Cr-rich magnesiokatophorite as an indicator of mantle metasomatism by hydrous Na-rich carbonatite

Abstract

Na- and Cr-rich amphibole and clinopyroxene are commonly observed in hydrous mantle xenoliths hosted by Pliocene-Quaternary basalts from Yemen. The amphibole is Cr-rich magnesiokatophorite high in Na₂O (3.66-5.65 wt%) and Cr₂O₃ (2.21-3.09 wt%), relatively low in K₂O (0.23-0.66 wt %) and almost free of Cl and F. The clinopyroxene is characterized by high Cr₂O₃ (up to 5.64 wt%) and Na₂O (up to 2.97 wt%), indicating appreciable amounts of kosmochlor. Both minerals show high REE concentrations as well as HFSE depletion, suggesting an involvement of carbonatitic melt in their formation. The magnesiokatophorites and kosmochlor-bearing diopside possibly formed through metasomatism by hydrous Na-rich carbonatite melt. Magnesiokatophorite is an indicator of metasomatism by hydrous Na-rich carbonatite in the upper mantle.