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

Abstract

Parent-daughter disequilibria between (230Th)/(238U), (231Pa)/(235U) and (226Ra)/(230Th) (parentheses refer to activities) have been measured by thermal ionization mass spectrometry and inductively coupled plasma-mass spectrometry in basalts from three tectonomagmatic settings of the East Pacific Rise (EPR) at 8°20′-10° N. Mid-ocean ridge basalts (MORB) from the Siqueiros Transform, the Lamont Seamounts, and the EPR ridge crest span a large compositional range from primitive, high-MgO basalts with strong incompatible element depletions (DMORB) to typical normal MORB (NMORB) to rare incompatible element enriched basalts (EMORB) derived from a more enriched source isotopically. Concentrations of U vary from <13 ppb in DMORB 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 (226Ra)/(230Th) that ranges from 3.2 to 4.2, while EMORB appear old being near secular equilibrium. Initial (231Pa)/(235U) are very high (>2.5) in all of the Siqueiros basalts. Three basalts from the Lamont Seamounts have low incompatible element concentrations and low Th/U and are in secular equilibrium for (226Ra)/(230Th) while the sample located closest to the ridge axis has significant 226Ra and 231Pa excesses and minor 230Th excess. DMORB lack 230Th excess, have high excesses of 226Ra and 231Pa, and resemble experimentally determined melts of peridotite at 1 GPa, implying derivation from relatively shallow level melting of spinel lherzolite at low residual porosity. Disequilibria for all three parent-daughter pairs are consistent with typical axial NMORB resulting from mixing of melts derived from heterogeneous sources, specifically 90-95% DMORB with 5-10% EMORB. The observation that all samples, regardless 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 230Th excess over short spatial scales imply that garnet-bearing mafic veins create all of the 230Th excess observed in typical NMORB. Copyright 1999 by the American Geophysical Union.