Discovery and Geochemistry of Off-Axis Volcanic Cones at East Pacific Rise 950N

Two groups of off-axis volcanic cones were sampled by ROV Jason dive 1322 in April 2021 as part of an investigation of the EPR axis focused on studying the run-up to the next seafloor eruption in the area, following those that occurred in 2005-2006. The cones, that are rare along this segment of the EPR, were discovered during AUV Sentry mapping of the off-axis terrain. The cones extend over a distance of ~2500 m and are located ~1500 m east of the EPR axis between 9 53.7N and 9 52.4N. The two primary groups of cones are each aligned along a ~350 trend, parallel to the EPR axis, and are ~800 m long and consist of at least 8-12 coalesced edifices, each with diameters of ~100-200 m and heights that vary from ~10 m to 40 m.Pillow and lobate lava flows were sampled from representative areas in and around 13 of the cones. The sampling traverse covered the entire N-S extent of the lineament and intervening terrain between the cones where glassy sheets and collapsed lobate flows dominate. These flows are presumed to be younger than the cones as flow textures mapped using the Sentry near-bottom sonar data suggest on-lap of the flows onto the base of the cones. Small cones of basaltic lava, common along the crests and flanks of other fast-spreading mid-ocean ridges (MORs) are often associated with fissures and ridge parallel faults, suggesting magmas stored in off-axis lenses may use roots of surficial structural features as pathways to deliver lava to the seafloor.Geochemical analysis of 13 glasses collected during the off-axis dive were compared to previously collected data (~1200 samples) from the adjacent section of the EPR. The samples all are N-MORB containing 7.2% to 8.2 % MgO (wt%) and with K/Ti values from 6.19 to 11.7; similar to the flows covering the ridge to the west of the cones. When plotted against MgO values the previously collected data and new samples show similar trends, consistent with a shallow level fractional crystallization. Eight of the samples taken directly from the cones are remarkably homogeneous suggesting they originated from a singular eruptive event and source similar to those lavas erupted in 1991-92 and 2005-6. The relationship of this type of volcanism along MORs to axially focused eruptions (like in 2005-06) and the subcrestal source of the magma, relative to axial melt lenses, are important and continuing topics of study.