Evolution of Bishop Tuff rhyolitic magma based on melt and magnetite inclusions and zoned phenocrysts

The evolution of large bodies of silicic magma is an important aspect of pl anetary differentiation. Melt and mineral inclusions in phenocrysts and zon ed phenocrysts can help reveal the processes of differentiation such as mag ma mixing and crystal settling, because they record a history of changing e nvironmental conditions. Similar major element compositions and unusually l ow concentrations of compatible elements (e.g. 0.45-4.6 ppm Ba) in early-er upted melt inclusions, matrix glasses and bulk pumice from the Bishop Tuff, California, USA, suggest eutectoid fractional crystallization. On the othe r hand, late-erupted sanidine phenocrysts have rims rich in Ba, and late-er upted quartz phenocrysts have CO2-rich melt inclusions closest to crystal r ims. Both features are the reverse of in situ crystallization differentiati on, and the might be explained by magma mixing or crystal sinking. Log(Ba/R b) correlates linearly with log(Sr/Rb) in melt inclusions, and this is inco nsistent with magma mixing. Melt inclusion gas-saturation pressure increase s with CO2 from phenocryst core to rim and suggests crystal sinking. Some i nclusions 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 conti nued to crystallize as they sank several kilometers through progressively l ess differentiated melts. Probable diffusive modification of Sr in sanidine phenocrysts and the duration of crystal sinking are consistent with an evo lutionary interval of some 100 ky or more. Crystal sinking enhanced the deg ree of differentiation of the early-erupted magma and points to the importa nce of H2O (to diminish viscosity and enhance the rate of crystal sinking) in the evolution of silicic magmas.