Thinning of cratons and diamond resorption - stories told by sheared peridotites

Dr. Catharina Heckel is currently a Feodor Lynen Postdoctoral Fellow at the Australian National University, working with Prof. Greg Yaxley and Prof. Olivier Alard. Catharina completed her PhD at Goethe University Frankfurt in 2023, focusing on sheared peridotites from the Kaapvaal craton and the link between deformation and metasomatism in the lithospheric mantle. Her work combines petrology, geochemistry, and microanalytical methods, and has been published in Lithos and the Journal of Petrology. Today, she will share insights from her current research on mantle xenoliths and metasomatic processes.

Kimberlite-hosted sheared peridotite xenoliths are highly deformed and exhibit a bimodal grain size distribution, consisting of coarse-grained porphyroclasts (mm-sized) and fine-grained neoblasts (down to <10 µm). Such textures cannot survive over 10’s of millions of years within the Earth’s mantle and provide a unique snapshot of processes ongoing in the cratonic, lithospheric mantle shortly before their entrainment. I present major and trace element compositions (porphyroclasts, neoblasts), along with measured H₂O-concentrations (porphyroclasts) and crystallographic preferred orientations (CPO) of olivine neoblasts, for low-, moderate- and high-T sheared peridotite xenoliths from the Late Cretaceous Kimberley and northern Lesotho kimberlite clusters (Kaapvaal craton). The results reveal a complex formation mechanism of these unique rocks.

The sheared xenoliths record a highly-dynamic environment within the Kaapvaal craton 90 Ma ago. (Proto-)Kimberlite pulses led to metasomatism, oxidation and triggered deformation of peridotites. Repeating metasomatism and deformation (a “short-lived metasomatism-deformation cycle” and positive feedback mechanism) caused mechanical, thermal and chemical perturbations of the lower Kaapvaal lithosphere. This 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, volatile-rich melts.