K. Bose et J. Ganguly, EXPERIMENTAL AND THEORETICAL-STUDIES OF THE STABILITIES OF TALC, ANTIGORITE AND PHASE-A AT HIGH-PRESSURES WITH APPLICATIONS TO SUBDUCTION PROCESSES, Earth and planetary science letters, 136(3-4), 1995, pp. 109-121
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.