Rare and unusual mineral inclusions in diamonds from Mwadui, Tanzania |
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| Authors: | Thomas Stachel Jeff W Harris Gerhard P Brey |
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| Institution: | Institut für Mineralogie, Universit?t Frankfurt, Senckenberganlage 28, D-60054 Frankfurt, Germany, DE
Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, Scotland, UK, GB
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| Abstract: | Syngenetic diamond inclusions from the Mwadui kimberlite reveal that an unusually fertile section of lithospheric mantle
beneath the Central African Craton was sampled. This is shown by a very high ratio of lherzolitic to harzburgitic garnet inclusions
(1:2) and low Mg/Fe-ratios in olivine and orthopyroxene. Geothermometry applied to the peridotitic inclusions indicates disequilibrium
between non-touching inclusion pairs to be common. Disequilibrium between garnet-olivine and garnet-orthopyroxene pairs suggests
successive iron enrichment during diamond formation, e.g. leading to the presence of harzburgitic garnet and lherzolitic olivine
in the same diamond. Apart from the dominant peridotitic inclusion suite (88%), rare eclogitic inclusions occur (2%) and a
number of uncertain paragenesis. Two diamonds, one with eclogitic garnets with moderate pyroxene solid solution and the other
with a single ferro-periclase inclusion, suggest the contribution of a small sub-lithospheric component. The finding of the
association Fe-FeO-Fe3O4 in one single diamond indicates diamond formation over a large range of f
O2 conditions, possibly along redox fronts. Steep compositional gradients may also be reflected by the joint occurrence of harzburgitic
garnet and a SiO2-phase in the same diamond. Alternatively the formation of the SiO2-phase may be due to extreme carbonation of the peridotitic source. Further unusual findings include the exsolution of a silicate
phase from magnetite inclusions, (i.e. primary solution of γ-olivine) and an ilmenite inclusion with an eskolaite (Cr2O3) component of 14.5 mol%, the latter together with harzburgitic paragenesis silicate inclusions.
Received: 23 August 1997 / Accepted: 7 January 1998 |
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