Experimental constraints on the origin and evolution of the Bishop Tuff.

The Bishop Tuff has benefited from extensive field, petrological and geochemical studies for more than 50 years to the point of becoming a classical example of a zoned magma chamber in many geological textbooks. The mechanism(s) leading to the development of geochemical zoning in such magmas are still vigorously debated, however. Fractionation mechanisms invoked so far call upon some sort of separation between early formed phenocrysts and liquid (Wallace et al., 1999; Anderson et al., 2000), mixing between various end-members, or on the establishment of chemical gradients within the liquid resulting from thermal gradients in the magma body (Hildreth, 1981). Early work rejected the possibility of fractionation being driven by crystal settling (Hildreth, 1981), but recent melt inclusion studies have resurrected some kind of crystal-liquid separation (Anderson et al., 2000). Interest in the petrogenesis of large silicic magma chambers revived in the early nineties when detailed isotopic work concluded that phenocryst crystallisation in rhyolitic magma chambers might precede by several hundred thousand years the time of eruption (Halliday et al., 1989), implying maintaining largely liquid for protracted periods huge amounts of relatively cold magma in upper crust. This model was questioned, mainly on physical grounds, on the basis that the thermal regime even of large silicic bodies would be unable to ensure magmatic lifetimes in excess of 100 kyr (Sparks et al., 1990), unless the heat supplied by putative underlying basalt strictly balances that resulting from conductive cooling atop the silicic magma body. Subsequent isotopic works have either supported the hypothesis of enhanced longevity (e.g., van den Bogaard and Schirnick, 1995; Reid et al., 1997; Davies and Halliday, 1998) or criticized it (Reid and Coath, 2000)