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
2017
The Ediacaran Doushantuo Formation at Weng9an, South China hosts well-preserved
phosphatized microfossils known as the Weng9an 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 Weng9an 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 La N /Sm N , Yb N /Sm N ) 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
La N /Yb N and La N /Sm N characteristics 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.
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