Diamantiferous diatremes [discussion]

In the discussion by A. P. Lebedev and A. S. Trofimov (p. 784-786), it is pointed out that Czechoslovakian diatremes are basaltic or basalt cementing detrital kimberlite and therefore not comparable to those of the Siberian platform. Discrepancies between geologic age and K-Ar ages of kimberlites are not real but are due to introduced argon. Diamonds form at temperatures above 200 degrees C, as indicated by experimental data and fluidal structures in host rock. Diamond-bearing kimberlite dikes without explosion or brecciation features suggest that shallow explosions are not essential. In the author's reply it is held that although differences between Czechoslovakian and Siberian kimberlites noted by Lebedev and Trofimov are real, xenoliths are similar, particularly in breccias. K-Ar dates for phlogopite cited in the original article have been supported by Pb-U dates and indicate emplacement in a relatively cold state. Diamond-bearing kimberlite dikes do display brecciation features. The upward increase in diamond content in an intrusive system suggests growth of diamonds during rather than prior to intrusion. Kyanite is another high-pressure mineral that has apparently formed by overpressures at high crustal levels. For reference to the paper under discussion, see this Bibliography Vol. 29, Davidson 1.

This record provided courtesy of AGI/GeoRef.