U-PB DATING OF CARBONATES: THE NEED FOR PETROGRAPHIC EVALUATION AND INTEGRATION OF UNDERGRADUATE AND GRADUATE STUDENTS IN THE RESEARCH

The first paper on U-Pb dating of carbonates appeared less than two decades ago (Smith and Farquhar, 1989). Since this contribution fewer than 20 papers have been published on U-Pb ages of carbonates and perhaps a few more than that have focused on Pb-Pb dating of carbonates. These results demonstrate that there is tremendous potential for improving time calibration in sections that otherwise do not have dateable material. However many sedimentary carbonates do not have a favorable spread in U/Pb ratios necessary to produce a precise age. Additionally, carbonates are vulnerable to diagenetic alteration because of their high solubility in most fluids. Therefore much of the effort to determine the viability of U-Pb dating of carbonates for dating the time of sedimentation needs to be accomplished with petrography. All Geology departments have petrographic microscopes and most have other equipment such as electron microprobes and microscopes, cathode-luminescence setups, fluid inclusion stages, etc. Since Th and Pb have low solubility in most natural fluids, the behavior and speciation of U is probably the most critical factor in determining the U/Pb ratio (and U/Th ratio) of sedimentary carbonates. Studies that focus on stratigraphically well constrained syn-sedimentary carbonates and combine phosphor imaging (autoradiography) and fission track mapping with detailed petrography offer the potential to dramatically improve our understanding of which carbonates offer the greatest chance of providing precise and reliable stratigraphic ages. This work can be accomplished by undergraduate and even high school researchers working in concert with graduate and post graduate researchers. Within the spirit of sharing and cooperation provided by the EARTHTIME initiative there is great scope for researchers at undergraduate institutions to connect with researchers that have the desire and facilities to do the U-Pb dating. Although not all carbonates can be dated, the fundamental understanding of the behavior of uranium in surface environments is also quite relevant to economic geology and the environmental problems of radioactive waste disposal and handling.