The discussions in that paper clearly demonstrate that the low Pb domains typically do preserve dates in accordance with the well-constrained depositional age of the host rock, and thus provide an accurate estimate of the fossilization date. C: Hadrosaur femur from San Juan River site of Fassett (2009); photograph by S. These authors accordingly concluded that these suites of bones had been mineralized while immersed in distinctly different mineral-bearing solutions, and had retained their original mineral fingerprints ever since, indicating that the Paleocene dinosaur bones of the Ojo Alamo had not been reworked from Cretaceous strata.
The fact that the date of 73.6 Ma that we obtained for control bone 22799-D is virtually the same as a single-crystal Ar age of 73.04 Ma obtained for sanidine crystals from a proximal, altered volcanic ash bed (Fassett and Steiner, 1997; Fassett, 2000) at the same stratigraphic level as bone 22799-D further validates our laser-ablation U-Pb dating procedures, and the dates we reported. B: 34 skeletal elements from a single hadrosaur (note hammer for scale); photograph from S. These results also confirmed that the sampled areas of the bones we analyzed represented closed geochemical systems from the time of their original mineralization to the present.
As noted in our Reply (Fassett et al., 2012) to Koenig et al.
Therefore, they provide different information about the source of sandstones than zircons such as low grade metamorphic rocks.
This provides further information about sediment input pathways to sedimentary basins and, when combined with detrital zircon analysis, provides a powerful tool to identify the provenance of sediments.
For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards.
Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA-ICP-MS determinations to the TIMS ages.
A manuscript now in preparation by Heaman, Simonetti, and Fassett (Heaman, 2012, personal commun.) will provide a more detailed description of the procedures used by Fassett et al. (2002) and Fassett (2009) tackled the possible reworking of Cretaceous Kirtland Formation dinosaur-bone fossils into Paleocene Ojo Alamo Sandstone strata by conducting chemical analyses of 15 bone samples from each of these formations.
(2011), and describes in detail the geochemistry and isotope systematics of some regions within bone 22799-D, disturbed by one or more post-fossilization uranium-enrichment events. It was found that these two suites of bones each had distinctive concentrations of U and REE: very low U and high REE for Kirtland samples, and high U and low REE for Ojo Alamo samples.
In summary, the laser ablation technique, using a magnetic sector ICP-MS, can be used for the U-Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques.
This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.
In this study we used LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) to determine U-Pb ages of 5 zircon samples of known age (˜1800 Ma to ˜50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique.