HR: 1340h
AN: P33A-1007    [Abstracts]
TI: Constraining the Thermochronological History of the IAB Parent Body: High Resolution Ar-40-Ar-39 Ages on Plagioclase Separates from Silicate Inclusions of IAB Meteorites
AU: Vogel, N
EM: nvogel@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: Vogel, N
EM: nvogel@bgc.org
AF: Department of Earth and Planetary Science, University of California Berkeley, 307 McCone Hall, Berkeley, CA 94720 United States
AU: * Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AU: * Renne, P R
EM: prenne@bgc.org
AF: Department of Earth and Planetary Science, University of California Berkeley, 307 McCone Hall, Berkeley, CA 94720 United States
AB: The processes that led to the assembly of primitive inclusions in a once molten metal matrix as represented by IAB meteorites have not yet been fully understood [1]. Ar-Ar dating of the inclusions provides important information about the thermal history of the IAB parent body [e.g., 2, 3], but the analysis of bulk inclusions, the standard procedure in the past, is often impaired by excess 40Ar and redistribution or loss of K and/or Ar during the history of the meteoriod and in the reactor. To minimize these problems, we prepared from silicate inclusions of four IABs pure plagioclase separates of different grain sizes and quality grades. On these we performed high resolution stepwise Ar-40-Ar-39 dating. Preliminary ages for the different separates of the inclusions are, in Ma, 4540(11) to 4459(12) for Caddo County, 4500(20) to 4380(30) for Landes, 4440(50) to 4340(30) for Ocotillo, and 4480(40) to 4200(30) and 4430(30) to 4300(30) for CDC2 and CDC1, respectively. The age ranges might reflect the residence time of each inclusion in the K-Ar blocking temperature range (ca. 600 K), and is narrowest for Caddo County, being also the oldest inclusion studied by us. Assuming that IABs resulted from a collision of a molten metal body with a chondritic planetesimal [4], Caddo County could represent a surface sample explaining the early and fast cooling, whereas the other samples might have been buried deeper within the IAB body, subject to prolonged residence at elevated temperatures. If IABs formed in impact metal melt pools peppered with chondritic host material [5] the different cooling ages, and age ranges recorded in each inclusion could reflect residence times in a certain metal melt pool, which indirectly would translate into pool sizes and the energies released by the previous impacts. Also, there may have been more than one IAB parent body. Whatever process led to the formation of IAB meteorites was active already very early in the history of the solar system, in particular taking into account the possibility of a slightly excessive decay constant for K-40 [6]. Increasing the ages accordingly by 0.5 percent would shift Caddo County to about 4560 Ma, indistinguishable from the age of the solar system [7]. [1] Mittlefehldt et al. (1998) In Planetary Materials, Vol. 36: 4-1-4-195. [2] Niemeyer (1979) GCA 43: 1829-1840. [3] Takeda et al. (2000) GCA 64: 1311-1327. [4] Benedix et al. (2000) MAPS 35: 1127-1141. [5] Wasson and Kallemeyn (2002) GCA 66: 2445-2473. [6] Renne (2000) EPSL 175: 13-26. [7] Amelin et al. (2002) Science 297: 1678-1683.
DE: 6205 Asteroids and meteoroids
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting