The 2008 eruption of Okmok Volcano, Alaska: Petrological and geochemical constraints on the subsurface magma plumbing system

The July-August 2008 phreatomagmatic eruption of Okmok Volcano produced ~ 0.26 km3 (DRE) of phenocryst-poor (1 to 2 vol.%) basaltic andesite ejecta, compositionally distinct from the basalt erupted during 1997 (51.90 wt.% SiO2). Analyzed juvenile products are tan to dark gray vesicular lapilli (scoria), and dense, purple-black bombs. Whole-rock compositions cluster tightly (54.97 ± 0.25 wt.% SiO2). The eruption also produced mafic ash containing basaltic groundmass glasses (52 wt.% SiO2) and olivine-hosted melt inclusions (down to 47 wt.% SiO2). The scoria and early-erupted ash contain compositionally similar plagioclase, clinopyroxene, and olivine phenocrysts. Olivine phenocrysts in the scoria and ash are not in equilibrium with the basaltic andesite whole-rock composition. Olivine-hosted melt inclusions yield 0.11 (± 0.04) to 3.61 (± 1.24) wt.% total H2O by μ-FTIR, with an average of 1.23 ± 0.68 (1σ) wt.%. Three inclusions contain CO2 = 37 to 49 ppm with the rest below detection. Solubility model-derived inclusion entrapment/re-equilibration depths extend from near surface to 4.6 (± 2.5) km, in agreement with recent geophysical studies. The 2008 eruption was triggered by an influx of melt-rich basalt originating from the 3 to 6 km storage region beneath the center of the caldera, which intersected a shallower, more evolved magma body beneath Cone D. Our study concludes that the Okmok magma system is “mush-column” like, containing multiple magma bodies with a common and frequent replenishment source, but segregated with unique geochemical signatures.