UPb dating of a carbonate subaerial exposure event

Abstract

Ten samples of the dolomite fraction from the Lower Ordovician Oneota Formation, directly below the Sauk-Tippecanoe unconformity in Wisconsin, define a 238U 206Pb age of 504 ± 14 Ma (MSWD = 190, 206 Pb 204 Pb = 27–85 , 238 U 204 Pb = 84–795 ), which is in agreement with the 500 ± 4 Ma depositional age based on conodont biostratigraphy. The UPb age is interpreted as recording U enrichment of the carbonate during the early stages of subaerial exposure, shortly following deposition. Variable initial Pb isotope ratios, prolonged interaction with meteoric fluids, and later recrystallization associated with the influx of fluids that produced the Permian Upper Mississippi Valley PbZn sulfide deposits are likely factors in producing scatter about the isochron. However, the substantial U enrichment that can be associated with subaerial exposure of carbonates apparently allows precise UPb ages to be obtained despite some isotope mobility that occurs during later diagenetic events. Dolomite samples have 232 Th 238 U < 0.3 , much lower than that of average silicate crust, consistent with the low solubility of Th in low-temperature systems and the high solubility of U in oxidized fluids. 204 Pb 206 Pb - 208 Pb 206 Pb variations in dolomite and oxide fractions from the Oneota Formation are linearly correlated ( 208 Pb 206 Pb = 38.955 204 Pb 206 Pb + 0.050 ; R 2 = 0.9996), and are interpreted as indicating that the U and common Pb were derived from Penokean (1.8 Ga) crust exposed to the north (i.e., the Wisconsin Dome) or from its derivative sediments.