Methane production from hydrothermal transformation of siderite to magnetite

Mumma et al. (2009) observed a methane (CH4) plume above the Nili Fossae region on Mars, a region rich in carbonate minerals. Morris et al. (2010) suggest this to be (Mg,Fe)-carbonate. McCollom (2003) demonstrated that the hydrothermal transformation of siderite (FeCO3), to magnetite (Fe3O4) produces CH4. This reaction may thus contribute to the formation of methane on Mars, but is also relevant in the context of such diverse topics as diagenesis of Precambrian banded iron formations, sources of prebiotic organic compounds on early Earth, oil and gas accumulations in Earth's crust, or geological sequestration and storage of CO2. However, neither the thermodynamics of this reaction nor the conditions of maximum CH4 yield have been investigated to date. In order to estimate how pressure and temperature influence CH4 yield we derived a thermodynamic model with a numerical solution implemented in MATLAB. We used the equation 12FeCO3 + 2H2O → 4Fe3O4 + 11CO2 + CH4 (Frost et al. 2007) and thermodynamic calculations of the stability field of FeCO3 by Thoms-Keprta et al. (2009) as a template. At 1 bar pressure, the Gibbs energy turns negative (favorable reaction conditions) at a temperature of 200°C. Increasing pressure to 1000 bar changes that temperature to 250°C. An increase in temperature has a larger effect on shifting the Gibbs energy to more negative values. We therefore chose ambient pressure and temperatures of 300°C, 400°C, and 500°C as experimental conditions. We added 100 mg of either natural or synthetic FeCO3 and 25 μL of MilliQ water into long tip Pasteur pipettes inside an anoxic glove box to avoid contamination by free oxygen. The Pasteur pipettes were sealed with butyl stoppers and then melted shut outside of the glove box. The glass capsules were heated for 48 hours in a muffle furnace at 300°C, 400 0C or 5000C. The composition of the gas phase and the formation of methane in particular were analyzed using gas chromatography with a flame ionization detector. We used Mössbauer spectroscopy, X-ray diffraction, X-ray fluorescence, and scanning electron microscopy with Energy-Dispersive X-ray spectroscopy to investigate changes in the solid phase. Synthetic FeCO3 was completely transformed to Fe3O4 and sometimes the further oxidized phases maghemite (γ-Fe2O3) and hematite (α-Fe2O3). Natural FeCO3 was not completely transformed, which can be explained by its larger particle size and therefore reduced reactivity. Methane yield was consequently higher from synthetic siderite. Our results show that hydrothermal activity invoked by either impact or volcanic activity could have transformed siderite and thereby released at least some of the CH4 observed on Mars. On Earth, long-term underground storage of CO2 as carbonate minerals has to avoid hydrothermal conditions. Otherwise not only CO2 will be released again, but some of it will potentially be transformed into the more potent greenhouse gas methane. References Frost et al., Contrib. Mineral. Pet. 153 (2006) 211; McCollom, Geochim. Cosmochim. Ac. 67 (2003) 311; Morris et al., Science 329 (2010), 421; Mumma et al., Science 323 (2009) 1041; Thomas-Keprta et al., Geochim. Cosmochim. Ac. 73 (2009) 6631, EA-4