IDENTIFYING GEOLOGICALLY MEANINGFUL U-PB DATES IN FOSSIL TEETH

Most attempts at direct age dating of fossils have been unsuccessful because bioapatite can be disturbed by post-fossilization processes, yielding ages younger than the age of the host strata. In order to develop a technique to identify relatively pristine parts of fossil teeth, LA ICP-MS trace element analyses were conducted on 3 alligatorid teeth from ~65.5 Ma strata of the Nacimiento Formation and 2 cf. T. rex teeth from the ~67 Ma Naashoibito Member, New Mexico. The trace element profiles were then compared to in situ U-Pb analyses. Two teeth showed enrichment at the edges, and concentrations in the center comparable to modern teeth. An alligatorid tooth yielded a date of 57.3 +/- 4.7 Ma, slightly younger than the depositional age of the host sediment. If analyses with the lowest REE and Y concentrations were selected, a U-Pb date of 63.8 +/- 6.5 Ma resulted. The two other alligatorid teeth have disturbed trace element profiles and yield dates younger than the host strata. These results indicate that geochemical screening of fossil teeth can identify pristine regions that are amenable to accurate U-Pb dating.