Studying the processes at the origin of "abiotic organic matter", i.e., of reduced carbon compounds produced without any biological intervention at any step, is of first importance to understand the origin of life building blocks. This issue was the main focus of my PhD research jointly supervised by the University of Rio de Janeiro (Brazil) and the Institut de Physique du Globe de Paris from September 2011 to September 2015. In the Solimões petroleum basin (Brazil) which underwent magmatic intrusions, we detected hydrocarbon gases with unconventional isotopic signatures inconsistent with a thermal evolution of fossil organic matter (Milesi et al., 2016a). The source rock of the Solimões petroleum was found to contain up to 7% wt. of siderite, the iron(II)-bearing carbonate (FeCO3). The siderite content was shown to decrease while, concurrently, the gas maturity and the occurrence of isotopic anomalies increased, which leads us to explore possible connections between siderite reactivity and abiotic production of reduced carbon species.

Laboratory experiments were carried out with synthetic siderite (FeCO3) and deionized water in the 200-300°C range and at 50 MPa (Milesi et al., 2015). Iron (II) from siderite and water reacted to yield magnetite and highly reducing fluids containing CO2 and H2. While the generation of CH4 is kinetically hindered, thermodynamic calculations suggest that the produced H2 and CO2 achieve equilibrium with a solid carbon phase with thermodynamic properties close to those of graphite. This was confirmed by transmission electron microscopy observations of carbonaceous material coating the grains of magnetite, which may act as catalyst for the reduction of CO2. The formation of solid reduced carbon compounds controls the H2 fugacity produced by the water-siderite interaction.

Additional thermodynamic calculations were performed to examine the potential for abiotic condensed carbonaceous material to be produced in the more general context of hydrothermal serpentinization of ultrabasic rocks (Milesi et al., 2016b). Recently, macromolecular carbonaceous deposits have been identified in serpentinites recovered from the seafloor (Menez et al., 2012; Pasini et al., 2013) but whether these deposits have a biologic or an abiotic origin is still unclear. Calculation of reaction pathways during serpentinization of olivine at 300°C showed that fluid compositions should equilibrate with condensed carbonaceous material (represented in the calculations by either graphite or anthracene), which regulates the H2 and CO2 activities. Fluids produced in serpentinization experiments as well as fluids sampled from ultramafic-hosted deep-sea hydrothermal systems at Lost City and Rainbow are shown to be consistent with the model, which indicates that carbonaceous material should be a major product of the reduction of CO2 in these hydrothermal settings.

Formation of carbonaceous material from CO2- and H2-rich hydrothermal fluids would have significant implications for a number of processes, including the cycling of organic carbon in the oceanic crust by creating a pool of relatively immobile reduced carbon, the extent of H2 production in ultrabasic environments and the emergence of a prebiotic chemistry. Indeed, similar phenomena of creation of carbon-carbon bonds could have played an important role for prebiotic reaction.

References

Ménez B., Pasini V. and Brunelli D. (2012) Life in the hydrated suboceanic mantle. Nat. Geosci. 5(2), 133-137.

Milesi V., Guyot F., Brunet F., Richard L., Recham N., Benedetti M. and Prinzhofer A. (2015) Formation of CO2, H2 and condensed carbon from siderite dissolution in the 200-300°C range and at 50MPa. Geochim. Cosmochim. Ac. 154, 201-211.

Milesi V., Prinzhofer A., Guyot F., Benedetti M. and Rodrigues R. (2016a) Contribution of siderite-water interaction for the unconventional generation of hydrocarbon gases in the Solimões basin, north-west Brazil. Mar. Petrol. Geol. 71, 168-182.

Milesi V., McCollom T. M. and Guyot F. (2016b) Thermodynamic constraints on the formation of condensed carbon from serpentinization fluids. Geochim. Cosmochim. Ac. 189, 391-403.

Pasini V., Brunelli D., Dumas P., Sandt C., Frederick J., Benzerara K., Bernard S. and Ménez B. (2013) Low temperature hydrothermal oil and associated biological precursors in serpentinites from Mid-Ocean Ridge. Lithos, 178, 84-95.