L. Franz et al., METASOMATIC REEQUILIBRATION OF MANTLE XEN OLITHS FROM THE GIBEON KIMBERLITE PROVINCE (NAMIBIA), Geologia i geofizika, 38(1), 1997, pp. 245-259
Mantle xenoliths from the Hanaus and Anis Kubub pipes of the Gibeon Ki
mberlite Province (Southern Namibia) were investigated with the electr
on microprobe. The xenolith suite consists of garnet lherzolites and h
arzburgites with coarse equant, porphyroclastic and mosaic-porphyrocla
stic textures, and coarse equant garnet spinel lherzolites, spinel lhe
rzolites and one ilmenite phlogopite orthopyroxenes. In xenoliths from
Hanaus, chemical zoning is restricted to part of the orthopyroxenes,
whereas most of the primary minerals from Anis Kubub reveal distinct z
oning. A steady state geotherm of about 44 mW/m(2) is displayed by mos
t coarse equant lherzolites, while some porphyroclastic and all mosaic
-porphyroclastic xenoliths plot along an elevated geothermal gradient
of more than 50 mW/m(2). These observations testify to transient heati
ng processes in the mantle coupled with ductile shearing as already ob
served in xenoliths from the Gibeon Townland 1 pipe [1]. Mantle xenoli
ths from Anis Kubub show prominent secondary reaction textures with wi
de spinel-pyroxene coronas around garnet and felty aggregates of amphi
bole and secondary olivine replacing pyroxene. Thermobarometric estima
tes for this thermal and metasomatic overprint reveal temperatures of
1100-1200 degrees C at pressures of about 20 kbar, pointing to a magma
chamber close to the boundary of upper mantle to lower crust. The res
idence time of the xenoliths within this intermediate magma chamber ca
n be estimated at about 60 (+/-20) days using the diffusion coefficien
ts of Jurewicz and Watson [2] and the Ca-zoning in olivine porphyrocla
sts and neoblasts. The absence of diamonds in the Gibeon Kimberlite Pr
ovince 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 gra
dient compared to the adjacent Kaapvaal craton. 3. Oxidation and disso
lution of diamonds in intermediate magma chambers as at Anis Kubub.