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Home , Chondrite, Chondrule, Enstatite chondrite, Grossite, H chondrite, L chondrite

1.07 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 Origins 3 1.07.1.4 Chondrites and the Solar 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 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 -Isotopic Compositions 10 1.07.4 , 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 14 1.07.5 CHONDRITIC COMPONENTS 15 1.07.5.1 - 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 -Rich Accretionary Rims around CAIs 19 1.07.5.3 Amoeboid Aggregates 20 1.07.5.3.1 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 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 material 31 1.07.5.5 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 and 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 OF CHONDRITIC COMPONENTS 55 1.07.7 HEATING MECHANISMS IN THE EARLY 55 1.07.7.1 Nebular Shocks 56 1.07.7.2 Jets and Outflows 56 1.07.7.3 Impacts on 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 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- . Chondrites are broadly ultramafic sions, lack volatile elements, and are the products in composition, consisting largely of , of high-temperature processes including conden- magnesium, silicon, and oxygen. The most sation, evaporation, and . 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 , composed of minerals such as , , commonly contain metallic Fe,Ni and are , , 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, , 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 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 , 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 . 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 classes, to understand the origin and evolution of the , the nature of POP POP planetesimals that accreted into terrestrial , and the reason for the dearth of plan- etary material between and . 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- 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 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 , 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 (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) 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 - 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, . 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). 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. Key isotopic anomalies, showing that most silicates to chondrule types: BO, barred olivine; C, crypto- in comets were thermally processed in the crystalline, PO, porphyritic olivine; POP, porphyritic solar nebula (Messenger et al.,2003; Wooden olivine-pyroxene; PP, porphyritic pyroxene; RP, et al.,2005). radial pyroxene. These maps were made with an electron microprobe from Mg Ka X-rays. 1.07.1.3 Historical Views on Chondrite Origins for developing theories for the formation of elements in evolved (see Chapters 1.01 At the beginning of the space age in the and 1.03). Presolar grains (see Chapter 1.02) 1960s, there were two views about the origin of provide additional clues to nucleosynthesis and chondrules and the variations in the concen- the subsequent growth of circumstellar grains. trations of the most volatile elements in chond- Chondrules, metal grains, refractory inclusions, rites. Ringwood, Urey, and others invoked and matrix materials formed under very diverse volcanism, impacts, and other planetary proc- conditions in the solar system and appear to esses, whereas Wood, Larimer, and Anders offer insights into processes that occurred invoked processes in the solar nebula (Wood, during the formation of the and planets 1963; Larimer, 1967; Larimer and Anders, 1967). from a collapsing cloud of interstellar dust and In the early 1970s, studies of CAIs by Grossman gas. The rocks themselves provide clues to the and others appeared to strongly favor an origin 4 Chondrites and Their Components for chondritic components as condensates from nebula location. However, it now appears that the solar nebula (Grossman and Larimer, 1974). chondritic components formed at various loca- However, this simple picture was disrupted by tions over a period of several million discoveries of ubiquitous isotopic anomalies in during which the nebula mass would have CAIs and the complexity of the mineralogical decreased by an order of magnitude or more. and isotopic record in chondritic components, Refractory inclusions probably formed close to which seemed inconsistent with the standard the protosun at an early stage when the disk model of a hot, monotonically cooling solar was most active and accretion rates onto the nebula. In addition, astronomical evidence and protosun were highest (Wood, 2000b, 2004). theoretical studies suggested that the solar Chondrules probably formed over several mil- nebula was not hot enough to vaporize silicates lion years in localized heating events near their where chondrites formed. The solar nebula was accretion site with the youngest and oldest envisaged as a place where there were dynamic, chondrules forming and accreting by different energetic, dust-rich zones with constantly mechanisms (see Kita et al.,2005; Scott and changing dust/gas ratios and fluctuating high Krot, 2005b; Krot et al., 2005a). However, temperatures. ‘‘Mineral material caught in this some chondrules, such as CAIs, may have maelstrom went through multiple cycles of formed at the inner edge of the disk prior to melting, evaporation, recondensation, crystal- injection into the asteroid belt by bipolar out- lization, and aggregation’’ (Wood, 1988). flows (Liffman and Brown, 1996a, b; Shu et al., Since the mid-1980s, much progress has been 1996, 2001). Alternatively, Alexander et al. made in understanding chondrites, though the (2001) have argued that CAIs and chondrules origin of their components has not been both formed in localized heating events in the resolved. Discoveries of new kinds of carbon- asteroid belt. aceous chondrites have shown that the Allende Astronomical studies of -forming regions , which is the most-studied chondrite, have provided considerable insights into the is not as pristine as once believed. Its compo- early evolution and dynamic environment of nents were modified in asteroids (e.g., Krot the Sun and its protoplanetary disk and et al., 1995, 1998b) or the solar nebula (e.g., possible origins for components in chondrites Palme and Fegley, 1990; Weisberg and Prinz, and comets (Reipurth, 2005; Alexander, 2005). 1998). Detailed isotopic and chemical data for However, the 71 difference between the plane- CAIs and chondrules from ion microprobe tary and solar angular momentum vectors studies have been invaluable in understanding (Hutchison et al., 2001), the multiplicity of how and where chondritic components were protostellar systems, and the discovery of altered, but have not yet resolved their origin. numerous ‘‘hot ’’ around other stars Most authors now accept that CAIs and most provide additional reminders of the complexity chondrules formed in some kind of solar nebula of disk evolution. environment, and several promising models have been developed, but some workers argue that planetesimals were essential for the forma- tion of chondrules (e.g., Sanders, 1996; Lugmair 1.07.2 CLASSIFICATION AND PARENT and Shukolyukov, 2001; Hsu et al.,2000). To BODIES OF CHONDRITES meteoriticists like Wood, who argued in 1963 1.07.2.1 Chondrite Groups, Clans, and that chondrites were aggregates of solar nebular Parent Bodies condensates, it has seemed that chondrite re- search has stagnated and that little progress has Many thousands of chondrites have been been made in developing plausible theories for classified into 15 groups: about 15 other chond- the origins of chondrules, CAIs, and chondrites rites do not fit comfortably into these groups (Kerr, 2001). However, most find the excitement and are called ungrouped (Table 1). Thirteen of of studying presolar grains, solar nebula parti- these groups comprise three classes: carbon- cles, and annual batches of new types of meteo- aceous (CI, CM, CO, CV, CR, CH, CB, and rites from Antarctica to be irresistible (Bischoff, CK), ordinary (H, L, and LL), and enstatite 2001). (EH and EL). The K and R chondrites do not belong to the three classes (see Chapter 1.05). Chondrites are also classified into petrologic 1.07.1.4 Chondrites and the Solar Nebula types 1–6, which indicate the extent of aster- oidal processing (see table 2 of Chapter 1.05). Chondrites used to be considered as products Type 3 chondrites are the least metamorphosed of a ‘‘minimum’’ solar nebula (containing just and least altered; type 6 are the most meta- enough material to make the planets) that morphosed. Type 1 chondrites, which lack accreted from components formed at a single chondrules, are composed almost entirely of Table 1 Abundances of refractory inclusions, chondrules, metallic Fe,Ni, and matrix and other key properties of the chondrite groups. Group Refract. lith./Mg CAI and AOA Chondrule Chondrules Metal Matrix Fall frequencyd Examples rel. CI a ðvol:%Þ average diameter ðvol:%Þb ðvol:%Þ ðvol:%Þc ð%Þ ðmmÞ Carbonaceous CI 1.00 o0.01 none o5 o0.01 95 0.5 Ivuna, CM 1.15 5 0.3 20 0.1 70 1.6 Murchison CO 1.13 13 0.15 40 1–5 30 0.5 CV 1.35 10 1.0 45 0–5 40 0.6 Vigarano, Allende CR 1.03 0.5 0.7 50–60 5–8 30–50 0.3 Renazzo CH 1.00 0.1 0.02–0.09 B70 20 5 0 ALH 85085 CBa 1.0 o0.1 B54060 o5 0 Bencubbin CBb 1.4 o0.1 B0.5 30 70 o5 0 QUE 94411 CK 1.21 4 0.8 15 o0.01 75 0.2 Karoonda Ordinary H 0.93 0.01–0.2 0.3 60–80 8 10–15 34.4 Dhajala L 0.94 o0.1 0.5 60–80 3 10–15 38.1 Khohar LL 0.90 o0.1 0.6 60–80 1.5 10–15 7.8 Semarkona Enstatite EH 0.87 o0.1 0.2 60–80 8 o0.1–10 0.9 Qingzhen, EL 0.83 o0.1 0.6 60–80 15 o0.1–10 0.8 Hvittis Other K 0.9 o0.1 0.6 20–30 6–9 70 0.1 Kakangari R 0.95 o0.1 0.4 440 o0.1 35 0.1 Rumuruti Sources: Scott et al. (1996); other data from Weisberg et al. (1996, 2001), Rubin (2000), Krot et al. (2002a), Kimura et al. (2002), and Bischoff et al. (1993). a Mean ratio of refractory lithophiles relative to magnesium, normalized to CI chondrites. b Includes chondrule fragments and silicates inferred to be fragments of chondrules. c Includes matrix-rich clasts, which account d for all matrix in CH and CBb chondrites (Greshake et al., 2002). Fall frequencies based on 918 falls of differentiated meteorites and classified chondrites (Grady, 2000). 6 Chondrites and Their Components minerals that formed during aqueous altera- Chapter 1.06) and the distribution of iron tion. Type 2 chondrites are partly altered. among silicate, metal, and sulfide phases (figure Elemental ratios are commonly used for com- 7 of Chapter 1.05). The latter requires classical paring chondrite compositions instead of con- methods of rock analysis (Jarosewich, 1990), centrations, as some chondrites have excessive which are no longer applied to chondrite. Many amounts of one component such as water or chondrite groups can also be distinguished by metallic Fe,Ni, for example, that dilute elements other parameters such as the sizes and propor- in other components. The elements silicon and tions of their components and the minerals in magnesium are commonly used for normalizing their chondrules, CAIs, or other components. lithophiles; nickel is used for comparing side- See Brearley and Jones (1998) (hereafter ab- rophile elements. CI chondrites, which have the breviated B&J), for a detailed account of highest concentrations of volatile elements, are chondrite mineralogy. used as the reference standard because they The chemical, mineralogical, and isotopic provide the best match for the composition of properties of chondrites distinguish closely the solar photosphere, neglecting the incom- related groups, which constitute ‘‘clans.’’ For pletely condensed elements hydrogen, helium, example, chondrules in the H, L, and LL carbon, nitrogen, oxygen, and the noble gases groups are chemically and isotopically similar, (see Chapter 1.03). though their average properties are different, Ratios of refractory and moderately volatile and these three groups form a clan. Four other lithophile (figure 3 in Chapter 1.05) or siderophile clans are recognized: the EH–EL, the CM–CO, elements (Figure 2)providethebestcriteriafor the CV–CK, and the CR–CH–CB. classifying chondrites. On these plots, the range Cosmic-ray exposure ages suggest that within each group is a small fraction of the total chondrites in many groups were exposed to range shown by all chondrites. Each group has a space as meter-sized objects in a limited unique chemical composition implying that its number of impacts (Chapter 1.13). For exam- members formed in a separate location. The CB ple, about one-third of the LL chondrites were group, which is split into two subgroups with exposed to cosmic rays 15 Ma and half the rather different properties, is an exception. Un- H chondrites have exposure ages of 7 Ma (Graf fortunately, many newly discovered, unusual and Marti, 1994, 1995). Three or four large chondrites have not been chemically analyzed, collisions appear to have generated two-thirds so their classification and the compositional of all H chondrites (Wieler, 2002). Since all limits of newly discovered groups are not well petrologic types are found in the 7 and 33 Ma defined. Two other properties are useful for peaks, it is possible that all H chondrites come classifying chondrites and understanding their from a single body. Some other groups show compositional diversity and genetic relation- distinctive impact ages or shock properties, ships: bulk oxygen-isotopic compositions (see suggesting that they were involved in a single impact. For example, many L chondrites, which are strongly shocked, have radiometric ages of B0.5 Ga (Bogard, 1995). These features suggest that each group comes from one, or possibly a 18 CV few bodies. CO We do not know for certain that two chond- 16 H CM rite groups did not originate from a single body but it is rather unlikely (except possibly for the 14 atom ratio

