SINGLE CRYSTAL HELIUM-LEAD DATING OF DETRITAL ZIRCON

Detrital chronometry provides some of the best constraints on basin provenance, orogenic exhumation, and, in some cases, depositional ages of sedimentary rocks themselves. These constraints are often hampered, however, by limitations of single-system ages for each crystal. Combined crystallization and cooling ages of single crystals would more effectively constrain thermal histories of orogenic sources, discriminate between candidate sources, and identify first-cycle detrital components providing depositional-age constraints. We have developed a method for obtaining both (U-Th)/Pb and (U-Th)/He ages of single zircon crystals, using laser-ablation ICP-MS followed by whole grain He and U-Th measurement. (U-Th)/Pb ages are measured by excimer-laser-ablation ICP-MS, from ~30 mm-diameter and ~15 mm-deep pits in unpolished zircons. ⁴He is then extracted by laser heating and measured by ³He-isotope dilution, and whole-grain U-Th contents measured by ²²⁹Th-²³³U-isotope dilution on a sector ICP-MS. 2s precision is typically 2% for U/Pb ages, and 8% for (U-Th)/He ages. Replicate analyses of zircons from the 28.0 Ma Fish Canyon Tuff yield ²³⁸U/²⁰⁶Pb ages of 28.08 ± 0.50 Ma (2s) and (U-Th)/He ages of 27.9 ± 2.6 Ma (2s).

            We measured (U-Th)/(He-Pb) ages of detrital zircons from the modern Mississippi delta, and two samples of the Cretaceous-Eocene Ukelayet flysch, Kamchatka. Mississippi delta zircons have U-Pb ages between 10-1300 Ma, and those that have been analysed thus far for (U-Th)/He have He ages between 8-93 Ma. Most zircons with U/Pb ages less than ~120 Ma have corresponding He ages that are only about 10-20% younger, whereas most zircons with older U/Pb ages have significantly younger He ages, consistent with increased tectonic and magmatic activity in the western U.S. beginning the mid-Mesozoic. Zircons from the Kamchatka flysch have U/Pb ages between ~50-485 Ma (with some cores up to 1.9 Ga), but a relatively limited range of (U-Th)/He ages between 30-80, with most He ages between 30-35 Ma. Zircon He ages have been partially to completely reset by heating during burial, but there are positive correlations between He and Pb ages. Forward diffusion models of these correlations suggest peak burial temperatures between 145-165 °C, consistent with apatite and zircon fission-track ages and illite crystallinity.