Thermal release characteristics of spallogenic He, Ne, and Ar from the Carbo iron meteorite

Abstract

The thermal release patterns of He, Ne and Ar from samples of the Carbo iron meteorite show that virtually no fractionation of 3He, 4He, 21Ne and 38Ar occurs. Thus, conclusions about iron meteorites based on measured noble gas ratios will be unaffected by gas loss, and measurement of these ratios cannot yield information about possible loss. Further, noble gas loss cannot explain the abnormal elemental and isotopic patterns observed in some iron meteorites, notably hexahedrites. The release of He and Ne was continuously observed throughout the stepwise heating sequence, and ratios of diffusion coefficients at two consecutive temperatures have been determined from measurements of the relative degassing rates at these temperatures at the time of temperature increase. Effective activation energies of 100 ± 20kcal/mol for both 3He and 21Ne are calculated from the diffusion coefficient ratios at temperatures above 1050°C. A sharp maximum at 750°C in both the effective activation energy and the gas release is correlated with the α, γ-phase transition.