Single-crystal elasticity of diopside to 14 GPa by Brillouin scattering

The single-crystal elastic moduli (Cij) of diopside have been measured up to 14 GPa using Brillouin spectroscopy, from which the aggregate compressional and shear velocities, adiabatic bulk modulus, shear modulus, and their pressure derivatives were obtained. A least-squares fit of the velocity–pressure data to third-order finite strain equation yields K S ′ = 4.8(2), G′ = 1.7(1) with ρ0 = 3.264(6) g/cm3, Ks = 114.6(7) GPa and G = 72.7(4) GPa. The current study provides the first high-pressure experimental values for the individual Cij’s of diopside and extends the range of direct measurements on the shear modulus to higher pressure. From the single-crystal moduli, the acoustic anisotropy of diopside is calculated and found to be higher than that of other major mantle minerals such as olivine, orthopyroxene, or garnet. The high anisotropy of diopsidic pyroxene is solely responsible for the observed acoustic anisotropy of eclogitic rocks. In mantle rocks containing both olivine and clinopyroxene, such as lherzolites, the diopside component could either enhance or diminish the bulk acoustic anisotropy, depending on the relationship of preferred orientation of these phases.