Chondrites oxygen isotopic composition

Different nebular isotopic reservoirs must have existed, since there are distinct differences in bulk meteoritic O-isotope composition. The carbonaceons chondrites display the widest range in oxygen isotope composition of any meteorite group (Clayton and Mayeda 1999). The evolntion of these meteorites can be interpreted as a progression of interactions between dust and gas components in the solar nebula followed by solid/fluid interactions within parent bodies. Yonng et al. (1999)... 

Oxygen isotopic compositions for bulk chondrites, after Clayton (2004). By convention, 170/160 and 180/160 ratios are plotted as deviations from the composition of standard mean ocean water (SMOW) in units of parts per thousand (permil). The 8 values are calculated as follows S170 = [((170/160)sampie/(170/160)SMOw)-1] x 1000, and similarly for S180. 

The systematic variations in oxygen isotopes provide an independent means of classifying chondrites that generates the same groups as the chemical compositions. The oxygen isotopes also work for classifying non-chondritic meteorites. Oxygen isotopic compositions are somewhat easier to obtain than detailed chemical data and so are often used to nail down a classification. 

Oxygen isotopic compositions of individual chondrules in various chondrite groups, after Clayton (2004). 

The degree of equilibrium isotopic fractionation among phases depends on temperature, so the isotopic compositions of co-existing phases can be used for thermometry. Oxygen is widely used in this way. For example, Clayton and Mayeda (1984) found that the oxygen isotopic compositions of calcite and phyllosilicates from Murchison lie on a mass-dependent fractionation line and differ in 6180 by 22%o. This difference requires a temperature of around 0 °C, which is interpreted to be the temperature of aqueous alteration on the Murchison parent asteroid. Similar measurements for Cl chondrites indicate that aqueous alteration for these meteorites occurred at higher temperature, 50-150 °C (Clayton and Mayeda, 1999). 

The oxygen isotopic compositions of comet silicate grains (Fig. 12.10) resemble those of chondrites, suggesting that they formed within the solar nebula (McKeegan et al., 2006). Only one grain was found with anomalous oxygen isotopes, demonstrating its extrasolar... 

The oxygen isotopes of terrestrial materials mostly fall along with what is termed a mass-fractionation line (MFL). The MFL is defined by a slope of 0.5 passing through VSMOW as shown in Fig. 4.5. The oxygen isotopic composition is unique for different types of meteorites, which are broadly classified as chondrites and achondrites. 

Figure 4.6 Bulk oxygen isotopic compositions of (a) achondrites and meteorites from Mars, the Moon, and Vesta (b) chondrites (after Yurimoto el al. 2006).

Figure 4 Bulk oxygen isotope compositions of chondrite groups and ungrouped chondrites (source Clayton and...

The recently described metal-rich Tafassasset meteorite has an oxygen isotopic composition similar to CR chondrites and an equigranular texture (Bourot-Denise et al., 2002). It is, however, characterized by uniformly ferrous ohvine (Fa3o) and orthopyroxene (FS25), and the presence of albitic plagioclase (An24 4s), chromite, and merrilite it may be the first metamorphosed CR chondrite. 

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