Ni partitioning between olivine and high-MgO silicate melts : Implications for Ni contents of forsteritic phenocrysts in basalts

Komatiites are volcanic ultramafic rocks with high MgO content (>18Wt.%), which make up the first few blocks of the Archean crust and are indicative of the earliest stages of the Earth mantle dynamics. The present study reports high-precision MC-ICP-MS measurements of Zn isotopic compositions in whole rocks and mineral separates from Fred’s Flow and Theo’s Flow, two thick Archean differenciated flows (2.7 Ga) located in the Abitibi greenstone belt (Munro Township, Canada). Fred’s Flow has a komatiitic affinity, and is classified as Al-undepleted type, whereas Theo’s Flow has an Fe-rich tholeiitic affinity and is classified as Aldepleted type. Their geographical relationship as well as their complementary geochemical compositions suggest that they are genetically related. Characterizing fractionation processes of their Zn isotope compositions would provide constraints on diffusion transport, crystallisation processes, and melting conditions involved in the komatiites petrogenesis. Small but significant shift in δZn values for whole rocks is systematically observed between Fred’s flow (mean δZn = +0.30±0.04‰ (2SD)) and Theo’s flow profiles (mean δZn = +0.39±0.03‰). For each flow, the Zn isotopic profile is relatively monotonous, except for the gabbroic units with slightly lower δZn (down to +0.28±0.04‰) and the basal ultrabasic units where enrichments in heavy Zn isotopes are clearly obvious (δZn up to +0.55±0.05‰). Zn isotopic compositions in mineral separates vary on a large range of 1.6 δ-unit, and their relative modal contributions can explain the whole rock isotopic trends. As a whole, mineral separates reproduce the same isotopic shift between the two flows, as previously shown by the bulk rocks, suggesting a fractionation control by the crystallisation/melting conditions. Individually, chromites show especially strong enrichments in light Zn isotopes, relative to the silicates. Olivine, clinopyroxene and plagioclase exhibit generally smaller isotopic fractionation with respect to each other, but are characterized by contrasted δZn values suggesting correlation with the polymerization degree. No relationship has been observed between the Zn isotope fractionation and (light) alteration degree of the lava flow or mineral. Further analyses are needed to discriminate secondary Zn mobilization, crystallisation or cooling rate effects, and establish interesting comparison with Mg and Fe isotopic results reported by Dauphas et al. (2010) on Alexo komatiites.