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

L.J Elkins,K.W.W Sims,J Prytulak,T Elliott,N Mattielli,J Blichert-Toft,J Blusztajn,N Dunbar,C Devey,D.F Mertz,J.-G Schilling,M Murrell

doi:10.1016/j.gca.2011.08.020

Abstract

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.

Highlights

Mid-ocean ridge basalt (MORB) chemistry varies systematically as a function of ridge depth, which was posited to be related to underlying crustal thickness (Klein and Langmuir, 1987)
Uranium-series analyses are reported in Table 2, and Sr, Nd, Hf, and Pb isotope results are presented in Table 3
Kolbeinsey Ridge samples are tholeiites characterized by extreme depletion in incompatible elements, with samples TRI0139-001-001G and POS185 1105B having, respectively, the lowest U and Th concentrations far measured in any MORB (Table 2)

Summary

Introduction

Mid-ocean ridge basalt (MORB) chemistry varies systematically as a function of ridge depth, which was posited to be related to underlying crustal thickness (Klein and Langmuir, 1987). Among global mid-ocean systems, the slow-spreading Kolbeinsey Ridge ($1.8 cm/yr; Mosar et al, 2002) north of Iceland represents Klein and Langmuir’s (1987) shallow end member, due to its shallow ridge axis (mean depth $1100 m). Relative to other MORB, the Kolbeinsey Ridge erupts basalts that are highly depleted in trace elements and have both low Na8, suggesting large degrees of melting, and high Fe8, suggesting deep initiation of melting (Klein and Langmuir, 1987). Using a limited global data set, and following on the model of Klein and Langmuir (1987), Bourdon et al (1996a) predicted that production of thick crust at shallow mid-ocean ridges, such as the Kolbeinsey Ridge, involves a long, deep melt column initiating in the garnet peridotite stability field, producing large melt fractions with high (230Th/238U) ratios.

Methods

Results

Conclusion