Cosmogenic neon from individual grains of CM meteorites: extremely long pre-compaction exposure histories or an enhanced early particle flux

Abstract

Meteoritic grains which contain solar flare VH ion tracks have clearly been individually exposed to energetic particles prior to assembly. In order to observe the effects of irradiation during the precompaction era, spallation-produced neon has been measured in individual grains, selected by the presence of solar flare VH tracks, from the CM regolith breccias Murchison, Murray, and Cold Bokkeveld. The presence of pre-compaction spallation neon correlates well with the presence of solar flare VH tracks (Z 20) and, in this study, detection of SF tracks is the critical parameter used to identify those grains where pre-compaction spallation effects are likely to be present. Only a few percent of the grains (at most) that do not contain solar flare VH tracks contain amounts of cosmogenic Ne larger than would be produced during the conventional cosmic-ray exposure age (and for them the excess is only marginal), whereas most of the grains with solar flare VH tracks contain spallation-produced Ne in significant excess of that due to the nominal cosmic-ray exposure. The magnitude of this excess, which clearly must have been produced prior to compaction, provides evidence for extensive energetic particle exposure during the pre-compaction era. If a contemporary energetic particle complex is assumed (galactic and solar cosmic rays: GCR and SCR), and if production is taken at the maximum present rates, minimum GCR pre-compaction exposure times can be found. The most heavily irradiated grains from Murray and Murchison would require a minimum GCR regolith exposure time of 145 Ma to accumulate the observed cosmogenic Ne. This is the lower limit because it is computed using the peak production rates from the GCR cascade, which occur at roughly 60 g/cm² and it requires that the grain spent its entire regolith residence time at that optimum depth. Studies of compaction constraints for CI and CM meteorites suggest that such long regolith residence times may be unlikely. The alternative to such long periods of parent body regolith activity is increased production rates in the early solar system from an enhanced energetic particle environment.