Melt Focusing and Assimilation Along the Lithosphere-Asthenosphere Boundary Beneath Axial Volcano

Abstract Beneath oceanic spreading centres, the lithosphere-asthenosphere boundary (LAB) acts as a permeability barrier that focuses the delivery of melt from deep within the mantle towards the spreading axis¹. At intermediate- to fast-spreading ridge crests, the multichannel seismic reflection technique has imaged a nearly flat, 1-2 km wide axial magma lens (AML)² that defines the uppermost section of the LAB³, but the nature of the LAB deeper into the crust has been more elusive with some clues gained from tomographic images, providing only a diffuse view of a wider halo of lower velocity material seated just beneath the AML⁴. Here we present three-dimensional (3-D) seismic reflection images of the LAB extending deep (5-6 km) into the crust beneath Axial Volcano, located at the intersection of the Juan de Fuca Ridge and Cobb-Eickelberg Hotspot. The geometry of the LAB is funnel-shaped, representing a magma assimilation front that is thermally-controlled. The morphology of the LAB, in combination with failure surfaces deep within Axial Volcano, highlights the focusing of hotspot- and mid-ocean spreading centre-related magmatism towards the centre of the volcano, controlling both eruption and hydrothermal processes, and the chemical composition of erupted lavas⁵. Our discovery of a funnel-shaped, crustal LAB suggests that thermally-controlled magma assimilation could be occurring along this surface at other volcanic systems such as Iceland.