Garnet EoS: a critical review and synthesis

All available volume and elasticity data for the garnet end-members grossular, pyrope, almandine and spessartine have been re-evaluated for both internal consistency and for consistency with experimentally measured heat capacities. The consistent data were then used to determine the parameters of third-order Birch-Murnaghan EoS to describe the isothermal compression at 298 K and a Mie-Gruneisen-Debye thermal-pressure EoS to describe the PVT behaviour. In a full Mie-Gruneisen-Debye EoS, the variation of the thermal Gruneisen parameter with volume is defined as gamma = gamma(0)(V/V-0)(q). For grossular and pyrope garnets, there is sufficient data to refine q which has a value of q = 0.8(2) for both garnets. For other garnets, the data do not constrain the value of q and we therefore refined a q-compromise version of the Mie-Gruneisen-Debye EoS in which both gamma/V and the Debye temperature theta(D) are held constant at all P and T, leading to (partial derivative C-V/partial derivative(P))(T) = 0, parallel isochors and constant isothermal bulk modulus along an isochor. Final refined parameters for the q-compromise Mie-Gruneisen-Debye EoS are: [GRAPHICS] for pyrope and grossular, the two versions of the Mie-Gruneisen-Debye EoS predict indistinguishable properties over the metamorphic pressure and temperature range, and the same properties as the EoS based on experimental heat capacities. The biggest change from previously published EoS is for almandine for which the new EoS predicts geologically reasonable entrapment conditions for zircon inclusions in almandine-rich garnets.

Automated summary

This study re-evaluated the volume and elasticity data of garnet end-members and determined the parameters of EOS to describe their compressibility and PVT behavior. The new EOS model showed significant improvement in predicting the properties of different garnets compared to previous models.