(U-Th)/He ages reflect the combined effects of He (a particles) loss due
to diffusion and He ingrowth due to radioactive decay of U and Th
series. Furthermore, the alpha particles are emitted with sufficient
kinetic energy that they can travel, in theory, up to 20 gm and as a
result, alpha 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 a-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. (C) 2007 Elsevier B.V. All rights
reserved.