Abiotic synthesis of volatile and condensed organic compounds in the deep oceanic lithosphere 

In nature, very few organic compounds are recognized as abiotic. Abiotic methane (CH4) is the most abundant, and can be accompanied by short-chain hydrocarbons (ethane, propane) or organic acids (formate, acetate) in fluids occurring in molecular hydrogen (H2)-enriched hydrothermal systems where olivine-bearing rocks are altered via serpentinization reactions, such as along slow and ultra-slow spreading ridges. In addition to those volatiles and dissolved organic species, studies of oceanic serpentinites have highlighted low temperature (T), abiotic formation of organic compounds such as amino acids or various carbonaceous compounds within the rock substrate. This suggests the availability of more diverse abiotic organic reactants than previously expected on Earth, notably in the subseafloor, and questions the reaction paths at their origin. Here we present an rocky road to abiotic organic synthesis and diversification in hydrothermal environments, which involves magmatic degassing and water-consuming mineral reactions occurring in olivine fluid inclusions. This combination gathers key gases (N2, H2, CH4, CH3SH) and various polyaromatic materials associated with nanodiamonds and mineral products of olivine hydration (serpentinization). This endogenous assemblage results from re-speciation and drying of cooling C-O-S-H-N fluids entrapped below 600°C-2kbars in rocks forming the present-day oceanic lithosphere. Samples have been drilled at the Atlantis Massif (30°N Mid-Atlantic Ridge) during IODP Expeditions 304-305, five km to the north of Lost City hydrothermal field where the discharge of abiotic H2, CH4 and formate have been observed in fluids. Fluid inclusions served as a closed microreactor in which serpentinization dries out the system toward macromolecular carbon condensation, while olivine pods keep ingredients trapped until they are remobilized for further reactions at shallower levels. Results greatly extend our understanding of the forms of abiotic organic carbon available in hydrothermal environments and open new pathways for organic synthesis encompassing the role of minerals and drying. Such processes are expected in other planetary bodies wherever olivine-rich magmatic systems get cooled down and hydrated.