The Search for Meterorites with Complex Exposure Histories Amoung Ordinary Chondrites with Low 3HE/21NE Ratios

Welton, K C ; Nishiizumi, K ; Caffee, M W

Lawrence Livermore National Laboratory

关键词: Rare Gases;    Meteoroids;    14 Solar Energy;    Shielding;    43 Particle Accelerators;   

DOI  :  10.2172/15013414
RP-ID  :  UCRL-ID-145418
RP-ID  :  W-7405-ENG-48
RP-ID  :  15013414

美国|英语

来源: UNT Digital Library

【 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 [1]. 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 [2]. 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 [e.g. 3,4]. 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 [4]. 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 > 1m [5]. To elucidate these issues, we selected 15 samples with known noble gas concentrations [6] 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. All samples have {sup 3}He deficiencies of >20% relative to the ''Bern-line'' (Table 1). Most of the selected samples also have low {sup 22}Ne/{sup 21}Ne ratios ({le}1.1), indicative of high shielding during most of their cosmic-ray exposure (Table 1), whereas one sample (Suizhou) was selected because of its relatively low {sup 81}Kr concentration [7]. In addition, we selected QUE 93021, for which initial radionuclide results suggested a short exposure age. Here we present cosmogenic {sup 10}Be, {sup 26}Al and {sup 36}Cl in stone and metal fractions for the 16 ordinary chondrites listed in Table 1.