Eruption ages of Las Tres Vírgenes volcano (Baja California): A tale of two helium isotopes

Abstract

La Virgen tephra is the product of the youngest and most voluminous explosive eruption of Las Tres Vírgenes volcano (Baja California). Combined U-Th and (U-Th)/He zircon dating allows accurate correction for uranium-series disequilibrium, and yields an eruption age for La Virgen tephra of 30.7-1.4+1.8ka (2σ). No difference in eruption ages was found between samples from proximal and distal La Virgen tephra. A previously undated tephra south of Las Tres Vírgenes volcano yielded U-Th and (U-Th)/He zircon ages overlapping those of La Virgen tephra, which suggests that both units are identical. 3He surface exposure ages (T3) of overlying mafic lavas are 25.5±4.4 and 22.0±2.5ka, and agree within uncertainties with 21Ne exposure ages (T21). T3 and T21 ages for dome lava south of La Virgen are slightly older (T3=42.5±3.8ka). (U-Th)/He and surface exposure ages are stratigraphically consistent, whereas a published Holocene 14C charcoal age from the distal site could not be confirmed. Instead, abundant zircon crystals with (U-Th)/He ages between ~1 and 22Ma are interpreted as undegassed detrital grains, suggesting reworking of pumice at the distal location. The results presented here underscore the potential of combined U-Th and (U-Th)/He zircon dating for Quaternary tephrochronology. © 2010 Elsevier B.V.