Pt-group metal anomalies in the Lower Mississippian of southern Oklahoma

Abstract

Four iridium (Pt-group elements) abundance anomalies have been found within a stratigraphic span of 3 m in the Lower Mississippian of Oklahoma. In ascending order, the first anomalies occur at the top of the Woodford Shale: Ir = 0.25 ppb, Pt = 48 ppb, Os = 7.5 ppb, and Au = 18 ppb. The anomalies occur just below a redox boundary and we suspect that the enriched elements were precipitated from sea water that contacted the organic- and sulfide-rick black shale. Two more anomalies occur in the Welden Limestone, the lower one weak and the upper one strong (Ir = 0.42 ppb, Pt = 50 ppb, Os = 0.075 ppb, and Au = 0.14 ppb). The excess Ir and Pt (also Co, As, and Ni) might have been enriched from sea water by bacteria at these two horizons. A 70-cm-thick interval of excess heavy siderophiles occurs in the overlying Caney Shale; the interval contains the following peak concentrations: Ir = 0.56 ppb, Pt =150 ppb, Os = 0.51 ppb, Co = 725 ppm, and Ni =1450 ppm. These elements vary in proportion to the Al (clay) content and we suspect that they were carried in with detrital material from erosion of ultramafic source rocks. We found no evidence of microspherules or shocked-mineral grains in any of these anomaly zones.