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

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 (delta 2H) of smectites. To evaluate this problem, delta 2H of different cationsaturated (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 delta 2H reproducibility for the determination of smectite hydroxyl hydrogen. The lowest adsorbed water contribution and the most reproducible and possibly accurate delta 2H for smectite hydroxyl hydrogen was obtained for Ksaturated smectites dried for both 4 and 24 h at 220 degrees 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 delta 2H and facilitated much greater sample throughput than classical methods for smectitic clays. This study proposes a protocol for hydroxyl delta 2H determination in smectitic clay minerals, and reveals the effect of H-isotope fractionation of adsorbed water during sample preparation.

Automated summary

The study investigated the impact of different interlayer cations on the hydrogen isotope composition of smectites, proposing a new protocol for determining the hydroxyl delta 2H in smectitic clay minerals. The findings also revealed the effect of H-isotope fractionation of adsorbed water during sample preparation.