Petrographic and geochemical characteristics of nodular carbonate-bearing fluorapatite in the lacustrine shale of the Shahejie Formation, Dongying Depression, Bohai Bay Basin

Abstract

A varying abundance of carbonate-bearing fluorapatite (CFA) nodules in Eocene lacustrine shale in the Bohai Bay Basin was observed; however, mineralogical and geochemical awareness is lacking, which limits the specific understanding of CFA formation mechanisms and paleoenvironmental proxies. We specified the petrology, trace elements (TE), rare earth elements and yttrium (REY) of the CFA nodules by using an integrated approach of cathodoluminescence, micro-Fourier transform infrared spectroscopy, electron microprobe analysis and laser ablation–inductively coupled plasma–mass spectrometry. The results suggest that CaO (25.85 to 39.95 wt%), P2O5 (17.19 to 29.35 wt%) and F (0.71 to 2.99 wt%) are the dominant components of the CFA nodules, with low Mn/Fe ratios and few incorporations of CO32−. They show selective concentrations of Sr (avg. 14,963.3 ppm), Ba (avg. 1831.6 ppm), U (avg. 314.2 ppm), Zr (avg. 263.5 ppm), and Th (avg. 185.4 ppm) and apparent negative Y anomalies with low Y/Ho ratios (18.40 to 28.90). These CFA nodules were classified into three types according to REY patterns normalized to the Post-Archean Australian Shale (PAAS). Type A nodules enclosed by micritic calcite laminae are middle rare earth elements (MREE) enriched which yield a typical “bell-shaped” REY pattern, nonanomalies of Ce and a good correlation of ΣREY with Ce/Ce* (R2 = 0.90) and Ba (R2 = 0.95). Type B nodules in the clay and organic-rich laminae were better phosphatized and display more heavy rare earth elements (HREE) depletion, minimal Ce anomalies and a good correlation between CaO and P2O5 (R2 = 0.84) than type A nodules. Type C nodules are rare but show light rare earth elements (LREE) enrichment relative to type A and B nodules. The adsorption of P, TE and REY by Fe-(oxyhydr)oxide and organic matter is significant for CFA formation through the “dissolution-recapture-reprecipitation” process in the ambient pore water of the Fe reduction zone during early diagenesis. The retention of HREE as HREE(CO3)2− in more reducing bottom water is critical to the HREE depletion for type B and C nodules, while the release of LREE from bacterial-mediated degradation of organic matter into ambient pore water enhances the concentration of LREE on type C nodules. Deviation of the REY patterns is probably related to the differential suboxic redoxclines for the deposition of calcite-rich and clay- and organic-rich laminae of shale, which are considered the depositional environment proxies of lacustrine shale.