HR: 11:50h
AN: V32B-07    [Abstracts]
TI: Combining Hf-W Ages, Cooling Rates, and Thermal Models to Estimate the Accretion Time of Iron Meteorite Parent Bodies
AU: * Qin, L
EM: lqin@ciw.edu
AF: Origins Laboratory, University of Chicago, Dept. of the Geophysical Sciences, 5734 S. Ellis Ave., Chicago, IL 60637, United States
AU: * Qin, L
EM: lqin@ciw.edu
AF: Dept. of Geology, the Field Museum, 1400 S. Lake Shore Dr., Chicago, IL 60605, United States
AU: Dauphas, N
AF: Origins Laboratory, University of Chicago, Dept. of the Geophysical Sciences, 5734 S. Ellis Ave., Chicago, IL 60637, United States
AU: Dauphas, N
AF: Dept. of Geology, the Field Museum, 1400 S. Lake Shore Dr., Chicago, IL 60605, United States
AU: Wadhwa, M
AF: Dept. of Geology, the Field Museum, 1400 S. Lake Shore Dr., Chicago, IL 60605, United States
AU: Masarik, J
AF: Komensky University, Dept. of Nuclear Physics, Mlynska dolina F/1, Bratislava, SK-842 15, Slovakia (Slovak Republic)
AU: Janney, P E
AF: Dept. of Geology, the Field Museum, 1400 S. Lake Shore Dr., Chicago, IL 60605, United States
AB: The ¹⁸²Hf-¹⁸²W short-lived chronometer has been widely used to date metal-silicate differentiation processes in the early Solar System. However the presence of cosmogenic effects from exposure to GCR can potentially hamper the use of this system for chronology purposes (e.g. [1,2]). These effects must be corrected for in order to calculate metal-silicate differentiation ages. In this study, high-precision W isotope measurements are presented for 32 iron meteorites from 8 magmatic and 2 non-magmatic groups. Exposure ages and pre- atmospheric size estimates are available for most of these samples [3]. Our precision is better than or comparable to the currently most precise literature data and our results agree with previous work [4]. All magmatic irons have ε¹⁸²W equal within error to or more negative than the Solar System initial derived from a CAI isochron [5]. Iron meteorites from the same magmatic groups show variations in ε¹⁸²W. These are most easily explained by exposure to cosmic rays in space. A correction method was developed to estimate pre-exposure ε¹⁸²W for individual iron meteorite groups. Metal-silicate differentiation in most iron meteorite parent bodies must have occurred within 2 Myr of formation of refractory inclusions. For the first time, we combine ¹⁸²Hf-¹⁸²W ages with parent body sizes inferred from metallographic cooling rates in a thermal model to constrain the accretion time of iron meteorite parent bodies. The estimated accretion ages are within 1.5 Myr for most magmatic groups, and could be as early as 0.2 Myr after CAI formation. This is consistent with the study of Bottke et al. [6] who argued that iron meteorite parent bodies could represent an early generation of planetesimals formed in the inner region of the Solar System. [1] Masarik J. (1997) EPSL 152, 181-185. [2] Markowski A. et al. (2006) EPSL 250,104-115. [3] Voshage H. (1984) EPSL 71, 181-194. [4] Markowski A. et al. (2006) EPSL 242, 1-15. [5] Kleine T. et al. (2005) GCA 69, 5805-5818. [6] Bottke W. F. et al. (2006) Nature 439, 821-824.
DE: 1155 Extinct radionuclide geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting