A hydrothermal pathway for dolomite precipitation

Many Phanerozoic dolostones provide evidences of high-temperature dolomitization. Buried limestones have been altered by hydrothermal fluids flowing through the Earth’s crust. The reactions have been reproduced in laboratories and are relatively well understood. This situation contrasts with dolostones that do not show signs of high-temperature dolomitization, but appear to have been precipitated on the seafloor or transported to form sedimentary beds. These dolomites are regarded as having formed at low temperatures (10 – 30°C, often described as warm), but this notion lacks support from laboratory experiments. These experiments have demonstrated that a kinetic energy barrier prevents dolomite formation. There are catalysts that enable the barrier to be overcome; notably the activity of sulphate-reducing microbes. The conceptual difficulty is that these catalysts are not capable of generating thick beds of dolomite, and most relate to niche environments that have not been shown to explain massive Precambrian and Phanerozoic deposits. This explanatory void is known as the “dolomite problem”. The aim of this study is to test the validity of a hypothesis that has the potential to solve the Dolomite Problem. The focus is on Precambrian dolomites, partly because there are no skeletal carbonates contributing to dolomite beds, and partly because Archaean and Proterozoic field evidences point strongly to precipitation. The hypothesis is that most of the dolomite precipitation took place when venting hydrothermal fluids interacted with seawater, with the implication that water in Precambrian seas was much hotter than today’s marine environments. Evidences supporting hot seawater and precipitation are discussed using white smokers as a modern analogue and illustrated using a case study of Precambrian dolomites found in the Garvellachs (islands in the Scottish Hebrides). First suggested in 1974, the hypothesis of hydrothermal-sedimentary dolomite has been used recently to explain lacustrine features associated with tectonic activity. Applying the hypothesis to all cases of precipitated dolomite leads to an alternative perspective that integrates the presence of sedimentary dolomite in marine settings and the process of dolomitization within the crust of the Earth.