Diagenesis of Ediacaran − early Cambrian phosphorite: Comparisons with recent phosphate sediments based on LA-ICP-MS and EMPA

Abstract

“Old phosphorite” deposits (Ediacaran to early Cambrian) located in Central Guizhou, South China, are hosted in carbonate and represent the earliest phosphogenesis. These old phosphorites, as well as Pliocene to Pleistocene and modern phosphate sediments (recent phosphorite) have been extensively studied. However, the possibility of recent phosphorite being the incipient stage of long-time phosphorite remains uncertain. Taking Weng’an (Ediacaran) and Zhijin (early Cambrian) phosphorites as examples, this study conducted in situ geochemical and mineralogical analyses of phosphate minerals (old apatites). Analogies between recent and old apatites were determined by comparing geochemistry based on rare earth elements (REE) and yttrium (REE + Y). Old phosphorite could be divided into authigenic grains and biodetritus, both comprising closely accumulated apatite nanocrystals. REE + Y concentrations (∑REE + Y) were much higher in the outer rim than in the inner core of biodetritus, whereas the authigenic grains showed relatively homogeneous REE + Y distributions. Three REE + Y patterns were observed: (1) “left-inclined” type, with no Ce anomalies; (2) shale-like pattern, with weak MREE enrichment and slightly negative Ce anomalies; (3) “hat-shaped” pattern, with notable MREE enrichment, evident HREE depletion, and remarkable negative Ce anomalies. Positive La, Gd, and Y anomalies, as well as MREE enrichment increased from type (1) to type (3), which can be explained by increasing degrees of oxidation. The REE + Y distributions of old apatites were similar to those of recent apatites, but exhibited MREE enrichment and HREE depletion and old authigenic apatites had no Ce anomalies. Furthermore, old apatite compositions lie in the diagenetic area on Y/Ho vs SmN/YdN plots. These characteristics indicate that old phosphorite could have been subjected to long-term diagenetic modification after burial, during which apatites recrystallized and absorbed REE + Y. Hence, it can be concluded that recent apatites could serve as predecessors of major ancient phosphorite, especially biogenic phosphorites. Globally, typical old phosphorites have quite variable REE + Y compositions, suggesting that the phosphorites experienced different depositional conditions and diagenetic alteration despite their contemporaneous formation. This study provides new data and insight on the diagenesis of old phosphorite and improves our deeper understanding of phosphogenesis in paleo- and modern environments.