Abstract
As the major component of garnet, the second most abundant phase in Earth's transition zone, MgSiO3-majorite plays a fundamental role in controlling the state and dynamics of Earth's mantle. However, due to challenges of experiments and simulations, there are still very limited data on the elastic properties of MgSiO3-majorite at simultaneously high temperatures and pressures. In this study, we have carried out extensive first principles calculations to determine the thermoelastic properties of MgSiO3-majorite up to 2000 K and 40 GPa. We find that the elastic constants of MgSiO3-majorite change significantly over the temperature and pressure range studied, with noticeable non-linearities in their pressure dependences. The seismic anisotropy of MgSiO3-majorite is high and generally increases with pressure. It is much higher than that of the other end-members of garnet and ringwoodite, which makes it the most anisotropic mineral in assemblages expected in the lower transition zone. Based on our calculated elastic moduli and with careful elimination of systematic errors, we establish a third-order Birch-Murnaghan-Mie-Grüneisen model for MgSiO3-majorite with the parameters: V0 = 114.1 cm3/mol, K0 = 163.6 GPa, G0 = 86.4 GPa, K0′ = 4.44, G0′ = 1.16, γ0 = 1.08, q0 = 0.48, ηS0 = 0.76, and θ0 = 822.5 K. Integrating our results into a thermodynamic model able to predict the properties of mantle assemblages, we find that a pyrolite composition produces velocities that agree with the seismic model AK135 in the upper transition zone. In the lower transition zone, a pyrolite composition fits well with some specific local observations, but a mechanical mixture with 18% basalt and 82% harzburgite is in better agreement with the global seismic model PREM. The much larger abundance of MgSiO3-majorite in the garnet phase of harzburgite suggests that the anisotropy in the lower transition zone may not be negligible and would be observable at least in the heterogeneous zones near subducting slabs.
Highlights
From the constraints provided by seismic observations and mineral physics, garnet is recognized to be the major phase in the Earth's upper mantle and transition zone (Stixrude and Lithgow-Bertelloni, 2012)
In this study we have performed first principles calculations to determine the thermoelastic properties of MgSiO3-majorite over a wide range of temperatures and pressures
The anisotropies in VP and VS of MgSiO3-majorite are found to be higher than the other major minerals such as pyrope, grossular, and ringwoodite, making MgSiO3majorite the most anisotropic mineral in assemblages expected in the lower transition zone
Summary
From the constraints provided by seismic observations and mineral physics, garnet is recognized to be the major phase in the Earth's upper mantle and transition zone (Stixrude and Lithgow-Bertelloni, 2012). One study reports data for the bulk modulus of MgSiO3-majorite at simultaneous high temperature and pressure but provides no values for the shear modulus or elastic constants (Yu et al, 2011). After carefully eliminating systematic errors in the simulation results, we obtain a third-order Birch-Murnaghan-MieGrüneisen model for MgSiO3-majorite Using this model to predict the properties of mineral assemblages in the transition zone, we find that a mechanical mixture of basalt and harzburgite may be a better interpretation than the equilibrium pyrolitic model for global seismic observations in the lower transition zone
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