6 EH and EL chondrites). Evidence from mete- L Cl

10 orites, impact modeling of asteroid collisions,

× 12 EL LL and asteroid density determinations suggests

Ir/Ni that all asteroids, except the few largest, were 10 well fragmented and mixed by collisions before EH were removed. Impacts also weld 8 3456789 rock fragments from different locations into Ga/Ni × 104 atom ratio coherent rocks that reach Earth as meteorites. Thus, if CV and CO chondrites, for example, Figure 2 Plot of Ga/Ni versus Ir/Ni showing bulk had formed in a single body, we might expect compositions of chondrites in nine groups. Each to find meteorites containing both types of group is well resolved. The proportions of these siderophiles are not correlated with other chemical material. Their absence makes it unlikely that properties showing that the chemical variations in one body ever contained both groups. Never- chondrites are complex. Reproduced by permission theless, contain a few volume of Verlag der Zeitschrift fu¨ r Naturforschuang from percent of foreign clasts showing that small Scott and Newsom (1989). amounts of material from different sources Classification and Parent Bodies of Chondrites 7 were mixed together. The close proximity of the single, aqueously altered body. However, bulk of an EL6 and an H5 chondrite, which chemical differences between these groups indi- might imply that they have a common parent cate fractionation during nebular processes, not (Spurny et al., 2003), appears to be coincidental aqueous alteration (see below), and the compo- (Pauls and Gladman, 2005). nents in CM and CV chondrites are quite Given the extraordinary diversity of chemical different. and mineralogical properties of the 15 chondrite Most carbonaceous chondrites are thought groups and the rapid rate at which new groups to come from the low-albedo, C-type asteroids, are being identified (only nine were listed by which are the most abundant type between Wasson, 1985), we should expect many more 2.7 and 3.4 AU (Bell et al., 1989), CM chond- unusual types to be discovered. Our sampling of rites may be derived from an altered C-like chondrites is also biased toward tough rocks asteroid called G-type (Burbine et al.,2002). that can survive the journey to Earth, and we The ungrouped C chondrite, , has cannot expect these to be representative sam- been linked with D asteroids, which appear to ples. Thus, it is important to study components dominate the asteroid population beyond 4 AU in all kinds of chondrites if we need to under- (Jones et al., 1990; Hiroi et al., 2001). CB stand how they formed, not just those in the chondrites have only very minor amounts of most common chondrite groups or those that phyllosilicates and may come from W-type are present in the largest falls, like Allende. asteroids (‘‘wet-M’’ asteroids). Lodders and Osborne (1999) discuss the possibility that CM and CI chondrites could be derived from a 1.07.2.2 Ordinary Chondrites small fraction of comets that evolve into near- Earth objects after losing . A small Since ordinary chondrites account for B80% fraction of near-Earth asteroids are derived of all meteorite falls (Table 1), it was once from Jupiter-family comets, and Lodders and believed that their parent bodies were common Osborne (1999) suggest that they are supplying in the main asteroid belt. However, spectral us with CI and CM chondrites. Gounelle et al. studiesshowthatmostasteroidsaredarkand (2006) also favor a cometary origin for Orgueil featureless (C and related types), and most of meteorite based on its inferred . However, the brighter S types are spectrally unlike ordi- Campins and Swindle (1998) argue that come- nary chondrites. H group chondrites probably tary meteorites, if they exist, probably resemble come from one or more S-type asteroids, pos- carbon-rich, unaltered chondrites without sibly (Burbine et al.,2002). Ordinary chondrules. The extremely fragile nature of chondrites are probably rare in the main part of chondritic porous IDPs, which are probably the asteroid belt (Meibom and Clark, 1999), derived from comets (Rietmeijer, 1998, 2002; though a significant fraction of the near-Earth Chapter 1.26), suggests that larger cometary asteroids including Eros are probably made of meteoroids would not survive atmospheric en- ordinary chondritic material (Binzel et al.,2002; try (Messenger et al., 2006). Chapman, 2004). A few ordinary chondrites Although most comets clearly accreted far including Tieschitz (Figure 1a) do not fit com- beyond the asteroid belt, similarities between fortably into the H, L, and LL groups and may the matrices of the most primitive carbon- be derived from separate bodies. aceous chondrites and the chondritic porous IDPs (Section 1.07.5.8) and the discovery that 1.07.2.3 Carbonaceous Chondrites some C-type asteroids show cometary activity (Hsieh and Jewitt, 2006) have blurred the dis- Some carbonaceous chondrites are rich in tinction between comets and asteroids. carbon (CI and CM chondrites have 1.5–6% carbon), but others are not. Carbonaceous chondrites are now defined on the basis of their 1.07.2.4 Enstatite Chondrites refractory elemental abundances, which equal or exceed those in CI chondrites. Carbonaceous The genetic relationships among E chondrites chondrites are derived from very diverse aster- are poorly understood, in part because of poor oids, which probably formed in very different sampling but also because the cause of their locations. The parent bodies of CI and CM chemical and mineralogical diversity is obscure. chondrites are highly altered, yet the parent EH and EL chondrites may come from one bodies of CH and CB chondrites are less altered (Kong et al.,1997)ortwobodies(Keil, 1989). than all other chondrite bodies. Young et al. Although there are compositional discontinui- (1999) infer from oxygen-isotopic compositional ties between the EH and EL groups (e.g., in gold data that CI, CM, and CV chondrites could and aluminum), Kong et al. concluded that have been derived from different zones in a elemental concentrations appear to vary 8 Chondrites and Their Components continuously through the sequence EH4, 5, 1,450 and 1,350 K, magnesium and silicon con- EH3, EL3, and EL6. Oxygen-isotopic composi- dense as forsterite and enstatite and iron con- tions of EH and EL chondrites show a large denses as metallic Fe,Ni along with cobalt. overlap, but do not resolve the issue (Newton Between 1,250 and 650 K, the alkalis and mod- et al.,2000). On the basis of their erately volatile siderophiles condense and at analyses, Patzer and Schultz (2002b) advanced 650 K, iron reacts to form FeS. Below 650 K, an entirely different proposal that ‘‘EH3 and the highly volatile elements like the halogens EL3 chondrites are derived from one body and and inert gases condense. The division of the EH4–6 and EL4–6 from a second.’’ They found elements into (1,800–1,450 K), that E3 and E4–6 chondrite types have different major elements (1,350–1,250 K), moderately trapped noble gas components (called ‘‘Q- volatile elements (1,250–650 K), and highly gases’’), and inferred that this difference could volatile elements (o650 K) is the key to under- not be attributed to asteroidal processing. In standing the chemical variations among chond- addition, about one-third of the E3 chondrites rites and their components. Note that only three contain solar noble gases, which are absent phases condense entirely from the gas phase— among E4–6 chondrites. Unfortunately, the cos- Al2O3,Mg2SiO4, and Fe,Ni—all other minerals mic-ray exposure age data do not yet provide form by reaction between solids and gas. definitive constraints. The 60-odd E chondrites At 103 atm, FeO concentrations in silicates have a wide range of exposure ages, 0.07–70 Ma are minimal above 650 K and all condensed with broad peaks as their precision is B20% (cf. phases are solid. If the total pressure increases, 10% for ordinary chondrites) (Patzer and condensation temperatures rise, but the sequence Schultz, 2002a; Okazaki et al.,2000). However, in which minerals condense does not change the data are not inconsistent with a single E significantly except that silicate melts become chondrite body, as Kong et al. (1997) proposed. stable at pressures above B102 atm. If conden- E chondrites may be derived from one or more sation occurs in systems that have been enriched M-type asteroids (Burbine et al.,2002), which in dust relative to gas, FeO concentrations in sil- are concentrated in the inner asteroid belt at icates become appreciable at high temperatures 2–3 AU (Bell et al.,1989). (up to B1,100 K for 103-fold dust enrichments), and liquids are also stabilized (Wood and Hashi- moto, 1993; Ebel and Grossman, 2000). Pres- sures and temperatures at the midplane of the 1.07.3 BULK COMPOSITION OF nebula vary considerably as the solar nebula CHONDRITES evolves, decreasing as the accretion rate falls (Wood, 2000a; Woolum and Cassen, 1999). 1.07.3.1 Cosmochemical Classification of Additional insights into chondrite properties Elements can be gained from fractional condensation The chemical fractionations observed among models in which condensates are continually chondrites and the compositions of many sequestered (Petaev and Wood, 2000). chondritic components are best understood in terms of quenched equilibrium between phases in a nebula of solar composition (Palme, 1.07.3.2 Chemical Compositions of 2001; Chapters 1.03 and 1.15). The equilibrium Chondrites model assumes that minerals condensed from, or equilibrated with, a homogeneous solar nebula at A large number of chondrites have been diverse temperatures. Isotopic variations among analyzed with instrumental and radiochemical chondrites and their components show that this neutron activation analysis using B250–350 mg assumption is not correct and detailed petrologic samples of chondrites (Kallemeyn and Wasson, studies have identified relatively few chondritic 1981; Wasson and Kallemeyn, 1988; Kalle- components that resemble equilibrium nebular meyn et al., 1991, 1994, 1996). Samples of this products. Nevertheless, the equilibrium model size can also be analyzed for many elements is invaluable for understanding the chemical with comparable accuracy using X-ray fluores- composition of chondrites and their components cence analysis (Wolf and Palme, 2001). Mean as the solar nebular signature is etched deeply compositions of chondrite groups are given by into their chemistry and mineralogy. Lodders and Fegley (1998). In a cooling nebula at a pressure of 103 atm, Chemical variations among chondrites are calcium, aluminum, and titanium condense as commonly attributed to the accretion of dif- oxides in the temperature range 1,800–1,450 K ferent proportions of five components: the along with refractory trace elements like the refractory component, magnesium silicates, rare earth elements (REEs) and platinum group metallic Fe,Ni, moderately volatile elements, (figure 2 of Chapter 1.15). Between and highly volatile elements (see chapters in Bulk Composition of Chondrites 9 Kerridge and Matthews, 1988; Palme, 2000). present in CAIs is generally small, except for In addition, variations in oxygen fugacity CVs, where it approaches B50%. Chond- are invoked to explain the diverse distribution rules, which are much more abundant and of iron among metallic and silicate phases have approximately CI-like levels of refrac- (figure 7 of Chapter 1.05). In general, chemical tories relative to silicon, account for most of fractionation patterns are simple: refractory the refractory elements in chondrites. Note elements, for example, are uniformly enriched that the depletions of refractory elements relative to CI chondrites in carbonaceous chond- in E and O chondrites require that their rites by factors of 1.0–1.4, whereas they are de- chondrules formed from a condensate that pleted in ordinary and enstatite chondrites by was depleted in refractories. factors of 0.8–0.95 (Figure 3). Moderately vola- 2. Magnesium silicates. Forsterite and ens- tile elements decrease in abundance with dec- tatite, which are the major minerals in reasing condensation temperature. (Note that chondrules, form in the temperature range enstatite chondrites do not follow these rules as 1,450–1,350 K, the latter by reaction bet- closely as the other groups.) Although the chem- ween forsterite and gaseous SiO. Thus, equi- ical fractionation patterns for most groups of librium condensation can produce a range of chondrites are known (Wasson and Kallemeyn, molar enstatite/forsterite ratios of 0–1.2 1988), their origins have proved to be surpri- (figure 2 of Chapter 1.15), provided that singly elusive. In particular, the relative impor- the condensates are sequestered from the gas tance of localized processes that made over a range of temperatures. To reach the chondrules and CAIs and nebula-wide thermal composition of E chondrites, which are gradients is controversial (see Cassen, 2001a): almost pure enstatite, would require removal of forsterite condensate. A correlation be- 1. Refractory elements. All the elements that tween the Mg/Si and refractory/Si ratios of condense above 1,450 K are precisely those the chondrite groups suggests that refracto- found in CAIs, and there is some correla- ries were partly associated with forsterite tion between the bulk concentrations of (Larimer and Wasson, 1988). refractory elements and refractory inclusions 3. Metallic Fe,Ni. which condenses over the (Table 1). This suggests that the fraction- same temperature range as the magnesium ation of refractory elements among chond- silicates, is closely associated with them in rites is simply caused by addition or loss of chondrules. At 105 atm, Fe,Ni condenses at CAIs. However, this is incorrect as the pro- slightly lower temperatures than forsterite, portion of the refractory elements that is but above 104 atm there is a reversal. Some

Al Sc Ca Lu Sm La Yb Eu V Mg Si Cr Mn K Na 1.4

1.2

1.0 0.9 0.8

0.7

0.6

0.5 Group: CI abundance ratio CI abundance Group: CM H EH 0.4 CO L EL

CV Si-normalized 0.3 Figure 3 Mean abundances of lithophile elements normalized to CI chondrites and silicon arranged in order of increasing volatility in seven chondrite groups. Refractories (elements condensing above V) are uniformly enriched in CO, CM, and CV chondrites and depleted in H, L, and EH chondrites. Moderately volatile elements, which condense below magnesium and silicon, are all depleted relative to CI chondrites. These fractionations are related in poorly understood ways to the formation of CAIs and chondrules. Reproduced by permission of The Royal Society from Wasson and Kallemeyn (1988). 10 Chondrites and Their Components groups such as the CV3 chondrites did not in carbonaceous chondrites and smaller anom- suffer metal/silicate fractionation (figure 2 of alies in chondrules transformed the study of Chapter 1.15), but the EH–EL, H–L–LL, chondrites (see Clayton, 1993; Chapter 1.06). It and CR–CH–CB clans clearly did. Fe/Si ra- galvanized the search for presolar grains, tios are B(2–5) CI levels in CH and CB provided another way to classify meteorites, chondrites because metal was enriched re- and offered numerous constraints on the origin lative to silicate; EL, L, and LL chondrites of chondrites and their constituents and sub- were depleted in metallic Fe,Ni. sequent asteroidal processing. In particular, the 4. Moderately volatile elements. The gradual oxygen-isotopic anomalies appeared to eliminate depletion of moderately volatiles with dec- the conviction that the meteorites and inner reasing condensation temperature can be planets were derived from a hot, gaseous, and attributed to loss of fine volatile-rich dust homogeneous nebula (e.g., Begemann, 1980). (Wasson and Kallemeyn, 1988), incom- However, the origin of the oxygen anomalies plete condensation due to isolation of con- remains a mystery, presolar grains were not densed phases (Palme et al.,1988; Cassen, discovered by searching for the source of the 2001b), or to evaporation prior to or during anomalous oxygen (Ott, 1993), and the oxygen chondrule formation (see, e.g., Young, 2000; anomalies are now known to be associated with Lugmair and Shukolyukov, 2001; see also nebular condensates, as well as evaporative res- Chapter 1.15). idues (see Scott and Krot, 2001; Krot et al., 5. Highly volatile elements may have failed to 2002b). The recent introduction of ion and laser accrete from the nebula because of high microprobe techniques has produced an explo- ambient temperatures during accretion or sion of new oxygen-isotopic analyses of minerals inefficient accretion of volatile-rich dust, in CAIs and chondrules, which have been espe- or they may simply have been lost during cially valuable in distinguishing asteroidal and asteroidal metamorphism. nebula processes (e.g., Choi et al.,1998; Yuri- moto et al., 1998; Yurimoto and Wasson, 2002). Differences between the bulk compositions of 1.07.3.3 Isotopic Compositions of Chondrites chondrites, planets and asteroids can be attributed to accretion from different batches Major and minor planetary bodies are of CAIs, chondrules and other components, essentially uniform isotopically (e.g., Palme, which are spread along 16O variation lines on the 2001). The great majority of elements are iso- standard three- plot. The preservation of topically uniform to within B0.01%. The oxygen-isotopic anomalies shows that there were exceptions are found for oxygen (discussed numerous oxygen reservoirs for the manufacture below), effects due to short-lived isotopes of chondrules and CAIs that were quite separate. (Chapter 1.16), slightly larger effects (per mil or The source ofthe anomalies hasbeenattributed less) due to anomalies in 54Cr and 50Ti in chond- to nucleosynthetic differences between batches rites and their components (e.g., Podosek et al., of presolar materials (Wasson, 2000), mass- 1997; Niemeyer, 1988), and isotopically anoma- independent partitioning among oxygen-bearing lous components in chondrites: interstellar molecules in the nebular gas (Thiemens, 1999), grains, certain rare CAIs called FUN inclusions and photochemical self-shielding of CO at the in- (containing fractionated and unidentified nuclear ner edgeof the solar nebula (Clayton, 2002), in the isotopic effects) and some hibonite inclusions presolar molecular cloud (Yurimoto and Kura- (see Chapter 1.08). The isotopic anomalies in moto, 2002),orinthe surface of the nebula(Lyons interstellar grains are orders of magnitude larger and Young, 2005). Whatever the source, the 16O than those in refractory grains in chondrites variations in CAIs and chondrules imply that the and the latter probably reflect incomplete anomalies can be understood in terms of mixing homogenization of the former (e.g., Ott, 1993, of 16O-rich and 16O-poor components in the solar 2001). Depending on one’s viewpoint, the nebula. The anomalies could have been preserved isotopic anomalies provide evidence that by enriching 16O-rich dust relative to gas prior to vaporization of presolar materials was limited vaporization (Cassen, 2001b; Scott and Krot, (Taylor, 2001) or that isotopic homogenization 2001). However, in the self-shielding model, the was remarkably complete (Palme, 2001). 16O-poor component was anomalously heavy and the nebula was initially 16O-rich, like the protosun. Luck et al. (2003) inferred that 16O 63 1.07.3.4 Oxygen-Isotopic Compositions excesses were correlated with Cu-isotopic anomalies, but the reason for this is not clear. It The discovery of large and ubiquitous oxygen- is hoped that the NASA Genesis mission will isotopic anomalies in refractory inclusions determine the oxygen-isotopic composition of the Metamorphism, Alteration, and Impact Processing 11 solar wind and establish the origin of the oxygen to the extent and nature of asteroidal process- anomalies (Wiens et al.,1999; Clayton, 2002). ing. This classification scheme, which has been invaluable (Anders and Kerridge, 1988), never- theless provides only a very rough guide as our ideas about asteroidal processes have changed 1.07.4 METAMORPHISM, ALTERATION, significantly since the scheme was first estab- AND IMPACT PROCESSING lished, and because one number cannot sum- marize all the effects of complex asteroidal All chondrites were modified in some way by geological processes in asteroids operating processing. As a result, the classification is not rigorous and may be ambiguous or even over 4.5 Gyr. If we want to understand how misleading. the components in chondrites were formed, we must understand how chondritic materials were The original division of chondrites by Van Schmus and Wood (1967) into six petrologic modified in asteroids. Three processes affected types was based on two underlying assump- chondrites: aqueous and hydrothermal altera- tion, thermal metamorphism, and impacts. tions that are now known to be incorrect: (1) that type 1 chondrites are the least altered The most important heat source for the chondrites that best represent the mineralogy alteration and metamorphism of chondrites 26 of the original materials that accreted into was heat from the decay of Al, and to a much lesser extent, 60Fe (Chapter 1.16). If chondrites asteroids, and (2) that types 2–6 represent increasing degrees of thermal metamorphism of come from the first generation of - 26 material that mineralogically resembled type 1 esimals that formed, one could exclude Al as a heat source for melting planetesimals chondrites. Although CI1 chondrites are 182 182 ‘‘chemically’’ most pristine as they are compo- (Kunihiro et al., 2004a). However, Hf– W sitionally similar to the solar composition, dating and thermal modeling show that an earlier generation of bodies accreted 0.5 Myr almost all of their ‘‘minerals’’ formed during o aqueous alteration. Type 3 chondrites are clos- after CAIs formed and melted due to the short- est mineralogically to the original materials lived radioistopes forming cores that are the sources of most iron meteorites (Kleine et al., that accreted into asteroids (McSween, 1979). An additional problem with the division into 2005; Bizzarro et al.,2005; Scherste´ n et al., 2006). 26 six petrologic types is that it ignores the role Evidence that Al was adequately mixed in the played by impacts in modifying and mixing solar nebula comes from the consistent chronol- ogy provided by 26Al–26Mg and Pb–Pb dating chondritic material. Many chondrites are brec- cias of materials with diverse alteration or (Zinner and Go¨ pel, 2002; Kita et al., 2005). metamorphic histories. Thus the petrologic Impacts also provide kinetic energy but for asteroids smaller than a few hundred kilome- type may provide no information about the metamorphic history of the whole chondrite, ters in size, they are not an effective heat source only an approximate guide to the history of for global metamorphism (Keil et al., 1997). For alternative views, see Rubin (1995, 2005), most constituents. The criteria used for assigning petrologic who favors heating by asteroidal impacts, and types were first given by Van Schmus and Wood Kurat (1988), who argues for nebular heating processes. Electrical induction heating of aster- (1967) and have not changed significantly since oids by plasma outflow from the protosun is (table 3 of Chapter 1.05). Table 2 shows the numbers of classified chondrites in petrologic not currently favored (see Scott et al., 1989). types 1–6 within each group. The unrepresented Arguments against the concept of hot accretion without radiometric heating (Hutchison et al., cells in Table 2 may reflect poor sampling (e.g., in the poorly represented K group). However in 1979) were given by Haack et al. (1992) and many cases, it is likely that the missing types Rubin and Brearley (1996). Alteration and metamorphism used to be were volumetrically insignificant. It is very un- likely, for example, that the ordinary chondrite considered as separate processes: the former parent bodies ever contained significant affecting carbonaceous bodies and the latter operating on enstatite and ordinary chondrites. amounts of type 1 and 2 material. However, it is now clear that fluids of some kind were present during thermal processing of virtually all chondrite classes. 1.07.4.1.1 Type 1 and 2 chondrites There is much ambiguity about the classifi- 1.07.4.1 Petrologic Types cation of type 1 and 2 chondrites that are not clearly associated with the major groups, CI Chondrites are classified into petrologic and CM. Different workers favor different cri- (or petrographic) types 1–6 to provide a guide teria, so that type 1, for example, may mean a 12 Chondrites and Their Components Table 2 Numbers of classified chondrites in petrologic types 1–6 by group. 1 2 3 4 5 6 Total no. Carbonaceous CI 5 5 CM 4 44 48 CO 31 31 CV 1 35 36 CR 1 13 1 15 CH 7 7 CBa 33 CBb 22 CK 2 13 6 1 23 Ordinarya H 187 1,371 3,319 1,784 6,661 L 316 415 1,220 4,053 6,004 LL 71 64 419 406 960 Enstatite EH 18 9 6 2 35 EL 8 0 2 19 29 Other K2 2 R3214 9 R3–6

Sources: Based largely on lists in Grady (2000) with additional data from Krot et al. (2002a) for CR, CH, and CB and Kallemeyn et al. (1996) and Bischoff (2000) for R chondrites. A small number of chondrites that are classified as intermediate types (e.g., 3–4 and 4–5) and mixed breccias (e.g., types 3–5) were omitted except for the R chondrites, which are mostly breccias of type 3–5 or 3–6 material. a Numbers of H, L, and LL chondrites are italicized as they have not been corrected for probable pairings: the number of individual meteorites is probably B2–4 times smaller than the number of specimens. chondrite with major minerals similar to those which are divided into subtypes 3.0–3.9, show in CI1, or carbon-rich like CI1, or simply that correlated variations in mineralogy and concen- all the chondrules have been almost completely trations of carbon and highly volatile elements altered. Since the original CI and CM chond- that can largely be attributed to metamorphism. rites were studied (see Zolensky and McSween, Subtypes were first defined for ordinary chond- 1988), CM1 chondrites have been described rites based largely on their thermoluminescence (Zolensky et al., 1997), as well as several CI or sensitivity, which is a measure of CM chondrites that were metamorphosed after growth in chondrule mesostases (Sears and alteration but have not been assigned a pet- Hasan, 1987; Sears et al., 1991, 1995a). Many rologic type (B&J, 1998, pp. 70–71, 240–244). other properties vary systematically with CR2 chondrites, which contain abundant progressive metamorphism and have been used Fe,Ni, are the least altered type 1 and 2 chond- to assign subtypes, for example, chemical rites and offer exceptional insights into the changes in chondrule (Figure 4), the nature of nebular components. Renazzo was chemical homogenization of olivines, FeO/ the first to be recognized, but other CR2s in- (FeO þ MgO) ratios in matrices (Huss et al., cluding PCA 91082 (Figure 1a) appear to be 1981), loss of trapped noble gases and loss of the least altered (Wood, 1967; Krot et al., carbon (Sears and Hasan, 1987), chromium 2002c). GRO 95577, a type 1 chondrite, is the levels in metallic Fe,Ni grains (Scott and Jones, most altered CR (Weisberg and Prinz, 2000). 1990), and growth of ferroan olivine in amoe- Since there are only two known CR1 and CR3 boid–olivine aggregates (Chizmadia et al., chondrites, the term ‘‘CR chondrite’’ is com- 2002). Especially in low-subtype chondrites, monly used as a synonym for CR2. Similarly, there is evidence that hydrous fluids promoted CM is used for CM2. chemical equilibration and alteration in chond- rule mesostases (B&J, 1998; Tomeoka and Itoh, 2004), CAIs (Russell et al.,1998), and matrices 1.07.4.1.2 Type 3 chondrites (Section 1.07.5.7). With increasing metamor- phism, fluids were lost from the parent bodies. Type 3 chondrites show very diverse miner- For CV3 chondrites, the effects of metamorphic alogies and in several groups they have been alteration are more complex: subtypes based on subdivided. CO3, H3, L3, and LL3 chondrites, thermoluminescence sensitivities appear to be Metamorphism, Alteration, and Impact Processing 13

Fa (mol.%) heated above 300 1C for more than a and 10 20 30 probably experienced much lower peak temper- 0.5 LL chondrites atures (Reisener et al., 2000).

