Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa
Phillips, D.; Harris Jeffrey, W.; Viljoen, K.S.
Lithos 77(1-4): 155-179
ISSN/ISBN: 0024-4937 DOI: 10.1016/j.lithos.2004.04.005
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+ or -chromite+ or -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 Cr (sub 2) O (sub 3) 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 Cr (sub 2) O (sub 3) (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63-67 wt.% Cr (sub 2) O (sub 3) and <0.5 wt.% TiO (sub 2) . 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 Cr (sub 2) O (sub 3) contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K (sub 2) O. 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 degrees C (average = 1197 degrees 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 degrees C (average = 1079 degrees 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 ( approximately 1200 degrees C; approximately 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle ( approximately 1050 degrees C) prior to kimberlite eruption at approximately 85 Ma. Pressure estimates for touching garnet-orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet-orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 degrees C).