Alfred T. Anderson, Jr., Professor Emeritus, Department of the Geophysical Sciences and the College
Editor, Journal of Geology

Abstracts

Gradients in H₂O, CO₂and exsolved gas in a large-volume silicic magma system: interpreting the record preserved in melt inclusions from the Bishop Tuff
Journal of Geophysical Research, Vol. 104, pages 20,097-20,122, 1999

Paul J. Wallace*
Ocean Drilling Program and Department of Geology and Geophysics
Texas A&M University, College Station, TX 77845

Alfred T. Anderson, Jr₁, Andrew M. Davis_1,2
1I>Department of the Geophysical Sciences, 2Enrico Fermi Institute
University of Chicago, Chicago, IL 60637

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 preeruptive crystallization of the magma body. Melt inclusions from the earliest-erupted samples contain lower H₂O (5.3±0.1 wt%) and CO₂ (62±11 ppm) than inclusions from the middle of the eruption (5.7±0.1 wt% H₂O; 117±10 ppm CO₂). Melt inclusions from late-erupted samples have much lower H₂O (4.1±0.1 wt%), and higher and variable CO₂ (150-1100 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 strongly suggests 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 downwards tapping of a zoned magma body during the eruption. Melt inclusion gas saturation pressures, magma volume estimates, and time- stratigraphic-compositional relations provide constraints on the subsurface geometry of the Bishop magma body before eruption. Early-erupted magma was probably stored in a relatively narrow cupola at the top of a downward widening magma body. This cupola was likely situated towards the southern end of Long Valley, subjacent to the vent for the initial plinian outbreak of the eruption. 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.

* Corresponding author: office (409) 845-0879; FAX (409) 845-0876; email paul_wallace@odp.tamu.edu

Evolution of Bishop Tuff rhyolitic magma based on melt and magnetite inclusions and zoned phenocrysts
Journal of Petrology, vol. 41, pages 449-473, 2000.

Alfred T. Anderson
Department of the Geophysical Sciences
The University of Chicago
5734 S. Ellis Ave.
Chicago, IL 60637

Andrew M. Davis
Enrico Fermi Institute
The University of Chicago
Chicago, IL 60637

Fangqiong Lu*
Department of the Geophysical Sciences
The University of Chicago
Chicago, IL 60637

*Present Address: 11304 Cedarcliff Dr., Austin, Texas 78750

Abstract-The evolution of large bodies of silicic magma is an important aspect of planetary differentiation. Melt and mineral inclusions in phenocrysts and zoned phenocrysts can help reveal the processes of differentiation such as magma mixing and crystal settling, because they record a history of changing environmental conditions.
Similar major element compositions and unusually low concentrations of compatible elements (e.g., 0.45­4.6 ppm Ba) in early-erupted melt inclusions, matrix glasses and bulk pumice suggest eutectoid fractional crystallization. On the other hand, late-erupted sanidine phenocrysts have rims rich in Ba, and late-erupted quartz phenocrysts have CO₂-rich melt inclusions nearest to crystal rims. Both features are the reverse of in situ crystallization differentiation, and they might be explained by magma mixing or crystal sinking. Log (Ba/Rb) correlates linearly with log (Sr/Rb) in melt inclusions, and this is inconsistent with magma mixing. Melt inclusion gas-saturation pressure increases with CO₂ from phenocryst core to rim and suggests crystal sinking. Some inclusions of magnetite in late-erupted quartz are similar to early-erupted magnetite phenocrysts, and this too is consistent with crystal sinking.
We argue that some large phenocrysts of late-erupted quartz and sanidine continued to crystallize as they sank several kilometers through progressively less differentiated melts. Probable diffusive modification of Sr in sanidine phenocrysts and the duration of crystal sinking are consistent with an evolutionary interval of some 100,000 years or more. Crystal sinking enhanced the degree of differentiation of the early-erupted magma and points to the importance of H₂O (to diminish viscosity and enhance the rate of crystal sinking) in the evolution of silicic magmas.