IN SITU GEOCHEMICAL, SR ISOTOPIC AND U-PB DATING OF DINOSAUR BONE: A RECORD OF FOSSILIZATION AND FLUID-FLOW HISTORY IN THE SAN JUAN BASIN, NEW MEXICO

The geochemistry and isotopic composition of fossil bone has been used with limited success in the past as both a proxy for paleoenvironmental conditions and as a chronometer to date the time of fossilization. Here we report geochemical, strontium isotope, and in situ U-Pb geochronology data for two dinosaur bones from the San Juan basin, New Mexico. One of the bone samples (22799-D) was collected from the Late Cretaceous Kirtland Formation in the southern part of the San Juan Basin. The U-Pb date obtained for the most pristine parts of this bone yield a weighted average 206Pb/238U date of 73.6±0.9 Ma, in perfect agreement with an independent 40Ar/39Ar age of 73.54±0.25 Ma obtained for a previously dated ash bed at virtually the same stratigraphic level in the Kirtland Formation, demonstrating the ability to obtain meaningful fossilization dates using the U-Pb technique. Disturbed parts of this bone reveal at least two patterns of element mobility post-fossilization; a Paleocene Fe-enrichment event and a Miocene U-enrichment event. Both events correlate with known periods of significant fluid-flow in the San Juan basin, indicating parts of the dinosaur bone may record the timing of tectonic events and major fluid-flow in sedimentary basins. The second bone sample BB-1, a fragment of a large sauropod femur (Alamosaurus sanjuanensis) was collected from the Paleocene Ojo Alamo Sandstone. This bone shows much less geochemical variation than bone 22799-D and is very well preserved. The weighted average 206Pb/238U date of 64.8±0.9 Ma is interpreted to record the time of bone fossilization. Considering that fossilization times are typically less than a few thousand years, the age result from BB-1 confirms the existence of Paleocene dinosaurs. The strontium isotopic composition of both bones are relatively unradiogenic (0.70811±3 and 0.70860±3, respectively). The strontium content of both bones is remarkably homogeneous, in contrast to the chemical variability displayed by most elements, therefore we interpret the strontium isotope values to reflect the indigenous bone composition. If true, these dinosaurs did not forage on vegetation growing on exposed Precambrian basement or sedimentary rocks derived from such basement in the vicinity of the San Juan basin.