Isotope-geochemical features of the Siberian-Platform kimberlites in connection with the problem of their origin

We report the results of studies of the isotopic composition of Sr and Nd a nd the contents of trace and dispersed elements in some Devonian kimberlite s of the Siberian Platform in connection with the problem of their origin. The isotopic composition of neodymium (epsilon(Nd) - 0.4-3.7) and the conte nts and ratios of trace elements suggest that these kimberlites originate f rom a slightly depleted source, as is the convectively mixing (asthenospher ic) mantle. On the other hand, some of them have a radiogenic isotopic comp osition of Sr. The calculations of miring, based on the isotopic compositio n of one xenolith of country rocks, argue against the considerable effect o f the latter on the isotopic composition of kimberlites. A positive correla tion between the contents of radiogenic Sr and the most incompatible elemen ts, such as Nb, Ce, and Th, evidences the presence of an ancient enriched c omponent in the mantle source of kimberlites. Kimberlite melt is suggested to be mixed with the melts from this enriched source, which is apparently s ituated in the basement of the continental lithosphere. Differences in the contents of incompatible elements, possibly, reflect regional differences i n the composition of enriched component and/or amount of this component inv olved in the formation of a kimberlite melt in this region.