Simultaneous determinations of the concentration and isotope ratio of sulfate- and sulfide-sulfur and carbonate-carbon in geological samples

Rock and mineral powders are decomposed at 280°C in vacuum with phosphoric acid containing Sn²⁺as a reducing agent (Kiba reagent). Sulfate and sulfide sulfur in the samples are extracted in liquid nitrogen-cooled traps as SO₂and H₂S, respectively, while carbonate carbon is collected as CO₂O The three gases are separated from each other by successive vacuum distillation. The H₂S is converted to SO₂by reaction with Cu₂O. The three gases, that is, two SO₂and one CO₂are measured volumetrically for the concentrations of these species and mass spectrometrically for their isotope ratios. The uncertainties in the measurements of concentration and isotope ratio of sulfur and carbon in rocks are ± 5% and ± 0, 3 ‰, respectively, in most cases. The technique is suitable for the analyses of micro-amounts of sulfur and carbon in rocks. A few grams of rock sample generally suffices for the chemical and isotopic analyses of sulfur and carbon (carbonate). Some of the results so far obtained on volcanic rocks are discussed briefly in order to demonstrate the usefulness as well as the limitations of the technique.