Systematic regional variations in diamond carbon isotopic composition and inclusion chemistry beneath the Orapa kimberlite cluster, in Botswana

摘要

The carbon isotepic composition, nitrogen content and aggregation state of 95 inclusion bearing diamonds, collected from the production of Damtshaa kimberlites, were investigated. The Damtshaa kimberlites belong to the group of southern African kimberlites showing ~(13)C enrichment in eclogitic compared to peridotitic: diamonds. The results were compared with similar studies of the nearby Orapa and Letlhakane kimberlites, providing the opportunity to examine variations in mantle composition on the kilometer scale. While Orapa,; Letlhakane and Damtshaa diamonds may share some overall similarities in their δ~(13)C distribution, there are significant differences among them. These differences support the concept that diamonds from different kimberlite pipes have distinctive features in their δ~(13)C distribution and that this is true for kimberlite occurrences which are spatially separated by only a few tens of km. Compared to Orapa, peridotitic garnet inclusions from Damtshaa and Letlhakane are derived, in part, from highly depleted dunitic-harzburgitic reservoirs extending to greater depth. In all three kimberlites diamonds with eclogitic garnet inclusions, displaying comparatively low Al2O3/Cr2O3 ratios, characteristic for mantle compositions, show a wide range in carbon isctopic composition from about -4 to -18 ‰vs. PDB. The observed relationship between δ~(13)C and Al2O3/Cr2O3 ratios indicates the presence of ~(13)C depleted mantle sources that cannot be explained by subduction. Estimates of possible pressure/temperature conditions, based on the Fe/Mg distribution coefficients of coexisting eclogitic garnets and clinopyroxenes and the Ca mole fraction in garnets, lead to the conclusion that beneath the Orapa, Letlhakane, and Damtshaa kimberlites there is a mantle zone depleted in (13)C, approximately 20-30 km thick and extending over an area of at least 200 km~2.