Volcanic evolution of the Otowi Member of the Bandelier Tuff, Jemez Mountains, New Mexico

The 1.61 Ma Otowi Member of the Bandelier Tuff is a compositionally-zoned, high-silica rhyolite tuff showing internal upward decreases in the concentrations of incompatible trace elements. It is associated with the first major episode of collapse at the Valles caldera. Using a comprehensive methodology, the minimum eruptive volume of the Otowi Member has been recalculated at 335 km 3 DRE. The plinian phase has been calculated at 45 km3 DRE. Based on these minimum calculations, it is likely that the original volume of the deposit was greater than 500 km3 DRE.;Prior studies suggested that the Otowi eruption began with a single, central vent and transitioned quickly to multiple ring vents as collapse ensued. This model has been evaluated by comparing vertical profiles in pumice chemistry and lithic abundances in outflow sheet sections around the caldera. Lithic distributions are surprisingly uniform around the caldera. Thus, a new model for eruption of the Otowi Member is presented here in which a single, central vent was responsible for the majority of the eruption. Lithic and chemical evidence indicate a vent shift late in the eruption towards the northeast as collapse ensued. However, multiple ring vents are not evident in the data. In addition, volume calculations of chemical types in the Otowi ignimbrites indicate a roughly equal tri-part distribution. This volume-constrained chemical data may prove useful in future petrogenetic studies on the Otowi Member.