(U–Th)/He thermochronometry: Mapping 3D geometry using micro-X-ray tomography and solving the associated production–diffusion equation

(U–Th)/He ages reflect the combined effects of He (α particles) loss due to diffusion and He ingrowth due to radioactive decay of U and Th series. Furthermore, the α particles are emitted with sufficient kinetic energy that they can travel, in theory, up to 20 μm and as a result, α particles may be ejected beyond the crystal edges. To circumvent this problem a correction factor must be applied, which is essentially based on the geometry of the crystals [Farley, K. A., Wolf, R., and Silver, L. (1996). The effect of long alpha-stopping distances on (U–Th)/He dates. Geochimica et Cosmochimica Acta, 60:4223–4229]. This strongly limits the application of the technique to crystals of appropriate geometries. We present here a method to compute the effects of α-ejection for (U–Th)/He dating by taking into account the 3D morphology of individual crystals determined with micro-X-ray tomography. We also develop a model which solves the diffusion equation for the crystals that have been imaged. We then couple this model with an inversion algorithm to extract the range of thermal histories that the rocks may have experienced. This technique is applied to detrital samples from rocks collected in the Southern Alps of New Zealand, a region of high exhumation rate.