A LA-ICP-MS analysis of rare earth elements on phosphatic grains of the Ediacaran Doushantuo phosphorite at Weng'an, South China: implication for depositional conditions and diagenetic processes

Abstract

The Ediacaran Doushantuo Formation at Weng'an, South China hosts well-preserved phosphatized microfossils known as the Weng'an biota. A laser ablation ICP-MS analysis of rare earth element (REE) characteristics of the fossil-bearing black phosphorite unit of the Doushantuo Formation at Weng'an was conducted, with the aim of unravelling the depositional conditions and diagenetic processes during formation of the phosphorites. Spherical phosphatized microfossils and phosphatic clasts were analysed, and the REE data display middle REE (MREE) -enriched shale-normalized REE patterns. The spherical phosphatized microfossils show an increase in total REE contents (∑REE) from core to rim. Negative correlations between ∑REE and the extent of MREE enrichment over the other REE (indicated by LaN/SmN, YbN/SmN) are observed for analysed spots within individual phosphatic grains, which may be due to complex diagenetic history of the phosphatic grains, with fluctuations in REE sources and chemical parameters in a high-energy shallow-water environment being additional factors. The LaN/YbNand LaN/SmNcharacteristics of the phosphatic grains suggest they were mostly influenced by early diagenetic alteration rather than late extensive recrystallization. The negative Ce anomalies in the samples suggest they formed under oxic-bottom-water conditions. Positive Eu anomalies are present in all samples, and are likely to reflect involvement of hydrothermal fluids rather than changes in redox conditions of porewater. Overall this study has major implications for phosphorites as important archives for the study of geochemical proxies, the Ediacaran period and also evolutionary changes.

In siturare earth element analysis of a lower Cambrian phosphate nodule by LA-ICP-MS

“…Based on the apparent black opaque appearance of our sample, the REE distributions are at least partially controlled by organic component. However, some organic-poor phosphorites from this time are also enriched in MREE (Xin et al 2015;Zhu & Jiang, 2017), suggesting that there are other REE sources in addition to the organic matter. Furthermore, the REE compositions of kerogen in the Niutitang black shales show diverse patterns instead of a single MREE bulge, which might be related to different contributions of marine biomass (Pi et al 2013).…”

“…Rare earth elements (REE) are reliable geochemical tracers and have been used successfully in oceanographic studies of redox conditions. Significantly, because of the substitution of REE for Ca, marine carbonates and phosphate particulates are often considered to be the archives of seawater or pore-fluid REE signals (Jarvis et al 1994;Shields & Stille, 2001;Jiang et al 2007;Hood & Wallace, 2015;Tostevin et al 2016b;Wallace et al 2017;Zhu & Jiang, 2017). However, because of the complexity of the controlling factors involved in this process, interpretations without careful assessment can be equivocal.…”

Evidence of Paleoproterozoic phosphogenesis in the Salvador-Curaçá Orogen (Tanque Novo-Ipirá Complex), northeastern São Francisco Craton, Brazil

“…However, REEY patterns experience limited change during high-grade metamorphism under anhydrous conditions (Cullers et al 1974, Bau 1991, 1993, Alexander et al 2008, thus providing information about protoliths under these conditions. Nevertheless, diagenetic and weathering processes can modify REEY patterns, and mixing with detrital sediments in the basin during deposition can also change the primary REEY signature in carbonate rocks (Shields and Stille 2001, Northdurft et al 2004, Ling et al 2013, Piper and Bau 2013, Zhu and Jiang 2017.…”

“…The mechanisms that may alter the REEY signature in marine chemical sediments, such as carbonates, are diagenesis, weathering, hydrothermalism, and detrital contamination. Terrigenous inputs contribute to the mixture of silicates and oxides within chemical sedimentary rocks (Northdurft et al 2004, Ling et al 2013, Piper and Bau 2013, Zhu and Jiang 2017. Terrigenous sediments typically have a higher content of REE and trace elements, such as Zr, Th, and Hf, than carbonates (Taylor and McLennan 1985).…”

Introduction: from snowball Earth to the Cambrian explosion–evidence from China

“…The data also indicate that the Fulu IF precipitated across an iron chemocline that separated a weakly oxic surface ocean from an oxygen-depleted deep ocean during the Neoproterozoic glaciation. Furthermore, the paper by Zhu & Jiang (2017: this issue) reports rare earth element (REE) data for the Ediacaran Doushantuo phosphorite, which contains the well-preserved phosphatized microfossils known as the Weng'an biota at Weng'an, central Guizhou. The results indicate a complex diagenetic history of the phosphatic grains, and suggest that the phosphorite formed under oxic bottom-water conditions with the involvement of hydrothermal fluids, providing new insight into the Ediacaran phosphorites and phosphatic fossils.…”