Major and minor element characteristics of olivine crystals from the Rungwe Volcanic Province (RVP), Tanzania and their mantle source implications

Abstract

Whole rock and olivine geochemistry, together with high 3He/4He (>MORB=8 R/RA, where RA denotes 3He/4He of air) found in the East African Rift System (EARS) suggests the presence of an underlying deep mantle plume contributing to the magmatism throughout the rift. This thesis contains new major, minor and trace elements of olivine crystals from a suite of 17 mafic lava and tephra samples from six volcanic series within the Rungwe Volcanic Province (RVP), Tanzania. The olivine chemistry shows considerable variations in their elemental concentrations and elemental ratios. Olivine chemistry proxies were used to identify the lithology of the source supplying magmas to the RVP and estimating the degrees of crustal contamination, partial melting and crystallization temperatures affecting the source. Variations in Ni content in the olivines suggest that the source has likely undergone a series of mafic recharge events and subsequent mixing. High Ni and low Mn found in the Ngozi -younger extrusives series, however, suggests the presence of pyroxenite in the source, which is attributed to a lithologically heterogenous (hybrid) source. Furthermore, our findings were compared to the whole rock chemistry, isotopic ratios and helium isotope systematics of the samples of Castillo et al (2014) and Hilton et al (2011). These findings concur with recent hypotheses where a primordial deeply sourced plume, characterized by high 3He/4He, is present underneath the RVP. Additionally, this hybrid plume component mixed with an old lithospheric component associated with the Tanzania Craton could result in the wide variations observed in the RVP lava chemistry.