Experimental study of quartz inclusions in garnet at pressures up to 3.0 GPa: evaluating validity of the quartz-in-garnet inclusion elastic thermobarometer
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Garnet crystals with quartz inclusions were hydrothermally crystallized from oxide starting materials in piston–cylinder apparatuses at pressures from 0.5 to 3 GPa and temperatures ranging from 700 to 800 °C to study how entrapment conditions affect remnant pressures of quartz inclusions used for quartz-in-garnet (QuiG) elastic thermobarometry. Systematic changes of the 128, 206 and 464 cm−1 Raman band frequencies of quartz were used to determine pressures of quartz inclusions in garnet using Raman spectroscopy calibrations that describe the P–T dependencies of Raman band shifts for quartz under hydrostatic pressure. Within analytical uncertainties, inclusion pressures calculated for each of the three Raman band frequencies are equivalent, which suggests that non-hydrostatic stress effects caused by elastic anisotropy in quartz are smaller than measurement errors. The experimental quartz inclusions have pressures ranging from − 0.351 to 1.247 GPa that span the range of values observed for quartz inclusions in garnets from natural rocks. Quartz inclusion pressures were used to model P–T conditions at which the inclusions could have been trapped. The accuracy of QuiG thermobarometry was evaluated by considering the differences between pressures measured during experiments and pressures calculated using published equation of state parameters for quartz and garnet. Our experimental results demonstrate that Raman measurements performed at room temperature can be used without corrections to estimate garnet crystallization pressures. Calculated entrapment pressures for quartz inclusions in garnet are less than ~ 10% different from pressures measured during the experiments. Because the method is simple to apply with reasonable accuracy, we expect widespread usage of QuiG thermobarometry to estimate crystallization conditions for garnet-bearing silicic rocks.
KeywordsQuartz Inclusion Garnet Experimental Barometer Thermobarometry
This work was supported by the Earth Sciences Division of the National Science Foundation through grant number EAR-1447468. JT thanks David Adams, Guiomar Camano and Anthony Crespo for assistance on numerous experiments, and Tara Kahan for usage of the Renishaw Raman Spectrometer. Numerous discussions with Mattia Bonazzi, Mateo Alvaro and Ross Angel greatly improved our understanding of inclusions with remnant pressures. Formal reviews by Matthew Kohn, Fabrizio Nestola and an anonymous reviewer greatly improved the final version of the manuscript.
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