0.4 3.0− Type IA chondrules 3.2 Type IIA chondrules 0.3 3.3−3.4 1.07.4.1.3 Type 4–6 chondrites 3.5−3.6 0.2 Type 4–6 ordinary chondrites, which are − CaO (wt.%) 3.7 3.9 also called equilibrated chondrites, are thought 3.0−3.2 to represent an isochemical, metamorphic 0.1 3.3−3.4 3.5− 3.7−3.9 sequence: types 4 and 5 were heated to 500– 3.6 4−6 1 1 0 800 C and type 6 to 800–1,000 C(seeKeil, 0 5 10 15 20 25 30 2000). However, Wlotzka (2005) found similar FeO (wt.%) olivine/Cr-spinel temperatures of 700–800 1Cfor type 4–6 ordinary chondrites and suggests that Figure 4 Plot showing effects of metamorphism on the mean CaO and FeO concentrations in olivine in these temperatures correspond to peak meta- two types of chondrules in LL3 chondrites (Scott morphism. The most noticeable metamorphic et al., 1994). Type 3 chondrites can be readily sub- effects are due to grain coarsening and growth divided into subtypes using these parameters or of , and phosphate grains. many others (e.g., Sears and Hasan, 1987). Repro- Some type 4–6 chondrites are not simple meta- duced by permission of Elsevier from Scott et al. morphic rocks: they are fragmental or regolith (1994). breccias of rock fragments that were metamor- phosed prior to assembly (Rubin, 1990). In these cases, the petrologic type may only be a measure less useful (Guimon et al.,1995; Krot et al., of the metamorphic history of most material in 1995), and the degree of crystallinity of carbon- the rock. In addition, Rubin (2002, 2003, 2004) aceous material appears to be a more reliable infers that practically all type 5 and 6 ordinary guide (Bonal et al., 2006). The latter parameter chondrites were shocked and later annealed and suggests that Allende was heated above type 3.6 that many experienced multiple episodes of levels and should no longer be considered as a shock and impact-induced annealing. Within pristine chondrite. each group of ordinary chondrites, there are Although type 3 chondrites are the least small but systematic increases in the FeO con- altered and metamorphosed chondrites, only a centrations of olivines and with inc- few of them are now thought to be relatively pris- reasing petrologic. These are probably due to tine samples, and all of these have components oxidation of metal by traces of water vapor that were tainted by their asteroidal environm- during metamorphism (McSween and Labotka, ent. Among the most pristine are the CBb3 and 1993). Fluid inclusions are present in halite in CH3 chondrites, Vigarano and Leoville (CV3), two regolith breccias and may have ALHA77307 (CO3.0), Semarkona (LL3.0), been derived from asteroidal fluids or icy pro- Kakangari (K3), and two ungrouped carbon- jectiles (see Rubin et al.,2002). aceous chondrites, Acfer 094 and Adelaide. To The criteria for classifying types 4–6 were aid in identifying the least metamorphosed based solely on the properties of H, L, and LL chondrites, subtypes 3.0 and 3.1 have been types 3–6 chondrites and most cannot be used divided into 3.00 to 3.15 on the basis of Cr2O for asteroidally processed carbonaceous and concentrations in chondrule olivines that enstatite chondrites. For these chondrites, the contain 42 wt.% FeO (Grossman and Brearley, type is based almost entirely on the degree to 2005). These data confirm that Acfer 094 and which the chondrules are delineated. If the ALHA77307 are among the least metamor- chondrules have been obscured by recrystalli- phosed chondrites. Chondrules in type 3 chond- zation during prolonged heating, the petrologic rites in the ordinary, K, CO, and CV groups type is a good measure of the metamorphic were probably heated to around 400–600 1C history. However, for the EH4–6 and EL4–6 for 4106 years, as they contain and chondrites, evidence for simple thermal meta- that equilibrated at these temperatures morphism is lacking. Rapid cooling rates (Keil, 2000). However, the CH3 and CB3 chond- recorded by sulfides and numerous examples rites contain martensite that has not exsolved to of impact heating suggest that impact process- kamacite and taenite and has retained chemical ing may have been more important for enstatite zoning patterns that were established prior to chondrites than heating by decay of 26Al accretion (Meibom et al., 1999, 2000; Krot et al., (Zhang et al., 1996; Rubin et al., 1997; Lin 2002c). Electron microscopic studies of the and Kimura, 1998). R chondrites experienced metal show that CH3 chondrites were not ubiquitous brecciation and although some have 14 Chondrites and Their Components been classified as R4 (or R3), all are probably no correlation between cooling rate and pet- breccias of type 3–5 or 3–6 material (Bischoff, rologic type (Taylor et al., 1987) suggesting 2000). that if the petrologic types were once arranged sequentially in layers, the bodies must have been catastrophically fragmented and reassem- 1.07.4.2 Thermal History and Modeling bled by impacts during cooling from peak metamorphic temperatures (Grimm, 1985). To understand geological processes on Regolith breccias show extreme variations in chondritic bodies and to estimate their sizes the cooling rates of their metal grains from 1 to we need to investigate the thermal history of 103 1CMyr1 implying that the ordinary chond- unshocked or only lightly shocked samples. The rite bodies were scrambled after metamorphism simplest metamorphosed chondrites are the so that material from all depths could be com- ordinary chondrites, for which there are three bined together (Williams et al.,1999). kinds of constraints: (1) mineral equilibration Estimates of the sizes of the ordinary chond- temperatures (e.g., McSween and Patchen, rite parent bodies can be derived from thermal 1989); (2) radiometric ages that date cooling models as the timescale for cooling is controlled below isotopic closure in minerals using by conduction for dry asteroids 410–20 km in isotope systems such as 207Pb–206Pb, 40K–39Ar, size (McSween et al., 2002). However, the ther- and 87Rb–87Sr (e.g., Trieloff et al.,2003); mal conductivity is very sensitive to the poros- and (3) cooling-rate determinations based on ity, which can be reduced by sintering and annealing of Pu fission tracks (Lipschutz et al., increased by impacts. Layers of regolith o1km 1989) and metallographic cooling rates (Taylor thick can drastically reduce the cooling rate of et al.,1987). The cooling rates determined asteroids and increase near-surface thermal by the metallographic technique, which are gradients (Haack et al., 1990). Akridge et al. mostly 1–103 1CMyr1 in the temperature range (1998) argue that H chondrites are derived of 400–500 1C, are compatible with cooling rates from depths of o10 km from a 100 km radius in the range 100–500 1C determined by the asteroid and that material at greater depths fission track technique. A third technique based has not been sampled. However, Bennett and on Fe–Mg ordering in orthopyroxene gives McSween (1996) infer complete sampling of cooling rates in the range 340–480 1Cthat bodies with radii of B80–95 km for the H and are systematically higher by several orders of parent bodies. magnitude (Folco et al., 1997), probably be- For volatile-rich carbonaceous chondrites like cause metamorphism was insufficient to reequil- CI and CM chondrites, constraints on thermal ibrate the ordering imposed during chondrule histories are derived from carbonate ages and formation (Artioli and Davoli, 1994). Cooling oxygen-isotopic data. The former indicate that rates can also be estimated from radiometric age alteration began soon after CAI formation and data for mineral grains that close isotopically at lasted B20 Myr (Endress et al.,1996; Brearley different temperatures (Bogard, 1995; Ganguly et al.,2001). Oxygen-isotopic compositions of and Tirone, 2001; Amelin et al., 2005). carbonates provide model-dependent tempera- Metamorphic ages of ordinary chondrites tures under 50 1C(seeMcSween et al.,2002). from Pb–Pb ages of phosphates in seven H4–6 Modeling metamorphism of wet asteroids chondrites range from 4.50 to 4.56 Ga (Go¨ pel requires more complex models to track fluid et al., 1994), while their Ar–Ar ages range from flow, chemical and mineralogical changes during 4.45 to 4.53 Ga (Trieloff et al., 2003). The two metamorphism, and exothermic serpentinizat- H4 chondrites, which have metallographic ion reactions (e.g., Wilson et al., 1999; Cohen cooling rates of 103 1C Myr1, have the oldest and Coker, 2000; Young, 2001; Young et al., ages while the H6 chondrites with cooling rates 2003). McSween et al. (2002) suggest that ther- of B10 1C Myr1 have younger ages. The mal buffering by ice and water prevented high- negative correlation between age and meta- temperature metamorphism in carbonaceous morphic type for these seven chondrites is chondrite bodies. However, some CV and CK compatible with an onion-shell model in which chondrites were clearly heated above 500 1C. burial depth correlates with petrologic type. However, Pb–Pb ages for two ordinary type 3 chondrites are significantly younger than the 1.07.4.3 Impact Processing of Chondritic phosphate ages for the H6 chondrites and are Asteroids not readily reconciled with this model (Go¨ pel et al., 1994), and the selected H4 chondrites Impacts fragmented, mixed, modified, melted, may not be representative (Wlotzka, 2005). devolatilized, and lithified chondritic material. Metallographic cooling rates for larger num- Asteroids were impacted throughout their his- bers of unshocked, ordinary chondrites show tory: when they accreted, during alteration and Chondritic Components 15 metamorphism, and during the 4.5 Ga after and chondrules, and finally matrix, which is a their parent bodies were heated. For reviews of complex mixture of many ingredients. impact processing of ordinary chondrites, see Although chondrules are volumetrically more Sto¨ ffler et al. (1991); carbonaceous chondrites, important, we start with CAIs as these formed Scott et al. (1992); and enstatite chondrites, first and their origins are better constrained Rubin et al. (1997). Experimental studies of by chemical and isotopic data. Chemical and impact metamorphism of ordinary chondrites isotopic variations in chondrules are much were made by Schmitt (2000) and Langenhorst more modest than those in CAIs but there are et al. (2002), CV chondrites by Nakamura et al. enough related features and components with (2000), and CM chondrites by Tomeoka et al. intermediate compositions to suggest that (1999). Meteorite evidence for impact proc- chondrules cannot be understood in isolation essing of chondritic and other asteroids was from CAIs and other components. reviewed by Keil et al. (1994) and Scott (2002). High-pressure minerals in shock veins in chondrites also provide clues to mineral 1.07.5.1 Calcium- and Aluminum-Rich stability at depth in the Earth (Sto¨ ffler, 1997). Inclusions Advocates for impact heating of chondrites point to Portales Valley, a of H6 clasts There are two types of refractory inclusions: embedded in a matrix of once-molten metallic calcium- and aluminum-rich inclusions (this Fe,Ni, which cooled slowly to form a Wid- section) and amoeboid olivine aggregates( Sec- mansta¨ tten pattern (Rubin et al.,2001). Gaffey tion 1.07.5.3). Since the mineralogy, chemistry, and Gilbert (1998) suggest that molten metal on and isotope chemistry of refractory inclusions the H parent body was derived from impact- were reviewed by MacPherson et al. (1988), formed melt sheets or residues of metal-rich many new analyses have been made of CAIs in projectiles and that the IIE were also CV, CM, CO, CR, CH, CB, ordinary, and derived from this melt (see section 1.12.2.5). enstatite chondrites that provide important However, the origin of Portales Valley and the constraints on physicochemical conditions, link with IIE irons remain obscure as asteroids time, and place of CAI formation. CAIs are are thought to be too small for impacts to gene- addressed in detail in Chapter 1.08, the role of rate melt sheets (Keil et al.,1997), and Portales condensation and evaporation in their forma- Valley may have been heated radiogenically tion in Chapter 1.15, and their clues to early prior to the impact (Ruzicka et al.,2005). solar system chronology in Chapter 1.16. CAIs are the oldest objects in chondrites excluding presolar grains (Amelin et al., 2002) 1.07.5 CHONDRITIC COMPONENTS and probably formed during the most energetic phase of protosolar disk evolution (e.g., Ireland Below we review the geochemical properties and Fegley, 2000; Wood, 2000b, 2004). They and possible origins of chondritic components are composed of the minerals corundum, hibo- focusing on what appear to be primary features nite, , perovskite, melilite, spinel, in the least altered type 2–3 chondrites. The Al–Ti-diopside, anorthite, and forsterite, which rich diversity of chondritic components even in are predicted to condense from a cooling unaltered chondrites and the vast differences gas of solar composition at temperatures between chondrite groups are key features of 41,200–1,300 K and total pressure of 105 bar chondrites. Several comprehensive reviews of (e.g., Ireland and Fegley, 2000; Ebel and Gross- chondrites and their components have been man, 2000). Some CAIs are thought to be published since Kerridge and Matthews (1988). condensates because they have irregular These include a comprehensive review of shapes, fluffy textures, and minerals that match chondrite mineralogy (B&J, 1998), edited books the sequence of mineral formation (Figure 5), on chondrules (Hewins et al.,1996), chondrites and trace-element chemistry (e.g., Simon et al., (Krot et al.,2005c), and constraints on early 2002). The strongest argument in favor of con- solar system processes (Lauretta and McSween, densation origin of some CAIs comes from 2006), and several reviews on chondrules their group II REE pattern, which is a highly (Hewins, 1997; Rubin, 2000; Jones et al., fractionated pattern with much lower abun- 2000a; Zanda, 2004; Connolly and Desch, dances of the heavy REEs Gd–Er and Lu, 2004) and CAIs (Ireland and Fegley, 2000; exhibited by B30% of all CAIs (Palme and Chapter 1.08). Boynton, 1993; Ireland and Fegley, 2000). This First we review CAIs and their forsterite-rich volatility-controlled pattern requires the prior accretionary rims, then amoeboid olivine in- removal of the ultrarefractory elements. How- clusions and aluminum-rich chondrules, which ever, the preservation of large isotopic anom- are intermediate in composition between CAIs alies (e.g., 48Ca, 50Ti) in platy hibonites 16 Chondrites and Their Components

Adelaide (PLACs) in CM chondrites and their lack of 26Al, plus mass fractionation effects and hib smaller nuclear anomalies in FUN inclusions (see Chapter 1.08), favor evaporation as the predominant process for other CAIs. CAIs may represent mixtures of solar nebula con- densates and refractory residue materials grs formed by evaporating material of presolar cor origin (Ireland and Fegley, 2000). Because of the diversity of CAIs, there is no universal classification scheme for all groups. The best studied CAIs from CV chondrites are (a) 25 µm divided into type A (melilite–spinel-rich), type Efremovka B (composed of pyroxene, melilite, spinel, 7plagioclase), forsterite-bearing type B, type sp C (melilite-poor, pyroxene–plagioclase-rich), and fine-grained spinel-rich (e.g., MacPherson et al., 1988). Type A CAIs are subdivided into mel di fluffy and compact subtypes. In other chondrite groups, CAIs are typically classified according di+sp to their modal mineralogy (e.g., corundum-, grossite-, and hibonite-rich). In CV chondrites, most coarse-grained CAIs were probably once mel molten. However, in other chondrite groups, (b) 10 µm CAIs were probably not molten, though defi- Acfer 094 nitive compositional data for comparison with melt–solid distribution coefficients are typically not available. fo met

1.07.5.1.1 Comparison of CAIs from different px di chondrite groups an A comparison of CAIs from carbonaceous, fo ordinary and enstatite groups (Table 3) shows that most inclusion types can be found in many 10 µm chondrite groups, and that the most common (c) varieties of CAIs are spinel-, pyroxene-, and Figure 5 Backscattered electron images of refractory melilite-rich (e.g., Lin et al.,2006). However, inclusions with clear condensation signatures: (a) CAI the relative proportions of CAI types and their in Adelaide (unique ), (b) CAI mean sizes, which are correlated with chond- in Efremovka (CV3), and (c) AOA in Acfer 094 rule size (May et al., 1999), differ among the (unique carbonaceous chondrite). (a) Corundum (cor) chondrite groups. For example, type B CAIs is replaced by hibonite (hib), which is corroded by are almost exclusively found in CV chondrites; grossite (grs). Reproduced by permission of University of Arizona, r The from Krot PLACs occur mostly in CM chondrites; gros- et al. (2001a). The inferred sequence of mineral crys- site-rich, hibonite-rich, and aluminum-diopside tallization is consistent with the predicted equilibrium spherules are common only in CH chondrites; condensation sequence at total pressure of r105 bar CAIs in enstatite chondrites are rich in spinel and CI chondritic dust/gas enrichment relative to solar and, in many cases, hibonite. There are also composition of B1,000 (Simon et al., 2002). (b) Re- mineralogical and isotopic differences between placement of melilite by a fine-grained mixture of CAIs of the same mineralogical type: for spinel and diopside, suggesting the following gas–solid example, (1) melilite in compact type A CAIs reaction: Ca2Al2SiO7 þ 3SiO(g) þ 3Mg(g) þ 6H2O(g) from CV chondrites is more a˚kermanitic than ¼ 2CaMgSi2O6 þ MgAl2O4 þ 6H2(g). After Krot that in compact melilite-rich CAIs from CO et al. (2004c). (c) Replacement of forsterite by low- calcium pyroxene (px), suggesting the following reac- and CR chondrites (Weber and Bischoff, 1997; Russell et al., 1998; Ale´ on et al., 2002a) and (2) tion Mg2SiO4 þ SiO(g) þ H2O(g) ¼ Mg2Si2O6 þ H2(g). Adapted by permission of Elsevier from Krot et al. the grossite-rich and hibonite-rich CAIs in CR 16 (2004e). chondrites are O-rich and have a canonical Chondritic Components 17 Table 3 Distribution of types of CAIs, AOAs, accretionary rims, and aluminum-rich chondrules in type 2 and 3 chondrites. Object CM CO CV CR CH CB CK Acfer 094 Adelaide H–L–LL EH–EL Melilite-rich (type A) r c c c c c — c c r — Al,Ti-px-rich (type B) — — c r — r r — — — — Fo-type B CAIs — — r — — r — — — — — Plagioclase-pyroxene-rich (type C) — r c r — — — r — — — Hibonite7spinel-rich c c c r c c — r c r r Grossite-rich — r — r c r — c — — — Spinel7pyroxene-rich c c c c c c r c c r r Pyx-hibonite spherules r r — — c c — r — — r Fo-rich accretionary rims — — c — — — — — — — — AOAs c c c c c r c c c r — Aluminum-rich chondrules c c c c c c c c c c c

Source: MacPherson et al. (1988) with additional data: CHs, Kimura et al. (1993); COs, Russell et al. (1998); CMs, MacPherson et al. (1984), Simon et al. (1994); CRs, Ale´ on et al., (2002a), Weisberg et al. (1993), Kallemeyn et al. (1994); CKs, Weber and Bischoff (1997), McSween (1977a), MacPherson and Delaney (1985), Kallemeyn et al. (1991), Keller et al. (1992), Noguchi, (1993); H-L-LLs, Bischoff and Keil (1984), Russell et al. (2000), Kimura et al. (2002), Itoh et al. (2004a); ECs, Bischoff et al. (1985), Guan et al. (2000), Fagan et al. (2000); CBs, Krot et al. (2001a); Acfer 094, Adelaide, Krot (unpublished). Note: r, rare; c, common; —, absent.

