| Summary: | 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°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, e Nd = 8.4–10.5, e 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.
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