THE DISTRIBUTION OF RARE EARTH AND TRACE ELEMENTS IN FRACTIONS OF MARINE SHALE

Shales have been extensively analyzed for total concentrations of rare earth (REE) and other trace elements (TE); however, little is known about partitioning of these elements between various mineral fractions. We have used the sequential extraction procedure of Terrier and Bisson (1979) to separate REE and other trace metals from five fractions of shale: 1) exchangeable, 2) bound to carbonates, 3) Fe and Mg oxides, 4) organics and 5) residual in crystal structures. REE and other trace elements (Y, Sr, Sc, Se, Cr, Pb, Th, and U) were analyzed by ICP-MS in sequential fractions extracted from four fossiliferous upper Cretaceous shale samples: Sharon Springs and Verendrye members of the Pierre Shale (Chamberlain, SD), and the Navesink and Hornerstown Formations (Inversand, NJ).

REE and TE have variable partitioning between the five chemical fractions. For example, Verendrye samples have little REE and Y in the exchangeable fraction; however, in Sharon Springs samples exchangeables represent approximately 40% to 60% of total REE and Y. The hydrous ferric oxide (HFO) and carbonate fractions contained variable proportions, and the organic fractions contained appreciable amounts of REE and Y in all shale samples. REE signatures were also variable in the fractions. Samples from the Verendrye are light-REE (LREE) depleted in the carbonate and HFO fractions whereas the Sharon Springs samples display LREE enrichment in these same fractions.

The different fractions in marine shale are sources of REE and TE which may be mobilized under different environmental conditions. Differential mobility of REE from the various phases will affect REE signatures in groundwater and potential uptake of REE and TE by secondary minerals, concretions, and vertebrate fossils. For example, significant amounts of REE and Y could be mobilized from the Sharon Springs Member under oxidizing conditions or even due to ion exchange, creating ground waters which are LREE- and MREE-enriched. Possible correlation of REE and other TE could provide a method for tracing release of toxic species.