The Effect of Iron and Aluminium Incorporation on the Single-Сrystal Elasticity of Bridgmanite at High Pressure.

Abstract:

MgSiO₃ bridgmanite constitutes about 70% by volume of the Earth’s lower mantle and likely governs the physical behavior of this region. Chemical substitutions in MgSiO₃ bridgmanite involving Al and Fe may explain seismic velocity anomalies observed in the Earth’s lower mantle [1-3]. However, the effects of these substitutions on the anisotropic elastic properties of bridgmanite at high pressure and temperature are still experimentally unconstrained.

Here, we present data of internally consistent measurements of the single-crystal elastic properties of Mg_0.88Fe_0.12Si_0.09Al_0.11O₃ bridgmanite at high-pressures. Two differently oriented single-crystals of brigmanite have been double-side polished and cut as two semi-disks using a FEI Scios Focused Ion Beam (FIB) machine [4]. Two semi-disks, one for each of the crystallographic orientations, were loaded together in the pressure chamber of a diamond anvil cell with helium as a pressure-transmitting medium. Simultaneous measurements of density and sound velocities have been made on both crystals at high pressures using single-crystal X-ray diffraction and Brillouin spectroscopy in order to obtain self-consistent data, which do not depend on a secondary pressure scale. The Brillouin data at each pressure were fitted for both crystals simultaneously in order to reduce correlations among the elastic constants C_ij. Our approach allows determining the single-crystal elastic properties of bridgmanite as a function of pressure, derived independently of a secondary pressure. We will use our results for Al-Fe-bearing bridgmanite to discuss the effects of chemical substitution on the high-pressure elasticity of bridgmanite and implications for the interpretation of seismic heterogeneities in Earth’s lower mantle.

[1] Ni et al. (2005), Geophys. J. Int. 161, 283–294. [2] Masters et al. (2000), AGU Monograph Series, 117, 63–87. [3] Garnero et al. (2005), The Geological Society of America Special Paper, 430, 79–101. [4] Marquardt et al. (2012), American Mineralogist, 97, 299-304.