In situ U-Pb ages of zircons from the Bishop Tuff: No evidence for long crystal residence times

Ion microprobe U-Pb isotope analyses of zircons from fallout and intercalated ash-flow deposits of the early, most chemically evolved Bishop Tuff (east-central California) yield a weighted mean age of 823 ± 11 ka (mean square of weighted deviates, MSWD = 0.8; n = 22). A similar mean age of 839 ± 36 ka (MSWD = 0.6, n = 15) obtained on unpolished zircon rims suggests that the difference between these ages and that of 760 ka for the Bishop Tuff obtained by recent Ar isotope determinations may be largely analytical in origin. Considerations based only on the effect of ²³⁰ Th- ²³⁴ U disequilibria on the U-Pb ages yield an apparent maximum mean crystal residence time for zircon of ~100 k.y. The zircon residence times are significantly shorter than those inferred from previous estimates for the timing of phenocryst growth and suggest that, once evolved, the early Bishop Tuff rhyolite was not stored for long before erupting.