Systematics of smectite hydrogen-isotope composition: Structural hydrogen versus adsorbed water

Abstract

Atmospheric water adsorbed onto clay mineral surfaces, of smectite group minerals in particular, can contaminate hydrogen released from structural hydroxyl groups during hydrogen-isotope analysis. Interlayer cation composition and cation hydration enthalpy, in particular, can affect the magnitude of this excess hydrogen yield and hence the hydrogen isotope composition (δ2H) of smectites. To evaluate this problem, δ2H of different cation-saturated (Ca2+, Na+, K+), dried forms of six smectite standards from the Clay Minerals Society Source Clays were measured using a modified sample drying and on-line High-Temperature-Conversion-Elemental-Analysis (TCEA) Continuous-Flow-Isotope-Ratio-Mass-Spectrometry (CF-IRMS) protocol. More negative interlayer cation hydration enthalpies (Ca2+ > Na+ > K+) led to higher residual adsorbed water contents, which produced poorer δ2H reproducibility for the determination of smectite hydroxyl hydrogen. The lowest adsorbed water contribution and the most reproducible and possibly accurate δ2H for smectite hydroxyl hydrogen was obtained for K-saturated smectites dried for both 4 and 24 h at 220 °C prior to isotopic analysis and transferred to a “Zero-Blank” autosampler in ≤2.5 min. This approach provided the lowest measurement error for hydroxyl δ2H and facilitated much greater sample throughput than classical methods for smectitic clays. This study proposes a protocol for hydroxyl δ2H determination in smectitic clay minerals, and reveals the effect of H-isotope fractionation of adsorbed water during sample preparation.