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Methane clumped isotope signatures of abiogenesis may be diagnostic of the origin of methane on Earth and other planetary bodies. We performed synthesis of abiogenic methane in hydrothermal conditions between 130 and 300°C and determined δ13C, δD, Δ13CH3D, and Δ12CH2D2. The experiments were performed by heating water in the presence of Fe0 powder and CO. The reduction of water on metallic iron led to the formation of H2. CO was reacted with both H2 and H2O, generating both CH4 and CO2. Methane δ13C values are isotopically depleted by ∼25‰ relative to the CO starting material. This is consistent with carbon isotopic equilibrium between methane, carbon monoxide and carbon dioxide in our experiments. In contrast, D/H ratios are inconsistent with equilibrium isotopic fractionation, as illustrated by δD values of methane fractionated by ∼500‰ relative to starting H2O. This suggests that under our experimental conditions, hydrogen additions to carbon may be governed by kinetics. Δ13CH3D values track experimental temperature, with values between +1.5‰ and +5.0‰ for most samples. In contrast, Δ12CH2D2 values are displaced from equilibrium. We find exclusively negative Δ12CH2D2 values, showing deficits down to 40‰ relative to thermodynamic equilibrium. We interpret the data as evidence for distinct, kinetically induced D/H pools contributing to methane assembly, that is, a combinatorial effect. The cumulative D/H fractionations associated with CO hydrogenation explain the direction and magnitude of Δ12CH2D2 values during abiotic methane formation. We suggest that near equilibrium Δ13CH3D with negative Δ12CH2D2 signatures will help identify methane formed abiotically in nature. Plain Language Summary: Methane is observed in various environmental settings on Earth, including but not limited to, hydrothermal fluids and sedimentary systems. It is also found on other worlds, as it was reported to occur on Mars, in Enceladus's geysers, and as of recently, in an exoplanet's atmosphere. Methane may be formed by life or by non‐biological processes, so the presence of methane alone on another world is not enough to declare a planet habitable. Geochemists have built methods to tell biogenic from abiotic methane, which ought to be useful for the identification of life‐derived methane on another planet. Those techniques are crucial but complex, based on integrated studies combining molecular and isotopic compositions of methane and associated gases. Here, we report on whether abiotic methane shows a unique signature for 13CH3D and 12CH2D2, the mass‐18 isotopologues of methane. We performed synthesis of abiotic methane in the laboratory and determined its composition. We observe unique signatures for methane clumping, where 12CH2D2 disequilibrium is associated with equilibrium 13CH3D abundances. We suggest that near equilibrium Δ13CH3D with negative Δ12CH2D2 signatures will help identify methane formed abiotically on Earth and on other worlds. Key Points: Synthesis of abiotic methane in the laboratory via a controlled Fischer‐Tropsch process under hydrothermal conditions carbon isotope equilibrium and hydrogen isotope disequilibrium during methane assembly13CH3D equilibration associated with substantial 12CH2D2 disequilibrium. Abiosignatures identified with combined 13CH3D and 12CH2D2 [ABSTRACT FROM AUTHOR]

We report the formation of minerals from the tochilinite-valleriite group (TVG) during laboratory serpentinization experiments conducted at 300 and 328 °C. Minerals in the TVG are composed of a mixture of sulfide and hydroxide layers that can contain variable proportions of Fe, Mg, Cu, Ni, and other cations in both layers. Members of this group have been observed as accessory minerals in several serpentinites, and have also been observed in association with serpentine minerals in meteorites. To our knowledge, however, TVG minerals have not previously been identified as reaction products during laboratory simulation of serpentinization. The serpentinization experiments reacted olivine with artificial seawater containing 34S-labeled sulfate, with a small amount of solid FeS also added to the 300 °C experiment. In both experiments, the predominant reaction products were chrysotile serpentine, brucite, and magnetite. At 300 °C, these major products were accompanied by trace amounts of the Ni-bearing TVG member haapalaite, Ni,Fe-sulfide (likely pentlandite), and anhydrite. At 328 °C, valleriite occurs rather than haapalaite and the accompanying Ni,Fe-sulfide is proportionally more enriched in Ni. Reduction of sulfate by H2 produced during serpentinization evidently provided a source of reduced S that contributed to formation of the TVG minerals and Ni,Fe-sulfides. The results provide new constraints on the conditions that allow precipitation of tochilinite-valleriite group minerals in natural serpentinites. [ABSTRACT FROM AUTHOR]

The properties of high-temperature lithospheric fluids within the early Earth are poorly known, yet many origin-of-life scenarios depend upon their characteristics. These fluids represent a key communication pathway between EarthÕs interior and hydrothermal pools. We use zircon chemistry, experiments, and modeling to infer the character of lithospheric fluids approaching 4 billion years. We constrain oxygen fugacity, chlorine content, and temperature, which allow us to model the solubility and transport of metals that are hypothesized to be crucial for the origin of life. We show that these fluids were more oxidized than the terrestrial mantle during this time and that they were interacting with near-surface aqueous systems, possibly subaerial hydrothermal pools, amplifying redox gradients in a location attractive for prebiotic molecular synthesis or sustained microbial activity. [ABSTRACT FROM AUTHOR]

Research indicates that hydrothermal plumes may generate sufficient biomass to make a significant contribution to organic matter found in both the deep ocean and in ocean basin sediments. Primary production was found to take place early on in plume development, and that oxidation of hydrogen provides the largest amount of energy among dissolved substrates.

The geological observations and experimental studies related to deep-sea hydrothermal systems where abiotic hydrocarbons are formed due abiotic organic synthesis are reviewed. The research provides insight into high abundance of abiotic methane present in the deep-sea hydrothermal fluids and the highly saline and low abundance of [H.sub.2]S-rich vent fluids.