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.