固物研究生论坛 - Thermodynamic and elastic properties of grossular

Thermodynamic and elastic properties of grossular ( Ca3Al2(SiO4)3, the calcium-aluminium end-member of garnet) at high pressures and temperatures are obtained using the first-principles calculations based on the density functional theory (DFT). And our calculated results show good agreements with available experimental data. While previous experiments show controversial pressure and temperature dependence of the elastic properties of grossular, the detailed pressure and temperature dependence can be obtained from our calculations. Compared to the thermoelastic properties of pyrope (Mg3Al2(SiO4)3), bulk moduli of grossular are similar those of pyrope, while its shear moduli are significantly larger than those of pyrope. Combining our calculated results with previously calculated elasticity of other upper-mantle minerals using the same method, we estimated density and wave velocity profiles for the pyrolitic and elcogitic compositions along different mantle geotherm. We find that a pyrolitic composition along the normal geotherm can respectably reproduce the reference velocities and densities for the upper mantle, supporting a pyrolitic upper mantle. Although a eclogitic composition also has similar velocities to reference seismic models above 350 km, it is significantly denser than the pyrolitic composition and the ambient mantle. However, the velocities of eclogite along the cold geotherm are relatively larger than those of the pyrolitic composition, indicating that the presence of eclogite in subduction zones could produce high velocity anomalies. In addition, the decomposition of grossular to calcium perovskite and corundum at the conditions of mantle transition zone can produce considerable velocity and density jumps, which probably provide interpretations for the seismically observed multiple reflections at ~ 660 km.