The involvement of diamond-forming fluids in the metasomatic ‘cocktail’ of kimberlite sources

Microinclusion-bearing diamonds offer the opportunity to investigate relationships between mantle metasomatism, diamond formation and kimberlite eruptions in intracratonic provinces. We have analyzed a suite of 7 microinclusion-bearing diamonds from the Finsch Group II kimberlite, South Africa, and identified two diamond populations: ‘Finsch IaA’ diamonds have nitrogen solely in A-centers and contain saline high-density-fluid (HDF) microinclusions, while ‘Finsch IaAB’ diamonds have nitrogen in both A- and B-centers (25–35% B-centers) and are characterized by carbonatite HDF compositions. Based on nitrogen aggregation states and estimates for mantle residence temperatures, we conclude that ‘Finsch IaA’ diamonds formed during a young saline metasomatic event that preceded kimberlite eruption by ~50 kyr to 15 Myr. The possible timing of metasomatism and formation of ‘Finsch IaAB’ diamonds by carbonatite HDFs is less constrained, and could have taken place between ~15 Myr and 2 Gyr before eruption. Two of the diamonds encapsulated omphacite microinclusions in association with saline or low-Mg carbonatitic-like HDF. We observe compositional differences for Al 2 O 3 vs. CaO between these metasomatised omphacites, and also compared to omphacites in mantle eclogites which were identified as metasomatised by kimberlite or high-Mg carbonatite; suggesting a possible relationship between Al 2 O 3 and CaO in metasomatised omphacite and the type of fluid/melt it interacted with. The combined data for microinclusion-bearing diamonds from the Finsch Group II kimberlite and the neighbouring Group I kimberlites at Koffiefontein and De Beers Pool indicate that a substantial volume of the southwest Kaapvaal deep lithosphere was impacted by saline metasomatism during Cretaceous time, and a direct relationship between saline metasomatism, diamond formation and the Kaapvaal late-Mesozoic ‘kimberlite bloom’. We therefore conclude that saline HDFs play a key role in the buildup of metasomatic mantle sources leading to kimberlite eruptions.