GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 324-9
Presentation Time: 9:00 AM-6:30 PM


METCALF, James R.1, BAUGHMAN, Jaclyn S.2 and FLOWERS, Rebecca M.1, (1)Department of Geological Sciences, University of Colorado Boulder, Boulder, CO 80309, (2)Department of Geological Sciences, University of Colorado Boulder, UCB 399, Boulder, CO 80309, James.Metcalf@Colorado.EDU

Refractory phases, especially zircon and titanite, are becoming increasingly popular targets for (U-Th)/He thermochronology due to their common occurrence in a variety of lithologies and their survivability during erosion, transport, and deposition. Zircon is also useful as a detrital thermochronometer, with studies that have double- (U-Pb and (U-Th)/He) or even triple- (U-Pb, (U-Th)/He, and fission-track) dated single grains. In addition, recent work has documented that zircon and titanite (U-Th)/He dates can provide thermal history information over a wide range of upper crustal temperatures (~200 °C to <50 °C) because of the effects of accumulated radiation damage on He diffusivity. However many of these studies, especially detrital investigations, require large numbers (~100) of individual analyses per sample. This demand for large datasets presents a unique problem for (U-Th)/He dating because of the time required to dissolve refractory silicate phases, especially zircon. The most common method to dissolve these silicates uses pressurized acid-vapor dissolution techniques, which is both time-consuming (~1.5 weeks to complete) and space limited (15-18 fractions per dissolution vessel).

This contribution evaluates a new method that uses ammonium fluoride to dissolve whole zircon and titanite crystals for (U-Th)/He dating. Instead of the 1.5 weeks needed to dissolve grains using standard acid-vapor dissolution methods, this new technique can completely dissolve grains in ~4 hours. In addition, it does not require pressure vessels, and instead uses screw-top Teflon vials, greatly increasing sample throughput. Finally, this method reduces the amount of ultra-pure reagents needed for dissolution (specifically ultra-pure HF), further decreasing the cost of analysis.

Zircons from the fish canyon tuff standard dissolved with the new method yield a date of 28.9 ± 1.7 Ma (n=10), indistinguishable from grains analyzed using the standard acid-vapor dissolution method (28.7 ± 1.8 Ma, n=178). Dates for 3 other zircon samples, as well as 1 titanite sample, are similarly indistinguishable between the new and conventional methods. These data strongly suggest that this new, higher-throughput method completely dissolves zircon and titanite and can be employed regularly in (U-Th)/He thermochronology.