Cosmic-ray-produced Helium and Neon in Chondrules in Allende and Murchison

Introduction: Finding evidence for an irradiation of chondrules by energetic particles in the solar nebula prior to their incorporation into meteoritic matter (pre-compaction exposure) might yield information about the early solar energetic particle environment and also constrain the lifespan of chondrules as freely floating objects in the nebula. In a pioneering study, [1] searched for unusual differences in the densities of nuclear tracks among chondrules from ordinary chon-drites. However, all observed track densities were entirely explained by exposure to galactic cosmic rays (GCR) during the journey of the meteoroid from its parent body to Earth. In contrast, Hohenberg and co-workers [2] reported large excesses of cosmogenic Ne in olivine grains with solar flare tracks in CM chon-drites compared to track-free grains. If caused by GCR only, some of the gas-rich olivines would have suffered a precompaction exposure in a parent body re-golith of several hundred million years. As this seemed excessively long, Hohenberg and coworkers suggested instead an irradiation by energetic particles from an early active sun. Smaller excesses of cosmogenic He, Ne and Ar relative to matrix samples were also reported for chondrules from some equilibrated chon-drites [3], whereas a similar study did not find unam-biguous differences [4]. This study reports concentrations of cosmogenic 3 He and 21 Ne and corresponding nominal GCR exposure ages of 23 chondrules from Allende (CV3) and 35 chondrules from Murchison (CM2) with individually determined major element concentrations. The low metamorphic grade as well as the short meteoroid exposure ages of these meteorites (Allende ~4 Ma and Murchison ~1 Ma) allow a relatively easy recognition of possible precompaction exposure records. Results: Small meteorite chips were disaggregated by repeated freeze-thaw cycling. Unbroken chondrules were handpicked under a stereomicroscope and individually abraded using a corundum abrasion cell to remove matrix materials and potential solar gas-rich rims. Abraded chondrules were cleaned in an ultra-sonic bath by acetone. Chondrules were then split under a stereomicroscope with a razor blade. The largest fragment was weighed and used for the mass spectro-metric measurement of He and Ne concentrations and isotopic compositions by IR-laser gas extraction. Up to three remaining fragments per chondrule were measured for major element concentrations by electron