Helium Diffusion in Natural Xenotime

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 20; no. 1; pp. 417 - 433
• Main Authors: Anderson, Alyssa J., Hodges, Kip V., Soest, Matthijs C., Hanchar, John M.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.01.2019
• Subjects: (U‐Th)/He dating; Ablation; Apatite; Composition; Cooling; Crystals; Diffusion; Dimensions; Dye dispersion; Heating; Helium; helium diffusion; Lasers; Phosphates; Temperature; thermochronology; xenotime
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2018GC007849

The typically high U and Th contents of xenotime ([Y,HREE]PO4) make this accessory mineral a promising candidate for (U‐Th)/He thermochronometry if the 4He diffusivity can be constrained well enough to estimate its closure temperature. We report new results for two 4He 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 % YPO4), yields a laser ablation 238U/206Pb 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(D0/a2) = 10.61 ± 0.20 ln(s−1). These results suggest that the bulk closure temperature for 4He 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. Key Points Kinetic parameters are reported for helium diffusion in natural xenotime The bulk closure temperature for helium in xenotime is ∼20 °C lower than that for helium in apatite Helium diffusion in xenotime is likely only moderately, not extremely, anisotropic