Processes of hydrothermal alteration in early Archaean oceanic crust

强硅化火山岩及其伴生沉积物是包括南非巴伯顿绿岩带在内的中太古代绿岩带中普遍存在的现象。硅化作用一般被解释为与海底浅对流池中太古宙海水相互作用的结果。热液系统最不寻常的方面之一是存在丰富的电气石，反映了异常高含量的溶解硼。然而，二氧化硅和硼的来源并没有得到明确的限制，其他潜在流体（如高温热液流体）在热液系统形成中的作用也没有得到探索。由于硅化带和电气石矿化带通常位于沉积物之下，这些沉积物含有地球上一些最古老的微生物生命证据，因此早期热液系统可能反映了太古宙海底微生物生命的重要栖息地。在ICDP“Barberton钻探项目：凝视生命的摇篮”的框架下，本项目旨在利用详细的岩石学、流体包裹体分析和电气石的原位b同位素分析来研究海底热液蚀变和硼交代过程。该研究结果将为相互作用流体的来源、组成和物理化学演化提供重要的约束，并将为中太古代海底热液蚀变机制提供详细的见解。

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.