Magnetotelluric evidence for a compositionally controlled lithosphere at the Gakkel Ridge, Arctic Ocean

In the summer of 2024, we conducted the first magnetotelluric (MT) profile survey beneath sea ice at the ultraslow spreading Gakkel Ridge. A total of 17 MT instruments were deployed for approximately 20 days along a 125 km profile across the ridge axis at 100°E. This profile spanned the 0–14 Ma lithosphere north of the Gakkel Ridge. Preliminary analysis reveals a zone of high electrical conductivity at depths of 30–50 km beneath the ridge axis, attributed to a high-degree partial melting zone. A more striking feature is the abrupt deepening of the electrical lithospheric base to ~65 km just north of the ridge axis, beyond which it flattens significantly. The flat lithospheric base likely represents a dehydration boundary, where water content sharply decreases above it due to melting processes. The dehydration could enhance mantle viscosity by 2–3 orders of magnitude, suggesting that the mechanical lithosphere near the ridge axis is governed more by compositions than by thermal structure. The depth of this boundary aligns with the seismic reflection boundary in the Atlantic Ocean, the Gutenberg discontinuity, and the top of the seismic radial anisotropy layer, indicating a possible global significance of this feature.