If diamonds are formed at depths of 150 to 200 kilometers in the upper mantle, how do they get to the surface of the earth? They are brought to the surface in a peculiar igneous rock called kimberlite (named after the diamond-bearing region of Kimberly, South Africa where these rocks were first identified). Kimberlites are intrusive bodies that originate in the upper mantle and are injected upward through the upper mantle and the lower and upper crust, eventually reaching the earth’s surface as a small volcanic complex.
Kimberlites have three facies correlative to their position in the mantle and crust: the root facies, the diatreme facies, and the crater facies. The shape of the kimberlite shown in the figure is similar to that of a carrot or a pipe; a comparatively wide upper zone, up to several hundred meters in diameter,in the diatreme and crater facies; to a lower zone, which narrows into a thin intrusive dike, possibly only a meter thick, in the root facies. The forceful intrusion of the kimberlite brecciates (that is: to break apart into smaller angular fragments. A rock composed of angular fragments is referred to as a breccia) the surrounding rocks of the upper mantle and crust and incorporates them as xenoliths (xenolith literally means “foreign rock”). Often these xenoliths of the upper mantle peridotites or eclogites contain diamonds.
Location map of State Line Kimberlite district and “Diamond Peak”.
Kimberlites, though rare, are widespread throughout the surface of the earth. Most well known diamond-producing pipes are small, 12 to 75 acres, and they generally occur in clusters of six to forty pipes. Almost all diamond-bearing kimberlites are found in the ancient stable cratons of the continents, never in oceanic crust or in younger mountain belts, like the Alps or the Sierra Nevada in California.
Generalized geologic map of Colorado’s State Line District.
Kimberlite intrusions are considered to be very gassy, so fragments from the walls can fall down into the pipe from where they were broken off. A fascinating case of this was discovered in a northern Colorado kimberlite. A large limestone fragment in a kimberlite had fossils showing that it was Silurian in age. Yet, no in-place layers ofSilurian-age strata had ever been found in Colorado, in fact not within 300 miles of this location. The previous interpretation was that Colorado had been a land area during the Silurian and no layers had been deposited. However, this fragment of marine limestone showed that such was not the case, at least for the area of the kimberlite.