Stable chlorine-isotope analysis of rock samples: New aspects of chlorine extraction

This paper presents refinements of the chlorine-extraction technique which allows to measure stable chlorine-isotope compositions (δ37Cl: per-mil deviation relative to standard mean ocean chloride, SMOC) of rock samples by isotope-ratio mass spectrometry (IRMS) with a precision of 0.12‰ (2σ). HF (17 M, 4 ml) was used for decomposition of gram amounts of several rock standard samples containing 220–16200 ppm Cl. Excess amount of F− was precipitated and removed as CaF2 by neutralizing with Ca(OH)2 and by cation-exchange chromatography with strongly acidic resin (Ca2+ form). Extracted Cl dissolved in 50–250 ml of water was concentrated on a strongly basic anion-exchange resin bed (OH− form) and was quantitatively recovered into 4.0 ml of 0.5 M KNO3 and then converted to CH3Cl. This CH3Cl was used for Cl isotope analysis by IRMS. Recovery yield of the Cl-extraction from standard rocks agreed well with Cl contents reported for them. Reproducibility of the measured δ37Cl values through the whole process of the Cl extraction was better than 0.10‰.