Murthy and three co-authors reported in 2004 that phosphate nodules from marine black shales and nonmarine gray shales from the Mississippian Fayetteville Shale of north-central Arkansas and northeastern Oklahoma are strongly enriched in middle rare-earth elements (MREEs) as is common in Paleozoic phosphate deposits. The nodules commonly preserve conspicuous europium anomalies that are not typical of phosphate deposits elsewhere in the geologic record, suggesting extremely reducing diagenetic conditions.
Contrastingly, minor cerium anomalies imply oxidizing conditions that are not unexpected in the light to medium gray nonmarine host shales. The most likely explanation for the Ce anomalies in the marine phosphate nodules is that bottom waters in the marine environment reverted from the anoxic waters that probably characterized the basal Fayetteville Formation to weakly oxidizing waters that permitted survival of the fauna that occurs in a well-defined zone. All of the Ce anomalies probably formed during an earlier diagenetic stage than the Eu anomalies that developed later, as decaying organic matter consumed what little oxygen was available.
There are three types of REE patterns in Mississippian organic-rich phosphate nodules. Middle rare-earth element-enriched patterns are common in both marine as well as nonmarine nodules. Negative cerium anomalies and positive europium anomalies were also observed. Negative cerium anomalies are probably caused by inherited seawater signatures and positive europium anomalies inherited from the source material. An interesting question is that if the negative Ce anomaly of the nodules is a direct record of the negative Ce anomaly common to seawater, why do most of these nodules have MREE-enriched patterns and not the HREE-enriched patterns observed in modern seawater?
Further investigation of the REE composition of these nodules may reflect what is suggested by Tang and Johannesson (2003) and others that organic ligands play an important role in the complexation of the REE, and that because of the relative abundance of organic matter in the surrounding shales and in the nodules themselves, the REE composition of the nodules may reflect reductive dissolution and complexation of the REE by organic ligands during deposition and diagenesis.
There is no relationship between phosphate-ion concentrations and MREE abundance in most samples. Although the nodules are primarily phosphatic, differences in phosphate abundance do not cause differential uptake of MREEs. Middle rare-earth element-phosphate relationships are not as straightforward as initially hypothesized. Middle rare-earth element enrichments are present in phosphatic material, but not all P2O5 is in the form PO4-3, and therefore, the chemistry of PO4 may not be as important as the relative abundance of P2O5. If P2O5 is accompanied by hydroxides, oxyhydroxides, and oxides, MREE enrichments could be observed. Diagenesis of nonmarine nodules may either have homogenized the chemistry over time, or the diagenetic history of nonmarine nodules and marine nodules may be quite different.
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