Oxygen isotope studies of carbonaceous chondrites

Abstract

The carbonaceous chondrites display the widest range of oxygen isotopic composition of any meteorite group, as a consequence of the interaction of primordial isotopic reservoirs in the solar nebula. These isotopic variations can be used to identify the reservoirs and to determine conditions and loci of their interactions. We present a comprehensive set of whole-rock analyses of CV, CO, CK, CM, CR, CH, and CI chondrites, as well as selected components of some of these meteorites. A simple model is developed which describes the isotopic behavior during parent-body aqueous alteration processes. The process of thermal dehydration also produces a recognizable effect in the oxygen isotopic composition.

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.