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

地幔中的不均匀性可能会随着时间的推移被对流混合所破坏。一些最强的对流运动发生在地幔柱附近，因此它们与周围地幔的内部成分分区的相互作用和混合的性质对于限制地幔混合过程很重要。以脊为中心的冰岛热点允许一个独特的采样的成分边界内和周围的假设地幔柱由于存在的扩展轴，它提供了一个采样剖面整个羽。本项目旨在对冰岛正北方Tjörnes地区的扩张轴进行取样，在那里，在近空中富集的热点状玄武岩和近海贫化的洋中脊状玄武岩之间存在明显的地球化学边界。我们的主要目标是找出这个地球化学边界有多尖锐，以及它现在和过去的位置。我们希望能由此推断出地幔边界的方向。为了实现这些目标，需要采取火山学、地球化学、地质年代学和地球物理学的联合方法。我们希望与该项目中的其他项目合作，确定：-Tjörnes岩浆类型的现在和过去分布; -Tjörnes扩展轴的喷发历史; -Tjörnes地震活动性，喷发和热液作用之间的关系。为了实现这些目标，我们建议在2002年和2003年夏天使用FS Poseidon对Tjörnes区进行船基测绘和采样。利用高分辨率ICP-MS和U、Th衰变系列开发定年方法，以及详细的侧扫描信息，对于限制该地区的演变将是重要的。

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.