Abstract

Mid-ocean ridges create oceanic lithosphere consisting normally of basaltic crust a few kilometers thick overlying a peridotitic mantle. However, lithosphere free of basaltic crust formed during the past ∼30 m.y. at an ∼50-km-long stretch of Mid-Atlantic Ridge south of the Romanche Fracture Zone, giving rise to a >500-km-long strip of ocean floor exposing mostly mantle peridotites that have undergone an unusually low (≤5%) degree of melting, mixed with peridotites that reacted with a small fraction of basaltic melt. This lithosphere contains <10% of scattered gabbroic pockets, representing melt frozen above 25 km depth within a relatively cold subaxial lithosphere. Numerical modeling excludes dry melting below this crust-free lithosphere, because of the cooling effect of the long- offset Romanche transform combined with a regional mantle thermal minimum; however, modeling allows a limited extent of hydrous melting. This unusual lithosphere, unable to expel the melt fraction, characterizes cold spots along mid-ocean ridges.

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