MOBILITY OF RARE-EARTH ELEMENTS (REE) IN MARINE CARBONIFEROUS PHOSPHATES AND HOST SHALES FROM THE MIDCONTINENT AND IMPLICATIONS FOR SHALE PROVENANCE ANALYSIS

The nature and extent of diagenetic movement of middle rare-earth elements (MREE) to phosphate nodules from pore waters of host muds can help determine the reliability of geochemical provenance analysis in shales. Paired analyses of phosphate concretions and black and gray host shales from the Eudora, Muncie Creek, and Stark Shales of eastern Kansas show that the phosphates are MREE-enriched and the host shales are MREE-depleted. Phosphate nodules from other Upper Carboniferous units in Kansas, Oklahoma, Iowa, and Ohio generally display either flat or MREE-enriched patterns with occasional cerium depletions. Other host shales most commonly display flat REE patterns. Cerium depletions in all three laminar phosphate samples from Nebraska suggest that genesis of phosphatic laminae was influenced by overlying seawater.

All of the MREE enrichment and depletion patterns are from outcrop samples, yet they are generally comparable to recent results from similar facies in core samples analyzed by Cruse et al. (2000). Although black shales are susceptible to chemical weathering, the similar chemistry between outcrop and core results suggests that the samples in our data set were not significantly altered by weathering.

The REE mobility discussed above appears consistent with movement of other elements used in provenance analysis. Preliminary results suggest that the chemistry of shale samples in which MREE have moved from host mud to phosphate differs from shales with flat REE patterns that presumably reflect less elemental migration. Ratios of Fe2O3/TiO2 , Al2O/(Al2O3 + Fe2O3),and values of La and Sc all appear to be grouped according to REE pattern. Shales with flat REE patterns are probably more reliable choices for provenance analysis than those in which REE have moved.