Composition of glass from high-temperature rhyolite of the Snake River Plain Yellowstone hotspot track: implications for crustal melting

Abstract

Large volume explosive eruptions comprising the members of the Cougar Point Tuff (≥ 7,000 km3) erupted from the Bruneau-Jarbidge volcanic center of the Yellowstone hotspot are melt-dominated with crystal contents ranging from 10-15%. Zircons from all of the units have low δ18O ratios indicative of a heterogeneous, hydrothermally altered protolith that underwent melting in the mid to upper crust as the result of injection of basalt. This study investigates detailed variations in melt compositions by measuring elemental concentrations of 51 elements in individual glass shards from basal airfall tuffs by EPMA and LA-ICP-MS. Some eruptive units contain two distinct compositional modes of glass, indicating the presence of discrete liquid volumes in the magma reservoir system at the time of eruption. Multi-modal behavior is also observed for minerals indicating that the erupted magmas are complex, non-equilibrium assemblages. The overall elemental pattern in glass is for younger units to become more mafic (higher Fe) and for zirconium saturation temperatures and εNd to likewise increase. A few elements (B, Rb, Cs, Th, U) decrease in concentration with Fe. Most trace elements however remain fairly constant in composition over the 2.2 myr eruption interval and exhibit no systematic variation with respect to Fe concentration (Li, Be, Zn, Nb and REE). Elements that increase systematically with Fe include Sr, Ba, Eu, P, Zr, Hf and Sc indicating feldspar, zircon and apatite control on the melt composition. Compositional variations can be modeled by batch melting, suggesting that magma reservoirs are constructed by incremental extraction of melts from a crystal-liquid mush with episodic eruption and replenishment. Fractionation may play a role in generating compositional diversity within units