Streaked Pumice From the Youngest Toba Tuff: New Constraints on Compositional and Temperature Gradients

Earth's largest Quaternary volcanic eruption (75 ka) expelled more than 2800 km3 of compositionally zoned silicic magma from Toba caldera, Indonesia, and produced the extensive Youngest Toba Tuff (YTT). Past petrologic studies identified that the YTT magma was zoned with respect to composition, temperature, and mineralogy. To understand the dynamics of differentiation leading to eruption of the YTT magma, we analyzed the geochemistry and mineralogy of rare mixed dacitic and rhyolitic pumice that extend the range of YTT compositions to less-evolved and apparently hotter compositions. Mixed pumices are streaked with light and dark domains mostly containing ca. 20-50% crystals of plagioclase, hornblende, quartz, biotite, and pyroxenes. In general, crystal size in the dark domains is smaller than in higher SiO2 pumice. Dark domains range in composition from dacite to low-silica rhyolite (63-72 wt.% SiO2, anhydrous); light domains typically contain higher SiO2 (65-74 wt.% SiO2, anhydrous) than their darker counterparts. SiO2 concentrations of the pumice domains correlate with trace element concentrations, including positive covariation with typically incompatible elements (e.g., Nb, Ta, U, Th) and negative covariation with typically compatible elements (e.g., Sr, Eu, Sc). Domain glasses mimic these same trace element trends. Within both domain types, plagioclase composition varies from An30 to An50. Hornblende composition generally covaries with pumice SiO2. Pressures calculated using the Johnson and Rutherford (1989) barometer range from ca. 2 kb to 5 kb. However, pressures calculated using the Anderson and Smith (1995) barometer range from ca. 0 kb to 2 kb. Hornblende-plagioclase pairs yield temperatures ranging from ca. 800° to 900°C, while equilibrium Fe-Ti oxide pairs yield temperatures ranging from ca. 760° to 800°C. In general, dark pumice domains yield the highest temperatures. The Pb and Nd isotope compositions of domains within single streaked pumice samples are variable, including different isotope compositions for glasses from adjacent streaks. Isotopic heterogeneity suggests mixing of melts that experienced different degrees of open-system evolution. The mixed pumice samples provide a new limit on the mafic end-member of YTT magma composition, and record mixing of magma with contrasting origins. Mixed pumice containing cm-scale compositional heterogeneity suggests mixing of contrasting magmas just prior to, or during, evacuation of the voluminous YTT reservoir, perhaps in response to heating after intrusion of new magma. Differences in hornblende composition between dacitic and rhyolitic pumice may reflect contrasting depths of crystallization for dacitic and rhyolitic end- members, or temperature-induced control of amphibole composition within a shallow reservoir beneath Toba.