Original ArticlesOxygen isotope studies of carbonaceous chondrites
Introduction
Carbonaceous chondrites play a central role in interpreting early solar system history and also provide a window for observation of presolar processes. The CI chondrites are taken to be the least chemically fractionated sample of the solar system which is available in the laboratory, and form the basis for detailed tables of the “cosmic” abundances of the elements (Anders and Grevesse, 1989). CI and CM chondrites contain abundant organic molecules, and thus provide our best guide to abiotic synthesis mechanisms (Cronin et al., 1988); large stable-isotope abundance variations in organic molecules suggest the action of low-temperature ion-molecule reactions in the solar nebula (Geiss and Reeves, 1981). Carbonaceous chondrites are probably samples of C-type asteroids and/or comets (Wetherill and Chapman, 1988). Most groups of carbonaceous chondrites contain refractory inclusions which appear to be the oldest preserved solid objects in the solar system, and which often carry signatures of presolar nucleosynthesis (MacPherson et al., 1988). Many of the carbonaceous chondrites have had minimal thermal metamorphism, so that they preserve presolar grains of diamond, graphite, and silicon carbide (Anders and Zinner, 1993). A characteristic of most of the carbonaceous chondrite groups, which sets them apart from all other meteorites, is the presence of abundant hydrous phases, primarily phyllosilicates, indicative of chemical reaction with water at low temperatures. The main theme of this paper is the elucidation of the hydration reaction conditions through measurement of oxygen isotope abundance variations among the various carbonaceous chondrite groups.
Oxygen isotope variations provide a very powerful tool for study of the origins of meteorites and their components (Clayton, 1993). Taylor et al. (1965) found that carbonaceous chondrites have 18O/16O ratios both greater and smaller than the narrow range observed for ordinary chondrites, and concluded that the former could not have been derived by aqueous alteration of the latter, but required a different source. The full significance of the observations of Taylor et al. could not be appreciated until later measurements of 17O/16O variations revealed that the early solar nebula was isotopically heterogeneous, with different meteoritic components carrying 16O-excesses derived from nucleosynthesis Clayton et al 1973, Clayton et al 1977. Thus isotopic variations in meteoritic oxygen are the combined results of two processes: (1) interaction between isotopically different reservoirs, and (2) mass-dependent isotope fractionation. In high-temperature processes in which mass-dependent fractionations are small, inter-reservoir exchange controls the isotopic variations, giving rise to linear mixing lines in the three-isotope plot, as exemplified by refractory inclusions and chondrules (Clayton, 1993). In low-temperature processes, fractionation effects are large, and may become dominant. In the CI, CM, and CR meteorite groups, the two effects are of comparable magnitude, leading to complicated isotopic patterns Clayton and Mayeda 1984, Rowe et al 1994. All nebular interactions affect the oxygen isotopic compositions of meteoritic materials in the same direction, to increase δ17O and δ18O; on the three-isotope diagrams shown below, this corresponds to an evolution from lower left toward upper right in all cases.
Brief summaries of oxygen isotopic compositions of carbonaceous chondrites were given by Clayton and Mayeda (1989) and by Clayton (1993). This paper includes data previously available only in conference abstracts, as well as some previously published results on specific groups Bischoff et al 1988, Bischoff et al 1991, Bischoff et al 1993, Clayton et al 1987, Endress et al 1994, Johnson et al 1990, Keller et al 1994, Olsen et al 1988, Weisberg et al 1993, Weisberg et al 1995.
Section snippets
Classes of carbonaceous chondrites
On a chemical basis, the three major chondrite groups can be distinguished by their Mg/Si ratios: 1.05 for carbonaceous chondrites, 0.95 for ordinary chondrites, and 0.80 for enstatite chondrites (Sears and Dodd, 1988). The subdivision of carbonaceous chondrites into CI, CM, CO, CV, CR, CH, and CK is based primarily on refractory and volatile element concentrations. For refractory lithophile elements, CV/CI is 1.33, whereas CO/CI and CM/CI are 1.11 (Kallemeyn and Wasson, 1981). CM can be
Experimental procedures
Whole-rock samples were analyzed isotopically by the methods of Clayton and Mayeda 1963, Clayton and Mayeda 1983. For some CM and CI chondrites, a physically separated phyllosilicate matrix sample was also analyzed. Data are reported in Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9 Table 8 as permil (‰) deviations from the SMOW standard for both 18O/16O and 17O/16O ratios. We also list values of Δ17O = δ17O–0.52 δ18O, a convenient measure of
Results and discussion
Results will be discussed by groups as defined above, in the sequence: CV, CO, CK, CM, CR + CH, and CI, a series which may reflect increasing degrees of low-temperature alteration.
Hydration model
The large oxygen isotopic fractionation between carbonates and phyllosilicates in Murchison requires a low temperature of equilibration, estimated to be 0–25°C. By material balance calculations, Clayton and Mayeda (1984) were able to calculate the oxygen isotopic composition of the aqueous reservoir, and from that to estimate the oxygen isotopic composition of the gas of the solar nebula. A modification of their material balance calculation is presented here, with emphasis on the effects of
Conclusions
Based on the systematic variations in oxygen isotopic compositions of the various types of carbonaceous chondrites and their constituent mineral and lithic components, the evolution 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. The most primitive materials are represented by refractory inclusions which are abundant in CV chondrites and which also occur in CO,
Acknowledgements
This research was supported by several NSF grants over many years, most recently EAR 95-26747. We gratefully acknowledge collaborations with many meteoriticists, especially Martin Prinz and Michael Weisberg of the American Museum of Natural History, Addi Bischoff and Dietmar Weber of the University of Münster, and John Wasson and Alan Rubin of U.C.L.A.
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