Phosphorite generative processes around the Precambrian-Cambrian boundary in South China: An integrated study of Mo and phosphate O isotopic compositions

摘要

Large phosphorite deposits in Central Guizhou, China, were formed around the Precambrian/Cambrian boundary (PC/C), including the Ediacaran (Doushantuo stage) and early Cambrian (Gezhongwu stage). Among them, Gezhongwu phosphorite from Zhijin are enriched in rare earth elements (REE) plus yttrium (REY), reaching 3.503 million tons. Although phosphorites have attracted great attention, the specific sources P and REY remained unclear. To determine the P and REY sources and establish a phosphogenic model of PC/C phosphorite, we present an integrated dataset of Mo and phosphate O isotopes for the first time, along with carbonate C and O isotopes, geology, petrology, and geochemistry. In all samples,δ18Op, Y/Ho, and Zr/Hf decreased from the Ediacaran to the early Cambrian, indicating increased terrigenous weathering fluxes while decreased upwelling water input. Furthermore, terrigenous weathering delivery significantly elevated marine REY concentrations in the Cambrian in Zhijin. The Ceanomandδ98/95Mo suggest that seawater was oxidized in the later Ediacaran and became entirely oxic in the early Cambrian. The positive feedback between oxygen levels in atmosphere and primary productivity caused progressive oxygenation in ocean–atmosphere system and enable phosphorites to be formed by different mechanisms. Results show that the Lower Doushantuo consist of abiotic intraclasts and exhibited “seawater-like” REY types, indicating abiological and mechanical reworking phosphogenesis. The Upper Doushantuo and Gezhongwu Formation contained mainly microbial debris and abiogenic intraclasts, and exhibit “hat-shaped” REY plots, suggesting microbially mediated phosphogenesis. Based on this data set, we developed a phosphogenic model illustrating formation of these two phosphorite deposits, wherein the Lower Doushantuo phosphorite formed through the reworking of pre-existing phosphatic sediments in anoxic and abiotic ocean, whereas the Upper Doushantuo and Gezhongwu phosphorite formed via microbial metabolisms in oxic and biotic conditions. Our study has implications on the PC/C phosphorite generative processes, as well as paleoenvironmental conditions.