TY - JOUR
T1 - Syngenetic inclusions in diamond from the Birim field (Ghana) - A deep peridotitic profile with a history of depletion and re-enrichment
AU - Stachel, Thomas
AU - Harris, Jeffrey W.
PY - 1997
Y1 - 1997
N2 - Diamonds and their syngenetic mineral inclusions from placer deposits (Akwatia mine) along the Birim River, Ghana were studied, thus providing the first detailed data collection for the West African Craton. Inclusion contents indicate an almost exclusively peridotitic diamond suite, with the vast majority being part of the harzburgitic paragenesis. Chemically the Akwatian diamond inclusions differ from those in our 1100 sample 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 best to be explained by re-fertilisation of a previously depleted source, similar to the metasomatic re-enrichment of deformed, Fe-Ti-rich and hot peridotites discussed by Harte (1983). Geothermometry shows Akwatian inclusions to be 140-190 °C hotter than the peridotitic average (1050 °C) given by Harris (1992). Since garnet-opx equilibria (1100 °C/ 50 kbar to 1370 °C/67 kbar) indicate a typical shield geotherm (40-42 mW/m2), these elevated temperatures imply an origin of the Akwatian 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 higher pressures than recorded for most peridotitic suites. 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 enstatite solid-solution component, which indicates crystallisation at pressures just below kbar. Akwatian diamond inclusions, therefore, represent the most complete cross-section through peridotitic subcontinental lithospheric upper mantle so far observed, down to a maximum depth between 200-240 km.
AB - Diamonds and their syngenetic mineral inclusions from placer deposits (Akwatia mine) along the Birim River, Ghana were studied, thus providing the first detailed data collection for the West African Craton. Inclusion contents indicate an almost exclusively peridotitic diamond suite, with the vast majority being part of the harzburgitic paragenesis. Chemically the Akwatian diamond inclusions differ from those in our 1100 sample 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 best to be explained by re-fertilisation of a previously depleted source, similar to the metasomatic re-enrichment of deformed, Fe-Ti-rich and hot peridotites discussed by Harte (1983). Geothermometry shows Akwatian inclusions to be 140-190 °C hotter than the peridotitic average (1050 °C) given by Harris (1992). Since garnet-opx equilibria (1100 °C/ 50 kbar to 1370 °C/67 kbar) indicate a typical shield geotherm (40-42 mW/m2), these elevated temperatures imply an origin of the Akwatian 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 higher pressures than recorded for most peridotitic suites. 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 enstatite solid-solution component, which indicates crystallisation at pressures just below kbar. Akwatian diamond inclusions, therefore, represent the most complete cross-section through peridotitic subcontinental lithospheric upper mantle so far observed, down to a maximum depth between 200-240 km.
UR - http://www.scopus.com/inward/record.url?scp=0001065291&partnerID=8YFLogxK
M3 - Article
AN - SCOPUS:0001065291
SN - 0010-7999
VL - 127
SP - 336
EP - 352
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 4
ER -