Use of rare earth element geochemistry to constrain the source of dolomitizing fluid for dolomitization of the Lower Triassic Feixianguan Formation, Jiannan area, China

Abstract

Abstract Rare earth element (REE) concentration and pattern may provide useful information to constrain the source of dolomitizing fluid. However, REE data measured from carbonates might be complicated by diagenetic alterations or clastic components. It is necessary to identify and evaluate such influences before interpreting REE data properly. REE determined in two types of dolostone, including the fine crystalline (D1) and the medium crystalline dolostone (D2), from the Lower Triassic Feixianguan Formation in the Jiannan area display different concentrations and PAAS-normalized patterns. The D1 dolostone that formed by an early near-surface dolomitization has the relatively higher total REE concentrations (ΣREE) and displays near-flat REE patterns similar to that of the PAAS-normalized North American Shale, implying some influences of the clastic components. This is supported by the fact that the D1 has extremely high Fe and Mn contents but dull to no cathodoluminescence. Contrarily, D2 dolostone has lower ΣREE values. Their REE patterns are characteristics of LREE depletion, negative Ce anomalies, and positive La anomalies consistent with the features of the host limestone that yields the seawater-like features. The lack of correlation between dolomite content and their ΣREE, Ce/Ce*, or (Nd/Yb) SN indicates that clastic components are minor and that dolomitization has no or only a minor impact on REE signatures of the precursor limestone. All those suggest that dolomitization associated with D2 was likely associated with seawater-derived fluids. The distinct REE features of the D1 dolostone are mainly attributed to the contamination of the clastic components rather than the indicators of the different dolomitizing fluid, and thereby provided no information on the sources of dolomitizing fluid. Therefore, the degree of contamination should be evaluated before interpreting the REE data properly, especially for dolostones that formed at or near-surface environments.