JpGU-AGU Joint Meeting 2020

Presentation information

[E] Poster

S (Solid Earth Sciences ) » S-IT Science of the Earth's Interior & Techtonophysics


convener:Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Dapeng Zhao(Department of Geophysics, Tohoku University), Michihiko Nakamura(Division of Earth and Planetary Materials Science, Department of Earth Science, Graduate School of Science, Tohoku University)

[SIT24-P02] Structure and density of H2O-rich Mg2SiO4 melts at high pressure from ab initio simulations

*Gerd Steinle-Neumann1, Liang Yuan1,2, Akio Suzuki2 (1.Bayerisches Geoinstitut, Universitaet Bayreuth, 2.Department of Earth Science, Tohoku University)

Keywords:mantle transition zone, water in silicate melts

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 of hydrous Mg2SiO4 melts with 6 wt.%, 16 wt.% and 27 wt.% H2O at pressure and temperature conditions relevant to the upper mantle and mantle transition zone. The results show that hydrogen not only bonds to network-forming cations Si, but also to network-modifying cation Mg which - nevertheless - remains the most important network modifier. There is no evidence to support the hypothesis inferred from experimental data that water may cause an increase in the melt polymerization for ultramafic magmas; the ratio of non-bridging oxygens per Si increases with the addition of the oxygen from H2O. The partial molar volume of water is independent on concentration in our simulations which allows us to examine the density of hydrous melt systemmatically. The critical water content - at which melts are neutrally buoyant compared to the surrounding mantle - is ~4 wt.% H2O for a pyrolite melt, 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.