Single-Crystal Elasticity of Iron-Bearing Bridgemanite in the Lower Mantle

Abstract:

Bridgemanite is believed to be the most abundant mineral in the Earth’s lower mantle. Knowing its elasticity is thus critical to our understanding of the lower-mantle seismology, geochemistry, and geophysics. Although single-crystal elasticity and elastic anisotropy of bridgemanite under high P-T have been reported theoretically, experimental results on the single-crystal elasticity of bridgemanite remain very limited[1, 2]. Published experimental results have been limited to ambient conditions due to technical challenges in high-pressure measurements to permit derivations of all nine elastic constants (C₁₁, C₂₂, C₃₃, C₄₄, C₅₅, C₆₆, C₁₂, C₂₃ and C₁₃) of the crystal. A thorough understanding of the elastic properties of bridgemanite at relevant lower mantle conditions, as well as the effects of iron, is essentially needed to interpret seismic observations and to construct a reliable mineralogical and geochemical model. In order to solve all individual elastic constants of bridgemanite at high pressures via Christoffel’s equations, we employed both Brillouin Light Scattering (BLS) which is sensitive to shear wave velocities (Vs) up to megabars, and Impulsive Stimulated Light Scattering (ISS) which is sensitive to compressional wave velocities (V_P) at lower mantle pressures. The BLS and ISS allowed us to measure V_P and V_S sound velocities as a function of the azimuthal angle from two orientated single-crystal iron bearing bridgemanite platelets under lower mantle pressures. These experimental results permit the derivations of full elastic constants of single-crystal bridgemanite that are consistent with previous theoretical studies [3, 4]. We will discuss how pressure-temperature, as well as the iron spin/valence states and minor element aluminum, affect the single-crystal elasticity and seismic parameters (e.g. V_P and V_S anisotropy AV_P, AV_S) at lower mantle conditions. Within a pyrolite mineralogical model, these results are extrapolated using a thermoelastic model and compared with seismic profiles of the lower mantle to better understand the deep-mantle geophysics and geochemistry.

References:

1. Sinogeikon,S.V., et al., 2004, GRL 31.
2. Yeganeh-Haeri, A., et al., 1994, PEPI 87.
3. Wentzcovitch, R.M., et al., 1998, EPSL 164.
4. Oganov, A.R., et al., 2001, Nature 411.