Distribution and significance of rare earth elements in cenomanian–turonian phosphate components and mudstones from the bohemian cretaceous Basin, Czech Republic

Abstract

Samples from the upper part of Pecínov Member of the Peruc-Korycany Formation (upper Cenomanian), lower part of the Bílá Hora Formation (upper Cenomanian–lower Turonian) and lower part of the Teplice Formation (upper Turonian) were analyzed for contents of rare earth elements (REE), P2O5, Y and U. The samples include glauconitic mudstones and sandstones, phosphate nodules, phosphate coprolites, sponges and tube-fills. The results indicate that carbonate-fluorapatite is the major host of REE in the Cenomanian–Turonian sequence, with REE most probably substituting for calcium in the apatite structure. The substantial proportion of REE was supplied from terrigenous material derived from highly weathered crystalline rocks of the Bohemian Massif and transported by rivers to the basin. The shale-normalized REE distribution shows a “bell-shaped” pattern, particularly developed in the phosphate-bearing samples. Such a pattern could have been produced by local environmental redox conditions, preferential substitutions in the apatite phase, and/or later diagenetic processes, in addition to a possible signature of source rocks. The results suggest that all investigated units were deposited under various degrees of oxygen availability. Redox conditions indicators, such as Ce-anomaly and Ce/La ratio, suggest decreased oxygenation of the depositional environment from bottom to top in the upper parts of the Pecínov Member and in the phosphate nodules at the base of the Bílá Hora Formation at Pecínov quarry. The phosphate nodules and coprolites were in contact with seawater for an extended period of time to accommodate higher concentrations of REE, suggesting slow deposition. Further REE enrichment could have taken place later during reworking which allowed further mixing with seawater. The phosphate sponges with less REE were deposited in shallow marine environment and had less contact time with seawater prior to burial. The large REE enrichments, detected in the tubular phosphatic deposits, may be linked to a higher terrigenous input of REE-bearing minerals associated with the proximity to the continental source area during events of extremesea level fall.