Mid-Ocean Ridge (MOR) Ash Dispersal on Axial Seamount, Juan de Fuca Ridge

Abstract

Rare eruption of primitive normal mid-ocean ridge basalt (N-MORB) magma (∼9.5 wt% MgO) on the summit of Axial Seamount generated abundant ash that was dispersed for several km. The unique geochemical signature of this ash deposit is distinct from otherwise typical MORB with more evolved compositions. As such, it is a key marker bed that can be used to track the dispersal of ash from an inferred source. The deposit rapidly fines over 1–4 km and becomes more chemically heterogeneous with distance. Toroidal bottom current circulation around Axial's summit caldera primarily constrained it to the southwest part of the summit with limited dispersal to the southeast and northern flanks. Computer simulations that best match the observed dispersal pattern suggest that ash was lofted to ∼250 masf by a moderate heat transfer rate (∼10⁹ W) from a small hydrothermal megaplume and co-genetic lava flow. Models invoking lower heat transfer rates from just a cooling lava flow could only loft the finest material to <225 m above the seafloor, and could not recreate the observed dispersal pattern, even under a strong bottom current regime. Radiocarbon ages and lithostratigraphy imply that the marker bed formed ∼600 years BP, after caldera formation, which occurred sometime between 1400 and 1000 years BP. Chemostratigraphic trends show that eruptions tapped more primitive magmas (8.0–9.7 wt% MgO) for several hundreds of years after caldera formation. This observation is interpreted to reflect catastrophic changes in crustal permeability that reduced the volume and magma storage times in crustal reservoirs, which in turn allowed magmas to rapidly ascend to the surface.