Constraining absolute deformation ages: the relationship between deformation mechanisms and isotope systematics

Isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size. Deformation must, therefore, be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape. We illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion. We consider three different thermal scenarios with deformation taking place during: (a) the prograde heating path, (b) at the closure temperature of the deforming mineral, and (c) at temperatures significantly below the closure temperature. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. This modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample. In the cases where deformation causes a change in grain size, it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation. The development of grains with Tc greater than the deformation temperature may record a deformation age. Examples of this condition include: (i) neocrystallisation; (ii) grain size reduction occurring at temperatures below Tc (of the reduced grain size) where the deformation mechanism has reset the grains; and (iii) deformation-induced grain coarsening.