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 + 1.8 ka (2 σ). 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 ( T 3) of overlying mafic lavas are 25.5 ± 4.4 and 22.0 ± 2.5 ka, and agree within uncertainties with 21Ne exposure ages ( T 21). T 3 and T 21 ages for dome lava south of La Virgen are slightly older ( T 3 = 42.5 ± 3.8 ka). (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 22 Ma 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.