Hydrothermal Experiments with Protonated Benzylamines Provide Predictions of Temperature-Dependent Deamination Rates for Geochemical Modeling

In fluids of sufficient temperature and residence time, certain organic species are observed to equilibrate, and their abundances become diagnostic of reaction conditions, including temperature, redox state, and pH. Organic species released from remote geologic and planetary settings can therefore serve as geochemical tracers for environments that are difficult to observe directly. Here, we provide a framework for selecting organic compounds as geochemical tracers based on kinetic modeling. We characterized temperature-dependent rates of deamination substitution reactions for aqueous protonated benzylamines, i.e., benzylaminiums, to form benzyl alcohols and ammonium. Hydrothermal experiments were conducted at 200-300 °C at liquid-vapor water saturation pressures with ring-substituted benzylaminiums expected to have comparable deamination rates to environmentally abundant aminiums, e.g., amino acids. We compared rates extrapolated from experiments to idealized natural systems, taking into account fluid temperatures and residence times. Our results indicate that reversible deamination/hydration reactions may equilibrate over geologic time scales across diverse environments, including those approaching freezing temperatures for the most reactive benzylaminium. Therefore, similar reaction constituents may be useful targets for the exploration of potentially habitable subsurface environments, such as icy ocean worlds of the solar system. Our investigation supports previous findings that aqueous deamination of benzylaminiums operates via two simultaneous substitution mechanisms, SN1 and SN2. We find that for certain benzylaminiums, rates of each mechanism should be modeled individually to improve extrapolation across temperatures. Extrapolations of observed (i.e., bulk) deamination kinetics to near-ambient temperatures (∼50 °C) without mechanistic considerations can produce discrepancies in reaction half-lives on the order of a billion years.