Pressure determination in HDAC experiments with haplogranite glass plus water by laser interferometry

Abstract

The pressure in a series of Hydrothermal Diamond Anvil Cell (HDAC) experiments with water plus haplogranite was determined via in-situ observation of the alpha to beta quartz transformation in the range 130–900MPa and 600–800°C, by means of laser interferometry.The pressures measured at the alpha-beta quartz transformation temperature are found to be similar or less than the values estimated using the EOS of pure water (including the error) for runs where the transition temperature is ≤665°C. On the contrary, at ~780°C the pressure in the HDAC is ~100MPa greater than that estimated using pure water isochore.A simplified model of haplogranite dissolution in water for an isochoric sample chamber is proposed to explain the negative sign and the variation of the observed discrepancy between measured and calculated pressure.We suggest that, beyond the change in the intrinsic properties of the fluid phase with increasing solute concentration, two factors control the P–T path in the HDAC: (1) hydration of the glass prior to the glass transition; and (2) changing volume of the aqueous pressure medium.The measurements made in this study demonstrate that laser interferometry on quartz inversion is an effective tool for precise pressure determination in HDAC experiments aimed at the investigation of physical and chemical properties of quartz-saturated hydrous melts in the mid and upper continental crust (i.e., P<1GPa).