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

Abstract

While it is accepted that the lower mantle phase assemblage is comprised primarily of Al-and Fe-bearing MgSiO 3 bridgmanite, the exact proportion and the bulk composition of the lower mantle as a whole remain uncertain. The abundance of bridgmanite relative to ferropericlase will be controlled by the bulk Mg/Si ratio. In a pyrolite composition, which is believed to represent the average composition of the upper mantle (Ringwood, 1962), this ratio is ∼1.3 and a pyrolytic lower mantle assemblage would be comprised of ∼78% bridgmanite, 15% ferropericlase, and 7% calcium perovskite by volume (e.g., Irifune, 1994;Irifune et al., 2010;Ishii et al., 2011). The Earth is frequently proposed to have formed from carbonaceous chondrites, however, which have a Mg/Si ratio much closer to 1.0 (Palme & O'Neill, 2004). Aside from Si potentially entering the Earth's core (e.g., Poirier, 1994;Wänke, 1981), one way to explain the Mg/Si ratio of the upper mantle is if the lower mantle is richer in bridgmanite as a result of fractional crystallization during the magma ocean stage of Earth's history (e.g., Ballmer et al., 2017;Elkins-Tanton, 2012;Tonks & Melosh, 1993). This scenario is supported by shear wave velocity measurements of MgSiO 3 and Al-bearing bridgmanite at pressures over 1 Mbar (Murakami et al., 2012), for which comparison with the PREM seismic velocity model indicates a lower mantle containing at least 93 vol% of bridgmanite.In the previous studies of Murakami et al. (2007Murakami et al. ( , 2012, performed in the DAC using Brillouin scattering measurements, it was not possible to directly measure values of v P , which were instead estimated using bulk modulus determinations from previous static compression measurements (Fiquet et al., 2000). This potential cause of uncertainty arises in high pressure Brillouin scattering measurements because the v P signal overlaps with the much more intense v S signal of the diamond anvils. This effect is particularly pronounced for polycrystalline samples (Murakami et al., 2007(Murakami et al., , 2012, where the sample peaks are broad and tend to overlap with diamond even at ambient pressure. To overcome this problem, Fu et al. (2019) combined Brillouin scattering measurements of v S with impulsive stimulated light scattering (ISLS) measurements for v P to examine bridgmanite single crystals at 25 and 35 GPa, because ISLS has the advantage that the signal originating from the sample does not overlap with that of the diamond anvils. Fu et al. (2019) studied two bridgmanite samples with different Fe and Al contents. The determined bridgmanite elastic anisotropy

Thermal Equation of State and Structural Evolution of Al‐Bearing Bridgmanite

“…As the axial compressibility is particularly sensitive to non‐hydrostatic stresses, we limit our comparison with previous work only to single crystals synthesized in a multi‐anvil press and compressed in a DAC under quasi‐hydrostatic conditions. The axial compressibility scheme determined in this study is in agreement with previous static compression (Boffa Ballaran et al., 2012) and elasticity measurements (Criniti et al., 2021; Sinogeikin et al., 2004) of single‐crystal MgSiO 3 bridgmanite. For the Al‐bearing experiments, the compressibility of the c ‐axis is higher than in MgSiO 3 , as also observed for the Fe 3+ AlO 3 substitution mechanism (Boffa Ballaran et al., 2012), confirming that the Al‐Si substitution at the B site plays a critical role in determining the stiffness of this direction.…”

High Sodium Solubility in Magnesiowüstite in Iron‐Rich Deep Lower Mantle

“…The phase fraction of ferropericlase in the pyrolite model is estimated to be ∼16% (Dorfman, 2016). While some high-pressure elasticity measurements support the pyrolite model for the composition of the lower mantle (Criniti et al, 2021), others suggest that a better match to average seismic wave speeds is obtained for a mantle composed of ∼7%-12% ferropericlase (Mashino et al, 2020;Murakami et al, 2012). In the lowermost mantle, higher sodium solubility implies that even less ferropericlase is needed to accommodate all available sodium.…”

Sound velocities and single-crystal elasticity of hydrous Fo90 olivine to 12 GPa

“…To obtain accurate and precise cij values, it is important that their correlation in the fit procedure is low. This is achieved by using two appropriate crystallographic orientations (Criniti et al, 2021). We used published cij data for Fo90 olivine (Abramson et al, 1997) Hydrous Fo90 olivine single crystals were first observed under a polarising microscope and selected based on the absence of inclusions and their sharp optical extinction.…”

“…This approach has the advantage that all velocity data from all pressure points are used to constrain the cijs, minimizing the effect of data scattering on the calculated cijs and thus reducing their estimated uncertainties (Buchen, 2018). Even though this fitting procedure is particularly suited for very high pressure data, where some of the cijs are poorly constrained due to the lack of observations (e.g., Criniti et al, 2021), it can be applied to any high-pressure sound velocity dataset and the two procedures should yield consistent results as long as the collected data are of high quality, the high-pressure evolution of all cij is well described by a third-order finite strain EoS, and no phase transition takes place in the investigated pressure interval. Therefore, we also applied the global fit procedure to our dataset, which yielded very consistent results compared to those obtained by individual fits.…”