FAVORABLE THERMODYNAMIC OF ANAEROBIC HYDROCARBONS OXIDATION IN HYDROTHERMAL SYSTEMS

The hydrothermal systems of mid-ocean ridges are characterized by the effects of endogenous origin hydrocarbons emission, accompanied by abiogenic products of their oxidation [1].
The С–Н system of aqueous alkanes, alkenes, and alkynes in hydrothermal environment presented as two-phase monovariant equilibria that divided the divariant areas of stability (the facies) of the hydrocarbons (Figure). The existence of those metastable equilibria of hydrocarbons in hydrothermal systems is experimentally proven [2].

 
Figure. Anaerobic oxidation of aqueous hydrocarbons on the phase diagram of the Н₂ (µ_Н2 = RTlnα_H2) and Н₂О (µ_Н2О = RTlnα_Н2О) chemical potentials under the 250 bar and 150°С.

 

The H₂O chemical potential increasing leads to the anaerobic oxidation of hydrocarbons with the formation of stable CO₂ facies and facies of HCOOH and CH₃OH at low and high chemical potential of H₂, respectively. The equilibria of hydrocarbons with CO₂ are "evolutionary death" of the С–Н–О system development process. Favorable thermodynamic conditions for the formation of organic compounds occur when the oxidation equilibrium are shifting in the region of higher chemical potentials of H₂O as result of the removal of CO₂ molecules from the hydrothermal solution with the formation of carbonate precipitates. In this case, the metastable equilibria are achieved (dashed lines) between the hydrocarbons and organic compounds (in particular, CH₃OH). The thermodynamics of the hydrocarbons aerobic oxidation in hydrothermal systems is considered in [3, 4].

 

References
1. Lang et al., 2010, Geochim. Cosmochim. Acta 74: 941–952. 2. Seewald, 2001, Geochim. Cosmochim. Acta 65: 1641- 1664. 3. Marakushev, 2008, Dokl. Biochem. Biophys. 418: 18–23. 4. Marakushev, Belonogova, 2009, J. Theoret. Biol. 257: 588–597.