Rare earth element geochemistry and tetrad effects of the Dalir phosphatic shales, northern Iran

The Dalir phosphatic shales (Mazandaran Province, northern Iran) were widely developed within the well-known Soltanieh Formation (Late Neoproterozoic-Early Cambrian). Calcite, dolomite, and fluor-apatite are the principal constituent minerals accompanied by accessory minerals such as quartz, illite, and pyrite. The results of whole-rock chemical analyses of the phosphatic shale samples display strong negative Ce anomalies (Ce/Ce* = 0.23-0.55), and negative to slightly positive Eu anomalies (Eu/Eu* = 0.89-1.11). The behavior of the geochemical pairs Y/Ho and Zr/Hf with depth indicate non-CHARAC (non charge-radius control) behavior which can be attributed to the tetrad-effect. PAAS-normalized REE patterns exhibit contemporaneous W-and M-type tetrad effects, which are likely related to the precipitation of the phosphatic shale from upwelling of reduced deep seawaters rich in ligands such as organic materials, F, and Cl and/or their admixture with shallower oxidizing seawaters. Plots of geochemical parameters such as Y/Ho, Zr/Hf, Ce anomaly, and Nb/Ta against the T-4 tetrad effect indicate various causes for the tetrad effects active during formation of the Dalir phosphatic shale. The main likely mechanisms for the tetrad effect were fluor-apatite crystallization, REE-complexation, upwelling seawaters, mixing of seawaters with different oxidation state, and/or combinations of these factors. The data indicate that the Dalir phosphatic shales probably precipitated from low-temperature, F-rich, and high-pH seawaters.