Understanding Melt Generation Beneath the Slow-Spreading Kolbeinsey Ridge Using (238)U, (230)Th, and (231)Pa Excesses
journal contributionposted on 01.11.2011, 00:00 by L. J. Elkins, Ken Sims, J. Prytulak, T. Elliott, N. Mattielli, J. Blichert-Toft, J. Blusztajn, N. Dunbar, C. Devey, D. F. Mertz, J. G. Schilling, M. Murrell
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° 05'-70 degrees 26'N) display (230Th/238U) < 1 and (230Th/238U) > 1 with (230Th/238U) 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. 87Sr/86Sr = 0.70272-0.70301, ε(Nd) = 8.4-10.5, ε(Hf) = 15.4-19.6 (La/Yb)(N) = 0.28-0.84) encompass a narrow range of (230Th/232Th) (1.20-1.32) over a large range in (238U/232Th) (0.94-1.32), producing a horizontal array on a (230Th/232Th) vs. (238U/232Th) diagram and a large variation in (230Th/238U). However, the (230Th/238U) of the Kolbeinsey Ridge basalts (0.96-1.30) are inversely correlated with (234U/238U) (1.001-1.031). Samples with low (230Th/238U) and elevated (234U/238U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium 234U/238U have high (230Th/238U) 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 (230Th/238U) and (231Pa/235U) ratios in uncontaminated Kolbeinsey lavas, but low (231Pa/235U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment.