Fundamentals of Noble Gas Thermochronometry

The ideal geochronometer would be a universally stable phase that quantitatively retains both parent and daughter isotopes. Though a few mineral systems such as zircon U-Pb dating come reasonably close to this ideal, most minerals are incompletely retentive of daughterproduct nuclides under crustal conditions. The mechanisms by which the daughter product can be lost from minerals include dissolution–reprecipitation reactions (e.g., salt; Obradovich et al. 1982), recrystallization (e.g., micas undergoing deformation; Chopin and Maluski 1980), and diffusive loss (e.g., 40Ar degassing of K-feldspar; Foland 1974). The latter mechanism is perhaps the most common source of discrepancy between a radiometric mineral date and the age of the rock from which it formed.