Cosmic‐ray exposure history of the Norton County enstatite achondrite |
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| Authors: | G F HERZOG David FINK Jeffrey KLEIN Donald D BOGARD L E NYQUIST C‐Y SHIH D H GARRISON Young REESE J MASARIK R C REEDY G RUGEL T FAESTERMANN G KORSCHINEK |
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| Institution: | 1. Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854–8066, USA;2. Institute for Environmental Research, Australian Nuclear Science and Technology Organisation, PMB 1, Sydney, New South Wales 2234, Australia;3. Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA;4. Johnson Space Center, Houston, Texas 77058, USA;5. ESCG, Jacobs‐Sverdrup, Houston, Texas 77058, USA;6. Mail Code JE‐23, ESCG/Muniz Engineering, Houston, Texas 77058, USA;7. Nuclear Physics Department, Comenius University, 842 15 Bratislava, Slovakia;8. Planetary Science Institute, Los Alamos, New Mexico 87544, USA;9. Fakult?t für Physik, TU‐München, 85748 Garching, Germany |
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| Abstract: | Abstract– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of 36Cl, 41Ca, 26Al, and 10Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N‐Particle codes. Assuming a one‐stage irradiation of a meteoroid with a pre‐atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of 10Be, 26Al, and 53Mn (<6%) but overestimates the 41Ca concentrations by about 20%. 3He, 21Ne, and 26Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, 36Arn, is attributed to production by thermal neutrons. From the values of 36Arn, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 1016 neutrons cm?2. We infer comparable values from ε149Sm and ε150Sm. Values calculated from 41Ca and a CRE age of 115 Ma, 0.2–1.4 × 1016 neutrons cm?2, are lower by a factor of approximately 2.5, indicating that nearly half of the 149Sm captures occurred earlier. One possible irradiation history places the center of proto‐Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma. |
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