Pre-eruptive storage conditions of the Huckleberry Ridge Tuff from quartz-hosted melt inclusions in the initial fall deposits

The Yellowstone Plateau volcanic field is one of the Earth's largest young silicic volcanic systems, erupting over 6,000 cubic kilometers of rhyolite in the Quaternary. The two most voluminous eruptions, the Huckleberry Ridge Tuff (HRT) (2.06 Ma, ~2500 km³) and the Lava Creek Tuff (0.62 Ma, ~1000 km³) are amongst the largest Quaternary eruptions on Earth. Geochemical studies of these deposits can reveal information on the accumulation and storage of silicic magma in this voluminous system and may also be useful for comparisons for current monitoring efforts. We have analyzed volatiles (H₂O, CO₂, F, Cl) and major elements in quartz-hosted melt inclusions (MIs) from near the top of the HRT fall deposit (the most coarse-grained part), which was deposited before eruption of the three major HRT ignimbrite units. We have combined these data with cathodoluminescence (CL) imaging on the host quartz crystals to obtain information on the evolution of the magmatic system before such a voluminous eruption. The major element compositions of the MIs indicate an evolved and homogeneous melt composition, with SiO₂ ranging from 75.0-77.0 wt.% on an anhydrous basis. The MIs fall into two different populations based on color. Clear MIs have H₂O concentrations of 0.5-1.5 wt.%, with CO₂ >400 ppm, whereas brown MIs have H₂O values falling mostly between 2.0-3.5 wt.%, with lower CO₂ (100-300 ppm). These two MI populations, however, do not appear to show any other compositional distinction, with similar Cl (1100-1700 ppm) and F (1500-5500 ppm) ranges, as well as indistinguishable major element compositions. Both populations give similar vapor saturation pressure ranges (1000-1300 bars), implying pre-eruptive storage at similar depths. CL imaging distinguishes at least three populations of quartz crystals that also appear to be independent of MI composition. These populations include: (a) crystals with fine-scale oscillatory zoning, (b) crystals with broader zones separated by diffuse boundaries, and (c) crystals that lack any zoning. Analyses of reentrants provide information on conditions during late-stage assembly that can be compared with fully sealed inclusions trapped during earlier crystallization stages. The reentrants have higher F concentrations than the majority of inclusions (~0.5-0.7 wt.%) and CO₂ below detection, documenting changing conditions in the system either during or shortly before the eruption.