Elastic thermobarometry of natural and experimental quartz inclusions in garnet (QuiG) under tension

Abstract

Elastic thermobarometry has been rarely applied to quartz inclusions entrapped in garnet (QuiG) in granulite and igneous terranes, in part, because there is uncertainty about the reliability of the thermobarometric results arising from the quartz inclusions being subject to tensile strain and stress when examined at room conditions. Here, we present QuiG results from high-temperature metapelites from the Adirondacks, NY, USA and piston-cylinder experiments that give insight into the deformation behavior of quartz inclusions under tension. Measured remnant pressures (P_inc) of experimental and natural samples calculated using the quartz phonon mode Grüneisen tensor are too tensile with respect to the expected P_inc values based on experimental and petrologic constraints. We show that these discrepancies are not related to non-elastic deformation nor inaccuracies in the quartz equation of state. Evaluation of previous density functional theory (DFT) results shows that the structural response of quartz is non-linear with increasing tensile strain. Therefore, because the available quartz phonon mode Grüneisen tensor was determined with a linear fit optimized for compressive strains, obtained tensile strains using this tensor are too large in magnitude. P_inc values obtained using the hydrostatic calibrations of the 128 and 464 cm⁻¹ peaks have better agreement with the expected values and return entrapment conditions that are consistent with petrologically constrained or known experimental pressures. P_inc values obtained through hydrostatic calibrations must nonetheless be treated with caution because the behavior of Raman phonon modes under tension has not been calibrated experimentally.