First Row Transition Metals in Olivine – Petrogenetic Tracers for the Evolution of Mantle-Derived Magmas
Abstract:Olivine is the most abundant mineral in the upper mantle and a major constituent of most mantle-derived rocks. However, despite its abundance, studies on the trace element chemistry of olivine are underrepresented in the literature.
We present the results of a comprehensive study on the contents of first-row transition metals (FRTM: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), Ga and Ge in olivines from 2.7-3.5 Ga old Munro- and Barberton-type komatiites from the Kapvaal and Zimbabwe Cratons in southern Africa, the Yilgarn Craton in Australia, and the Superior Craton in Canada. Komatiitic olivines are compared to olivines from a Devonian-Carboniferous mantle peridotite (Finero, Italy) and contemporary ocean Island basalts (OIB, from St. Helena, South Atlantic Ocean).
The olivine major element chemistry was determined using a JEOL JXA-8900 Superprobe at the University of Maryland. Trace element contents were determined using a Photon Machines Analyte G2 193 nm Excimer laser ablation system coupled to a Nu Instruments AttoM single collector ICP-MS at NASA Goddard Space Flight Center. Medium resolution mass discrimination (m/Δm = 2500, at 5% peak intensity) was leveraged to separate isobaric interferences and support accurate quantitation of elemental abundances.
The results show that olivines from komatiites are largely depleted in FRTM, Ga and Ge relative to the composition of the primitive mantle (FRTMPM-norm = ~0.01 - 1). All komatiitic olivines have similar mantle-normalized trace element patterns, regardless of age and/or locality. Olivines from the Finero mantle peridotite and the St. Helena OIB are similarly depleted. However, compared to komatiites, grains from Finero are enriched in Ge and distinctly depleted in Ti, V, Cr, and Ga, whereas olivines from St. Helena have overall flatter normalized trace element patterns. The distinct patterns show that olivine chemistry can be used to identify and understand the source and evolution of mantle-derived rocks. Furthermore, variable redox sensitive element ratios (e.g. V, Sc), and chalcophile element abundances (e.g. Cu) indicate a potential use of olivine FTRM chemistry to constrain mantle redox conditions and the sulfur saturation history of a magma.