26Al/27Al ratio (Marhas et al., 2002; Ale´ on CAIs in CV chondrites appear to have experi- et al., 2002a), whereas those in CH chondrites enced multiple heating events in nebular are 16O-rich, but generally lack radiogenic 26Mg environments that fluctuated from 16O-rich, to (26Mg*) (MacPherson et al.,1989; Kimura et al., 16O-poor, and back to 16O-rich (Yoshitake 1993; Weber et al.,1995; McKeegan et al., et al.,2005).16O-poor compositions of second- 2000a). The grossite- and hibonite-rich CAIs in ary minerals (nepheline, sodalite, andradite, and CB chondrites are 16O-poor (Krot et al.,2001a); ) in CV CAIs (Cosarinsky et al., no Al–Mg-isotopic measurements have been 2003) and oxygen-isotope heterogeneity in fine- published yet. grained spinel-rich CAIs that were probably not melted suggest that some oxygen-isotopic he- terogeneity resulted from secondary alteration 1.07.5.1.2 Oxygen-isotopic compositions of in asteroids (Ale´ on et al.,2002b; Fagan et al., CAIs 2002). CAIs from CO chondrites (Figure 6)showa In situ ion microprobe analyses of oxygen- correlation between oxygen-isotopic composi- isotopic compositions of individual minerals in tion and degree of thermal metamorphism: CAIs in the chondrite groups provide very im- spinel, hibonite, melilite, diopside, anorthite, portant constraints on the origin of CAIs. CAIs and olivine in most CAIs from CO3.0 chon- from CV chondrites typically exhibit oxygen- drites Colony and Y-81020 are uniformly isotope heterogeneity within an individual 16O-enriched, whereas melilite and secondary inclusion, with spinel, hibonite, and pyroxene nepheline in CAIs from metamorphosed CO enriched and anorthite and melilite depleted in chondrites Kainsaz (3.2) and Ornans (3.2) are 16O. At the same time, several coarse-grained 16O-depleted relative to spinel and diopside. igneous CAIs containing 16O-rich and 16O-poor These observations favor oxygen-isotope melilite, anorthite and Al–Ti-diopside, and Al– exchange during fluid–rock interaction in an Ti-diopside with oxygen-isotope zoning have asteroidal setting (Itoh et al., 2004a, b; Wasson been described in the Allende and Efremovka et al.,2001). However, the 16O-poor composi- meteorites (e.g., Kim et al.,1998; Yurimoto tions of some CAIs in CO3.0 chondrites (e.g., et al.,1998;Itoet al.,1999;Imai and Yurimoto, Yurimoto et al., 2001) suggest that some 2000). Based on these observations and exper- isotopic exchange also occurred in the solar imental data on oxygen-isotope self-diffusion in nebula in the presence of 16O-poor gas. CAI minerals (Ryerson and McKeegan, 1994), CAIs in type 2 and 3 CR, CH, CB, CO, CM, Yurimoto et al. (1998) concluded that coarse- ordinary, and enstatite chondrites are typically grained CAIs in CV chondrites were isotopically uniform (within 3–4%): both initially 16O-rich and subsequently experienced 16O-rich and 16O-poor CAIs have been found. oxygen-isotope exchange with 16O-poor nebular Most CAIs in CR and CH chondrites are gas while partly molten during reheating. A few 16O-rich (Figure 7); the 16O-poor CAIs in CR, 18 Chondrites and Their Components 26 27 41 40 20 canonical ( Al/ Al)0 and ( Ca/ Ca)0 ratios CO CAIs and group II REE patterns, whereas platy 10 hibonites (PLACs), which generally have group III REE patterns, large (410%) stable-isotopic 48 50 26 0 ne anomalies in Ca and Ti, and lack Al and 41Ca, are similarly 16O-enriched. No obvious −10 correlation between hibonite morphology and (‰) radiogenic 26Mg, Ca (d48Ca) or Ti (d50Ti) − 20 Terrestrial fractionation li isotopic anomalies has been found (Sahijpal SMOW

O sp et al., 2000; Goswami et al., 2001). 17 −30  hib To summarize, variations in oxygen-isotopic mel compositions within a single CAI in CV and −40 px an CO chondrites require isotopic exchange, Allende CAI line ol −50 however, the exact mechanism of exchange nph remains poorly understood. Proposed mecha- nisms include gas–solid and/or gas–melt ex- −50 −40 −30 −20 −10 0 10 20 change in the solar nebula (e.g., Clayton, 1993; 18 (a) OSMOW (‰) Yurimoto et al., 1998, 2000, 2001; Krot et al., 2005c), and solid–fluid exchange in an aster- 0 oidal setting (e.g., Wasson et al., 2001; Fagan et al., 2002, 2004). The presence of 16O-poor CAIs with igneous textures in primitive CR, −10 CH, and CB chondrites suggests that in these cases oxygen-isotopic exchange occurred dur- Kainsaz 3.2 −20 Ornans 3.3 ing melting. It remains unclear whether this Kainsaz 3.2 (‰) exchange occurred in the CAI- or chondrule- Kainsaz 3.2 − forming regions and whether this exchange was Kainsaz 3.2

SMOW 30 Ornans 3.3 Ornans 3.3 O late or early. Ornans 3.3 Y-81020 3.0 Y-81020 17  Y-81020 3.0 Y-81020 − 3.0 Y-81020 The oxygen-isotopic compositions of 40 chondritic components are generally assumed Colony 3.0 Colony to reflect isotopic variations among the differ- −50 ent locations within the meteorite-forming regions of the solar nebula. However, since −60 CAIs in most chondrite groups have similar (b) CAIs 16O-rich isotopic compositions, it was inferred Figure 6 Oxygen-isotopic compositions of that CAIs formed in a single, restricted nebular individual minerals in CAIs from the CO chondrites locale and were then unevenly distributed to Y-81020, Colony, Kainsaz, and Ornans. Primary the regions where chondrites accreted after sor- minerals in CAIs from the least metamorphosed CO ting by size and type (McKeegan et al., 1998; chondrites Y-81020 (type 3.0) and Colony (3.0) are Guan et al., 2000). Although formation in a uniformly 16O-enriched, whereas CAIs from Kainsaz single restricted region appears to be generally (3.2) and Ornans (3.3) tend to show oxygen-isotopic accepted, sorting by size and type cannot heterogeneity with spinel and high-calcium pyroxene explain the observed variations in mineralogy enriched in 16O and melilite and secondary nepheline 16 and isotope chemistry of CAIs in different depleted in O. Based on these observations, chondrite groups. We suggest instead that CAI Wasson et al. (2001) inferred that oxygen-isotope exchange took place during thermal metamorphism formation occurred multiple times in the CAI- and alteration in an asteroid. Data from Itoh et al. forming region under diverse physicochemical (2004b) and Wasson et al. (2001). conditions (e.g., dust/gas ratio, peak heating temperature, cooling rate, and number of re- cyclings) and that CAIs were subsequently iso- lated from the hot nebular gas at various ambient nebular temperatures. The isolation CH, and CB chondrites show petrographic could be due to removal by stellar wind (e.g., evidence for extensive melting (Ale´ on et al., Shu et al., 1996, 2001). The 16O-rich and 16O- 2002a; Krot et al., 2001a; McKeegan et al., poor CAIs reflect either temporal variations in 2006). CAIs in CM, ordinary, and enstatite oxygen-isotopic compositions of the nebular chondrites are 16O-rich (D17Oo20%). In gas in the CAI-forming region(s) (e.g., Yuri- CM chondrites, hibonites in spinel–hibonite moto et al., 2001) or radial transport of some spherules (SHIBs), which generally lack stable- CAI precursors into an 16O-poor gas prior to isotopic anomalies in 48Ca and 49,50Ti, have their melting (see also Scott and Krot, 2001; Chondritic Components 19

20 20 20 sp CR CAIs CB CAIs mel CH CAIs 10 px 10 10 an 0 0 0 ion line ion line ion line (‰) −10 −10 −10 ial fractionat ial fractionat

SMOW −20 Terrestr −20 Terrestr −20 Terrestrial fractionat

O hib hib

17 grs − hib − grs −  30 30 30 mel grs mel mel sp Allende CAI line sp −40 sp − − sp+pk+an 40 px 40 px Allende CAI line px pv Allende CAI line an fo −50 −50 fo −50 −50 −40 −30 −20 −10 0 10 20 −50 −40 −30 −20 −10 0 10 20 −50 −40 −30 −20 −10 0 10 20 18 18 18 (a) OSMOW (‰) (c)OSMOW (‰) (e) OSMOW (‰)

0 0 0

−10 −10 −10

(‰) −20 −20 −20

SMOW − − −

O 30 30 30 17  −40 −40 −40

−50 Non-igneous(?) Igneous −50 −50

(b) CAIs (d) CAIs (f) CAIs Figure 7 (a, b) Oxygen-isotopic compositions of individual minerals in CAIs from CR chondrites. Most CAIs are 16O-rich and isotopically homogeneous; CAIs, which show petrographic evidence for extensive melting are less 16O-rich. Secondary phyllosilicates replacing 16O-rich melilite plot along terrestrial fractionation line. Data from Ale´ on et al. (2002a). (c, d) Oxygen-isotopic compositions of individual minerals in CAIs from CH chond- rites show a bimodal distribution. The most refractory CAIs, which are composed of grossite, hibonite, spinel, perovskite, and melilite, are enriched in 16O; aluminum-diopside rims around these CAIs, pyroxene-spinel CAIs, and two melilite-rich CAIs surrounded by a forsterite layer are 16O-poor. Data from McKeegan et al. (2006). (e, f) Oxygen-isotopic compositions of individual minerals in CAIs from CB chondrites Hammadah al Hamra 237 and QUE 94411. The CAIs are rather homogeneous and 16O-poor. Data from Krot et al. (2001c).

Ale´ on et al., 2002a; Krot et al., 2005c; in all layers. Based on the observed disequilib- McKeegan et al., 2006). rium mineral assemblages, abundant euhedral crystals with pore space between them, and rim thicknesses controlled by the underlying 1.07.5.2 Forsterite-Rich Accretionary Rims topography of their host inclusions, MacPher- around CAIs son et al. (1985) concluded that the layered accretionary rims in Allende are aggregates of Olivine-rich, accretionary rims around CAIs, gas–solid condensates which reflect significant which have grain sizes 45–20 mm, were first de- fluctuations in physicochemical conditions in scribed in the altered CV chondrite Allende by the solar nebula and grain/gas separation proc- MacPherson et al. (1985). In this meteorite, the esses. Hiyagon (1998) analyzed the oxygen-iso- accretionary rims have four layers that differ in tope compositions of minerals in accretionary texture and mineralogy. The innermost layer rims around two type A CAIs from Allende. consists of either pyroxene needles, olivine, Olivines in both accretionary rims are charac- clumps of hedenbergite, and andradite or olivine terized by 16O-rich-isotopic compositions ‘‘doughnuts’’ (i.e., crystals with central cavities). (d18O ¼ –28%, d17O ¼ –30%), whereas wollasto- The next two layers outward both contain nite, andradite, and hedenbergite show much olivine plates and laths. The final layer separa- heavier oxygen-isotopic compositions (d18O ting accretionary rims from the Allende matrix from þ 2% to þ 10%, d17Ofrom–3% to contains clumps of andradite and hedenbergite þ 4%). surrounded by salitic pyroxene needles. Nephe- In the reduced CV chondrites, which are less line and Fe,Ni-sulfides are common constituents altered than Allende, accretionary rims are 20 Chondrites and Their Components

Efremovka, E38 AR mel

CAI

fas CAI (b) 200 µm

AR mel cpx ol cpx an mel sp ol pv an sp WLR sp CAI mel µ (c) 50 m

CAI sp AR ol cpx AOA WLR

1 mm CAI 50 µm (a) (d)

Figure 8 (a) Combined elemental map in magnesium (red), calcium (green), and Al Ka (blue) X-rays of a type B CAI E38 from Efremovka with an accretionary rim (AR); (b)–(d) backscattered electron images from regions marked in elemental map. This CAI consists of melilite (mel), aluminum, titanium-diopside (fas), spinel (sp), and anorthite (an) and is surrounded by a multilayered Wark–Lovering rim (WLR) composed of \spinel, perovskite (pv), anorthite, aluminum-diopside, and forsterite, and a thick outer forsterite-rich accretionary rim (red in (a)). The accretionary rim consists of forsterite (ol) and refractory components composed of spinel, aluminum-diopside, and anorthite.

not layered and consist of coarse-grained rims (see Wark and Boynton, 2001), which must (20–40 mm), anhedral forsterite (Fa1–8 versus therefore have occurred in the CAI-forming Fa5–50 in Allende), Fe,Ni-metal nodules, and region as well. Andradite, wollastonite, he- fine-grained refractory components composed denbergite, and fayalitic olivine in the Allende of aluminum-diopside, anorthite, spinel, and, rims formed by alteration under oxidizing con- in some cases, forsterite (Krot et al., 2001f, ditions in the presence of 16O-poor fluid (Krot 2002b). These rims surround different types of et al., 1995, 1998a, b, c, 2001e). CAIs: types A, B, and fine-grained spinel-rich (Figure 8). Forsterite grains in the accretionary rims are 16O-enriched similar to spinel, alumi- num-diopside, and hibonite of the host CAIs 1.07.5.3 Amoeboid Olivine Aggregates (Figure 9). Krot et al. (2001f, 2002b) concluded that AOAs are irregularly shaped objects with fine forsterite-rich rims formed by accretion of high- grain sizes (5–20 mm) that occupy up to a few temperature condensates in the CAI-forming percent of type 2 and 3 carbonaceous chondrites regions. The rims were emplaced after high-tem- (Table 3). AOAs are similar in size to co- perature events experienced by the CAIs, such accreted chondrules and CAIs being mostly as melting and formation of Wark–Lovering 100–500 mminsizeinCO(Chizmadia et al., Chondritic Components 21

20 (Grossman and Steele, 1976), are best observed Efremovka E104 in the least altered chondrites, CO3.0, CR2, 10 CH, CB chondrites, Adelaide, and Acfer 094 (Krot et al., 2004d and references therein). 0 Here they consist of anhedral, fine-grained (1–20 mm) forsterite (Fao1–3), Fe,Ni-metal, −10 and a refractory component composed of alu- minum-diopside, spinel, anorthite, and rare

O (‰) −20 melilite, or CAIs of similar mineralogy. AOAs

17 Terrestrial fractionation line

 in CO3.0 chondrites contain B8 vol.% metallic −30 Allende CAI line AR, ol Fe,Ni: troilite is rare or absent (Chizmadia WLR, sp et al., 2002). The refractory objects in AOAs CAI, cpx −40 CAI, sp are characterized by significant variations in CAI, mel size (1–250 mm), shape (irregular, rounded), dis- −50 CAI, hib tribution (uniform, heterogeneous), modal mineralogy (spinel-rich, spinel-poor), and abun- −50 −40 −30 −20 −10 02010 (a) dance (rare, abundant) (Figure 10). Forsterite grains typically contain numerous pores and tiny 20 inclusions of aluminum-diopside. However, Vigarano 1623-2 some AOAs show triple junctions between 10 forsterite grains and coarse-grained shells sur- rounding finer-grained cores, indicating high- 0 temperature annealing (Komatsu et al., 2001). About 10% of AOAs contain low-calcium pyr- −10 oxene replacing forsterite at grain boundaries in AOA peripheries (Figure 5c), or rimming

O (‰) − 20 Terrestrial fractionationI lineline the AOA (Krot et al., 2004e). Although most 17  AOAs show no clear evidence of being melted, −30 those with low-Ca pyroxene rims are compact Allende CA AR, ol rather than porous and may have experi- − 40 CAI, sp enced small degrees of melting. Some AOAs ap- CAI, cpx pear to contain CAIs that have been melted −50 CAI, mel (Figure 10). −50 −40 −30 −20 −10 02010 Forsterite grains in AOAs in pristine chond- rites show diverse compositions. Forsterite (b) 18O (‰) grains in Adelaide AOAs that lack low-Ca Figure 9 Oxygen-isotopic compositions of minerals pyroxene are nearly pure (FeOo1 wt.%) with in the Efremovka CAI E104 (a) and Vigarano CAI only small amounts of Cr2O3 (o0.3 wt.%) and 1623-2 (b) and their forsterite-rich accretionary rims MnO (o0.07 wt.%) whereas those in pyroxene- (AR) and Wark–Lovering rims (WLR) showing that bearing AOAs are richer in FeO (1–4 wt.%), both formed in an 16O-rich reservoir. Data from Krot et al. (2002b). Cr2O3 (0.2–0.6 wt.%), and MnO (up to 0.8 wt.%) (Krot et al., 2004d). Mn and Cr con- centrations in AOAs in a CO3.0 chondrite 2002), up to 5 mm in size in CV (Komatsu et al., (Yamato 81020) appear to be higher in frac- 2001), and typically o500 mm in CR chon- tured or peripheral regions and lowest in AOAs drites (Ale´ on et al., 2002a). In the least altered with high CaO concentrations (0.2–0.8 wt.%) chondrites, they are porous aggregates and (Sugiura et al., 2004, 2006), This suggests that their mineralogy matches that expected for both Mn and Cr were introduced by gas–solid high-temperature nebular condensates (Krot reactions after Ca-rich forsterite condensed, et al., 2004d). Unlike CAIs and chondrules, consistent with thermodynamic models (e.g., AOAs do not appear to show mineralogical and Weisberg et al.,2004). isotopic differences between groups and thus Secondary nepheline, sodalite and ferrous provide an excellent guide to the alteration his- olivine are common in AOAs in the altered tory of the meteorite and its constituents. and metamorphosed CV and CO chondrites, but are absent in AOAs from the unaltered chondrites. Olivines in AOAs from CV chond- 1.07.5.3.1 Mineralogy and petrology of AOAs rites are generally more fayalitic than in the unaltered chondrites, with fayalite con- AOAs, which were first described in centrations increasing from Leoville and Vigar- the heavily altered Allende CV3 chondrite ano to Efremovka to Allende (Figure 11). 22 Chondrites and Their Components

Efremovka

AOA1 50 µm JSM 5900 (b)

Ol

Met Pores

cpx AOA2 an

10 µm ×2, 000 10 µm JSM-5900 (c)

Ol cpx

an AOA3 Ol sp mel sp+cpx+an

µ (a) 250 µm 100 m (d)

Figure 10 Combined elemental map in magnesium (red), calcium (green), and Al Ka (blue) X-rays (a) and BSE images (b)–(d) of AOAs in the reduced CV chondrite Efremovka. They consist of fine-grained forsteritic olivines (Ol), Fe,Ni-metal (Met), and CAIs composed of aluminum-diopside (cpx), anorthite (an), spinel (sp), and rare melilite (mel); melilite is replaced by anorthite. Although AOAs show no clear evidence of being melted, some CAIs inside them appear to have experienced melting. Regions outlined in (a) and (b) are shown in detail in (b)–(d).

