THE HIGH-PRESSURE STABILITY OF TALC AND 10 ANGSTROM PHASE - POTENTIALSTORAGE SITES FOR H2O IN SUBDUCTION ZONES

The pressure-temperature conditions of the reactions governing the hig h-pressure stability of talc were investigated in experiments on the b ulk composition Mg3Si4O10(OH)(2) + H2O at 2.9-6.8 GPa, 650-820 degrees C, using piston-cylinder and multianvil apparatus. The reaction talc = enstatite + coesite + vapor was bracketed between 800 and 820 degree s C at 2.90-2.95 GPa and between 770 and 780 degrees C at 3.77-4.02 GP a. 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 o f the talc dehydration reaction calculated using THERMOCALC v2.4 (Powe ll and Holland, 1988; Holland and Powell, 1990; Holland, personal comm unication). This revised version of THERMOCALC incorporates new compre ssibility and thermal expansivity data for talc (Pawley, Redfern, and Wood, in preparation). Agreement between experimental and calculated c urves continues up to 4 GPa, but at higher pressures the talc dehydrat ion reaction occurs at lower temperatures than calculated, so that by 4.6 GPa the thermal stability of talc is at <730 degrees C. At similar to 5 GPa, 710 degrees C, there is an invariant point involving talc, 10 Angstrom phase, enstatite, coesite, and vapor. This point marks the highest pressure at which talc is stable. Above it, the thermal stabi lity of 10 Angstrom phase expands with increasing pressure. Its maximu m stability is unknown. Talc is a common hydrothermal alteration produ ct of peridotite and may transport H2O in subducting slabs from shallo w depths to depths of similar to 150 km. It may also crystallize in ov erlying mantle-wedge peridotite after infiltration of fluid from the s lab, and its dehydration in the mantle wedge may lead to partial melti ng. The 10 Angstrom phase may transport H2O in silica-enriched hydrate d peridotite to depths of at least 200 km.