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Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa |
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| Authors: | D Phillips JW Harris KS Viljoen |
| Institution: | a School of Earth Sciences, The University of Melbourne, Parkville, VIC, 3010, Australia b Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK c GeoScience Centre, De Beers Consolidated Mines Ltd., P.O. Box 82232, Southdale, 2135, South Africa |
| Abstract: | Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr2O3 contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr2O3 (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr2O3 and <0.5 wt.% TiO2. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr2O3 contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K2O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86). Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation ( |
| Keywords: | Mineral chemistry Thermobarometry De Beers Pool inclusions |
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1200 °C;