Magma mingling and ascent in the minutes to hours before an explosive eruption as recorded by banded pumice

Abstract

High-threat explosive silicic eruptions commonly contain banded pumice, reflecting magma mingling in the conduit prior to or during eruption. Heterogeneities in tuffs have been attributed to the draw-up of compositionally distinct magmas, in which low-viscosity magmas ascend more quickly than high-viscosity magmas. The Rattlesnake Tuff of the High Lava Plains in Oregon (northwestern United States) represents a zoned magma reservoir where at least five different rhyolite compositions are preserved in banded pumice samples in variable mingled combinations. Geochemical gradients recorded across band boundaries in pumice were modeled using a Monte Carlo least-square minimization procedure to find the complementary error function that best fit observed Si and Ba diffusion profiles by iteratively varying the concentration of each plateau (i.e., the concentration on either side of the band boundary), the center and spacing of the diffusion profile, diffusion length scale, and temperature. Modeling indicates maximum time scales between mingling and conduit ascent from minutes to hours. Viscosity calculations for each rhyolite composition confirm that highly viscous rhyolites have longer ascent times than low-viscosity magmas, strongly supporting a model of sequential tapping of a zoned chamber controlled by viscosity.