Steady-state creation of crust-free lithosphere at cold spots in mid-oceanridges

Mid-ocean ridges create oceanic lithosphere consisting normally of basaltic crust a few kilometers thick overlying a peridotitic mantle. However, lith osphere free of basaltic crust formed during the past similar to 30 m.y. at an similar to 50-km-long stretch of Mid-Atlantic Ridge south of the Romanc he Fracture Zone, giving rise to a > 500-km-long strip of ocean floor expos ing mostly mantle peridotites that have undergone an unusually low (less th an or equal to5%) degree of melting, mixed with peridotites that reacted wi th a small fraction of basaltic melt. This lithosphere contains < 10% of sc attered gabbroic pockets, representing melt frozen above 25 km depth within a relatively cold subaxial lithosphere. Numerical modeling excludes dry me lting below this crust-free lithosphere, because of the cooling effect of t he long-offset Romanche transform combined with a regional mantle thermal m inimum; 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.