Purified Na2CrO4–H2SO4: A New Low‐Blank Medium for Re‐Os Dating of Black Shale

Abstract

The H2SO4–CrO3 method has been widely used in Re‐Os isotope dating of organic‐rich sedimentary rocks. However, the high Re blank of this method inhibits the collection of high‐precision Re‐Os data, especially for organic‐rich sedimentary rocks with low Re and Os mass fractions. Here, we report a new H2SO4–Na2CrO4 method in which the Re blank of the Na2CrO4 reagent can be greatly reduced after efficient purification by acetone. A proportion of 0.6 g Na2CrO4/8 ml H2SO4 was selected as the best digestion combination based on tests of black shales with different organic carbon contents in three digestion media (inverse aqua regia, H2SO4–CrO3, and H2SO4–Na2CrO4). The H2SO4–Na2CrO4 method has an ultra‐low total procedural blank of 1–2 pg for Re and 0.6–1 pg for Os, respectively, which is one order of magnitude lower than the H2SO4–CrO3 method and is comparable with to inverse aqua regia method. The new method was applied to reference material SBC‐1 and two series of black shale samples, and the results were consistent with those obtained via the inverse aqua regia method. These findings confirm the accuracy and feasibility of the H2SO4–Na2CrO4 method, and demonstrate its ability to accurately determine the depositional age of organic‐rich sedimentary rocks with low Re and Os mass fractions.