The present study provides a comprehensive geochemical dataset on ultra-/high pressure (UHP/HP) eclogites and enclosed, fluid-deposited metamorphic veins. It targets to broaden our understanding of the nature and extent of fluid-rock interaction in deep subduction environments, and to evaluate the capability of metamorphic fluids at high pressures to producing elemental fractionations which may account for global mass imbalances such as the enigmatic lead-and niobium paradoxes. This is

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... d by bringing together geochemistry, isotope geology and a fluid inclusion study to reconstructing fluid origin and compositional evolution as well as fluid-assisted element transport and fractionation in subducting basaltic rocks. The occurrence of different vein generations records multiple stages of fluid flow during deep subduction and exhumation of continental crust. 1 st generation quartz-rutile veins are identified to represent internal remnants of prograde, virtually "dry" eclogite dehydration. The precipitating Si-, and HFSE-bearing aqueous fluids solidify in a closed system at UHP peak conditions, without evidence of retrogression in the host eclogite or detectable compositional fluid evolution. Fluids that precipitate mineralogical complex 2 nd generation veins are found to form at the onset of exhumation from UHP eclogite facies depths. Based on chemical and isotopic fingerprinting, their initial source is determined to be external fluids that are derived from prograde dehydration of bedrock gneisses. A doctoral thesis is always the final outcome of constructive interaction of various persons with both scientific and non-scientific background, whose contributions shall be acknowledged at this point. First of all, I convey my sincere gratitude to my supervisor Gerhard Wörner for raising the funding (DFG project WO 362/45-1) and for providing me with this topic. His passion for science and his continuous and encouraging support with fresh ideas and optimism paved my way to working on this thesis with curiosity and confidence. Yilin Xiao (USTC Hefei) is thanked for initiating this project in the course of a collaboration with the Geoscience Center Göttingen and for conducting the field work in China that left a deep and memorable impression on me. Andreas Pack is thanked for co-supervising this thesis and for providing me with a research assistant sideline position in his working group alongside my doctoral studies. Working in the stable isotope lab has always been a welcomed change from my PhD project. My research would not have been possible without the help and reliable support of my colleagues: