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Science 16 July 1993:
Vol. 261. no. 5119, pp. 315 - 320
DOI: 10.1126/science.261.5119.315
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Articles

A Cold Suboceanic Mantle Belt at the Earth's Equator

Enrico Bonatti 1, Monique Seyler 2, and Nadia Sushevskaya 3

1 Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 and the Istituto Geologia Marina CNR, Bologna, Italy
2 Laboratoire de Petrologie, Université Lille, Lille, France
3 Vernadsky Institute of Geochemistry, Russian Academy of Sciences, Moscow, Russia

An exceptionally low degree of melting of the upper mantle in the equatorial part of the mid-Atlantic Ridge is indicated by the chemical composition of mantle-derived mid-ocean ridge peridotites and basalts. These data imply that mantle temperatures below the equatorial Atlantic are at least sim150°C cooler than those below the normal mid-Atlantic Ridge, suggesting that isotherms are depressed and the mantle is downwelling in the equatorial Atlantic. An equatorial minimum of the zero-age crustal elevation of the East Pacific Rise suggests a similar situation in the Pacific. If so, an oceanic upper mantle cold equatorial belt separates hotter mantle regimes and perhaps distinct chemical and isotopic domains in the Northern and Southern hemispheres. Gravity data suggest the presence of high density material in the oceanic equatorial upper mantle, which is consistent with its inferred low temperature and undepleted composition. The equatorial distribution of cold, dense upper mantle may be ultimately an effect of the Earth's rotation.


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