Rare earth element and SrNd isotope geochemistry of phosphate nodules from the lower Cambrian Niutitang Formation, NW Hunan Province, South China

• REE and Sr-Nd isotopes are studied in phosphate nodules from Niutitang Formation.
• The seawater-like REE patterns record a seawater signature.
• The hat-shaped REE patterns reflect diagenetic fluid influence.
• The decay of organic matter is an important supplier for REE in the nodules.

Phosphate nodules are widely distributed in the early Cambrian black shale sequence in Yangtze platform, South China. In this study, we carried out X-ray diffraction, major element, rare earth element, and SrNd isotopic studies on phosphate nodules from two PC-C sections in Hunan Province, South China. Two different types of phosphate nodules can be recognized based on REE and Nd isotopic characteristics: type 1 with seawater-like REE patterns, higher εNd(t) values; and type 2 with hat-shaped REE patterns and more negative εNd(t) values. We suggest that the difference in REE patterns and Nd isotopic compositions between the two types reflects changes in pore water geochemistry from seawater-dominated towards diagenetic fluid-dominated conditions during early diagenesis. Positive Eu anomalies are displayed in most samples, and are likely to record not only the anoxic environments but also the involvement of submarine exhalative hydrothermal fluids which prevailed in South China during early Cambrian.