U–Th/He age of phenocrystic garnet from the 79 AD eruption of Mt. Vesuvius

The U–Th/He system can potentially be used for dating volcanic rocks with ages as young as a few thousand years and as old as several million years, thus providing a valuable supplement to radiocarbon and K–Ar dating. Garnet phenocrysts from the 79 AD eruption of Mt. Vesuvius were dated to evaluate the accuracy with which the necessary measurements and corrections can be made. The determined age, corrected for diffusive loss of He, alpha ejection, and initial U-series disequilibrium, is 1885±188 yr which compares well with the known age of 1923 yr. U and Th concentrations were measured by isotope dilution on different aliquots than were used for He concentration measurements. Step-wise degassing yielded an Arrhenius relationship for He diffusion in garnet with an activation energy of 91.31±5.76 kJ/mol and ln D0/a2=−2.00±0.56. The uniformity of U and Th concentrations in garnet was checked by ion microprobe analysis. The 234U/238U and 230Th/238U activity ratios were measured by MC-ICPMS. The results suggest that with proper analysis and corrections, the U–Th/He method can be used to date young volcanic minerals with useful precision and accuracy, and may therefore be valuable for dating volcanic rocks that have low K or are otherwise difficult to date accurately with Ar–Ar or radiocarbon.