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Cosmic‐ray exposure history of the Norton County enstatite achondrite
Author(s) -
HERZOG G. F.,
ALBRECHT Achim,
MA Peixue,
FINK David,
KLEIN Jeffrey,
MIDDLETON Roy,
BOGARD Donald D.,
NYQUIST L. E.,
SHIH C.Y.,
GARRISON D. H.,
REESE Young,
MASARIK J.,
REEDY R. C.,
RUGEL G.,
FAESTERMANN T.,
KORSCHINEK G.
Publication year - 2011
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2010.01154.x
Subject(s) - nuclide , achondrite , accelerator mass spectrometry , enstatite , isotope , isotopes of argon , neutron , neutron temperature , radiochemistry , thermal ionization mass spectrometry , analytical chemistry (journal) , cosmic ray , meteoroid , irradiation , nuclear physics , chemistry , meteorite , ionization , mass spectrometry , physics , chondrite , astrobiology , environmental chemistry , chromatography , ion , organic chemistry
– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of 36 Cl, 41 Ca, 26 Al, and 10 Be 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 10 Be, 26 Al, and 53 Mn (<6%) but overestimates the 41 Ca concentrations by about 20%. 3 He, 21 Ne, and 26 Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, 36 Ar n , is attributed to production by thermal neutrons. From the values of 36 Ar n , an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 10 16 neutrons cm −2 . We infer comparable values from ε 149 Sm and ε 150 Sm. Values calculated from 41 Ca and a CRE age of 115 Ma, 0.2–1.4 × 10 16 neutrons cm −2 , are lower by a factor of approximately 2.5, indicating that nearly half of the 149 Sm 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.