EXPERIMENTAL AND THEORETICAL-STUDIES OF THE STABILITIES OF TALC, ANTIGORITE AND PHASE-A AT HIGH-PRESSURES WITH APPLICATIONS TO SUBDUCTION PROCESSES

We have experimentally determined the equilibrium talc reversible arro w enstatite + quartz/coesite + H2O to 40 kbar in the system MgO-SiO2-H 2O (MSH) using both synthetic and nearly pure Mg end-member natural ta lc and other synthetic starting materials for the other solid phases. At 40 kbar, the equilibrium dehydration boundary lies similar to 150 d egrees C higher than that calculated using data from the existing inte rnally consistent thermochemical data bases. The reason for this discr epancy lies in the erroneous compressibility data of talc in the data bases. We have retrieved the compressibility of talc from the experime ntal phase equilibrium data, and have also calculated several other eq uilibria in the MSH system involving talc, antigorite and the dense hy drous magnesium silicate (DHMS), commonly referred to as phase A. Comp arison of these equilibria with selected thermal profiles at the leadi ng edge of young and old subducting oceanic slabs, along with the dehy dration condition of basaltic amphibole and solidus of mantle peridoti te, provides an explanation for the observed heights of the volcanic f ronts above subducting oceanic lithosphere. Further, it is found that in cold oceanic slabs (greater than or equal to 50 Ma with subduction velocity of greater than or equal to 10 cm/y), antigorite will transfo rm to the DHMS phase A through a vapor conserved reaction at a depth o f similar to 200 km. Phase A will then serve as a carrier of water int o the deeper mantle.