Syngenetic Mineral Inclusions in Diamonds From Ghana - Unique Samples From the Base of the Peridotitic Mantle Lithosphere

Thomas Stachel Department of Geology & Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK

Jeffrey W. Harris Department of Geology & Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK

Diamonds and their syngenetic mineral inclusions from placer deposits along the Birim River, Ghana were studied, thus providing the first detailed data collection for the West African Craton.

Inclusion contents indicate an almost exclusive peridotitic diamond suite, with the vast majority being part of the harzburgitic paragenesis. Chemically the Ghanaian diamond inclusions differ from the world-wide data-base mainly by shifts towards lower Mg/Fe-ratios for harzburgitic olivines and orthopyroxenes, extremely high Ni-contents in both harzburgitic and lherzolitic olivines, and a higher mean
Cr-content in chromites. The inconsistency between the
low Mg/Fe-ratios and the highly refractory compatible
trace element signature seems to be best explained by re-fertilization of a previously depleted source, similar to the metasomatic re-enrichment of deformed, Fe-Ti-rich and hot peridotites discussed by Harte (1983).

However, ion probe trace element analyses of garnet and clinopyroxene inclusions indicate not only different degrees but also different modes of enrichment for the lherzolitic and harzburgitic parageneses. Calculated melt compositions in equilibrium with lherzolitic garnet inclusions show REE fractionation similar to kimberlitic, carbonatitic or lamproitic melts, with LREE contents of about 100 times and HREE contents of about 1 times primitive mantle (Hofmann 1988). Melts in equilibrium with harzburgitic garnets, however, would show much higher degrees of REE fractionation (LREE about 300 times and HREE about 0.1 times primitive mantle) together with very low concentrations in the HFS-elements Hf, Zr, Y and Ti. This suggests fractionation of a percolating fluid by chromatographic processes (Navon and Stolper 1987, Bodinier et al. 1990), leading to enrichment of the harzburgitic source only in the most incompatible trace elements (LREE, Rb, Sr).

Geothermometry shows equilibration temperatures for Ghana to be 140-190°C higher than the peridotitic average (1050 °C) given by Harris (1992). Since garnet-opx equilibria (1100 °C / 50 kbar to 1370°C / 67 kbar, Harley 1984, Brey & Köhler 1990) indicate a typical shield geotherm (40-42 mW/m2), these elevated temperatures imply an origin of the Ghanaian diamonds unusually deep for a peridotitic suite. This is consistent with the presence of extraordinary amounts of silicate spinel component in chromite inclusions, indicative of crystallisation under unusually high pressure.
In addition, one garnet showed the highest knorringite component (66.4 mol.%) so far observed in an inclusion in diamond. The same garnet also contained a minor entstatite solid-solution component, which indicates crystallisation at pressures just below 80 kbar. Ghanaian diamond inclusions, therefore, represent the most complete cross-section through the subcontinental lithospheric upper mantle so far observed, down to a maximum depth between 200-240 km.


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