—Oxygen isotope, mineral trace element, and measured and reconstructed whole rock compositions are reported for low MgO (6–13 wt.% MgO in the whole rock) eclogite xenoliths from the Koidu kimberlite complex, Sierra Leone. The δ18O values of garnet (4.7–6.8‰), determined by laser fluorination on clean mineral separates, extend beyond the range for mantle peridotites. All low MgO eclogites have reconstructed trace element patterns that are depleted in Ba, Th, U, and light rare earth element (LREE), with jadeite-rich samples having more variable trace element patterns than jadeite-poor samples. These observations, coupled with low SiO2 contents, and Nb-rich but LREE-depleted reconstructed whole rock compositions, suggest the low MgO eclogites are remnants of altered oceanic crust that was partially melted during subduction. Partial melting of a mafic protolith at high pressure (leaving a garnet-bearing residue) is the preferred model to explain the origin of Archean tonalite-trondhjemite-granodiorite (TTG) suites, which make up large portions of the crust in Archean cratons. We therefore suggest that the Koidu low MgO eclogites may be residues from Archean continental crust formation.