Radiometric ages of Siberian kimberlite breccias are consistently in the range 610-695 million years, yet on the basis of sound geologic evidence, the breccias were intruded sometime between the Ordovician and early Cretaceous. These data, together with the absence of contact metamorphic effects, indicate that the breccias were intruded as a slurry of solid rock fragments at temperatures below 200 degrees C. The pipes appear to be genetically related to areas of late Precambrian alkalic-ultrabasic central intrusions. From a review of the Russian and Czechoslovakian literature, it is concluded that the diamonds and associated olivine, pyrope, and chrome diopside were formed within the crust, rather than in the mantle, during the Mesozoic reactivation and emplacement of the pipes.

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