COMBINED TRACE ELEMENT AND PB-ND-SR-O ISOTOPE EVIDENCE FOR RECYCLED OCEANIC CRUST (UPPER AND LOWER) IN THE ICELAND MANTLE PLUME

Abstract

We present the results of a comprehensive major element, trace element and Sr-Nd-Pb-O isotopic study of post-glacial volcanic rocks from the Neovolcanic zones on Iceland. The rocks studied range in composition from picrites and tholeiites, which dominate in the main rift systems, to transitional and alkalic basalts confined to the off-rift and propagating rift systems. There are good correlations of rock types with geochemical enrichment parameters, such as La/Sm and La/Yb ratios, and with long-term radiogenic tracers, such as Sr-Nd-Pb isotope ratios, indicating a long-lived enrichment/depletion history of the source region. 87Sr/86Sr vs 143Nd/ 144Nd defines a negative array. Pb isotopes define well-correlated positive arrays on both 206Pb/ 204Pb vs 207Pb/204Pb and 208Pb/204Pb diagrams, indicating mixing of at least two major components: an enriched component represented by the alkali basalts and a depleted component represented by the picrites. In combined Sr-Nd-Pb isotopic space the individual rift systems define coherent mixing arrays with slightly different compositions. The enriched component has radiogenic Pb (206Pb/204Pb > 19.3) and very similar geochemistry to HIMU-type ocean island basalts (OIB). We ascribe this endmember to recycling of hydrothermally altered upper basaltic oceanic crust. The depleted component that is sampled by the picrites has unradiogenic Pb (206Pb/ 204Pb < 17.8), but geochemical signatures distinct from that of normal mid-ocean ridge basalt (N-MORB). Highly depleted tholeiites and picrites have positive anomalies in mantle-normalized trace element diagrams for Ba, Sr, and Eu (and in some cases also for K, Ti and P), negative anomalies for Hf and Zr, and low δ18Oolivine values (4.6-5.0‰) below the normal mantle range. All of these features are internally correlated, and we, therefore, interpret them to reflect source characteristics and attribute them to recycled lower gabbroic oceanic crust. Regional compositional differences exist for the depleted component. In SW Iceland it has distinctly higher Nb/U (~68) and more radiogenic 206Pb/204Pb ratios (18.28-18.88) compared with the NE rift (Nb/U ~47; 206Pb/ 204Pb = 18.07-18.47). These geochemical differences suggest that different packages of recycled oceanic lithosphere exist beneath each rift. A third and minor component with relatively high 87Sr/86Sr and 207Pb/204Pb is found in a single volcano in SE Iceland (Öræfajökull volcano), indicating the involvement of recycled sediments in the source locally. The three plume components form an integral part of ancient recycled oceanic lithosphere. The slope in the uranogenic Pb diagram indicates a recycling age of about 1.5 Ga with time-integrated Th/U ratios of 3.01. Surprisingly, there is little evidence for the involvement of North Atlantic N-MORB source mantle, as would be expected from the interaction of the Iceland plume and the surrounding asthenosphere in form of plume-ridge interaction. The preferential sampling of the enriched and depleted components in the off-rift and main rift systems, respectively, can be explained by differences in the geometry of the melting regions. In the off-rift areas, melting columns are truncated deeper and thus are shorter, which leads to preferential melting of the enriched component, as this starts melting deeper than the depleted component. In contrast, melting proceeds to shallower depths beneath the main rifts. The longer melting columns also produce significant amounts of melt from the more refractory (lower crustal/lithospheric) component. © 2006 Oxford University Press.