Cl-derived argon isotope production in the CLICIT facility of OSTR reactor and the effects of the Cl-correction in 40Ar/39Ar geochronology

The production of 36Ar from Cl during neutron irradiation of samples for 40Ar/39Ar dating can compromise the determination of radiogenic versus atmospheric 40Ar, resulting in spuriously young computed ages. An accurate correction for Cl-derived 36Ar requires knowing the nucleogenic production ratio P(36Cl/38Cl) resulting from (n, γ) reactions on 35Cl and 37Cl. P(36Cl/38Cl) can be derived from Ar isotope analysis of irradiated Cl-bearing compounds. Results from NaCl and KCl irradiated in the Cd-lined facility of OSTR reactor yield P(36Cl/38Cl) = 262.8 ± 1.7 and 252.7 ± 1.8 (respectively), distinctly lower than 2 previous determinations from other reactors without Cd-shielding. The Cl-correction for samples irradiated in this reactor facility is consequently smaller than would be made using previous P(36Cl/38Cl) values, but large enough to be non-negligible in some cases.