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

Abstract

The shifts in wavenumber of the _ν3(SiO₄) (~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 °C and 10 GPa. The relationship between wavenumber (ν) of this band and T from 22 to 950 °C is described by the equation ν (cm^-1) = 7.54·10⁻⁹·T³ - 1.61·10⁻⁵·T² - 2.89·10−2·T + 1008.9, where T is given in °C.

The pressure dependence is nearly linear over the studied range in P. At ~25 °C, the ∂ν/∂P slope to 6.6 GPa is 5.69 cm^-1/GPa, and that to 2 GPa is 5.77 cm^-1/GPa. The ∂ν/∂P slope does not significantly change with temperature, as determined from experiments conducted along isotherms up to 700 °C. Therefore, this pressure sensor has the advantage that a constant ∂ν/∂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 °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.