First-row transition elements in pyroxenites and peridotites: A promising tool for constraining mantle source mineralogy

Abstract

Mantle heterogeneity has a first-order control on the petrological and geochemical differences of erupted mafic lavas worldwide. Because of their contrasted distributions between mantle phases, First Row Transition Elements (FRTEs) have been considered potential lithological tracers. Using a combination of published data on natural and experimental samples and new high-precision analyses on a variety of pyroxenite samples, we investigate the parameters that control FRTE partition (Ds) and exchange (KDs) coefficients between common mantle minerals. We demonstrate that mineral-clinopyroxene exchange coefficients are independent of composition and temperature and that coefficients obtained from natural samples can be accurate as long a sufficiently high number of compositionally diverse samples are considered, making them reliable input parameters in mantle melting models. As a proof of concept, we use the exchange coefficients determined from natural mantle lithologies in this study, along with published experimental clinopyroxene/melt partitioning coefficients, to perform simple inverse modeling on two basalt suites from the Western Volcanic Zone in Iceland and Samoa, selected for their contrasted Mn/Fe and Zn/Fe ratios. Our results show that a given FRTE ratio in basalt can be explained by a large range of modal proportions in the source. However, when combined, FRTE ratios become a powerful tool to constrain the nature of the source.