Silicic microinclusions in a metasomatized eclogite from Roberts Victor mine, South Africa

We report results of a systematic search for fluid/melt microinclusions (<1 μm) in “dusty” garnets from a heavily metasomatized Type-I kyanite-bearing eclogite from the Roberts Victor mine, South Africa. FTIR analyses of “dusty” zones indicate the presence of molecular water, CO2 and carbonate in the inclusions and of hydroxyl groups in the garnet. EPMA analysis of 136 microinclusions reveals the bulk composition of the microinclusions. Relative to the host garnet, they are enriched in TiO2, FeO, CaO, Na2O and K2O and depleted in SiO2, Al2O3 and MgO. Most of the elements form compositional mixing arrays of microinclusion + clear garnet. The arrays trend away from the compositions of large melt pools or secondary minerals found in the xenolith. They point, with some notable differences, towards the array of silicic to low-Mg carbonatitic high-density fluids trapped in diamonds. The main differences are the presence of molecular CO2 in addition to carbonate, the lower concentration of incompatible major constituents such as water, K2O and TiO2 and higher levels of Al2O3 and FeO. Still, the resemblance is close, indicating the role of diamond forming fluids in mantle metasomatism in general. The trapped melts are secondary and penetrated late in the history of the eclogite. They did not take part in the main metamorphic or metasomatic reactions, however they can account for introduction of Al2O3, FeO and incompatible trace elements into the ecologies.