Step-wise de-gassing diffusion experiments, performed on 39 different apatite samples using radiogenic 4He and proton-induced 3He, revealed closure temperatures (Tc) ranging from 50 to 115C, for a cooling rate of 10C/Myr. It was found that there was no correlation between He diffusion and apatite chemistry, including the F/Cl ratio. However, the closure temperature was positively correlated with the radiogenic 4He concentration, [4He], in each sample. It was suggested that [4He] was a proxy for natural exposure to actinide radioactivity below the closure temperature and that He diffusion in apatite was impeded by radiation-induced damage. The kinetics were therefore a changing function of time, and measured diffusivities alone could not be applied to the thermochronometric interpretation of a given sample. The effect of radiation damage upon He diffusion appeared to exceed other known effect upon He diffusivity; including the grain size. The diffusion data were well-described by a previously proposed quantitative model that consisted of 2 Arrhenius relationships; one for volume diffusion through an undamaged mineral structure, and another for the release of He from radiation-damage traps. The unknown parameters in the trapping model were determined from diffusion experiments and permitted the development of a tentative mathematical function that related the diffusivity to temperature and to [4He]. By inserting this function into a 4He-production/diffusion model, it was shown how these results affected the interpretation of apatite (U–Th)/He thermochronometry. The model predicted that the effective 4He closure temperature of apatite would vary with the cooling rate and the effective U concentration, and could differ from the commonly assumed Tc of 70C by up to ±15C. The 4He partial retention zone would therefore look similar to previous estimates, but its depth would depend upon the accumulation time and the effective U concentration. It was particularly noted that samples subjected to re-heating, after the accumulation of substantial radiation damage, would be more retentive than was previously expected.

The Influence of Natural Radiation Damage on Helium Diffusion Kinetics in Apatite. D.L.Shuster, R.M.Flowers, K.A.Farley: Earth and Planetary Science Letters, 2006, 249[3-4], 148-61