Meteorites oxygen isotopes

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. 

Most non-chondrule solids in the inner Solar System experienced thermal processing (see Chapter 8) that could have modified their initial oxygen isotopic composition (Yurimoto Kuramoto 2004). The complicated structure of meteoritic oxygen isotopes is difficult to reproduce simply by physical mixing of different reservoirs. Apart from thermal processing (e.g. melting, vaporization, condensation), a large mass-independent chemical process is required. The exact mechanism for this likely photochemical process is yet unknown, but the available constraints leave only a few pathways open. 

Meteorites "map the Solar System by having specific oxygen isotopic compositions, a major element in all but the irons. Since its high chemical reactivity causes oxygen to form numerous compounds, it exists in many meteoritic minerals, even in silicate inclusions in iron meteorites. Elsewhere in this volume, Clayton (5) discusses meteoritic oxygen isotopic differences and processes that established these [cf (/)]. 

Eig. 4. The bulk oxygen isotopic composition of different meteorite classes where (—) is the terrestial fractionation line. The 5 notation refers to the normalized difference between or ratios to those in standard mean ocean water (SMOW) in relative units of parts per thousand. The... 

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. 

R. C. Wiens, G. R. Huss, and D. S. Burnett. The Solar Oxygen Isotopic Composition Predictions and Implications for Solar Nebula Processes. Meteoritics and Planetary Science, 34(1995) 99-108. 

Oxygen Isotope Fractionation in Earth, Moon, and Meteorite Samples... 

Romanek CS, Grossman EL, Morse JW (1992) Carbon isotopic fractionation in synthetic aragonite and calcite Effects of temperature and precipitation rate. Geochim Cosmochim Acta 56 419-430 Rowe MW, Clayton RN, Mayeda TK (1994) Oxygen isotopes in separated components of Cl and CM meteorites. Geochim Cosmochim Acta 58 5341-5347... 

Clayton RN, Onuma N, Mayeda TK (1976) A classification of meteorites based on oxygen isotopes. Earth Planet Sci Lett 30 10-18... 

Hashimoto A, Hinton RW, Davis AM, Grossman L, Mayeda TK, Clayton RN (1986) A hibonite-rich Murchison inclusion with anomalous oxygen isotopic composition. Lunar Planet Sci XVII 317-318 Heydegger HR, Foster JJ, Compston W (1979) Evidence of a new isotopic anomaly from titanium isotopic ratios in meteoritic material. Nature 278 704-707... 

Clayton DD (1999) Radiogenic iron. Meteor Planet Sci 34 A145-A160 Clayton RN (1993) Oxygen isotopes in meteorites. Ann Rev Earth Planet Sci 21 115-149 Clayton RN, Mayeda TK, Hurd JM (1974) Loss of oxygen, silicon, sulfur, and potassiimi from flie limar regolith. Proc Lunar Sci Conf 5 1801-1809... 

Mass-independent isotopic fractionations are widespread in the earth s atmosphere and have been observed in O3, CO2, N2O, and CO, which are all linked to reactions involving stratospheric ozone (Thiemens 1999). For oxygen, this is a characteristic marker in the atmosphere (see Sect. 3.9). These processes probably also play a role in the atmosphere of Mars and in the pre-solar nebula (Thiemens 1999). Oxygen isotope measurements in meteorites demonstrate that the effect is of significant importance in the formation of the solar system (Clayton et al. 1973a) (Sect. 3.1). 

Bulk meteorites, the Moon and Mars lie within a few percentile above or below the terrestrial fractionation line on a three-oxygen isotope plot (see Fig. 3.1). Therefore, the oxygen isotope composition of the Sun has been assnmed to be the same as that of the Earth. This view has changed with the suggestion of Clayton (2002) that the Sun and the initial composition of the solar system are 0-rich com-... 

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)... 

In addition to oxygen isotopes, the volatile elements H, C, N, and S also show extremely large variations in isotope composition in meteorites. In recent years, most investigations have concentrated on the analyses of individual components with more and more sophisticated analytical techniques. 

Fig. 3.3 Three oxygen isotope plot of lunar and Martian rocks and HED meteorites supposed to be fragnments of asteroid Vesta (after Wiechert et al. 2003)...

Franchi lA, Wright IP, Sexton AS, Pilhnger T (1999) The oxygen isotopic composition of Earth and Mars, Meteorit Planet Sci 34 657-661... 

Onuma N, Clayton RN, Mayeda TK (1970) Oxygen isotope fractionation between minerals and an estimate of the temperature of formation. Science 167 536-538 Ott U (1993) Interstellar grains in meteorites. Nature 364 25-33... 

The oxygen isotopic composition of the solar system is a key constraint for understanding the environment in which the Sun formed, the environment in the early solar system, and the processes that operated in the early solar system. Oxygen isotopic composition is also a key property by which we classify meteorites. Yet until very recently, we have not been able to make any measurement that could unambiguously tell us what the oxygen isotopic composition of the Sun, and by extension the solar system, really is. 

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. 

---