Characterizing fluid microinclusions in eclogite from Roberts victor RSA

We report 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 "dusty" zones indicate the presence of molecular water and carbonate in the inclusions and hydroxyl groups in the garnet. EPMA measurements of 136 microinclusions reveal the bulk composition of the microinclusions. Compared to the host garnet, they are enriched in TiO_2, FeO, CaO, Na₂O and K₂O and depleted in Al₂O₃ and MgO while the silica contents seem to be similar to that of the host garnet.

Most of the elements form compositional mixing arrays of microinclusion+garnet. The arrays trend away from the compositions of large melt pools or secondary minerals found in the xenolith. They point towards the array of silicic to low-Mg carbonatitic high density fluids (HDFs) trapped in diamonds, with ~40% SiO₂, ~6% MgO and ~10% CaO. K₂O (~4%) and Na₂O (~1%) are lower compared to the diamond HDFs while the FeO (~ 20%) and Al₂O₃ (~17%) are higher (all oxides are on a H₂O and CO₂ free basis). Such compositions may be the result of melting of eclogites or subducted sediments. The HDFs trapped in the garnet are enriched enough to play a role in mantle metasomatism.

[1] Gréau et al. (2011) GCA 75, 6927-6954.