Experimental study of the interaction between garnets of eclogitic and lherzolitic parageneses and H₂O-CO₂ fluid under the P-T parameters of the lithospheric mantle

Experimental modeling of the interaction between garnets of mantle parageneses with H2O-CO2 fluid was performed in order to determine the characteristic features and trends in garnet compositions due to metasomatic alterations, and to study the patterns of formation of carbon and carbon-bearing phases in these processes. Experiments were carried out in the lherzolitic garnet-CO2-H2O-C and eclogitic garnet-CO2-H2O-C systems at 6.3 GPa in the temperature range of 950-1550 degrees C using double graphite+platinum ampoule on a multi-anvil high-pressure apparatus of a split-sphere type (BARS). The interaction of mantle garnets with H2O-CO2 fluid was found to cause garnet recrystallization and formation of new phases, namely magnesian carbonate, coesite, kyanite, corundum-eskolaite solid solution and silicate-carbonate melt, as well as metastable graphite (950-1550 degrees C) and diamond growth on seeds (1250-1550 degrees C). It was established that the garnet-fluid interaction was accompanied by the bivalent cations' redistribution between recrystallized garnet, carbonate and carbonate-silicate melt. In these processes calcium was preferentially partitioned in the melt, while magnesium was partitioned in garnet and carbonate. Recrystallization of garnets of lherzolitic and eclogitic parageneses under effect of H2O-CO2 fluid resulted not only in compositional changes, but in the formation of carbonate, kyanite, coesite, graphite, corundum-eskolaite solid solution and carbonate-silicate melt inclusions in these garnets. The revealed patterns of variations in garnet compositions, as well as the typical inclusions in them, can be regarded as indicators of metasomatic alteration of garnets of mantle parageneses involving H2O-CO2 fluid. The obtained results allow one to infer that H2O-CO2 fluids play a crucial role in redistribution of divalent cations under the P-T parameters of the lithospheric mantle and control over the composition of carbonate-silicate melts coexisting with the eclogitic and lherzolitic assemblages. Another important result of this study is the experimental demonstration that graphite capsules on their own fail to ensure reliable hermeticity with respect to the fluid and melt phases in the high-pressure, high-temperature experiments. It was substantiated that the procedure involving a hermetically sealed platinum ampoule with inner graphite capsule prevents uncontrollable loss of matter in long-term petrological experiments and is optimal for modeling systems containing melts and fluids..