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Chondritic Meteorites and the High-Temperature Nebular Origins of Their Components

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Chondritic Meteorites and the High-Temperature Nebular Origins of Their Components Chondrites and the Protoplanetary Disk ASP Conference Series, Vol. 341, 2005 A. N. Krot, E. R. D. Scott, & B. Reipurth, eds. Chondritic Meteorites and the High-Temperature Nebular Origins of Their Components Edward R. D. Scott and Alexander N. Krot Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA Abstract. We present an overview of the chondrite groups focusing on the major con- straints that can be derived on thermal history of silicates in the solar nebula from the min- eralogical, chemical, and isotopic properties of the components in the least-altered chon- drites. Recent advances in developing a chronology for the formation of CAIs and chon- drules and plausible models for their oxygen isotopic compositions provide the basis for understanding their origin. Evidence from short-lived and long-lived isotopes, oxygen iso- topes, nuclear isotopic effects and petrologic studies all suggest that refractory inclusions and grains were the first solids to form in the protosolar disk, probably within a period of <0.3 Myr, when the protosun was accreting rapidly (possibly as a class 0 or I protostar). Refractory inclusions formed in an 16O-rich, reducing environment of near-solar composi- tion, <10-4 bar pressure, and temperatures >1300 K. Most chondrules appear to have formed 1-3 Myr after refractory inclusions, when the protosun was accreting more slowly. Chondrules in a single chondrite group probably formed over a much shorter period. Sev- eral types of chondrules formed under diverse conditions that were generally more oxidiz- ing with lower ambient temperatures and higher total pressures or high dust/gas ratios, so that liquids were stable for hours. Formation of type I chondrules involved melting, evapo- ration, condensation, and accretion of solid, partly melted and completely melted materials. Chondrite matrices are mixtures of materials that probably formed in diverse locations in the solar nebula and traces of presolar materials. Matrices in pristine carbonaceous chon- drites are largely composed of crystalline Mg-rich silicates (forsterite and enstatite) and amorphous Fe-Mg silicate. The cooling rates, composition, and structure of the Mg-rich silicates suggest that they probably condensed during heating events that formed chon- drules. The near-solar composition of the matrix implies that the amorphous silicates have similar origins and that gas and dust were not separated during condensation. Comets and chondritic, porous interplanetary dust particles have more pre-solar material than chon- drites, but also contain abundant forsterite and enstatite crystals resembling those in matri- ces of primitive chondrite matrices. The associated amorphous silicate is Fe-rich suggesting that the Mg-rich silicates may have formed by nebular condensation rather than by anneal- ing. Since forsterite and enstatite are abundant around many protostars, the processes that heated silicate dust in the solar nebula may be common to other protostellar disks. 15 16 Scott and Krot 1. Introduction Chondrites are the meteorites that provide the best clues to the composition and ori- gin of the solar system. They are derived from the subset of asteroids that did not melt and are capable of supplying tough rocks that can survive the journey to Earth and atmospheric entry. Additional clues come from the porous, anhydrous, inter- planetary dust particles (IDPs), which have broadly chondritic compositions and probably come from comets. Chondrites and the chondritic, porous IDPs both contain a large fraction of crystalline silicates that formed at high temperatures. Since the crystalline silicates, with few exceptions, have solar isotopic compositions and 99.8±0.2% by mass of the interstellar silicates are amorphous (Kemper, Vriend, & Tielens, 2004), we can infer that nearly all of the crystalline silicates in asteroids and comets formed in the solar system. Our purpose here is to review with a minimum of jargon what can be inferred from chondrites, and to a much lesser extent, comets, about the high-temperature processes in the protoplanetary disk (or solar nebula) that converted amorphous silicate into crystals. Chondrules, which are the major constituents of most chondrites, are roughly millimeter-sized particles that were wholly or partly molten in the solar nebula and crystallized in minutes to hours between ~1800 and ~1300 K prior to accretion (Rubin 2000; Zanda 2004; Jones, Grossman, & Rubin, this volume). The two major minerals that crystallized in chondrules are olivine, (MgxFe1-x)2SiO4, and low-Ca py- roxene, MgxFe1-xSiO3, where x is the Mg/(Mg+Fe) ratio. The