HR: 0800h
AN: P41C-06    [Abstracts]
TI: The Acapulco Parent Planetesimal: An Early Collisional History in the Solar System
AU: * Marti, K
EM: kmarti@ucsd.edu
AF: Univ. of California, San Diego, Dept. Chemistry (m/c 0317) 9500 Gilman Drive, La Jolla, CA 92093-0317, United States
AU: Kim, Y
EM: y1kim60@yahoo.com
AF: Korea Institute of Geology, Mining and Minerals, 30 Gajungdong, Taejon, 305-350, Korea, Republic of
AB: The Acapulco, Mexico, meteorite was recovered in 1976 from a crater of approx. 30 cm diameter. An old crystallization age of 4.60 (error 0.03) Ga (Prinzhofer et al., 1992) establishes that its parent object is one of the oldest known planetesimals in the solar system, although not in a pristine form. Other dating systems indicated somewhat younger ages and isotopic variabilities in several elements documented a complex early history. The younger ages date the closure times in secondary minerals. The initial parent object was in a partially molten state when isotopically distinct foreign matter invaded the chondritic parent and some of the isotopic signatures survived. Nitrogen in the primitive achondrite Acapulco was found to have distinct isotopic signatures for the metal and silicate phases and also in different morphologies of graphites (El Goresy, 1995, 2005). The delayed collisional event probably disrupted the parent object, as Acapulco cooled very rapidly. Nitrogen in the injected metal and graphite did not isotopically exchange with the host silicates. We observed nitrogen isotopic signatures of several separated mineral phases which cover a range of delta 15N values from -150 permil to +13 permil. The lightest nitrogen signatures observed in metal separates are comparable to those in some morphologies of Acapulco graphites. The heavy N signatures observed in several silicate minerals are consistent with each other, while nitrogen in chromite is distinctly light (delta 15N of -80 permil), intermediate between those of metal and silicates. The incipient rapid cooling history is well documented down to approx. 120° C, as recorded by U/Th-4He ages in phosphates (Min et al., 2003). The history of the Acapulco parent object was uneventful after its early evolution in an environment where no perturbation by collisions occurred, until the meteorite's recent (6.0 Ma ago) injection into an earth-crossing orbit. References: El Goresy, A., Zinner, E., and Marti, K. (1995), Nature 373, 496-499. El Goresy A., Zinner E., Pellas P., and Caillet C. (2005), Geochim. et Cosmochim. Acta 69 (18), 4535. Min K., Farley K. A., Renne P. R. and Marti K. (2003), Earth Planet. Sci. Lett. 209, 323-336. Prinzhofer, A., Papanastassiou, D.A., Wasserburg, G.J. (1992), Geochim. et Cosmochim. Acta 56: 797-815.
DE: 5749 Origin and evolution
DE: 6205 Asteroids
DE: 6240 Meteorites and tektites (1028, 3662)
SC: Planetary Sciences [P]
MN: 2007 Joint Assembly