Cosmic‐ray exposure age and preatmospheric size of the Bunburra Rockhole achondrite |
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| Authors: | Kees C WELTEN Matthias M M MEIER Marc W CAFFEE Matthias LAUBENSTEIN Kunihiko NISHIZUMI Rainer WIELER Phil A BLAND Martin C TOWNER Pavel SPURNÝ |
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| Affiliation: | 1. Space Sciences Laboratory, University of California, Berkeley, California 94720–7450, USA;2. Department of Earth Sciences, ETH Zürich, CH‐8092 Zürich, Switzerland;3. PRIME Laboratory, Purdue University, West Lafayette, Indiana 47907, USA;4. Laboratori Nazionali del Gran Sasso, I.N.F.N., I‐67100, Assergi (AQ), Italy;5. Impacts & Astromaterial Research Centre (IARC), Department of Earth Science & Engineering, Imperial College, London SW7 2AZ, UK;6. IARC, Department of Mineralogy, Natural History Museum, London SW7 5BD, UK;7. Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia;8. Astronomical Institute of the Academy of Sciences, Fri?ova 298, CZ‐251 65 Ond?ejov Observatory, Czech Republic |
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| Abstract: | Abstract– Bunburra Rockhole is the first meteorite fall photographed and recovered by the Desert Fireball Network in Australia. It is classified as an ungrouped achondrite similar in mineralogical and chemical composition to eucrites, but it has a distinct oxygen isotope composition. The question is if achondrites like Bunburra Rockhole originate from the same parent body as the howardite‐eucrite‐diogenite (HED) meteorites or from several separate, differentiated parent bodies. To address this question, we measured cosmogenic radionuclides and noble gases in the Bunburra Rockhole achondrite. The short‐lived radionuclides 22Na and 54Mn confirm that Bunburra Rockhole is a recent fall. The concentrations of 10Be, 26Al and 36Cl as well as the 22Ne/21Ne ratio indicate that Bunburra Rockhole was a relatively small object (R approximately 15 cm) in space, consistent with the photographic fireball observations. The cosmogenic 38Ar concentration yields a cosmic‐ray exposure (CRE) age of 22 ± 3 Myr, whereas 21Ne and 3He yield approximately 30% and approximately 60% lower ages, respectively, due to loss of cosmogenic He and Ne, mainly from plagioclase. With a CRE age of 22 Myr, Bunburra Rockhole is the first anomalous eucrite that overlaps with the main CRE peak of the HED meteorites. The radiogenic K‐Ar age of 4.1 Gyr is consistent with the U‐Pb age, while the young U,Th‐He age of approximately 1.4 Gyr indicates that Bunburra Rockhole lost radiogenic 4He more recently. |
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