EXPERIMENTAL DEMONSTRATION OF REFRACTORY CARBONATE-BEARING ECLOGITE AND SILICEOUS MELT IN THE SUBDUCTION REGIME

Calcitic carbonate is a very common low temperature alteration mineral in vugs and veins in oceanic basalts. Transport of altered basaltic crust through high PT conditions during subduction may involve loss of fluids and possible melting reactions but the behaviour of carbonate is quite unknown. In order to constrain this behaviour, a series of fO(2)-buffered piston-cylinder experiments were performed at pressures from 1.5 to 3.5 GPa and temperatures of 700-1000 degrees C. A composition representative of altered oceanic basalt (synthesised with excess H2O at 2.0 GPa and 700 degrees C, producing a quartz + rutile-bearing garnet-amphibolite assemblage) was run with 10 wt% calcite. These experiments demonstrate that carbonate remains as a refractory phase in rutile eclogite residue, in equilibrium with hydrous dacitic, in andesitic melts to temperatures > 900 degrees C over the pressure interval 1.5-3.5 GPa. At low pressures (P = 1.5-2.0 GPa) and high temperatures (T greater than or equal to 850 degrees C) calcite decarbonates, producing amphibolite or eclogite residue in equilibrium with hydrous siliceous melt and a CO2-rich fluid. Representative slab geotherms traverse the residual carbonate stability field, under both sub- and super-solidus (silicate solidus) conditions. Examination of compositions of co-existing phases in the experiments reveal PT-sensitive garnet-carbonate exchange equilibria which could, with further experimentation, be developed as geothermometers or barometers applicable to carbonate-bearing eclogites.