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Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
The East Pacific Rise axis, 10–12°N, consists of three discrete topographic or tectonic units each of which has erupted compositionally distinct magmas. The two major units have an axial bathymetric high, are bounded by transform or overlapping spreading centres and are shown to be discrete volcanoes. One of the units shows systematic variation in magma composition along the rift consistent with injection of lava down-rift and concomitant low-pressure fractional crystallization with distance from the topographic high. Compositional variation within and between magmatic units is largely controlled by fractional crystallization at low and intermediate pressures. The eastern and western limbs of a large overlapping spreading centre at about 11°45'N have compositionally distinct lavas, apparently from separate magma batches. Axial eruptions in the vicinity of a large transform fault (Clipperton Fracture Zone) show a large variation in composition and at least one independent magma injection. Highly fractionated lavas with compositions up to 59% silica are included in these eruptions. One of the axial tectonic units has, in addition to normal tholeiitic eruptions, some lavas highly enriched in incompatible elements such as potassium, titanium, strontium, barium, zirconium and lanthanum. These lavas are also isotopically enriched with respect to 87Sr/86Sr, suggesting that the mantle beneath some portions of the axis is compositionally heterogeneous. The presence of compositionally distinct magma batches, often in close proximity, suggests that magma chambers underlying the East Pacific Rise axis are small and probably intermittent.
