Insights into the age of the Mono Lake Excursion and magmatic crystal residence time from (U‐Th)/He and 230Th dating of volcanic allanite

We present new data for the age of the Mono Lake Excursion at its type locality. Using the (U-Th)/He system on allanite, we dated Wilson Creek Ash 15 (Lajoie, 1968) to 38.7 ± 1.2 ka (2 SE). The new age for this ash supports the hypothesis (Kent et al., 2002; Zimmerman et al., 2006) that the Mono Lake Excursion is coincident with, and probably the same event as, the Laschamp Geomagnetic Excursion (40.4 ± 2 ka), an event that shares similar magnetic characteristics with the excursion identified at Mono Lake. We also estimate an allanite magma residence time of slightly less than 30 ka based on 230Th/238U disequilibrium and the (U-Th)/He-based eruption age.

► Allanite (U-Th)/He age of Wilson Creek Ash 15 of 38.7 ± 1.2 ka. ► Disequilibrium dating of allanite reveals a crystallization age of 66.0 ± 3.3 ka. ► The Mono Lake Excursion type locality is an expression of the Laschamp Excursion. ► Ash 15 allanite is from late Pleistocene lake sediments around Mono Lake. ► Combined dates suggest magmatic crystal residence time of ~ 30 ka.