Cretaceous thinning of the Kaapvaal craton and diamond resorption: Key insights from a highly-deformed and metasomatized ilmenite-dunite xenolith

Abstract

Sheared peridotite xenoliths have been entrained by kimberlites in most global cratons but typically constitute only a minor proportion of mantle xenolith suites. Nevertheless, sheared peridotites are important because they record the short-lived cycles of metasomatism, deformation and rheological weakening that occur in Earth's ancient cratonic mantle immediately prior to their entrainment. Our work is focused on a highly-deformed ilmenite-dunite entrained from the south-east margin of the Kaapvaal craton by a Late Cretaceous kimberlite from Thaba Putsoa (northern Lesotho). The compositions of olivines and ilmenites show that this dunite is enriched in Fe and Ti and not simply a fragment of highly-refractory mantle. Two populations of olivine porphyroclasts are present: (i) Olivine porphyroclasts with moderate forsterite (Fo) contents (Fo_85–86) and minor and trace element concentrations comparable to those in many other sheared peridotites and also Cr- and Fe-poor olivine megacrysts (Fo_83–88) found in kimberlites from northern Lesotho and adjacent South Africa; and (ii) olivine porphyroclasts (Fo_78–79) with compositions that are enriched in Mn, Zn and Ge and depleted in Ni, Ca, Cr, Al, V and Cu, and overlap with Cr-poor, Fe-rich olivine megacrysts (Fo_78–82). Olivine neoblasts in the sheared ilmenite-dunite xenolith reveal the full range of Fo contents (Fo_79–86) exhibited by the porphyroclasts whereas the orthopyroxene neoblasts have Mg# of 86.5, CaO contents of ∼1 wt% CaO and variable Ti, Al and Cr contents. Ilmenite neoblasts have variable concentrations of TiO₂ (45–55 wt%), MgO (9–12 wt%), and calculated Fe₂O₃ (2–12 wt%).

This sheared fragment of mantle material is relatively unique in that it experienced multiple oxidizing magmatic/metasomatic events accompanied by deformation, giving important insights into a highly-dynamic environment within the Kaapvaal craton 90 Ma ago. The olivine megacrysts are thought to have crystallized in the Kaapvaal mantle from percolating proto-kimberlite melts and their chemical similarities with the porphyroclasts in the ilmenite-dunite imply that they both crystallized from a similar melt, but the two populations of porphyroclasts were subsequently mechanically mixed during deformation and contemporaneous oxidizing, Ti-rich metasomatism. Oxidation is implied by heterogeneous V/Sc ratios in olivine porphyroclasts and neoblasts and heterogeneous Fe₂O₃ contents in ilmenite neoblasts. We propose that multiple proto-kimberlitic pulses were widespread in northern Lesotho during the Late Cretaceous and led to the crystallization of megacrysts, metasomatism, oxidation and deformation of the surrounding mantle wall-rock. This short-lived metasomatism-deformation cycle caused mechanical, thermal and chemical perturbations in the lower Kaapvaal lithosphere, which had several consequences: (i) chemical and physical pre-conditioning of the lithosphere that facilitated subsequent kimberlite pulses to reach the surface; (ii) an overall mechanical weakening and destabilization of the lower lithosphere; and (iii) resorption of any diamonds present due to interaction with oxidizing melts.