Snapshots of a supereruption: multiphase insights into the Tuff of Elevenmile Canyon from glass, feldspar, biotite, and zircon

Understanding the architecture of magmatic plumbing systems, including the coexistence, depth, and connectivity of magma reservoirs leading to eruption, guides where and how we probe the crust for hazardous magma bodies. Given the scarcity of volcanic events at a given volcano and challenges in resolving magmas in the subsurface with geophysical techniques, products from past eruptions provide an alternate avenue to reconstructing this architecture. We present major and trace element data from matrix glass and minerals (feldspar, biotite, zircon) from the Tuff of Elevenmile Canyon supereruption (≤5000 km³; 60-78 wt.% SiO₂) in western Nevada, USA. Matrix glass geochemistry and geobarometry reveal that the eruption tapped multiple discrete, coexisting magma reservoirs within a vertically extensive magmatic system. Glasses fall into at least five compositional groups distributed over pressures ranging from ~50 to 1000 MPa (2-37 km), determined by multiple geobarometers, including rhyolite-MELTS, MagMaTaB, Al in hornblende, and the haplogranitic ternary. Glass compositions vary along at least two independent crystallization paths (Paths 1 and 2) which are associated with distinct feldspar textures and compositions. Path 1 feldspars are dominated by simple, oscillatory zoning, while Path 2 feldspars dominantly display complex resorption and exsolution-like textures (e.g., feathery lamellae at zone boundaries). Compositionally, Path 1 plagioclase records more extensive variation (An_20-65) from core to rim while alkali feldspar is compositionally constrained (Or_60-70). One distinctive sample group (Path 2) includes a feldspar population characterized by oligoclase and anorthoclase cores rimmed by alkali feldspar (anti-rapikivi). Biotite grains are ubiquitous and euhedral throughout the eruption, with no apparent breakdown or reaction rims. Their compositions generally fall into discrete groups associated with distinct pressure horizons in our geobarometry results. In BSE imaging, higher Mg# (47-60) biotites commonly contain dark, resorbed cores with bright rims while lower Mg# (40-45) biotites display the opposite trend and are either unzoned or oscillatory zoned. In some cases, single biotite grains may contain cores that fall into one sample group, and rims that plot in another, illuminating links between melts in different pressure horizons. Other samples bridge compositional gaps between populations. Thus, biotite captures melt interactions and dynamics not recorded by other phases. Zircon crystals from all glass compositional groups overlap in trace element composition. This suggests that this phase captures either a distinct period in the system’s evolution or an alternate process. This work highlights the power of a multiphase approach to capturing snapshots into a dynamic magmatic system’s history, leveraging both compositional and mineral textural information.