SOLAR-PROTON-PRODUCED NEON IN SHERGOTTITE METEORITES AND IMPLICATIONS FOR THEIR ORIGIN

Measured Ne isotopes in samples of shergottite ALHA77005 show variations in Ne-21/Ne-22 ratios and Ne-21 abundances that are consistent with the presence of two cosmogenic components: a component produced by nuclear interactions of galactic cosmic rays (GCR) and a component produced at shallow shielding depths (similar to 0-3 cm) by energetic solar flare protons (SCR). We suggest that the Ne-21/Ne-22 ratio generally can be used to distinguish between SCR and GCR components in many meteorite types. Analysis of cosmogenic Ne produced in chondrite mineral separates, eucrites, and anorthositic lunar rocks, all having diverse major element compositions, indicate that the GCR Ne-21/Ne-22 ratio increases modestly with relative Mg content. Data for hundreds of chondrite analyses suggest that SCR Ne is present in no more than a very small fraction of chondrites. Examination of literature data for other shergottites, however, indicate that all of these meteorites contain SCR Ne but that it is apparently absent in other SNC meteorites. The ubiquitous presence of SCR Ne in shergottites, in contrast to most other types of meteorites, suggests that the martian origin of shergottites gave them different orbital parameters compared to other meteorites. This in turn may have contributed to slower entry velocities and lesser surface ablation in the atmosphere or even to higher SCR production rates.