Structure and Density of H₂O‐Rich Mg₂SiO₄ Melts at High Pressure From Ab Initio Simulations

Abstract

Water has a strong effect on silicate melt properties, yet its dissolution mechanism in depolymerized melts, typical for mantle composition, remains poorly understood. Here we report results of first-principles molecular dynamics simulations for hydrous Mg 2 SiO 4 melts with 6, 16, and 27 wt% H 2 O at pressure and temperature conditions relevant to the upper mantle and mantle transition zone. The results show that hydrogen bonds not only to the network-forming cation Si but also to Mg which-nevertheless-remains the most important network modifier. There is no evidence to support the hypothesis based on experimental data that water may cause an increase in melt polymerization for ultramafic magmas; the ratio of non-bridging oxygen per Si increases with the addition of the oxygen from H 2 O. The partial molar volume of water is independent on concentration in our simulations which allows us to examine the density of hydrous melt systematically. The critical water content-at which melts are neutrally buoyant compared to the surrounding mantle-is~4 wt% H 2 O for a pyrolite composition, much lower than the high water content (>10 wt%) observed in petrological experiments and estimated thermodynamically for low-degree partial melts formed in the vicinity of the mantle transition zone. Plain Language Summary Geophysical observations indicate abrupt changes in the material properties of the Earth's mantle right below and above the transition zone, at a depth between 410 and 660 km. The existence of low-velocity and high conductivity layers near this region is often taken as an indication for melting caused by the presence of water. However, the density of such hydrous melts is not well characterized over the relevant pressure-temperature range. Here we present new simulations on Mg 2 SiO 4 melts with a wide range of water concentrations to address this issue. The water content at which hydrous melts have the same density as the surrounding mantle is about 4 wt%, much lower than the high water content (>10 wt%) estimated for melts occurring in the Earth's mantle, suggesting they cannot experience long-term gravitational stability in the mantle transition zone and its vicinity. We further find that water does not cause a significant change in melt structure as proposed previously.

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

“…We find that hydrogen is highly mobile in all melts simulated here, in general agreement with previous computational results 26 , 46 , 47 . At each temperature, the diffusivity decreases with increasing pressure implying positive activation volume and all log D H – P isotherms tend to be linear (Fig.…”

“…6 ). The predicted pressure evolution of the hydrous speciation is generally consistent with previous computational studies of hydrous silicate melts 6 , 7 , 24 – 26 .…”

“…Similarly, we estimate that up to 9 wt% water may be accommodated in a buoyantly neutral hydrous melt at the 660 km depth discontinuity corresponding to its large density jump 50 of 0.38 g/cm 3 . The implied presence of gravitationally trapped hydrous partial melts at these depths have been previously suggested 6 , 12 , 26 , 51 . The water content of possible melts at these depths also depends on the iron content because water and iron change the density in an opposite way.…”

“…On the other hand, a relatively few computational studies of hydrous silicate melts have so far been reported using first-principles approach 6 , 7 , 24 – 26 . These studies have covered relatively high temperatures and chemically simple compositions.…”

“…For example, the use of the local density approximiation (LDA) for the exchange–correlation functional tends to overbind the structure leading to significant underestimation of the volume 6 . While the calculated partial molar volume of water appears to be mostly insensitive to water concentration 26 and melt composition 6 , 24 , 25 , the issue is yet to be fully resolved. Similarly, it is not clear how hydrous contributions to electrical conduction are sensitive to pressure, temperature, and composition.…”