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

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