Chapter 5 - Hydrothermal circulation in oceanic crust

Abstract

This chapter discusses the nature, chemical changes, and hydrothermal activities during hydrothermal circulation in the oceanic crust. Hydrothermal systems have two basic characteristics: they occur in regions of high geothermal gradient where hot rocks lie near the surface, and they have a “plumbing system” of fractures so that the cold water can percolate downwards into the crust and then rise convectively to the surface after being heated. All hydrothermal systems require a permeable layer of rock that allows cold water to percolate slowly downwards over a wide area, a localized heat source below the permeable layer, and channels for the heated buoyant plume of hot water to escape more rapidly at the surface above the heat source. Rock metamorphism during hydrothermal activity involves hydration, the addition of large amounts of magnesium, the loss of calcium and sometimes sodium and potassium, and gains and losses of numerous minor and trace elements. During hydrothermal circulation, seawater loses all its magnesium and sulfate and gains significant amounts of calcium and sometimes potassium and sodium too. Sea-floor weathering occurs at bottom-water temperatures and involves hydration, alteration of feldspars and glass to clay minerals and zeolites, as well as oxidation, especially of iron and manganese to form oxide and hydroxide coatings.