Quartz-Hosted Melt Inclusions as Windows Into Magma Assembly and Storage Processes in the Youngest Toba Tuff

Quartz-hosted melt inclusions are windows into the assembly and storage of compositionally zoned magmas systems like the giant magma body responsible for the 2,800 km³ Youngest Toba Tuff (YTT), Indonesia. Feldspar-sensitive element concentrations and interelement ratios in YTT quartz-hosted melt inclusions are found to cluster into three discrete melt inclusion populations. Each population represents a small fraction of the overall range in YTT melt compositions and no more than ∼12% to ∼21% fractional crystallization at mostly eutectoid conditions accounts for the trace elements variations within them. Kinships between host matrix glasses and co-hosted phases are interpreted as evidence of spatially discrete chemical domains of magma within a more broadly zoned magma system. Quartz growth in the dominant portion of the system apparently occurred after extraction of melt from mush, whereas chemically distinct crystal cores in the more crystal-rich, low-silica magmas may have been derived from mush during extraction or remobilized by mush rejuvenation. Collectively, the chemical zoning of the YTT appears to have developed prior to most quartz growth, likely “bottom-up” due to mush-derived melt heterogeneity.