A new HNO₃ digestion procedure for Re-Os isotope analysis and its implications for organic-rich sedimentary rocks and mafic rocks

The precision of available Re-Os isotope analysis for organic-rich sedimentary (ORS) rocks is influenced by the coexisting clastic and organic components and the considerable differences between Re and Os mass fractions in such rocks. Here, we present a new method to analyse Re-Os isotopes of organic-rich sedimentary (ORS) rocks by thermal ionization mass spectrometry. Nitric acid (HNO3) was used as the single digestion medium, and this method mainly relies on HNO3 at the optimum digestion condition, which can both equilibrate Os isotope between spike and sample and prevent the dissolving of clastic particles as many as possible especially the non-aqueous elements inside. After a series of condition experiments, our data demonstrate that digestion temperatures and durations play important roles not only on the equilibrium between sample and spike, but also on the dissolving-out quantity of non-aqueous Re and Os elements. The optimum condition of 24 h and 230 degrees C (the obtained Re and Os concentrations for the oil shale reference material SGR-1b, and the Os-187/Os-188 ratios are 33.44 +/- 0.09 ng/g, 0.4452 +/- 0.0107 ng/g and 1.7822 +/- 0.0439, n = 3, 1SD) can produce the Re-Os isotopic ratios of ORS at a precise level, and restrain the liberation of clastic Re and Os. This method at the digestion condition of 24 h and 230 degrees C, therefore, will increase the accuracy of Re-Os isotope analysis of ORS on the basis of low blank and high efficiency. When this method is used for olivine tholeiite reference materials (BIR-1a), the result demonstrates that the Os isotopes of mafic rocks do not produce fractionation when the temperature is higher than 200 degrees C during the HNO3 digestion.