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

correlation between CO 2 and incompatible elements. This pattern indicates that the magma was gas-saturated during crystallization, with CO 2 partitioning into a coexisting gas phase. Quantitative modeling using H20-CO 2 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 C1, B, Li, and Be in melt inclusions correlate negatively with CO 2. Mass balance modeling of C1 loss to exsolving H20-rich gas during crystallization provides strong corroborating evidence for the mass fractions of exsolved gas estimated from H20, CO 2, and trace element data. Pressures of quartz crystallization and melt inclusion entrapment calculated from inclusion H20-CO 2 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 H20, CO 2 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.

Gas‐saturated crystallization and degassing in large‐volume, crystal‐rich dacitic magmas from the Altiplano‐Puna, northern Chile

“…It is reasonably assumed that inclusions located in the interior of a crystal were trapped earlier than inclusions closer to the rim [Wallace et al, 1999]. Thus the zonal arrangement of inclusions can provide further information on magma evolution.…”

“…Crystals within an individual pumice clast are thought to have resided within the adjacent magma batch for a significant time prior to eruption [Wallace et al, 1999]. However, it should be noted that evidence for eruptive magma mingling and fragmentation of crystals during decompression and eruption was found in APVC pyroclastic deposits.…”

“…Melt inclusions trapped during phenocryst growth provide information on the volatile contents in the melt during crystallization and preeruptive magma storage. Constraints on the pressure conditions under which inclusions were trapped, preeruptive gas saturation, and the mass fraction of exsolved gas in magmas can also be inferred [Wallace et al, 1995[Wallace et al, , 1999.…”

Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics

“…The rhyolites at Long Valley have been shown to be volatile rich (e.g., Wallace et al, 1999). In the subvolcanic environment at Long Valley, rising vapor pressure could cause rapid exsolution of a gas phase.…”

Accessory mineral U–Th–Pb ages and 40Ar/39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California

“…The Coso magma system is likely to have waxed and waned over time, as suggested for Long Valley by Reid and Coath (2000), Simon and Reid (2005), and Simon et al (2007). Wallace et al (1999) for quartz hosted melt inclusions for Long Valley rhyolites. Solid arrows (Coso) and dashed arrow (Long Valley) used to clarify temporal trends and should not be mistaken for an implied genetic linkage between sequential eruptions (cf.…”