Radiogenic isotopes and the interpretation of granitic rocks

Abstract

Three decades ago, radiogenic isotopes in geological studies were used primarily for obtaining crystallisation ages for igneous rocks, but developments in isotope geochemistry now allow a range of processes to be investigated that both pre- and postdate crystallisation. Many early studies, based on whole-rock analyses, have generated erroneous crystallisation ages as subsequently revealed by high-precision chronometry of accessory phases. Of these, zircon is the most widely used, but monazite and xenotime can also generate precise ages from peraluminous magmas, whereas allanite has been used with success in metaluminous granitoids. Complex cooling histories can be traced by the use of minerals with appropriate closure temperatures. Single crystal zircon evaporation techniques and in-situ electron microprobe analysis of monazites provide new methods for rapid and inexpensive dating of some granites, but both approaches are limited by the range of granites that can be dated and by the assessment of realistic uncertainties. Accessory phase dissolution in felsic melts underpins the use of these minerals in chronometry and also in the interpretation of model Nd ages in constraining the time of crust formation. Ongoing research suggests that disequilibrium studies can potentially constrain the rates of melt formation and extraction although more precise measurements on diffusion rates of critical trace elements are now required.