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

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=1.8cm/yr; 67°05′–70°26′N) display (²³⁰Th/²³⁸U)<1 and (²³⁰Th/²³⁸U)>1 with (²³⁰Th/²³⁸U) from 0.95 to 1.30 and have low U (11.3–65.6ppb) and Th (33.0ppb–2.40ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e.g. ⁸⁷Sr/⁸⁶Sr=0.70272–0.70301, εNd=8.4–10.5, εHf=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.