Textural, geochemical, and volatile evidence for a Strombolian-like eruption sequence at Lō`ihi Seamount, Hawai`i

Abstract

Despite recent advances and observations, many questions remain about the range of eruption styles that are possible at submarine volcanoes. We report deposit characteristics, clast microtextures, and geochemical and volatile data for a ~ 22 m thick pyroclastic succession on the summit plateau of Lō`ihi Seamount, Hawaii, with the goal of determining eruption style directly from features preserved in the submarine deposits. Deposit stratigraphy indicates emplacement of lapilli-dominated ejecta from a nearby vent or series of vents. Lapilli contain heterogeneous vesicle (N v = 1.6 × 10 4 to 9.5 × 10 5 cm − 3 ) and gradational microlite populations that matured below the vent, before fragmentation. The homogeneous, tholeiitic matrix glasses are degassed of CO 2, but retain variable concentrations of dissolved H 2O (0.41 to 0.82 wt.%). Olivine crystals are mostly highly magnesian (> Fo 88) crystals, bearing inclusions entrapped at ~ 1300 ± 37 °C that suffered post-entrapment Fe-loss in contact with cooler (~ 1136 ± 50 °C) bulk melt over calculated time periods up to ~ 3 months, and a small subset of lower-Mg (< Fo 85.5) phenocrysts bearing glass inclusions entrapped at ~ 1217 ± 12 °C. Inclusions are also mostly degassed of CO 2, and retain variable H 2O concentrations (0.36 to 0.74 wt.%). No single data type definitively proves what style of eruption formed this deposit, and each is treated in-turn with realistic discussion of their respective limitations. Textural features indicate Strombolian-like (e.g., episodic) ejection of magma, in pulses that allowed recharging magma to pond and partially cool in the shallow conduit between bursts. Evidence against H 2O exsolution between source and vent suggest that the decoupled volatile slugs driving the eruption(s) must have been dominated by CO 2 derived from unerupted magma deeper in Lō`ihi' s plumbing system. This study advances our catalog of analyses for submarine pyroclastic rocks, but also highlights that we are still far from a full understanding of submarine explosive volcanic processes.