Comment on 'A Metamodel for Crustal Magmatism: Phase Equilibria of Giant Ignimbrites' by S. J. Fowler and F. J. Spera

Abstract

The generation and evolution of giant, supereruptionforming magma bodies is a contentious topic that has prompted much recent work, much of which has appeared in this journal. In a recent paper, Fowler & Spera (2010) summarize the state of the discussion (and disagreement) in very useful terms. On one side, Bindeman & Valley (2001) and Bindeman et al., (2008), using evidence from geochronology and oxygen-isotope geochemistry of Yellowstone zircons, argue in favor of a selfcannibalization, crustal melting origin of large volumes of low-dO rhyolites. On the other side, Bachmann & Bergantz (2004, 2008) emphasize the challenges posed by heat-flow constraints to an upper crustal melting scenario, and advocate step-wise crystal fractionation of mafic parental magmas. Interestingly, Fowler & Spera (2010) use phase equilibria information derived from MELTS simulations to place themselves in a more extreme position, and suggest that single-step fractional crystallization from mafic parental magmas is sufficient to explain the origin and evolution of giant bodies of rhyolitic magma. There are many critical assumptions in the treatment of Fowler & Spera (2010) that need careful scrutiny, but we concentrate on those that we feel better equipped to address. We focus on the evolution of the Bishop magma body, and we comment on (1) the compatibility of the proposed scenario with existing evidence from minerals, melt inclusions and pumice from the Bishop Tuff, and (2) the estimates for the longevity of the Bishop magma body derived from heat-balance considerations. It seems significant to point out that the model proposed by Fowler & Spera (2010) is essentially a transposition of the model they developed for the evolution of the Campanian Ignimbrite using similar methods (Fowler et al., 2007). However, there are significant differences between the trachyte^phonolite system characteristic of the Campanian Ignimbrite and the high-silica rhyolite systems discussed by Fowler & Spera (2010). Probably cognizant of some of the difficulties in transposing their model from one type of system to another, Fowler & Spera (2010) present what they call a ‘metamodel’, an abstract construct that is useful to understand the generation and evolution of large volumes of high-silica rhyolite. By doing so, they purposely distance themselves from the details of the various magmatic systems and focus on the first-order features of the geochemical evolution of the systems of interest. Importantly, they postulate evolution from parental mafic^intermediate magmas erupted in the same regions and with similar ages to the high-silica rhyolites, and carry forward the exercise of seeking inconsistencies between modeled evolutionary paths and data for the natural systems.