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Fig. 7 | Journal of Palaeogeography

Fig. 7

From: Growth mechanisms and environmental implications of carbonate concretions from the ~ 1.4 Ga Xiamaling Formation, North China

Fig. 7

BSE images and chemistry of calcites in the carbonate concretions of Xiamaling Formation. a and b BSE images, showing a calcite crystal with dark nucleus (low Mn–Fe contents) and light rim (high Mn–Fe contents) (Zhaojiashan section); c and d EDS spectrums showing that the Mn–Fe contents of the nucleus are lower than the detection limits of EDS, while the rim has higher Mn–Fe contents; e A cross plot showing that the nuclei of calcite crystals have low Mn and Fe concentrations, while the rims have high Mn and Fe concentrations; f Average Mn/Fe ratios under different redox conditions. Mn/Fe ratios are enriched in our samples compared with those of average global carbonate (0.29; Turekian and Wedepohl 1961) and of manganous samples (Tostevin et al. 2016). Bars represent median values; arrows indicate samples with exceptionally high Mn/Fe ratios that lie above the limit of the Y axis; g A cross plot of δ13C vs. δ18O; h REE patterns of the carbonate concretions, showing prominent positive Ce anomalies and slight enrichment of MREE

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