U/Th and ⁴⁰Ar/³⁹Ar dating provides eruption ages and magmatic evolution for the East Fork Member of the Valles Rhyolite

Abstract

U/Th and Ar/Ar dating was conducted to determine the age of the youngest eruptions at the Valles caldera. The youngest series of eruptions, collectively known as the East Fork Member of the Valles Rhyolite, includes the co-erupted El Cajete pumice and Battleship Rock ignimbrite, and overlying Banco Bonito lava flow. Despite the importance for hazard assessment, previous geochronology investigations have not produced conclusive eruption ages for these units. Ages range from less than 30 ka to more than 1 Ma. Ar/Ar dating was conducted using the high-sensitivity ARGUS VI mass spectrometer, which is capable of dating late Quaternary samples with the single crystal method. Ar/Ar ages indicate that the El Cajete pumice and Battleship Rock ignimbrite erupted at 74.7 ± 1.3 ka, whereas the Banco Bonito lava erupted at 68.3 ± 1.5 ka. To test the reproducibility of these eruption ages, as well as to investigate the crystallization of the East Fork Member, the U/Th method was used to date zircon. Unpolished zircon surfaces were dated using the high-spatial resolving power of the SHRIMP-RG. By analyzing the surface of zircon crystals we are able to determine the timing of crystal growth immediately prior to eruption. U/Th ages indicate that the youngest phase of crystal growth occurred at 70 ± 4 ka for the El Cajete pumice and Battleship Rock ignimbrite, and 68 ± 4 ka for the Banco Bonito lava flow. These ages agree with the Ar/Ar eruption ages. In addition to zircon that crystallized near the time of eruption, U/Th ages indicate an abundance of inherited crystals. Most antecrystic or xenocrystic zircon yield ages ranging from near the eruption age to 97 ± 11 ka, and from 149 ± 16 ka to 160 ± 20 ka. These older zircon populations reveal a previously unknown period of protracted magmatism prior to the eruption of East Fork Member.