HR: 0800h
AN: V51C-0583    [Abstracts]
TI: Isotopic Geochronology by Means of a Dynamic Simulation Model --- Part I. Uranium Series Method
AU: * Hung, C
EM: chengandme@aol.com
AF: Retired/US Environmental Protection Agecy, 6178 Hardy Drive, McLean, VA 22101 United States
AB: This paper presents the hydrodynamic implications of traditional approaches to dating minerals and rocks by the U-Pb method, the development of an alternative dynamic simulation model, and the comparison of these two models. The proposed alternative model simulates dynamic mass transport and radioactive decay of the decay-chain members in a crystalline mineral deposit and in the surrounding host rocks. In conjunction with the results of mass spectrometer measurements, the simulation results can be used for geochronological dating of minerals and rocks. Because of the dynamic simulation approach, this model can avoid imposing two crude assumptions that are normally required by the traditional modeling approach. They are: (1) mineral deposits are confined in a closed system and (2) the decay chain is in secular equilibrium at the time of dating. Currently, these crude assumptions still remain controversial amongst geochronological scientists. The proposed simulation model is verified for the cases of diffusion and radioactive decay. Comparison of the simulation results and analytical solutions for the radioactive decay and the 3-D diffusion cases indicates that they are in excellent agreement. Eight scenarios are analyzed to demonstrate the effects of preexisting daughter nuclides, closed system assumptions, and secular equilibrium assumptions. By selecting the most conservative-values as model input parameters, all eight scenarios are analyzed. The results indicate that (1) analysis with preexisting daughter nuclides is always greater than those without, (2) closed system assumptions may considerably overestimate the age of minerals and rocks, and (3) secular equilibrium assumptions will result in an underestimation of the age of minerals and rocks. The results of an existing radiometric dating study conducted for the Alder Creek rhyolite by Getty and Depaolo are compared with the dynamic simulation model using the same mass spectrometer data. The results indicate that the age of the rhyolite is on the order of 11,300 to 11,900 years, as compared to the age of 1,030,000 years reported in Getty and Depaolo's study. This huge discrepancy in the Alder Creek rhyolite chronology indicates that it is necessary to reevaluate the accuracy of the conventional model, which utilizes a closed system assumption.
DE: 3200 MATHEMATICAL GEOPHYSICS (New field)
DE: 1800 HYDROLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting