Process-related isotope variability in oceanic basalts revealed by high-precision Sr isotope ratios in olivine-hosted melt inclusions

Genske F, Stracke A, Berndt J, Klemme S

Abstract

Melting of the proto-Iceland Plume produced the intra-plate lavas of the Faroe Islands. The lava series of theMalinstíndur Formation contains picrite flows which have accumulated up to 50% olivine pheno- and antecrysts.Whole-rocks, minerals and olivine-hosted melt inclusions (MI) from this formation, in addition to other Faroe Islandlavas, were analysed for their major-trace element and Sr isotope composition. In comparison to other lavasfrom the Faroe Islands, the depleted incompatible trace-element composition of the population of inclusions investigatedhere is equivalent to that of its host-lava, and thus minorly influenced by post-entrapment crystallisation(PEC). High-precision Sr isotope ratios of silicate melt inclusions in olivine, binned by their Fo contents(Fo⁠93–78, in 1mol% Fo bins), reveal large variations in ⁠87Sr/⁠86Sr within a single lava flow. Both the ⁠87Sr/⁠86Sr andtheir variability increase with decreasing Fo content, suggesting progressive incorporation of different componentsduring magma evolution, that is, concurrent mixing and crystallisation. The total variation in Sr isotoperatios in the MI population covers about 60% of the variation recorded in the erupted lavas of the entire Faroe Islandarchipelago, but is slightly biased towards more radiogenic ⁠87Sr/⁠86Sr. The variation of Sr isotope ratios andincompatible trace element concentrations is best explained by mixing-in of melts with different trace elementand isotope compositions. Clinopyroxene barometry indicates that mixing of melts likely occurred at pressureshigher than 0.6GPa, in sub-MOHO magma conduits and sills. Hence the chemical and isotopic variability of themelt inclusions is influenced by magmatic processes during melt evolution, which must therefore also influencehow source heterogeneity is reflected in the composition of the erupted basalts