Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

• Main Authors: KOKFELT, THOMAS FIND, HOERNLE, KAJ, HAUFF, FOLKMAR, FIEBIG, JENS, WERNER, REINHARD, GARBE-SCHÖNBERG, DIETER
• Language: English
• Published: 2017
• Subjects: Iceland; Ocean Island
• Online Access: http://doc.rero.ch/record/300044/files/egl025.pdf

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 ...