Chondrite parent bodies

The chondrite parent bodies obviously could not have accreted before their constituent chondrules formed. Based on the formation times of chondrules, accretion of the ordinary chondrite parent bodies began 2.5-3 Myr after CAIs (4565.7—4565.2 Ma). The end of accretion can be inferred from the metamorphic history of the chondrite parent bodies. Isotopic data from metamorphic assemblages, coupled with thermal modeling of the chondrite parent bodies, suggest that the time of peak metamorphism for the H chondrite parent body was at-4563 Ma. As will be discussed in Chapter 11, it is likely that the source of heat for metamorphism on chondrite parent bodies was the decay of26 Al, perhaps with a contribution from 60Fe. Thermal evolution models indicate that accretion of chondritic asteroids could not have occurred earlier than -2 Myr after CAI formation, or they would have melted. 

Once formed, the chondrite parent bodies experience a variety of processes, including thermal metamorphism, aqueous alteration, shock metamorphism due to impacts, and even disruption from large impacts. Several radiochronometers can provide information on the timing of metamorphism and aqueous alteration. The chronology of this processing is summarized in Figure 9.11. 

As with the ordinary chondrites, the accretion times for CV and CO chondrite parent bodies can be estimated from the times of chondrule formation. The 26Al-26Mg and129I-129Xe data suggest chondrule formation extended to 4565 Ma, 3.2 Myr after CAIs. This is slightly later than, but within the uncertainties of, the time indicated for the accretion of ordinary chondrite parent bodies. 

As already noted, spectral similarities between the various asteroid classes and specific types of meteorites provide a way to identify possible meteorite parent bodies. The Tholen and Barucci (1989) asteroid taxonomy has been interpreted as representing the types of meteorites shown in Table 11.1. Using the Bus et al. (2002) taxonomy, the C-complex asteroids are probably hydrated carbonaceous chondrites (e.g. Cl or CM). These carbonaceous chondrite asteroids probably accreted with ices and will be considered in Chapter 12. Some S-complex asteroids are ordinary chondrite parent bodies, but this superclass is very diverse and includes many other meteorite types as well. The X-complex includes objects with spectra that resemble enstatite chondrites and aubrites, and some irons and stony irons, although other X-complex asteroids are unlike known meteorite types. A few asteroid spectra are unique and provide more definitive connections, such as between 4 Vesta and... 

Grimm, R. E. andMcSween, H. Y. (1989) Water and the thermal evolution of carbonaceous chondrite parent bodies. Icarus, 82, 244-280. 

Young, E. D., Ash, R. D., England, P. and Rumble, D. (1999) Fluid flow in chondrite parent bodies deciphering the compositions of planetesimals. Science, 286, 1331-1335. 

Kleine, T., Mezger, K., Palme, H., Scherer, E. and Munker, C. (2005) Early core formation in asteroids and late accretion of chondrite parent bodies Evidence from Hf- W in CAIs, metal-rich chondrites and iron meteorites. Geochimica et Cosmochimica Acta, 69, 5805-5818. 

Nevertheless, all abnormal isotopic ratios are not necessarily related to an initial heterogeneity of the protosolar nebula. Magnetic isotope effects could have played a role during the accretion and even later, during the long life-time of the chondrites parent bodies. 

Refractory inclusions and chondrules are embedded in fine-grained (0.1 to 1 pm) silicate-rich dust that was present in the proto-solar nebula at the time of the accretion of the chondritic parent bodies ( 2 Myr). This compacted dust, known as matrix, may offer the best comparison to the micron-sized dust grains studied in protoplanetary disks around other stars (see Chapters 6, 7, and Section 8.2). The matrix carries highly heterogeneous dust from different locations in the proto-solar... 

Nagahara H. (1992) Yamato-8002 partial melting residue on the unique chondrite parent body. Proc. NIPR Symp. Antarct. Meteorit. 5, 191-223. 

Van Schmus and Wood, 1967). This metamorphism occurred in an almost closed system with respect to oxygen, resulting in less than 0.5%o variation in 5 0 for different metamorphic grades within each iron group (H, L, and LL) (Clayton et al., 1991). This observation is in accord with the inferred anhydrous state of the O-chondrite parent bodies. 

Bennett M. E. and McSween H. Y., Jr. (1996) Revised model calculations for the thermal histories of ordinary chondrite parent bodies. Meteorit. Planet. Sci. 31, 783-792. 

Kong P., Mori T., and Ebihara M. (1997) Compositional continuity of enstatite chondrites and implications for heterogeneous accretion of the enstatite chondrite parent body. Geochim. Cosmochim. Acta 61, 4895-4914. 

Rubin A. E., Zolensky M. E., and Bodnar R. J. (2002) The halite-bearing Zag and Monahans (1998) meteorite breccias shock metamorphism, thermal metamorphism and aqueous alteration on the H-chondrite parent body. Meteorit. Planet. Sci. 37, 124-141. 

Young E. D. (2001) The hydrology of carbonaceous chondrite parent bodies and the evolution of planet progenitors. Phil. Trans. Roy. Soc. London A 359, 2095-2110. 

Mittlefehldt D. W. and Lindstrom M. M. (2001) Petrology and geochemistry of Patuxent Range 91501, an impact melt from the L chondrite parent body, and Lewis Cliff 88663, an L7 chondrite. Meteorit. Planet. Sci. 36, 439-457. 

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