From cold seeps to hot vents on the Nubian/Eurasian plate boundary in the North Atlantic Ocean

Hot and cold fluid expulsions at the seabed are common phenomena in various tectonic settings. Both temperature endmembers play a major role in the element exchange between the lithosphere and the ocean. Understanding the formation and processes involved in fluid generation is the key to understand the impact of fluid discharge on global element cycles. Hot and cold vents normally appear in different tectonic settings, where hot vents are most common in spreading centers and cold vents are present in active and passive continental margin settings. This study focuses on the western and eastern end of the Nubian/Eurasian plate boundary in the North Atlantic Ocean. The Nubian/Eurasian plate boundary and its specific tectonic characteristics with a spreading environment in the western part (Azores Plateau and Terceira Rift) and a compressional and strike-slip stetting at its eastern end (Gulf of Cadiz) allows for, both, hot and cold fluid venting. The first part of this study compromises a novel fully-coupled, basin-scale, reaction-transport model to simulate the fluid genesis of five mud volcanoes in the Gulf of Cadiz, at the eastern end of the Nubian/Eurasian plate boundary. The study was designed to investigate fluid formation processes that were previously postulated by Hensen et al. (2015). An advantage of this model is the coupling of a realistic geophysical setting with geochemical processes, considering a growing sediment column over time together with compaction of sediments as well as diffusion and advection of dissolved pore water species and chemical reactions. The modeled processes affecting the fluid genesis include the dehydration of clay minerals and the recrystallization of calcium carbonate. In addition to that, we tested if a circulation of fluids through aged (>140 Ma) oceanic crust could also affect fluid genesis. The model is capable to reproduce the fluid signatures (chloride, strontium and 87Sr/86Sr) of all mud volcanoes. With this modeling approach we confirm the hypothesis that fluid circulation along deeply rooted strike slip faults old oceanic crust is likely affecting the fluid composition at the studied mud volcanoes. The second chapter of this study gives a first comprehensive overview of pore water compositions for the magmatically active Azores Plateau, at the western end of the Nubian/Eurasian plate boundary. The data set compromises 22 gravity cores with various different geochemical fluid patterns. Most gravity cores are affected by early diagenetic processes like AOM, carbonate precipitation and recrystallization. Furthermore, also weathering of volcanic compounds could affect the pore water patterns. Despite the ongoing volcanic activity no deep submarine hydrothermal systems at the Azores plateau are described to date. Here we report for the first time clear indications for the existence of sediment hosted hydrothermal systems on the Azores Plateau. The geochemical signals of the four identified systems are typical mid-oceanic-ridge signals, with increased Ca and depleted Mg and SO4 fluids, indicating a hydrothermal alteration of the basement. In order to distinguish those signals from ash alteration we underline our findings by seismic profiles showing sill intrusions and fluid conduits as well as free gas in the subsurface. The last chapter provides a multi element and isotope analyses of a hydrothermal vent in the Terceira Rift, at the western end of the Nubian/Eurasian plate boundary. Fluids sampled in the Terceira Rift show elevated Mg, SO4 and total alkalinity values. Those fluids do not show the typical trends known from hydrothermal vents at mid-oceanic spreading centers. The most straightforward interpretation of those findings is the dissolution of the hydrothermally formed mineral caminite, an Mg-sulfate-hydroxide-hydrate. Caminite is a rare mineral but proposed to form under specific conditions such as high temperature and inhibited Mg-smectite formation in hydrothermal recharge zones. In general, those conditions are met in the Terceira Rift. We propose the hydrothermal system is in a waning state and caminite dissolves as it is shifted out of the temperature stability field by progressive cooling of the hydrothermal system. Enriched fluids are rising to the seafloor via slow advection through listric faults.


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