Depletion and refertilisation of the lithospheric mantle below the Kapsiki plateau (Northern Cameroon Volcanic Line) deduced from trace element and H2O systematics in mantle xenoliths

Abstract

Clinopyroxene bearing spinel harzburgites recovered from Cenozoic alkaline basalts from the Kapsiki plateau were investigated with the aim of understanding depletion and enrichment processes in the subcontinental mantle below a major tectonic line. Concentrations of major and trace elements (including H) were determined in-situ in olivine, orthopyroxene, clinopyroxene and spinel by EPMA, LA-ICP-MS and FTIR. The xenoliths are refractory with high olivine (0.91–0.92) and orthopyroxene (0.92–0.93) Mg# coupled with relatively high modal proportions of clinopyroxene (5–8%) and a remarkably wide range in clinopyroxene Mg# (0.91–0.94), interpreted to reflect high degrees of partial melting followed by modal and chemical metasomatic refertilization. A fractional partial melting model of orthopyroxene trace elements in a depleted mantle indicates 20–30% of melting. Clinopyroxenes show two groups based on their CaO contents (18.7–18.9 and 20.6–22.6 wt %) suggesting a secondary origin and modal metasomatism. The low Ca clinopyroxenes were the latest to be formed, and the wide range of clinopyroxene Mg# shows a lack of equilibrium with the coexisting minerals. The high Ca clinopyroxenes record ancient metasomatic events that are also documented in orthopyroxene. Cryptic metasomatism is shown by strong progressive enrichments in LREEs and MREEs of orthopyroxene and clinopyroxene. Th vs La and Sr vs Nd of clinopyroxene are used to identify different generations of metasomatism. Ti/Eu vs (La/Yb)N and Zr/Hf systematics indicate that the metasomatising agent has both a carbonatite and a silicate component. The structural hydroxyl contents of the different mineral phases are very low (<1 ppm for olivine, 40–48 ppm for orthopyroxene and 164–277 ppm for clinopyroxene). The very low water contents compared to typical sub-continental peridotites could be attributed to the depleted nature of the xenoliths or reflect lower water activities of carbonatitic melts relative to alkaline melts. Estimated fO2 values expressed as ΔlogFMQ, (where FMQ corresponds to the fayalite magnetite-quartz oxygen buffer) range from -0.42 to -0.26 and are consistent with minor OH stretching regions of 3520 cm-1 in orthopyroxene, related to a ferric iron defect.The combination of textural investigations, major, and trace element and H2O data suggests that a refractory subcontinental mantle was affected by several episodes of intraplate metasomatism dominated by silicate-carbonatite melts above the North-Eastern end of the Cameroon Volcanic Line.