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Abstract: No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of 238U–230Th–226Ra–210Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens (Kent et al., 2000). We report 210Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222Rn from 226Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8ka, and likely as young as 1ka, despite being located on crust that has a spreading age of ~75ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies. [Copyright &y& Elsevier]

The discovery of chemically and isotopically enriched mid-ocean ridge basalts (E-MORB) has offered substantial insight into the origin, time scales, and length scales of mantle heterogeneity. However, the exact processes involved in producing this E-MORB enrichment are vigorously debated. Additionally, because the ages of E-MORB are not well constrained, the petrogenetic, temporal, and geological relationships between E-MORB and normal (N)-MORB are not known. To investigate these relationships and to explore how melting and melt transport processes contribute to or modify enriched mantle source compositions and generate E-MORB melts beneath mid-ocean ridges, we measured major and trace elements, and Sr, Nd, Hf, Pb, and U–Th–Ra isotopes for a suite of lavas that were collected off-axis, including several E-MORB, at 9–10°N along the East Pacific Rise (EPR). These data show coherent mixing trends among long-lived radiogenic isotopes, U-series nuclides, and incompatible trace elements, implying that mixing of melts from different sources occurs at different depths. Our results are consistent with previous studies that show that melting occurs in a two-porosity melting regime, with high-porosity channels forming deeply in the presence of garnet and transporting enriched melts with large 230Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large 226Ra excesses at the expense of diminished 230Th excesses. Forward modeling of the trace element data also is consistent with mixing of melts in a two-porosity melting regime. U-series age constraints suggest that E-MORB neither erupt at systematically different times from N-MORB, nor necessarily through different pathways. Previous studies of E-MORB at 9–10°N have suggested that E-MORB compositions could be explained by off-axis eruption. However, when considered in light of previously published magnetic paleointensity and U-series age constraints, recent geological studies, and the most widely accepted contemporary understanding of volcanic construction at 9–10°N EPR, the asymmetric, off-axis distribution of E-MORB at 9–10°N EPR is consistent with, and more simply explained by, a model in which E-MORB erupted within the axial summit trough (AST) and flowed down the ridge flanks (∼0–3 km). These E-MORB subsequently spread away from the AST, and, finally, were preserved on the seafloor through asymmetric construction of the extrusive layer. Taken together, the range of ages of E-MORB at 9–10°N EPR and the geochemical and isotopic mixing trends suggest that enriched melts are continuously supplied to the ridge axis, but because of their small proportions relative to the volumetrically and volcanically dominant N-MORB, E-MORB preservation and exposure is comparatively scarce. [ABSTRACT FROM AUTHOR]