Formation mechanisms of lanthanide tetrad effect in limestones: an example from Arbanos district, NW Iran

Abstract

The Arbanos Limestone is a part of the Ruteh Formation of Upper Permian age located in NW Iran. No detailed geochemical characterizations of these limestones have been carried out to date. In this research, formation mechanisms of tetrad effect phenomenon in the lanthanide distribution patterns in these limestones are surveyed by applying different geochemical techniques. The mineralogical and geochemical characteristics of the limestones were examined using X-ray diffractometry (XRD) and chemical (ICP–AES and –MS) methods. Mineralogical studies indicate that calcite is the principal constituent accompanied by some minor accessory minerals such as quartz, kaolinite, plagioclase hematite, and smectite in the limestones. Geochemical parameters such as Ca/Mg, Sr, V/Cr, V/(V + Ni), U/Th, Ni/Co, and Mn* reveal that the limestones of the Ruteh Formation were probably deposited in a shallow, oxic, and low salinity environment. The distribution pattern of REE normalized to PAAS (Post Archean Australian Shale) display positive Ce and Eu anomalies in the limestones. The concentration of major and trace elements such as Si, Al, V, Cr, Ni, Rb, Th, U, Y, Zr, and Hf as well as Er/Nd ratio reveal contamination of the limestones by terrigenous materials. Furthermore, strong positive correlations between REE with elements such as Si and Al suggest detrital origin for REE, which is supported by non-seawater like PAAS-normalized REE distribution curves. Some geochemical rations such as Th/Co, Cr/Th, La/Co, Th/Cr, Eu/Eu*, and (La/Lu)N displayed that the terrigenous materials in the limestones were derived from shales of the Ruteh Formation. The zigzag forms in normalized REE patterns and non-CHARAC behavior of Y/Ho and Zr/Hf ratios are another geochemical characteristic of the limestones. According to calculated values for T1, T3 and T4 tetrad effect values reveal the occurrence of tetrad effect phenomenon in REE distribution pattern. Strong positive correlations between T1 and elements such as Hf, U, and V as well as negative correlation with Ca reveal that amount of detrital input and redox potential were likely be responsible for occurrence of lanthanide tetrad effect in the limestone. Meanwhile, strong positive correlations between T3-P, T4-Al, and T4-Nb suggest the important role of diagenetic fluids and contaminations in occurrence of lanthanide tetrad effect in limestones of the Arbanos district.