Hydrogen Isotope Exchange in Clay Mineral–Water Systems: Geochemical Insights From an Improved Kinetic Model

Hydrogen (H) isotopic compositions of clay minerals have long been used to reconstruct the climate, environment, and hydrology of Earth’s past, and more recently, on Mars. Though these approaches often rely on the assumption that primary H isotopic composition is preserved through time, rates and mechanisms of post-formational isotopic exchange with secondary waters are not well understood. The kinetics of isotopic exchange are critical to assessing the fidelity of paleoclimatic reconstructions from clay minerals, with further applications in geoengineering such as the use of clays to effectively immobilize tritium from radioactive waste. Here, we use controlled laboratory experiments coupled with isotopic and mineralogical analyses to assess the kinetics of H exchange in kaolinite–water and montmorillonite–water systems, demonstrating that self-diffusion dominates in the absence of mineralogical alteration – we propose this occurs via hydrogen hopping. We find that H exchange between montmorillonite and water exhibits a lower activation energy (18.7 kJ/mol) than kaolinite (35.0 kJ/mol), likely due to increased structural defects and isomorphous substitutions lowering the energetic barrier for self-diffusion of H. We present an improved model of isotopic exchange kinetics that accurately captures the observed behavior, enabling more realistic extrapolations to geological timescales. The moving r model incorporates a time-dependent rate of exchange that accounts for the progressive decline in exchange rates observed in many heterogeneous systems. By applying this to our results and building a forward model, we demonstrate that some amount of isotopic exchange between secondary fluid and clay mineral H is inevitable over geological timescales – however, this may be analytically indistinguishable and insignificant within paleo-reconstructions depending on system specifics (e.g., the difference in isotopic composition between primary and secondary fluids). We use clay mineral-based thermometry as a case study to quantitatively evaluate the ramifications of post-formational H isotope exchange on paleoclimate reconstructions.