PRE-ERUPTIVE TIMESCALES OF DEVILS WASHBASIN BASALT AND HOGBACK MOUNTAIN BASALTIC-ANDESITE DEPOSITS IN THE GOAT ROCKS AREA, CASCADE ARC, WA

Understanding timescales related to pre-eruptive processes provides critical information to support eruption forecasting and volcanic hazard assessment. Diffusion chronometry has allowed for the calculation of magma recharge-to-eruption timescales by modeling the chemical gradients between zone boundaries preserved in crystals in erupted material. We apply this tool using iron and magnesium interdiffusion in clinopyroxene crystals to investigate the timing of pre-eruptive processes in the Goat Rocks area in the Cascade Arc in Washington state. The area comprises several composite volcanoes built between 3.1 and 0.1 Ma and several nearby mafic volcanoes active between 3.5 Ma and 65 ka. Here, we focus on the Devils Washbasin basaltic volcano (~3.0 Ma) and Hogback Mountain basaltic shield complex (~1.0 Ma), to begin to explore relationships between pre-eruptive timescales and characteristics of volcanoes such as eruptive style, composition, and lifespan. Backscattered electron images of zoned clinopyroxenes show normal zoning textures, dark and more mafic cores, and bright and more evolved rims. Greyscale profiles were extracted perpendicular to the zone boundaries nearest the crystal edge using ImageJ. Using a misfit minimization approach in Python, these greyscale profiles—using greyscale value as a proxy for Fe/Mg ratio—were modeled at a range of temperatures from 1050ºC to 1210ºC using the analytical solution to the diffusion equation. Preliminary results for Devils Washbasin basalt and Hogback Mountain basaltic andesite suggest that for both units, the timing between the final magmatic event prior to eruption (i.e., potential recharge event mobilizing stored magma) is on the order of years and no longer than decades. These timescales agree with diffusion studies conducted on other mafic eruptive products of the Cascade Arc, including from Mt. Baker, Mt. St. Helens, and Lassen. Our results provide insight into pre-eruptive timescales for polygenetic mafic eruptive centers of Cascade volcanoes, and more so, provide the first ever pre-eruptive timescales for the Goat Rocks area. Future research will include comparing pre-eruptive timescales between mafic and felsic centers in the Goat Rocks area and utilizing additional crystal phases for diffusion chronometry.