The K isotopic signatures of coals: A reconnaissance study

Potassium (K) isotopes are significantly fractionated in sediments, with biological samples exhibiting even greater K isotopic variability than geological samples. Coal is a unique type of sedimentary rock composed of organic matter and minerals, both of which are K-rich. Therefore, K isotopes hold the potential for tracing the origin of coal and understanding coal-forming processes. However, our knowledge of K isotopes in coals is limited due to the scarcity of K isotopic studies on coals. In this study, we conducted the first detailed investigation of K isotopes in 12 coal samples of varying ranks by analyzing the abundance and isotopic compositions of K in different phases. We ashed the coals and sequentially leached K from the ashes with deionized water (DW), ammonium acetate (AA), HCl, and HF, followed by elemental and isotopic analyses. The majority of K occurs in DW-leached and HF-leached fractions of these coal samples, representing the K associated with organic matter and the K that exists in the silicate phase, respectively. The bulk K content of the twelve studied coals ranges from 0.004 to 1.308 wt%, with the HF-leached K holding the dominant K budget. The δ⁴¹K values vary from −1.19 ‰ to 0.52 ‰ in the DW-leached phase, and from −1.27 ‰ to −0.22 ‰ in the HF-leached phase of the low temperature coal ashes. The large variations in δ⁴¹K values of DW leachate are attributed to the plants decay and diffusion in pore fluids in the early stage of diagenesis. The K isotope compositions of the HF-leached K in coals are variable and lighter than average igneous rocks (δ⁴¹K_BSE = −0.43 ‰). Pore fluid exchanges likely caused the preference for ³⁹K in the silicate phase of coal. The K isotope compositions of coals overall reflect ³⁹K enrichment during diagenesis or organic matter inheritance.