Elasticity of Majorite and a Majorite-Pyrope solid solution to high pressure: Implications for the Transition Zone

[1] High seismic velocity gradients in the transition zone have been attributed to large pressure derivatives of the elastic moduli of transition zone minerals, such as majorite-garnet. Here we present Brillouin scattering measurements of the elasticity of polycrystalline Mg-majorite (Mj(100), Mg4Si4O12) and a Mj(50)Py(50) majorite - pyrope solid solution to pressures similar to those at the top of the transition zone (15.1(1) GPa and 12.8(1) GPa, respectively). The pressure derivatives of the adiabatic bulk (K-S) and shear (mu) moduli of both Mj(50)Py(50) and Mj(100) are 4.2(3) and 1.4(2), respectively, and are equal to those of pyrope within the experimental uncertainties. We conclude that neither the adiabatic compression of majorite-garnet nor any other likely transition zone phase is able to produce the high velocity gradients observed seismically. High velocity gradients are consistent with gradual transformations between minerals with very different elastic properties. The transformation of similar to25% or more of clinopyroxene into majorite above 520 km, and the formation of Ca silicate perovskite below 520 km could produce the observed seismic gradients.