Density variations of Cr-rich garnets in the upper mantle inferred from the elasticity of uvarovite garnet

Abstract

The thermoelastic parameters of Ca3Cr2Si3O12 uvarovite garnet were examined in situ at high pressure up to 13GPa and high temperature up to 1100K by synchrotron radiation energy-dispersive X-ray diffraction within a 6-6-type multi-anvil press apparatus. A least-square fitting of room T data to a third-order Birch–Murnaghan (BM3) EoS yielded K0 = 164.2±0.7GPa,V0 = 1735.9±0.3 Å3 (K’0 fixed to 4.0). P–V–T data were fitted simultaneously by a modified HT-BM3 EoS, which gave the isothermal bulk modulus K0 = 163.6±2.6GPa, K’0 = 4.1±0.5, its temperature derivative (∂K0,T/∂T)P =–0.014±0.002 GPa K−1, and the thermal expansion coefficients a0 = 2.32±0.13 ×10−5 K−1 and b0 = 2.13±2.18 ×10−9 K−2 (K’0 fixed to 4.0). Our results showed that the Cr3+ enrichment in natural systems likely increases the density of ugrandite garnets, resulting in a substantial increase of mantle garnet densities in regions where Cr-rich spinel releases chromium through a metasomatic reaction.