Processes in Caldera-Forming High-Silica Rhyolite Magma: Rb-Sr and Pb Isotope Systematics of the Otowi Member of the Bandelier Tuff, Valles Caldera, New Mexico, USA

The 1·60Ma Otowi Member of the BandelierTuff is a chemically and isotopically zoned high-silica rhyolite ignimbrite with subchondritic concentrations of Sr. Sanidine phenocrysts and glasses from early erupted to late-erupted tuff exhibit systematic large variations in Sr/Sr (0·70519^0·70947) and small ranges in Pb isotope ratios (Pb/Pb1⁄417·795^17·835). In all but the earliest-erupted tuff, sanidine phenocrysts are zoned with relatively Srand Ba-rich overgrowths on Srand Ba-poor cores.We estimate sodic sanidine/ melt partition coefficients for Rb, Cs, Sr, Ba and Pb to aid in understanding the origins of the elemental and isotopic variations. Isotopic heterogeneity is observed within single pieces of pumice, single quartz^sanidine glomerocrysts and single crystals. At the observed high values of Rb/Sr ( 1000) caused by the extreme Sr depletion, radiogenic ingrowth or contamination of the magma by small amounts of country-rock could both be responsible for elevated Sr/Sr.The Pb isotope variations cannot be due to ingrowth and unequivocally indicate open-system behavior. Among the sanidine phenocrysts, positive correlations of Sr/Sr vs Rb/Sr superficially resemble isochrons but are better modelled as mixing arrays; at least three such arrays, corresponding to three distinct magmatic recharge events, can be identified.The latest event may have played a role in destabilizing the system and triggering eruption. In all cases the recharging magma was another high-silica rhyolite only slightly less depleted in Sr.These relationships are explained by a model that starts with a crystal-poor high-silica rhyolite melt lens overlying a sanidineþ quartz crystal pile. Successive melting events within the crystal pile and mixing of new and old melts (recharge) causes isotopic and chemical zoning in the main melt lens and short length scale disequilibrium in the crystal pile. A late episode of quartz crystallization may have captured inclusions of melt that had been additionally contaminated by Precambrian granitoid country-rock as the magma began the upward journey that culminated in eruption.