SUBCONTINENTAL MANTLE EXPOSED IN THE ATLANTIC-OCEAN ON ST-PETER-PAUL ISLETS

ST PETER-PAUL (Penedos de São Pedro e Paulo), tiny islets in the Atlantic ocean just north of the Equator, have interested Earth scientists for more than a century. Darwin1 was the first to recognize their peculiarity: he landed on St Peter-Paul (SPP) from the Beagle in 1831 and noticed that, unlike most oceanic islands, SPP are not volcanic. Subsequent studies confirmed Darwin's view, and established that SPP are made mostly of peridotites and represent an uplifted body of upper mantle2-7. As a result, SPP have been cited frequently as providing a representative sample of sub-oceanic mantle. Here I argue that, although they are located on a major transform zone less than 200 km from the axis of the Mid-Atlantic Ridge, SPP are not a sample of sub-oceanic mantle. Mineral chemistry, elemental and isotopic chemistry and geother-mometry suggest that SPP are a relict of sub-continental mantle, which was left behind during the opening of the equatorial Atlantic, and was then tectonically uplifted to its present position. © 1990 Nature Publishing Group.