Implications of pre-eruptive magmatic histories of zircons for U–Pb geochronology of silicic extrusions

Zircons in silicic magmas begin to crystallize 10's to 100's of thousands of years (ka) prior to their eruption. U/Pb and U/Th dating for zircons from 32 young extrusions indicate a range of mean pre-eruption ages from ∼ 33 to 247 ka, with a median of 92 ka. Explosive large volume (widespread) eruptions show a narrower range and a younger median of 70 ka. These results indicate that the difference between initial zircon growth and eruption age measured by other (e.g., 40Ar/39Ar) techniques is in excess of the achievable analytical uncertainty for eruptions < 30 million years (Ma), and likely for eruptions of up to 300 Ma. Protracted pre-eruption zircon growth is well established by an increasing number of studies on magma process time scales, but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high-precision U/Pb data. Until intrinsic properties of zircons or their host deposits enable a priori estimates of pre-eruption age bias, it is prudent to expand the uncertainties attending all U/Pb zircon ages interpreted as deposition ages (in e.g., chronostratigraphic studies) by + 0/− 200 ka.