- © The Mineralogical Society Of America
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 daughter-product 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.
Geochronologists have learned to turn this non-ideal behavior to their advantage and we now understand that most mineral ages from exhumed crustal rocks act in effect as kinetic thermometers sensitive to geologically-induced thermal effects. Such apparent ages are a measure of the temperature range over which daughter product ceased to be lost from a crystal, with intracrystalline diffusion usually acting as the rate-limiting process.
Consider the case in which a mineral sample containing a radioactive parent element experiences a complex thermal evolution, possibly involving heating as well as cooling. Within the sample, daughter product is continually produced by radioactive decay and lost by diffusion at natural boundaries. Although random at the scale of an individual atom, both diffusion and radioactive decay are highly predictable processes over longer and larger scales involving many particles. Coupled with the strong temperature dependence of diffusion, the elegant mathematics of the production-diffusion relationship make it possible to recover information about the thermal history experienced by such a sample, simply by knowing the amount of daughter product remaining in the mineral following cooling, or even better, by knowing the distribution of daughter product within the …