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.

The application of sophisticated numerical calculations to understanding complex natural phenomena is now widespread in earth science. Recently, Proussevitch et al. [1993] illustrated some of the advantages in the power of such methods to investigate complex processes and also some of the difficulties. Calculations are presented on the diffusive growth of bubbles in magmas following a sudden and large decrease in pressure. In terms of theory the approach is an advance over a previous numerical model [Sparks, 1978]. The new study calculates the full diffusion problem of spherical bubble growth, whereas the earlier study used empirical data to calculate a growth rate constant. The new model is only a beginning, because even more complex problems relevant to gas bubble growth in magmas can be tackled, such as concentration dependent diffusivity, correlation of viscosity with water content, and variable decompression rates. Before going to even more elaborate calculations, however, it is important that the present ones are well understood. A difficulty with numerical results of this kind is that the results themselves have to be interpreted, and this can be of difficulty comparable to interpreting the complex natural systems. Another difficulty is that there are no independent data with which to compare the calculated bubble growth rates, due to the experimental difficulties in studying violent bubble growth. In this comment I draw attention to the need for fuller explanation and verification of these models. Some of the results are not easily reconciled with the interpretations offered by Proussevitch et al. and indicate that a more detailed analysis of the physics of diffusive bubble growth is needed. I propose an alternative explanation of certain aspects of their results which would have important volcanological implications. Many of the numerical results shown by Proussevitch et al. display a characteristic sigmoidal growth curve. This result contrasts with the classical result [e.g., Scriven, 1959] where radius increases as the square root of time. At large bubble sizes toward the end of growth, the bubble growth rate should depart from the parabolic growth law, because the model involves a shell of finite volume. The departure from the parabolic law at small times and bubble sizes is substantial and is hard to reconcile with the interpretations offered. This period of growth shows radius increasing as some power n of time which is significantly larger than 1. For brevity, I use the term accelerating growth to describe this phase. This period is interpreted by Proussevitch et al. in terms of a time delay due to the effects of surface tension. The time delay in each calculation is listed in their Table 2,

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Supereruptions violently transfer huge amounts (100 s–1000 s km3) of magma to the surface in a matter of days and testify to the existence of giant pools of magma at depth. The longevity of these giant magma bodies is of significant scientific and societal interest. Radiometric data on whole rocks, glasses, feldspar and zircon crystals have been used to suggest that the Bishop Tuff giant magma body, which erupted ∼760,000 years ago and created the Long Valley caldera (California), was long-lived (>100,000 years) and evolved rather slowly. In this work, we present four lines of evidence to constrain the timescales of crystallization of the Bishop magma body: (1) quartz residence times based on diffusional relaxation of Ti profiles, (2) quartz residence times based on the kinetics of faceting of melt inclusions, (3) quartz and feldspar crystallization times derived using quartz+feldspar crystal size distributions, and (4) timescales of cooling and crystallization based on thermodynamic and heat flow modeling. All of our estimates suggest quartz crystallization on timescales of <10,000 years, more typically within 500–3,000 years before eruption. We conclude that large-volume, crystal-poor magma bodies are ephemeral features that, once established, evolve on millennial timescales. We also suggest that zircon crystals, rather than recording the timescales of crystallization of a large pool of crystal-poor magma, record the extended periods of time necessary for maturation of the crust and establishment of these giant magma bodies.

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