PHREATOMAGMATIC ASH FROM DEEP-MARINE CALDERA ERUPTIONS ON AXIAL SEAMOUNT, JUAN DE FUCA RIDGE

Axial Seamount is located on the active Juan de Fuca mid-ocean ridge off the coast of Oregon. Volcaniclastic stratigraphy preserved on caldera flanks includes a hydrothermal muddy tuff (HMT) facies that is believed to have formed from the fall-out of ash plumes generated by phreatomagmatic eruptions along caldera ring-faults. In this study we use the term “phreatomagmatic” to imply sub-seafloor interaction between magma and seawater. Sediment push cores were collected along a transect perpendicular to the eastern caldera ring-fault. Visual observations of push cores reveal that the HMT facies is approximately 40 cm thick near the caldera rims and extends up to 4 km away from the caldera margin before thinning out suggesting the source was near its ring-fault system. A phreatomagmatic origin for HMT lithofacies is tested with grain-size and componentry analysis using laser particle diffraction, point-counts (n=500 grains/sample), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDX). Particle morphology of juvenile vitriclasts was measured with a Morphologi G3S (MGS) instrument and supports qualitative assessments from visual point-counts and scanning electron microscopy (SEM). HMT beds have peak modes ranging from 40 - 163 µm based on 70 subsamples. Point-counts show that the 125 – 250 µm size fraction is composed of 60% vitriclasts (95% angular, 4% fluidal, 1% vesicular), 28% volcanic lithics (e.g. plagioclase, crystalline basalt, olivine), and 13% hydrothermal lithics (e.g. smectitic clay, sulfide, zeolites). Morphologic analysis of the 250-500 µm fraction indicates that fluidal vitriclasts in distal cores are as much as 20% more abundant than in proximal cores. XRD results show the presence of upper zeolite and greenschist grade minerals (e.g. laumontite, clinochlore, actinolite) indicative of altered oceanic crust. The presence of these alteration minerals coupled with very fine-grained volcanic lithics and fresh vitriclasts suggest that deeper parts of the oceanic crust were mixed with an ascending magma that was undergoing intense fragmentation. Such an eruption style from a ring-fault source implies that subseafloor magma-water interaction (phreatomagmatism) likely accompanied caldera formation on Axial Seamount.