NOBLE GAS STUDIES IN DIAMONDS SYNTHESIZED BY SHOCK LOADING IN THE LABORATORY AND THEIR IMPLICATIONS ON THE ORIGIN OF DIAMONDS IN UREILITES

Abstract

We synthesized diamonds by shock transformation from natural graphite using a single-stage propellant gun and examined the noble gas concentrations in them. The diamond yields correlated with the shock pressure and with the porosity of the starting material. Considerable amounts of noble gases were trapped in the shock-produced diamonds when the starting material was sealed inside the container (closed system experiment), but were not trapped when employing vented containers (open system experiment). The noble gases were tightly retained within the diamonds and were released only during graphitization of the diamonds. The elemental abundance patterns of noble gases are similar to that of air in the closed system experiments, suggesting that noble gases in the pore spaces within the starting material were driven into and/or dissolved in the diamonds. In the open system experiments, the elemental abundance patterns of noble gases were highly fractionated, indicating that only the gases adsorbed on the preshocked graphite surfaces were trapped in the diamonds. There was no large variation of the noble gas concentrations related to shock pressure or porosity of the starting material in the closed system experiments. These results imply that the vapor-growth model is better than the shock model as the origin of ureilite diamonds.