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

Abstract 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.