Search for Meteorites with Complex Exposure Histories Among Ordinary Chondrites with Low (sub)HE/(sub 21) NE Ratios

摘要

In calculating cosmic-ray exposure ages of meteorites it is generally assumed that the meteoroids were expelled from a shielded position within their parent body and then experienced a single stage exposure before colliding with Earth. The combination of noble gas and radionuclide measurements in several large meteorites, such as Jilin and Bur Ghelaui, have revealed complex exposure histories: i.e. an initial exposure on the surface of an asteroid (or within meter-sized meteoroid), followed by a second exposure as a smaller object. In fact, orbital dynamics calculations predicted that at least 30% of the meteorites arriving on Earth experienced two- or multiple-stage exposure histories. More recently, after the recognition that the Yarkovsky effect plays an important role in delivering meteorites from the asteroid belt to Earth-crossing orbits, it was confirmed that complex exposure histories should be common. Nevertheless, despite the ability to measure a wide range of radionuclides with accelerator mass spectrometry (AMS), only a few meteorites with complex exposure histories have been identified. The question is whether the relatively paucity of complex exposure histories is real or have we simply overlooked complex-exposure histories. In this work we focus on meteorites with low (sup 3)He/(sup 21)Ne ratios, since it is known that most meteorites with complex exposure histories have relatively low (sup 3)He/(sup 21)Ne ratios, i.e. the (sup 3)He/(sup 21)Ne ratio is below the 'Bern-line'. Several hypotheses have been suggested for these low (sup 3)He/(sup 21)Ne ratios, including solar heating in low-perihelion orbits, shock-related diffusion of He during the collision that ejected the meteoroid, or an artifact of high shielding conditions. The first two hypotheses seem to be supported by low radiogenic (sup 4)He concentrations in samples with low (sup 3)He, whereas Monte Carlo calculations have shown that some of the low (sup 3)He/(sup 21)Ne ratios may be due to high shielding conditions in objects with radii greater than 1 m. To elucidate these issues, we selected 15 samples with known noble gas concentrations for radionuclide studies and obtained aliquots of the samples adjacent to those measured for noble gases. The specific goal is the identification of complex exposure histories among samples having low (sup 3)He/(sup 21)Ne ratios.