Mantle eclogites: evidence of igneous fractionation in the mantle

Abstract

Despite recent geochemical studies advocating an origin of eclogite xenoliths in kimberlite by subduction of altered sea-floor basalts and crustal cumulates, there are numerous examples showing abundant exsolution of garnet and kyanite from clinopyroxene that could only have occurred on cooling from near-solidus temperatures at pressures in excess of 3 GPa. Samples showing extensive exsolution tend to have more calcic garnets and sodic pyroxenes than the average eclogite, but do not form a group that is geochemically distinct in major, minor, or trace elements. Also, composite samples containing both kyanite and bimineralic eclogites demonstrate the co-genetic relation of extremes of composition of the eclogite suite. Accordingly, we present an igneous fractionation model to account for the major and trace element variations observed in these rocks: grospydites and kyanite eclogites are early pyroxene, or pyroxeneplus-corundum, cumulates that have undergone extensive subsolidus exsolution of kyanite and garnet. These were then followed by more Fe-rich eclogites which had pyroxene plus garnet as solidus phases and fractionated towards more magnesian bimineralic eclogites.