Mineral inclusions under non-ambient conditions

Abstract

Mineral inclusions represent a real treasure in earth sciences because of their large range of applicability to unravel metamorphic and geodynamic processes. For instance, the contrast in the thermal expansion and compressibility coefficients between a mineral inclusion and its surrounding host leads to a residual pressure in the inclusion (Pinc) that, once determined at ambient conditions, can be used to back-calculate the pressure and temperature of entrapment (e.g. Rosenfeld and Chase, 1961; Angel et al. 2015). The Pinc can be quantified via in situ Raman spectroscopy, using ab initio calibrations (e.g. Murri et al. 2018). In addition, Raman spectroscopy can be used to explore the evolution of residual pressure in mineral inclusions under non-ambient conditions. We have analysed quartz-in-garnet (QuiG) host-inclusion systems in situ under high external pressure applied with a diamond-anvil cell at room temperature. The evolution of quartz Pinc calculated from the Raman data collected on fully encapsulated inclusions at high pressure agrees with the predictions calculated from the equations of state and confirm that the garnet host acts as a pressure shield to the softer quartz inclusion. The pressure-dependent sharpening of the A1 mode near 207 cm-1 indicates that quartz inclusions become metastable against coesite at values of Pinc of ~ 2.4 GPa, which corresponds to the pressure of the quartz-coesite phase boundary at room temperature of free crystals (Bose and Ganguly 1995). However, the external applied pressure may exceed the Pinc of more than 2 GPa. Finally, we show that “partially” encapsulated inclusions undergo significant non-hydrostatic stress with evident symmetry-breaking due to heterogeneous host-shielding effects. At room temperature, such deviatoric stresses do not affect the metastability of quartz inclusions with respect to coesite. Financial support by the Alexander von Humboldt Foundation to N. Campomenosi., and by the European Research Council (ERC) grant agreements 714936 to M. Alvaro Angel, R. J., et al. (2015).  Journal of Metamorphic Geology, 33(8), 801-813. Bose, K., & Ganguly, J. (1995). American Mineralogist, 80(3-4), 231-238. Kaminsky, F. (2012). Earth-Science Reviews, 110(1-4), 127-147. Murri, M., et al. (2018). American Mineralogist, 103(11), 1869-1872. Rosenfeld, J. L., & Chase, A. B. (1961). American Journal of Science, 259(7), 519-541.