Abstract
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 2000–4000 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 almost 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.
Highlights
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We evaluate the partial molar volume of water in silicate melt ( V H2O ) from the direct volume difference between hydrous melt (Vhymelt) and dry melt (Vmelt): V H2O(P, T) = Vhymelt(P, T) − Vmelt(P, T) NA/n where n is the number of H2O units in the supercell and NA is the Avogadro’s number
Silicate melts contain some water. Whether such hydrous melt sinks or rises or stagnates at a depth controls the amount of water that may be entertained in the interior and its subsequent contributions to global water cycle at all times of Earth’s history
Summary
First-principles molecuar dynamics simulations of several silicate melt + water systems were performed using the generalized gradients approximation (PBE exchange–correlation functional)[55] and project augmented wave method[56] as implemented in VASP57. All simulations were based on canonical NVT ensembles where the number of atoms N, volume V, and temperature T are fixed. The supercells consisted of 32MgSiO3 + 16H2O (N = 208), 16Mg2SiO4 + 8H2O (N = 136), and 16Mg2SiO4 + 16H2O (N = 160) for hydrous silicate melts containing 8.2, 6.0, and 11.4 wt% water (equivalently, 20.0, 14.3 and 25.0 mol% water), respectively. The corresponding water-free supercells involved 32MgSiO3 and 1 6Mg2SiO4. To simulate iron-bearing melts, Mg atoms were substituted with Fe atoms for x = Fe/(Mg + Fe) = 0.25. A plane wave cutoff of 400 eV and Gamma Point Brillouin zone sampling were used.
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