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

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