Properties of dolomitizing fluids indicated by rare earth elements: A case study of the Upper Miocene to Pliocene dolostone on the Xisha Islands, South China Sea

Abstract

Rare earth elements and yttrium (REE + Y), with stable and predictable chemical characteristics, serve as important proxies for diagenetic fluids, but the effect of dolomitization on REE + Y signatures is still unclear. To determine the factors that control their REE + Y signatures, this study presents a detailed investigation of the dolostone on the Xisha Islands, South China Sea. Samples from the Huangliu Formation (Upper Miocene) and the Yinggehai Formation (Pliocene), 339 m thick in well CK-2, are characterized by low REE + Y concentrations (typically <10 ppm), with LREE depletion, HREE enrichment, negative cerium (Ce) anomalies, and high Y/Ho values that are similar to modern seawater. There is no correlation between sedimentary facies and the REE + Y profile of the dolostone, which indicates that the facies did not impact the fractionation and incorporation of REE + Y during dolomitization. The deviation from the Ce and praseodymium (Pr) anomalies of seawater in the Huangliu Formation suggests that the dolomitizing fluid was modified by evaporation or fluid–rock interaction. Positive europium (Eu) anomalies in the Huangliu Formation may reflect the higher temperature of dolomitization or the preferential dissolution and input of Eu-rich plagioclase into the South China Sea. Sequentially digested samples of dolomitic limestone from the Yinggehai Formation, where both the bulk rock and the pure dolomite were analyzed, indicate that dolomitization has a minor but discernible effect on REE + Y concentrations, Y/Ho, and Ce anomalies. The similar REE + Y signatures of the dolostone on islands in the South China Sea, the Pacific Ocean, and the Caribbean Sea suggest that the dolomitizing fluid was sourced from seawater and that there may have been global paleoceanographic controls on dolomitization during the Cenozoic. This study also provides an invaluable reference to determine how the REE + Y signatures of Cenozoic “island dolostone” can be used to interpret the paleoenvironmental significance of ancient dolostone in the geological record.