Gradients in H2O, CO2, and exsolved gas in a large‐volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff

Abstract

Infrared spectroscopic analyses of ∼140 melt inclusions in quartz phenocrysts from the zoned Bishop rhyolitic tuff demonstrate that systematic gradients in dissolved magmatic H2O and CO2 concentrations were present during preemptive crystallization of the magma body. Melt inclusions from the earliest erupted samples contain lower H2O (5.3±0.4 wt %) and CO2 (62±37 ppm) than inclusions from the middle of the eruption (5.7±0.2 wt % H2O; 120±60 ppm CO2). Melt inclusions from late erupted samples have much lower H2O (4.1±0.3 wt %) and higher and variable CO2 (150–1085 ppm). Trace element analyses of melt inclusions by ion microprobe show that inclusions within single pumice clasts from the early and middle Bishop Tuff have an inverse correlation between CO2 and incompatible elements. This pattern indicates that the magma was gas‐saturated during crystallization, with CO2 partitioning into a coexisting gas phase. Quantitative modeling using H2O‐CO2 solubility relations reveals a preeruptive gradient in exsolved gas, with gas contents varying from ∼1 wt % in the deeper regions of the magma body to nearly 6 wt % near the top. Dissolved Cl, B, Li, and Be in melt inclusions correlate negatively with CO2. Mass balance modeling of Cl loss to exsolving H2O‐rich gas during crystallization provides strong corroborating evidence for the mass fractions of exsolved gas estimated from H2O, CO2, and trace element data. Pressures of quartz crystallization and melt inclusion entrapment calculated from inclusion H2O‐CO2 data are consistent with progressive downward tapping of a zoned magma body during the eruption. Melt inclusion gas saturation pressures, magma volume estimates, and time‐stratigraphic‐compositional relations suggest that early erupted magma was stored at the top of a downward widening magma body. Melt inclusion data and the inferred gradients in dissolved H2O, CO2 and exsolved gas in the Bishop magma body suggest that gas saturation plays an important role in the formation and subsequent preservation of compositional gradients in silicic magma reservoirs.