METASOMATIC REEQUILIBRATION OF MANTLE XEN OLITHS FROM THE GIBEON KIMBERLITE PROVINCE (NAMIBIA)

Citation
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
Citations number
28
Categorie Soggetti
Geosciences, Interdisciplinary
Journal title
ISSN journal
00167886
Volume
38
Issue
1
Year of publication
1997
Pages
245 - 259
Database
ISI
SICI code
0016-7886(1997)38:1<245:MROMXO>2.0.ZU;2-1
Abstract
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.