We are developing an approach for improving time resolution in the sedimentary rock record with the combination of sampling individual sedimentary features in a well constrained geologic context and the potential for reducing age uncertainties by at least a factor of two with the 238U/207Pb- 206Pb/207Pb isochron. A number of studies have demonstrated the potential for U/Pb dating of calcite, but few of the studies have used samples from a well constrained relative petrographic context. This is because previous studies have used 100's of mg to gram sized samples.
The late Paleozoic is noted for frequent sea-level fluctuations that resulted from changing ice sheet volumes of Gondwana continental glaciers. The duration of these cycles is between 200 to 400 ka based on the number of cycles and the best estimates of the duration of the interval over which they are counted. Duration of exposure of a cycle is only some fraction of the time represented by the cycle. Thus
the age of a mineral formed at an exposure surface will be the age of deposition within the time resolution of current techniques for dating rocks that are Mesozoic or older. Low Mg calcite is commonly precipitated at subaerial exposure surfaces. This calcite has great potential for U/Pb dating because U is commonly concentrated by meteoric fluids at the exposure surfaces and low Mg calcite is quite stable.
We sampled from a core acquired by UNOCAL from the Central Basin Platform of the Permian basin of Texas and New Mexico, which sampled an almost continuous section of the lower Permian and upper Pennsylvanian. Cycle caps almost all show evidence of subaerial exposure and some of them have well developed calcite nodules. U/Pb data from eight 5 to 20 mg samples of calcrete nodules from a cycle top at the top of the Pennsylvanian section give an age of 289±27 Ma (2s) using the 238U/207Pb-206Pb/207Pb isochron approach. This age is consistent with the fusulinid biostratigraphy as the surface is 5 m lower than the Permian boundary.
The uncertainty on the radiometric age is reduced by a factor of two relative to the standard isochron approach of 238U/204Pb-206Pb/204Pb. The uncertainty on the age is mainly a function of the small range in U/Pb. The 238U /204Pb ranges from 40 to 91. Other examples from exposure surfaces have a much greater range of U/Pb and it will be possible to get 2s age uncertainties of 2-3% using the new isochron approach (Hoff et al., 1995; Winter and Johnson, 1995).
This isochron differs from the 235U/207Pb- 206Pb/207Pb concordia first introduced by Tatsumoto et al. (1972). The concordia approach requires subtracting out common Pb,
but for low to intermediate 238U/204Pb systems this is not practical. Instead, the isochron approach allows evaluation of the initial isotopic composition, but does not assume any age significance and the uncertainty on the slope is not influenced by the large percent of common Pb that must be subtracted. Because it is not necessary to measure 206Pb/204Pb for either subtracting common Pb for concordia, or for one of the axes of the isochron, this approach also
has great potential for dating samples with very low Pb concentrations where it is an analytical feat to precisely measure the 206Pb/204Pb.
Hoff, J.A. et al., Journal Sed. Research A65, 225-233 (1995).
Tatsumoto, M. et al., Supplement 3, Geochim. Cosmoschim. Acta 2, 1531-1555 (1972).
Winter, B.L. & Johnson, C.M., Earth Planet. Sci. Lett. 131, 177-187 (1995).