Rare-earth element chemistry of Mississippian–age phosphate nodules in the Fayetteville Shale of Oklahoma and Arkansas

Abstract

Phosphate nodules from marine black shales and nonmarine gray shales from the Mississippian Fayetteville Shale of north-central Arkansas and northeastern Oklahoma are strongly enriched in middle rare-earth elements (MREEs) as is common in Paleozoic phosphate deposits. The nodules commonly preserve conspicuous europium anomalies that are not typical of phosphate deposits elsewhere in the geologic record, suggesting extremely reducing diagenetic conditions. Contrastingly, minor cerium anomalies imply oxidizing conditions that are not unexpected in the light to medium gray nonmarine host shales. The most likely explanation for the Ce anomalies in the marine phosphate nodules is that bottom waters in the marine environment reverted from the anoxic waters that probably characterized the basal Fayetteville Formation to weakly oxidizing waters that permitted survival of the fauna that occurs in a well-defined zone. All of the Ce anomalies probably formed during an earlier diagenetic stage than the Eu anomalies that developed later, as decaying organic matter consumed what little oxygen was available. Murthy is currently the assistant coordinator for the Upward Bound Program at Arkansas State University. She completed her B.A. degree in journalism at Arkansas State University (2000) and her M.S. degree in chemistry at Arkansas State University (2003).Kidder is currently associate professor of sedimentary geology and earth systems history and chair of the Department of Geological Sciences at Ohio University. He received his Ph.D. in geology at University of California, Santa Barbara (1987). His research interests include sedimentary petrology, phosphates, cherts, and earth system history. Mapes is currently a full professor of invertebrate paleontology in the Department of Geological Sciences at Ohio University. He earned his Ph.D. in paleontology at University of Iowa (1977). His research interests include fossil cephalopods of the Carboniferous, involving analyses of their evolution, phytogeny, biostratigraphy, and more recently, paleobiology and taphonomy of fossil terrestrial plants in the upper Paleozoic in the marine environment. Hannigan is currently an associate professor at the Arkansas State University. She received her B.S. degree in biology and chemistry from the College of New Jersey (1988), her M.S. degree in geology from the State University of New York at Buffalo (1994), and her M.S. degree and her Ph.D. in geochemistry from the University of Rochester (1995). She is a National Science Foundation Research Experiences for Undergraduates Leadership Council awardee and has been an Aldo Leopold Leadership Fellow since 2001.