Calibration of zircon as a Raman spectroscopic pressure sensor to high temperatures and application to water-silicate melt systems

The shifts in wavenumber of the v(3)(SiO4) (similar to 1008 cm(-1)) Raman band of fully crystalline synthetic zircon with changing pressure (P) and temperature (T) were calibrated for application as a Raman spectroscopic pressure sensor in optical cells to about 1000 degrees C and 10 GPa. The relationship between wavenumber (v) of this band and T from 22 to 950 degrees C is described by the equation v (cm(-1)) = 7.54.10(-9).T-3 - 1.61.10(-5).T-2 - 2.89.10(-2).T + 1008.9, where T is given in degrees C. The pressure dependence is nearly linear over the studied range in P. At similar to 25 degrees C, the partial derivative v/partial derivative P slope to 6.6 GPa is 5.69 cm(-1)/GPa, and that to 2 GPa is 5.77 cm(-1)/GPa. The partial derivative v/partial derivative P slope does not significantly change with temperature, as determined from experiments conducted along isotherms up to 700 degrees C. Therefore, this pressure sensor has the advantage that a constant a partial derivative v/partial derivative P slope of 5.8 +/- 0.1 cm(-1)/GPa can be applied in experiments to pressures of at least about 6.6 GPa without introducing a significant error. The pressure sensor was tested to determine isochores in experiments with H2O+Na2Si3O7 and H2O+NaAlSi3O8 fluids to 803 degrees C and 1.65 GPa. These pressures were compared to pressures calculated from the equation of state (EoS) of H2O based on the measured vapor dissolution or ice melting temperature for the same experiment. Pressures determined from the zircon sensor in runs in which NaAlSi3O8 melt dissolved in aqueous fluid were close to or lower than the pressure calculated from the EoS of H2O using the vapor dissolution or ice melting temperature. In experiments with H2O+Na2O+SiO2 fluids, however, the pressure obtained from the Raman spectrum of zircon was often significantly higher than that estimated from the EoS of H2O. This suggests that the pressures along some critical curves of water-silicate melt pseudobinary systems should be revised.