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Galactic cosmic ray‐produced 129 Xe and 131 Xe excesses in troilites of the Cape York iron meteorite
Author(s) -
MATHEW K. J.,
MARTI K.
Publication year - 2009
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.1945-5100.2009.tb00721.x
Subject(s) - meteorite , troilite , nuclide , radiogenic nuclide , cosmic ray , flux (metallurgy) , cosmogenic nuclide , meteoroid , isotopes of xenon , xenon , radiochemistry , physics , isotope , chondrite , astrobiology , chemistry , astrophysics , nuclear physics , organic chemistry
— The flux of galactic cosmic rays (GCR) in the solar system appears to change with time. Based on the abundances in iron meteorites of cosmogenic nuclides of different half lives, Lavielle et al. (1999) found that the GCR flux increased in recent times (<100 Ma) by about 38% compared to average flux in the past 150 Ma to 700 Ma ago. A promising technique for calibrating the GCR flux during the past ˜50 Ma, based on the 129 I and 129 Xe pair of nuclides, was discussed earlier (Marti 1986; Murty and Marti 1987). The 129 I‐ 129 Xe n chronometer provides a shielding‐independent system as long as the exposure geometry remained fixed. It is especially suitable for large iron meteorites (Te‐rich troilite) because of the effects by the GCR secondary neutron component. Although GCR‐produced Xe components were identified in troilites, several issues require clarifications and improvements; some are reported here. We developed a procedure for achieving small Xe extraction blanks which are required to measure indigenous Xe in troilites. The 129 Xe and 131 Xe excesses ( 129 Xe n , 131 Xe n ) due to neutron reactions in Te are correlated in a stepwise release run during the troilite decomposition. Our data show that indigenous Xe in troilite of Cape York has isotopic abundances consistent with ordinary chondritic Xe (OC‐Xe), in contrast to a terrestrial signature which was reported earlier. Two methods are discussed which assess and correct for an interfering radiogenic 129 Xe r component from extinct 129 I. The corrected 129 Xe n concentration in troilite D4 of Cape York yields a cosmic ray exposure (CRE) age of 82 ± 7 Ma consistent, within uncertainties, with reported data (Murty and Marti 1987; Marti et al. 2004).