The eruptive and magmatic history of the youngest pulse of volcanism at the Valles caldera: Implications for successfully dating late Quaternary eruptions

New 40Ar/39Ar and U/Th ages provide insight into the youngest eruptions at the Valles caldera and also reveal previously unknown pulses of magmatism. The youngest eruptive units, collectively termed the East Fork Member of the Valles Rhyolite, include the El Cajete pyroclastic beds and co-erupted Battleship Rock ignimbrite, and the disconformably overlying Banco Bonito lava flow. Previous attempts to date these units using a variety of techniques yielded ages ranging from < 40 ka to > 1 Ma. New 40Ar/39Ar ages were generated using the high-sensitivity, multi-collector ARGUS VI mass spectrometer, which provides more than an order of magnitude increase in precision compared to most single-detector mass spectrometers. 40Ar/39Ar dating of single crystals yields a range of ages, of which the youngest populations are interpreted to represent the eruption age. Sanidine ages indicate that the El Cajete pyroclastic beds and Battleship Rock ignimbrite erupted at 74.4 ± 1.3 ka, whereas the Banco Bonito lava erupted at 68.3 ± 1.5 ka. Populations of older crystals represent variably degassed xenocrysts, explaining why previous 40Ar/39Ar bulk step-heating analyses yielded spuriously old and irreproducible results. U/Th dating of unpolished zircon surfaces also yield multiple age populations, which range from the eruption age to > 350 ka, indicating protracted magmatism during the 453-ka-long eruptive hiatus prior to the eruption of the East Fork Member. 40Ar/39Ar and U/Th ages indicate that the East Fork Member represents a short (6.1 ± 2.8 ka) eruptive cycle, from a longer-lived magmatic system beneath the southern caldera. Equally short repose periods, similar to the interval between the El Cajete-Battleship Rock and Banco Bonito eruptions, are possible during future volcanism at the Valles caldera. Results demonstrate that detailed geochronology using single-crystal and in-situ techniques is necessary for understanding the eruptive history and magmatic evolution at some young volcanic systems.