UC San Diego

Years of studies show that most of the ocean island basalts (OIB) that comprise the bulk of prominent linear volcanic chains are geochemically different from mid-ocean ridge basalts (MORB). However, the cause of the geochemical differences between OIB and MORB as well as the origin of geochemical enrichment and heterogeneity in OIB are highly controversial. Volcanic glasses and lavas with OIB-like composition from isolated intraplate volcanoes in the northern Lau Basin in the southwestern Pacific and from abandoned spreading centers in the eastern Pacific were investigated for their petrology and geochemistry to indirectly evaluate the man-tle source and origin of the enriched geochemical signature of OIB that form prominent linear volcanic chains. The study of the OIB-like lavas from isolated intraplate volcanoes in the Pacific suggests that (1) the mantle source of the OIB- like lavas is the compositionally heterogeneous upper mantle; (2) in the northern Lau Basin, the heterogeneity was created by the introduction into a pre-existing Indian Ocean-type suboceanic mantle of subduction components from the Tonga Trench and later by OIB components from the nearby Samoan mantle plume; (3) in the eastern Pacific, the geochemically enriched, OIB-like compositional signature of post-spreading lavas on fossil spreading axes were produced by low degrees of partial melting of the compositionally heterogeneous Pacific suboceanic mantle; and (4) OIB comprising prominent linear volcanic chains could not have been formed through the same mechanism of partial melting of random, enriched heterogeneities in the upper mantle; instead they appear to be coming from distinct, relatively large reservoirs that reside deeper in the mantle