The influence of natural radiation damage on helium diffusion kinetics in apatite

Stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4He and proton-induced 3He reveal a range in closure temperature (Tc) from ∼ 50 to 115 °C, for a cooling rate of 10 °C/Myr. There is no correlation between helium diffusion and apatite chemistry including F/Cl ratio, but the closure temperature is positively correlated with the radiogenic 4He concentration ([4He]) in each sample. We argue that [4He] is a proxy for a sample's natural exposure to actinide radioactivity below the closure temperature, and that helium diffusion in apatite is impeded by radiation-induced damage to the apatite structure. The kinetics must therefore be an evolving function of time; measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample. The effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity, including grain size.Our diffusion data are well described by a previously proposed, quantitative model that consists of two Arrhenius relations, one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage “traps.” The unknown parameters in this “trapping model” were determined from the diffusion experiments, and allow us to develop a tentative mathematical function that relates diffusivity to temperature and [4He]. By inserting this function into a 4He production-diffusion model we have explored how these results affect the interpretation of apatite (U–Th)/He thermochronometry. The model predicts that the effective 4He closure temperature of apatite will vary with cooling rate and effective U concentration (eU) and may differ from the commonly assumed Tc of 70 °C by up to ± 15 °C. The 4He partial retention zone will look similar to previous expectations, but its depth will depend on accumulation time and on eU. Most notably, samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected. These predictions are consistent with recent observations of unexpected apatite (U–Th)/He ages in some settings.