Elasticity of San Carlos olivine, (Mg0.9, Fe0.1)2SiO4, 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 KS0 = 130.3(4) GPa, G0 = 77.4(2) GPa, K'S0 = 4.61(11), G'0 = 1.61(4), ∂KS/∂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 ¿-(Mg, Fe)2SiO4 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.