may exsolve carbon as complex reduced species possibly to the core during core-mantle differentiation while retaining a majority of hydrogen as hydroxyls-associated species. 

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4. Melting of Bridgmanite Under Hydrous Shallow Lower Mantle Conditions

   https://doi.org/10.1029/2021JB022222  

   Amulele, George ; Karato, Shun‐ichiro ; Girard, Jennifer ( September 2021 , Journal of Geophysical Research: Solid Earth)

   High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24–25 GPa and 1673–1873 K with 5–10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO₃, (Mg, Fe)SiO₃, (Mg, Al, Si)O₃, and (Mg, Fe, Al, Si)O₃. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO₂in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO₂ratio than the initial material. (Mg + Fe)O‐rich hydrous melt could explain the low‐velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid‐lower mantle and may explain the “stishovite paradox. Since stishovite‐rich materials are formed only when silica‐rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates.

    

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5. Hydration-driven stabilization and volume collapse of grain boundaries in Mg2SiO4 forsterite predicted by first-principles simulations

   https://doi.org/10.2138/am-2021-7732  

   Ghosh, Dipta B. ; Karki, Bijaya B. ; Wang, Jianwei ( April 2022 , American Mineralogist)

   Abstract Grain boundaries in mantle minerals are of critical importance to geophysical and geochemical processes of the Earth’s interior. One of the fundamental issues is to understand how the water (H2O) component influences the properties of grain boundaries in silicate materials. Here, we report the results of the structure and stability of several tilt grain boundaries in Mg2SiO4 forsterite over the pressure range 0 to 15 GPa using density functional theory-based first-principles simulations. The results suggest greater energetic stability and hydration-driven volume collapse (negative excess volume) at zero pressure for the majority of hydrous grain boundaries relative to the anhydrous (dry) ones. All the hydrous grain boundaries become increasingly favorable at elevated pressures as the calculated hydration enthalpy systematically decreases with increasing pressure. The hydrous components at the interfacial regions are predominantly in the hydroxyl form and, to a lesser extent, in the molecular H2O form. Their calculated ratio ranges from 1.6 to 8.7 among the different grain boundary configurations. Our structural analysis also reveals that the hydroxyls are bound to either both Mg and Si or to Mg only. In comparison, the molecular species are bound only to Mg sites. Besides direct oxygen-hydrogen bonding, intermolecular hydrogen bonding becomes important with compression. On the basis of our results, we suggest that local atomic rearrangements caused by dissociative adsorption of water facilitate efficient compaction of the boundary interfaces, which, in turn, results in greater relative stability of hydrous grain boundaries. This means that water prefers to be incorporated within the grain boundaries over the bulk of silicate materials. 

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