U-Th-He age determinations on volcanic rocks: A new tool for Quaternary geochronology.

The U-Th/He system, which has been used successfully for thermochronology, also has potential for dating young volcanic rocks. To test this possibility the age of garnet phenocrysts from the 79 AD eruption of Mt. Vesuvius was measured. The samples consisted of 600-1000 μm diameter andradite-grossular garnet crystals separated from a volcanic tuff previously dated by Ar-Ar (Renne et al, 1997). Our age calculation accounts for diffusive loss of radiogenic He since eruption, loss of He by alpha ejection from the garnet grains, U series disequilibrium, and non-uniform U-Th distribution within the mineral grains. The effect of diffusion was investigated using existing diffusivity and activation energy data for almandine rich garnet (Dunai and Roselieb, 1996), and by step-wise degassing of the Vesuvius garnets. Diffusivity was measured over 7 temperature steps ranging from 800^oC to 1300^oC with one additional 1500^oC step for complete extraction. Even up to the 1300^oC step the diffusivity remains linear, indicating an Arrhenius relationship with an activation energy of approximately 427 kJ/mol and a frequency factor, D_o/a² = 0.578 s^-1. Alpha emission is accounted for by modeling the garnet grains as spheres with the injection and ejection of alphas from the sphere depending on the concentration and distribution of the U and Th inside and outside of the mineral grain. Distribution of U, Th was determined by ion-probe analysis across garnet grains in thin section. While the overall concentration of U and Th within the garnets is quite high (approximately 16-19ppm and 23-27ppm, respectively), there is a deficiency in ²³⁰Th and ²²⁶Ra. The ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity we measure directly, while the ²²⁶Ra/²³⁸U activity ratio was estimated by assuming Ra follows barium. Due to machine limitations, U, Th concentrations and He concentrations were determined on different aliquots. The measured concentration of ⁴He in the garnet grains varies from 3.72x10^-9 to 4.16x10^-9 cc STP/g, giving us a range in the calculated age of approximately 1820 +/- 140 yr. These results suggest that with the proper analysis and corrections, we can apply the U-Th/He method to dating young volcanic minerals. The potential value of the method is that it can provide ages for rocks as young as a few thousand years and as old as several million years, supplementing the radiocarbon and K-Ar approaches in this critical age range. Renne, P.R., W.D. Sharp, A.L. Deino, G. Orsi, L. Civetta, { \it Science} 277 (5330), 1279-1280, 1997 Dunai, T.J., K. Roselieb, { \it Earth and Planetary Science Letters} 139 (3-4), 411-451, 1996