Accretion and history of chondritic 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. 

Chronology of the solar system from radioactive isotopes 

Chronology of secondary processes in the early solar system. Plot format and anchor points are the same as in Fig. 9.9. Dates related to thermal metamorphism are shown as open symbols, and dates related to aqueous alteration are shown as filled symbols. Both thermal metamorphism and aqueous alteration continued for tens to as much as 100 Myr after CAI formation. Data from Flohenberg and Pravdivtseva (2008), Flutcheon et al. (1998), Flua etal. (2005), Trinquier etal. (2008), Endress et al. (1996), Hoppe et al. (2004), and Zinner and Gopel (2002). 

Information about the timing of metamorphism in ordinary chondrites is derived from the 40 - 39 53 - 53 - 26 A1 26, —J 129t 129Xe systems and by fission-track 

4UAr-39Ar, 53Mn-53Cr, 26Al-26Mg, and 129I- 

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