Quantifying interference of krypton produced from neutron irradiation of inclusion-hosted and lattice-coordinated bromine with 40Ar/39Ar geochronology. (15th August 2017) - Rutte, Daniel
Abstract: 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 40 Ar/ 39 Ar geochronology. Interference of double charged 80 Kr ( 80 Kr ++ ) with 40 Ar has not yet been considered. Significant amounts of 80 Kr are produced during neutron irradiation through the reaction 79 Br(n, β − ) 80 Kr. While previous workers reported a computed fission spectrum averaged cross section of ∼48 mb—compared to ∼113 mb for 39 K(n, p) 39 Ar—we determined a ∼33-fold higher production rate of 80 Kr from Br compared to 39 Ar 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 80 Kr ++ interference biasing 40 Ar/ 39 Ar dates in the ‰ to % range. 80 Kr ++ resembles excess 40 Ar in step-heating experiments. The interference can be corrected for by using the parallel reaction 81 Br(n, β − ) 82 Kr.