I–Xe dating: From adolescence to maturity

The I–Xe chronometer is based upon decay of now-extinct 129I where the ratio of accumulated daughter 129Xe to stable 127I reflects the iodine isotopic ratio at closure of the host mineral. Since none of the parent remains, I–Xe is by nature a relative chronometer but, when referenced by a standard mineral of known age, the I–Xe system becomes an absolute chronometer reflecting true closure times. Most iodine hosts are secondary minerals so the I–Xe system is unique in providing details of post-formational chronometry not readily available with other chronometers. The short half-life of 129I gives it exceptional precision. However, the secondary nature of iodine host minerals, combined with the inherent precision of I–Xe, were responsible for a large database of “whole-rock” I–Xe ages that were not easily interpreted. As this problem evolved historically, doubts were cast upon the viability of the I–Xe system as a chronometer which persisted until it was tested against other chronometers in single-mineral systems. Properly calibrated, absolute I–Xe ages reflect the true closure time of the host minerals, and sequences of closure times in different hosts provide cooling rates for the parent object.