Effect of serpentinization on carbon speciation: an experiment with formic acid

Abstract

<p>One of the principal theories about the origin of life is based on the abiotic reduction of carbon oxides to various organic molecules in hydrothermal systems. This synthesis is most favored in ultramafic environments undergoing hydrothermal alteration where the serpentinization reaction efficiently produces H<sub>2</sub>. Nevertheless, decades of hydrothermal experiments have hardly succeeded in producing abundant organic volatiles such as CH<sub>4</sub> and short-chain hydrocarbons. On another hand, natural observations have shown the occurrence of other abiotic compounds such as organic acids in fluids and carbonaceous matter (CM) within serpentinized rocks. But organic acids as carbon source and CM as product have not been investigated so far in experiments reproducing hydrothermal peridotite alteration. Here, we explored the effect of formic acid (HCOOH) on the serpentinization reaction and possible feedback effects on carbon speciation in both fluid and solid. We performed reactions at 300°C and 250 bar using peridotite powder (<40 microns) in the presence of  0.1 M formic acid. A temperature of 300°C has been shown to be optimal for olivine serpentinization, while formic acid should partly decomposed into H<sub>2</sub>, CO, and CO<sub>2</sub>. After 4 months, H<sub>2</sub>, CO, CO<sub>2</sub>, CH<sub>4</sub> and short-chain alkanes (mainly ethane) were measured in the fluid, and the powder was completely indurated. The solidified powder displayed a black and white layering perpendicular to fluid diffusion. Its analysis showed the advancement of the serpentinization reaction, and the incorporation of carbon compounds into the solid phase. XRD analysis indicated 70% of serpentinization. SEM-EDX observations showed peculiar texture with large and localized euhedral magnetite grains alternating with larger magnetite grains mixed with C-enriched areas of long chrysotile fibers. FT-IR measurement attested of the widespread formation of carbonaceous material in the solid. Liquid analyses are under progress. Those first results suggest that serpentine formation not only provides additional H<sub>2</sub> to the system, but also mineral surfaces that could play a role in the precipitation of carbonaceous material and carbon speciation in natural systems. The nature and formation mechanisms of this latter remain to be addressed but this opens new paths for abiotic organic synthesis under hydrothermal conditions. In addition to their implications as an abiotic carbon source for deep hydrocarbon degraders ecosystems, it could have important implications for the total carbon cycle.</p>