Assimilation, contamination and felsic melt generation at the top of the axial melt lens at fast-spreading oceanic crust: Combined geochemical and experimental investigations at the dike/gabbro transition from IODP Site 1256 (East Pacific Rise)

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The formation and evolution of oceanic lithosphere at mid-ocean ridges is one of the dominant processes in the chemical differentiation and physical evolution of our planet. The about 60.000 km long mountain chain of mid-ocean ridges is associated with 90% of Earth´s volcanism and about 25 % of heat release of the Earth. Although so important, major issues on the mode of magmatic accretion, the way how the lower crust is hydrothermal cooled, or the geological meaning of the seismic Layer 2-3 boundary are still poorly understood. To solve critical questions related to the geodynamics of fast-spreading mid-ocean ridge systems, the IODP drilling campaign "Superfast Spreading Crust" (Leg 206, Expeditions 309, 312, 335) at Site 1256 was initiated. Site 1256 is located on 15 million year old crust formed at the East Pacific Rise during an episode of superfast ocean spreading (>200 mm/yr full rate). It is one of the major achievements of this campaign that, for the first time in IODP, a complete section through intact crust from lavas via sheeted dikes into the uppermost gabbros was drilled. One characteristic of the lowermost sheeted dikes as well as in the gabbroic section of the 1256 drill core, is the abundance of felsic rocks which show significant differences in compositions, modes of emplacement, and relations to the various background rocks. They provide the key for understanding the complex and intimate coupling between magmatic, metamorphic and hydrothermal processes at the top of the axial melt lens in fast-spreading mid-ocean ridge systems. In order to understand the different mechanisms of evolved melt formation within a spectrum of magmatic processes from MORB differentiation to assimilation-partial melting of previously hydrothermally altered material and its role in contaminating MORB, we attempt to start a systematic investigation on the geochemistry and petrology of these rocks. For this, major and trace element geochemical methods will be applied to bulk rocks as well as to mineral phases. The results will enable us to unravel the principal magmatic endmember processes involved, and their role as contaminator for primary MORB. In a second part of the project we plan to apply experimental techniques - crystallization and partial melting experiments using natural systems recovered at Site 1256 - in order to test, confirm, or modify those models derived by our geochemical and petrological results. The combination of both approaches will offer considerable insight into the complex interfering magmatic, metamorphic, and hydrothermal processes ongoing at the top of axial melt lens in fast-spreading mid-ocean ridge systems. Details on this are poorly known up to now, and the exceptional rocks of the dike-gabbro transition at Site1256 drilled by IODP provide the key for improving our understanding.