Dehydration kinetics of antigorite at high pressure and high temperature

Antigorite is the most abundant hydrous phase in subducted slab, and the descending slab transports water into the Earth's interior. The dehydration of antigorite at certain depth supplies free water, and the upward migration contributes to the generation of magma in the mantle wedge. To evaluate the ability of H2O transportation into the deep Earth's mantle, it is important to know the dehydration rate of antigorite at high P-T conditions. We have conducted the dehydration experiments of antigorite at high pressure and high temperature by in situ X-ray diffraction using MAX-80 at PF, Japan. Natural antigorite was used as a starting material, and the sample was sealed by Au lids plus diamond sleeve, which enabled us to get high quality of diffraction because of high X-ray transparency of diamond. Temperatures were measured by WRe thermocouple, and pressures were calculated by equation of state of NaCl proposed by Decker (1971). Experiments were conducted between 3 and 9 GPa and temperature up to 700°C. We gradually increased temperature to 500°C, and then the diffraction data were collected at every 50°C with increasing temperature. In each temperature, the diffractions were collected in every 50 seconds for certain duration with checking the peak change. Antigorite dehydrated to forsterite + enstatite + fluid via talc formation with increasing temperature. These results were almost consistent with Perrillat et al. (2005). At 3 GPa, the rapid dehydration was occurred at 700°C in 15 minutes, whereas at 5 GPa, that was occurred at 650°C in 50 minutes. Comparing with the previous phase equilibrium studies of antigorite, our result shows that the dehydration of antigorite quickly occurs when P-T path goes into the region of forsterite + enstatite + fluid. This shows that antigorite in subducted slab should be quickly dehydrated when the slab across the dehydration boundary of antigorite. Thus metastable antigorite can not exist in subducted slab at around 650-700°C and 3-5 GPa.