Zircon and quartz inclusions in garnet used for complementary Raman thermobarometry: application to the Holsnøy eclogite, Bergen Arcs, Western Norway

Abstract

Mineral inclusions are common and have been widely used to investigate complex geological history. When a rock undergoes cooling and decompression after the entrapment of an inclusion into a host mineral, residual pressure may develop within the inclusion because of the differences in thermal expansivity and compressibility between the inclusion and host. By combining laser Raman spectroscopy and experimental data relating hydrostatic pressure and Raman shift, it is possible to estimate the entrapment pressure–temperature (P–T) conditions using an isotropic elastic model. In this study, we report Raman spectroscopic data on both zircon and quartz inclusions in garnet host from the Holsnoy eclogite, Bergen Arcs, Norway. Averaged residual pressures based on different Raman peaks for zircon and quartz inclusions are obtained to be ca. 0.6 GPa and ca. 0.65 GPa respectively. Using the equation of state for zircon and quartz, the entrapment P–T conditions are constrained to be 1.7–1.9 GPa, 680–760 °C, consistent with previous estimates based on phase equilibria. Heating/cooling experiments are performed on an entrapped zircon inclusion. A clear trend is found between the residual zircon inclusion pressure and the externally controlled temperature. We show that the residual zircon inclusion pressure sealed in garnet host is very sensitive to the entrapment temperature, and can be used as a Raman thermometer. The effects of laser heating and the thermo-elastic anisotropy of zircon inclusion are quantified and discussed.