Single‐crystal elasticity of γ‐(Mg0.91Fe0.09(2SiO4 to high pressures and to high temperatures

Sound velocities and elastic moduli have been measured on γ‐(Mg0.91Fe0.09)2SiO4 (ringwoodite) single‐crystals to 16 GPa and to 923K by Brillouin spectroscopy. A third‐order finite‐strain fit to the data yields pressure derivatives of 4.1(2) and 1.3(1) for the adiabatic bulk modulus (KS) and shear modulus (μ) respectively. Mg‐rich α, β, and γ (Mg,Fe)2SiO4 have identical values of ∂KS/∂P and of ∂μ/∂P within the experimental uncertainties. ∂KS/∂T and ∂μ/∂T for γ‐(Mg,Fe)2SiO4 are −0.021 and −0.016 GPa/K respectively. Our results indicate that the shear and compressional impedance contrasts associated with β‐ → γ‐(Mg,Fe)2SiO4 transition are sufficient to produce a visible discontinuity at 520 km even with a moderate (30–50%) amount of olivine. The generally low visibility of the 520 km discontinuity is likely due to its width and large amplitude topographic variations.