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Stability of phlogopite-quartz and sanidine-quartz: A model for melting in the lower crust |
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| Authors: | Steven R Bohlen A L Boettcher V J Wall J D Clemens |
| Institution: | 1. Institute of Geophysics and Planetary Physics, University of California, 90024, Los Angeles Los Angeles, California, USA 3. Institute of Geophysics and Planetary Physics and Department of Earth and Space Sciences, University of California, 90024, Los Angeles Los Angeles, California, USA 4. Department of Earth Sciences, Monash University Clayton, 3168, Victoria, Australia |
| Abstract: | The melting of phlogopite-quartz and sanidine-quartz under vapor-absent conditions and in the presence of H2O-CO2 vapor have been determined from 5–20 kbar. In the lower crust (P=6–10 kbar), phlogopite + quartz melts incongruently to enstatite + liquid at temperatures as low as 710° C in the presence of H2O. When the activity of water is sufficiently reduced by addition of CO2, phlogopite + quartz undergoes a dehydration reaction to enstatite + sanidine + vapor, for example at 790±10° C, 5 kbar, with \(X_{H_2 O}^V\) =0.35. In the absence of vapor, phlogopite + quartz is stable up to a maximum temperature of 900° C in the crust; at higher temperatures this assemblage melts incongruently to enstatite + sanidine + liquid. The melting of sanidine-quartz in the presence of H2O-CO2 vapor shows marked topological differences from melting in the system albite-H2O-CO2, and as a result, apparent activity coefficients for water calculated from sanidine-quartz H2O-CO2 are less than those calculated from albite-H2O-CO2 by up to a factor of five. These data shed light on anatexis in the lower crust, but uncertainties related to ordering of Al and Si in natural and synthetic micas forestall a more rigorous analysis. Nevertheless, maximum temperatures for some granulite terranes can be established. |
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