Do Off-ridge Volcanoes on the East Pacific Rise Originate from the Moho Transition Zone?

Abstract

A large number of intraplate volcanoes erupted two to several hundred kilometers off the fast-spreading East Pacific Rise（EPR）. These volcanoes consist of large lava fields, monogenet-ic volcanoes, and linear chains of monogenetic volcanoes and volcanic ridges. Large lava fields of 7⊖26 km 3 in volume are known at 8° N, 14° S, and 16° S within 2⊖19 km from the rise axis and from the top 75⊖100 m of ODP Site 1256 on the 15 Ma Cocos plate. Monogenetic volcanoes form within ～ 20 km from the rise axis or on the basement ＜ 200 kyr, and are evenly distributed over the rise axis. Linearly aligned volcanoes and volcanic ridges occur farther from the rise axis than large lava fields and monogenetic volcanoes, and run subparallel to the direction of the Pacific plate motion. The Sojourn Ridge, the largest volcanic ridge, extends up to 440 km in length and is several hundred cubic kilometers in volume. Eruptive ages along a volcanic ridge and a volcano chain contradict the hot-spot origin of these volcanic features. Negative free-air and residual mantle Bouguer anomalies correlate well with the linearly aligned volcanoes and volcanic ridges, suggesting excess magma supply beneath the volcanic edifices. Seismic experiments show volcanic ridges have no keel below the Moho, indicating compensation of surface loading by plate flexure and underplating. Whole rock compositions of off-ridge volcanoes have a much wider spectrum than the adjacent axial lavas, spanning from depleted NMORB through TMORB to isotopically fertile EMORB. Some off-ridge lavas could be produced by the fractional crystallization of the same parent magma as the adjacent axial lavas. However, most off-ridge lavas originate from different parent magmas than the neighboring axial lavas. Some TMORB magmas including the 14° S large lava field are the mixing product of the NMORB and EMORB magmas. Copious differentiated lavas of the large lava fields require a large magma chamber as a the site for crystallization differentiation and magma mixing. The lava geochemistry of off-ridge volcanoes strongly suggests the presence of a magma source that is independent of the axial magma plumbing system. Seismic tomography and seafloor compliance measurements beneath the northern EPR indi-＊ 静岡大学理学部地球科学教室 ＊＊ 産業技術総合研究所地質調査総合センター ＊＊＊ 海洋研究開発機構地球内部変動研究センター