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 com plexity of the volcanic and magmatic processes occurring along and acr oss 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 yo ung basalts using mass spectrometric uranium-series disequilibria meth ods8-11. 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 youn ger than would be expected based on off-axis distance and spreading ra te. 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 resi dence times and magmatic supply systems that differ from those of axia l lavas.