RARE EARTH ELEMENT FRACTIONATION DURING PHOSPHATE NODULE DIAGENESIS

Phosphate nodules commonly occur in the marine Mississippian black shales of Oklahoma and Arkansas. These nodules often contain, at their cores, exceptionally well preserved fossils and fecal pellets. The rare earth element (REE) chemistry of these nodules was analyzed to examine the conditions of diagenesis, which ultimately lead to ideal fossil preservation. Because REE abundance patterns in phosphate nodules are commonly enriched in the middle REE (MREE; Sm-Dy) the variations in this enrichment may provide clues as to the diagenetic history and perhaps to the paleoenvironment.

The ICP-MS based REE chemistry of phosphate nodules collected from the marginal marine to possibly non-marine Fayetteville Shale Formation in Arkansas differs from deeper marine facies in Arkansas and Oklahoma, particularly with regard to cores vs. rims of the phosphate nodules. In all of the nodules from the deeper facies, the cores are enriched in MREE when compared to the rims. The opposite trend occurs in the coastal nodules with greater MREE enrichments in the rim compared to the core. The relative loss of MREE is generally not accompanied by a commensurate loss in the light or heavy REE.

Several factors may explain the trends observed in our data. Mineralogical differences between the cores and rims may control the fractionation of MREE. Rims on near-shore nodules are generally richer in silicate material (clays) than their deeper marine counterparts. The higher electronegativity of the clays may encourage movement of the MREE away from the core in coastal nodules or towards the core in open marine nodules that are richer in lower electronegativity carbonates. High phosphate content may characterize clay-poor nodules, and this would probably favor higher overall REE abundance. MREE enrichment may correlate positively to total REE abundance. Redox variations may also be important in controlling intensity of MREE enrichment. Many of these hypothesized diagenetic factors may favor enhanced fossil preservation, but further exploration of the chemical and mineralogical differences within and among these phosphate nodules is needed.