Thermodynamics of Organic Transformations in Hydrothermal Fluids

Hydrothermal fluids obtain organic compounds through diverse pathways. In submarine systems organic compounds are already dissolved in seawater that is heated and transformed into hydrothermal fluids through water-rock reactions. Microbes inhabiting hydrothermal systems produce metabolites that enter the fluids, and cells can be carried into the reaction zones by circulating fluids and pyrolyzed. Analogous sources of organic compounds can be anticipated in continental systems with the possible addition of novel plant- and soil-derived organic compounds from the surface. In addition, hydrothermal systems possess large potentials for abiotic organic synthesis that may add a novel suite of compounds. When sedimentary rocks are present, ancient biogenic organic matter can be mobilized or transformed by hot fluids. These transformations accompany the generation of petroleum, coal, and other fossil fuels, suggesting that expectations for hydrothermal transformations can be built on those that occur in sedimentary basins. Likewise, some types of ore deposition are accompanied by transformations of organic compounds, and metal-organic complexes may be involved in enhancing the transport of metals in ore-forming and other crustal fluids. With these thoughts in mind, this review starts with an inventory of the types of organic compounds found in hydrothermal systems and some ways that hydrothermal organic compounds are transformed. Methane can be generated biotically and abiotically from organic or inorganic reactants, and since it lacks a carbon-carbon bond, some researchers would not consider it to be an organic compound. Nevertheless, more data exist for methane in hydrothermal fluids than for any organic compound that fits the definition. Methane has been quantified in continental and submarine hydrothermal fluids, fumarolic gases associated with hydrothermal systems, oil-field brines, deep fluids in sedimentary basins and igneous basement rocks, fluids associated with active serpentinization, and fluid inclusions in minerals from ore deposits, sedimentary basins, and deep crustal settings (recent examples include: …