Organic Sulfides in Hydrothermal Solution: Standard Partial Molal Properties and Role in Organic Geochemistry of Hydrothermal Environments

Thermodynamic properties for aqueous alkyl sulfides have been compiled and/or estimated through established methods. These properties are used to investigate reactions among various sulfur compounds in a variety of geological environments, ranging from sea floor hydrothermal systems to organic-rich sludge. Using thermodynamic data and the revised Helgeson-Kirkham-Flowers (HKF) equations of state, along with geochemical constraints imposed by the environment, it is possible to estimate the abiotic production of this class of organic sulfur compounds. For example, in hydrothermal systems in which H₂ and H₂S concentrations are buffered by the pyrite–pyrrhotite–magnetite (PPM) mineral assemblage, calculated equilibrium activities of dimethyl sulfide (DMS) are as high as 10⁻³ through formation reactions in which the environment contains millimolal concentrations of CO₂. Higher activities are obtained when DMS formation from CO is considered and when more reducing mineral assemblages are present.