The Application of Scanning Electron Microscopy-Energy Dispersive Spectrometer in Paleoenvironmental Studies: Methods, theories and corrections to geochemical parameters

Key words: 
• SEM-EDS /
• Genesis identification of clay minerals /
• Chemical weathering /
• Paleoclimate /
• Meso-Neoproterozoic

Abstract: Geochemical proxies such as the Chemical Index of Alteration (CIA) are essential for reconstructing paleoweathering intensity and climate regimes. However, their reliability is fundamentally constrained by conventional analytical methods due to their inability to distinguish mineral phases with identical chemical compositions yet possessing distinct genetic origins—such as detrital mica and diagenetic illite—often leading to significant misinterpretations of continental weathering and paleoclimatic conditions. To address this persistent challenge, this study applied the advanced SEM-EDS-Nanomin system, which integrates high-resolution field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and an innovative mixed pixel deconvolution (Mixel) algorithm. This cutting-edge technology achieves nanoscale (<0.5 μm) mineral mapping and automatically correlates resolved compositional data with critical morphological characteristics (e.g., particle orientation, boundary sharpness, spatial relationships) to objectively classify clay minerals into detrital, authigenic, or diagenetic categories with high accuracy. The elevated bulk CIA values of the organic-rich shales from the Late Neoproterozoic intervals in South China (Cryogenian interglacial Datangpo Formation and Ediacaran Doushantuo Formation) and the Mesoproterozoic Velkerri Formation in northern Australia reflected intense chemical weathering under warm and humid climatic condition. However, the new generation of the SEM-EDS coupled with the Nanomin system in contrast revealed that these seemingly apparent chemical weathering signatures were substantially overprinted by the dominant contribution of diagenetic clay minerals to key element budgets. Specifically, aluminum-rich phases previously assumed to reflect pedogenic processes were identified as diagenetic products, especially illite and kaolinite formed via pseudomorphic replacement of precursor feldspar or mica grains during burial and diagenesis. After systematically excluding these secondary phases identified through Nanomin genetic discrimination and recalculating CIA based on primary mineral assemblages, the data in fact indicated physically dominated weathering operating under colder, more arid climatic conditions. This critical revision resolved long-standing paradoxes where high bulk CIA values conflicted with independent sedimentological, mineralogical, and isotopic evidence. Furthermore, beyond CIA correction, the technique demonstrated exceptional capability in identifying authigenic minerals such as saponite formed in restricted marine evaporitic settings during the Ediacaran biotic emergence. By providing essential mineralogical context, the SEM-EDS-Nanomin methodology fundamentally enhances the reliability of diverse geochemical proxies and analytical techniques including accurate neodymium isotope(εNd) analysis and high-precision Rb-Sr geochronology based on unaltered minerals (e.g., glauconite). By establishing the unambiguous determination of clay mineral origin as an indispensable prerequisite for robust paleoenvironmental interpretation, this SEM-EDS-Nanomin approach—through its unique synergy of high-resolution imaging, spectral deconvolution, and automated morphological correlation—provides a revolutionary framework for deciphering Earth's environmental dynamics during critical transitional periods marked by extreme climate shifts, tectonic reorganizations, and biological innovations. Its capacity to bridge nano-scale mineralogical observations with macro-scale geochemical signals represents a paradigm shift in reconstructing deep-time Earth system processes.