New Geochemical and Geodynamic Insights into the Cullinan Kimberlite, Kaapvaal Craton: Mantle Processes, Magmatic Heterogeneity, and Diamond Potential.

The Cullinan kimberlite, also known as ‘Premier’, is located 40 km northeast of Pretoria on the Archean Kaapvaal craton, in South Africa. Dated at ca 1150 Ma, it provides a unique opportunity to explore the geochemistry and geodynamics of the deep mantle, and its interactions with the lithosphere.

This study integrates geochemistry of whole-rock and kimberlite indicator minerals with geothermobarometric evaluations to unravel the mantle components of the kimberlite magmas, and to disclose the diamond potential of the kimberlitic pulses. Major element thermobarometry on olivine, garnet, and pyroxene reveals pressure and temperature conditions of crystallization along the paleo-geotherm, as well as the correspondent mineral stability fields.

Cullinan consists of several distinct kimberlite types, four of which - Fawn, Pale Piebald, Black Transitional, and Blue - are the focus of this study. Geothermobarometric calculations demonstrates that each of these kimberlite eruptions formed under distinct chemical conditions, with three of the four types intersecting the diamond stability field. We highlight their potential to carry diamonds and we emphasize the influence of varying mantle conditions on their formation.

Preliminary geochemical classification confirms Fawn and Pale Piebald as Group I kimberlites, while Black Transitional and Blue kimberlites show evidence of contamination and metasomatic alteration, suggesting a more complex petrogenetic history. These variations in composition testify to the various mantle processes that contributed to the unicity of the Cullinan kimberlite pipe.

This study represents an advancement into the understanding of how mantle-derived melts evolve as they ascend and interact with the lithosphere. It provides critical insights into the geochemical fingerprints of the mantle and contributes to a better understanding of the dynamic processes that drive diamondiferous kimberlite formation, not only at Cullinan but also at a global scale.