Isotopic signatures in seafloor hydrothermal systems: from crust creation to subduction

The geochemical exchange between fluids and rocks in seafloor hydrothermal systems has large impacts on the composition of seawater and the increasingly altered oceanic crust. The characteristics of hydrothermal fluid-rock exchange are indeed highly diverse in differing geological settings due to variations in fluid temperature and acidity, host rock and alteration phase compositions, and basement structures. The investigation of seafloor hydrothermal systems is therefore of major importance to learn more about the processes that drive changes in the mode of fluid-rock exchange and to make constraints on global geochemical budgets.

In the scope of this dissertation, hydrothermal fluids and fresh as well as altered rocks from four diverse settings of fluid-rock interaction were investigated by using boron and/or lithium elemental and isotopic compositions, partly combined with strontium and oxygen isotopic systematics. The studied locations range from high-temperature hydrothermal systems that are (1) hosted in mantle rocks at the slow-spreading Mid Atlantic Ridge, over (2) a low-temperature ridge flank hydrothermal system at the eastern flank of the East Pacific Rise, up to high-temperature hydrothermal activity that is associated with (3) an active arc volcano in the southern Kermadec Arc and (4) with a vent system at the eastern Manus back-arc basin. In addition, an experimental study was conducted to investigate the boron partitioning and isotopic fractionation between seawater and mantle materials.

The conducted research underlines the effectiveness of the used multi-proxy approach to identify variations in fluid-rock interaction temperatures, to distinguish between different host rock lithologies and also to determine differences in the water-to-rock-ratios that are sensitively coupled to permeability contrasts or structural heterogeneities within the rock column of seafloor hydrothermal systems. Furthermore, the study highlights the impact on ocean chemistry of the high diversity in total hydrothermal mass flux of lithium and boron and its isotopic composition between varying settings of hydrothermal fluid-rock exchange.