Figure 3

Results of U–Th dating of zircon cores ((A,B), by LAICPMS) and outermost rims ((C–E), depth profiling by SIMS). Notice significant inheritance of older zircons. In (C), average ages, where no significant age variability was detected with depth (~ 3 µm total depth profile); (D) ages calculated for ~ 1 µm outermost rim penetration in Holocene lavas, see Methods for further explanation. Numbers next to error ellipses (displayed at 1 s uncertainty) correspond to zircon number; gray lines connect progressively deeper zircon layers in each profile. (E) Histogram of all model ages of dated zircon rim indicating protracted zircon crystallization even for the outermost 1–3 µm of individual zircon crystals ranging between 13.9 ka and secular equilibrium (> 300 ka). Model ages are displayed for an initial (²³⁰Th)/(²³²Th) corresponding to whole rock Th and U abundances, assuming secular equilibrium. The younger isochron age in A and B is based on the youngest zircon cores, but these are ~ 20 to 37 kyr older than the presumed Late Holocene eruption age of these lavas, as is the zircon surface age in (D), suggesting that zircons were dissolving prior to the eruption (see Fig. S1).