INSIGHTS FROM CRYSTAL CHEMISTRY INTO THE PLUMBING SYSTEM OF A RAPIDLY ERUPTED SHIELD VOLCANO, CASCADES VOLCANIC ARC, CALIFORNIA

Crater Mountain is a ~11 km³ volcano erupted ~365-370 ka that is located ~40 km to the east of Lassen Peak in the southern Cascades Volcanic Arc. Recent paleomagnetic work has revealed that the edifice was emplaced in a maximum of 50 to 200 years and possibly much less, which implies rapid ascent and sustained eruption of its lavas. Additionally, most of the edifice is composed of phenocryst-poor lavas, implying short, rapid transit through the shallow crust. The duration of this eruptive sequence is typical, yet the shield edifice is larger, than the classic monogenetic volcano. This study investigates the plumbing system that supported such rapid eruption of the Crater Mountain shield volcano though mineral compositions, thermobarometry, and diffusion chronometry of Fe and Mg in orthopyroxene. Erupted lavas are andesitic with considerably mixed antecrystic and equilibrium mineral assemblages, implying variable mixing of magmatic batches sourced from different crustal levels. Compositions of olivine, spinel, orthopyroxene, clinopyroxene, and plagioclase are investigated, with multiple mineral and mineral-melt pairs leveraged to constrain pressures and temperatures of formation. Orthopyroxene phenocryst cores mostly range from Wo_2.5–3.5 En_79–82 Fs_15–17 and have high Cr₂O₃, up to (and rarely higher than) ~0.5 wt.%, and Al₂O₃ to ~2.4 wt.%. Antecrysts of ortho- and clinopyroxene and plagioclase phenocrysts fractionated in the lower crust from a basaltic liquid and were brought to the surface in otherwise mostly aphyric magmas. This study highlights an exemplar of the widespread yet under-investigated shield volcanoes in the Cascades Volcanic Arc, providing some of the first assessments of the plumbing system dynamics that support the eruption of large amounts of lava in a human lifespan.