Processes of hydrothermal alteration in early Archaean oceanic crust

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Strongly silicified volcanic rocks and associated sediments are a common phenomenon in Mesoarchaean greenstone belts, including the Barberton greenstone belt in South Africa. The silicification is generally interpreted as a result of interaction with Archaean seawater in shallow subseafloor convection cells. One of the most unusual aspects of the hydrothermal systems is the presence of abundant tourmaline, reflecting exceptionally high contents of dissolved boron. However, the source of silica and boron has not been constrained unequivocally, and the role of other potential fluids in the formation of the hydrothermal systems (e.g. higher-temperature hydrothermal fluids) has not been explored. Because the zones of silification and tourmaline mineralization commonly underlie sediments that contain some of the oldest evidence for microbial life on Earth, the early hydrothermal systems potentially reflect an important habitat for microbial life on the Archaean seafloor. Within the framework of the ICDP “Barberton Drilling Project: Peering into the Cradle of Life” this project aims to investigate the processes of seafloor hydrothermal alteration and boron metasomatism, using detailed petrology, fluid inclusion analysis, and in situ B-isotope analysis in tourmaline. The results will provide important constraints on the source, composition and physico-chemical evolution of the interacting fluid(s), and will give detailed insights into the mechanisms of subseafloor hydrothermal alteration in the Mesoarchaean.