PA-231 AND TH-230 CHRONOLOGY OF MIDOCEAN RIDGE BASALTS

Mass spectrometric measurements of U-235-Pa-231 and U-238-Th-230 disequilibria are used to further constrain processes producing U-series disequilibria in young mid-ocean ridge basalts (MORB) and to determine eruption ages for samples from 9-10-degrees-N East Pacific Rise and Juan de Fuca-Gorda ridges. Pa-231/U-235 activity ratios for axial basalts are large and relatively uniform within ridge segments. The systematically large Pa-231/U-235, Th-230/U-238, and Ra-226/Th-230 activity ratios in MORB are inconsistent with simple equilibrium melting models and crustal contamination or seawater alteration processes. More complicated melting processes involving disequilibrium or heterogeneous melting of mantle sources and disequilibrium transport of melts to the crust could produce the large U-series disequilibria in MORB. Pa-231 and Th-230 model ages were determined by estimating initial Pa-231/U-235 and Th-230/Th-232 ratios from axial samples and by assuming constancy of these ratios over time. All Pa-231 and Th-230 ages agree to +/-0-7 ka for samples with ages of 0-130 ka. These concordant ages validate the model assumptions of the two dating methods and suggest that mantle sources and melt generation processes have been uniform for these areas during the past 130 ka. Consequently, Pa-231 and Th-230 age-dates can be used to quantify the temporal and spatial dependence of volcanism at oceanic spreading centers during the past 375 ka.