Neon in gas-rich samples of the carbonaceous chondrites Mokoia, Murchison, and Cold Bokkeveld

Abstract

High-resolution stepwise-release patterns of neon extracted from gas-rich whole-rock samples of the carbonaceous chondrite meteorites Murchison, Cold Bokkeveld, and Mokoia are characterized by multiple release peaks and complex isotopic variations. Mokoia contains abundant ancient solar wind neon with 20Ne/ 22Ne = 13.7, identical in composition to the modern solar wind. For Murchison and Cold Bokkeveld, 20Ne/ 22Ne ratios of low-temperature release fractions are about 11 and 12.4, respectively. In higher-temperature fractions 20Ne/ 22Ne of trapped neon migrates toward neon-E, reaching a minimum of 6.9 for Murchison. Features in the Murchison thermal release and isotopic variation patterns can be associated with the release of neon from the gas-rich trace phases chromite/carbon and “Q”. The characteristic high-temperature release of neon-E is closely related to the release of neon from “Q” in Murchison. For simplicity, in this paper we consider a model in which neon in the early solar system consisted originally of two components, one component in the gas phase and the second trapped in dust grains. Neon present in solar system reservoirs today is then described as a mixture of galactic cosmic ray spallation neon and two primary trapped components, neon with ( 20Ne/ 22Ne) t = 13.7 and neon-E. The composition of neon in the gas phase is determined from the composition of neon in the sun. Intermediate neon compositions observed in condensed materials such as the earth and meteorites require mechanisms such as gravitational collection, ion implantation or adsorption to combine gas-phase neon with dust-phase neon. The present detailed release patterns show no clear evidence of preferred intermediate compositions such as neon-A. The apparently uniform composition of neon in gas-rich chromite/carbon residues may represent a special mixture of gas-phase and dust-phase neon, but requires detailed investigation. The composition of neon in “Q” is variable and different from the composition in chromite/carbon. It is suggested that the total neon composition in a solar system object will be determined by the relative proportions of various gas-rich trace phases accreted, plus the amounts incorporated of galactic cosmic ray spallation neon and pure gas-phase neon such as the solar wind.