This sequence correlates with the degree of sec- and anorthite) in AOAs from CM, CR, CB, ondary alteration and thermal metamorphism and unmetamorphosed CO chondrites are experienced by CV chondrites (e.g., Krot et al., uniformly enriched in 16O, whereas anorthite, 1995, 1998a, b, c, 2001e; Komatsu et al., 2001). melilite, and secondary minerals (nepheline and In CO3 chondrites, fayalite contents are well sodalite) in AOAs from CVs and metamor- correlated with petrogic subtype (Chizmadia phosed COs are 16O-depleted to various degrees et al., 2002). (Hiyagon and Hashimoto, 1999a; Imai and Yurimoto, 2000, 2001; Itoh et al.,2002; Krot et al., 2001f, 2002b; Fagan et al.,2004) (Figure 12). Melilite grains in AOAs from the 1.07.5.3.2 Trace elements and isotopic 16 composition of AOAs reduced CV3 chondrite Efremovka are O- enriched relative to those from Allende, sugges- Trace element concentrations have been ting exchange during alteration (Fagan et al., measured in 10 AOAs from Allende (Grossman 2004). Imai and Yurimoto (2000, 2001) et al.,1979). Most AOAs have unfractionated reported two generations of olivines in Allende abundances of refractory lithophile and side- AOAs: primary forsteritic olivines, which are rophile elements (2–20 CI); two AOAs analy- 16O-rich (D17OB–20%), and secondary fayali- zed have group II REE patterns. tic olivines, which are 16O-poor (D17OB–5%) Oxygen-isotopic analysis of primary min- and probably grew on the parent asteroid in the erals (forsterite, aluminum-diopside, spinel, presence of 16O-poor fluid. Grain-boundary abncoscodie dlie() n ce 9 h.Oiie nCs dlie n ce 9 are from 094 are AOAs Acfer in Allende alteration and for increase secondary Data of AOAs unique Adelaide, degree chondrites. CV the (f), CV CRs, from with by chondrites correlated olivines in experienced is in CB metamorphism this Olivines contents Allende; thermal and Efremovka, Fayalite and (h). Vigarano, CH AOAs. and 094 CV Leoville (e), order in Acfer chondrites the olivines and CR to compared (d), (g), magnesium-rich Allende Adelaide chondrite chondrites CV carbonaceous oxidized (c), Efremovka 11 Figure rsmn(1987) Grossman (c) ( (b) (a) d

) Frequency (%) Frequency (%) Frequency (%) Frequency (%) 10 15 20 25 30 10 15 20 25 30 10 15 20 opstoso lvn nAA rmterdcdC hnrtsVgrn a,Loil (b), Leoville (a), Vigarano chondrites CV reduced the from AOAs in olivine of Compositions 0 5 0 1 2 3 4 5 6 0 5 0 5 0 1216202428323640 8 4 0 21 02 83 640 36 32 28 24 20 16 12 8 4 0 1216202428323640 8 4 0 4 812 n o feok,Loil,adVgrn Osfrom AOAs Vigarano and Leoville, Efremovka, for and ; CV CV CV CV red oxA red red , Efremovka ( 62 42 32 28 24 20 16 , Allende( , Vigarano ( , Vigarano Fa Fa , Leoville ( ( mol.% n n n = 451) ) n = 21) = 64) = 96) hnrtcComponents Chondritic Amoeboid olivineaggregates 36 40 (g) (e) ( (f) h ) 10 20 30 40 50 10 20 30 40 50 60 70 10 20 30 40 50 60 10 20 30 40 50 0 0 0 0 0 0 0 0 1 1 1 1 Adelaide ( Acfer 094( CH &CB( Fa Fa 2 2 2 2 CR ( ( mol.% n Komatsu n = 99) n n = 49) = 45) = 17) 3 3 3 3 ) tal. et 4 4 4 4 ahmt and Hashimoto (2001) 5 5 5 5 . 23 24 Chondrites and Their Components

20 CR AOAs CV AOAs 10

0 e e

nation lin nation lin −10 ial fractio ial fractio restr

O (‰) −20 Ter Terrestr 17  Allende CAI line Allende CAI line −30 ol cpx −40 fo an cpx sp an phyl − 50 sp nph −50 −40 −30 −20 −10 0 10 20 −50 −40 −30 −20 −10 0 10 20 (a) (b)

20 CM AOAs CO AOAs 10

0 e e

−10 nation lin nation lin ial fractio ial fractio −20 restr O (‰) Ter Terrestr 17

 fo −30 Allende CAI line Allende CAI line cpx an sp −40 fo relict fo fo −50 px pig sp an −50 −40 −30 −20 −10 0 10 20 −50 −40 −30 −20 −10 0 10 20 (c) 18O (‰) (d) 18O (‰) Figure 12 Ion microprobe analyses of oxygen-isotopic compositions of individual minerals in AOAs from CR2 (a), CV3 (b), CM2 (c), and CO3 (d) chondrites. Key: fo, forsterit; cpx, Al-diopside; an, anorthite; sp, spinel; fas, Al–Ti-diopside; phyl, phyllosilicates; nph, nepheline; fo relict, relict AOA forsterite in chond- rules; pig, chondrule pigeonite. Data for CR AOAs from Ale´ on et al. (2002a); CV AOAs are from Imai and Yurimoto (2000, 2001), Fagan et al. (2004), Hiyagon and Hashimoto (1999a), and Krot et al. (2002b); CM AOAs from Hiyagon and Hashimoto (1999b); CO AOAs from Itoh et al. (2002); CB AOA from Krot et al. (2001c).

low-Ca pyroxene in AOA peripheries in prim- 26Mg excesses corresponding to initial 26Al/27Al itive carbonaceous chondrites is 16O-rich ratios of (2.770.7) 105,(2.971.5) 105, (D17Oo–20% ) whereas low-Ca pyroxene igne- and (3.271.5) 105. Comparable excesses of ous rims on AOAs are 16O-poor (D17OB5% ) 26Mg were not detected in the AOA-bearing (Krot et al., 2005b). Thus AOAs appear to chondrule (Itoh et al.,2002). The lower have been modified in two different nebular 26Al/27Al ratios in AOAs compared to a canon- environments, in addition to the final, pre- ical value of B5 105 found in most CAIs sumably asteroidal environment where Na-rich (e.g., MacPherson et al.,1995) suggest that phases were introduced. either AOAs formed 0.1–0.5 Ma after CAIs or Three AOAs in the CO3.0 chondrite Y-81020 that 26Al was heterogeneously distributed in the that were studied for Al–Mg systematics showed CAI-forming region (Itoh et al.,2002). Chondritic Components 25 1.07.5.3.3 Origin of amoeboid olivine according to their bulk composition and miner- aggregates alogy into three groups: ‘‘plagioclase-rich,’’ 16 which plot in the anorthite ( þ spinel) field on The mineralogy, petrology, O-rich compo- the liquidus and contain abundant euhedral ca- sitions, depletion in moderately volatile lcic plagioclase crystals, ‘‘olivine-rich,’’ which elements, such as manganese, Cr, and Na, plot in the forsterite ( þ spinel) field and com- and the general absence of low-calcium pyrox- monly contain large euhedral olivine pheno- enes associated with pristine AOAs strongly crysts, and ‘‘glassy,’’ which plot near the suggest that AOAs are aggregates of grains of corundum–forsterite side of the ternary forsterite, aluminum-diopside, spinel, anort- and are characterized by abundant transparent hite, and Fe,Ni-metal that condensed in the 16 Na–Al-rich and calcium-poor that encloses nebula from an O-rich gaseous reservoir and a phenocryst assemblage of olivine, pyroxene, aggregated with refractory objects (Krot et al., and spinel. In carbonaceous chondrites, 2004d, e, 2005b; Weisberg et al., 2004). however, Krot and Keil (2002) and Krot Mineralogical and chemical similarities bet- et al. (2002a) found a continuum between the ween AOAs and forsterite-rich accretionary plagioclase-rich and olivine-rich aluminum- rims suggest that AOAs formed contempora- rich chondrules: glassy aluminum-rich neously with these rims. Some refractory chondrules are absent (Figure 14). Because al- objects inside AOAs were melted prior to uminum-rich chondrules in carbonaceous aggregation. AOAs subsequently experienced chondrites all have similar mineralogy (magne- high-temperature annealing and solid-state sian low-calcium pyroxene and forsterite phe- recrystallization with only very minor melting nocrysts, Fe,Ni-metal nodules, interstitial and nebular alteration resulting in formation anorthitic plagioclase, Al–Ti–Cr-rich low- of anorthite and, in some cases, low-calcium calcium and high-calcium pyroxenes, and crys- pyroxene. The AOAs formed in the CAI- talline mesostasis composed of silica, anorthite, forming region(s) and they were either largely and high-calcium pyroxene), Krot and Keil absent from chondrule-forming region(s) when (2002) and Krot et al. (2002a) called them all chondrules formed, or the scales of the chond- plagioclase-rich chondrules, irrespective of mo- rule-forming events were small and their fre- dal mineralogy. quency low. Their preservation can be Because the aluminum-rich chondrules are attributed to removal from the CAI-forming compositionally most similar to CAIs and region prior to formation of low-calcium pyr- often contain aluminum-rich phases with high oxene by gas–solid reactions and/or kinetic in- Al/Mg ratios (plagioclase and, occasionally, hibition of these reactions. Nevertheless, AOAs glass) which make them suitable for Al–Mg have preserved a record of nebular processing 16 16 isotope studies, these chondrules received spe- in O-rich and O-poor environments and cial attention (e.g., Kring and Boynton, 1990; may have been important precursor materials Sheng et al.,1991; Hutcheon and Jones, 1995; in the formation of several kinds of igneous, Russell et al., 1996, 2000; Hutcheon et al., coarse-grained chondritic components such as 1994, 2000; Marhas et al.,2002; Srinivasan refractory forsterite-rich objects and type I et al., 2000a, b; Huss et al.,2001; Krot and chondrules. Keil, 2002; Krot et al., 2001b, 2002a, c; Guan et al.,2006).

1.07.5.4 Aluminum-Rich Chondrules 1.07.5.4.1 Isotopic and trace element studies of Aluminum-rich chondrules are a diverse aluminum-rich chondrules group of objects with igneous textures and 410 wt.% Al2O3 (Bischoff and Keil, 1984; Huss Oxygen-isotopic data for seven aluminum-rich et al.,2001; MacPherson and Huss, 2005; Guan chondrules from ordinary chondrites (Russell et al.,2006),whichappeartohavedistinct et al.,2000) are continuous with the ordinary origins from ferromagnesian chondrules (Sec- chondrite ferromagnesian chondrule field tion 1.07.5.5). Their mineralogy, bulk chemical above the terrestrial line, but extend it to more compositions, oxygen, and Al–Mg isotope 16O-enriched values (d18O ¼ –15.771.8%, systematics suggest a close link between d17O ¼ –13.572.6%) along a mixing line CAIs and aluminum-rich chondrules. Using of slope ¼ 0.8370.09 (Figure 15a). Two chond- the CaO–MgO–Al2O3–SiO2 phase diagram rules exhibit significant internal-isotopic (Figure 13), Huss et al. (2001) subdivided alu- heterogeneity indicative of partial exchange with minum-rich chondrules in ordinary chondrites a gaseous reservoir (Russell et al., 2000). 26 Chondrites and Their Components

Al-rich chondrules Al O 2 3 OCs Hib Cor CVs Adelaide CRs AOAs in Acfer 094, CRs

Cor+L

Hib+L Mull+L An Saph+L Cord+L Grs+L An+L En Geh Pyx+L

Me+L Fo+L Mo+L Mw+L La+L Per+L La Fo Ca2SiO4 Mw Mo Mg2SiO4

Figure 13 Bulk compositions of aluminum-rich chondrules plotted in the system Al2O3–Mg2SiO4–Ca2SiO4 and projected from spinel (MgAl2O4). Mineral abbreviations: An, anorthite; Grs, grossite; Cor, corundum; Cord, cordierite; Di, diopside; Fo, forsterite; Geh, gehlenite; Hib, hibonite; La, ; L, liquid; Mel, melilite solid solution; Mull, mullite; Mo, monticellite; Mw, merwinite; Per, periclase; Pyx, pyroxene solid solution; Saph, sapphirine. Data from Bischoff and Keil (1984); Sheng et al. (1991); Huss et al. (2001); Krot and Keil (2002); and Krot et al. (2002c, 2004a).

Porphyritic aluminum-rich chondrules are con- McKeegan et al., 2000b; Mostefaoui et al., sistently 16O-rich relative to nonporphyritic 2000, 2002). ones, suggesting that degree of melting was a Correlated in situ analyses of the oxygen- key factor during nebular exchange (Russell and magnesium-isotopic compositions of Al- et al.,2000). rich chondrules from unequilibrated enstatite Oxygen-isotopic compositions of alumi- chondrites show that among 11 Al-rich chond- num-rich chondrules from CR and CH chond- rules measured for 26Al–26Mg systematics, only rites (Krot et al., 2006a) are plotted in Figure one chondrule contains 26Mg* corresponding 15b. Three out of six chondrules analyzed ex- to the initial 26Al/27Al ratio of (6.872.4) 106 hibit large internal-isotopic heterogeneity, (Guan et al., 2006). The oxygen-isotopic com- whereas others are isotopically uniform; all positions of five Al-rich chondrules on a three- chondrules have porphyritic textures. In con- oxygen-isotope diagram define a line of slope trast to aluminum-rich chondrules in ordinary B0.670.1 and overlap with the field defined by chondrites, oxygen-isotopic heterogeneity is ferromagnesian chondrules in enstatite chond- due to the presence of relict CAIs (Krot and rites but extend to more 16O-rich compositions Keil, 2002). with a range in d18O of about 12% (figure 3 in Aluminum–magnesium isotope measure- Guan et al., 2006). ments of aluminum-rich chondrules in ordi- The REEs in most aluminum-rich chond- nary and carbonaceous chondrites were rules from carbonaceous chondrites studied summarized by Huss et al. (2001). About 10– are unfractionated (Kring and Boynton, 1990; 15% of all aluminum-rich chondrules studied Weisberg et al., 1991). However, some have 26 26 27 show Mg*. The inferred ( Al/ Al)0 ratio of highly fractionated REE patterns, similar to these chondrules is B(0.5–2) 105, which is volatility-controlled group II and ultrarefracto- consistent with the values found in ferromagne- ry patterns seen in refractory inclusions (Kring sian chondrules from type 3.0–3.1 unequili- and Boynton, 1990; Misawa and Nakamura, brated ordinary chondrites (Kita et al., 2000; 1988, 1996). Chondritic Components 27

pl pv sil pl

pl sp sil opx CAI pl met cpx

chd w

sp

50 µm (a) (b) 20 µm (c) Mg K Si K met px

an

p

µ cpx 300 m (d) (e) (f) Mg K Si K opx

ol

met

cpx 200 µm an (g) (h) (i)

Figure 14 Backscattered electron images (a, c), combined elemental map in magnesium (red), calcium (green) and Al Ka (blue) X-rays (b, d, g), and elemental maps in magnesium (e, h) and Si Ka X-rays (f, i) of the CAI-bearing aluminum-rich chondrule #2 in the CH chondrite Acfer 182 (a–c) and plagioclase-bearing chondrules #1 (d–f) and #17 (g–i) in the CR chondrites EET92042 and PCA91082, respectively. (a–c) Relict CAI consists of irregularly-shaped spinel (sp) core surrounded by anorthite (pl); tiny perovskite (pv) inclusions occur in spinel. The chondrule portion consists of low-calcium pyroxene (opx), high-calcium pyroxene (cpx), Fe,Ni-metal nodules (met) and silica-rich crystalline mesostasis composed of silica (sil), anorthite and high- calcium pyroxene. Region outlined in (b) is shown in detail in (c). White veins (w) in (a) and (c) are veins of terrestrial weathering products. (d–f) Chondrule #1 consists of anorthite poikilitically enclosing spinel grains, which are relict, high-calcium pyroxene, low-calcium pyroxene, Fe,Ni-metal nodules, minor forsterite and silica-rich crystalline mesostasis composed of silica, anorthite, and high-calcium pyroxene. (g–i) Chondrule #17 consists of forsteritic olivine, low-calcium pyroxene, anorthitic plagioclase, high-calcium pyroxene, Fe,Ni-metal nodules (black), and silica-rich crystalline mesostasis composed of anorthite, silica, and high-calcium pyroxene. Low-calcium pyroxene occurs as elongated grains overgrown by high-calcium pyroxene in the chondrule cores and as phenocrysts poikilitically enclosing forsterite grains in chondrule peripheries.

