A relativelyhigh-temperature oxidizing digestion using aqua regia has been developed for< 0.1-5gsize 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 conven-tional double distillation from sulfuric acid, and Re is separated by anion exchange chromatography. Compari-son of conventional Teflon vessel digestions and Carius tube digestions for a diverse suite of geological samples shows that the Carius tube digestionboth liberates more Os from most matrices and also is a much more robust method for reproducibly measuring the isotopic composi-tion of a sample.

The Re-Os isotopic system (187Re decays to 1870s via-/3 decay with a~ 42 Ma half-life) has seen increasing use in solid Earth geochemistry as a tracer with chalcophilic and siderophilic properties and thus forms a unique complement to the other longlived, lithophilic radioisotope systems. The system has proven useful for a variety of applications including constraining the ages of subcontinental lithospheric mantle12 and magmatic iron mete-orite groups, 3 examining the recycling of oceanic crust into the mantle, 1 234 determining the origin of platinum-group elements in certain ores, 5 and constraining the influx of extraterrestrial and continental components into the oceans. 6 The increased use of the Re-Os system has been spurred by advances in measurement sensitivity and analytical accuracy. The