Workshop on Parent-Body and Nebular Modification of Chondritic Materials 4019.pdf

PRESOLAR GRAINS AS TRACERS OF NEBULAR AND PARENT BODY PROCESS- ING OF CHONDRITIC MATERIAL. Gary R. Huss, Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, 170-25, California Institute of Tech- nology, Pasadena, CA 91125

Presolar grains such as diamond, SiC, These observations show that a single wide- graphite, and Al2O3 are present in the least al- spread reservoir of presolar grains was sam- tered members of all chondrite classes [1,2]. pled by all chondrite classes. This reservoir is These grains reside in the fine-grained “matrix” most plausibly the dust from the solar system’s and did not experience the chondrule-forming parent molecular cloud. process. Because most types of presolar grains The CI chondrite, Orgueil, and the matrices are chemically unstable in putative nebular of CM2 chondrites have the highest matrix- conditions and in chondritic meteorites, they normalized abundances of silicon carbide, and serve as sensitive monitors of nebular and par- graphite; have among the highest diamond ent-body conditions. abundances; and have diamonds with high ni- Tracers of Metamorphism: Isotopic and trogen contents and the highest contents of P3 trace element characteristics of some presolar noble gases [1-4]. These characteristics indi- material and the abundances of presolar grains cate that CI chondrites and the matrices of in “matrix” are functions of the metamorphic CM2 chondrites contain the least processed history of the host meteorite [1–4]. Currently sample of the presolar-grains reservoir from recognized presolar grains show a range of the sun’s parent molecular cloud. The CI resistance to metamorphism. If all meteorite chondrites and the CM2 matrices also have the classes acquired similar initial mixtures of pre- bulk chemical compositions that are most like solar grains, then the relative abundances of the composition of the sun [e.g., 9, 10]. As the the different types of presolar grains can be bulk compositions of the meteorite classes be- used as probes of metamorphism in the host come more fractionated relative to the CI meteorite. In most meteorite classes, graphite composition, the abundances and characteris- and the diamond fraction that carries P3 noble tics of presolar grains also become increasingly gases are least resistant; SiC is moderately re- different from those in CI chondrites. For ex- sistant; and diamond and Al2O3 are most resis- ample, EH chondrites are much enriched in tant to parent-body metamorphism. However, total iron and are much more reduced than CI in the highly reduced EH chondrites, SiC and chondrites. The EH chondrites also acquired diamond are both more stable, and SiC be- higher matrix-normalized abundances of dia- comes more resistant to metamorphism than mond and SiC than CI chondrites and no de- diamond [1, 2]. These relationships provide a tectable graphite [1,2]. The CV3 chondrites relative scale of metamorphic intensity. An ab- are enriched in refractory elements such as solute temperature scale for low-grade meta- calcium and aluminum and in 16O relative to morphism may be provided by the P3 noble- CI chondrites. They also have higher diamond gas carrier in presolar diamonds, which ap- abundances and much lower SiC and graphite pears to release its gases over a limited tem- abundances than CI chondrites. In both of perature range, independent of the nature of these classes, the most resistant presolar grains the surroundings [3]. (diamonds and, under reducing conditions, Tracers of Nebular Processing: In the SiC) have higher abundances in the more least metamorphosed meteorites of each class, chemically fractionated material, while the matrix-normalized abundances of presolar more reactive graphite and the carrier of P3 diamonds vary by only a factor of ~2.2 [1,2]. noble gases are strongly depleted. These corre- There is also increasingly compelling evidence lations suggest that the same processes were that each chondrite class acquired the same responsible for both the abundance differences mixtures of diamonds [3–5] and SiC [6–8]. Workshop on Parent-Body and Nebular Modification of Chondritic Materials 4019.pdf

PRESOLAR GRAINS AS TRACERS: G. R. Huss

and the differences in bulk composition and interstellar amorphous material and dirty ices. provide insight into those processes. Similar abundances of refractory presolar A nebular model in which the bulk compo- grains would be expected in all classes because sitions of chondrite classes reflect different de- chondrites would be modified samples of a grees of nebular condensation [e.g., 9,11] is single initial mixture. Nebular evaporative inconsistent with presolar-grain abundance processing acting on the initial presolar mix- data. Presolar grains cannot survive in a solar- ture could have produced the chemical frac- composition gas hot enough to evaporate most tionations that define the meteorite classes nebular dust [e.g., 12]. If presolar grains repre- while at the same time producing the variations sent a minor component added to the nebula in abundances and properties of presolar grains after condensation, then very efficient mixing [1,2]. If so, then the characteristics and relative in the meteorite-formation region would have abundances of different kinds of presolar ma- been required to produce the near-constant terials in unmetamorphosed chondrites are diamond abundances in meteorite matrices. potential probes of nebular processing. This mixing would have to have taken place without destroying the chemical fractionations Acknowledgments: Supported by NASA produced by condensation. Not only is such a grants NAGW 3040 and 3296 to G. J. Was- mixing scenario unlikely, this model does not serburg. Caltech Div. Contrib. #5821 (965). predict or explain the observed correlations between bulk compositions of chondrites and References: [1] Huss G. R. and Lewis R. S. abundances and characteristics of presolar (1995) GCA, 59, 115–160. [2] Huss G. R. grains. Efficient mixing apparently occurred (1997) in Astrophysical Implications of the prior to nebular chemical fractionations and Laboratory Study of Presolar Grains, (T. Ber- processing of the presolar-grains complex. If natowicz ed.), AIP Press. [3] Huss G. R. and so, then chondrite groups could not have re- Lewis R. S. (1994) Meteoritics, 28, 811–829. sulted from partial condensation of hot nebular [4] Russell S. S. et al. (1996) Meteoritics and gas followed by isolation of the condensates Planet. Sci., 31, 343–355. [5] Huss G. R. and from the nebula [e.g., 9, 11]. Lewis R. S. (1994) Meteoritics, 28, 791–810. The currently known presolar grains are [6] Huss G. R., Hutcheon I. D. and Wasser- only the most recognizable part of a much burg G. J. (1997) GCA, 61, submitted. [7] larger complex of presolar material. This pre- Huss G. R. et al. (1994) LPSC XXV, 585–586. solar dust, processed in various ways in the [8] Huss G. R. et al. (1995) LPSC XXVI, 645– solar system, but not evaporated and recon- 646. [9] Larimer J. W. and Anders E. (1967) densed, may have been the principal raw ma- GCA, 31, 1239–1270. [10] Anders E. and terial for meteorites and planets. Bulk presolar Grevesse N. (1989) GCA, 53, 197–214. [11] matter would have had CI (= solar) bulk com- Grossman L. and Larimer J. W. (1974) Rev. position, with the elements partitioned between Geophys. Space Phys., 12, 71–101. [12] Men- different kinds of solids ranging from stellar dybaev R. A. et al. (1997) LPSC XXVII, 937– condensates like SiC and graphite to 938.