Planetary and pre-solar noble gases in meteorites |
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| Institution: | 1. University of West Hungary, Faculty of Natural Sciences, Savaria Campus, H-9700 Szombathely, Hungary;2. Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany;1. Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich, Switzerland;2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 2015-1305, USA;3. Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany;1. CRPG-CNRS, Université de Lorraine, 15 rue Notre Dame des Pauvres, 54500 Vandoeuvre-les-Nancy, France;2. LATMOS, Université Versailles St. Quentin, UPMC Univ. Paris 06, CNRS, 11 Bvd. d’Alembert, 78280 Guyancourt, France;1. Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015, USA;2. Kingsborough Community College of the City University of New York (CUNY), 2001 Oriental Blvd., Brooklyn, NY 11235, USA;3. Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, DC 20015, USA;1. Physics Department, Washington University, Saint Louis, MO 63130, USA;2. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA;1. Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS, Université de Lorraine, UMR 7358, 15 rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre-lès-Nancy, France;2. School of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK;3. Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa;4. Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, UMR 7154, Paris F-75238, France;5. Géosciences Montpellier, Université de Montpellier, UMR 5243 CNRS, Place Eugène Bataillon, 34095 Montpellier, France;6. NASA Ames Research Center, Moffett Field, CA 94035, USA;7. Department of Earth Sciences, University of Oregon, Eugene, OR 97403, USA |
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| Abstract: | Noble gases are not rare in the Universe, but they are rare in rocks. As a consequence, it has been possible to identify in detailed analyses a variety of components whose existence is barely visible in other elements: radiogenic and cosmogenic gases produced in situ, as well as a variety of “trapped” components – both of solar (solar wind) origin and the “planetary” noble gases. The latter are most abundant in the most primitive chondritic meteorites and are distinct in elemental and isotopic abundance patterns from planetary noble gases sensu strictu, e.g., those in the atmospheres of Earth and Mars, having in common only the strong relative depletion of light relative to heavy elements when compared to the solar abundance pattern. In themselves, the “planetary” noble gases in meteorites constitute again a complex mixture of components including such hosted by pre-solar stardust grains.The pre-solar components bear witness of the processes of nucleosynthesis in stars. In particular, krypton and xenon isotopes in pre-solar silicon carbide and graphite grains keep a record of physical conditions of the slow-neutron capture process (s-process) in asymptotic giant branch (AGB) stars. The more abundant Kr and Xe in the nanodiamonds, on the other hand, show a more enigmatic pattern, which, however, may be related to variants of the other two processes of heavy element nucleosynthesis, the rapid neutron capture process (r-process) and the p-process producing the proton-rich isotopes.“Q-type” noble gases of probably “local” origin dominate the inventory of the heavy noble gases (Ar, Kr, Xe). They are hosted by “phase Q”, a still ill-characterized carbonaceous phase that is concentrated in the acid-insoluble residue left after digestion of the main meteorite minerals in HF and HCl acids. While negligible in planetary-gas-rich primitive meteorites, the fraction carried by “solubles” becomes more important in chondrites of higher petrologic type. While apparently isotopically similar to Q gas, the elemental abundances are somewhat less fractionated relative to the solar pattern, and they deserve further study. Similar “planetary” gases occur in high abundance in the ureilite achondrites, while small amounts of Q-type noble gases may be present in some other achondrites. A “subsolar” component, possibly a mixture of Q and solar noble gases, is found in enstatite chondrites. While no definite mechanism has been identified for the introduction of the planetary noble gases into their meteoritic host phases, there are strong indications that ion implantation has played a major role.The planetary noble gases are concentrated in the meteorite matrix. Ca-Al-rich inclusions (CAIs) are largely planetary-gas-free, however, some trapped gases have been found in chondrules. Micrometeorites (MMs) and interplanetary dust particles (IDPs) often contain abundant solar wind He and Ne, but they are challenging objects for the analysis of the heavier noble gases that are characteristic for the planetary component. The few existing data for Xe point to a Q-like isotopic composition. Isotopically Q-Kr and Q-Xe show a mass dependent fractionation relative to solar wind, with small radiogenic/nuclear additions. They may be closer to “bulk solar” Kr and Xe than Kr and Xe in the solar wind, but for a firm conclusion it is necessary to gain a better understanding of mass fractionation during solar wind acceleration. |
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| Keywords: | Meteorites Noble gases Planetary noble gases Phase Q Presolar noble gases |
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