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1.07 Chondrites and Their Components E.R.D 1.07 Chondrites and Their Components E.R.D. Scott and A.N. Krot University of Hawai’i at Manoa, Honolulu, HI, USA 1.07.1 INTRODUCTION 2 1.07.1.1 What Are Chondrites? 2 1.07.1.2 Why Study Chondrites? 2 1.07.1.3 Historical Views on Chondrite Origins 3 1.07.1.4 Chondrites and the Solar Nebula 4 1.07.2 CLASSIFICATION AND PARENT BODIES OF CHONDRITES 4 1.07.2.1 Chondrite Groups, Clans, and Parent Bodies 4 1.07.2.2 Ordinary Chondrites 7 1.07.2.3 Carbonaceous Chondrites 7 1.07.2.4 Enstatite Chondrites 7 1.07.3 BULK COMPOSITION OF CHONDRITES 8 1.07.3.1 Cosmochemical Classification of Elements 8 1.07.3.2 Chemical Compositions of Chondrites 8 1.07.3.3 Isotopic Compositions of Chondrites 10 1.07.3.4 Oxygen-Isotopic Compositions 10 1.07.4 METAMORPHISM, ALTERATION, AND IMPACT PROCESSING 11 1.07.4.1 Petrologic Types 11 1.07.4.1.1 Type 1 and 2 chondrites 11 1.07.4.1.2 Type 3 chondrites 12 1.07.4.1.3 Type 4–6 chondrites 13 1.07.4.2 Thermal History and Modeling 14 1.07.4.3 Impact Processing of Chondritic Asteroids 14 1.07.5 CHONDRITIC COMPONENTS 15 1.07.5.1 Calcium- and Aluminum-Rich Inclusions 15 1.07.5.1.1 Comparison of CAIs from different chondrite groups 16 1.07.5.1.2 Oxygen-isotopic compositions of CAIs 17 1.07.5.2 Forsterite-Rich Accretionary Rims around CAIs 19 1.07.5.3 Amoeboid Olivine Aggregates 20 1.07.5.3.1 Mineralogy and petrology of AOAs 21 1.07.5.3.2 Trace elements and isotopic composition of AOAs 22 1.07.5.3.3 Origin of amoeboid olivine aggregates 25 1.07.5.4 Aluminum-Rich Chondrules 25 1.07.5.4.1 Isotopic and trace element studies of aluminum-rich chondrules 25 1.07.5.4.2 Origin of aluminum-rich chondrules 27 1.07.5.4.3 Relict CAIs and AOAs in ferromagnesian chondrules 29 1.07.5.4.4 Relict chondrules in igneous CAIs and igneous CAIs overgrown by chondrule material 31 1.07.5.5 Refractory Forsterite-Rich Objects 33 1.07.5.6 Chondrules 34 1.07.5.6.1 Chondrule textures, types, and thermal histories 34 1.07.5.6.2 Compositions of chondrules 36 1.07.5.6.3 Compound chondrules, relict grains, and chondrule rims 37 1.07.5.6.4 Closed-system crystallization 38 1.07.5.6.5 Open-system crystallization 38 1.07.5.6.6 Chondrules that formed on asteroids 39 1.07.5.7 Metal and Troilite 40 1.07.5.7.1 CR chondrite metallic Fe,Ni 41 1.07.5.7.2 CH and CB chondrite metallic Fe,Ni 42 1.07.5.7.3 Troilite 43 1 2 Chondrites and Their Components 1.07.5.8 Matrix Material 43 1.07.5.8.1 CI1 chondrites 45 1.07.5.8.2 CM2 chondrite matrices 46 1.07.5.8.3 CR2 chondrite matrices 46 1.07.5.8.4 CO3 chondrite matrices 46 1.07.5.8.5 CK3 chondrite matrices 50 1.07.5.8.6 CV3 chondrite matrices 50 1.07.5.8.7 Matrix of ungrouped C chondrites, Acfer 094, and Adelaide 51 1.07.5.8.8 H–L–LL3 chondrite matrices 51 1.07.5.8.9 K3 chondrite matrix (Kakangari) 52 1.07.5.8.10 Heavily altered, matrix-rich lithic clasts 52 1.07.5.8.11 Origins of matrix phases 53 1.07.6 FORMATION AND ACCRETION OF CHONDRITIC COMPONENTS 55 1.07.7 HEATING MECHANISMS IN THE EARLY SOLAR SYSTEM 55 1.07.7.1 Nebular Shocks 56 1.07.7.2 Jets and Outflows 56 1.07.7.3 Impacts on Planetesimals 58 ACKNOWLEDGMENTS 58 REFERENCES 59 1.07.1 INTRODUCTION rounded or droplet chondrules in the so-called ordinary chondrites (Figure 1a), studies of the 1.07.1.1 What Are Chondrites? origin of chondrules have commonly been Chondrites are meteorites that provide the based on these chondrites. best clues to the origin of the solar system. They Chondrites contain diverse proportions of are the oldest known rocks—their components three other components: refractory inclusions formed during the birth of the solar system (0.01–10 vol.%), metallic Fe,Ni (o0.1–70%), and ca. 4,567 Ma—and their abundances of non- matrix material (1–80%). Refractory inclusions volatile elements are close to those in the solar are tens of micrometers to centimeters in dimen- photosphere. Chondrites are broadly ultramafic sions, lack volatile elements, and are the products in composition, consisting largely of iron, of high-temperature processes including conden- magnesium, silicon, and oxygen. The most sation, evaporation, and melting. Two types are abundant constituents of chondrites are chond- recognized: calcium- and aluminum-rich inclu- rules, which are igneous particles that crystal- sions or Ca–Al-rich inclusions (CAIs), and am- lized rapidly in minutes to hours. They are oeboid olivine aggregates (AOA). CAIs are composed largely of olivine and pyroxene, composed of minerals such as spinel, melilite, commonly contain metallic Fe,Ni and are hibonite, perovskite, and