Kinetics of hydrogen isotope exchange in kaolinite and the prediction of δD signature retention over geological time

Abstract

Retention of the pristine hydrogen isotope composition (expressed as δD) formed during mineral formation in equilibrium with water is the basis for any paleoenvironmental reconstructions and for tracing mineral reactions using hydrogen (or H and O) isotope composition in kaolinite. Not only do post-formation reactions cause partial equilibration with ambient water, altering the pristine signature, but also result in the decoupling of H and O diffusion.

In this study, kaolinite samples of different structural order (expressed, for example, as the Hinckley Index) were tested for their susceptibility to H isotope exchange under D₂O vapor in an open system. The H isotope exchange was detected by recording the kaolinite’s structural OH and OD stretching mode using infrared spectroscopy. A number of different in-situ and ex-situ testing protocols under diverse temperature ranges (90 °C, 125–275 °C, 300–700 °C), show consistent kinetics and mechanism. In each kaolinite sample, the H isotope exchange rate increases with temperature; however, the major control on the reaction rate is the kaolinite crystallite’s structural order, not the particle (agglomerated crystallites) size. The hydrogen isotope composition in kaolinite can be altered independently of oxygen atoms, likely via proton hopping mechanism through structural defects. The H isotope exchange reaction under vapour shows a two-mode kinetics: the time-to-the-quarter (TTTQ; t^1/4) dependence dominates in the first part of the reaction or in low-temperature reactions, transitioning to the log₁₀t (Elovich) relationship and with lower activation energy, in advanced stages and in higher temperatures. Extrapolating the TTTQ model to temperatures < 90 °C allows estimating the rate of H isotope exchange between water vapor and kaolinite. The paper presents the prediction of preservation of pristine δD signature and thus reliability of using isotope data in kaolinites of different origins and structural order when exposed to pore water / water vapor over geological timescales.