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

J. L. Elkins (1, 2) , K. W. W. Sims (1, 3) , J. Prytulak (4, 5) , N. Mattielli (6) , Janne Blichert-Toft (7) , J. Blusztajn (1) , N. Dunbar (8) , C. Devey (9) , D. F. Mertz (10) , J.-G. Schilling (11) , M. Murrell (12)

1 WHOI - Woods Hole Oceanographic Institution
2 Department of Geology
3 Department of Geology and Geophysics [Laramie]
4 AOPP - Department of Atmospheric, Oceanic and Planetary Physics [Oxford]
5 School of Earth Sciences [Bristol]
6 Département des Sciences de la Terre et de l'Environnement
7 LGL-TPE - Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement
8 NMT - New Mexico Institute of Mining and Technology [New Mexico Tech]
9 IFM-GEOMAR - Leibniz-Institut für Meereswissenschaften
10 Institut für Geowwisenschaften
11 University of Rhode Island
12 LANL - Los Alamos National Laboratory

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

Mots clés: 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

Domaines: Géochimie; Milieux et Changements globaux; Sciences de la Terre

J. L. Elkins, K. W. W. Sims, J. Prytulak, N. Mattielli, Janne Blichert-Toft, et al.. Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using (238)U, (230)Th, and (231)Pa excesses. Geochimica et Cosmochimica Acta, 2011, 75 (21), pp.6300-6329. ⟨10.1016/j.gca.2011.08.020⟩. ⟨insu-00674633⟩