23-6 Pre-Eruptive Magmatic Processes and Eruption Initiation Timescales for the Multi-Phase Eruption of Sunset Crater, San Francisco Volcanic Field, Arizona, USA

Session: Distributed volcanic fields of the southwestern U.S. (Posters)

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Abstract:

Understanding pre-eruptive processes and their associated timescales is critical for future eruption forecasting and hazard mitigation. Currently, there is a limited understanding of magmatic processes and timescales for the San Francisco Volcanic Field, where the most recent eruption occurred at Sunset Crater (ca. 1085 CE). Sunset Crater erupted in several phases, with effusive activity throughout: fissure opening, cone-building and subplinian (VEI 3–4) eruptions, and waning explosive eruptions. We focus on the first three (of five) Sunset Crater tephra units: Unit I (fissure phase) and Units II and III (subplinian phase).

This study aims to (1) constrain pre-eruptive magmatic processes and (2) determine their timescales using diffusion modeling. Specifically, we examine olivine, plagioclase, and clinopyroxene textures and chemistry from each of the selected tephra units. Similar textures are observed across all tephra units. Resorption textures are common (i.e., extensive spongy sieving of plagioclase, rounded clinopyroxene morphologies, melt embayments in all phases). Olivine crystals have skeletal rim morphologies, indicating undercooling and rapid growth before eruption.

Notably, olivine crystals can be divided into four populations by size fraction and geochemical zoning patterns. Major and trace element chemical transects across olivine core to rim zones were collected via EPMA and LA-ICP-MS. Across olivine zones in all populations, distinct major and trace element concentrations indicate interaction between multiple magma compositions before eruption. Rim zone compositions are similar across all olivine populations and have the lowest Fo content relative to interior zones (i.e., normal zoning). The combination of plagioclase and clinopyroxene resorption with skeletal, normally zoned olivine indicate mafic recharge as a likely eruption initiation mechanism.

For modeling Fe-Mg and Ni diffusion in olivine across the outermost rim zone boundary, temperature of diffusion is estimated by olivine-liquid thermometry, with liquid composition from Sunset Crater melt inclusion analyses¹. The resulting distribution of eruption initiation timescales is similar for Fe-Mg and Ni diffusion modeling (n= 41 transects), with a range of 2 days to 1 year, average of 1 month, and mode at 3 and 1.6 weeks for Fe-Mg and Ni results, respectively. Timescales are similar across successive tephra units. Overall, olivine diffusion timescales indicate that eruption initiation (likely mafic recharge) occurred within weeks of the Sunset Crater eruption.

1.       Allison, CM et al. (2021). Nat Commun, https://doi.org/10.1038/s41467-020-20354-2

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

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