Precise and accurate Re–Os isotope dating of organic-rich sedimentary rocks by thermal ionization mass spectrometry with an improved H2O2-HNO3 digestion procedure

Abstract

This contribution presents a new method for Re–Os isotope dating organic-rich sedimentary (ORS) rocks by thermal ionization mass spectrometry using an H2O2–HNO3 solution as the digestion medium, rather than CrO3–H2SO4 or inverse aqua regia. The main underlying principle of this method is that H2O2–HNO3 digestion would preferentially liberate hydrogenous Re and Os, and minimize the dissolution of non-hydrogenous detrital Re and Os, thereby providing more accurate and precise ages. A series of tests were performed, and the experimental data demonstrate the fundamental controls on spike–sample equilibrium and that the amount of detrital Re and Os incorporated into the system are subjected to the volumetric ratio of H2O2 to HNO3. The optimum method is a H2O2:HNO3 ratio of 5 to complete spike–sample equilibration, and to minimize the amount of detrital Re and Os in the system. A comparison of our new method with inverse aqua regia and CrO3–H2SO4 showed that the three techniques yield indistinguishable Re–Os results, suggesting complete spike–sample equilibrium was achieved by all of the digestion techniques. Moreover, the data show that our new technique leaches out the least amount of detrital Re and Os isotopes relative to conventional methods Thus, we propose the H2O2–HNO3 method may increase the precision and accuracy of Re–Os depositional ages of organic-rich sedimentary systems.