Reequilibration of Ultramafic Xenoliths from Namibia by Metasomatic Processes at the Mantle Boundary

Abstract

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°C at 44 kbar in garnet harzburgites and a minimum of about 630°C at 19 kbar in spinel lherzolites. 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 ...