22-EPN3-124: Unfolding Geochemical Evolution of the Subcontinental Lithospheric Mantle
April 26, 2024

22-EPN3-124: Unfolding Geochemical Evolution of the Subcontinental Lithospheric Mantle Recorded by Diamond-Forming Carbon and Water Rich (C-O-H) Mantle Fluids Throughout Time

Virtual visit by Yael Kempe and Ofir Tirosh (The Hebrew University of Jerusalem, Israel) to TA2.1 VU Geology and Geochemistry radiogenic and non-traditional stable Isotope Facility (GGIF).
Dates of visit: 9 November 2023 – 25 January 2024.

Report Summary: Fibrous diamonds from the Voorspoed, Venetia and Koffiefontein mines record deep mantle events involving C-O-H fluid types, alongside gem diamonds containing mineral inclusions that were related to modification episodes of the Kaapvaal lithospheric region. Although a connection has been made, the longstanding debate between diamond formation in the mantle and the relationship between gem diamonds and fibrous diamonds is yet to be resolved.

While we have extensive radiometric dating of mineral inclusions trapped in gem diamonds, alongside knowledge about the major and trace elements of C-O-H fluid microinclusions in diamonds, their radiogenic isotopic data is insufficient (e.g. Sr, Nd, and Pb isotopes). High-precision Sr-Nd-Pb isotope TIMS analyses of C-O-H mantle fluids in diamonds from these three prime locations in the Kaapvaal craton were preformed using a novel laser ablation diamond-in-water technique, combined with ultra-low blank column chromatography and 1013 Ohm resistors.

The team successfully processed and analysed 12 samples from Voorspoed, 5 from Venetia and 5 from Koffiefontein mines, as well as standards and blanks. The collected data show intriguing Sr-Nd-Pb relationships that vary between diamonds carrying different C-O-H fluids. The team has further data processing and calculations to complete, as well as correlate the isotopic ratios with trace element compositions to fully understand the results and their geological significance. The outcome of this Europlanet project is expected to provide new insights into the complex tectonic history of this lithospheric province, the fluids themselves and the connection between different diamond types and their formation mechanism.