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 in cluding silicates, sulfides, and metals prior to Re-Os isotopic analys is. Reactions are accomplished in sealed, thick-waded Pyrex tubes (Car ius tubes) at 240 degrees C and elevated pressures for similar to 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 p rocedure also leads to the oxidation of Re and Os to their highest val ences and, therefore, promotes the complete chemical equilibration of sample Re and Os with enriched isotopes of Re and Os that are added fo r isotope dilution analysis. Once sample digestion and equilibration a re complete, subsequent Os separation is accomplished by conventional double distillation from sulfuric acid, and Re is separated by anion e xchange chromatography. Comparison of conventional Tenon vessel digest ions and Carius tube digestions for a diverse suite of geological samp les shows that the Carius tube digestion both liberates more Os from m ost matrices and also is a much more robust method for reproducibly me asuring the isotopic composition of a sample.