A ZIRCON-BASED INVESTIGATION INTO TEPHRAS OF UNCERTAIN ORIGINS: A CASE STUDY WITH THE OLD CROW TEPHRA IN CENTRAL ALASKA

Knowing the eruptive frequency of explosive volcanoes is important for anticipating future risks and prioritizing monitoring efforts of active and dormant centers. Eruptive frequency can be constrained by studying the tephra record for an individual system, but this can be difficult in areas with complex volcanic histories. Several regional tuffs preserved in Fairbanks, AK originate from unconfirmed sources, including (in stratigraphic order, oldest to youngest): Sheep Creek-Fairbanks; Old Crow (OCT); and Halfway House tephras. The OCT represents a particularly significant eruption, traveling > 1000 km from its likely source (the Aleutians), and ejecting > 200 km³of material.

This study, which uses volcanic glass (major elements) and zircon (trace elements), is the first step in an ongoing project that will use zircon ages, trace elements, and isotopes as provenance tools to link tephras to their system of origin. Samples of OCT were collected from the Halfway House trench ~20 miles WSW of Fairbanks. Compositions of volcanic glass and associated minerals were measured using EDS with a variable pressure SEM (Vanderbilt). Glass compositions and mineral assemblages are consistent with published OCT data, with average SiO₂ wt% of 75.36%; Al₂O₃, Na₂O, and CaO wt% deviate less than 0.2% from the average published values.

Cathodoluminescence (CL) imaging of polished zircon, separated from bulk OCT, guided high spatial resolution trace element analysis of 45 crystals (48 spots) with the SHRIMP-RG (Stanford-USGS). Chondrite normalized REE diagrams (with mean elemental concentrations spanning two orders of magnitude from Gd to Lu) and bivariate plots of trace elements (particularly U/Yb vs Nb/Yb; Ti vs Yb; Sc/Yb vs. Nb/Yb) are consistent with zircons originating from a continental arc setting (Grimes et al., 2015). Concentrations of Hf and Ti in zircon range from 7,800 - 14,000 ppm and 2 - 17 ppm respectively, corresponding to crystallization temperatures of ~700 to 800°C for ~80% of analyzed grains. In the future, we hope to acquire more zircon-based evidence (ages and isotopic data) and compare OCT zircons to those from Emmons Lake, a proposed OCT source volcano. If successful, this provenance technique could be used in other areas where complex volcanic histories make it difficult to trace tephra deposits to their source.