Elsevier

Geochimica et Cosmochimica Acta

Volume 67, Issue 24, 15 December 2003, Pages 4823-4848
Geochimica et Cosmochimica Acta

Presolar diamond, silicon carbide, and graphite in carbonaceous chondrites: implications for thermal processing in the solar nebula

https://doi.org/10.1016/j.gca.2003.07.019Get rights and content

Abstract

We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 (Q-Xe) in eight carbonaceous chondrites by measuring the abundances of noble gas tracers in acid residues. The meteorites studied were Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CV3ox), Axtell (CV3ox), and Acfer 214 (CH). These data and data obtained previously by Huss and Lewis (1995) provide the first reasonably comprehensive database of presolar-grain abundances in carbonaceous chondrites. Evidence is presented for a currently unrecognized Ne-E(H) carrier in CI and CM2 chondrites.

After accounting for parent-body metamorphism, abundances and characteristics of presolar components still show large variations across the classes of carbonaceous chondrites. These variations correlate with the bulk compositions of the host meteorites and imply that the same thermal processing that was responsible for generating the compositional differences between the various chondrite groups also modified the initial presolar-grain assemblages. The CI chondrites and CM2 matrix have the least fractionated bulk compositions relative to the sun and the highest abundances of most types of presolar material, particularly the most fragile types, and thus are probably most representative of the material inherited from the sun's parent molecular cloud. The other classes can be understood as the products of various degrees of heating of bulk molecular cloud material in the solar nebula, removing the volatile elements and destroying the most fragile presolar components, followed by chondrule formation, metal-silicate fractionation in some cases, further nebula processing in some cases, accretion, and parent body processing. If the bulk compositions and the characteristics of the presolar-grain assemblages in various chondrite classes reflect the same processes, as seems likely, then differential condensation from a nebula of solar composition is ruled out as the mechanism for producing the chondrite classes. Presolar grains would have been destroyed if the nebula had been completely vaporized. Our analysis shows that carbonaceous chondrites reflect all stages of nebular processing and thus are no more closely related to one another than they are to ordinary and enstatite chondrites.

Introduction

Presolar grains were incorporated into all classes of chondrites and survive today in the most primitive members of each class Huss and Lewis, 1995, Huss, 1997. Known types of presolar materials include carbonaceous phases such as diamond, silicon carbide, graphite, and, probably, organic materials, as well as silicon nitride and oxide phases such as corundum, spinel, and hibonite Lewis et al., 1987, Tang and Anders, 1988, Amari et al., 1990, Hutcheon et al., 1994, Huss et al., 1994, Nittler et al., 1994, Nittler et al., 1995, Nittler et al., 1997, Choi et al., 1998, Choi et al., 1999. Most recently, presolar silicates were identified in interplanetary dust particles (Messenger et al., 2002). Because these recognized types of presolar grains do not add up to the bulk composition of the solar system, it is clear that they constitute only a small fraction of the presolar matter that provided the building blocks for the solar system.

The known types of presolar material have different thermal and chemical resistance. If all types of chondrites inherited the same initial mixture of presolar grains, then the abundances patterns found in the meteorites today would be expected to reflect the thermal and chemical history of the host meteorites. Good correlations between the abundance patterns within a meteorite class and the petrologic type of the host meteorites show that this expectation is largely satisfied (e.g., Huss, 1990, Huss and Lewis, 1995. Presolar diamonds contain three isotopically distinct noble gas components (P3, HL, and P6) that are released at different temperatures, both in the laboratory and in nature Huss and Lewis, 1994a, Huss and Lewis, 1994b. The carriers of these components have not been effectively separated in the laboratory and show no evidence of being separable in nature except by thermal destruction (Huss and Lewis, 1994a). Thus these components provide independent evidence about the history of the host material. The abundance of the P3 component, which is released at low temperatures, seems to be a function only of the maximum temperature experienced by the diamond and thus is independent of the nature of the surroundings (Huss and Lewis, 1994b). In contrast, the P6 component releases its xenon only at very high temperatures and thus can serve as a tracer of the original diamond abundance in thermally processed material. The relative abundances of the three noble gas components in diamonds within a class also correlate well with other indicators of the metamorphic grade of the host meteorite (Huss and Lewis, 1994b).

The metamorphism model works well within a chondrite class, but comparisons of the least metamorphosed samples of each class show that the different classes of chondrites did not inherit the same initial mixture of presolar grains. For example, the least metamorphosed CV3 chondrites (e.g., Leoville and Vigarano) and the least metamorphosed EH chondrite (Qingzhen) show abundance patterns for presolar grains and noble-gas characteristics of diamonds that indicate significant heating, but the diamond abundances are quite high and the abundances of the most thermally resistant component in the diamonds (Xe-P6) is significantly higher than in the supposed CI-chondrite-like starting material (Huss and Lewis, 1995). These observations led Huss and Lewis (1995) to postulate that the presolar grains in the CV and EH chondrites were recording preaccretionary thermal processing. In this paper, we investigate the abundances and characteristics of presolar grains from CI and CM2 chondrites and some of the least metamorphosed members of the CR, CH, CO, and CV chondrites. The goal is to identify characteristics of the presolar-grain assemblage in each class that could be indicators of preaccretionary nebular processing. We also investigate the bulk compositional characteristics of each meteorite class to follow up the suggestion by Huss and Lewis (1995) that the abundances and characteristics of presolar grains correlate with the bulk compositional properties of the host meteorites. The implications of such a correlation would be profound, because it would imply that the bulk composition of each chondrite class and the modified suite of presolar grains originated from the same nebular processing. If this were true, then nebula-scale vaporization and recondensation of presolar dust cannot be responsible for the bulk chemical properties of chondrites because the presolar grains would have been destroyed (once vaporized they cannot come back). Instead, thermal processing in the nebula of the bulk dust inherited from the sun's parent molecular cloud is implicated, with the known types of presolar grains providing direct evidence of this processing while the bulk compositions provide information about the majority of the presolar material, the nature of which has not yet been determined.

