Spallation recoil and age of presolar grains in meteorites

We have determined the recoil losses from silicon carbide (SiC) grain-size fractions of spallation Ne produced by irradiation with 1.6 GeV protons. During the irradiation, the SiC grains were dispersed in paraffin wax in order to avoid reimplantation into neighboring grains. Analysis for spallogenic Ne-21 of grain-size separates in the size range 0.3 to 6 mu m and comparison with the Na-22 activity of the SiC + paraffin mixture indicates an effective recoil range of 2-3 mu m with no apparent effect from acid treatments, which are routinely used in the isolation of meteoritic SiC grains. Our results indicate that the majority of presolar SIC grains in primitive meteorites, which are micrometer-sized, will have lost essentially all spallogenic Ne produced by cosmic-ray interaction in the interstellar medium. This argues against the validity of previously published presolar ages of Murchison SiC (similar to 10 to similar to 130 Ma, increasing with grain size; Lewis et al., 1994), where recoil losses had been based on calculated recoil energies. It is argued that the observed variations in meteoritic SiC grain-size fractions of Ne-21/Ne-22 ratios are more likely due to the effects of nucleosynthesis in the He-burning shell of the parent AGE stars which imposes new boundary conditions on nuclear parameters and stellar models. It is suggested that spallation-Xe produced on the abundant Ba and REE in presolar SiC, rather than spallogenic Ne, may be a promising approach to the presolar age problem. There is a hint in the currently available Xe data (Lewis et nl., 1994) that the large (> 1 mu m) grains may be younger than the smaller (< 1 mu m) grains.