Metasomatic reequilibration of mantle xenoliths from the Gibeon kimberlite province (Namibia)

Abstract

Mantle xenoliths from the Hanaus and Anis Kubub pipes of the Gibeon Kimberlite Province (Southern Namibia) were investigated with the electron microprobe. The xenolith suite consists of garnet lherzolites and harzburgites with coarse equant, porphyroclastic and mosaic-porphyroclastic textures, and coarse equant garnet spinel lherzolites, spinel lherzolites and one ilmenite phlogopite orthopyroxenite. In xenoliths from Hanaus, chemical zoning is restricted to part of the orthopyroxenes, whereas most of the primary minerals from Anis Kubub reveal distinct zoning. A steady state geotherm of about 44 mW/m2 is displayed by most coarse equant lherzolites, while some porphyroclastic and all mosaic-porphyroclastic xenoliths plot along an elevated geothermal gradient of more than 50 mW/m2. These observations testify to transient heating processes in the mantle coupled with ductile shearing as already observed in xenoliths from the Gibeon Townland 1 pipe [1]. Mantle xenoliths from Anis Kubub show prominent secondary reaction textures with wide spinel-pyroxene coronas around garnet and felty aggregates of amphibole and secondary olivine replacing pyroxene. Thermobarometric estimates for this thermal and metasomatic overprint reveal temperatures of 1100-1200°C at pressures of about 20 kbar, pointing to a magma chamber close to the boundary of upper mantle to lower crust. The residence time of the xenoliths within this intermediate magma chamber can be estimated at about 60 (±20) days using the diffusion coefficients of Jurewicz and Watson [2] and the Ca-zoning in olivine porphyroclasts and neoblasts. The absence of diamonds in the Gibeon Kimberlite Province may be related to the following facts: 1. The shallow position of the asthenosphere under southern Namibia and the sampling of mantle by the kimberlite in shallower levels. 2. The elevated geothermal gradient compared to the adjacent Kaapvaal craton. 3. Oxidation and dissolution of diamonds in intermediate magma chambers as at Anis Kubub.