Abstract

Alkalic basalt dredged from Yuryaku Seamount in the southernmost Emperor Seamount chain and from the western Hawaiian Ridge at Pearl and Hermes Reef and at two unnamed seamounts 160 and 380 km west of Midway is similar to the alkalic basalt that caps the volcanoes in the Hawaiian Islands. Conventional and 40Ar/39Ar K-Ar analyses give best weighted mean ages of 42.3 ± 1.6 m.y. for Yuryaku Seamount, 27.3 ± 0.4 m.y. and 26.7 ± 0.5 m.y. for the two unnamed seamounts, and 20.1 ± 0.5 m.y. for the volcano that forms Pearl and Hermes Reef. The data show that the age of the Hawaiian-Emperor bend is about 41 to 43 m.y. Although the volcanoes in the Hawaiian-Emperor chain generally increase in age to the north and west of the island of Hawaii, the measured age-distance relations along the chain are not linear in detail. A phonolite, possibly a differentiated member of a posterosional nephelinic suite and the first found on the Hawaiian Ridge, was recovered from Pearl and Hermes Reef. Samples of analcime tephrite recovered from the unnamed seamount 380 km west of Midway may also be derived from a posterosional nephelinic suite.

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