Oxygen isotopes in chondrites

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. 

Each meteorite class has a unique oxygen composition. The origin of these unique compositions is not fully understood, but is known to reflect two types of isotopic variations. Mass-dependent isotope variations, introduced in Chapter 2, result from physical and chemical processes that slightly favor one isotope over another due to the difference in mass. Mass-independent variations probably reflect mixing of components with isotopic compositions that originated either through nucleosynthesis or through processes that do not depend on mass. We will discuss these processes in more detail in Chapter 7. 

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). 

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Yurimoto A, Krot A, Choi BG, Aldon J, Kunihiro T, Brearly AJ (2008) Oxygen isotopes in chondritic components. Rev Miner Geochem 68 141-186 Yurtsever Y (1975) Worldwide survey of stable isotopes in precipitation. Rep Sect Isotope Hydrol IAEA, November 1975, 40 pp... 

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 ... 

Oxygen isotopes in achondrites (above) and primitive achondrites (below). The 8 notation and units are explained in the caption for Figure 6.4. Most achondrites define mass fractionation lines parallel to, but slightly offset from the terrestrial line. Aubrites and lunar samples plot squarely on the terrestrial line. Primitive achondrites generally do not define oxygen mass fractionation lines, but are scattered and resemble their chondrite precursors. 

Effects of secondary processes in modifying volatile elements (Grossman et al, 2000, 2002) and oxygen isotopes in chondrules, especially in ordinary chondrites (Bridges et al, 1998 Franchi et al, 2001) require further study (see Chapter 1.09). 

Choi B. G., Krot A. N., and Wasson J. T. (2000) Oxygen isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia. Meteorit. Planet. Sci. 35, 1239-1248. 

Guan Y., McKeegan K. D., and MacPherson G. J. (2000) Oxygen isotopes in calcium—aluminum-rich inclusions from enstatite chondrites new evidence for a single CAI source in the solar nebula. Earth Planet. Sci. Lett. 181, 271-277. 

Hiyagon H. (2000) An ion microprobe study of oxygen isotopes in some inclusions in Kainsaz and Y-81020 C03 chondrites. Symp. Antarct. Meteorit. 25, 19-21. 

Figure 24 Oxygen isotopes in CV3 chondrite CAIs. Data in (a, c) taken from the summary table of Clayton et al. (1988) and references therein ion probe data (b) from McKeegan et al. (1996).

Figure 25 Oxygen isotopes in CM and CO chondrite CAIs. (sources Fahey etai, 1987b Wasson a/., 2001).

Figure 27 Oxygen isotopes in minerals within individual CR chondrite CAIs. Data taken from the much larger data set of Aleon et al (2002) (reproduced by permission of the Meteoritical Society from Meteorit. Planet. Sci. 2002, 37,...

Ash RD, Rumble D, III, Alexander CMOD, MacPherson GJ (1998) Oxygen isotopes in isolated chondrules from the Tieschitz ordinary chondrite Initial compositions and differentia parent body alteration. Lunar Planet Sci XXIX, 1854 (CDROM)... 

Gooding JL, Mayeda TK, Clayton RN, Fukuoka T (1983) Oxygen isotopic heterogeneities, their petrological correlations, and implications for melt origins of chondrules in unequilibrated ordinary chondrites. Earth Planet Sci Lett 65 209-224... 

A test of the hypothesis that the linear trends in Figures 10-12 are due to mixing comes from comparisons with oxygen isotopes. It has been conventional wisdom that Mg does not correlate in detail with O excesses in primitive chondrite components, including CAIs. However, combining the new highly precise MC-ICPMS data with isotope ratio data for the... 

Figure 14. Comparison of A Mg (DSM3) and (SMOW) values for various chondrules and whole-rock chondrite samples. The sources of the oxygen isotope data are given in Table 3. Aqueous alteration on this diagram will move points approximately horizontally on this diagram.

Chondrites are the oldest and most primitive rocks in the solar system. They are hosts for interstellar grains that predate solar system formation. Most chondrites have experienced a complex history, which includes primary formation processes and secondary processes that inclnde thermal metamorphism and aqneons alteration. It is generally very difficult to distinguish between the effects of primary and secondary processes on the basis of isotope composition. Chondrites display a wide diversity of isotopic compositions including large variations in oxygen isotopes. 

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)... 

Clayton RN, Goldsmith JR, Karel KJ, Mayeda TK, Newton RP (1975) Limits on the effect of pressure in isotopic fractionation. Geochim Cosmochim Acta 39 1197-1201 Clayton RN, Onuma N, Grossman C, Mayeda TK (1977) Distribution of the presolar component in Allende and other carbonaceous chondrites. Earth Planet Sd Lett 34 209-224 Clayton RN, Goldsmith JR, Mayeda TK (1989) Oxygen isotope fractionation in quartz, albite, anorthite and caldte. Geochim Cosmochim Acta 53 725-733 Cliff SS, Thiemens MH (1997) The 0/ 0 and 0/ 0 ratios in atmospheric nitrous oxide a mass independent anomaly. Science 278 1774-1776 Cliff SS, Brenninkmeijer CAM, Thiemens MH (1999) First measurement of the 0/ 0 and ratios in stratospheric nitrous oxide a mass-independent anomaly. J Geophys Res 104 16171-16175... 

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