EGU2020-12297, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-12297
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Water in MgSiO3-majorite at high temperatures and pressures: Incorporation mechanisms and thermoelastic properties

Zhigang Zhang1,2, Yancheng Lou1,2,3, Stephen Stackhouse3, and Andrew Walker3
Zhigang Zhang et al.
  • 1Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, China
  • 2College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, China
  • 3School of Earth and Environment, University of Leeds, UK

As the major component of garnet, the second most abundant phase in Earth’s transition zone, MgSiO3-majorite (Mgmj) may be an important reservoir of water. Previous works at ambient conditions show that water is incorporated in Mgmj by substituting Si with H in the Si-O tetrahedra. Due to the challenges of experiments and simulations, there are still very limited data on hydrous MgSiO3-majorite at simultaneously high temperatures and pressures.

In this study, we have carried out extensive first principles calculations to determine the properties of hydrous MgSiO3-majorite up to 2000 K and 40 GPa. We systematically considered all possible incorporation mechanisms with seven substitutional reactions. By solving the equations of equilibrium constants of the reactions, we obtained the ratios of various polyhedral substitutions and their temperature and pressure dependences. Our results show that the occupations of water (hydrogen) in Mgmj change mainly with temperature and are generally pressure-independent. Almost all of hydrogen occupy in the Si-O tetrahedra at 300 K, but the ratio of Mg-O dodecahedra increases remarkably with temperature and is the highest at 2000 K, which suggests that water in Mgmj under mantle conditions would be different from that observed at ambient conditions and implies the importance of in-situ measurements of high-TP experiments.

We have also calculated the elastic velocities and anisotropies of two types of hydrous Mgmj which caused by substitutions of Mg and Si. Our results show that ~1 wt% water in Si vacancy would decrease wave velocities of Mgmj by ~3% in both VP and VS, while the same amount of water in Mg vacancy would more effectively decrease the velocities by ~6%. In addition, water in Mg vacancy would noticeably improve the seismic anisotropy and the dlnVs/dlnVP of Mgmj while water in Si vacancy shows much smaller effects.

How to cite: Zhang, Z., Lou, Y., Stackhouse, S., and Walker, A.: Water in MgSiO3-majorite at high temperatures and pressures: Incorporation mechanisms and thermoelastic properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12297, https://doi.org/10.5194/egusphere-egu2020-12297, 2020

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