A complex patchwork of magma bodies that fed the Bishop Tuff supereruption (Long Valley Caldera, CA, United States): Evidence from matrix glass major and trace-element compositions

Abstract

The geologic record reveals events in which enormous volumes (100–1000s of km3) of magma were erupted in a matter of days to months. Yet, the architecture of magmatic systems that feed supereruptions can only be investigated through the study of ancient systems. For more than 40 years, the Bishop Tuff (Long Valley, California) has been the archetypal example of a single, zoned magma body that fed a supereruption. Early-erupted material is pyroxene-free and crystal poor (< 20 wt%), presumably erupted from the upper parts of the magma body; late-erupted material is orthopyroxene and clinopyroxene-bearing, commonly more crystal rich (up to 30 wt% crystals), and presumably tapped magma from the lower portions of the magma body. Fe-Ti oxide compositions suggest higher crystallization temperatures for late-erupted magmas (as high as 820°C) than for early-erupted magmas (as low as 700°C). Pressures derived from major-element compositions of glass inclusions were used to suggest an alternative model of lateral juxtaposition of two main magma bodies—each one feeding early-erupted and late-erupted units. Yet, this interpretation has proven controversial. We present a large dataset of matrix glass compositions for 227 pumice clasts that span the stratigraphy of the deposit. We calculate crystallization pressures based on major-element glass compositions using rhyolite-MELTS geobarometry and crystallization temperatures based on Zr in glass using zircon-saturation geothermometry. Additionally, we apply the same methods to 1,538 major-element and 615 trace-element analyses from a dataset from the literature. The results overwhelmingly demonstrate that the variations in crystallization temperature and pressure are not consistent with vertical stratification of a single magma body. All crystallization pressures and temperatures are very similar, with modes of ∼150 MPa and ∼730°C. Our results support lateral juxtaposition of three main magma bodies. Magmas represented by smaller stratigraphic units crystallized at similar pressures as the main bodies, which suggests coexistence of larger and smaller magma bodies at the time of eruption. We compare our findings with results for other very large eruptions and supereruptions. We argue that supereruptions typically mobilize a complex patchwork of magma bodies that reside within specific levels of the crust. These eruptions reveal the architecture of the crust during moments of high abundance of eruptible magma, revealing crustal states that differ from what is inferred for magmatic systems currently present on Earth.