Meteorites CAIs

There now exist numerous observations of mass-independent isotopic compositions in nature. Most of these have recently been reviewed and will not be repeated here. When the first laboratory measurements of the mass-independent isotope effect were reported by Thiemens and Heidenreich (1983), their occurrence in nature was not expected, except possibly for the early solar system to produce the observed meteoritic CAI data. It is significant to note that, at present, all oxygen-bearing molecules in the atmosphere (except water) possess mass-independent isotopic compositions. These molecules include O2, O3, CO2, CO, N2O, H2O2, and aerosol nitrate and sulfate. Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. These studies have been reviewed recently (Thiemens et al., 2001 Thiemens, 1999). 

Another feature of meteorites that proves to be important is the calcium-aluminium inclusions (CAIs), which, as the name suggests, show regions of enhanced Ca and Al. These micron- to centimetre-sized particles are some of the oldest objects known and have a similar temperature history. They probably formed at temperatures in the region 1700-2400 K and so are close to the centre line of the solar nebula. Although it is hard to be sure about the origin of these objects, there is agreement on their age based on radioisotope dating. 

Meteorites present an opportunity to look at geological time or the time told by radionucleotides within rocks. The oldest rocks found on Earth are not as old as the age of the Earth due to continual reprocessing of the Earth s surface. The oldest discovered rocks so far are the Acasta gneisses from Northwestern Canada, which are 4.03 Gyr, but these are young compared with the CAIs found in the Allende meteorite, which are 4.566 0.002 Gyr or 4.556 billion years. The ages of these species are derived from the relative abundances of radioisotopes and their daughter species, as seen in Table 6.3. 

Substantial abundance anomalies occur among the heavy oxygen isotopes 170 and 180, which are underabundant by up to about 4 per cent relative to 160 in oxide grains of certain of the CAIs, compared with the bulk composition in which the isotope ratios are closer to a terrestrial standard. The intriguing feature of these anomalous ratios is that, in common with some other meteorites, but in contrast to terrestrial and lunar samples, the relative deviations of the two heavy isotopes are equal most normal fractionation processes would cause 180 to have twice the anomaly of 170, as indeed is observed in terrestrial samples and more differentiated meteorites, where the anomalies are also usually much smaller. While there has been speculation that there might be a substantial admixture of pure 160 from a supernova, there are fractionation mechanisms that may be able to account for the effect, e.g. photo-dissociation of molecules affected by selfshielding (R. Clayton 2002). In this case, it is possible that the terrestrial standard is enriched in the heavy O-isotopes while the inclusions have more nearly the true solar ratio. 

Figure 10. A Mg vs. 5 Mg plot showing that the chondrite meteorite data define a trend inconsistent with mass-dependent fractionation. A single whole-rock CAI sample with the lowest A Mg value plots off the diagram to the lower right.

Young et al. (2002a) showed that ultraviolet laser ablation combined with MC-ICPMS (LA-MC-ICPMS) can offer advantages over other methods of spatially resolved Mg isotopic analysis of meteorite materials. They collected data for chondrules and a CAI from the Allende meteorite. Each datum in that study represents approximately 2.8 pg of material (based on a laser spot diameter of 100 pm and laser pit depth of 30 pm depths are uncertain to + 20%). 

Variability in Mg isotope ratios among chondritic meteorites and their constituents is dominated by mixing between a radiogenic CAI-like reservoir and a reservoir resembling ordinary chondrites. The mixing is evident in 5 Mg and 8 Mg, Al/Mg, and A 0 values but... 

The amoeboid descriptor for amoeboid olivine aggregates refers to their irregular shapes. AOAs tend to be fine-grained and porous, and have comparable sizes to CAIs in the same meteorite. They consist mostly of forsterite and lesser amounts of iron-nickel metal, with a refractory component composed of anorthite, spinel, aluminum-rich diopside, and rarely melilite. The refractory component is sometimes recognizable as a CAI embedded within the AOA. The AOAs show no evidence of having been melted, but some contain CAIs that have melted. 

Al)0 ratio of (5.1+0.6) x 10 (Fig. 8.24). Since then, a ratio of 5 x 10 "5 has been found repeatedly in CAIs of different types and from different classes of meteorites and has become known as the canonical ratio. 

Ion probe (SIMS) measurements of (26Al/27Al)0 for chondrules from several meteorite groups. The right axis shows the formation time relative to CAIs, the oldest solids formed in the solar system. These data indicate that chondrule formation started at least 1 Myr after CAIs formed and continued for 1-2 Myr. For CR chondrites, in addition to the plotted data, a similar number of chondrules show no resolvable evidence for 26Al. Data for unequilibrated ordinary chondrites (UDC) from Kita ef al. 

Taken together, the data summarized on Figure 9.9 indicate that CAIs were the first solids to form in the solar system, that they formed relatively quickly, and that many were subsequently altered in the nebula and in the host meteorites. Most chondrules probably formed 1-3 Myr after CAIs, although those in metal-rich chondrites formed considerably later. 

---