Equation of state and phase transition of antigorite under high pressure and high temperature

Abstract

The equation of state of natural antigorite has been determined up to ∼10GPa and 500°C by in situ X-ray diffraction in a cubic type multi-anvil apparatus, MAX80, located at the Photon Factory-Advanced Ring at the High Energy Accelerator Research Organization (KEK), and the temperature dependence of the bulk modulus was determined for the first time by high pressure and high temperature X-ray diffraction based on a series of direct measurements. No dehydration occurred during the entire experimental process, and no significant deviatoric stress was observed after heating. The room temperature P–V data below 7GPa have been fit to the Birch-Murnaghan equation of state, yielding V0,300K=366.9(7)Å3, K0, 300K=65.2(31)GPa, and the pressure derivative K′ fixed to 6.1. The high pressure phase transition was observed through volume softening behavior at around 7GPa, and the dP/dT slope seems to be flat or slightly positive, which is consistent with the recent report by Bezacier et al. (2013). The high temperature Birch-Murnaghan equation of state was used to fit the P–V–T data below 7GPa. Since the present experimental data was obtained by energy dispersive X-ray diffraction at high pressure and high temperature, the resolution was slightly lower than that obtained by the angle dispersive X-ray method. So the bulk modulus K0, 300K and the pressure derivative K′ were fixed to 62.9GPa and 6.1, respectively, which was obtained by single crystal X-ray diffraction in a diamond anvil cell by means of the angle dispersive method (Nestola et al., 2010) during fitting. From the fitting, we obtained V0, 300K=367.3(2)Å3, dK/dT=−0.0265(41)GPa/K, thermal expansion α0=3.92(50)×10−5/K. The temperature dependence of the bulk modulus was larger than the value calculated empirically (Holland and Powell, 1998). The thermal expansion of antigorite is larger than the results from the previous study. The compression of antigorite is very anisotropic along three axes, with a ratio of 1.15:1.00:3.33 at room temperature. Considering the P–T conditions of the subducting slab, the phase transition of antigorite may occur only in a limited area in the slab.