Resolving mantle and magmatic processes in basalts from the Cameroon volcanic line using the Re–Os isotope system

This study presents major-, trace element and Re–Os isotope and elemental data for young alkaline basalts (20) possess 187Os/188Os isotope compositions between 0.14 and 0.18 (e.g., basalts from Mt Cameroon and Sao Tomé) which reflect the chemical characteristics that are more likely to be primary features of CVL, and are close to the value of 0.153 attributed to the HIMU end-member (Tubuai–Mangaia). However, most of the lavas from the continental sector show highly radiogenic initial 187Os/188Os ratios (0.36 to 0.56) that are outside the range previously observed for ocean island basalts, with shifts to radiogenic Os isotope compositions accompanied by less radiogenic 206Pb/204Pb and increasing SiO2 contents. The increase in 187Os/188Os is also associated with the decrease of Os, Ni, MgO and phenocryst abundances. These data can be explained by fractional crystallisation and assimilation of continental crust by the ascending magma. The systematic shift to unradiogenic lead isotope compositions from the COB into the oceanic sector is positively correlated with variations in 187Os/188Os isotope composition (from 0.140 to 0.128). At first sight this covariation might be attributed to the mixing of HIMU material with the ambient upper mantle (DMM). However, there is a clear covariation of the Os isotope and elemental composition, best explained with contamination of the oceanic basalts by the physical entrainment of xenoliths and xenocrysts of mantle origin. Overall, these results indicate that Os in CVL basalts is highly susceptible to contamination from both oceanic and continental lithospheres, under these circumstances covariations with other isotopes and elements must be interpreted with caution. •Re and Os elemental and isotope data in Cameroon volcanic line basalts•Evidence of fractional crystallisation–assimilation of continental crust in continental sect