Chondritic meteorites oxygen isotope composition

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. 

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)... 

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 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 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. 

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... 

Figure 6 Oxygen isotopic compositions of individual chondrules from ordinary chondrites, with symbols showing the H, L, or LL group of the parent meteorite. The large gray symbols show the mean compositions of H, L, and LL whole rocks. There is no correlation between chondmle composition and parent composition, indicating that all ordinary chondrite chondrules are drawn from the same population. The TF line is shown for reference. The least-squares line fit to the chondmle data has a slope of 0.69 (source Clayton et al, 1991).

Krot A. N., Brearley A. J., Ulyanov A. J., Biryukov V. V., Swindle T. D., Keil K., Mittlefehldt D. W., Scott E. R. D., and Nakamura K. (1998d) Mineralogy, petrography, bulk chemical, iodine-xenon, and oxygen isotopic compositions of dark inclusions in the reduced CV3 chondrite Efremovka. Meteorit. Planet. Sci. 34, 67-89. 

Krot A. N., Libourel G., and Chaussidon M. (2003c) Oxygen isotopic compositions of chondrules in CR chondrites. Meteorit. Planet. Sci. (in preparation). 

Clayton R. N. and Mayeda T. K. (1977) Oxygen isotopic compositions of seperated fractions of the LeoviUe and Renazzo carbonaceous chondrites. Meteoritics 12, A199. 

Sahijpal S., McKeegan K. D., Krot A. N., Weber D., and Ulyanov A. A. (1999) Oxygen isotopic compositions of Ca—Al-rich inclusions from the CH chondrites, Acfer 182 and Pat91546. Meteorit Planet Sci. 34, AlOl. 

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