REY enrichment mechanisms in the early Cambrian phosphorite from South China

Abstract

Worldwide phosphogenesis during the early Cambrian occurred coevally with localized Rare Earth Element plus Y (REY)-rich phosphorite in Zhijin, South China. Although characteristics of the Zhijin phosphorite have been well constrained, the major factors controlling REY enrichment remain unknown. To address this, the research studied the REY occurrence forms and enrichment mechanisms. Accordingly, carbonate C and O isotopes, micropetrographic, and in situ geochemical analyses of Zhijin phosphorites were conducted. Results show that the phosphates are composed of closely-packed nano to μm-scale apatites, which were spherulitic and short cylindrical, comprising mainly fluorapatites and minor francolites. Element mapping verifies that REYs were hosted in apatites by isomorphous substitution, with minor REY being adsorbed by clay minerals. Further, REY was incorporated into apatite crystals during the early diagenesis according to LaN/SmN vs. LaN/YbN plots. Apatites show Medium REE (MREE)-rich post-Archean Australian shale-normalized distribution patterns, attributed to selective adsorption of MREEs by Fe-/Mn oxides/hydroxides or organic matters. Plots of Nd and Ceanom manifest lower sedimentation rates of REY, providing adequate time for REY absorption in the seawater. Excluding secular variations of REEs in the ocean chemistry, we suggest that constrained paleogeography provided a suitable space for REEs entering apatite crystals during early diagenesis. This is possibly the primary cause for the vast REE reserves of the Zhijin phosphorite deposit. In addition, redox conditions, temperature, biological activities, and weathering played non-critical roles in REY enrichment in apatites. Our findings can inform on multi-factor constraints of REY-rich marine phosphorites in the early Cambrian and provide a further understanding of REY enrichment in worldwide sedimentary marine phosphorites.