Magnetotelluric evidence for highly focused mantle melting along the ultraslow-spreading Gakkel Ridge, Arctic Ocean.

It is well known that ultraslow-spreading mid-ocean ridges display significant variations in axial magmatism and tectonics. Yet, the processes governing mantle melting and melt transport remain a subject of ongoing debate. A key limitation has been the lack of contrasting observations of mantle melting beneath axial segment centers versus segment ends, particularly through electromagnetic methods, which are highly sensitive to partial molten mantle. Here, we present the first one-dimensional magnetotelluric observation conducted along the ultraslow-spreading Gakkel Ridge in the Arctic Ocean. Our findings reveal prominent low-resistivity zones at depths of 20-45 km beneath segment centers, which are indicative of shallow melting zones. We propose that the robust magma supply and associated repeated magma intrusions lead to a thin thermal lithosphere and associated shallow mantle melting. In contrast, such electrical resistivity anomalies are absent at comparable depths beneath the magma-poor deep valley, where the electrical lithosphere extends to depths of >50 km. The extremely thick lithosphere restricts mantle melting to greater depths and facilitates melt migration toward adjacent segment centers. Our study highlights the critical role of highly variable lithospheric thickness in regulating melting depth and focusing melt flow along ultraslow-spreading ridges. We propose that the significant variation in lithospheric thickness and the associated focused melting result in the recently observed highly variable crustal thickness along the Gakkel Ridge.