1.07.5.4.2 Origin of aluminum-rich chondrules aluminous (evolved) parent body surface ma- terial, mixing of aluminous components with MacPherson and Russell (1997) and Mac- ferromagnesian chondrule precursors prior to Pherson and Huss (2005) discussed several chondrule formation, and volatilization. They possible models for the origin of aluminum- concluded that (1) no single model appears rich chondrules including impact melting of capable of explaining the entire spectrum of 28 Chondrites and Their Components

20 Al-rich chondrules in OCs, heterogeneous Chainpur 14-1, sp 10 Chainpur 14-1, px Chainpur 14-1, ol Sharps 10-1, sp 0 Sharps 10-1, ol+px+pl Sharps 10-1, nph+pl Sharps 10-1, ol −10 ation line

O (‰) −20 17

 Terrestrial fraction

− 30 line Al-rich chondrules in OCs, homogeneous −40 Allende CAI Chainpur 10-1, gl, sp Chainpur 14-2, mix Chainpur 20-1, ol, px, sp Inman 1-1, mix −50 Quinyambie 5-2

−50 −40 −30 −20 −10 0 10 20 (a)

20 Al-rich chondrules in CRs & CHs, with relict grains Acfer 182 #2, sp Acfer 182 #2, px 10 Acfer 182 #2, pl EET92042 #1, sp EET92042 #1, opx 0 ation line EET92042 #1, cpx EET92042 #1, pl EET92147 #1, sp Terrestrial fraction −10 EET92147 #1, cpx EET92147 #1, pl

O (‰) −20 17 

−30

Al-rich chondrules in CRs, −40 Allende CAI line without relict grains GRA95229 #9, pl, cpx, opx MAC87320 #1, ol, pl, opx −50 GRA95229 #7, pl, cpx, ol

−50 −40 −30 −20 −10 01020 (b) 18O (‰) Figure 15 Oxygen-isotopic compositions of individual minerals in (a) aluminum-rich chondrules from ordinary chondrites (Russell et al., 2000) and (b) CR carbonaceous chondrites (Krot et al., 2006a). Abbreviations: cpx, clinopyroxene; gl, glass; nph, nepheline; ol, olivine; opx, orthopyroxene; pl, plagioclase; px, pyroxene; sp, spinel. Chondritic Components 29 aluminum-rich chondrule bulk compositions, 1984; Misawa and Fujita, 1994; Krot et al., (2) aluminum-rich chondrules are not inter- 2006b). Relict CAIs are also present in AOAs mediate in an evolutionary sense between (Itoh et al.,2002; Hiyagon, 2000). These rel- known CAIs and chondrules, and (3) alumi- ict CAIs are very refractory, unlike those in num-rich chondrules are more closely related to aluminum-rich chondrules, being composed of normal chondrules, either by addition of an hibonite, grossite, spinel, melilite, perovskite, anorthite-like component to chondrule precur- and platinum group metals (Figure 16). The sors or possibly, in some cases, by volatilization relict CAIs in chondrules are corroded by host of chondrule or chondritic material of unusual chondrule melts, but have largely preserved oxygen-isotopic composition. Russell et al. the 16O-rich isotopic signatures of typical (2000), MacPherson and Huss (2005),and CAIs (Figure 17a). The only relict AOA Guan et al. (2006) concluded that ‘‘if alumi- observed inside a chondrule has also preserved num-rich chondrules were mixtures of fer- its 16O-rich isotopic signature (Figure 17b). romagnesian chondrules and CAI material, These observations indicate that some CAIs their bulk chemical compositions would re- were present in the chondrule-forming regions quire them to exhibit larger 16O enrichments and were melted together with chondrule than we observe. Therefore, aluminum-rich precursors, suggesting that at least some CAIs chondrules are not simple mixtures of these formed before chondrules. two components.’’ Magnesium isotope measurements of the However, Krot and Keil (2002) and Krot relict CAIs and CAI- and AOA-bearing chond- et al. (2001b, 2002a) showed that B15% of al- rules typically show no resolvable 26Mg* (Krot uminum-rich chondrules from different car- et al., 2002a; Itoh et al., 2002), implying upper 26 27 5 bonaceous chondrites are associated with relict limits for ( Al/ Al)I of o1 10 and CAIs (Figure 14). These relict CAIs are min- o5 106. These data suggest either these eralogically similar to CAIs in AOAs and con- CAIs formed without 26Al or their Al–Mg sys- sist of spinel, anorthite, 7aluminum-diopside, tems were reset during chondrule formation. and 7forsterite; most of them were extensively If these CAIs formed initially with a canonical 26 27 melted during chondrule formation. Krot and ( Al/ Al)I that was reset during chondrule colleagues inferred that aluminum-rich chond- melting, the host chondrules must have formed rules formed by melting of the ferromagnesian at least B2 Myr after the CAIs. chondrule precursors (magnesian olivine and The only exception is the hibonite-rich pyroxenes, Fe,Ni-metal) mixed with the refrac- relict CAI #9d from Adelaide (Figure 16d) tory materials, including relict CAIs, composed that shows clear excesses of radiogenic 26Mg in of anorthite, spinel, high-calcium pyroxene, hibonite that are correlated with 27Al/24Mg and forsterite. This mixing explains the relative ratio. The observed 26Mg* in hibonite together 16 26 27 enrichment of anorthite-rich chondrules in O withspineldefineanisochronwith( Al/ Al)I compared to typical ferromagnesian chond- of (3.770.5) 105. Melilite in this CAI rules (Figure 12), and the group II REE pat- shows no 26Mg*, while a mixture of hibonite terns of some of the aluminum-rich chondrules and melilite shows an intermediate 26Mg ex- (Misawa and Nakamura, 1988, 1996; Kring cess that appears to plot on a mixing line and Boynton, 1990). connecting melilite and the most 26Mg*- enriched hibonite. Two measurements of plagioclase in the host chondrule, which were made in the rim around the CAI, show 1.07.5.4.3 Relict CAIs and AOAs in 26 26 27 6 ferromagnesian chondrules no resolvable Mg* [( Al/ Al)Io5 10 ]. Krot et al. (2006b) concluded that when Mineralogical observations indicate that CAI #9d was incorporated into the surround- CAIs outside chondrules show no clear evid- ing chondrule melt, diopside, melilite (or ence for being melted during a chondrule- anorthite), and spinel in its Wark–Lovering forming event (Krot et al.,2002a,b,c,2005d). rim sequence began reacting with the sur- Relict CAIs inside ferromagnesian chondrules rounding chondrule melt to produce a layer of are exceptionally rare. Only six relict CAIs plagioclase, and melilite in the interior of the have been described in ferromagnesian chond- inclusion-exchanged magnesium with the sur- rules: in the unique carbonaceous chondrites rounding chondrule melt. The newly formed Acfer 094 and Adelaide, the CH chondrites plagioclase and the melilite inside the CAI Acfer 182 and Patuxent Range 91546, the CV no longer contain evidence of 26Mg*. This chondrite Allende, and the H3.4 ordi- constrains the timing of the chondrule melting nary chondrite Sharps (Bischoff and Keil, event to be at least 2 Myr after the CAI 30 Chondrites and Their Components

Acfer 094 #17

Zr-ph o1 chd

hib

PGE pv sf

CAI ol pl sp

µ µ 100 m (a) 20 m (b) Adelaide #9d Sharps #9

hib px mel met cpx mes CAI px chd pv hib nph met chd sp CAI ol sp ol

25 µm (c) 25 µm mes (d) PAT91546 #16 Y-81020 #1

cpx met

opx ol mes sf AOA grs sp ol mantle

pl

20 µm (e) 100 µm (f)

Figure 16 Backscattered electron images of chondrules containing relict refractory inclusions: #17 in Acfer 094 (a, b), #9d in Adelaide (c), #9 in Sharps (d), and #16 in PAT91546 (e). Relict CAI Acfer 094 (a, b) consists of hibonite (hib) with inclusions of perovskite (pv), a Zr-bearing phase (Zr-ph), and Re-, Ir-, Os-bearing (PGE) nuggets. It is surrounded by a shell of fine-grained, iron-spinel (sp). The host chondrule consists of ferrous olivine (ol), plagioclase mesostasis (pl), Fe,Ni-sulfides (sf), Cr-spinel, and relict grains of forsteritic olivine (fo). The relict CAI in Adelaide (c) consists of hibonite, perovskite, and melilite (mel); it is surrounded by a magnesium-spinel rim. Its host chondrule consists of forsteritic olivine, low-calcium pyroxene, glassy me- sostasis, and kamacitic metal. The relict CAI in Sharps (d) consists of hibonite surrounded by iron-spinel and a nepheline-like phase (nph). Its host chondrule consists of forsteritic olivine, low-calcium pyroxene, high-calcium pyroxene, anorthitic mesostasis, and kamacitic metal. The relict CAI in PAT91546 (e) has a grossite (grs) core surrounded by a spinel (sp) rim. This CAI is surrounded by an igneous chondrule-like material composed of plagioclase (pl), low-Ca pyroxene (opx), high-Ca pyroxene (cpx), olivine (ol), and Fe,Ni-metal (met), mesostasis (mes) (after Krot et al., 2005d). (f) Relict AOA #1 in a type II chondrule from the CO3.0 chondrite Y-81020 (after Yurimoto and Wasson, 2002). Chondritic Components 31

20 20 Adelaide CAI #9d Host chondrule 10 hib+mel 10 chd #9d ol Host chondrules 0 mes 0

Acfer 094 −10 CAI #17 −10 hib CCAM

O, (‰) O, sp±plag 17 − Terrestrial fractionation line −  20 chd #17 20 Terrestrial fractionation line ± Y-81020 CCAM plag sp ol AOA #1 −30 Acfer 182 −30 ol, core CAI #2 ol, mantle sp chd #1 −40 chd #2 −40 ol, Relict CAIs plag Relict AOA mes px −50 −50 −50 −40 −30 −20 −10 0 10 20 −50 −40 −30 −20 −10 0 10 20 (a) (b) 20 20

10 10 Relict chondrules Relict chondrules 0 0

−10 −10 Allende

O, (‰) O, Host CAI ABC 17 − an −  20 Terrestrial fractionation line 20 Terrestrial fractionation line fas Y-81020 mel Host CAI A5 −30 sp −30 mel Cr-sp mes di Relict chd A5 −40 Relict chd ABC −40 en CCAM en pgn ol CCAM −50 −50 −50 −40 −30 −20 −10 0 10 20 −50 −40 −30 −20 −10 0 10 20 (c) 18O (‰) (d) 18O (‰) Figure 17 Oxygen-isotopic compositions of chondrules with relict CAIs (a) and relict AOA (b), and type C CAIs with relict chondrules (c, d). (a) The relict CAIs are 16O-enriched compared to their host chondrules. (b) The core of the relict AOA (see Figure 16f) is 16O-enriched compared to the olivine phenocrysts and mesostasis of the host chondrule; olivine mantle overgrowing AOA has intermediate oxygen-isotopic com- positions. (c) Spinel and Al–Ti-diopside of the host CAI ABC are 16O-enriched; whereas other minerals are 16O-depleted to various degrees. Relict olivine and low-Ca pyroxene grains have 16O-poor compositions. (d) Spinel of the CAI core is 16O-rich, whereas other minerals are 16O-depleted to various degrees. The Al– Ti-diopside grains in the core are less 16O-depleted compared to those in the mantle. Relict olivine and high-Ca pyroxene halo around it have 16O-poor compositions. Al–Ti-di, Al–Ti-diopside; an, anorthite; cpx, high-Ca pyroxene; di, diopside; mel, melilite; mes, mesostasis; ol, olivine; plag, plagioclase; px, low-Ca pyroxene; sp, spinel. Data from Yurimoto and Wasson (2002) and Krot et al. (2005d).

formed, because 480% of 26Al had decayed. 1.07.5.4.4 Relict chondrules in igneous CAIs Thus, Adelaide CAI #9d and its surrounding and igneous CAIs overgrown by chondrule provide clear evidence of a Z2Myr chondrule material time gap between the formation of the CAI and its incorporation into the chondrule, Relict chondrules have been reported in three and this conclusion does not require an as- igneous CAIs—compact type A CAI A5 from sumption of homogeneously distributed 26Al Y-81020, coarse-grained, igneous, anorthite-rich in the chondrule-forming region (MacPherson (type C) CAIs ABC, and TS26 from Allende et al.,1995). (Itoh and Yurimoto, 2003; Krot et al., 2005d). 32 Chondrites and Their Components No relict chondrules have been found inside compositions (Figure 17c). The CAI anorthite AOAs or nonigneous CAIs. has a resolvable 26Mg* corresponding to the 26 27 6 CAI A5 has a melilite-rich core surrounded ( Al/ Al)I ratio of (4.771.4) 10 . by a very fine-grained mantle composed of TS26 is a mineralogically zoned type C CAI Al-diopside, spinel, and anorthite; the mantle (Figures 19c and 19d). The coarse-grained core is contains igneously zoned pyroxene grains surrounded by a thick, fine-grained mantle con- ranging in composition from enstatite to augite taining abundant relict grains of forsteritic oli- (Figure 18). The melilite and pyroxene grains vine and low-Ca pyroxene; the Wark–Lovering are similarly 16O-depleted, whereas the mantle rim layers are absent. Spinel and Al,Ti-diopside shows a wide compositional range and is of the CAI core are 16O-rich, whereas melilite, 16O-enriched (Figure 17d). Itoh and Yurimoto anorthite, Al,Ti-poor diopside of the CAI (2003) concluded that the pyroxene grains are mantle, and relict olivine and low-Ca pyroxene relict chondrule fragments incorporated into are 16O-depleted to various degrees (Figure 17c). the host CAI prior to its final melting in the Anorthite shows no resolvable 26Mg*; the 26 27 6 CAI-forming region. Alternatively, the lack of inferred ( Al/ Al)I ratio is o2.5 10 . Wark–Lovering rim layers around the host The relict nature of the olivine–pyroxene CAI, the estimated fast cooling rate fragments in the periphery of the CAIs does (4100 1Ch1) during crystallization of the not necessarily mean that they predated the CAI mantle, and the wide range of oxygen-iso- CAIs. Instead, the 16O-poor oxygen in the topic compositions of the CAI minerals over- periphery of the CAIs and resetting of their lapping with those of relict chondrule 26Al–26Mg systematics, provide evidence that fragments, may indicate that this compound they experienced a late remelting event. During CAI-chondrule object experienced late-stage, this event, incorporation of the chondrule-like, incomplete melting and oxygen-isotopic ex- olivine–pyroxene material could have occurred change in an 16O-rich gaseous reservoir during (Krot et al., 2005d). This interpretation is chondrule formation (Russell et al., 2005). consistent with some igneous CAIs being over- Type C CAI ABC in its peripheral zone con- grown by chondrule-like material (Figures 19e tains a relict olivine-pyroxene fragment that is and 19f). We note that the apparent discrep- corroded by Al-diopside and surrounded by a ancy in interpretation of chondrule-bearing halo of high-Ca pyroxene (Figures 19a and igneous CAIs (Itoh and Yurimoto, 2003; Krot 19b). Spinel and Al,Ti-diopside of the host CAI et al., 2005d) is largely due to differences in are 16O-enriched, whereas melilite, anorthite, definitions of CAI-formation. According to olivine, and low-Ca pyroxene are 16O-depleted; Itoh and Yurimoto (2003), all high-tempera- Cr-spinel, high-Ca pyroxene, and Al,Ti-poor ture processes that affected CAIs should be diopside have intermediate oxygen-isotopic called CAI-forming events, whereas Krot et al.

oI

ol tr mes

en CAI mel chd

aug

tr pg mx 20 µm

Figure 18 Backscattered electron image of the chondrule-bearing CAI A5. The CAI consists of a polycrys- talline melilite core surrounded by a porous mesostasis composed of a fine-grained mixture of Al-rich clinypyroxene (lighter area) and Al-rich glass (darker area). The mesostasis encloses a pyroxene assemblage dominated by enstatite with minor pigeonite and augite; troilite and Fe,Ni-metal (white spots) are scattered in the mesostasis near the pyroxene assemblage. Chondritic Components 33

ABC Mg:Ca:Al cpx sf sec di di px an nph ol

sod sp di 200 µm (a) X700 20 µm TS26 Mg:Ca:Al 20 µm (b) Mantle ol

ol di fa+sod cpx core px an sod sf di sp 200 µm (c)

93 cpx Mg:Ca:Al X850 µ 20 µm 20 m (d)

an cpx

px mel sp sp

di an

di

nph fa+sod sod µ µ 50 µm 500 m (e) 50 m X300 (f)

Figure 19 Combined elemental maps in Mg (red), Ca (green), and Al Ka (blue) X-rays (a, c, e) and back- scattered electron images (b, d, f) of the Allende type C CAIs associated with chondrule-like ferromagnesian silicates. (a, b) CAI ABC. (c, d) CAI TS26. (e, f) CAI 93. Regions outlined in ‘‘a,’’ ‘‘c,’’ and ‘‘d’’ are shown in detail in ‘‘b,’’ ‘‘d,’’ and ‘‘e,’’ respectively. an, anorthite and anorthitic plagioclase; cpx, high-Ca pyroxene; di, diopside; mel, melilite; nph, nepheline; px, low-Ca pyroxene; sec, secondary sod, sodalite; sp, spinel. Reproduced by permission of Astronomical Society of the Pacific from Russell et al. (2005).

