Fluid-induced transition from Kyanite- to bimineralic Eclogite: A petrological, geochemical and mass balance approach to mantle eclogites

Abstract:

In this study, we present the phase transition from gabbro into granulite and finally into kyanite- bearing and bimineralic eclogite. The investigated rock sample is a heterogeneous kyanite- bearing and bimineralic eclogite from the Earth’s mantle collected at the Roberts Victor Diamond mine in South Africa. Plagioclase of the former granulite reacted completely out under low H₂O activity (fH₂O) to form this kyanite- bearing and bimineralic eclogite. To quantify the phase transitions of the original gabbroic precursor, which was first metamorphosed under H-T granulite facies conditions followed by metamorphism under Earth’s mantle conditions into both types of eclogite, a petrological, geochemical and a mass balance approach has been made. i) The results from our petrological approach show that Ca-rich garnet, which is coexistent with Ca-rich omphacite are the metastable phases from the original granulite in the kyanite-bearing relict while Mg-rich garnet, coexistent with Na-rich omphacite are the stable phases in the bimineralic eclogite part which shows equilibration conditions of ~5.5 Gpa and ~1200°C. ii) Our geochemical results show a positive Eu anomaly in garnet from the kyanite-bearing part, which indicates that the igneous precursor of the granulite was a gabbro, probably oceanic crust. Most of the rare earth elements show an excellent correlation with the major elements of the rock forming minerals during the plagioclase-out reaction of the former granulite. The LREE in garnet are removed during the formation of the bimineralic eclogite due to loss of the anorthite component in plagioclase of the former granulite. Contrary, the HREE are enriched in garnets in the bimineralic part of the eclogite compared to those in the kyanite zone, and correlate with the Mg-Ca exchange between both garnet generations. iii) Garnet d18O values ranging between ~5.27 and ~5.90 in the kyanite part and systematically decrease to values of ~5.81 to ~6.29 in the bimoneralic part. The difference in the d18O values is indicative for two different fluid pulses. iv) The following kyanite bearing and bimineralic eclogite forming reaction from the original granulite was calculated from our mass balance approach: 0.625 Cpx + 0.45 Plag + 0.925 Grs + 0.89 MgO + 1.15 CO₂ = 1 Pyp + 1 Omph + 0.04 Ky + 1.15 Cc.