Determination of K Isotope Compositions in Sedimentary Rocks and Their Implications for Discriminating Sediment Origin

Abstract

The advent of a new generation of collision-cell multicollector inductively-coupled-plasma mass-spectrometers (CC–MC–ICP–MS), the Nu sapphire, has provided a new venue in achieving higher precision K isotopic compositions compared to the older generation of instruments. Here, we take advantage of this new technology to report the K isotopic compositions of eight sediment reference materials. The K isotopic compositions (reported as the δ⁴¹K representing the ⁴¹K/³⁹K ratios) for these sediment reference materials span a range of 0.2‰: −0.42 ± 0.04‰ (BCSS-1), −0.51 ± 0.07‰ (MESS-1), and −0.43 ± 0.01‰ (MESS-4) for three marine sediments, −0.35 ± 0.07‰ (NIST-SRM 1646a) for estuarine sediment, −0.40 ± 0.06‰ (NIST-SRM 2704) and −0.40 ± 0.05‰ (SWR-3) for two river sediments, −0.55 ± 0.04‰ (NIST-SRM 1d) for limestone, and −0.46 ± 0.00‰ (SBC-1) for marine shale. The high-precision K isotopic data presented herein provide a valuable reference for future quality control and interlaboratory comparisons. Data compilation reveals that biogenic sediments show extremely low K concentrations (K₂O = 0.001–0.048 wt.%) and large K isotopic variations (δ⁴¹K = −1.88–0.94‰) with an average value of −0.001‰, whereas the abiogenic sediments are featured with light K isotopes (average δ⁴¹K value of −0.47‰) and high K concentrations (K₂O = 0.52 ~ 4.29 wt.%). This finding suggests that the variation of K isotopes may serve as a useful tool for discriminating the various geneses of sediments.