Phreatomagmatic Eruption Deposits on the Seafloor Record Cataclysmic Caldera Formation on Axial Seamount, Juan de Fuca Mid-Ocean Ridge

Abstract

The physical and compositional characteristics of a unique ash-rich hydrothermal sediment, hydrothermal muddy tuff (HMT), on Axial Seamount, Juan de Fuca Ridge, suggest that it formed by phreatomagmatic eruptions during caldera formation. Deposit thickness trends away from the seamount summit and the presence of 68% primary volcaniclasts and 32% secondary lithics suggest a source from along caldera ring-faults deep in the crust. Lithic geothermometry indicates that the source was subject to low-temperature (<150>°C) alteration, consistent with a provenance as deep as 600 – 800 m below the seafloor. At this depth, just above the critical point of seawater, the brittle fragmentation of magma upon interaction with deep-circulating hydrothermal fluids triggered molten fuel coolant interaction (MFCI) and the production of highly angular and very fine grained (100 µm) glass shards. The resulting phreatomagmatic eruption was dispersed over 3.5 km away from the caldera by eruption plume fall-out, ocean bottom currents, and dilute gravity-flows. The result is a unique volcaniclastic deposit that has never been described in mid-ocean ridge settings. This study provides the framework for future studies to further explore fragmentation and dispersal mechanisms within the context of deep-marine volcanic setting.