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

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.

Hydrous silicate melts and the deep mantle H2O cycle

“…A full description of the melt structure computed from the simulation trajectories is provided in the Supplementary Material. Consistent with previous studies (Mookherjee et al, 2008;Karki and Stixrude, 2010a;Karki et al, 2010;Bajgain et al, 2019;Yuan et al, 2020;Karki et al, 2020;Solomatova and Caracas, 2021;Karki et al, 2021), H 2 O dissolution proceeds via depolymerisation of the melt structure with little influence on the Si and Mg local coordination environment. Dissolved H 2 O leads to a substantial reduction in both melt density and viscosity, with the hydrous melts highly inviscid over all P -T space.…”

“…Green diamonds are the experimental bulk compositions of Sakamaki et al (2006) and the yellow diamond shows the extrapolated 0.7% melt fraction model melt of Freitas et al (2017). Blue, cyan and green squares show the melt compositions used in previous DFT calculations (Karki et al, 2021;Mookherjee et al, 2008;Yuan et al, 2020). The green field shows hydrous silicate melts at 13.5 GPa reported by Melekhova et al (2007), and the blue field shows melt compositions from hydrous peridotite at 11-24 GPa and 1100-1400 • C (Kawamoto, 2004;Kawamoto and Holloway, 1997) with H 2 O contents estimated from deficiency in microprobe totals.…”

“…H 2 O dissolution in silicate melts proceeds by depolymerisation of the melt structure via reaction with bridging oxygen atoms to form hydroxyl (OH − ) units resulting in a profound reduction in viscosity (Kohn, 2000). Various ab initio MD simulations have been reported for hydrous silicate melts under a wide range of P -T conditions (Mookherjee et al, 2008;Karki and Stixrude, 2010a;Karki et al, 2010;Bajgain et al, 2019;Yuan et al, 2020;Karki et al, 2020;Solomatova and Caracas, 2021;Karki et al, 2021). Despite the lower density of hydrous melts relative to anhydrous compositions, Mookherjee et al (2008) argued that a buoyantly stable melt at the upper boundary of the transition zone would contain approximately 3 wt% H 2 O.…”

“…Despite the lower density of hydrous melts relative to anhydrous compositions, Mookherjee et al (2008) argued that a buoyantly stable melt at the upper boundary of the transition zone would contain approximately 3 wt% H 2 O. Very recently, Karki et al (2020Karki et al ( , 2021 reported calculations of hydrous MgSiO 3 and Mg 2 SiO 4 melts with up to 25 mol% FeO and predicted that partial melts containing a few weight per cent of H 2 O may be gravitationally stable above and below the mantle transition zone. However, as noted by Yuan et al (2020), even with iron enrichment, the critical H 2 O content (∼4 wt%) required for neutral buoyancy of hydrous silicate melts relative to pyrolytic solid mantle is much lower than expected for small-degree mantle melts from experimental phase relations (section 1.1), with the consequence that natural hydrous melts cannot experience long-term gravitational stability above, below or within the transition zone.…”

Thermoelasticity of Water in Silicate Melts: Implications for Melt Buoyancy in Earth's Mantle

“…The melts in the FPMD simulations are typically annealed at very high temperatures (1,000's of K above T melt ) to ensure thermodynamic equilibrium, and then are typically cooled to lower temperatures. The (Mg,Fe)-silicate and pyrolite melt simulations at 2,000 K from Karki et al (2021) and Solomatova and Caracas (2021) include the lowest temperature simulations of the FPMD data set that we consider. There is thus overlap in temperature between the hottest experimental measurements (2,473 K from Sakamaki et al, 2006) and the FPMD studies of Bajgain et al (2015), Karki et al (2021), and Solomatova and Caracas (2021).…”