OFF-AXIS VOLCANISM AT THE EAST PACIFIC RISE DETECTED BY URANIUM-SERIES DATING OF BASALTS

RECENT detailed surveys of the East Pacific Rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast-spreading ocean ridge crests1-7. In parallel with geological and geochemical investigations, it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium-series disequilibria methods8-11. Here we use U-238-Th-230 and U-235-Pa-231 ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9-degrees 31' N on the East Pacific Rise. Most of the ages are younger than would be expected based on off-axis distance and spreading rate. We infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0.5-2 km outside the axial summit caldera, with some volcanism occurring as far as 4 km off-axis. Melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas.