Melt focusing along lithosphere-asthenosphere boundary below 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-spreading 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 3D 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 the Cobb-Eickelberg hotspot. The 3D shape of the LAB, which is coincident with a thermally controlled magma assimilation front, focuses hotspot-related 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. In this context, the LAB can be viewed as the upper surface of a 'magma domain', a volume within which melt bodies reside (replacing the concept of a single 'magma reservoir')⁶. 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.