Helium Diffusion in Natural Xenotime

The typically high U and Th contents of xenotime ([Y,HREE]PO ₄ ) make this accessory mineral a promising candidate for (U-Th)/He thermochronometry if the ⁴ He diffusivity can be constrained well enough to estimate its closure temperature. We report new results for two ⁴ He step-heating experiments on different-sized fragments of a natural xenotime crystal from the Torghar district of Pakistan (FPX-1). This material, which has a composition within the range of most natural xenotimes (72 mol % YPO ₄ ), yields a laser ablation ²³⁸ U/ ²⁰⁶ Pb date of 28.82 ± 0.13 Ma and a (U-Th)/He date of 15.32 ± 0.61 Ma (2σ). Results for our more detailed diffusion experiment display excellent linearity on an Arrhenius diagram and indicate kinetic parameters of E = 131.4 ± 1.1 kJ/mol and ln(D ₀ /a ² ) = 10.61 ± 0.20 ln(s ⁻¹ ). These results suggest that the bulk closure temperature for ⁴ He in the degassed crystal fragment is ∼75 °C for the nominal cooling rate of 10 °C/Ma. At equivalent cooling rates and for crystals with equivalent diffusion dimensions, the closure temperature for helium in xenotime is ∼20 °C lower than the closure temperature for helium in apatite. Because xenotime typically has high U and Th contents, it may serve as a high-precision method for dating young, low-temperature cooling histories of rocks in which it crystallized. Helium diffusion in xenotime is likely to be moderately anisotropic and moderately dependent on crystal chemistry, so closure temperature interpretations should be made cautiously.