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VKC February 2021 - Nicole Meyer: Diamonds and Their Inclusions From The Koffiefontein Pipe

Nicole Meyer (University of Alberta, PhD candidate) presents: "Diamonds and Their Inclusions From The Koffiefontein Pipe Provide Insights Into the Formation and Evolution of the Kaapvaal Craton" Seminar was live on Feb 24, 2021 at 6 PM (PST) hosted by Katherine Landoni and convened by Barbara Scott Smith. Sponsored by SRK Consulting (https://cdn-web-content.srk.com/uploa.... Seminar Abstract: Diamonds and their mineral inclusions preserve mantle processes over space and time. Forming over a protracted period, diamonds also provide snapshots of early craton formation and mantle evolution over much of Earth’s history. The lithosphere beneath Koffiefontein is extremely depleted and is characterised by high-Mg# olivine and low-Ca garnet. In addition to garnet LREE enrichment, Koffiefontein experienced a unique K-Nb-Ta-rich metasomatism event that resulted in new minerals. The lack of clinopyroxene and co-existing garnet-orthopyroxene assemblages lead to the use of the electron microprobe for trace element analysis of Al in olivine. Geothermobarometry indicates that upper mantle diamond formation conditions are 1100-1300 °C and 4-7 GPa. Koffiefontein diamonds have a main δ13C mode for both peridotitic and eclogitic diamonds similar to mantle carbon. Relationships of δ15N-[N] and δ13C-δ15N indicate that nitrogen was derived from subducted sources and suggests that formation of not only eclogitic but also peridotitic diamonds involved fluids derived from altered oceanic crust. Lower mantle diamonds with coexisting ferropericlase and former bridgmanite indicate formation at or below 660 km. The high bulk Mg# of this assemblage is consistent with the diamond substrate originating from the depleted lithospheric mantle portion of an oceanic slab. Diamond formation at Koffiefontein dominantly takes place in depleted peridotite at both lithospheric and lower mantle depths. The δ13C-δ15N systematics suggest the same subducted source for both peridotitic and eclogitic diamonds. Subduction has played an important role in the formation and evolution of the Kaapvaal Craton and subsequent diamond formation.