Basaltic phreatomagmatic fissure at 71 Gulch Part 2: unusual pyroclasts from sediment magma mingling and melting
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RESEARCH ARTICLE
Basaltic phreatomagmatic fissure at 71 Gulch Part 2: unusual pyroclasts from sediment magma mingling and melting Alison Graettinger 1
&
K.L. Bennis 1,2 & B. Brand 3 & E. Reynolds 1 & Joseph Nolan 1
Received: 20 April 2020 / Accepted: 26 October 2020 # International Association of Volcanology & Chemistry of the Earth's Interior 2020
Abstract 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. Keywords Sediment magma mixing . Melting . Phreatomagmatism . Basalt . Subaqueous fissure
Introduction The host environment and magma transport conditions (velocity, overpressure, thermal, and degassing history) that dictate whether magma will pass through unaffected or interact with unconsolidated sediments en route to the eruptive surface are not quantitatively constrained. Such constraints are an important first step to determining what conditions lead to phreatomagmatic explosions. Field examples of sediment and basaltic magma interaction in the form of cored bombs (Houghton and Smith 1993; Troll et al. 2012; Graettinger et al. 2016; Lanzafame et al. 2018), lithic-rich pyroclastic deposits (White 1991; Valentine and van Wyk de Vries 2014), peperite (Busby-Spera and White 1987; Kano 2002; Squire and Editorial responsibility: P-S. Ross * Alison Graettinger [email protected] 1
Department of Earth and Environmental Science, University of Missouri-Kansas City, Kansas City, MO, USA
2
Global Volcanism Program, Smithsonian National Museum of Natural History, Washington, DC, USA
3
Department of Geoscience, Boise State University, Boise, ID, USA
McPhie 2002; Jutras et al. 2006; Graettinger et al. 2012), convoluted dikes (Befus et al. 2009; Muirhead et al. 2016), and debris-filled vents (Martin and Németh 2007; Nemeth and White 2009; Lefebvre et al. 2013) are numerous and cover a range of depths below the eruptive surface. 71 Gulch Volcano in Idaho, USA represents a subaqueous basaltic fissure syst
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