DIVERSE MAGMAS ERUPTED OVER SHORT TIME SCALES IN THE SAN FRANCISCO VOLCANIC FIELD, ARIZONA

Distributed volcanic fields (DVFs) are abundant in western North America, and future unrest at one or more of these areas in the coming decades is possible given known recurrence intervals. To conduct volcanic hazard assessments for DVFs, we investigate past eruptive behavior (e.g., eruption styles, volumes), magmatic processes (e.g., melt sources and storage conditions), and timescales of eruptions (e.g., ages/recurrence intervals, eruption duration, and magma storage and ascent timescales). This study contributes field observations, bulk rock and mineral compositions, and paleomagnetic directions to better characterize the recent eruptive history of the San Francisco Volcanic Field near Flagstaff, AZ. We focus on unraveling the progression of the ~19 ka eruptions of The Sproul (a shallow spatter vent and lava lake) and adjacent Merriam cluster (comprising two scoria cones, Merriam Crater and Merriam-A, and uplifted lava pond Merriam-B), which together produced at least five lava flows (including the Grand Falls lava flow). The Sproul and Merriam cluster each represent a distinct compositional suite: The Sproul lavas are more alkalic ol + cpx basalt (1.0–1.3% K₂O, 9.2–8.3% MgO), while Merriam lavas and scoria are less alkalic, variably evolved ol +cpx ± plag basalts (0.7–0.9% K₂O, 9.6–7.4% MgO). Distinct trace element ratios such as Nb/Ta and Dy/Yb suggest that these were independent magma batches from different mantle sources, despite having erupted in close proximity. The two suites display similar clinopyroxene textures (euhedral with cyclical zoning) and compositions, which may indicate similar crustal storage and magma recharge patterns. We show that a previously uncharacterized lava flow, partly buried by the Merriam cluster, matches The Sproul in composition; thus, we interpret that The Sproul erupted first in this sequence. Ground-penetrating radar and tephra pits confirm that lavas from Merriam-B were the last erupted products. Overlapping paleomagnetic directions for Merriam-B and The Sproul from this and previous studies suggest that all erupted within a matter of centuries. Continued work will refine conditions and timescales of magma storage, which will provide a basis for assessing whether similar eruptions in the future can be anticipated using geophysical monitoring data.