Quartz under stress: Raman calibration and applications to geobarometry of metamorphic inclusions

An experimental calibration of the Raman shifts of quartz with pressure up to ~2 GPa and differential stress up to ~0.6 GPa parallel and perpendicular to the c-axis is presented. The position of the 206 cm⁻¹ peak depends only on hydrostatic pressure P, and its pressure dependence is recalibrated with a peak fitting procedure that is more adequate for Raman barometry than previous calibrations. The position of the 128 and 464 cm⁻¹ peaks depends on P and also on differential stress , which can be determined from the position of these two peaks knowing hydrostatic pressure from the position of the 206 cm⁻¹ peak. This calibration provides direct relationships between Raman shifts and stress, with a simple formulation of residual pressure and differential stress assuming uniaxial stress along the c-axis of quartz inclusions. It is tested on data from experimental and natural inclusions. Pressures from the present calibration are similar within uncertainties to those obtained with previous experimental calibration within uncertainties, and experimental inclusions yield residual pressures consistent with synthesis pressure. Inconsistent residual differential stresses are obtained from the 128 and 464 cm⁻¹ peaks on some experimental inclusions, providing a criterion for identifying inclusions under complex stress conditions that are not appropriate for geobarometry. Recent data on natural inclusions show self-consistent differential stress, consistent with the assumption of major stress along symmetry axis of the inclusion crystals and with values expected from elastic models. The average pressure values from the 128 and 464 cm⁻¹ peaks is similar to the residual pressure from the 206 cm⁻¹ peak that depends only on hydrostatic pressure. It can be used to obtain pressure when the 206 cm⁻¹ peak position cannot be used due to interference with host mineral peaks. Using the 128 and 464 cm⁻¹ peaks alone, or averaging either 128 and 206 or 206 and 464 cm⁻¹ peaks can induce systematic bias in the residual pressure determination. Applications of the present results to natural inclusions suggest that combined determination of residual pressure and differential stress may be used both for barometry and thermometry pending further calibration.