The other isotopes: Research avenues based on 36Ar, 37Ar and 38Ar

The earliest publications on 40Ar/39Ar dating recognized the usefulness of 39Ar/37Ar in providing chemical clues to the species releasing argon during stepped heating, while 37Ar measurements are required for interference corrections. Aside from these essential roles, 36Ar, 37Ar and 38Ar have other useful applications. The dominance of Ca as the target nucleus for the production of 38Ar and 36Ar from spallation by cosmic rays has led to the determination of cosmic ray exposure ages from 38Ar/37Ar. 38Ar production from Cl has an important role in the study of ore minerals containing saline fluid inclusions, both in dating and in understanding their genesis. Combining 38Ar/36Ar ratios with micro-thermometric determinations of salinity provides a way to determine absolute concentrations of noble gases in ore fluids. Absorption of cosmic-ray-produced secondary neutrons by 35Cl and 37Cl provides another means to study cosmic ray exposure of meteorites. 36S excesses in meteoritic sodalite provide evidence of now-extinct 36Cl in the early Solar System. Surprisingly, there is little or no evidence of excess 36Ar beyond what can be accounted for by secondary neutron exposure. We have recently devised a method to identify monoisotopic 36Ar from 36Cl decay. © The Geological Society of London 2014.