Perspective on the Genesis of E-MORB from Chemical and Isotopic Heterogeneity at 9-10 degrees N East Pacific Rise

Autor: Michael R. Perfit, Janne Blichert-Toft, Kenneth W.W. Sims, Christopher L. Waters, Jurek Blusztajn
Přispěvatelé: Department of Geology and Geophysics [Laramie], University of Wyoming (UW), Woods Hole Oceanographic Institution (WHOI), Department of Geological Sciences [Gainesville] (UF|Geological), University of Florida [Gainesville] (UF), Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure de Lyon (ENS de Lyon)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Université Jean Monnet - Saint-Étienne (UJM)-Centre National de la Recherche Scientifique (CNRS), National Science Foundation (NSF) : OCE-0623838, OCE-0137325, OCE-9402360, 9403773, 0138088, Institut National des Sciences de l'Univers : INSU, Laboratoire de Sciences de la Terre (LST), Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Claude Bernard Lyon 1 (UCBL), Université de Lyon-Université de Lyon-École normale supérieure - Lyon (ENS Lyon), Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement [Lyon] (LGL-TPE), Université de Lyon-Université de Lyon-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)
Jazyk: angličtina
Rok vydání: 2011
Předmět:
Zdroj: Journal of Petrology
Journal of Petrology, 2011, 52 (3), pp.565-602. ⟨10.1093/petrology/egq091⟩
Journal of Petrology, Oxford University Press (OUP), 2011, 52 (3), pp.565-602. ⟨10.1093/petrology/egq091⟩
ISSN: 0022-3530
1460-2415
DOI: 10.1093/petrology/egq091⟩
Popis: International audience; 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 degrees 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 (230)Th excesses to the crust, whereas low-porosity channels transport melts more slowly, allowing them to equilibrate at shallow depths and develop large (226)Ra excesses at the expense of diminished (230)Th 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 degrees 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 degrees N EPR, the asymmetric, off-axis distribution of E-MORB at 9-10 degrees 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 (similar to 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 degrees 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.
Databáze: OpenAIRE
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