Reequilibration of ultramafic xenoliths from Namibia by metasomatic processes at the mantle boundary

Mantle xenoliths from the Hanaus and the Anis Kubub pipes in the Gibeon Kimberlite Province of southern Namibia show evidence for intensive heating in lower as well as intermediate lithospheric levels. The investigated samples are garnet- and spinel-bearing peridotites (as well as one orthopyroxenite) with granular, partly sheared, and sheared textures. While granular and partly sheared xenoliths reveal distinct zonation patterns, most of the sheared xenoliths display perfect mineral equilibria. Thermobarometric estimates for the primary mineral assemblages of these samples plot in the graphite stability field with maximum P-T conditions of about 1320 degrees C at 44 kbar in garnet harzburgites and a minimum of about 630 degrees C at 19 kbar in spinel Iherzolites. The initial steady-state geotherm of 44 mW/m(2) is only preserved in a few granular xenoliths, whereas the sheared samples plot along an elevated geothermal gradient of about 50 mW/m(2), testifying to transient heating processes in the mantle coupled with kimberlitic activity. Mantle xenoliths from Hanaus pipe only display small kelyphitic rims around garnet, whereas xenoliths from the Anis Kubub diatreme show prominent secondary reaction textures with wide spinel-pyroxene coronas replacing garnet as well as felty aggregates of K-richterite/Mg-arfvedsonite and secondary olivine growing at the expense of ortho- and clinopyroxene. For the secondary mineral assemblages in the coronas, temperatures of 1100-1200 degrees C at pressures of about 20 kbar can be estimated using several independent mineral geothermobarometers and the TWEEQ program. Thus, this thermal and metasomatic overprint of the mantle xenoliths must have occurred within a magma chamber located close to the boundary of upper mantle/lower crust. The residence time of the xenoliths in the magma chamber can be estimated at about 8 weeks using the diffusion coefficients for Ca in olivine and the Ca-zoning of the olivine porphyroblasts and neoblasts. Beside the shallow position of the asthenosphere and the elevated geothermal gradient in the off-craton area of southern Namibia, such a thermal overprint and contemporaneous metasomatic processes could also be a reason for the absence of diamonds in the Gibeon Kimberlite Province.