Basaltic Phreatomagmatic Fissure at 71 Gulch Part 2: Unusual Pyroclasts from Sediment Magma Mingling and Melting

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The behavior of magma as it encounters unconsolidated sediment can be studied in fortuitous exposures of incised volcanic systems to help determine the conditions that control the transport, arrest, or mingling of that magma in the sediment. The Pliocene subaqueous basaltic fissure at 71 Gulch, Idaho, USA contains unusual light-colored glassy mingled clasts that were produced through the incorporation of domains of siliciclastic sands and silts into basalt. These clasts contain textures reflective of melting and mechanical mixing of these sediments with the rising basalt along intrusions at depths > 30 m beneath the pre-eruptive surface and transport into and through a shallow subsurface debris-filled vent. Mingled clasts comprised colorless glass with locally high silica contents and visible swirls of sideromelane along with crystals derived from both the host sediment and the 71 Gulch basalt. The unique combination of textures, minerals, plus major and trace element geochemistry within the geologic context of 71 Gulch points to a two-phase mixing that did not contribute directly to, nor impede, the phreatomagmatic explosions that erupted the mingled clasts.