73-4 Geochemical and Magma Storage Evolution at Sunlight Peak Stratovolcano, Absaroka Volcanic Province, Wyoming

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

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Abstract:

Volcanic centers evolve throughout their lifetimes in composition, magma storage conditions, and eruptive tempos. Understanding magma storage durations before eruptions and how magma compositions change over time are key to improving our interpretations of volcanic hazards. However, much of the eruption history is buried within active volcanoes, making it difficult to assess long-term evolution of their magma systems. This study focuses on Sunlight Peak, a deeply eroded volcanic center active between 49 and 44 Ma. Sunlight Peak is part of the Eocene Absaroka Volcanic Province, which spans from northwestern Wyoming to southwestern Montana. By comparing magma storage timescales, mineral chemistry, and bulk lava compositions across multiple lava flows in stratigraphic sequence, this study addresses how magma composition and storage conditions influenced volcanic behavior at Sunlight Peak. 

Here, we integrate field observations, bulk lava compositions, and mineral geochemistry to investigate changes over time in the magma system at Sunlight Peak. The lavas in the stratigraphic section studied here are classified predominantly as shoshonites based on high Na₂O+K₂O (4-8 wt.%). Bulk lava compositions trend from higher silica in the older lavas (54-56 wt.% SiO₂) to less silicic younger lavas (50-54 wt.% SiO₂). Preliminary hypotheses support thermal priming of the magma storage system over time, allowing for eruption of less evolved lavas later in the lifetime of the volcano. Mineral thermometry results, using mineral chemistry measured via EPMA, will further test this hypothesis. 

In parallel, we will present results of diffusion chronometry and crystal size distributions (CSD) from the sampled lava flows to assess magma storage timescales prior to eruption. Diffusion chronometry exploits the gradual blurring of compositional boundaries in zoned minerals to estimate how long phenocrysts resided in the magma chamber prior to eruption. These timescales will be quantified using backscattered electron imaging (BSE) and experimentally calibrated diffusion models for Mg, Fe, and Ca in pyroxenes. CSD analysis will provide insight into crystal growth histories and magma cooling rates. By quantifying the size and abundance of feldspar crystals, CSDs distinguish between steady-state crystal growth in long-lived magma chambers and rapid crystallization during short lived storage or eruption-triggering events, complementing the timescales derived from diffusion chronometry. 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7435

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