Coupled lithium- and iron isotope fractionation during magmatic differentiation

Abstract

In this study, we investigated Fe and Li isotope fractionation between mineral separates of olivine pheno- and xenocrysts (including one clinopyroxyene phenocryst) and their basaltic hosts. Samples were collected from the Canary Islands (Teneriffa, La Palma) and some German volcanic regions (Vogelsberg, Westerwald and Hegau). All investigated bulk samples fall in a tight range of Li and Fe isotope compositions (δ56Fewr=0.06–0.17‰ and δ7Lima=2.5–5.2‰, assuming δ7Li of the olivine-free matrix is virtually identical to that of the bulk sample for mass balance reasons). In contrast, olivine phenocrysts display highly variable, but generally light Fe and mostly light Li isotope compositions compared to their respective olivine-free basaltic matrix, which was considered to represent the melt (with δ56Feol=−0.24 to 0.14‰ and δ7Liol=−10.5 to +6.5‰, respectively). Single olivine crystals from one sample display even a larger range of δ56Feol between −0.7 and +0.1‰. One single clinopyroxene phenocryst displays the lightest Li isotope composition (δ7Licpx=−17.7‰), but no Fe isotope fractionation relative to melt. The olivine phenocrysts show variable Mg# and Ni (correlated in most cases) that range between 0.89 and 0.74 and between 300 and 3000μg/g, respectively. These olivines likely grew by fractional crystallization in an evolving magma. One sample from the Vogelsberg volcano contained olivine xenocrysts (Mg#>0.89 and Ni>3000μg/g), in addition to olivine phenocrysts. This sample displays the highest Li- and the second highest Fe-isotope fractionation between olivine and melt (Δ7Liol-melt=−13; Δ56Feol-melt=−0.29).Our data, i.e. the variable olivine- at constant whole rock and matrix isotope compositions, strongly indicate disequilibrium, i.e. kinetic Fe and Li isotope fractionation between olivine and melt (for Li also between cpx and melt) during fractional crystallization. Δ7Liol-melt is correlated with the Li partitioning between olivine and melt (i.e. with Liol/Limelt), indicating Li isotope fractionation due to preferential (faster) diffusion of 6Li into olivine during fractional crystallization. Olivine with low Δ7Liol-melt, also have low Δ56Feol-melt, indicating that Fe isotope fractionation is also driven by diffusion of isotopically light Fe into olivine, potentially, as Fe–Mg inter-diffusion. The lowest Δ56Feol-melt (−0.40) was observed in a sample from Westerwald (Germany) with abundant magnetite, indicating relatively oxidizing conditions during magma differentiation. This may have enhanced equilibrium Fe isotope fractionation between olivine and melt or fine dispersed magnetite in the basalt matrix may have shifted its Fe isotope composition towards higher δ56Fe. The decoupling of Li- and Fe isotope fractionation in cpx is likely due to faster diffusion of Li relative to Fe in cpx, implying that the large investigated cpx phenocryst resided in the magma for only a short period of time which was sufficient for Li- but not for Fe diffusion. The absence of any equilibrium Fe isotope fractionation between the investigated cpx phenocryst and its basaltic host may be related to the similar Fe3+/Fe2+ of cpx and melt. In contrast to cpx, the generally light Fe isotope composition of all investigated olivine separates implies the existence of equilibrium- (in addition to diffusion-driven) isotope fractionation between olivine and melt, on the order of 0.1‰.