The northern boundary of the Iceland plume: Petrology and volcanology of the Tjörnes "fracture zone"

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Heterogeneities in the Earth´s mantle are probably destroyed through time by convective mixing. Some of the strongest convective movements occur close to mantle plumes, and so the nature of their interaction and mixing with the surrounding mantle of their internal compositional zoning is important for placing constraints on mantle mixing processes. The ridge-centred Iceland hotspot allows a unique sampling of compositional boundaries both within and around the supposed mantle plume due to the presence of the spreading axes, which provide a sampling profile across the plume. The present project aims to sample the spreading axis in the Tjörnes region directly north of Iceland, where a clear geochemical boundary occurs between enriched hotspot-like basalts subaerially and depleted mid-ocean ridge-like basalts offshore. Our main aims are to find out how sharp this geochemical boundary is and where it lies at present and back through time. From this we hope to be able to deduce the orientation of the boundary in the mantle. To achieve these aims a joint volcanological, geochemical, geochronological and geophysical approach is required. We wish, in collaboration with the other projects in this package to determine: - the present-day and past distribution of magma-types at Tjörnes; - the eruption history of the Tjörnes spreading axis; - the relationships between seismicity, eruptions and hydrothermalism at Tjörnes. To achieve these aims we propose a ship-based mapping and sampling of the Tjörnes zone using the FS Poseidon in the summers of 2002 and 2003. The development of a dating method using high-resolution ICP-MS and the U and Th decay series will be important, together with detailed side-scan information, for constraining the evolution of the area.