^4He/^3He thermochronometry

Abstract

We demonstrate a technique that uses classical diffusion theory to determine ^4He concentration profiles within minerals. This approach should prove useful for constraining the low temperature cooling histories of individual samples and for correcting (U-Th)/He ages for partial diffusive loss. To validate the method, we investigated apatites with independently constrained cooling histories. The minerals of interest were irradiated with 220 MeV protons to induce an artificial distribution of spallation ^3He. Our results indicate that proton-induced ^3He is uniformly distributed and that radiation damage associated with a proton fluence of ~5x10^(14) protons/cm^2 does not cause noticeable changes in He diffusion behavior. Proton-induced ^3He can therefore be used to establish He diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural ^4He distribution is inhomogeneous. In addition, step heating ^4He/^3He analysis of a mineral with a uniform synthetic ^3He concentration provides a means to constrain its natural ^4He distribution that arose during ingrowth and diffusion over geologic time.