magnesian end- members are called forsterite, Mg2SiO4, and enstatite, MgSiO3. Olivine and low-Ca pyroxene are also major minerals in the chondrite matrix—the fine-grained silicate material that coats chondrules and other coarse chondritic ingredients and fills the interstices between them (Scott & Krot, 2003; Nuth et al. and Huss et al., this vol- ume). The other important ingredient of chondrites are refractory inclusions, which are composed almost entirely of crystalline silicates and oxides that are rich in Ca, Al, and Ti and formed above 1300 K (MacPherson, 2003; MacPherson et al., this volume). Since Wood (1962, 1963) first proposed that chondrules formed in the solar nebula and were a major component of the material that accreted into the terrestrial planets, there have been three major workshops devoted to understanding chondrule origins. The workshop in Kauai in 2004 differed from the first two as a broader range of questions was addressed including the role of high-temperature processing in the formation of all the ingredients in chondrites, the detailed timescales for the forma- tion of chondrules and refractory inclusions, constraints from grains that formed around other stars, and relationships between chondrites, comets and chondritic IDPs. The Kauai workshop also benefited considerably from vastly improved astronomical and astrophysical insights into star formation. Here, we review the major constraints from chondrites on the high temperature processing of silicates in the solar nebula prior to their accretion into planetesimals. We review the mineralogy of the refractory inclusions, chondrules, and matrices, the chondrite groups, the chemical and isotopic compositions of chondrites and their components, and attempt to develop a plausible scenario for their formation. We con- clude that thermally processed dust was ubiquitous in the solar nebula and was a ma- jor ingredient in asteroids and comets. Chondritic Meteorites and Their Components 17 2. Chondritic Components The nature and abundances of the three major chondritic ingredients—refractory in- clusions, chondrules and matrix material—vary widely among the different chondrite groups. Table 1 lists the proportions of these ingredients in each group, the mean Table 1. Concentrations of chondritic components in the chondrite groups. Group Type Refract. Chondr. Chondr. Fe,Ni Matrix Fall freq. Refract. Examples incls. (vol.%)+ mean metal (vol.%)§ (%)^ lith./Mg (vol. %) diam. (vol.%) rel. CI# (mm) Carbonceous CI 1 <0.01 <5 - <0.01 95 0.5 1.00 Orgueil CM 1-2 5 20 0.3 0.1 70 1.6 1.15 Murchison CO 3 13 40 0.15 1-5 30 0.5 1.13 Ornans CV 2-3 10 45 1.0 0-5 40 0.6 1.35 Vigarano, Allende CK 3-6 4 15 0.8 <0.01 75 0.2 1.21 Karoonda CR 1-2 0.5 50-60 0.7 5-8 30-50 0.3 1.03 Renazzo CH 3 0.1 ~70 0.05 20 5 0 1.00 ALH 85085 CBa 3 <0.1 40 ~5 60 <5 0 1.0 Bencubbin CBb 3 <0.1 30 ~0.5 70 <5 0 1.4 QUE 94411 Ordinary H 3-6 0.01-0.2 60-80 0.3 8 10-15 34.4 0.93 Dhajala L 3-6 <0.1 60-80 0.5 3 10-15 38.1 0.94 Khohar LL 3-6 <0.1 60-80 0.6 1.5 10-15 7.8 0.90 Semarkona Enstatite EH 3-6 <0.1 60-80 0.2 8 <0.1-10 0.9 0.87 Qingzhen EL 3-6 <0.1 60-80 0.6 15 <0.1-10 0.8 0.83 Hvittis Other K 3 <0.1 20-30 0.6 6-9 70 0.1 0.9 Kakangari R 3-6 <0.1 >40 0.4 <0.1 35 0.1 0.95 Rumuruti Sources of data: Scott & Krot (2003) and references listed therein. ALH = Allan Hills; QUE = Queen Alexandra Range. # Mean ratio of refractory lithophiles relative to Mg, normalized to CI chondrites. + Includes chondrule fragments and silicates inferred to be fragments of chondrites. ^ Fall frequencies based on 918 falls of differentiated meteorites and classified chondrites (Grady 2000). § Includes matrix-rich rock fragments, which account for all the matrix in CH and CB chondrites. chondrule sizes, and the abundances of Fe,Ni metal grains, which are located within chondrules or probably formed with them. All but two of the 15 chondrite groups fall into the ordinary, carbonaceous, or enstatite classes (Table 1). A critical step in understanding the origin of chondritic components was to iden- tify the effects of metamorphism, aqueous alteration, shock, and brecciation in aster- oids and to establish which chondrites could have been derived from a common source (Wood 1962; Zolensky & McSween 1988; Scott et al. 1989; Scott 2002). Van Schmus & Wood (1967) inferred that most chondrites had been heated in asteroids 18 Scott and Krot and devised various mineralogical and chemical criteria to divide the chondrite groups into six metamorphic (or petrologic) types (Keil, this volume). These criteria and the development of the electron microprobe quickly led to the establishment of a small group of type 3 chondrites as the least equilibrated or metamorphosed of the ordinary chondrites and the precursors to the strongly metamorphosed types 4-6 (Dodd et al.
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