Galactic cosmic ray-produced ¹²⁹Xe and ¹³¹Xe excesses in troilites of the Cape York iron meteorite

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 &tild;50 Ma, based on the ¹²⁹I and ¹²⁹Xe pair of nuclides, was discussed earlier (Marti 1986; Murty and Marti 1987). The ¹²⁹I-¹²⁹Xe_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 ¹²⁹Xe_n and ¹³¹Xe excesses (¹²⁹Xe_n, ¹³¹Xe _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 129Xer component from extinct 129I. The corrected ¹²⁹Xe_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).