Rare earth element geochemistry of kimberlite

Abstract

Abundances of rare earth elements (REE), Sc, Co, Hf, Ta and Th have been determined by neutron activation analysis in kimberlites from the Wesselton pipe, in micaceous kimberlites from the Swartruggens fissure and kimberlite from the Monastery Mine, Ison Creek and Somerset Island pipes. Kimberlites are characterized by a high REE content and have chondrite normalized REE distribution patterns which show extreme fractionation of the REE group, e.g. kimberlite La Yb = 100 ; micaceous kimberlite La Yb = 140 . Distribution patterns are linear, with weak negative Eu anomalies evident in the micaceous types. It is considered that the major sites of REE are apatite, perovskite and carbonate and that Eu anomalies are due to the presence of perovskite or mica. REE distributions are analysed in terms of partial melting and eclogite crystallization models by means of REE crystal-liquid distribution coefficients. Partial melting models indicate that the La Yb ratios of kimberlites and micaceous kimberlites can be produced independently by differing amounts of partial melting (< 1%) of a garnet lherzolite mantle. Eclogite crystallization models indicate that kimberlites can be produced by greater than 96% crystallization of an REE poor parent magma(basaltic) with a low La Yb ratio (ca. 5) which was produced by extensive (15–20%) partial melting of garnet lherzolite mantle. REE distributions do not provide evidence in favour of any one petrogenetic model, especially with regard to La and Yb abundances; this is considered to be a reflection of errors in the magnitude of the distribution coefficients. Evidence bearing on the possibility of eclogite crystallization from kimberlitic magmas is reviewed (i.e. eclogite distribution, age, mineralogy of kimberlite and of inclusions in diamonds) and is interpreted to indicate that eclogite fractionation is the least likely means of generating kimberlite liquids. The relation of kimberlite magmatism to continental basaltic magmatism is considered in terms of a partial melting model in which the extent of partial melting of the mantle is dependent upon heat flow variations with time. The small volumes of liquid required in partial melting hypotheses are thought to be concentrated into kimberlitic magmas by shearing processes.