Elasticity of mantle minerals: Composition, pressure and temperature dependence

The elastic properties of candidate mantle minerals are important for constraining mantle mineralogy through comparison of seismic velocities with laboratory measurements. I investigated the compositional, pressure and temperature dependence of several major mantle minerals by means of Brillouin spectroscopy. I measured the single crystal elasticity of chondrodite at ambient conditions and found that the bulk elastic wave velocities of chondrodite and olivine are very similar. It may be difficult to distinguish chondrodite-bearing rocks from “anhydrous” mantle on the basis of seismic velocities. I studied the effect of Fe on the elasticity of Al-free majorite, and β- and γ-(Mg,Fe)₂SiO₄. I found that the dependence of bulk modulus, K_s, and shear modulus on Fe content in majorites is small. K_s of Fe-bearing β-Mg₂SiO₄ is indistinguishable from that of the Mg end-member within experimental uncertainty, whereas K_s of γ-(Mg,Fe)₂SiO₄ increases rapidly with increasing iron content. The velocity and impedance contrasts between olivine and β-(Mg,Fe)₂SiO₄ are independent of Fe content for Mg-rich compositions, but the contrast for the β− → γ-(Mg,Fe) ₂SiO₄ transition increases significantly with increasing Fe content.; I measured the elasticity of pyrope, periclase, and γ-(Mg,Fe) ₂SiO₄ in a diamond anvil cell (DAC) to pressures of the transition zone (16–20 GPa). If the scattering geometry and hydrostaticity of the pressure transmitting medium in a DAC are carefully controlled, Brillouin scattering is an accurate method for high-pressure density and elastic modulus measurements. I designed a compact ceramic high-temperature cell for single-crystal Brillouin spectroscopy and measured the single-crystal elastic constants of MgO to 15 10 K, and γ-(MgFe)₂SiO₄ to 923 K.; An extrapolation of our data to transition zone pressure and temperature indicates that the shear and compressional impedance contrasts associated with β− →γ-(Mg,Fe)₂SiO₄ transition are sufficient to produce a resolvable discontinuity at 520 km even with a moderate (30–50%) amount of olivine. A single olivine content can explain both the 400 and 520 km discontinuities. High velocity gradients in the transition zone is explained by pyroxene → majorite transition rather than adiabatic compression of minerals.