We report preliminary results of a systematic search for fluid/melt microinclusions in mantle minerals. “Dusty” garnets from xenolith XRV6 [1], a heavily metasomatised Type I eclogite from Roberts Victor mine, SA, carry many microinclusions (<1 μm). FTIR analyses of the “dusty” zones indicate the presence of molecular water in the inclusions and hydroxyl groups in the garnet. EPMA analysis begins with identification of shallow, subsurface inclusions in backscattered electron imaging and minimal expression in the secondary electron image. Once found, the inclusions (along with the host garnet) are analysed by wavelength dispersion x-ray spectrometry (WDS). Measurements of 136 microinclusions reveal their bulk composition. Compared to the host garnet, they are enriched in TiO2, FeO, CaO, Na2O and K2O and depleted in Al2O3 and MgO. The silica contents seem to be similar to that of the host garnet. Most of the elements form compositional mixing arrays of microinclusion+garnet. Since the proportion of the garnet component is unknown, the composition of the microinclusions cannot be derived directly. The arrays trend away from the compositions of large secondary melt pools or secondary minerals found in healed cracks the xenolith. They point towards the array of silicic to low-Mg carbonatitic high density fluids (HDFs) trapped in diamonds and we derive their composition by intersecting the garnet-inclusions mixing lines with the corresponding arrays of the diamond HDFs. The results indicate close proximity to the fluids known from the diamonds but with lower content of K2O, indicating the important role of HDFs in mantle metasomatism.
[1] Gre´au et al. (2011) GCA 75, 6927–6954.
