Olivine chemistry from Cameroon: evidence of carbonate metasomatism along the ocean-continental boundary of the Cameroon volcanic line

Abstract

We report olivine chemistry in basaltic rocks from the Mt. Cameroon Volcanic area, which is used as a proxy to understand mantle and early igneous processes along the ocean-continent segment of the Cameroon Volcanic Line. Most primitive olivine phenocrysts in this study have forsterite contents of 86–87.5%. Their Li concentrations are below 3.5 ppm and occur in similar concentrations in all the samples. Ni has a large range in concentration, with some values as low as 742 ppm and others as high as 2500 ppm. The concentrations of Ca are in the range 2666–3755 ppm and that of Mn in the range 1283–2358 ppm. Olivine Ca/Al ratios are higher than those from both MORB and some within plate settings (e.g., Hawaii). Bulk rocks have low SiO2 (39.3–40.4 wt%) and high CaO/Al2O3 (0.72–1.03). The high olivine Ca/Al ratios, low bulk rocks SiO2 contents, and high bulk rocks CaO/Al2O3 ratios require a peridotite mantle source modified by carbonate-rich fluids. The plot of bulk rocks SiO2 vs. CaO similarly indicates CO2- enrichment. This metasomatic reaction may be regarded as the main source of volcanic hazardous CO2 along the CVL; such as in lakes Nyos and Monoun in 1986 and 1984 respectively, where approximately 2000 human and countless animal deaths were recorded.