A Lithosphere-Asthenosphere Boundary and Partial Melt Estimated Using Marine Magnetotelluric Data at the Central Middle Atlantic Ridge

The differential motion between the lithosphere and the asthenosphere is aseismic, so the magnetotelluric (MT) method plays an important role in studying the depth and nature of the lithosphere-asthenosphere boundary (LAB). In March 2016, we deployed 39 marine MT instruments across the Middle Atlantic Ridge (MAR), 2,000 km away from the African coast, to study the evolution of the LAB with ages out to 45 million years (My). The MT acquisition time was limited to about 60 days by battery life. After analyzing dimensionality and coast effects for the MT data, determinant data were inverted for two-dimensional resistivity models along two profiles north and south of the Chain Fracture Zone (CFZ). The imaged thickness of the lithospheric lid (>100 omega m) ranges from 20 to 80 km, generally thickening with age. In the north of CFZ, punctuated low-resistivity anomalies (<1 omega m), likely associated with potential partial melts, occur along its base. In the south of CFZ, the base of the resistive lid is demarcated by a low-resistivity channel (<1 omega m) most likely fed by deeper melts. Sensitivity analyses and structural recovery tests indicate the robustness of these features. Resistivity models are in good agreement with results of seismic data. These results imply that partial melt is persistent over geologic timescales and that the LAB is dynamic features fed by upward percolation of mantle melt. The melt fraction is about 1-7% based on the resistivity, temperature, pressure, and hydrous basalt models, which is consistent with petrophysical observations.