High-pressure single-crystal elasticity of corundum: Implication for multiple seismic structure of 660-km discontinuity

Abstract

The complex multi-discontinuity structure at 660‐800 km depth is likely attributable to lateral mantle composition heterogeneities, which are closely related to the mid-ocean ridge basalts (MORB). To decipher the impact of varying composition on the seismic properties of MORB, detailed knowledge of the elasticity of candidate minerals is thus important. Here we employed Brillouin scattering coupled with diamond anvil cells to determine the single-crystal elasticity of corundum up to 14 GPa and 300 K. Using third-order finite strain equation of state, we calculate the pressure derivatives of adiabatic bulk modulus and shear modulus of corundum, which yields K_S0’ = 3.8(1), G₀’ = 1.8(1) with K_S0 = 256(1) GPa and G₀ = 163(1) GPa and ρ₀ = 3.987(1) g/cm³. Combined with previous high-temperature data, the velocity and anisotropy of corundum have been calculated at 300 K or along normal geotherm. Our results are applied to model the density and velocity profiles of normal and alkali-depleted MORB. Our modeling demonstrates that varying the alkali content and temperature of MORB can significantly impact the discontinuity depth and velocity jump at 660–800 km depth. For normal MORB, reducing the temperature by 300 K from normal mantle geotherm results in a shift of the velocity jump from 670–710 to 650–710 km depth but hardly affects the magnitude of the velocity jump (5.8–6.8(3)% for V_P and 10.0–10.8(5)% for V_S). By contrast, in alkali-depleted MORB, the discontinuity will occur at a greater depth from 705–730 to 720–745 km depending on temperature with a V_P jump of 3.8–4.6(2)% and V_S jump of 6.3–7.4(4)%.