[S-IT17] TRANSPORT PROPERTIES AND PROCESSES IN THE EARTH

[SIT17-P04] Pressure and temperature dependence of the Raman spectra of zircon and its application as a pressure scale for studies of geological fluids using a DAC

*Naoko Takahashi¹, Hiroki Kobayashi¹, Hiroyuki Kagi¹ (1.Graduate School of Science, The University of Tokyo)

In diamond anvil cell (DAC) experiments, pressure determination is essential for investigating the physical and chemical properties of solids or liquids at high-pressure and high-temperature. Because zircon (ZrSiO₄) has high chemical resistance to fluid media, the B_1g Raman-active mode near 1008 cm^-1 (ν₃) of zircon has been recently used as a pressure scale for studies of geological fluids using DACs. Despite its stability up to pressures of about 9 GPa, the calibration of the scale has been carried out at limited pressures and temperatures, in particular, at simultaneous high-pressure and high-temperature conditions up to 1.2 GPa and 973 K, based on liquid-vapor homogenization temperature and the equation of state of H₂O (Schmidt et al., 2013 [1]). In the present study, pressure and temperature dependence of Raman spectra of zircon were obtained up to 9 GPa and 773 K using an externally-heated DAC. Ruby fluorescence and the equation of state of gold were used as internally consistent pressure references for the calibration of the scale. Our results showed a linear pressure dependence for the ν₃ Raman band of 5.47(3) cm^-1/GPa up to 8 GPa at room temperature. Simultaneous high-pressure and high-temperature measurements newly confirmed the pressure dependence of 5.47 cm^-1/GPa along isotherms from 373 K to 673 K up to 9 GPa. Moreover, there is negligible cross-dependence of the pressure and temperature on the Raman shift beyond the previously covered range. The new results also allowed an analysis of the mode Grüneisen parameters for each mode of zircon, which can be compared with the mode Grüneisen tensors determined by the density-functional theory calculations (Stangarone et al., 2019 [2]). We further investigated the persistence of the zircon, c-BN, and quartz scales in alkaline silicate solutions at elevated pressures and temperatures, and confirmed that the pressures determined with these scales reasonably agreed. The applications of the zircon pressure scale with our calibration for studies of subcritical and supercritical electrolyte fluids to elucidate the causative links between macroscopic properties and local structures of the fluids will be discussed.