The H2O component of sheared mantle peridotites from the Kaapvaal craton: evidence for hydrous deformation with implications for the stability of cratons

Sheared peridotite xenoliths entrained by kimberlites provide snapshots of metasomatism and deformation within the cratonic mantle. Here, we present H 2 O concentrations of nominally anhydrous minerals (olivine, orthopyroxene and clinopyroxene) in 18 low-, moderate- and high-T sheared peridotites from the Kimberley and northern Lesotho kimberlite cluster (Kaapvaal craton). H 2 O contents range from < 1–80 µg/g (olivine), 70–310 µg/g (orthopyroxene) and 120–300 µg/g (clinopyroxene), implying hydration through metasomatism. Core to rim heterogeneities in H 2 O concentrations indicate that ortho- and clinopyroxene cores preserve an older metasomatic/deformation event in the lithospheric mantle while olivine cores record the youngest metasomatic event. The latter must have happened immediately prior to xenolith entrainment and transport to the surface. Calculated H 2 O contents for the metasomatic agents range from 1 to 3 wt% H 2 O. We speculate that highly-reactive hydrous (proto-)kimberlitic melts caused oxidizing metasomatism that triggered deformation prior potentially enhanced craton destabilization as well as the local resorption of diamonds.