VARIABLE REDOX SIGNALS RECORDED BY RARE-EARTH ELEMENTS IN CARBONIFEROUS PHOSPHATIC BLACK SHALE FROM KANSAS, OKLAHOMA, AND ARKANSAS AND THE INFLUENCE OF AUTHIGENIC PHOSPHATE ON PROVENANCE ANALYSIS

Shale-normalized rare-earth element (REE) patterns record variable redox conditions in coprolitic phosphorite concretions and their host shales. Patterns enriched in middle REE (MREE) developed under reducing conditions in pore waters of organic-rich marine muds. Rare and weak negative cerium depletions in both reworked and unreworked phosphorite concretions suggests exposure to oxygenated waters. Positive europium anomalies in thick phosphorite coatings on coprolitic phosphorite concretion cores suggests extreme reduction during lithification of the phosphorite coating.

Phosphate diagenesis can affect the trace element chemistry of black shales enough to alter provenance signals. Flat REE patterns in black and gray shales that host the phosphorite concretions as well as some phosphorite concretions probably reflect detrital influence. Host shales depleted in MREE suggest mobility of REE from early diagenetic pore waters to REE- and MREE-enriched phosphorite concretions. The effect of REE mobility on geochemical provenance information is illustrated in La-Th-Sc relationships. Typical La-Th-Sc distribution in host shale results when P2O5 values are below 0.5%. High La abundance substantially skews the La-Th-Sc distribution when P2O5 is above 5.0%, but when P2O5 is between 0.5% and 5.0%, the extent of the effect is unclear. Detrital influence and relative proximity to the shoreline is suggested by phosphorite concretions that have both flat REE patterns and elevated values of Al2O3/(Al2O3 + Fe2O3).