Al–Ti-diopside,which 0.01–10 mm in size. Some chondrules are are absent in other chondritic components (see rounded as they were once entirely molten but Chapter 1.08). Amoeboid olivine aggregates many are irregular in shape because they were consist of fine-grained olivine, Fe,Ni metal, and a refractory component largely composed only partly melted or because they accreted of aluminum-diopside, anorthite, spinel, and other particles as they solidified. Chondrites rare melilite. Grains of metallic Fe,Ni occur in- themselves were never molten. The definition of side and outside the chondrules as grains up to a a chondrite has expanded recently with the millimeter in size and, like the chondrules and discovery in Antarctica and the Sahara Desert refractory inclusions, formed at high tempera- of extraordinary meteorites with chondrules tures. Matrix material is volatile-rich, and fine- 10–100 mm in size, and chondrites so rich in grained (10 nm–5 mm) and forms rims on other metallic Fe,Ni that they were initially classified components and fills the interstices between as iron meteorites with silicate inclusions. Thus, them. Chondrite matrices have diverse miner- in meteoritics, as in other fields of planetary alogies: most are disequilibrium mixtures of hy- science, new discoveries sometimes require drated and anhydrous silicates, oxides, metallic definitions to be modified. Fe–Ni, sulfides, and organic material and con- Chondrites are so diverse in their miner- tain rare presolar grains. alogical and textural characteristics that it is not possible to describe a typical chondrite. We 1.07.1.2 Why Study Chondrites? show one with diversely textured chondrules including prominent, esthetically pleasing, Goldschmidt, Suess, and Urey showed that rounded chondrules (Figure 1a), and another chondrites provide the best estimates for the with more uniformly textured chondrules mean abundances of condensable elements in (Figure 1b). Owing to the high abundance of the solar system. These estimates were essential Introduction 3 Mg K PCA91082, CR geological processes including impact processes that affected asteroids over 4.5 Ga. Studies of chondrites help us to match chondrite groups with asteroid classes, to understand the origin and evolution of the asteroid belt, the nature of POP POP planetesimals that accreted into terrestrial planets, and the reason for the dearth of plan- etary material between Mars and Jupiter. Fi- POII nally, chondrite studies help us to understand POI the physical and mineralogical structure of un- melted asteroids and to assess what should be done about rogue near-Earth objects that 1 mm PO (a) I threaten Earth. Mg K Tieschitz, H/L3.6 In this chapter, we focus on the insights that chondrites offer into the earliest stages in PO I the formation of asteroids and planetesimals in the solar nebula—the protoplanetary disk POP of dust and gas that evolved into the plane- tary system. Other chapters review many other BO RP aspects of chondrite studies. Since the first edition of the Treatise on Geochemistry, there have been considerable advances in understan- ding the chronology of chondrule and CAI C PO II formation by reconciling data from short- and PP long-lived isotopes (see Kita et al., 2005 and Chapter 1.16), possible mechanisms responsible for the oxygen-isotopic anomalies in chond- rites, and in the use of detailed isotopic meas- urements to distinguish and date nebula and 1 mm (b) parent body alteration effects. A surprise to Figure 1 Maps showing magnesium concentrations many is the discovery that planetesimals al- in two chondrites: (a) PCA91082, a CR2 carbon- ready existed in the nebula when chondrules aceous chondrite, and (b) Tieschitz, an H/L3.6 were forming, and that in some cases at least, ordinary chondrite. In CR chondrites, as in most chondrules may have formed from preexisting carbonaceous chondrites, nearly all chondrules have bodies (Kleine et al., 2005; Hevey and Sanders, porphyritic textures and are composed largely of 2006; Krot et al., 2005a). Comets too are not forsterite (white grains), enstatite (gray), and metal- as pristine as once believed. Chondritic porous lic Fe,Ni (black). The subscripts show type I chond- interplanetary dust particles (IDPs), like pris- rules, which are common, and type II, which are tine chondrite matrices, contain abundant FeO-rich and rare in this chondrite. Tieschitz, like forsterite and enstatite grains that lack large other ordinary chondrites, is composed of all kinds of chondrules with diverse FeO concentrations.
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