We determined equilibrium Mg isotope fractionation between olivine and melt (Δ 26/24 Mg Ol/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 Δ 26/24 Mg Ol/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 Δ 26/24 Mg Ol/melt = (−1.46 ± 0.26) × 10 5 /T 2 . The samples analysed have variable H 2 O content from 0.1 to ∼1.2 wt. %, yet no discernible difference in Δ 26/24 Mg Ol/melt was evident. We have used this Δ 26/24 Mg Ol/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 Δ 26/24 Mg Ol/melt .

In Figure S-6 in the Supplementary Information, the δ 26 Mg value of BHVO-2 was mislabelled as the δ 25 Mg value of BHVO-2 (−0.103 ± 0.014 ‰, 2 s.d.). The rest of the information in Figure S-6 is correct, including the figure caption. The correct text should be "BHVO-2: −0.202 ± 0.027 ‰ (2 s.d.)". The figure has now been corrected in the Supplementary Information, and the corrected figure is shown below.