(2005d) define CAI-forming events as those in size in type 2 and 3 ordinary and carbon- which occurred in the CAI-forming regions, aceous chondrites. Although most isolated that is, regions with high ambient temperatures olivines are probably formed by disaggregat- 16 (41350 K), relatively low pressure, O-rich ion of chondrules (McSween, 1977b; Roedder, nebular gas, and approximately solar 1981), Steele (1986, 1989) argued that the conditions. extremely forsteritic grains (Fao2) in carbon- aceous chondrites, which have a blue cathodo- 1.07.5.5 Refractory Forsterite-Rich Objects luminescence color, high concentrations of CaO and Al2O3 (typically 0.5 and 0.25 wt.%, A long-running controversy has existed over respectively) and comprise a few percent of all the origin of refractory forsterites 0.1–0.5 mm olivines, did not crystallize in chondrules. 34 Chondrites and Their Components Steele suggested that the refractory forsterites (Chaussidon et al.,2006), could not have may have condensed from the nebula, formed formed in the solar nebula and may be derived relict grains in chondrules, and were an impor- from planetesimals that were disrupted. How- tant component of chondrule precursor mate- ever, if these planetesimals existed, they have not rial. Weinbruch et al. (2000) showed that supplied us with meteorites, as differentiated isolated refractory forsterites were more 16O- meteorites have D17O4–3% (Chapter 1.05). rich than olivines that crystallized in chond- Whatever their origin, refractory 16O-enriched rules and also favored a condensation origin. forsterites appear to be an important compo- However, Jones et al. (2000b) showed that nent of type I chondrule precursor materials. there are rounded objects composed largely of a single large forsterite crystal with traces of low-Ca pyroxene and very little mesostasis that 1.07.5.6 Chondrules have compositions like those of the isolated forsterites. Jones et al. (2000b) inferred that Chondrules are igneous-textured particles these objects were chondrules, that relict fors- composed mainly of olivine and low-calcium terites from such objects were present in other pyroxene crystals set in a feldspathic glass or type I chondrules, and that refractory 16O-rich microcrystalline matrix. (Chondrules with 410 forsterites were primitive nebular components. wt.% Al2O3, which have closer links with CAIs are reviewed in Section 1.07.5.4.) Many chond- Pack et al. (2004, 2005) found o0.01– 0.4 vol.% luminescent refractory forsterites in rules with FeO-poor silicates also contain me- a variety of ordinary, carbonaceous, and R tallic Fe,Ni grains, often clustered near the chondrites, and found no difference in their periphery. Metal grains outside chondrules that chemical and isotopic compositions. Highly re- are of comparable sizes to those inside have fractory forsterites with 40.6 wt.% CaO and geochemical properties suggesting they were 16 17 once ejected from or broken out of chondrules, o0.5 wt.% FeO are enriched in O with D O between –4% and –10%, whereas moderately or in the case of the CB group (where reduced refractory and nonrefractory olivines range be- chondrules lack metal), that metal and chond- tween –5% and þ 2%. They inferred that the rules formed in the same environment. A close refractory forsterites come from a single com- link between metal and reduced silicates is con- mon reservoir and that they formed in a red- sistent with mineral–gas equilibration in the ucing nebula gas of solar composition. The solar nebula as forsterite, enstatite, and metal- oxygen-isotopic and chemical compositions of lic Fe,Ni are stable over similar temperature the refractory forsterites lie beyond those at the ranges (figure 2 of Chapter 1.15). The miner- FeO-poor end of the sequence defined by alogical, chemical, and isotopic properties of olivine-rich, FeO-poor type IA chondrules chondrules have been reviewed most recently and pyroxene-rich type IB chondrules (see by Jones et al. (2005). Jones, 1994; Section 1.07.5.6.1). Melt inclu- Links between asteroids and chondrites sions in refractory forsterites are not in equi- suggest that chondrules are abundant in the librium with the olivine hosts—either because inner part of the asteroid belt and that the forsterite crystallized outside its stability field terrestrial planets probably formed from (McSween, 1977b), or because the melt reacted chondritic materials. Thus, much of the mass more readily with nebular gas than the crystal- that accreted in the inner solar system would lizing solids (Varela et al., 2005; Pack et al., have been derived from chondrules. In the 2004, 2005). absence of sticky organics, dry grains of silicate Refractory forsterite-rich objects are clearly do not readily adhere so that chondrule forma- a fascinating component of chondrites and may tion may have been the process that triggered provide a possible link between refractory accretion in the inner solar system (e.g., Wood, inclusions, especially AOAs, and chondrules. 1996a; Cuzzi et al.,2001; Scott, 2002). Under- Their 16O-enriched composition points to an standing the origin of chondrules has been a early solar nebula origin, though their forma- major goal of meteorite studies since chondrites tion ages have not been determined. Rounded, were first described over 200 years ago. refractory forsterite-rich objects may be a kind of proto-type-I chondrules, as normal 1.07.5.6.1 Chondrule textures, types, and type I chondrules show evidence for gas–solid thermal histories reactions during cooling( Section 1.07.5.6.5). However, Libourel and Krot (2006) described Chondrule textures are conveniently divided coarse-grained forsteritic aggregates with into two types: porphyritic, which have large granoblastic textures inside type I chondrules crystals of olivine and/or pyroxene in a fine- in CV chondrites and inferred that these igne- grained or glassy matrix, and nonporphyritic, ous objects, which are also 16O-enriched which include those with cryptocrystalline, Chondritic Components 35 radial-pyroxene, and barred-olivine textures Laboratory experiments constrain the initial (Figures 1 and 20). Table 4 shows how the conditions and thermal history required for proportions of different types vary considera- each textural type, but there are too many bly among the groups. Carbonaceous chond- variables (e.g., initial state of chondrule, peak rites tend to have the highest fraction of temperature, time at peak temperature, and porphyritic chondrules, but CH and CB groups cooling rate) to make very quantitative state- are exceptions. Mean sizes and abundances of ments (Lofgren, 1996; Hewins, 1997; Hewins chondrules in each group are given in Table 1. et al., 2005). Porphyritic chondrules crystallized from melts with many nuclei after incomplete melting of fine-grained, precursor materials (Lofgren, 1996; Connolly et al., 1998). Peak temperature and duration of heating are linked as porphyritic textures can be reproduced by heating fine-grained material to B100 1C above the liquidus temperature (generally 1,350– mes 1,800 1C) for 5 min, or just below the liquidus temperature for hours (Hewins and Connolly, ol met 1996; Lofgren, 1996). Nonporphyritic chond- rules crystallized from melts that were super- Dusty ol heated above their liquidus for long enough to destroy all solids that could act as nuclei. For barred olivine and radial pyroxene chondrules, 400 µm (a) nucleation was probably induced by collision with solid grains (Connolly and Hewins, 1995). Radial textured chondrules crystallized while 1 1 ol cooling at 5–3,000 Ch , barred olivine chondrules at 500–3,000 1Ch1, and porphyri- sf tic chondrules at 5–100 1Ch1 (Desch and mes Connolly, 2002). Cooling rates above the liq- uidus may have been much higher; below the solidus, chondrules cooled more slowly at rates of 0.1–50 1Ch1 (Weinbruch et al., 2001). Porphyritic chondrules are classified into type I, which are FeO-poor in unmetamorphosed µ (b) 200 m chondrites, and type II, which are FeO-rich. Type I chondrules account for 495% of chond- rulesinCO,CV,CR(Figure 1a), and CM chondrites and olivine-rich varieties were prob- ably abundant in the material that accreted to form the Earth (Hewins and Herzberg, 1996). Types I and II are both subdivided into A, which are olivine-rich (490 modal percent olivine), and B, which are pyroxene-rich (Jones, 1990, 1994, 1996a, b). Olivines in type IA chondrules in unmetamorphosed chondrites are mostly Fa0.3–5 and Fa2–10 in IB; type II chondrule µ olivines are Fa in ordinary chondrites and (c) 100 m 10–20 up to Fa50 in carbonaceous chondrites. Figure 20 Backscattered electron images of three Rounded type I chondrules commonly have chondrules in Tieschitz (H/L3.6) chondrite. (a) Type low-calcium pyroxene rims, glassy mesostases, IA porphyritic chondrule composed largely of fors- and grains of metallic Fe,Ni, whereas type II teritic olivine (ol), mesostasis (mes), metallic Fe,Ni chondrules lack these features, contain small droplets (met) (white). Dusty olivines are relict, FeO- and commonly have larger olivines rich olivines that crystallized elsewhere and formed (Figure 20; Scott and Taylor, 1983). Type I and tiny metallic Fe,Ni particles when heated in the II chondrules can be identified from these fea- chondrule melt. (b) Type IIA porphyritic chondrule containing large euhedral, FeO-bearing, olivine tures in many type 2–6 chondrites so that meta- phenocrysts, dark mesostasis (mes), and white sulfide morphic and alteration effects can be tracked droplets (sf). (c) Nonporphyritic chondrule contain- (McCoy et al.,1991; Hanowski and Brearley, ing fine pyroxene crystals that appear to radiate from 2001). Type I chondrules also occur as finer the upper edge of the chondrule. grained, irregularly shaped aggregates, which 36 Chondrites and Their Components

Table 4 Proportions of chondrule types in chondrite groups.

Type/group CM CO CV CR CH CBb CK H–L–LL EH EL R Porphyritic Olivine 8 23 0.1 Oliv-pyx. 69 48 4 Pyroxene 18 10 77 Granular oliv-pyx. o0.1 o0.1 3 1 Total porphyritic B95 95 94 96–98 20 B1 499 84 82 B87 B92 Nonporphyritic Barred olivine 2 6 1 4 0.1 Radial pyx. 2 0.2 7 13 Cryptocrystalline 1 0.1 79a 55 Total nonporph. B5 5 6.3 2–4 80 B99 o11618B13 B8 Sources: Rubin (2000), Kallemeyn et al. (1994), Weisberg et al. (2001), Scott (1988). a Includes radial pyroxene and glass chondrules. represent partly melted dust aggregates or aggregates of partly melted particles (e.g., Rubin and Wasson, 2005). In ordinary chondrites, which have very diverse textured chondrules, all chondrules having olivines with Fao10 1 may be referred to as type I by some authors (Hewins, 1997). Chondrules have also been classified using their cathodoluminescence properties and the compositions of olivines and mesostases (Sears 0.1 et al., 1995a). For unmetamorphosed chond- rites, groups A and B in this scheme correspond IAB,B

to types I and II. During metamorphism, the to Cl and Si relative Abundance cathodoluminescence properties and chondrule IA group change. The merits of the two schemes 0.01 were reviewed by Scott et al. (1994) and Sears Al Ca Ti Mg Si Cr Mn K Na et al. (1995b). Grossman and Brearley (2005) caution that the classification criteria based on Figure 21 Elemental abundances in type IA chond- the compositions of chondrule mesostases need rules, which are olivine-rich and type IB and IAB to be revised because of Na loss during electron chondrules, which are pyroxene rich, from the Se- markona (LL3.0) chondrite normalized to CI chond- probe analysis. rites and silicon. Type IA chondrules are enriched in refractories relative to IAB and IB. For moderately volatile elements, all type I chondrules are depleted, 1.07.5.6.2 Compositions of chondrules with most IA showing larger depletions than types IB and IAB. Reproduced by permission of Elsevier Chemical compositions of chondrules have from Jones (1994). been determined from extracted samples using neutron activation analysis and by in situ anal- ysis in polished sections using electron micro- silicon-poor type IA chondrules (Figure 21). probe and ion probe analysis (e.g., see Gooding The source of the fractionations in type I et al., 1980; Grossman et al., 1988; Alexander, chondrules is discussed below. 1995). Chondrules typically show flat refrac- Oxygen-isotopic compositions of large tory abundances that are relatively close to the individual chondrules suggest that chondrules mean chondrite value and abundances of mod- formed from several different reservoirs that were erately volatile elements that scatter more poorer in 16O than the CAI source (Figure 22). widely about the mean. Type II chondrules Chondrules in ordinary chondrites generally are relatively unfractionated with near-CI plot above the terrestrial fractionation line, levels of refractories and moderately volatile chondrules plot on or near elements. However, type I chondrules show the line, and carbonaceous chondrite chond- systematic depletions of moderately volatile rules lie below. Ion probe analyses for oxygen elements and a broader spread of refractory isotopes in olivine show that most ferromagne- abundances with the silicon-rich type IB chond- sian chondrules have relatively homogeneous rules being poorer in refractories than the compositions due to exchange between Chondritic Components 37

6 composition between that of the Sun and the R OC major trapped component in type 1–3 chondrites (‘‘Q-type’’ gases) and occurs in E4–6 chondrites 4 (Patzer and Schultz, 2002b), CR2, CH3, CBa3, EH and K3 chondrites (see Bischoff et al., 1993). 2 Okazaki et al. (2001) inferred that solar wind CR had been implanted into chondrule precursor TF 0 material near the protosun and then fractionated CCAM during melting to leave a subsolar pattern. CV However, subsolar noble gases are present in − 2 CM several other sites suggesting that the subsolar O (‰ rel. SMOW) O (‰ rel.

17 gases were redistributed (Busemann et al., 2002).

 CO −4 Further laser microprobe analyses of chondrules and other components in the most pristine chondrites are clearly needed to establish which −6 chondrules, if any, acquired solar gases from the nebula or were irradiated by high-energy parti- −8 cles near the inner edge of the disk. −4 −2 02468 18O (‰ rel. SMOW) Figure 22 Oxygen-isotopic plot showing fields of 1.07.5.6.3 Compound chondrules, relict grains, analyses of large chondrules in chondrite groups. and chondrule rims The data show that chondrules are derived from many isotopically distinct reservoirs. Reproduced by Chondrules in all groups except CB and CH permission of Elsevier from Rubin (2000). show evidence for collisions: with each other while partly molten and with solid grains while molten and solid. Compound chondrules form chondrule melt and nebula gas; but some type I by collisions of partly molten chondrules and 16 chondrules have O-rich forsterites that were their abundance gives some constraint on the probably present in precursor materials (see number density of chondrules in the region references in Scott and Krot, 2001; Jones et al., where they formed. Unfortunately, there are 2004; Krot et al., 2006a). Chondrules are iso- large uncertainties in chondrule speed and topically homogeneous in comparison to CAIs: the provenance of the joined chondrules, 50 Ti anomalies (see Niemeyer, 1988) and mass which make estimates of the chondrule density fractionation effects (e.g., Clayton et al., 1985; very uncertain. About 1–2% of chondrules in Misawa and Fujita, 2000; Alexander and ordinary and CV chondrites are compound Wang, 2001; Alexander et al., 2000) are much (Wasson et al., 1995; Akaki and Nakamura, smaller than those in many CAIs. Inferred 2005). In over half of these, the chondrules that 26 initial concentrations of Al in chondrules and collided have such similar compositions that 207 206 Pb/ Pb dating suggest that chondrules they could have been derived from a single formed 1–4 Ma after CAIs (Kita et al., 2000; large droplet. Other compound chondrules Mostefaoui et al., 2002; Amelin et al., 2002). may have formed by part melting of a porous See Gilmour (2002), Gilmour et al. (2006), Kita aggregate containing an embedded chondrule: et al. (2005), and Chapter 1.16 for reviews of relatively few compound chondrules probably chondrule formation ages. formed by random collisions. However, Ciesla Effects of secondary processes in modifying et al. (2004b) infer that at least 5% of chond- volatile elements (Grossman et al., 2000, 2002) rules are compound and that chondrules and oxygen isotopes in chondrules, especially formed where solids were concentrated by in ordinary chondrites (Bridges et al., 1998; 445 times above the canonical solar nebula Franchi et al., 2001) require further study (see value, possibly at the nebula midplane. Chapter 1.09). Foreign grains in chondrules appear to have Trapped and cosmogenic noble gases in been derived from other chondrules (Jones, chondrules offer clues to their irradiation his- 1996b). Relict low-FeO olivines from type I tory and formation but not definitive constraints chondrules can be found in high-FeO, type II (see Chapter 1.14). Trapped noble gases are chondrules and vice versa, indicating that chond- generally absent or rare in chondrules, but signi- rule formation was a repeated process and that ficant amounts of trapped 36Ar, 84Kr, and 132Xe material was recycled through the chondrule from a so-called subsolar component were found furnace (e.g., Kunihiro et al., 2004b, 2005a). in chondrules in an EH3/4 chondrite (Okazaki Chondrules may have two types of rims: fine- et al., 2001). This component is intermediate in grained matrix rims and coarse-grained igneous 38 Chondrites and Their Components rims: both are missing in CH and CB chond- of dust aggregates during melting events that rites. Matrix rims vary in composition but the lasted seconds to minutes, or by high dust–gas variations are uncorrelated with chondrule ratios or gas pressures. composition (Section 1.07.5.7). However, the The major difficulty with closed-system mod- compositions of igneous rims are related to els is that they do not explain why some those of the host chondrule: type I chondrules chondrules are olivine-rich and others are have igneous rims that are FeO poor, while pyroxene-rich. It is very difficult to see how type II chondrules have FeO-rich rims (Krot fine-grained nebula dust could have been sorted and Wasson, 1995). The igneous rims formed into high Mg/Si dustballs for olivine chond- by collisions between chondrules and smaller rules and low Mg/Si dustballs for pyroxene- partly molten particles, or solid particles with rich chondrules (e.g., Wood, 1996a). Although diverse oxygen-isotopic compositions that were matrix contains both phases, they are not accreted and later incompletely melted (Naga- segregated into chondrule-sized volumes. The shima et al., 2003). only plausible way to produce both olivine- and From the abundances of igneous rims, com- pyroxene-rich chondrules is by high-tempera- pound chondrules, and relict grains and the ture reactions between condensed material and dearth of unmelted, chondrule-sized dustballs, nebular gas. New evidence, especially from we infer that collisions between partly melted CR–CH–CB clan chondrites, supports this objects and remelting of aggregates from such thesis. collisions were important processes that ena- bled chondrules to form from fine particles. 1.07.5.6.5 Open-system crystallization

