Nine small seamounts located on young crust (< 10 m.y.) near the East Pacific Rise were surveyed in detail and sampled by multiple dredges. Preliminary results indicate that most seamounts (closed elevations on the sea floor with about 1 km of relief) are volcanoes, but that some, especially those near fracture zones, may be fault blocks. The flanks and summits of the volcanoes are mostly composed of basalt sheet flows, hyaloclastite, and hyaloclastite breccia, whereas fault blocks contain mostly pillow lava. The rocks from the volcanoes show great chemical diversity. Most are large-ion-lithophile–depleted tholeiitic basalt similar to normal mid-ocean ridge tholeiite, with a wide range of 100 Mg/ (Mg + Fe+2) and other chemical abundances indicative of low-pressure fractionation. As a group, the samples also show large differences in (La/Sm)N, even in rocks from a single volcano. This provides evidence that some chemical heterogeneities in the source region of these basalts are small.

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