Huss and Lewis (1995) demonstrated that abundances of presolar diamond, silicon carbide, and graphite can be estimated from lightly processed acid residues of bulk meteorite by means of noble-gas tracers. Diamond abundances can be accurately estimated from HF-HCl residues that have been “etched” by an oxidant (e.g., chromic acid, nitric acid) to remove the dominant planetary gas component in the meteorite by measuring the Xe-HL content of the residue and of the diamond separate from the same meteorite. If the acid residues are prepared using high-yield procedures, these abundance estimates are accurate to ∼10%, with the main uncertainties being small sample losses associated with preparing the etched residue and the noble gas abundances in the samples (Huss and Lewis, 1995). Silicon carbide abundances can be estimated from Ne-E(H) in the etched residues. We did not prepare silicon carbide separates because this is quite difficult to do. Instead we used the Ne-E(H) content for silicon carbide from Murchison separates prepared by Amari et al. (1994) to estimate the silicon carbide content in the meteorite. Thus, these abundance estimates are less accurate than those for presolar diamond, but should still be reliable at the ∼20% level (Huss and Lewis, 1995). Graphite abundances can be estimated from Ne-E(L) in the etched residues. However, Huss and Lewis (1995) found that some Ne-E(L) is lost during etching. In addition, the Ne-E(L) content of presolar graphite is not known with any certainty because graphite separates prepared by Amari et al. (1994) contain both isotopically anomalous (presolar) and isotopically normal (presumably of local origin) graphite (Zinner et al., 1995). Thus the true graphite abundances in chondrites are poorly known, but comparisons of Ne-E(L) content in a series of etched residues processed the same way can be used to estimate relative abundances among meteorites.

In this paper, we present noble gas data for HF-HCl residues, chromic-acid-etched residues, and diamond separates from eight meteorites from five classes of carbonaceous chondrites (CM2, CR2, CV3, CO3, and CH). These data are then converted to abundance information for diamond, silicon carbide, and graphite in the meteorites. The residues were prepared using the high-yield methods of Huss and Lewis (1995). The data for CI, CV3, and CO3 chondrites from Huss and Lewis (1995) and the new data presented in this paper provide the first consistent set of abundance data for the carbonaceous chondrites. The abundance data and the noble gas data for diamond separates are evaluated in the context of the petrography and the bulk compositions of the host chondrites and reveal some significant insights into the preaccretionary history of the meteorites and the relationships between them. The combined data show that carbonaceous chondrites span the range from the least processed material available for study (CI chondrites and CM2 matrix) to some of the most highly processed material known in chondrites (CV chondrites). Thus, carbonaceous chondrites are not closely related and should not be considered as a group that is distinct from ordinary or enstatite chondrites.

Section snippets

Sample selection

The details of the samples used in this study are given in Table 1. Meteorites were chosen to produce a self-consistent database that includes most types of carbonaceous chondrites. Huss and Lewis (1995) collected data for Orgueil (CI), Leoville (CV3red), Vigarano (CV3red), Allende (CV3ox), and Kainsaz (CO3.1). Previous work indicated that CM2 chondrites contain a very primitive assemblage of presolar grains (Amari et al., 1994; Lewis, unpublished), but high-yield chemical processing designed

Results

The neon and xenon data are presented in Appendix 1. The abundances of the noble gas components (per gram of meteorite) used to estimate the presolar grain abundances are given in Table 5. The inferred abundances of the presolar grains in meteorites are given in Table 6 and matrix-normalized abundances are given in Table 7.

Discussion

The abundances and characteristics of presolar grains can be combined with bulk compositional information for the host meteorites to infer relationships among chondrite groups and to infer something about the nebular processing that produced those groups. In the discussion below, we will first attempt to establish the characteristics of the most primitive (least fractionated) material in chondritic meteorites. Then we will show that the abundances and characteristics of presolar grains and the

Summary and conclusions

We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 in the Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CVox), Axtell (CVox),and Acfer 214 (CH) chondrites. These data, combined with data obtained previously by Huss and Lewis (1995), provide a reasonably comprehensive data set of presolar grain abundances in carbonaceous chondrites. The abundance data together with bulk compositional data for the host meteorites

Acknowledgements

This work was supported by NASA grants NAG5-8173, NAG5-10449, and NAG5-11770 (GRH) and NAG5-9414 (CMH). We thank the Center for Meteorite Studies at ASU, the Naturhistorisches Museum in Vienna, G. J. Wasserburg, and the Antarctic Meteorite Curatorial Facility for providing samples for this study. Reviews by Roy Lewis, Jamie Gilmour, Monica Grady, and an anonymous reviewer resulted in significant improvements to this paper.

Associate editor: M. M. Grady

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