Oxygen isotopes chondritic meteorites

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)... 

Because of the isotopic variability and the high cosmic abundance of oxygen, oxygen isotopes are very useful for meteorite classification. Below the condensation temperature of silicates and above the condensation temperature of ices, approximately 25% of the oxygen in the solar nebula is predicted to have occurred in condensed solids, with the remainder in gaseous molecules. Chondrites provide samples of the condensed oxygen in the early solar system. 

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. 

How are oxygen isotopes in chondrites and achondrites illustrated graphically, and how are the isotopic compositions useful in classifying meteorites and recognizing relationships between them ... 

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). 

This isotope of oxygen does not always occur in all natural samples in its usual proportion. Because ofmass-dependentchemical fractionation itis notpossible to distinguish an anomaly in this one isotope from measurement of a single isotope ratio. One must use the the three-isotope plot (see Glossary). Using it one concludes that the different classes of chondritic meteorites differ in their ratio l80/170. 

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).

The MIF phenomenon was first observed by Clayton in 1973 for the isotopic oxygen content in the earliest solids in the solar system, the so-called calcium-aluminum-rich inclusions (CAIs) in carbonaceous chondritic meteorites [1]. The slope of versus plot for the CAIs was close to unity, the CAIs being equally deficient in the heavy O isotopes, deficient in the S notation sense, while the ozone is equally enriched in those isotopes in that sense, as in Figure 2.2. Both are examples of an MIF. Interest in this striking phenomenon for the CAIs is motivated by what it may reveal about the formation of the early solar system. Standard reaction rate transition state theory [3], and behavior of oxygen an other isotope fractionation in many other systems, would have led, instead, to the slope... 

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. 

Fagan T. J., McKeegan K. D., Krot A. N., and Keil K. (2001) Calcium, aluminum-rich inclusions in enstatite chondrites (2) oxygen isotopes. Meteorit. Planet. Sci. 36, 223 —230. 

Figure 2 Oxygen isotopic compositions of chondmles from all classes of chondritic meteorites ordinary (O), enstatite (E), carbonaceous (C), and Rumumti-type (R). The TF line and carbonaceous chondrite anhydrous mineral (CCAM) line are shown for reference in this and many subsequent figures. Equations for these lines are TF—5 = 0.525 and CCAM—5 = 0.9415 -4.00 (sources Clayton et al, 1983, 1984, 1991 Weisberg et al., 1991).

The most abundant stony meteorites are the ordinary chondrites (H, L, and LL). Whole-rock oxygen isotopic compositions are similar, but resolvable, for the three iron groups (Clayton et al., 1991). Remarkably, analyses of individual... 

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