Elastic geothermobarometry on multiple inclusions in a single host

The characterization of the pressure and temperature (P-T) histories of subducted rocks is of key importance to unravel geological processes at all scales. Conventional element-exchange geothermobarometers are challenged in ultra-high-pressure metamorphic terranes as the subduction temperatures may exceed their closure temperature and minerals may undergo re-equilibration along their path. Elastic geobarometry applied to host-inclusion systems is a complementary method to determine P and T conditions of metamorphism that does not rely upon chemical equilibrium. Recent development of elastic geobarometry (Angel et al., 2019; Campomenosi et al., 2018; Murri et al., 2018) allows us to retrieve entrapment pressures for host-inclusion pairs from the residual strains acting on the inclusion. Because only a single measurement, the inclusion strain, is made, only a line in PT space of possible entrapment conditions, the entrapment isomeke, can be determined. Thus, the entrapment pressure along an isomeke can only be determined if the entrapment temperature is known.

An alternative is to calculate entrapment conditions for two types of inclusions that are believed, from petrological evidence, to have been entrapped at the same time. In this study we performed micro-Raman measurements on quartz and zircon inclusions trapped in garnets from a garnet-kyanite gneiss and a quartz-garnet vein from the Fjørtoft UHP terrane, Norway. From the micro-Raman data, using the program stRAinMAN (Angel et al., 2019), we calculated the strains at room conditions (Murri et al., 2018) and thus the entrapment conditions. The intersection between the two sets of isomeke calculated on multiple quartz and zircon inclusions demonstrates that measuring different inclusion phases trapped inside a single host allows unique P-T conditions for the host rock to be determined.

This work was supported by ERC-StG TRUE DEPTHS grant (number 714936) to M. Alvaro

Angel R.J., Murri M., Mihailova B. & Alvaro M. (2019) - Stress, strain and Raman shifts. Zeitschrift für KristallographieCrystalline Materials, 234(2), 129-140.

Campomenosi N., Mazzucchelli M.L., Mihailova B., Scambelluri M., Angel R.J., Nestola, F., Reali A. & Alvaro M. (2018) - How geometry and anisotropy affect residual strain in host-inclusion systems: Coupling experimental and numerical approaches. American Mineralogist, 103(12), 2032-2035.

Murri M., Mazzucchelli M.L., Campomenosi N., Korsakov A.V., Prencipe M., Mihailova B.D., Scambelluri M., Angel R.J. & Alvaro M. (2018) - Raman elastic geobarometry for anisotropic mineral inclusions. American Mineralogist, 103(11), 1869-1872.