Cosmic-ray exposure history of the Norton County enstatite achondrite

G. F. Herzog, Achim Albrecht, Peixue Ma, David Fink, Jeffrey Klein, Roy Middleton, 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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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 50cm, 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 115Ma. 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 4ppm, and a CRE age of 115Ma, we estimate thermal neutron fluences of 1-4×1016neutronscm-2. We infer comparable values from ε149Sm and ε150Sm. Values calculated from 41Ca and a CRE age of 115Ma, 0.2-1.4×1016neutronscm-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 88cm in a large body for 140Ma prior to its liberation as a meteoroid with a radius of 50cm and further CRE for 100Ma.

Original languageEnglish (US)
Pages (from-to)284-310
Number of pages27
JournalMeteoritics and Planetary Science
Issue number2
StatePublished - Feb 2011

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Space and Planetary Science


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