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 δ26Mg varying from –2.15‰ to –0.46‰ in garnets and from –0.49‰ to +0.35‰ in clinopyroxenes. Bulk δ26Mg values (–1.38‰ to +0.05‰), constructed from garnet and clinopyroxene data, are similar to results from rock powders (–1.60‰ to +0.17‰), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The δ26Mg values of low-MgO eclogites (–0.80‰ to +0.05‰) exceed the range of mantle peridotite xenoliths (–0.25‰ ± 0.04‰), consistent with the eclogite’s derivation from recycled altered oceanic crust. Similarly variable δ26Mg values in high-MgO eclogites (–0.95‰ to –0.13‰), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO 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.