Potassium (K) is one of the major elements in both the upper continental crust and seawater. The marine K cycle is closely coupled with the surface carbon cycle and involves significant K isotope fractionation. Therefore, reconstructing seawater K isotope compositions provides a new approach for quantitatively constraining the evolution of marine K cycling and its links to climate system.  Over the past five years, marine authigenic clay minerals have attracted increasing attention as potential archives of seawater K isotope compositions. Glauconite, a typical marine authigenic clay mineral, is characterized by high concentrations of Mg and K. Its distinctive features, including green to emerald-green coloration, ferromagnetic properties, and relatively large grain size (up to ~0.5–1 mm), allow it to be distinguished from terrigenous clay minerals.
      In this study, we investigate Mg and K isotope systematics in glauconite-bearing limestones and dolostones to evaluate the potential of glauconite as a recorder of seawater geochemical signals. Petrographic and mineralogical observations indicate that glauconitization in carbonate rocks mainly occurs through the replacement of calcite by glauconite, whereas dolomite appears to be relatively resistant to glauconitization. During this process, Al, Fe, and K contents increase, accompanied by decreasing Ca concentrations. The molar ratio of (Al+Fe+K)/Ca is therefore used as an indicator of glauconitization intensity. The δ²⁶Mg values increase from −3.5‰ to −0.3‰ as the (Al+Fe+K)/Ca ratio increases from 0.08 to ~0.2, and then remain relatively constant despite further increases in the (Al+Fe+K)/Ca ratio (0.2–0.7). These observations suggest that the Mg isotope compositions of glauconitized carbonates are controlled by mixing between calcite and glauconite end-members. Once glauconite becomes the dominant Mg-bearing phase, δ²⁶Mg values become relatively homogeneous at approximately −0.3‰. This value is remarkably similar to the reconstructed Mg isotope composition of early Cambrian seawater (−0.25‰ to −0.35‰), suggesting that glauconite may preserve primary seawater Mg isotope signals.
        Because calcite contains negligible potassium, K in marine glauconite is generally assumed to be derived from seawater. However, measured δ⁴¹K values show substantial variability, ranging from −0.82‰ to −0.39‰, indicating that additional processes may influence K isotope incorporation during glauconitization. Further investigation of the controls on K isotope fractionation during glauconite formation is therefore essential for evaluating the potential of authigenic clay minerals as archives of past seawater K isotope evolution.
 

暂无