Equilibrium olivine-melt Mg isotopic fractionation explains high δ26Mg values in arc lavas

Abstract

We determined equilibrium Mg isotope fractionation between olivine and melt (Δ26/24MgOl/melt) in five, naturally quenched, olivine-glass pairs that were selected to show clear textural and chemical evidence of equilibration. We employed a high-precision, critical mixture double-spiking approach to obtain a weighted mean of Δ26/24MgOl/melt = -0.071 ± 0.010 , for values corrected to a common olivineglass temperature of 1438 K. As function of temperature, the fractionation can be expressed as Δ26/24MgOl/melt = (-1.46 ± 0.26) × 105/T2. The samples analysed have variable H2O content from 0.1 to ∼1.2 wt. %, yet no discernible difference in Δ26/24MgOl/melt was evident. We have used this Δ26/24MgOl/melt to revisit the puzzling issue of elevated Mg isotope ratios in arc lavas. In new Mg isotope data on sample suites from the Lesser Antilles and Mariana arcs, we show that primitive samples have MORB-like Mg isotope ratios while the evolved samples tend to have isotopically heavier compositions. The magnitude of this variability is well explained by olivine fractionation during magmatic differentiation as calculated with our new equilibrium Δ26/24MgOl/melt.