Crust-Free “Constipated” Lithosphere created at cold spots in Mid Ocean Ridges

A ~50 km stretch of the MAR south of its intersection with the Romanche transform lacks a basaltic crust and exposes mantle ultramafics. We tested the hypothesis that the absence of a basaltic crust reflects a temporary phase of a-magmatic extension by sampling the ocean floor along ridge-parallel profiles taken at increasing distances from the MAR axis, in lithosphere of increasing age. The basaltic crust is nearly absent in a ~50 Km wide strip of ocean floor on the southern side of the Romanche FZ extending from ridge axis for over 400 km along a sea floor spreading flow line. Assuming a spreading half rate of 1.7 cm/yr, crust-free lithosphere formed and spread for roughly 30 m.y., excluding for this area a model of alternating cycles of igneous injection and a-magmatic extension at ridge axis. A ~30 m.y. steady-state formation and spreading of lithosphere that lacks a basaltic crust can be explained by the effect of a regional mantle thermal minimum combined with the thermal effect of a long-offset, slow-slip transform. The basaltic crust-free lithosphere created at the MAR close to the Romanche FZ consists of "normal" mid ocean ridge peridotites (N-MORP) that, however, have undergone an unusually low (<5%) degree of melting, mixed with "metasomatic" peridotites (M-MORP) that have reacted with a small fraction of basaltic melt. This lithosphere contains <10% of scattered gabbroic pockets. These melt impregnations and gabbroic pockets represent melt produced by small degree of melting, frozen within the top ~20 Km of the relatively cold subaxial lithosphere. This unusual lithosphere, unable to expel the melt fraction, ("constipated lithosphere") should be common in "cold spots" along mid ocean ridges.