A PROLONGED AND EPISODIC OPENING TO A MAGMATICALLY COMPLEX SUPERERUPTION: HUCKLEBERRY RIDGE TUFF, YELLOWSTONE

Resolving the pre-eruptive storage configuration, initiation and release of magma in large-scale silicic eruptions can be achieved through linking field studies with microanalytical approaches. Initial fall deposits of the ~2.08 Ma, 2500 km³ Huckleberry Ridge eruption, Yellowstone are multiply bedded and contain reworked intervals, suggesting that the opening phases of the eruption were episodic and lasted for weeks to months. Here we present data from a 2.5-m section of these fall deposits, sampled at multiple intervals, and from the basal vitric zone of the ignimbrite at locations around a 220° sector around the subsequent caldera. Although major element glass compositions are relatively uniform, trace elements span a large range (e.g. Ba 10–900 ppm, Sr/Rb = 0.005–0.09), with highly evolved glasses dominating in the fall deposits. Several trace elements (e.g. Ba and light rare earth elements) in the glass samples define compositional clusters in the fall deposits and basal ignimbrite. These clusters are interpreted to represent multiple, discrete melt-dominant domains that were systematically tapped by multiple vents in early stages of the eruption. Timescales of magma ascent feeding the initial fall deposits are interpreted from scatter in H₂O concentrations from enclosed melt inclusions. The wide H₂O variations (1.0-4.7 wt.%) are interpreted to reflect partial loss of H₂O by diffusion through the quartz host, implying ascent times as long as ~14 days from storage, reflecting highly variable and slow decompression conditions, with total durations comparable with those inferred from field evidence. The transition to ignimbrite deposition is marked by an increase in the number of erupted melt compositional clusters from four in the fall deposits to eight in the ignimbrite, representing nine distinct melt-dominant domains at depth. The onset of widespread ignimbrite deposition, inferred to relate to caldera collapse, occurred after ~ 50 km³ of magma had been discharged. Our data collectively highlight the complex initial evacuation of an array of unusually heterogeneous magma systems. Although external controls were important in modulating initial processes (start-stop deposits, slow ascent, etc.), depressurization of the system then controlled the onset of widespread caldera collapse.