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Geology; November 2001; v. 29; no. 11; p. 979-982; DOI: 10.1130/0091-7613(2001)029<0979:SSCOCF>2.0.CO;2
© 2001 Geological Society of America
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Steady-state creation of crust-free lithosphere at cold spots in mid-ocean ridges

Enrico Bonatti1, Daniele Brunelli2, Paola Fabretti2, Marco Ligi2, Rosana Asunta Portaro2 and Monique Seyler3

1 Dipartimento di Scienze della Terra, Università La Sapienza, Rome, Italy, and Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA
2 Istituto di Geologia Marina, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129, Bologna, Italy
3 Laboratoire de Petrologie, ESA 7058, Université de Paris 6-7 and Université de Lille 1, Villenueve d'Ascq, France

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.

Key Words: oceanic lithosphere • mid-ocean ridges • mantle cold spots • mantle flow numerical model




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