Isotopic closure

The closure of isotopic chronometers to loss or exchange of radiogenic daughters is best analysed in terms of phenomena that induce recrystallization, i.e. fluid circulation and strain. In the presence of these fast phenomena, temperature os not a rate-controlling parameter for isotope transport. Only in those rare instances where no fast mechanism can be petrographically documented, do minerals survive very high temperatures without appreciable modification of their isotopic record. For these (admittedly unlikely) rocks, a self-consistent upward recalibration of the ‘closure temperature’ is defined.