Evolution of Bishop Tuff Rhyolitic Magma Based on Melt and Magnetite Inclusions and Zoned Phenocrysts

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. 0·45–4·6 ppm Ba) in early-erupted melt inclusions, matrix glasses and bulk pumice from the Bishop Tuff, California, USA, 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 closest 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 ky 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.