Magnesium isotope evidence for a recycled origin of cratonic eclogites

The Mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from the Koidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa, provide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxene in both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mg isotope partitioning, with delta Mg-26 varying from -2.15 parts per thousand to -0.46 parts per thousand in garnets and from -0.49 parts per thousand to + 0.35 parts per thousand in clinopyroxenes. Bulk delta Mg-26 values (-1.38 parts per thousand to + 0.05 parts per thousand), constructed from garnet and clinopyroxene data, are similar to results from rock powders (-1.60 parts per thousand to + 0.17 parts per thousand), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The delta Mg-26 values of low-MgO eclogites (-0.80 parts per thousand to + 0.05 parts per thousand) exceed the range of mantle peridotite xenoliths (-0.25 parts per thousand +/- 0.04 parts per thousand), consistent with the eclogite's derivation from recycled altered oceanic crust. Similarly variable delta Mg-26 values in high-MgO eclogites (-0.95 parts per thousand to -0.13 parts per thousand), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten lowMgO eclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneity of the mantle.