Plume-related diamond formation in the central Kaapvaal craton

Despite its long-term lithospheric stability, the Kaapvaal craton hosts several large igneous provinces (LIPs) that generated large amounts of magma over short time intervals during the Archaean and Proterozoic. These include the Ventersdorp at 2791–2683 Ma, the Hekpoort at 2247–2224 Ma, the Bushveld at 2056–2054 Ma, and the Umkondo at 1112–1106 Ma.

In some cratons, plume magmatism and heating in the lithospheric mantle is destructive to diamonds. For example, in the Superior craton plume magmatism and heating related to the 1.1 Ga Midcontinent Rift caused lithosphere thinning below the Attawapiskat region with a significantly narrower ‘diamond-window’ (Smit et al., 2014, Stachel et al., 2018). In the Slave craton, the 1.27 Ga Mackenzie Plume thinned the lithospheric mantle to only 80 km, which is not sufficient for diamond stability (Liu et al., 2021). In contrast, the Kaapvaal craton hosts diamonds that formed during times of plume magmatism. For example, 1930 ± 40 Ma lherzolitic diamonds from Cullinan mine formed within 100 My of the Bushveld and likely represent plume-related heating in the lithospheric mantle and melt mobilisation in peridotite (Richardson et al., 1993)

New Sm-Nd isotopic data on garnet and clinopyroxene inclusions in diamonds from the Voorspoed mine reveal two distinct age populations. For the peridotitic suite (28 inclusions) diamond formation occurred at 2736 ± 27.4 Ma, while inclusions (n = 16) from eclogitic diamonds define a 2196 ± 61 Ma Sm-Nd isochron. Growth of these diamonds correlates with the emplacement of plume-related Ventersdorp and Hekpoort LIPs, at ~2.7 Ga and 2.2 Ga, respectively.

Geothermobarometry of Voorspoed diamonds and their mineral inclusions shows that at the time of Ventersdorp and Hekpoort plume magmatism, the Kaapvaal lithospheric mantle experienced high temperatures >1250–1300 °C, yet there was preservation of a thick lithospheric mantle with conditions favourable for diamond growth. Additionally, peridotites from across the Kimberley and Witwatersrand blocks have a mode of osmium TRD ages between 2.8 and 2.4 Ga (e.g., Carlson et al., 1999, Pearson et al., 2004), likely reflecting melt depletion and lithospheric mantle creation due to the Ventersdorp event. In the lower crust, Mesoarchaean granulite xenoliths also preserve ultra-high temperature metamorphic assemblages (Schmitz and Bowring, 2003).

To reconcile thermal reactivation in the crust with the preservation of coeval diamond-favourable conditions in the mantle, one plausible model is that crustal extension was asymmetric, e.g., the simple-shear model from Wernicke (1985), and modelling in Zwaan et al. (2022). This is consistent with the proposed Ventersdorp plume impingement site being below the present-day eastern Bushveld (Humbert et al., 2019), whereas the voluminous surface volcanism extends much further west of the Colesberg magnetic lineament