Variations in the H2O Content and H2O/Ce Ratio of Mantle Pyroxenites: Implications for Enriched Components in the Mantle

Mantle pyroxenite xenoliths were collected from Hannuoba, East China, and investigated for their H2O content and trace element contents, with particular regard to the H2O/Ce ratio. The pyroxenites were divided into three groups: Cr-pyroxenites, Al-pyroxenites, and garnet pyroxenites. The H2O contents and H2O/Ce ratios of the pyroxenites in this study showed much wider ranges than those previously reported for the whole rock pyroxenites. The Cr-pyroxenites had an H2O content ranging from 23 to 339 ppm and an H2O/Ce ratio between 32 and 212; the Al-pyroxenites had an H2O content ranging from 42 to 106 ppm and an H2O/Ce ratio between 1 and 22, and the garnet pyroxenites had an H2O content ranging from 37 to 69 ppm and an H2O/Ce ratio between 5 and 54. These systematic variations in the H2O contents and H2O/Ce ratios for the Hannuoba pyroxenites are associated with their origins. Both the H2O content and H2O/Ce ratio of the Al-pyroxenites were low compared to those of Hawaiian pyroxenite, enriched mantle, and depleted MORB mantle sources. The high H2O content and H2O/Ce ratio of the melt equilibrated with the Cr-pyroxenites indicate that a water-rich component was involved in its formation. Their large variations in the H2O contents and H2O/Ce ratios may indicate that pyroxenites are essential enriched components in the sources of continental basalts.