Elemental geochemistry of lower Cambrian phosphate nodules in Guizhou Province, South China: An integrated study by LA-ICP-MS mapping and solution ICP-MS

摘要

The late Ediacaran to early Cambrian transition was a critical interval in Earth history, during which both the environment and life experienced dramatic change. Relationships between these events, however, are widely debated. This time interval also witnessed the occurrence of massive phosphate deposits. Studies on the characteristics, origins, and depositional context of this geochemical archive may provide important insights into not only the ore-forming process but also the coevolution of the Earth surface system. Here, phosphate nodules from two sections (Jinsha and Songtao) are investigated using in-situ LA-ICP-MS mapping and solution ICP-MS methods. Elemental images and factor analyses of the LA-ICP-MS dataset show that Ca, P, and Sr are evenly distributed and co-enriched within the nodule, whereas iron-related elements (Fe, As, and Mo) are roughly present as concentric layers. The PAAS-normalized REE patterns of the Jinsha samples exhibit moderately negative Ce anomalies and pronounced MREE enrichments, possibly related to uptake of REE released by iron oxyhydroxides during early diagenesis. In contrast, the Songtao samples are characterized by more seawater-like signals with significantly lower Ce/Ce* and higher Eu/Eu* values. These positive Eu anomalies are ascribed to active submarine hydrothermal vents along the Yangtze Platform margin. Based on a compilation of published Ce/Ce* data, we suggest that the prominent negative Ce anomalies during the Ediacaran Cambrian phosphogenesis give evidence of possible link between P dynamics and ocean oxygenation.