Isotopic signatures in hydrothermal vent fluids and the oceanic crust : tracing of sub-seafloor magmatic and hydrothermal processes

Abstract

The circulation of seawater through the oceanic crust plays an essential role for the heat and element budgets on our planet. A huge variety of processes such as interaction with the oceanic crust and marine sediments at low and high temperatures, phase separation and segregation, and magmatic fluid influx influence submarine hydrothermal vent fluids on their pathway through the oceanic crust. Since the discovery of first hydrothermal vent fields about 50 years ago, much effort in their exploration has been invested. Because it has long been stated that vent fluids from mid-ocean ridge systems control oceana s chemistry, most of the research was conducted on mid-ocean ridge vent systems. Since several years, vent fluids from arc- and back-arc basins have been the subject of increasing investigation. Several studies revealed that vent fluids from these subduction-related settings also have a huge impact on the oceana s chemistry and that their compositions differ from fluids venting along mid-ocean ridges. Although some of these differences have been identified, their individual impact on vent fluidsa composition remains still poorly understood. The thesis aims to unravel and understand the individual processes on the composition of hydrothermal vent fluids from back-arc basins. For this purpose vent fluids and volcanic rocks from the Manus Basin, Papua New Guinea, and Nifonea volcano, Vanuatu were analysed for their Li, B, Sr and Mg isotopic composition. Vent fluids from these settings have a high variability due to different host rock compositions, different proportions of altered oceanic crust in the hydrothermal circulation cell and variable influx of magmatic fluids. Further, some of the fluids from the Manus Basin and Nifonea indicate phase separation at different pressure and temperature conditions. In accordance with studies on mid-ocean ridge fluids, B in black-smoker fluids from the Manus Basin displays mainly the interaction of seawater with the oceanic crust. B concentrations appear to be slightly affected by phase separation close to the two-phase curve of seawater. In contrast, B concentrations in the boiling fluids from Nifonea volcano are highly enriched suggesting that B preferentially partitions into the low-salinity, high-vapour phase. However, as the low B isotope ratios from Nifonea volcano cannot be explained by phase separation, we rather propose that B is enriched due to a preferential mobilisation from the oceanic crust during water-rock interaction with vapour-rich fluids. This is in accordance with the findings from the gas-rich acid-sulfate fluids from the Manus Basin. Nevertheless, it remains unclear whether B is added by magmatic fluids to the vent fluids or not. Further, the results also show that B in vent fluids might be used to assess the proportion of altered crust in the hydrothermal circulation cell. Because I 7Li values in vent fluids from back-arc basins have lower values compared to mid-ocean ridges although their host rocks have a similar isotopic composition, Li behaviour during water-rock interaction at back-arc basins appears to be distinct from mid-ocean ridges. Furthermore, the results demonstrate that the Li isotope ratios in the vent fluids characterized by the lowest water/rock ratios during water-rock interaction from the Manus Basin match the isotopic composition of their host rocks. This implies that the proposed isotope effect during water-rock interaction is not applicable to the vent fluids from the Manus Basin. It is rather suggested that Li in these vent fluids reflect simple leaching of Li from the oceanic crust with no or negligible isotope effect. This might be valid also for vent fluids from other arc- and back-arc environments. The third part of the dissertation shows that the high Mg concentrations in acid-sulfate fluids have their source in unmodified seawater rather than in the oceanic crust. This supports the theory that they reflect submarine analogues of fumaroles. Nevertheless, the combination of Li, B and Sr isotopes in all acid-sulfate fluids shows a considerable amount of water-rock interaction. The data implies that multi-proxy isotope studies in these fluids offer the potential to trace the progressive alteration of the oceanic crust. The results of this dissertation extended the existing databases on Li, B, Sr and Mg isotope ratios of vent fluids from back-arc basins. Furthermore, the findings of this project yield valuable insights into subduction-related hydrothermal processes and showed that water-rock interactions are distinct from those at mid-ocean ridges.