Lithological and compositional diversity of diamond substrates beneath the Koidu kimberlite reveals addition of subducted sediments

Abstract

Six diamond-bearing eclogite xenoliths with oceanic crust protoliths and 370 mineral inclusions in 104 diamonds recovered from the Koidu kimberlite complex in Sierra Leone provide insight into the lithological and compositional diversity of the lithospheric mantle beneath the West African Craton. Diamond formation beneath Koidu is predominantly associated with eclogitic substrates that originated from subduction and high-pressure metamorphism of oceanic crust, as indicated by a dominance of eclogitic (78%) over peridotitic (17%) and mixed paragenesis diamonds (5%). Peridotitic diamonds contain olivine inclusions with very high Mg# (92.2–94.7; median = 94.2), indicative of derivation from dunite or harzburgite protoliths. Moreover, a peridotitic spinel with Cr# = 50.9 suggests that it equilibrated with orthopyroxene-free dunite. 44% of Koidu diamonds contain coesite, of which some coexist with omphacite, eclogitic garnet, and/or kyanite. Most analysed eclogitic garnet inclusions have extremely high δ¹⁸O values ( ≥ + 9.9‰) and occur with clinopyroxene inclusions that have very high jadeite components (~ 70 mol%). These high jadeite components are a close match to clinopyroxenes in high-pressure metapelites, which have a phase assemblage that includes coesite and kyanite. Our data suggest that the eclogitic mineral inclusions in most Koidu diamonds have oceanic basalt protoliths that were mingled with pelagic sediments, which may have increased δ¹⁸O values to levels much higher than observed for other eclogites at Koidu and shifted the originally basaltic bulk compositions closer to that of pelites. Most eclogitic mineral inclusions in Koidu diamonds have elemental compositions not observed for Koidu eclogite xenoliths, which have clear oceanic crust protolith (oceanic lavas and cumulates) signatures without significant crustal sediment contamination. These findings suggest the subduction of distinct packages of oceanic crust into the Koidu lithospheric mantle through time.