T164 Gem Minerals, Localities and Geological Significance

Tuesday, 24 September 2024
8:00 AM - 12:00 PM
207A

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

Paper number: T164-11
Program: Technical Sessions
Time: 10:25 AM – 10:40 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-402387

© Copyright 2024 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.