Empirical and experimental constraints on Fe-Ti oxide-melt titanium isotope fractionation factors

The Titanium (Ti) isotope compositions of felsic rocks are heavier than their mafic counterparts, and alkaline magmas develop Ti compared to other magma series during magmatic differentiation. Both observations interpreted reflect the preferential sequestration light isotopes in Fe-Ti oxides, such as rutile, ilmenite titanomagnetite. However, interpretations so far rely on whole rock studies cogenetic samples detailed mechanics oxide-melt equilibrium composition is poorly constrained. To address this, we have measured co-existing oxides groundmass or silicate melt both natural lavas from contrasting tectonic settings (Heard Island Santorini), experimental run products (rutile-melt). All oxide phases consistently isotopically lighter respective host melt, with magnitude Δ49/47Tioxide-melt increasing rutile ilmenite, observed difference fractionation between rutile-melt experiments ilmenite-groundmass pairs primarily reflective small differences TiO bond length, being (Δ49/47Tiilmenite-melt extrapolated 1000 K = −0.600 ± 0.035‰) (Δ49/47Tirutile-melt at −0.404 0.099‰) due slightly longer bonds ilmenite. In contrast, variation Δ49/47Ti titanomagnetite-groundmass increases a function TiO2 content (increasing ulvöspinel component) titanomagnetite, Δ49/47Tititanomagnetite-melt values ranging −0.811 −1.451‰ Ti-rich titanomagnetite (21–23 wt.% TiO2; Usp66-73) Heard −0.673‰ −0.863 Santorini (14–15 Usp45-49). We interpret this result weaker distorted crystal lattice changes local cationic environment resulting exchange smaller Fe3+ ions larger Fe2+ magnetite-ulvöspinel solid solution. Our results consistent factors inferred isotopic analyses mineral separates ab-initio calculations. use these combination for minerals recalculated previous studies, along petrologic information model behaviour partial melting Earth’s mantle well reproducing δ49/47Ti differentiated distinct geodynamic settings.