Oxygen isotopes differentiated meteorites

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 17 Bulk oxygen isotopic compositions of primitive achondrites and differentiated meteorites (sources...

Figure 10 Oxygen isotopic compositions of whole-rock meteorites from differentiated bodies HED, possibly asteroid Vesta SNC, possibly Mars and Moon. Each body produces a slope-1/2 mass-dependent fractionation line, with values of characteristic of the whole source planet. The isotopic compositions of...

Differentiated, or evolved, achondrite types are rare. Other than the HED group, there are only two other known asteroidal sources, one for the angrites (six known meteorites) (Mittlefehldt et al., 2002) and one for the unique basalt NWA Oil (Yamaguchi et al., 2002). Oxygen isotopic... 

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