The high-pressure stability of talc and 10 Aa phase; potential storage sites for H2O in subduction zones

Abstract

The pressure-temperature conditions of the reactions governing the high-pressure stability of talc were investigated in experiments on the bulk composition Mg3Si40IO(OH)2 + H20 at 2.9-6.8 GPa, 650-820 °C, using piston-cylinder and multianvil apparatus. The reaction talc = enstatite + coesite + vapor was bracketed between 800 and 820°C at 2.90-2.95 GPa and between 770 and 780°C at 3.77-4.02 GPa. The lower-pressure bracket, which is just above the quartz-coesite phase transition, is consistent with some of the previous brackets on the reaction talc = enstatite + quartz + vapor and with the position of the talc dehydration reaction calculated using THERMOCALCv2.4 (Powell and Holland, 1988; Holland and Powell, 1990; Holland, personal communication). This revised version of THERMOCALC incorporates new compressibility and thermal expansivity data for talc (Pawley, Redfern, and Wood, in preparation). Agreement between experimental and calculated curves continues up to 4 GPa, but at higher pressures the talc dehydration reaction occurs at lower temperatures than calculated, so that by 4.6 GPa the thermal stability of talc is at <730°C. At -5 GPa, 710 °C, there is an invariant point involving talc, 10 A phase, enstatite, coesite, and vapor. This point marks the highest pressure at which talc is stable. Above it, the thermal stability of 10 A phase expands with increasing pressure. Its maximum stability is unknown. Talc is a common hydrothermal alteration product of peridotite and may transport H20 in subducting slabs from shallow depths to depths of -150 km. It may also crystallize in overlying mantle-wedgeperidotite after infiltration of fluid from the slab, and its dehydration in the mantle wedge may lead to partial melting. The 10 Aphase may transport H20 in silica-enriched hydrated peridotite to depths of at least 200 km.