Elemental mobility in phosphatic shales during concretion growth and implications for provenance analysis

Abstract

Shale-normalized rare-earth element (REE) patterns record variable redox conditions in coprolitic phosphorite concretions and their host shales from the Pennsylvanian-age cyclothems in the U.S. midwestern states of Oklahoma, Kansas, Nebraska, Iowa, and Ohio as well as the Mississippian-age Fayetteville Shale of Arkansas. Shale-normalized enrichments in middle REE (MREE) developed as the phosphatic concretions lithified under reducing conditions in pore waters of organic-rich muds. Depletions in MREE (Sm–Ho) in host shales correspond to the MREE enrichments in some nearby concretions. Rare and weak negative cerium depletions in both reworked and unreworked phosphorite concretions suggest exposure to oxygenated waters. Positive europium anomalies in thick phosphorite coatings on coprolitic phosphorite concretion cores suggest extreme reduction during lithification of the phosphorite coating. Flat REE patterns in both host shales and phosphorite concretions probably reflect detrital influence. Phosphate diagenesis can affect the trace element chemistry of black shale enough to alter provenance signals. Overall enrichment in REE in phosphorite relative to host shale suggests movement of REE as does the MREE-enriched phosphorite near the MREE-depleted host shale. Diagenetically mobile REE can affect La–Th–Sc relationships used in provenance analysis. The extent of the diagenetic modification of La–Th–Sc distribution is related to the amount of P 2O 5 present. Typical La–Th–Sc distribution in host shale results when P 2O 5 values are below 0.5%. High La abundance substantially skews La–Th–Sc distribution when P 2O 5 is above 5.0%, but when P 2O 5 is between 0.5% and 5.0% the extent of the effect is unclear. Oxides of aluminum and other major elements may be useful in recording detrital influence and proximity to the shoreline in phosphorite concretions. Most of the concretions nearest to shoreline sources of detrital input have both flat REE patterns and elevated values of Al 2O 3/(Al 2O 3+Fe 2O 3).