ocean-island basalt

views updated May 08 2018

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.