HR: 13:55h
AN: V22E-02    [PDF]
TI: Dating Earth Core and Atmospheric Formation Through Hf-W and I-Pu-Xe Clocks
AU: * Yin, Q
EM: yin@geology.ucdavis.edu
AF: Department of Geology, University of California,Davis,, One Shields Avenue, Davis, CA 95616 United States
AU: Ozima, M
EM: EZZ03651@nifty.ne.jp
AF: Graduate School of Earth and Planetary Science, University of Tokyo, Tokyo, 113-0033 Japan
AB: It was the discovery of the radioactivities and nuclear energy at the beginning of last century that settled the long lasting debate about the ages of the Earth and the Sun, between elegant physical arguments advanced by Lord Kelvin favoring short timescale vs. Charles Darwin's trouble for not having enough time for evolution of species, hence resorted to geological observation of sedimentation rate favoring longtime scale. Claire Patterson­_s landmark work on Pb isotopes (1956) establishes the age of meteorites and the Earth at 4.56 Ga, albeit with somewhat wrong half-life of U and wrong sample (one ocean sediment landing on meteorite isochron). Modeling of planetary accretion rate via statistical approach pioneered by Safranov suggested planet formation lasted over 100 Ma. This long timescale was shaken by modern computer simulation. When actual orbital characteristics of the accreting bodies were considered (aided by ever-increasing computing power), the timescale for the inner planet formation is typically around 30 Ma. Discoveries of extrasolar planets place demanding constraints for the timescale of planet formation, i.e. gaseous giant planets must form before the disk dissipation (typically less than 10 Ma). No conventional long-lived isotopic systems are likely to place constraints for the planet formation with sufficient precision and resolution (30/4567), nor do we have 4.56 Ga-old terrestrial sample to work with. Modern approach to the problem is to exploit the now-extinct radioactive isotopes that were once extent at the beginning of the solar system, and look for radiogenic signatures of its daughter isotopes affected by planet wide fractionation. In this sense, we treat the Earth as one piece of whole rock; metallic core, silicate mantle and atmosphere are its mineral constituents. Both Hf-W and I-Pu-Xe clocks are uniquely affected by large-scale processes, core-mantle segregation in the Hf-W system and atmosphere-solid Earth segregation in the I-Pu-Xe system. And the ruler sizes are just right: 182Hf (9Ma); 129I (15.6 Ma); 244Pu (80 Ma). The first-discovered extinct-radionuclide (129I) by Reynolds (1960) played important role in planetary chronometry over the last four decades. The persistent timescale of ~100 Ma provided by terrestrial I-Pu-Xe system all goes back to the influential paper by Wetherill (1975). We will show that the atmospheric retention age (Xe closure) is only 30 Ma, in remarkable agreement with the radiogenic 182W signature of the silicate Earth that argue for rapid core-mantle segregation of 30 Ma at most (Yin et al., 2002). Missing Xe event lasted another 90-120 Ma, possibly associated with early continental crust formation, a timescale consistent with 146Sm-142Nd clock. It is interesting to note in this regard that Xe could form silicate compound under lower crustal pressure and temperature, as shown by recent experiments at Geophysical Lab. What need to be shown are Kr, Ar, and Ne do not form silicates under the same condition, or released readily, in order to explain the fact the only Xe is missing.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 5705 Atmospheres--evolution
DE: 7207 Core and mantle
DE: 8147 Planetary interiors (5430, 5724)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting