Single-Crystal Elasticity of MgSiO3 Bridgmanite to Mid-Lower Mantle Pressure

摘要

The combination of seismic observations and mineral physics data represents a unique tool to understand the structure and evolution of the deep Earth's interior. However, to date, elasticity data on both compressional (v(P)) and shear (v(S)) wave velocities of MgSiO3 bridgmanite are limited to shallow mantle conditions, hampering the resolution of mineral physics models. Here, we report the first single-crystal measurements of v(P) and v(S) of MgSiO3 bridgmanite up to similar to 79 GPa using high-pressure Brillouin scattering and single-crystal X-ray diffraction in a diamond anvil cell. At shallow lower mantle pressures, the elastic anisotropy of MgSiO3 bridgmanite was found to be similar, albeit smaller than that of Fe,Al-bearing bridgmanite of Kurnosov et al. (2017) but differed significantly from that proposed in the recent study of Fu et al. (2019). Using the elastic stiffness coefficients of bridgmanite obtained in this study at different pressures, we calculate the pressure dependence of the adiabatic bulk modulus, K-S0 = 257.1(6) GPa, K'(S0) = 3.71(4), and of the shear modulus, G(0) = 175.6(2) GPa, G'(0) = 1.86(1). These elastic parameters are included in a self-consistent thermodynamic model to calculate seismic wave velocities along a lower mantle adiabat for a primitive upper mantle bulk composition in the FeO-CaO-MgO-SiO2 system, which is currently the most complex system for which sufficient data exist. This preliminary model provides a good match to the v(S) and v(P) of 1D seismic models, implying that the composition of the lower mantle may be closer to pyrolite, rather than being more bridgmanite rich.