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

1999; American Geophysical Union; Volume: 104; Issue: B6 Linguagem: Inglês

10.1029/1999jb900081

2156-2202

Craig Campbell Lundstrom, Daniel E. Sampson, M. R. Perfit, James B. Gill, Quentin Williams,

High-pressure geophysics and materials

Parent‐daughter disequilibria between ( 230 Th)/( 238 U), ( 231 Pa)/( 235 U) and ( 226 Ra)/( 230 Th) (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 ( 226 Ra)/( 230 Th) that ranges from 3.2 to 4.2, while EMORB appear old being near secular equilibrium. Initial ( 231 Pa)( 235 U) 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 ( 226 Ra)/( 230 Th) while the sample located closest to the ridge axis has significant 226 Ra and 231 Pa excesses and minor 230 Th excess. DMORB lack 230 Th excess, have high excesses of 226 Ra and 231 Pa, 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 230 Th excess over short spatial scales imply that garnet‐bearing mafic veins create all of the 230 Th excess observed in typical NMORB.