COMBINING PHYSICAL VOLCANOLOGY AND MAGMA GEOCHEMISTRY TO UNDERSTAND THE COMPLEX HISTORY OF A YOUNG, MONOGENETIC VOLCANO: BLUE LAKE CRATER, OREGON (Invited Presentation)

Monogenetic volcanoes have increasingly been shown to be complex, with a wide range of magma compositions, magma storage processes, eruptive volumes and eruptive styles. Additionally, these common volcanoes are a likely source of future eruptions in the western US, and thus an understanding of their potential eruption styles and hazards is crucial.

This research presents an overview of the eruptive history of Blue Lake crater, a young (<3 ka), basaltic, monogenetic volcano in the central Oregon Cascades. Utilizing a range of techniques, including field mapping, deposit componentry and granulometry, and geochemical analysis of melt inclusions and minerals, this work assembles a comprehensive history of this volcano and, more broadly, illustrates the complexity of monogenetic volcanoes.

Mineral and melt geochemical analyses were used to investigate the pre-eruptive history of Blue Lake. Olivine-hosted melt inclusions are relatively volatile-rich (≤3.6 wt% H₂O, ≤850 ppm CO₂). Pressures of melt inclusion entrapment (185 ±14 MPa) suggest the Blue Lake magma stalled at ~6-7 km depth and crystallized. Zoning within olivine crystals was used to investigate eruption processes and magma ascent. Olivine are normally zoned with Fo₈₄cores and Fo₇₉rims; diffusion models indicate a more-evolved magma intruded into the magma storage region ~1-2 years prior to eruption. Ascent must have been relatively rapid (likely >0.1 m/s) given the lack of H⁺diffusion out of the melt inclusions; estimates of ascent rates are being refined using H₂O zoning in olivine.

Field mapping and deposit granulometry and componentry reveal changes in eruption style. Although called a “maar” volcano, Blue Lake had a complex eruptive history. The results of this work demonstrate that the eruption began with a dominantly magmatic episode, but quickly shifted to a phreatomagmatic phase, with surge deposits up to 0.3 m thick. Activity then transitioned to dominantly magmatic for the remainder of the eruption, with thick tephra fall deposits. This final explosive magmatic phase was likely driven by high melt H₂O contents, as shown in the melt inclusion analyses.