Mg o

O Normal CAIs • Achondrites 0 ■ O & E chondrite objects À FUN & UN CAIs

200 400 600 800

27Al/ 24 Mg

1000

1200

Fig. 10.7 The 26Al-26Mg systematics in CAI and other objects from various meteorites. The reference lines correspond to initial ratios (26Al/27Al)INI of 5 x 10-5 and 0 respectively. Most normal CAIs show (26Al/27Al)iNI values similar to the canonical SOS ratio 5 x 10-5. The inset shows the same evolution diagram but for very high 27Al/24Mg ratios. From MacPherson et al. (1995), © Meteoritical Society 1995, reproduced by permission.

Table 3.3). Somewhat higher initial ratios, up to 6 x 10-5, were recently observed in the CAIs separated from several chondrites (Young etal., 2005). According to Eqn (10.11), this corresponds to the formation of these inclusions ~ 0.25 Myr before the 26Al-26Mg systematics became closed for the majority of the inclusions. The authors consider that especially intense heating events during this interval reset the cosmochronometer, and the data on Mg diffusion rates in the CAIs validate this interpretation. In the following discussions we will use the "canonical" ratio, which fits the great majority of CAIs, as a time marker for the short-time-scale dating of other nebula objects.

It should be emphasized that Al is one of the most refractory elements and Mg is also not a volatile element (Table 3.1). Therefore the 26Al-26Mg chronometer works well for high-temperature objects such as CAIs and some chondrules, because only high-temperature condensation fractionates these elements, enhancing the Al/Mg ratio in such minerals as hibonite (Table 10.1). Further, Al is immobile in aqueous metamorphic and alteration processes, although Mg can be mobilized. Some CAI minerals, such as hibonite, are highly resistant to alteration and therefore appear to be reliable carriers of fingerprints of in situ 26Al decay. Because of these and other promising features of the 26Al-26Mg systematics, a large data base has been generated (Fig. 10.7). Inspection of this data base shows immediately that (26Al/27Al)INI ratios similar to those discovered in Allende are also observed in the great majority of CAIs from different carbonaceous and ordinary chondrites. Some samples, however, have lower values of (26Al/27Al)INI and some do not indicate the presence of live 26Al at all.

The fact that these (26 Al/27 Al)INI ratios in the great majority of CAIs do approach the canonical value, ~ 5 x 10-5, leads to several important conclusions. (1) The nucleosynthetic and transport processes responsible for live 26 Al in the solar nebula must have yielded at least the above initial ratio. (2) Material with an initial ratio ~ 5 x 10-5 was abundant in the early solar nebula in the zone where CAIs were formed, and it originated within a short time scale of a few hundred thousand years. (3) If there was an initial26 Al/27 Al heterogeneity in the solar nebula, this must have been small compared with the time-related isotope effect, so that a chronological interpretation of the 26Al-26Mg data is possible.

Correlated petrologic and isotopic studies show that the majority of canonical CAIs have almost never been disturbed by secondary processes. In contrast, samples in which (26Al/27Al)INI approaches zero demonstrate reheating or secondary alteration, which could have happened at any time after most 26Al had decayed (MacPherson et al., 1995; Swindle et al., 1996; Huss et al., 2001).

The (26Al/27Al)INI ratios can vary according to the distinct petrographic setting even within one inclusion: in late second-generation igneous phases and rims a 26Mg/24Mg anomaly may be absent (MacPherson and Davis, 1993). In some cases the high time resolution of the 26Al chronometer allows the dating of multilayered CAIs. In one Allende CAI the spinel-free inclusions (within a spinel-rich core), whichhave an almost canonical (26Al/27AlINI) value, (5 ± 0.1) x 10-5, were formed first whereas the spinel core and the melilite-rich mantle show lower ratios (4.3 ± 0.1) x 10-5 and (3.3 ± 0.8) x 10-5 respectively (Fig. 10.6, inset). Substitution of these values into Eqn (10.11) using the inclusion as the reference sample gives the relative (post-inclusion) formation times for the core and the mantle as ~ 0.1 and ~ 0.4 Myr respectively. This shows that fractionation events, which occurred at least 4567 Myr ago, can be resolved on a scale of ~ 0.1 Myr and suggests that the high-temperature processes of CAI formation continued for as long as ~ 0.5 Myr (Hsu et al., 2000). Chondrules in ordinary chondrites show lower (26Al/27AlINI) ratios than those for CAIs, which means that chondrules (and chondrites) were formed 1 or 2 Myr later than CAIs; this follows, for example, from an elegant investigation of a CAI core hosted by a chondrule rim (Krot et al., 2006). The core and rim have different O-isotope compositions (used for the identification; see the next section) and the 26Al-26Mg systematics indicate formation of the rim at least 2 Myr later. The Pb/Pb chronology suggests a similar time difference (Fig. 11.6).

The interval between CAI and chondrite formation conflicts somewhat with model estimates of the residence time of CAI-like objects in the solar nebula, ~ 105 yr: during this time such an object would be expected to drift into the Sun (Cameron, 1995). A mechanism must have operated that prevented loss of the CAIs from the nebula for this relatively long interval, up to ~ 2 Myr, and this constrains models of CAI formation (Section 10.6).

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