In-situ Micro-scale Analysis of Pyrite Morphology and Sulfur Isotopes in the Cretaceous Lacustrine Shale

Pyrite exhibits diverse morphological types and sulfur isotopic compositions, each corresponding to distinct genetic pathways.Bulk rock analysis may fail to provide accurate information; therefore, it is crucial to develop new methods for insitu micro-scale analysis of pyrite.Herein, we developed an integrated approach to analyze the morphology and sulfur isotopes of pyrite using Scanning electron microscope energy-dispersive X-ray spectroscopy (SEM-EDS) and Nanoscale second ion mass spectrometry (NanoSIMS).The optimized spatial resolution of sulfur isotope analysis was 1-2 m with an analytical precision lower than 0.5 .For the analyzed sample from the Cretaceous Qingshankou Formation, Songliao Basin, most pyrite are euhedral pyrite crystals (EPCs) in both ostracods and host rocks, exhibiting cubic and octahedral forms with an average size from 4.5 1.1 m to 6.0 1.4 m.The 34 S values of pyrite in the ostracods are ranging from 7.7 to 19.1 (mean 13.52.9),which are slightly lighter than those in host rocks (ranging from 12.2 to 23.4, mean 17.73.2).The absence of framboidal pyrite and the lack of negative 34 Spy values suggest low-sulfate levels in both the water column and diagenetic water.These findings are consistent with the previously reported results from in-situ fine-scale analysis of dolomite in the Qingshankou shale.Our study not only provides an in-situ micro-scale analytical method for pyrite but also offers new evidence for understanding the sulfur cycle in the Cretaceous lacustrine environment.