Insights into mid-ocean ridge basalt petrogenesis: U-series disequilibria from the Siqueiros Transform, Lamont Seamounts, and East Pacific Rise

Parent-daughter disequilibria between (Th-230)/(U-238), (Pa-231)/(U-235) an d (Ra-226)/(Th-230) (parentheses refer to activities) have been measured by thermal ionization mass spectrometry and inductively coupled plasma-mass s pectrometry in basalts from three tectonomagmatic settings of the East Paci fic Rise (EPR) at 8 degrees 20'-10 degrees N. Mid-ocean ridge basalts (MORB ) from the Siqueiros Transform, the Lament Seamounts; and the EPR ridge cre st span a large compositional range from primitive, high-MgO basalts with s trong incompatible element depletions (DMORB) to typical normal MORE (NMORB ) to rare incompatible element enriched basalts (EMORB) derived from a more enriched source isotopically. Concentrations of U vary from <13 ppb in DMO RB to >400 ppb in EMORB while Th/U ranges from 2 in DMORB up to 3 in EMORB. The young-looking high-MgO basalts have (Ra-226)/(Th-230) that ranges from 3.2 to 4.2, while EMORB appear old being near secular equilibrium. Initial (Pa-231)/(U-235) are very high (>2.5) in all of the Siqueiros basalts. Thr ee basalts from the Lament Seamounts have low incompatible element concentr ations and low Th/U and are in secular equilibrium for (Ra-226)/(Th-230) wh ile the sample located closest to the ridge axis has significant Ra-226 and Pa-231 excesses and minor Th-230 excess. DMORB lack Th-230 excess, have hi gh excesses of Ra-226 and Pa-231, and resemble experimentally determined me lts of peridotite at 1 GPa, implying derivation from relatively shallow lev el melting of spinel Iherzolite at low residual porosity. Disequilibria for all three parent-daughter pairs are consistent with typical axial NMORB re sulting from mixing of melts derived from heterogeneous sources, specifical ly 90-95% DMORB with 5-10% EMORB. The observation that all samples, regardl ess of tectonomagmatic setting, lie on the same mixing trend suggests that melting beneath seamounts and transforms is similar to melting beneath the ridge axis. Variations in Th-230 excess over short spatial scales imply tha t garnet-bearing mafic veins create all of the Th-230 excess observed in ty pical NMORB.