Determination of halogen abundances in terrestrial and extraterrestrial samples by the analysis of noble gases produced by neutron irradiation

The lack of a reliable database for heavy halogens (bromine and iodine) in terrestrial and extraterrestrial samples is mainly due to the analytical challenges of determining their very low abundances (< 1 ppm) in the materials of interest. The neutron irradiation noble gas mass spectrometric (NI-NGMS) technique initially developed in the 1960s is the only viable technique currently capable of determining concentrations below 1 ppb of iodine for small (< 10 mg) sample sizes. We describe in detail the analytical protocols and provide a comprehensive and transparent overview of the data reduction procedures in order to fully explore the uncertainties of the technique. We demonstrate how the capabilities of modern mass spectrometers used for Ar-Ar dating, can be readily extended to incorporate halogen measurements. A new and critical assessment of the use of standards is presented based on results from multiple irradiations, including a meteorite (Shallowater aubrite), scapolite minerals introduced by Kendrick (2012) and a novel internal calibration method based on using barium.