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Phase relations of an aphanitic kimberlite from the Wesselton mine, Kimberley, South Africa have been determined at 10-40 K bar and 1000-1525°C.This kimberlite is fine-grained with low macrocrystic olivine content which distinguishes it from other macrocrystic Type I kimberlites from the same locality.From this study, crystalliza¬ tion paths in the aphanitic kimberlite at depth can be compared with those inferred from its petrography and some suggestions made as to the source material for this unusual kimberlite. PETROGRAPHY AND GEOCHEMISTRYThe petrography and geochemistry of aphanitic and macrocystic Wesselton kimberlites (Shee 1984) indicate that the aphanitic varieties can be distinguished by olivine microphenocrysts in a groundmass of calcite, monticellite, ilmenite, spinel, perovskite, apatite, serpentine and rare phlogopite.Xenocrysts of olivine and phlogopite and xenoliths only occur in trace amounts. Whole rock analyses indicate Mg/Mg + Fe, K/K + Na and Ca/Ca + Mg ratios range from 0.80-0.86, 0.13-0.81 and 0.24-0.29 respectively.Olivine microphenocrysts have Mg/Mg + Fe of 0.83-0.95. Groundmass monticellites have Mg/Mg + Fe of 0.92-0.94, spinels have chromite-rich cores and titanomagnetite-rich rims. EXPERIMENTAL METHODSExperiments were done in a 1.27 cm piston-cylinder apparatus in Fe-soaked Pt, Pt and Ag5oPd5o capsules.Pressures and temperatures are considered accurate to jfO.5 K bar and _+ 5^C respectively.Runs were buffered at fo2S equivalent to >QFM300®C at 10 K bar. Below about 13 K bar, 1000-1250OC monticellite occurs with olivine + spinel + perovskite jf calcite. Clinopyroxene is absent in all runs with monticellite, suggesting some type of reaction relationship. Distinction between primary and quench perovskite is difficult but appears to lie at temperatures below "calcite in" curve ( Fig. la).The effect of increasing CO2 to \qq2=0,5Z is shown in Fig. 1b.In these runs identification is more difficult due to the abundance of fine-grained quench products.The liquidus is at a slightly higher temperature and the olivine + spinel + calcite near liquidus assemblage gives way to clinopyroxene + olivine + spinel + calcite...

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March , 19o8.] Notes and Comments. 215