Trace element behaviour in soils developed along the slopes of Mt. Cameroon, West Africa

A geochemical study of trace elements in soil profiles developed on basaltic parent materials of unknown ages along the slopes of Mt. Cameroon was carried out. The study objectives included vertical description of element dispersion, element mobility and relating element distribution to weathering intensity. Twenty five soil samples from 6 sites (between 30 to 1017 m a.s.l.) were collected and analysed for trace element concentrations by ICP-MS technique. The weathering status of these soils, (derived from chemical weathering indices) indicate that the most weathered profiles are located at higher elevations with lower mean annual precipitation. Higher losses in Cu, Co and Zr in the moderately weathered profiles in the zones of high leaching are linked to the comparatively higher dissolution of Fe-bearing minerals such as magnetite to which resulted in higher Fed/FeT ratios. Organic matter may account for low mobility of Cu and Pb, while for Zn, the Mn-Al system may control Zn retardation at low elevations. Ni and Sr were highly mobilised at all elevations while Pb, Rb and Sr showed evidence of atmospheric deposition. Comparatively higher inputs of dust-derived minerals in the lower elevation could also affect the distribution of TE. Suggestions are therefore made for the existence of an inflection point within this elevational bracket marked by incongruent dissolution of primary silicate minerals (particularly Feminerals)

Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line

Debunscha Maar is a monogenetic volcano forming part of the Mt. Cameroon volcanic field, located within the Cameroon Volcanic Line (CVL). Partly glassy cauliflower bombs have primitive basanite-picrobasalt compositions and contain abundant normally and reversely zoned olivine (Fo 77–87) and clinopyroxene phenocrysts. Naturally quenched melt inclusions in the most primitive olivine phenocrysts show compositions which, when corrected for post-entrapment modification, cover a wide range from basanite to alkali basalt (MgO 6.9–11.7 wt.%), and are generally more primitive than the matrix glasses (MgO 5.0–5.5 wt.%) and only partly fall on a common liquid line of descent with the bulk rock samples and matrix glasses. Melt inclusion trace element compositions lie on two distinct geochemical trends: one (towards high Ba/Nb) is thought to represent the effect of various proportions of anhydrous lherzolite and amphibole-bearing peridotite in the source, while the other (for example, high La/Y) reflects variable degrees of partial melting. Comparatively low fractionation-corrected CaO in the melt inclusions with the highest La/Y suggests minor involvement of a pyroxenite source component that is only visible at low degrees of melting. Most of the samples show elevated Gd/Yb, indicating up to 8% garnet in the source. The range of major and trace elements represented by the melt inclusions covers the complete geochemical range given by basalts from different volcanoes of the Cameroon volcanic line, indicating that geochemical signatures that were previously thought to be volcano-specific in fact are probably present under all volcanoes. Clinopyroxene-melt barometry strongly indicates repeated mixing of compositionally diverse melts within the upper mantle at 830 ± 170 MPa prior to eruption. Mantle potential temperatures estimated for the primitive melt inclusions suggest that the thermal influence of a mantle plume is not required to explain the magma petrogenesis