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Starting the recycling engine: how far back in time can we fingerprint crust in Earth’s mantle?

The nature of the tectonic processes that shaped the early Earth remain unresolved, hampered not only by the sparse Early Archaean crustal rock record, but also by the dearth of tangible mantle samples (e.g., xenoliths and diamonds) older than 3 Ga. Investigating the Archaean mantle provides a complementary foil to the knowledge gleaned from the early Earth crust, and can be used to trace the onset of crustal recycling, but also to evaluate the secular evolution of Earth’s mantle regarding its temperature and composition including redox state. We have conducted an in-situ carbon and nitrogen isotope study of “confirmed” Archaean diamonds from the 3.0 – 2.8 Ga Witwatersrand Supergroup of the Kaapvaal craton in South Africa [1]. While the absolute formation age of the placer diamonds is unknown, nitrogen aggregation suggests diamond residence within the upper mantle for 10 - 400 Myr. Coupled with the depositional age of the Archaean basin, the Witwatersrand diamonds may have formed in the mantle as early as 3.5 Ga, before their transport via kimberlite-like magmatism to Earth’s surface during formation of the Kaapvaal craton. The d15N values of 0.5 to +2.7 ‰ determined for the Witwatersrand diamonds are higher than both the ancient and modern mantle (-5 ‰), and overlap with positive d15N values shown by >3 Ga old Kaapvaal sedimentary rocks. The diamond carbon isotope ratios (d13C of -5.7 to -3 ‰) are mantle-like, but increases in d13C values from core to rim suggest that the Witwatersrand diamonds formed from relatively oxidised fluids containing CO2 rather than CH4. It follows that oxidised CHO-fluids containing recycled crustal nitrogen were present in the upper mantle possibly prior to 3.5 Ga. This observation suggests operation of subduction-style tectonics during the inception of craton formation in the Eo- to Palaeoarchaean. It also implies that the Early Archaean upper mantle was not more reducing than at the present, in alignment with new evidence for an oxidised CO2-rich early Earth atmosphere created by mantle outgassing. [1] Smart KA, Tappe S, Stern RA, Webb SJ and Ashwal LD. 2016. Early Archaean tectonics and mantle redox recorded in Witwatersrand diamonds. Nature Geoscience, v. 9, p. 255–259.


Sebastian Tappe1, Katie A. Smart2, Richard A. Stern3
1University of Johannesburg, South Africa; 2University of the Witwatersrand, South Africa; 3University of Alberta, Canada
GeoKarlsruhe 2021
South Africa