Solution and shock-induced exsolution of argon in vitreous carbon

Abstract

Carbon has been shown by several researchers to be the main noble-gas carrier in chondritic meteorites. To add to our knowledge of noble gas solution and exsolution in carbonaceous material, experiments were performed on vitreous carbon, a type of amorphous carbon. Ar-rich vitreous carbon samples were prepared under vapor-saturated conditions using argon as the pressurizing medium. Solubility data were obtained for temperatures of 773 to 973K and pressures of 250 to 1500 bars. Up to 7 wt.% Ar was dissolved in the carbon. The solubility data were compared to a thermodynamic model of argon atoms dissolving into a fixed population of “holes” in the carbon. Two variations of the model yielded estimates of the enthalpy of solution of Ar in vitreous carbon (Δ H° Ar) of ≈ −4700 cal/mole. Preliminary shock experiments showed that 28% of the total argon was released by driving 4 GPa * shocks into the argon-rich carbon. It was demonstrated that shock-induced argon loss is not simply caused by the impact-induced diminution of grain size. The present value of shock pressure required for partial impact devolatilization of Ar from carbon is below the range (5–30 GPa) that H 2O is released from phyllosilicates.