The garnets in kimberlite and in the associated griquaite and ultramafic nodules |
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| Authors: | C Frick |
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| Institution: | (1) School of Geology, University of Melbourne, Parkville, Australia |
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| Abstract: | The mineralogy, chemistry and occurrence of the garnets in kimberlite and in the associated griquaite and ultramafic nodules have been investigated. It was found that the garnets in the ultramafic nodules have a composition similar to those in kimberlites. On the other hand, the garnets in the griquaite nodules have a completely different chemical composition from the former ones, and become extremely enriched in grossular in the grospydite and kyanite griquaite nodules.The O'Hara and Yoder hypothesis (1967) for the genesis of kimberlite seems to be in conflict with these findings. The distribution of diamonds in the different types of kimberlites and nodules, which have been investigated, also appears to contradict the above hypothesis.An alternative hypothesis, which considers the griquaite as the source material and the ultramafic nodules as the cumulus phases during the genesis of kimberlites, appears to be more feasible in the light of the present investigation. It is shown that the objections raised against  eclogite as source material (Rickwood et al., 1968, p. 298) can be accommodated by considering the kyanite griquaite and grospydite nodules as residual fractions, derived from the partial melting of the griquaite parent. This model is used to explain the distribution of the diamonds, garnets and the different types of nodules in the different types of kimberlites. |
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