Coupled Uranium-Series and (U-Th)/He Zircon Geochronology of the Emmons Lake Volcanic Center (ELVC): Dating the Record of Voluminous Tephra Production in Quaternary Eastern-Beringia.

Abstract:

Tephrochronology is an invaluable tool used to date, link, and reconstruct paleo-environments, climates, and landscapes. Single tephra layers represent isochronous markers across broad regions, thus accurate and precise temporal constraints on the timing of eruption are critical to their utility. If a U-bearing accessory phase such as zircon is present, U/Pb, U-series, and (U-Th)/He geochronometers may be selectively applied. Application of multiple geochronometers to the same sample corroborates accuracy, can potentially resolve mineral crystallization and volcano eruption dates, and can define an eruption age from inherited crystals, assuming complete thermal resetting of the (U-Th)/He system upon crystal incorporation into magma prior to eruption. The Emmons Lake Volcanic Center is one of the largest Quaternary volcanic systems in the Aleutian volcanic arc, and is characterized by at least two major caldera-forming eruptions. C1 has been dated by ⁴⁰Ar/³⁹Ar at ~238 ka, and was originally proposed as the source for the Old Crow tephra, the largest and most widespread Quaternary tephra in eastern Beringia, and a critical time horizon for reconstruction of Pleistocene paleo-environment and climate. C2 produced the widespread Dawson tephra, and has been dated indirectly by radiocarbon at ~27 ka. We present in-situ grain-surface ion microprobe (SHRIMP-RG) ²³⁸U-²³⁰Th and/or U/Pb data on a suite of autocrysitc zircon grains from a C1 sample, the Old Crow, and from the Dawson. On these same zircon crystals, we utilize a noble gas sector mass spectrometer to make sensitive, low blank, single crystal ⁴He measurements. With these datasets, we investigate the temporal and potential genetic relationship between C1 and Old Crow, and place absolute radiogenic time constraints on the C2 eruption. Coupled ²³⁸U-²³⁰Th and sector field (U-Th)/He application shows significant promise for generating accurate, precise dates for Quaternary tephra bearing a U-rich accessory mineral phase.