40AR/39AR GEOCHRONOLOGY PROVIDES NEW INSIGHTS INTO ERUPTIVE DURATIONS OF POST-CALDERA LAVA DOMES AT THE VALLES CALDERA, NEW MEXICO, USA

High-precision ⁴⁰Ar/³⁹Ar geochronology is assessing the frequency and duration of post-caldera activity at the Valles Caldera in northern New Mexico. Initial dating has focused on the two oldest post-resurgent domes, Cerro del Medio and Cerros del Abrigo. A new age for the third oldest unit of Cerro del Medio is 1158.2 ± 3.4 ka. When coupled with the published age of 1237 ± 17 ka for the oldest unit of Cerro del Medio, the new age indicates an eruptive lifespan of at least 78.8 ± 17 ka for the dome. The oldest unit of Cerros del Abrigo yields a new age of 1010.3 ± 1.6 ka. The published age of the second oldest lobe of Cerros del Abrigo is 983 ± 20 ka, suggesting a repose period of 27.3 ± 20 ka for successive pulses of Cerros del Abrigo activity. This preliminary data indicates that at least two domes were constructed during numerous protracted events, rather than a single eruptive episode. A critical aspect to improving our understanding of post-Valles volcanism is avoiding sanidines that contain melt inclusions. For example, melt inclusion-rich sanidines from the Cerro del Medio unit yield multiple age populations between ~1140 and 1290 ka, indicating that melt inclusion-hosted excess ⁴⁰Ar is detrimental to establishing accurate eruptive histories. Ongoing research is focused on assessing whether the younger post-caldera Valles domes have protracted eruptive histories with significant repose periods. The ages of all 33 post-caldera units will be used to establish the eruptive lifespans of the post-caldera domes, as well as the duration of resurgence. Additionally, ages will be combined with a volumetric assessment in order to yield the post-caldera eruptive flux at Valles that will be compared to other Quaternary caldera systems. We anticipate that the comprehensive, precise, and accurate ages of the post-caldera units at Valles Caldera will have implications for volcanic hazard assessment of the region.