ISOTOPE DISTRIBUTION IN HYDROGEN OF DIAMOND

Abstract

Hydrogen is a common impurity in diamonds, where its average concentration is 0.01 percent; but it may reach levels as high as 0.5 percent in carbonadoes and bort. Determining the isotopic composition of this hydrogen is of interest both in connection with the isotopic makeup of hydrogen in the mantle and in connection with the isotopic makeup of hydrogen in the mantle and in connection with the mechanism by which diamond is generated. In the present paper we present the first determinations of the isotopic composition of hydrogen in diamonds. In principle, two situations are possible: fractionation of hydrogen isotopes in the initial magma or fluid, and fractionation of isotopes in the course of crystallization of the diamond. If we assume that diamonds can be generated by cavitation-induced compression of methane, then it is obvious that this process will always be accompanied by out-diffusion of hydrogen from the evolving protodiamond. The residual impurity hydrogen should then become enriched in deuterium. This idea was the basis for our experiment. To summarize, the anomalous isotopic composition of hydrogen that we found in diamond indicates, first, that the generation of diamonds is associated in some way with mantle fluids; second, that isotope fractionation occurs during the crystallization of the diamond, most probably from a hydrocarbon-containing material; and third that the observed isotopic fractionation and the possible mechanism giving rise to it are consistent with a cavitation-induced mechanism of diamond nucleation.