The role of element distribution for the isotopic dating of metamorphic minerals

Abstract

The daughter element of geochronologically relevant parent-to-daughter pairs may develop with time a wide range of isotopic compositions in different minerals. Metamorphic recrystallization, involving continuous and discontinuous reactions, redistributes the geochronologically relevant elements between reactant and product minerals. In a sequence of reactions, changing reactant minerals may diminish but are unlikely to remove the heterogeneous isotopic composition of the daughter element in the product minerals. A heterogeneous initial isotopic composition of the daughter element in metamorphic minerals, however, may result in ages that are not accurate and eventually yield distorted time scales and incorrect rates for geologic processes. Minerals that do not incorporate the daughter element are not prone to chemical inheritance and may yield ages that can be both precise and accurate. However, minerals that include the daughter element unavoidably are affected by inheritance and, thus, yield ages that may be precise but not necessarily accurate. To which extent initial isotopic heterogeneity affects the apparent isotopic age depends (1) on the parent-to-daughter ratio of the product mineral and (2) on the parent-to-daughter ratio of the reactant minerals that dominate the budget of the daughter element. Apparent ages of metamorphic minerals tend to be too young if reactant minerals strongly exclude the parent element and too old if reactant minerals dominate the budget of the parent element. Furthermore, product minerals that form by multiple reactions over a wide range of pressure and temperature are likely to show initial isotopic heterogeneity. Thus, the more petrologic information is recorded in a mineral, the less likely is it to yield accurate ages.