CAIs formation

The highest of these ratios are the same as those measured in CAIs, which suggests that chondrule formation was contemporaneous with CAI formation in Allende. It is currently not clear why the ICPMS measurements of the 26Al-26Mg system give older ages for Allende chondrules than other types of measurements. 

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 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). 

These models provide an explanation for the thermal structure of the asteroid belt that is probably correct in principle but not in its details. The recognition that differentiated asteroids formed earlier than chondrites, perhaps within the terrestrial planet region, requires models in which asteroid accretion was initiated earlier than 2 Myr after CAI formation. 

The observation that bulk chondrites are isotopically homogeneous - with the exception of H, C, N, and O - is evidence for a very thorough mixing in an early phase in the hot nebula (> 2000 K). Chondrules themselves provide a variety of constraints on the dust and gas content of their natal environment. Their chemical and isotopic compositions, size, and shape distributions all suggest dusty gas reservoirs during their formation epoch, which lasted 1-3 Myr after CAI formation, possibly with a peak at 2 Myr. 

The key uncertainty in relating the astronomical observations to the Solar System constraints is at the zero point. However, the different constraints seem to line up well if CAIs formed less than 1 Myr after the protostellar collapse. If so, chondrules would have formed within 3 Myr, consistent with the presence of fine dust in many astronomical analogs. The presence of millimeter- and centimeter-sized objects at a few million years after CAI formation is also broadly consistent with the astronomical constraints, as is the timing for planetesimal collisions indicated by the freed planetary debris. This phase is likely to have started by 3-5 Myr after CAI formation and would have lasted for tens to hundreds of million years, until the final planetary architecture was reached. 

Marcus (2004) hypothesized that CAI formation through surface oxidation can lead to 160 enhancements in solid products. This silicate-forming oxidation process bypasses the drawback of very low densities (and thus long timescales) for gas-phase recombination reactions under nebular conditions. The reaction scheme is displayed in Fig. 4.10. 

Figure 4.10 Schematic of a surface-assisted 160-rich CAI formation scheme of Marcus (2004).

The FU Ori outbursts may have repeatedly heated the solar nebula, but the typical duration of these events ( 50—100 yr) make them unlikely sources for chondrule or CAI formation. However, free-floating refractory grains or pebbles, such as CAIs or chondrules, may have survived these episodes and may now carry clues on these events. That CAIs have been repeatedly heated (Connolly et al. 2006, and references therein) - some after an epoch of alteration (Beckett et al. 2000) - appears to be consistent with such episodes of global disk heating. 

For volatile-rich carbonaceous chondrites like Cl and CM chondrites, constraints on thermal histories are derived from carbonate ages and oxygen isotopic data. The former indicate that alteration began soon after CAI formation and lasted —20Myr (Endress et al, 1996 Brearley et al, 2001). Oxygen-isotopic compositions of carbonates provide model-dependent temperatures... 

There are two types of refractory inclusions calcium- and aluminum-rich inclusions (this section) and amoeboid olivine aggregates (Section 1.07.5.3). Since the mineralogy, chemistry and isotope chemistry of refractory inclusions were reviewed by MacPherson et al. (1988), many new analyses have been made of CAIs in CV, CM, CO, CR, CH, CB, ordinary and enstatite chondrites that provide important constraints on physicochemical conditions, time, and place of CAI formation. CAIs are addressed in detail in Chapter 1.08, the role of condensation and evaporation in their formation in Chapter 1.15, and their clues to early solar system chronology in Chapter 1.16. 

The chronology of alteration in CM chondrites is imperfectly understood. Iodine-xenon dating of mineral separates from Murchison, Murray, Mighei, and Cold Bokkeveld (Lewis and Anders, 1975 Niemeyer and Zaikowski, 1980) indicate an extended alteration history for the CMs. Relative to the formation of Murchison magnetite, Murray, and Cold Bokkeveld samples yield closure ages of 10.1 3.2 Myr and at least 11 Myr, respectively, after CAI formation. 

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