SUBDUCTED SEDIMENTS RECORDED BY MINERAL INCLUSIONS IN KOIDU DIAMONDS FROM THE WEST AFRICAN CRATON

The Koidu kimberlite complex on the West African Craton is one of the few mined kimberlites worldwide where eclogite is the exclusive mantle xenolith suite. Previous studies on Koidu mainly focused on the eclogite xenoliths, but little is known regarding its diamonds. To understand the formation and evolution of eclogites beneath Koidu, we studied 81 diamonds containing eclogitic mineral inclusions that were sampled from the Koidu kimberlites. We compare the compositions of the eclogitic mineral inclusions from these diamonds with those in Koidu eclogite xenoliths.

Koidu eclogitic diamonds contain an unusually high abundance of kyanite (22% of 81 diamonds) and coesite (54%) inclusions. Worldwide, only a small proportion of inclusion-bearing diamonds contain these minerals. The high abundance of kyanite inclusions indicates highly aluminous eclogitic diamond substrates. Kyanite in mantle eclogite typically forms through prograde metamorphism of plagioclase-rich oceanic crust during subduction. Consequently, garnets in kyanite-bearing eclogites tend to have positive Eu-anomalies, high CaO contents, and mantle-like or lower δ¹⁸O values (e.g., ≤ 5.5±0.4 ‰) associated with high-temperature seawater alteration of oceanic crust. However, the coexisting garnet inclusions in kyanite-bearing diamonds from Koidu have low CaO contents (≤ 6.6 wt%), high δ¹⁸O (> 11 ‰), and lack positive Eu-anomalies. These elemental and isotopic signatures indicate that the garnet inclusions do not have plagioclase-rich oceanic protoliths. Instead, high jadeite (~70 mol% NaAlSi₂O₆) in omphacite inclusions in Koidu diamonds is within the range of omphacite in modeled metapelites at pressures within the diamond-stability field. Thus the high abundances of kyanite and coesite inclusions can be explained by the subduction of a mixture of sea floor sediment (with elevated δ¹⁸O values) and igneous oceanic crust.

Eclogite xenoliths from Koidu do not have abundant kyanite or coesite, and the jadeite components of the xenolith omphacites are also significantly lower than in omphacite inclusions in Koidu diamonds. The significant compositional differences between eclogite xenoliths and the eclogitic mineral inclusions in diamond suggest that the eclogites beneath Koidu have at least two distinct protoliths, with the latter containing a crustal sediment component.