Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using (238)U, (230)Th, and (231)Pa excesses

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Elkins, J. L., Sims, K. W. W., Prytulak, J., Mattielli, N., Blichert-Toft, Janne, Blusztajn, J., Dunbar, N., Devey, C., Mertz, D. F., Schilling, J.-G., Murrell, M.
• Other Authors: Woods Hole Oceanographic Institution (WHOI), Department of Geology, Bryn Mawr College, Department of Geology and Geophysics Laramie, University of Wyoming (UW), Department of Atmospheric, Oceanic and Planetary Physics Oxford (AOPP), University of Oxford, School of Earth Sciences Bristol, University of Bristol Bristol, Département des Sciences de la Terre et de l'Environnement, Université libre de Bruxelles (ULB), Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement (LGL-TPE), École normale supérieure - Lyon (ENS 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), New Mexico Institute of Mining and Technology New Mexico Tech (NMT), Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR), Institut für Geowwisenschaften, Johannes Gutenberg - Universität Mainz = Johannes Gutenberg University (JGU), University of Rhode Island, University of Rhode Island (URI), Los Alamos National Laboratory (LANL), NSF : OCE-0422278, OCE-1061037/1060434, INSU : French Institut National des Sciences de l'Univers
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2011
• Subjects: EAST PACIFIC RISE; U-SERIES DISEQUILIBRIA; MID-ATLANTIC RIDGE; NORWEGIAN-GREENLAND SEA; ND-PB ISOTOPE; TH-PA-RA; HETEROGENEOUS ICELAND PLUME; UPWELLING RATES BENEATH; TRACE-ELEMENT EVIDENCE; JUAN-DE-FUCA; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDE.MCG]Environmental Sciences/Global Changes; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Greenland; Jan Mayen; Kolbeinsey; Kolbeinsey Ridge; Mid-Atlantic Ridge; Pacific; Greenland Sea; Iceland
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-00674633; https://doi.org/10.1016/j.gca.2011.08.020

International audience To examine the petrogenesis and sources of basalts from the Kolbeinsey Ridge, one of the shallowest locations along the global ridge system, we present new measurements of Nd, Sr, Hf, and Pb isotopes and U-series disequilibria on 32 axial basalts. Young Kolbeinsey basalts (full-spreading rate = 1.8 cm/yr; 67 degrees 05'-70 degrees 26'N) display ((230)Th/(238)U) < 1 and ((230)Th/(238)U) > 1 with ((230)Th/(238)U) from 0.95 to 1.30 and have low U (11.3-65.6 ppb) and Th (33.0 ppb-2.40 ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e. g. (87)Sr/(86)Sr = 0.70272-0.70301, epsilon(Nd) = 8.4-10.5, epsilon(Hf) = 15.4-19.6 (La/Yb)(N) = 0.28-0.84) encompass a narrow range of ((230)Th/(232)Th) (1.20-1.32) over a large range in ((238)U/(232)Th) (0.94-1.32), producing a horizontal array on a ((230)Th/(232)Th) vs. ((238)U/(232)Th) diagram and a large variation in ((230)Th/(238)U). However, the ((230)Th/(238)U) of the Kolbeinsey Ridge basalts (0.96-1.30) are inversely correlated with ((234)U/(238)U) (1.001-1.031). Samples with low ((230)Th/(238)U) and elevated ((234)U/(238)U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium (234)U/(238)U have high ((230)Th/(238)U) values (>= 1.2), which are consistent with melting in the presence of garnet. This is in keeping with the thick crust and anomalously shallow axial depth for the Kolbeinsey Ridge, which is thought to be the product of large degrees of melting in a long melt column. A time-dependent, dynamic melting scenario involving a long, slowly upwelling melting column that initiates well within the garnet peridotite stability zone can, in general, reproduce the ((230)Th/(238)U) and ((231)Pa/(235)U) ratios in uncontaminated Kolbeinsey lavas, but low ((231)Pa/(235)U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment