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

摘要

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.