EVALUATING A NEW, RAPID ZIRCON AND TITANITE DISSOLUTION METHOD FOR (U-TH)/HE THERMOCHRONOLOGY

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.