Influence of crystal size on apatite (U–Th)/He thermochronology: an example from the Bighorn Mountains, Wyoming
Section snippets
Introduction and theoretical considerations
The decay of U and Th to 4He in common accessory minerals provides a low-temperature (low-T) thermochronometer because of the relatively high diffusivity of He at temperatures above ∼70°C for apatite [1], and ∼200°C for titanite [2] and zircon [3]. Thus measured (U–Th)/He ages reflect the competing effects of in-situ radiogenic He production and diffusive loss. In apatite and titanite, the length scale of He diffusion co-varies with crystal size [1], [2]. Extrapolated to geologic conditions
The Bighorn Mountains and (U–Th)/He dating procedures
To test these theoretical expectations, we measured apatite (U–Th)/He ages from samples of granitic basement from the Bighorn Mountains of north-central Wyoming. Previous apatite (U–Th)/He dating [6] has shown that most exposed basement rocks in the Bighorn Mountains have not been subjected to temperatures higher than about 60–90°C since the Precambrian, and represent a fossil He PRZ that was uplifted and exhumed during Laramide orogenesis, at about 65 Ma. We analyzed apatites from two (∼20 cm
Results and thermal history modeling
Table 1 and Fig. 3 show the (U–Th)/He ages of the Shell Canyon samples as a function of MWAR. Apatites from both samples show a wide range of He ages that correlate positively with MWAR: from 98 to 348 Ma across the range 32–99 μm in sample PRBH12, and from 107 to 232 Ma across the range 42–103 μm in sample PRBH17. These relationships between crystal size and He age are consistent with thermal histories involving long time periods in the He partial retention zone (approximately 30–70°C). The
Discussion and conclusions
Previous apatite (U–Th)/He dating studies have not reported crystal size–age correlations, which is probably due to either restricted crystal size distributions of most dated apatite aliquots, or the lack of significant crystal size effects for some thermal histories (e.g. rapid, recent cooling). However in certain cases, including those involving rocks from both modern and fossil He PRZs in both borehole samples and exposures such as the Bighorns, crystal size can have a measurable influence
Acknowledgments
We gratefully acknowledge the collaboration of the 1999 Keck–Bighorn participants (especially Peter Crowley, Joanna Reuter, and Grant Kaye), and Cathy Manduca. Thanks also to Charles Knaack for valuable technical assistance at WSU. We appreciate helpful reviews by Terry Spell and Peter Zeitler. This work was supported in part by NSF Grant EAR 0073576 to P.W.R.[RV]
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