Elasticity of San Carlos olivine to 8 GPa and 1073 K

Elasticity of San Carlos olivine, (Mg 0.9 , Fe 0.1 ) 2 SiO 4 , has been measured at simultaneous high pressure and high temperature to 8.2 GPa and 1073 K using ultrasonic interferometry in conjunction with synchrotron X‐radiation. The elastic moduli and their pressure and temperature derivatives are precisely determined using a pressure standard free fit using third‐order finite strain equations to the velocity and unit cell volume data in the entire pressure and temperature range, yielding K S 0 = 130.3(4) GPa, G 0 = 77.4(2) GPa, K ′ S 0 = 4.61(11), G ′ 0 = 1.61(4), ∂ K S /∂ T = −0.0164(5) GPa/K, and ∂ G /∂ T = −0.0130(3) GPa/K. Combined with previous thermoelastic data on wadsleyite, the velocity contrasts between α‐ and β‐(Mg, Fe) 2 SiO 4 at 410‐km depth are calculated along a 1673 K adiabatic geotherm with plausible iron partition between the two phases. The fraction of olivine consistent with a ∼5% seismic discontinuity in an anhydrous mantle is constrained to be less than ∼50% with the possibility that a hydrous or a cooler mantle increases the olivine content towards pyrolitic composition.