Assessment of the reliability of elastic geobarometry with quartz inclusions

Elastic geobarometry makes use of the contrast in elastic proprieties between host and inclusion crystals to determine the entrapment conditions of the inclusions from the residual stress and strain measured in the inclusion when its host is at ambient conditions. The theoretical basis has been developed extensively in the past few years, but an experimental validation of the method is still required. We performed two syntheses experiments with quartz inclusions in pure almandine garnet at eclogitic conditions. Experiment labelled Alm-1 with synthesis performed at P = 3.0 GPa and T = 775 °C and Alm-2 at P = 2.5 GPa and T = 800 °C. All the experiments have been carried out in a piston-cylinder press. Isolated, fully-enclosed quartz inclusions in the recovered garnets have been then measured using micro-Raman spectroscopy. All fully-buried inclusions exhibit Raman peaks at higher frequencies and wavenumbers than those obtained from quartz crystals at ambient pressure. If these peak shifts are interpreted as a remnant pressures by use of hydrostatic calibrations of the Raman shifts of quartz with pressure, the remnant pressures show a large spread in values and lead to significant errors in back-calculated entrapment pressures, of up to 1.4 GPa for inclusions synthesised at 3.0 GPa. These results confirm that quartz inclusions trapped inside garnet are not subject to hydrostatic pressure. We therefore used the phonon-mode Grüneisen tensors of quartz to calculate the full strain state of each inclusion, from which the full anisotropic stress state can be calculated by using the elastic properties of quartz. The mean residual remnant stress of the inclusions determined in this way show a much smaller spread in values. Entrapment pressures calculated from this mean stress with the isotropic model for host-inclusion systems differ from the known experimental values by <0.2 GPa, which is of the order of the combined experimental uncertainties. These results show that the most significant effect of the elastic anisotropy of quartz is on the Raman shifts of the inclusion, and not on the subsequent calculation of entrapment conditions.