High-pressure and high-temperature Raman spectroscopic study of zircon as a pressure scale in hydrothermal DACs

Raman spectra of zircon have recently been used as a pressure scale for studies of geological fluids at high temperatures and high pressures using diamond anvil cells (DACs). The zircon scale is advantageous in high chemical stability and the large pressure response of the B_1g mode. Despite its excellent applicability, the calibration of the scale has been carried out only in a narrow pressure–temperature range, especially under limited high-temperature and high-pressure conditions. In this study, the pressure and temperature dependence of the Raman modes of synthetic zircon was investigated up to 9.5 GPa and from room temperature to 776 K using an externally heated diamond anvil cell. Ruby and gold were used as the reference pressure scales. The Raman shift of the B_1g mode for the antisymmetric stretching of the SiO₄ structure in zircon showed a linear pressure dependence of 5.48(4) cm⁻¹/GPa up to 8 GPa at room temperature, in agreement with the previous studies. Measurements under high-pressure and high-temperature conditions confirmed that the pressure dependence up to 9.5 GPa along the isotherms from 373 to 675 K was consistent with the room-temperature value; the wavenumbers can be well deduced from the sum of the individual effects of pressure and temperature, obtained at ambient temperature and pressure, respectively. A comparison of the zircon scale with the c-BN Raman spectroscopic scale confirmed that the pressures determined with these scales were in reasonable agreement. The present results provide a confident use of the zircon Raman spectroscopic scale in a wider pressure–temperature range than previous studies for the internally consistent pressure determination.