BANDED PUMICE IN THE ~180 KA DACITE TUFF OF ANTELOPE WELL, MEDICINE LAKE VOLCANO, CALIFORNIA

Magmatic systems at ocean-continental convergent boundaries undergo a number of magma differentiation processes. Over time, systems may evolve into viscous, silica-rich magmas that can erupt in a violent and explosive manner. Magma mixing by basaltic recharge is one way that explosive eruptions may be triggered where the injection of hot, mafic magma into a cold, felsic magma in the shallow crust remobilizes the host magma into an eruptible state. The ~180 ka dacite tuff of Antelope Wells is a non-welded to welded ash-flow tuff that erupted from Medicine Lake Volcano (MLV) in the California Cascades. It is the only ash-flow tuff to erupt from MLV and is thought to have erupted during the formation of the volcano's caldera (Donnelly-Nolan et al., 2008). In addition to previously described juvenile clasts - white, vesicular, and crystal-poor felsic pumice and dark gray vesicular mafic scoria – the tuff contains banded pumice clasts where mm-scale streaks of the white pumice and dark gray scoria occur in the same clast. Also, some banded pumice clasts contain white felsic clasts wholly encased by mafic scoria. The abrupt contacts between white and dark gray pumice streaks in banded pumice are evidence of melt disequilibrium caused by the mingling of at least two physically and chemically distinct magma compositions. In locations where the stratigraphy of the dacite tuff is preserved, as at Antelope Well located 13 km northeast of Medicine Lake Volcano, the degree of welding and proportion of mafic scoria and banded pumice steadily increases with height compared to white felsic pumice clasts, which are generally limited to the basal portions of the ash-flow tuff. All juvenile clasts are dominated by plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides. Future in situ analyses of minerals and glass via SEM will be completed to characterize compositions and assess compositional variation. Results of these analyses will advance our understanding of the compositional and thermal history of the dacite magma prior to eruption, which will provide insight into the ability for the Medicine Lake Volcano to produce explosive eruptions with juvenile material formed through magma mixing processes.