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Cunha, Tristão da

Tristão da Cunha (trēshtouN´ dä kōō´nyə), c.1460–1514?, Portuguese navigator. His most important voyage was undertaken in 1506, when he set out with 15 ships for India. He discovered three volcanic islands in the S Atlantic, one of which is named for him. After taking Socotra off Arabia, in the hope of establishing control over the Red Sea, he went on to India, while Afonso de Albuquerque, under secret royal orders, detached part of the fleet. On his return to Portugal, Tristão da Cunha carried out a diplomatic mission at the papal court. A son, Nuno da Cunha, 1487–1539, was governor of India and captured Basra in 1529.

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ocean-island basalt

ocean-island basalt (OIB) Quartz tholeiites, alkali basalts, and nephelinites found on volcanoes which build up from the ocean floor to form the ocean islands away from ocean ridges. Examples include the Cape Verde Islands, Ascension Island, Tristan da Cunha Island, and Gough Island in the Atlantic, and the Hawaiian-Emperor seamount chain in the Pacific. Compared to MORBs (see MID-OCEAN-RIDGE BASALT), OIBs are enriched in (a) large-ion lithophile (LIL) elements, (b) light rare-earth elements (LREE) relative to heavy rare-earth elements (HREE), and (c) incompatible elements such as Ti, Ga, Li, Nb, V, Zn, Zr, and Y. Radiogenic-isotope and trace-element evidence suggests that OIBs are formed by partial melting of enriched mantle.

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