Elasticity of single-crystal iron-bearing pyrope up to 20 GPa and 750 K

Abstract

Elastic properties of the major constituent minerals in the Earth's upper mantle at relevant high pressure–temperature (P–T) conditions are crucial for understanding the composition and seismic velocity structures of this region. In this study, for the first time, we have measured the single-crystal elasticity of natural Fe-bearing pyrope, Mg2.04Fe0.74Ca0.16Mn0.05Al2Si3O12, using in situ Brillouin spectroscopy and X-ray diffraction at simultaneous high P–T conditions up to 20GPa and 750K in an externally-heated diamond anvil cell. The derived aggregate adiabatic bulk and shear moduli (KS0, G0) at ambient conditions are 168.2 (±1.8)GPa (the value in parentheses represents propagated uncertainties ±1σ) and 92.1 (±1.1)GPa, respectively, consistent with literature results. Using the third-order Eulerian finite-strain equation to model the high P–T data, the derived pressure derivatives of the bulk and shear moduli at constant temperature are (∂KS/∂P)T=4.4 (±0.1) and (∂G/∂P)T=1.2 (±0.1), respectively, whereas the temperature derivatives of these moduli at constant pressure are (∂KS/∂T)P=−16.8 (±1.3)MPa/K and (∂G/∂T)P=−5.1 (±1.1)MPa/K, respectively. Compared to literature values, our results show that addition of 25mol% Fe in pyrope increases the pressure derivative of the bulk modulus by 7%, but has a negligible effect on other elastic parameters. Extrapolation of our results shows that Fe-bearing pyrope remains almost elastically isotropic at relevant P–T conditions of the upper mantle, indicating that it may not have a significant contribution to seismic VP and VS anisotropy in the upper mantle. Together with the elasticity of olivine and pyroxene minerals in the upper mantle, we have constructed new velocity profiles for two representative compositional models, pyrolite and piclogite, along a representative geotherm of the Earth's upper mantle. These velocity models show VS profiles consistent with seismic observations, although VP profiles are slightly lower than global seismic observations. Our analyses also show that approximately 30% garnet in a mineralogical model is needed to best match both seismic VP and VS profiles of the region, although such high garnet content needs to be reconciled with our current geochemical understanding. Our results here provide new insights into seismic profiles and mineralogical models of the upper mantle region.