Clinopyroxene and olivine-hosted spinel inclusions from the Mt Cameroon volcanic area, west-central Africa

Abstract

Compositions of clinopyroxenes and spinels from the ocean/continent boundary of the Cameroon Line are presented. The spinels occur as minute inclusions in olivine phenocrysts from alkali basaltic bombs collected from the Debunscha maar deposit at the foot of Mt Cameroon. The inclusions are mostly subhedral to euhedral in shape, measuring generally less than 70 μm in diameter. The studied spinels are chemically characterized by: Fe2+/(Fe2++Mg) = 0.47–0.64, Fe3+/(Fe3++Al+Cr) = 0.13–0.18, Cr# = Cr/(Cr+Al) = 0.47–0.65, and Mg# 43.69–56.94. The lack of a systematic correlation between these spinels’ composition and their positions within the host olivine phenocrysts, is in line with the quenching nature of the rocks. The compositions of these spinels show that they are chromites. Comparisons of these chromites with those from other settings show that the Debunscha chromites have Fe3+/(Fe3++Al+Cr) (0.13–0.23) and TiO2 (1.96–5.63 wt %) higher than chromites from most lavas (Fe3#: 0.01–0.16, TiO2: 0.37–2.14). The clinopyroxenes have the following major element compositions SiO2 (45.97–51.62 wt %), MgO (13.84–16.45 wt %), CaO (21.16–23.52 wt %), Al2O3 (3.25–7.45 wt %), FeO (5.50–8.17 wt %), MnO (0.09–0.16 wt %), TiO2 (0.99–2.88 wt %) including Mg# in the range 83.8–96.3 and plot in the diopside-augite boundary on the conventional Wo-En-Fs diagram. These clinopyroxenes are, variably enriched in trace elements. The concentrations of Co are low and are in the range 32.75–42.38 ppm while Cr and Ni concentrations are high and in the 80.60–3557.19 ppm and 38.55–228.62 ppm respectively. Many other important trace elements, for instance, the rare earth elements, Sc, Zr, Sr, Nb, Ba also show variations in the studied clinopyroxenes. The clinopyroxenes equally show very high Zr/Nb ratios (104–708), and plot on the depleted mantle domain in the Zr/Y–Nb/Y graphs. However, primitive mantle normalized trace element patterns for all the clinopyroxene samples are similar with noticeable depletions in Sr and Ba, which is also suggestive of a depleted mantle source. This similarity is an indication that all the samples are genetically related, and have crystallized from petrogenetically related melts as part of fractional crystallization and mixing sequences.