FOSSIL PRESSURES OF FLUID INCLUSIONS IN MANTLE XENOLITHS EXHIBITING RHEOLOGY OF MANTLE MINERALS: IMPLICATIONS FOR THE GEOBAROMETRY OF MANTLE MINERALS USING MICRO-RAMAN SPECTROSCOPY

Abstract

Micro-Raman spectroscopic analysis allows us to estimate the internal pressure of small fluid inclusions. We applied this method to CO2-dominated fluid inclusions in mantle-derived xenoliths. The pressures estimated from the equilibration temperature and density of the fluid range from 0.96 to 1.04 GPa corresponding to depths of up to 30 km, which confirms that these rocks and fluids are of uppermost mantle origin. Furthermore, the inclusions show pressures specific to the individual host minerals (spinel≥orthopyroxene≈clinopyroxene≫olivine). In particular, the densities of CO2 in pyroxenes are 10% higher than in olivine. Such an enormous difference cannot be explained by elastic deformation of the minerals during ascent of the xenoliths, although the process may explain the slightly higher density of CO2 in spinel. During the ascent, the strain rate of orthopyroxene calculated using the ‘constitutive equation’ is several orders of magnitude lower than that of olivine. The difference in densities of CO2 among the host mineral species is therefore attributable to the rheological properties of the minerals. Present internal pressures of fluid inclusions can be a sensitive strength marker of mantle minerals. Conversely, the density of CO2 inclusions in pyroxene (and spinel) may be a useful geobarometer.