Isotopic and physical evidence for persistently high eruption temperatures for Yellowstone-Snake River Plain rhyolites

Abstract:

Low crystallinity rhyolite lavas and tuffs from the Yellowstone-Snake River plain system were long-thought to erupt at high 800-900 °C temperatures with evidence derived from experimental work and geothermometry (e.g., QUILF, Ti-in-quartz). Despite this evidence, newer experimental phase equilibria studies as well as a reformulation of zircon saturation temperatures support lower temperature magma eruption conditions. Here we present two independent lines of evidence for 850 °C and greater temperatures.

We present high precision oxygen isotope thermometry for coexisting quartz, glass, pyroxene, and magnetite in order make temperature estimates independent of phase equilibria. For all analyzed Snake River Plain-Yellowstone rhyolites, we determine 800-1100 °C temperatures for clinopyroxene and 850-1100 °C temperatures for magnetite. Extremely slow oxygen diffusion in pyroxene will preserve oxygen isotope crystal composition for millions of years stored at magmatic temperatures. Interestingly, oxygen in magnetite will reequilibrate in <1000 years, so systematically higher magnetite-quartz temperatures suggests a short lifespan of magmas from liquidus crystallization to eruption.

In an alternative approach, we have modeled the physical emplacement of the large volume (up to 70 km³) rhyolite lavas of the recent Central Plateau Member group. Using simple solutions to gravity-driven viscous fluid flow, we have made first-order estimates for extremely high discharge rates in order to enable effusion of sufficient volume in relatively axisymmetric morphologies—where glacial ice caps or prexisiting topography did not otherwise restrict flow. Using these results and simple conductive cooling models, we show that flows erupted at >800 °C and probably ~850 °C in order to be emplaced before cooling below the melt-glass transition and forming a more dome-like and lobate morphology.