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 & delta;26Mg in outside analytical uncertainty and yield a value of -0.236 & PLUSMN; 0.006%o (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 & delta;26Mg between olivine and pyroxenes, & UDelta;26/24Mg ol/cpx = -0.118 & PLUSMN; 0.018%o and & UDelta;26/24Mg ol/opx = -0.056 & PLUSMN; 0.018%o, in excellent agreement with ab initio calculations for temperatures -1000 degrees C, as recorded by mineral thermometry in the peridotites. The differences in & delta;26Mg between olivine and spinel (& UDelta;26/24Mg ol/sp) 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 & delta;26Mg of residual peridotites. This helps account for the minimal variability of & delta;26Mg in fresh, mantle peridotites. However, the & delta;26Mg of primary mantle melts are predicted to be discernibly higher than their sources (& UDelta;26Mg - 0.06%o and -0.123%o for representative partial melts of peridotitic and pyroxenitic sources respectively) across a wide range of melting conditions. Such elevated & delta;26Mg 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.