THE REACTION TALC PLUS FORSTERITE = ENSTATITE PLUS H2O - NEW EXPERIMENTAL RESULTS AND PETROLOGICAL IMPLICATION

The reaction talc + forsterite = enstatite + H2O has been investigated between 6 and 20 kbar. Previous high-pressure experimental studies su ggested various reaction positional mostly with positive P-T slopes. T he new results show that the reaction has a negative slope in the pres sure range studied. It was bracketed between 640 and 680 degrees C at 10 kbar. between 620 and 640 degrees C at 15 kbar, and between 580 and 600 degrees C at 20 kbar. The growth of antigorite at high pressures prevented the reaction from being determined above 20 kbar. The reacti on position is consistent with the previous low-pressure data of Chern osky et al. (1985), is in very good agreement with the position calcul ated using the thermodynamic database of Berman (1988), and is in reas onable agreement with the calculation using the thermodynamic database of Holland and Powell (1990). The high reaction temperatures suggeste d by previous high-pressure experiments may reflect metastable or quen ch growth of talc in those experiments. Talc in equilibrium with forst erite is stable over a wide P-T range in hydrothermally altered and me tamorphosed ultramafic rocks and may be important for carrying H2O int o subduction zones. However, talc dehydration occurs at too shallow a depth for the H2O to contribute directly to subduction zone volcanism.