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 c rust through high PT conditions during subduction may involve loss of fluids and possible melting reactions but the behaviour of carbonate i s quite unknown. In order to constrain this behaviour, a series of fO( 2)-buffered piston-cylinder experiments were performed at pressures fr om 1.5 to 3.5 GPa and temperatures of 700-1000 degrees C. A compositio n representative of altered oceanic basalt (synthesised with excess H2 O at 2.0 GPa and 700 degrees C, producing a quartz + rutile-bearing ga rnet-amphibolite assemblage) was run with 10 wt% calcite. These experi ments demonstrate that carbonate remains as a refractory phase in ruti le 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 g reater 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-exis ting phases in the experiments reveal PT-sensitive garnet-carbonate ex change equilibria which could, with further experimentation, be develo ped as geothermometers or barometers applicable to carbonate-bearing e clogites.