MELT INCLUSION AND CRYSTAL TEXTURAL INSIGHTS INTO THE ASSEMBLY, STORAGE AND LATE-STAGE EVACUATION OF THE BISHOP TUFF MAGMA SYSTEM

Major insights into how large silicic magma systems are assembled, stored in the crust, and ultimately erupted are derived via information gathered from crystal-scale features like melt inclusion composition and trace element zoning patterns, especially when linked to controls from field studies. Here, for the c. 0.76 Ma Bishop Tuff, California, we have targeted the stratigraphically latest fall unit (F9) to investigate the late-stage dynamics in the deeper regions of the compositionally and thermally zoned magma body. The spatial and temporal connection between the crystal-poorer, more evolved ignimbrite to the east of Long Valley caldera and the crystal-richer, less evolved ignimbrite to the north is still debated. New trace-element and volatile data (supplemented by data from Wallace et al. 1999, JGR 104, 20097; Roberge et al. 2013, CMP 165, 237; Chamberlain et al. 2015, J Pet 56, 605) from quartz-hosted melt inclusions (MIs) show that unit F9 is the condensed fall equivalent of the northern and eastern ignimbrite lobes, incorporating material from a range of levels in the magma chamber. This result confirms that these major ignimbrite packages, once considered as sequential, were erupted coevally during the later stages of the eruption. The compositions of some F9 matrix glasses and MIs have been affected by late-stage mixing with less evolved melts that link to the source of mixed-composition swirly and dacite pumices present in several Bishop Tuff units. To test these relationships, we present new volatile data from MIs in unit F9, as well as recalculated water values for MIs from previously published work (Wallace et al. 1999; Roberge et al. 2013; Myers et al. 2019, JVGR 369, 95) that may have been affected by post-entrapment diffusive H exchange, to estimate pressures/depths of entrapment during the interactions with the less-evolved inputs. In addition, diffusion chronometry using Ti in quartz and Ba in sanidine from F9 samples is used to provide estimates of the timing of these mixing events. These and future studies will continue to elucidate the complex processes at work in the formation and evacuation of supervolcanic-scale magma bodies in the upper crust.