PRELUDE TO A SUPERVOLCANO: REU INVESTIGATIONS IN THE MIOCENE VOLCANIC FIELD OF THE SOUTHERN BLACK MOUNTAINS, ARIZONA

Ten undergraduates are investigating magmatism leading up to and including the >640 km³, 18.8 Ma Peach Spring Tuff (PST) supereruption in the southern Black Mountains western AZ (Pamukcu et al. 2013; Ferguson et al. 2012) through an REU led by Vanderbilt and Mercyhurst Universities. The supereruption was shortly preceded by voluminous trachytic lavas, thin mafic lavas, an enigmatic ignimbrite, the Cook Canyon Tuff (CCT), and locally important sedimentary rocks. In year 1 of this 3-year REU, students characterized and explored potential relationships between these units in an effort to advance our understanding of supervolcano construction and evolution. Students worked with leaders to design and implement original, collaborative research that included field work and some combination of remote sensing, optical microscopy, SEM, XRF, Excel-MELTS modeling, mineral thermometry and trace element modeling. Using satellite imagery, students identified and mapped units, and identified compositional variability (Beckens et al., 2014; Gibson et al., 2014; McGuiness et al., 2014). The voluminous pre-PST trachyte lavas are relatively homogeneous, and intrigued us as potential parent magmas for subsequent, more evolved magmas (Rice et al., 2014). The trachyte lavas contain 3 types of enclaves, representing 3 phases of related magmatism (Rentz et al. 2014). Field relations suggest that the lavas were locally coeval with wet sediments and, along with a local unconformity, reveal a dynamic environment leading up to the eruption of the PST, including a transition from trachytic to rhyolitic volcanism (Lee et al. 2014; Williams et al. 2014). Mafic lavas and a late, nearly aphyric trachyte document an influx of heat, potentially priming the crust for the PST magma body (Flansburg et al. 2014). Evaluation of crystal-rich trachytic pumice that marked the final stage of the dominantly high-SiO₂rhyolite PST eruption and included mafic enclaves suggests heating may have triggered the supereruption, remobilizing a crystal mush at the base of the magma chamber (Foley et al. 2014; Flansburg et al. 2014). Considering its spatial and temporal relationship to the PST, the CCT presents interesting questions of dynamics and timescales of magma production, evolution, and storage (Pratt et al. 2014).