Studies have demonstrated that 238U-206Pb isochrons derived from carbonate rocks yield dates consistent with depositional ages despite evidence of diagenesis in the host carbonates. The spread of observed isotope ratios in carbonates is sufficient to give a potential age resolution of + 1 Ma for the Paleozoic; however, studies to date have achieved 2o error limits of no better than roughly + 10 Ma due to scatter about the isochron ('errorchron') beyond what is attributable to analytical error. The aim of this study is to gain an understanding of the causes of this scatter by studying U-Pb systematics in pristine and near-pristine low- Mg calcite fossils from the Lower Jurassic and in a number of well-preserved marine and diagenetic carbonate phases from the Permian Reef Complex. The importance of diagenesis and possible trace phases (Fe oxides, phosphate minerals, organic matter) will be assessed through the use of microsampling methods, partial leaching and, as applicable, the monitoring of a variety of isotopic and chemical tracers. A better understanding of the processes that control l the initial variability of carbonate 238U/204Pb ratios and that subsequently alter U-Pb isotopic systems will lead to more reliable and precise carbonate 238U-206Pb and 207Pb-206Pb isochrons. The direct dating of carbonates has immediate application to a number of problems in the earth sciences, including the calibration of the Phanerozoic time scale, the dating and correlation of Precambrian sediments, and the dating of diagenesis.
