Carius tube digestion for low-blank rhenium-osmium analysis
A relatively high-temperature oxidizing digestion using aqua regia has been developed for <0.1-5 g size samples of various types of rocks including silicates, sulfides, and metals prior to Re-Os isotopic analysis. Reactions are accomplished in sealed, thick-walled Pyrex tubes (Carius tubes) at 240 °C and elevated pressures for ∼12 h. This digestion technique dissolves platinum-group element minerals, metals, and sulfides and evidently sufficiently reacts with silicates to release most or all Re and Os contained in a silicate matrix. The procedure also leads to the oxidation of Re and Os to their highest valences and, therefore, promotes the complete chemical equilibration of sample Re and Os with enriched isotopes of Re and Os that are added for isotope dilution analysis. Once sample digestion and equilibration are complete, subsequent Os separation is accomplished by conventional double distillation from sulfuric acid, and Re is separated by anion exchange chromatography. Comparison of conventional Teflon vessel digestions and Carius tube digestions for a diverse suite of geological samples shows that the Carius tube digestion both liberates more Os from most matrices and also is a much more robust method for reproducibly measuring the isotopic composition of a sample.