Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with 40Ar/39Ar dating

U-Th)/He chronometry of zircon has a wide range of potential applications including thermo- chronometry, provided the temperature sensitivity (e.g., closure temperature) of the system be accurately constrained. We have examined the characteristics of He loss from zircon in a series of step-heating diffusion experiments, and compared zircon (U-Th)/He ages with other thermochronometric constraints from plutonic rocks. Diffusion experiments on zircons with varying ages and U-Th contents yield Arrhenius relationships which, after about 5% He release, indicate Ea 163-173 kJ/mol (39 - 41 kcal/mol), and D0 0.09 -1.5 cm 2 /s, with an average Ea of 169 3.8 kJ/mol (40.4 0.9 kcal/mol) and average D0 of 0.46 0.87 0.30 cm 2 /s. The experiments also suggest a correspondence between diffusion domain size and grain size. For effective grain radius of 60 m and cooling rate of 10°C/myr, the diffusion data yield closure temperatures, Tc, of 171- 196°C, with an average of 183°C. The early stages of step heating experiments show complications in the form of decreasing apparent diffusivity with successive heating steps, but these are essentially absent in later stages, after about 5-10% He release. These effects are independent of radiation dosage and are also unlikely to be due to intracrystalline He zonation. Regardless of the physical origin, this non-Arrhenius behavior is similar to predictions based on degassing of multiple diffusion domains, with only a small proportion (2- 4%) of gas residing in domains with a lower diffusivity than the bulk zircon crystal. Thus the features of zircon responsible for these non-Arrhenius trends in the early stages of diffusion experiments would have a negligible effect on the bulk thermal sensitivity and closure temperature of a zircon crystal. We have also measured single-grain zircon (U-Th)/He ages and obtained 40 Ar/ 39 Ar ages for several minerals, including K-feldspar, for a suite of slowly cooled samples with other thermochronologic constraints. Zircon He ages from most samples have 1 reproducibilities of about 1-5%, and agree well with K-feldspar 40 Ar/ 39 Ar multidomain cooling models for sample-specific closure temperatures (170 -189°C). One sample has a relatively poor reproducibility of 24%, however, and a mean that falls to older ages than predicted by the K-feldspar model. Microimaging shows that trace element zonation of a variety of styles is most pronounced in this sample, which probably leads to poor reproducibility via inaccurate -ejection corrections. We present preliminary results of a new method for characterizing U-Th zonation in dated grains by laser-ablation, which significantly improves zircon He age accuracy. In summary, the zircon (U-Th)/He thermochronometer has a closure temperature of 170 -190°C for typical plutonic cooling rates and crystal sizes, it is not significantly affected by radiation damage except in relatively rare cases of high radiation dosage with long-term low-temperature histories, and most ages agree well with constraints provided by K-spar 40 Ar/ 39 Ar cooling models. In some cases, intracrystalline U-Th zonation can result in inaccurate ages, but depth-profiling characterization of zonation in dated grains can significantly improve accuracy and precision of single-grain ages. Copyright © 2004 Elsevier Ltd