Quantifying interference of krypton produced from neutron irradiation of inclusion-hosted and lattice-coordinated bromine with 40Ar/39Ar geochronology

Various interfering reactions producing Ar isotopes during neutron irradiation from Cl, Ar, K, and Ca have been previously detailed with the significant ones being routinely corrected for in 40Ar/39Ar geochronology. Interference of double charged Kr-80(Kr-80(++)) with 40Ar has not yet been considered. Significant amounts of 80 Kr are produced during neutron irradiation through the reaction Br-79(n, beta(-)) Kr-80. While previous workers reported a computed fission spectrum averaged cross section of similar to 48 mb-compared to similar to 113 mb for K-39(n, p) 39Ar-we determined a similar to 33-fold higher production rate of Kr-80 from Br compared to 39Ar from K in the CLICIT facility of the OSU reactor. Low-K, high-Br phases, e. g., some amphiboles, and fluid or melt inclusion rich samples are prone to Kr-80(++) interference biasing 40Ar/(39) Ar dates in the parts per thousand to % range. Kr-80(++) resembles excess 40Ar in step-heating experiments. The interference can be corrected for by using the parallel reaction Br-81 (n, beta)Kr-82. (C) 2017 Elsevier Ltd. All rights reserved.