The magnesium isotopic composition of the mantle

In order to better constrain the Mg isotopic composition of the mantle, we have analysed twenty-eight samples of both oceanic and continental peridotite using a high-precision, critical mixture double spiking approach. The unaltered samples show no variability ?Mg in outside analytical uncertainty and yield a value of -0.236 ± 0.006? (2 s.e.) for the accessible mantle, substantiating its non-chondritic composition. We have also determined inter-mineral Mg isotopic fractionations for a sub-set of samples. We document small but significant differences in ?Mg between olivine and pyroxenes, ?Mg = -0.118 ± 0.018? and ?Mg = -0.056 ± 0.018?, in excellent agreement with ab initio calculations for temperatures ~1000 °C, as recorded by mineral thermometry in the peridotites. The differences in ?Mg between olivine and spinel (?Mg ) are more variable and generally higher than theoretical calculations at corresponding temperatures, likely due to incomplete Fe-Mg diffusive exchange during post-eruptive cooling of the xenoliths. Using these data, together with a recently determined olivine-melt fractionation factor for Mg isotopes, we show that partial melting has a negligible influence on the ?Mg of residual peridotites. This helps account for the minimal variability of ?Mg in fresh, mantle peridotites. However, the ?Mg of primary mantle melts are predicted to be discernibly higher than their sources (?Mg ~ 0.06? and ~0.123? for representative partial melts of peridotitic and pyroxenitic sources respectively) across a wide range of melting conditions. Such elevated ?Mg values are not generally observed in the current dataset of mantle derived melts. We propose that this inconsistency is likely a consequence of diffusive fractionation during partial re-equilibration between low Mg/Fe melts migrating through high Mg/Fe mantle en route to the surface.