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的普遍现象。硅化通常被解释为在浅海底对流单元中与太平洋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，反映了溶解硼的含量异常高。然而，二氧化硅和硼的来源并没有明确的限制，其他潜在的流体在形成热液系统（如高温热液流体）的作用还没有得到探讨。由于硅化和电气石矿化区通常位于含有地球上微生物生命最古老证据的沉积物之下，因此早期热液系统可能反映了大洋底微生物生命的重要栖息地。在ICDP "巴伯顿钻探项目：窥视生命的摇篮"的框架内，该项目旨在利用详细的岩石学、流体包裹体分析和在图尔库的原位硼同位素分析，调查海底热液蚀变和硼交代作用的过程。研究结果将对相互作用的流体的来源、成分和物理化学演化提供重要的制约，并将详细了解中太古代海底热液蚀变的机制。

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.