New Perspectives on Abiotic Organic Synthesis and Processing during Hydrothermal Alteration of the Oceanic Lithosphere

The main known organic compounds on Earth are biologically derived, whether they are direct products of biological activity or the result of thermal degradation of bio-derived material. While the synthesis of organic compounds from inorganic reactants is a common process in the chemical industry, it remains an unverified component of the deep carbon cycle on Earth and possibly on other planetary bodies. Abiotic organic synthesis is central to life emergence and sustainability, and possibly to “geo-inspired” resources. Intensive efforts are still needed to unravel the possible forms, sources, quantities, and formation mechanisms of abiotic carbon compounds under geologically relevant conditions. An improved knowledge of their processing within the lithosphere is also mandatory to better quantify their impact on biogeochemical cycles and their contribution to C fluxes between Earth’s external and internal envelops. Their presence in fluids and rocks may also affect the kinetics of fluid–silicate reactions and the fates of other elements, particularly the redox-sensitive ones (e.g. transition metals, S). Abiotic organic compounds in the lithosphere can have two main origins: either rising from a deep volcanic source in the mantle or formed in situ in the upper lithosphere during hydrothermal processes from mantellic or seawater inorganic carbon compounds (see Refs. 1 and 2 for reviews). In the present chapter, we focus on lithospheric hydrothermal processes that include late magmatic stages and fluid–rock reactions. In fluids, natural occurrences of recognized abiotic organic volatiles of hydrothermal origin include methane (CH4), short-chain alkanes, and small organic acids. They have mainly been observed in geothermal systems or continental seepages within ophiolites and Precambrian shields and at hydrothermal vents near mid-ocean ridges and subduction forearcs. Hence, they are not necessarily associated with deep active volcanism, and they attest to the contribution of lithospheric hydrothermal processes to abiotic organic synthesis. In most cases, these occurrences of organic volatiles are associated with high concentrations of H2 reached by reduction of water during aqueous alteration of ferrous iron-bearing minerals. The latter are particularly abundant in mantle and olivine-rich lithologies whose alteration process is known as serpentinization.