Identifying Geologically Meaningful U-Pb Dates in Fossil Teeth

Abstract

Dating attempts on fossil material have thus far yielded mixed results. While successes have been reported on single specimens, most fossils are thought to have experienced a complex history that obscures a simple determination of age. For this thesis, transects along the cross-sections of 14 fossil alligorid teeth from the Arroyo Chijuillita Member of the Nacimiento Formation, New Mexico, and tyrannosaur and crocodilian teeth from the Dinosaur Park Formation, Alberta were analysed for their trace elements and U-Pb isotope composition using laser ablation ICP-MS. The reliability of this method is supported by good agreement in the TIMS (61.5±1.6 Ma) and laser ablation ICP-MS (64.9±3.6 Ma) dates for a targeted tooth region. The cross-sectional profile of Y concentration in fossil teeth was found to be a good indicator of the extent of post-fossilisation alteration: the highest-gradient Y profiles (Patterns 1 and 2: 2-3 orders of magnitude variation) tend to indicate preservation of pristine geochemical profiles where meaningful fossilisation dates can be obtained, while lower Y gradients (Patterns 3-4) are associated with progressive disturbance of the fossilisation-age signature. In the Arroyo Chijuillita Member, the U-Pb system preserved in the fossil teeth analyzed can be considered a mixing between the fossilisation-age, low-Y center of a Pattern 2 sample (64.2±5.4 Ma) and a Pattern 4 sample preserving an alteration age (25.7±2.1 Ma) corresponding to the end of large-scale volcanic activity in the Four Corners region. In the Dinosaur Park Formation, the U-Pb data yield a date of 71.8±9.4 Ma from a Pattern 1 sample at low 238U/206Pb and altered analyses skewing towards higher 238U/206Pb values. Disturbed dates are ~49 Ma, possibly related to the last pulse of Rocky Mountain formation, and ~33 Ma, which has not been tied to a known geological event. A simple geochemical screen using Y profiles can be used to monitor the extent of post-fossilisation alteration. Using this geochemical screen, regions of bioapatite that preserve U-Pb fossilisation and post-fossilisation alteration dates can be discerned.