1.07.5.6.4 Closed-system crystallization Unlike the archetypal chondrules, type I chondrules show a variety of evidence for Understanding whether chondrules acted volatility-controlled chemical fractionations as open or closed systems during chondrule that may have occurred during chondrule formation is crucial for elucidating their origin formation. Hewins et al. (1997) studied micro- (Sears et al., 1996). Type II chondrules appear porphyritic and cryptoporphyritic type I to be good candidates for closed system crys- chondrules in Semarkona (LL3.0), which have tallization, as chemical zoning profiles across irregularly shaped interstitial metal grains their phenocrysts are consistent with fractional rather than the rounded droplets in mesostasis crystallization (Jones, 1990; Jones and Lofgren, common in coarser type I chondrules. They 1993). Type II chondrules contain significant found an inverse correlation between grain size concentrations of sodium and other volatile and abundance for the moderately volatile elements that would have been readily lost if elements, sodium, potassium, and , as the chondrules had been molten for more than well as iron and nickel (Figure 23). Hewins a few minutes (Yu and Hewins, 1998). Such et al. inferred that volatilization increased with chondrules are commonly considered as arche- the extent of melting of a fine-grained precursor typal chondrules formed by closed-system mel- having a chondritic composition. Alternatively, ting. Although most workers infer that type II these very fine-grained chondrules may have chondrules resulted from single heating events, formed by aggregation of partly melted parti- Wasson and Rubin (2003) argue for multiple cles that scavenged volatiles with an efficiency events with low degrees of melting, which that depended on their grain size. Despite the would help to explain how voltatiles were large increase in alkali concentrations from retained. type IA to type IB chondrules, they do not Many workers once argued that all chond- show any systematic variation in K isotopic rules formed by isochemical melting of precur- composition (Alexander and Grossman, 2005). sor material and that the chemical fractionations Mesostases in some type IA chondrules in among chondrules, except for metal loss, result Semarkona show volatility-related chemical from processes that predate chondrule forma- zoning (Matsunami et al., 1993; Nagahara tion (e.g., Gooding et al.,1980). Few workers et al., 1999). Mesostasis near the rims is higher hold this view now, nevertheless it is commonly in volatiles and lower in refractories than held that chemical fractionations prior to mesostasis near the core. Matsunami et al. chondrule formation were much more impor- invoked recondensation and reduction proc- tant than any fractionation during chondrule esses in the solar nebula. The chemical varia- formation, besides loss of metal (e.g., Gross- tions across type I chondrules and those in man, 1996). Thus, the standard model for microporphyritic and cryptoporphyritic type I chondrule formation envisages that volatile loss chondrules (Figure 23) mimic the fractionation from chondrules was minimized by flash melting patterns shown by the bulk chemical data for Chondritic Components 39

10

Cl 1

3−4

0.1 5−20

20−30 0.01 − Chondrule grain size (µm) 30 1,000 Abundance relative to CI and Mg

0.001 Ca Al Ti Mg Si Cr Mn K Na Fe Ni P S Figure 23 Elemental abundances in microporphyritic and cryptoporphyritic type I chondrules in Semarkona showing an inverse correlation between grain size (in micrometer) and abundance for the moderately volatile elements (chromium to sulfur). Hewins et al. (1997) attribute this to volatilization during melting. Reproduced by permission of National Institute of Polar Research from Hewins et al. (1997).

chondrules in the IA–IB sequence (Figure 21), was proposed for chondrules in CBb chond- suggesting single-process-controlled chemical rites, which have unusual textures and compo- variations within and among type I chondrules. sitions (Krot et al., 2001d, 2006c). The The concentration of pyroxene phenocrysts chondrules are cryptocrystalline or skeletal around the periphery of many type I chond- olivine, lack igneous or fine-grained rims and rules was reproduced experimentally by Tis- relict grains, and do not form compound ob- sandier et al. (2002), who allowed melt to react jects (Figure 25). They clearly formed from to- with gaseous SiO during crystallization. They tal melts in a dust-free environment. Some inferred that such gas–melt interactions were cryptocrystalline chondrules are embedded in prevalent during chondrule formation, and zoned metal grains, which have compositional invoked high dust–gas ratios to stabilize chond- profiles consistent with nebular condensation rule melts in the nebula (Ebel and Grossman, (Krot et al., 2001d, 2006c). Chondrules have 2000). Further evidence for gas–melt interac- uniform flat refractory element abundances but tions is provided by silicon-rich, igneous rims the variation ((0.02–3) CI) is far wider than in on many type I chondrules in CR2 chondrites other chondrites (Figure 26). The depletions of (Krot et al., 2004b). These rims contain high- refractory elements relative to silicon in the calcium and low-calcium pyroxene, glassy cryptocrystalline chondrules indicate that silicon mesostasis, Fe,Ni, and silica, with high con- or the refractory elements, or both, condensed centrations of chromium and manganese and into chondrules or their precursors. The near-CI low concentrations of aluminum and calcium bulk abundances of refractory elements in in high-calcium pyroxene (Figure 24). Meso- the chondrites favor closed-system fractional stasis zoning follows that in the ordinary condensation. Moderately volatile elements in chondrites with silicon, sodium, potassium, chondrules show large depletions (Figure 26) and manganese increasing and calcium, magne- that are inversely correlated with condensation sium, aluminum, and chromium decreasing temperature, as for bulk chondrites (Figure 3) from the olivine-rich core, through the pyrox- and type I chondrules (Figure 21). Chondrule ene-rich rim to the silicon-rich rim. FeO textures are consistent with condensation of remains nearly constant. Krot et al. (2004b) melts. infer that gas–melt fractional condensation or gas–solid condensation followed by remelting 1.07.5.6.6 Chondrules that formed on asteroids produced these chemical fractionations. Isola- tion of early-formed forsterite condensates may Chondrites contain a small fraction of have resulted naturally from crystallization, as chondrules that may have formed on asteroids. forsterite would have equilibrated with the One rare type of chondrule (B0.1%) in type nebula more slowly than the melt. 4–6 ordinary chondrite breccias is composed Formation of chondrules from a gas–melt largely of plagioclase (or mesostasis of plagio- impact plume, some by gas–melt condensation, clase composition) and chromite (Krot et al., 40 Chondrites and Their Components

Mg K Si K Ni K

250250 µm (a) (b) (c)

SIR

POP − − µ 200200 mmmm JSM 5900 190190 30mm30mm JSM 5900 220000 m (d) 100 µm (e)

Figure 24 X-ray elemental scanning maps using Mg Ka (a), Si Ka (b), and Ni Ka (c), plus backscattered electron images (d, e) of a typical type I porphyritic chondrule in the QUE 99177 CR2 chondrite, which has a forsterite-rich core, a porphyritic olivine–pyroxene rim (POP) and an outermost silica-rich rim (SIR). The Ni Ka map (c) shows that metallic Fe,Ni grains in the chondrule core are richer in nickel than those in the outer portions. The zoned structure of the chondrule can be explained by gas–melt fractional condensation or gas– solid condensation followed by remelting (see Krot et al., 2003d).

1993). Krot and Rubin (1993) suggest that refractory oxides above B1,600 K at 104 atm, these chondrules may have formed by impact and grains composed predominantly of iron, melting as they find impact melts with similar cobalt, and nickel, which condense with forste- compositions inside shocked ordinary chond- rite and enstatite at B1,350–1,450 K (Campbell rites and such chondrules are absent in type 3 et al., 2005). The former are associated with chondrites. In addition, some chromite-rich CAIs (Palme and Wlotzka, 1976) and the latter chondrules contain chromite-rich aggregates, with chondrules, typically type I or FeO-poor which appear to be fragments of equilibrated chondrules (B&J, 1998, pp. 244–278). Unfortu- chondrites. Other possible impact products nately, few chondrites preserve a good record of were described in LL chondrites, which are the formation history of their metal grains mostly breccias, by Wlotzka et al. (1983). They because subsequent low-temperature reactions found microporphyritic potassium-rich clasts commonly formed oxides and sulfides and ther- with highly variable K/Na ratios that are mal metamorphism allowed kamacite to exsolve totally unlike chondrules in type 3 chondrites. from taenite. Most refractory nuggets have been Large chondrules and clasts in Julesberg (L3) studied in Allende CAI, where rampant oxida- may also be impact melt products according to tion and sulfurization and redistribution of Ruzicka et al. (1998). volatile tungsten and molybdenum compounds have severely compromised the high-tempera- ture nebular record (Palme et al., 1994). 1.07.5.7 Metal and Troilite Most grains of metallic Fe,Ni and troilite outside chondrules were probably lost from Two kinds of metal are found in chondrites: chondrules when they were molten. The sizes of grains composed of refractory elements (iridium, prominent grains are roughly correlated with osmium, ruthenium, molybdenum, tungsten, chondrule size (Skinner and Leenhouts, 1993; and rhenium), which condense along with the Kuebler et al., 1999; Schneider et al., 2003), Chondritic Components 41

cpx mes ol cpx

ol px 200 µm mes (a) 200 µm (c)

cpx ol ol px mes cpx ol

µ µ 25 m mes (b) 50 m (d)

Ni Kα

CC

CC µ (e) 150 m (f)

Figure 25 Combined elemental map in magnesium (red), calcium (green), and Al Ka (blue) X-rays (a, c) and backscattered electron images (b, d) of two barred or skeletal olivine chondrules, and a combined X-ray map (e) and NiKa scanning map (f) of a zoned Fe,Ni metal grain with an enclosed cryptocrystalline chondrule (CC) in the CBb chondrite HaH 237. The chondrules, which contain forsteritic olivine (ol), low-calcium pyroxene (px), high-calcium pyroxene (cpx), and mesostasis (mes), lack rims and relict grains and clearly formed in a dust-free environment from total melts. Reproduced by permission of the University of Arizona on behalf of The Meteoritical Society from Krot et al. (2002c). but were probably modified by shock after coexisting low-nickel kamacite and high-nickel compaction. Grains of metallic Fe,Ni in most taenite showing that these chondrites were not unshocked type 3–6 chondrites provide a heated above B300 1C for more than a year record of slow cooling at B1–1,000 K Myr1 (Reisener et al., 2000). Metal grains are not through the temperature range B550–350 1C, present in most chondrules in CH chondrites, when kamacite and taenite ceased to equili- which are cryptocrystalline, and are entirely brate (Wood, 1967). In most type 2 and 3.0–3.3 absent from chondrules in CB chondrites. chondrites, metallic Fe,Ni grains typically contain concentrations of 0.1–1% chromium, silicon, and phosphorus, which are not found 1.07.5.7.1 CR chondrite metallic Fe,Ni in type 4–6 chondrites, and reflect high- temperature processing prior to accretion. CR2 chondrites contain 5–8 vol.% metallic The most pristine metallic Fe,Ni grains are Fe,Ni grains in type I chondrules and between found in the CR–CH–CB clan (Krot et al., them, which are not associated with troilite. 2002c). Most metal grains with 48% Ni lack Grains outside chondrules were probably once 42 Chondrites and Their Components

16 Ni 12 Co Cr 1 8 4 0.4 0.2 0.1 0.0

Concentration (wt.%) Concentration 0 100 200 300 400 500 (a) 0.01 3.5 Pd Elements/Si/relative to CI SO 3.0 Os CC 2.5 Ir Pt 0.001 2.0 Ca AI Ti Mg Si Cr Mn Na 1.5 1.0 Figure 26 Bulk concentrations of lithophile ele- Element/Fe/Cl 0.5 ments normalized to CI chondrites and silicon in 0 100 200 300 400 500 skeletal-olivine and cryptocrystalline chondrules in- cluding cryptocrystalline inclusions in metallic Fe,Ni (b) in two CBb chondrites (HH 237 and QUE 94411). 5 Mo Shaded regions show compositional range of chond- 4 Ru rules in other carbonaceous chondrites, excluding Rh the CH group chondrules that are closely related to 3 CB chondrules. The wide range of refractory abun- 2 dances in CBb chondrules ((0.02–3) CI levels) 1 appears to reflect fractional condensation with Element/Fe/Cl skeletal-olivine chondrules condensing at higher 0 100 200 300 400 500 temperatures than cryptocrystalline chondrules. (c) Distance (µm) Both types are more depleted in moderately volatile Figure 27 Concentration profiles of siderophile el- elements than other carbonaceous chondrites. After ements in a radially zoned Fe,Ni grain in the CBb Krot et al. (2002c). chondrite, QUE 94411: (a) electron microprobe data; (b, c) trace element data from laser ablation associated with chondrules. Metal grains have ICPMS (Campbell et al., 2001). The nickel, cobalt, bulk concentrations of 4–15 wt.% Ni, solar and chromium profiles can be matched by nonequi- B librium nebular condensation assuming an enhanced Co/Ni ratios and up to 1% chromium and dust–gas ratio of B36 solar, partial condensation phosphorus (Weisberg et al., 1993). Grains at of chromium into silicates, and isolation of 4% of chondrule cores have higher nickel concentra- condensates per degree of cooling (Petaev et al., tions than those at the rims (Figure 24c)andall 2001). Concentrations of the refractory siderophile grains are depleted in moderately volatile ele- elements, osmium, iridium, platinum, ruthenium, ments (Connolly et al., 2001). Kong and Palme and rhodium, are enriched at the center of the grain (1999) noted that Renazzo metal grains are by factors of 2.5–3 relative to edge concentrations, covered with layers of carbon, which appear to which are near CI levels after normalization to iron. have evolved at low temperatures. Two kinds of Reproduced by permission of University of Arizona processes have been invoked to account for the on behalf of The Meteoritical Society from Krot et al. (2002c). origin and composition of the metal grains: con- densation as nebular solids or recondensation during chondrule formation (Grossman and all the zoned metal grains in CBb chondrites Olsen, 1974; Weisberg et al., 1993; Connolly (Meibom et al.,2001; Campbell et al.,2001) et al.,2001) and silicate–metal equilibration and have radial chemical zoning patterns that are reduction during chondrule formation (Scott consistent with formation as solid nebular con- and Taylor, 1983; Zanda et al.,1994). Given densates above 1,200 K (Figure 27). Fractional that the silicate portions of type I chondrules in condensation models suggest that grains in CR2 chondrites have retained a record of QUE 94411 formed under a variety of oxida- condensation processes, it seems likely that the tion conditions with enhanced dust–gas ratios metallic portions have also, and that both kinds of 10–40 times solar prior to vaporiza- of processes were involved in making CR metal. tion (Petaev et al.,2001). Kinetic growth models suggest growth timescales of 1–4 days 1.07.5.7.2 CH and CB chondrite metallic at cooling rates of a few degrees per hour and Fe,Ni removal from hot gas before temperatures fell below B1,200 K (Meibom et al.,2001). The tiny Between 0.01% and 1% of the metal grains in fraction of appropriately zoned grains in CH CH chondrites (Meibom et al., 1999, 2000)and chondrites might have condensed in convective Chondritic Components 43 updrafts in the nebula and been transported to 5 mm in size that rims chondrules, CAIs, and outward to cooler regions by convection other components and fills in the interstices (Meibom et al.,2000). However, the large frac- between them (Scott et al.,1988). Matrix grains tion of zoned grains in CBb chondrites requires are generally distinguished from fragments of a more efficient transport mechanism such as an chondrules, CAIs, and other components by X-wind (Meibom et al.,2001; Shu et al.,2001). their distinctive sizes, shapes, and textures. CB chondrites also contain relatively homo- Minerals found in matrices include silicates, geneous metal grains with micrometer-sized oxides, sulfides, metallic Fe,Ni, and espe- troilite droplets. Large grains in CBa chondrites cially in type 2 chondrites, phyllosilicates, and have near-solar Co/Ni ratios and 0.1–1 wt.% Cr carbonates (Table 5). Matrices are broadly and P and were among the first metallic grains chondritic in composition though richer in FeO to be tagged as nebular condensates (Newsom than chondrules and have refractory abun- and Drake, 1979; Weisberg et al., 1990). Mod- dances that deviate more from bulk chondrite erately volatile siderophile elements gallium, values (McSween and Richardson, 1977; Brear- germanium, arsenic, and antimony are depleted ley, 1996; Bland et al., 2005; Huss et al., 2005). by factors of up to 103 from CI levels (Campbell Matrix typically accounts for 5–50 vol.% of the et al., 2002). Condensation models suggest that chondrite (Table 1). the precursor gas was enriched in metals by a In the least altered chondrites, matrix rims, factor of 107 above solar levels prior to con- which are also called fine-grained rims, are densation. Hypervelocity impacts between as- similar in chemical composition and mineralogy teroids or disruption of molten planetesimals to the interstitial matrix material, though matrix might provide an appropriate environment for rims may be finer-grained and lack clastic the condensation of CBa metal as molten sulfur- material. Compositions of rims on chondrules bearing droplets (Campbell et al.,2002; Rubin and rims on CAIs appear indistinguishable. et al., 2003; Krot et al., 2005a). However, the Rims are not uniform in width (Figure 28), but presence of CAIs in CBa chondrites and the rim thickness is correlated with chondrule size intimate mixture of zoned and sulfur-bearing within a chondrite group (Metzler et al.,1992; metallic Fe,Ni grains in CBb chondrites suggest Paque and Cuzzi, 1997). However, matrix rims that components from diverse sources were are absent in the CH, CB, and K chondrite accreted to form the CB chondrites. groups. Clearly, matrix rims around chondritic components were acquired after the components formed and before final lithification of these 1.07.5.7.3 Troilite components into a rock. (Note that some rims Troilite in chondrites may have condensed around altered mineral grains appear to have below B650 K by reaction of nebular gas with formed in situ by cementation of interstitial metallic Fe,Ni, or crystallized from Fe–Ni–S material during aqueous alteration.) Most melts in chondrules or during secondary proc- authors envisage that the chondritic compo- essing in asteroids. Whether any metal grains in nents acquired their rims by accreting dust in the chondrites have preserved nebular signatures is nebula (e.g., Morfill et al.,1998; Liffman and questionable as sulfur is readily mobilized Toscano, 2000; Cuzzi et al.,2001; Cuzzi, 2004), during metamorphism and shock (Lauretta though some argue that rims form in asteroidal et al.,1997). Criteria for identifying nebular (e.g., Symes et al.,1998; Tomeoka and sulfide condensates (Lauretta et al.,1998)and Tanimura, 2000; Trigo-Rodriguez et al.,2006). igneous sulfides (Rubin et al.,1999; Kong et al., Although chondrules and CAIs may have ac- 2000) have been proposed. Rubin et al. (1999) quired transient, fluffy rims in the nebula, the found that 13% of the chondrules in Semarkona identical nature of well-compacted rims on (LL3.0) contain metal-troilite nodules with igne- objects that formed at different times and places ous textures inside phenocrysts. They inferred suggests that rim acquisition and compaction that these chondrules were heated only briefly in may have been delayed until the nebula, otherwise sulfur would have been accretion. lost. Lauretta et al. (2001) infer that troilite and Millimeter-to-submillimeter lumps of matrix- fayalite rims on metal grains in Bishunpur rich material are present in many chondrites. (LL3.1) formed in a totally different way at Lumps that are mineralogically similar to B1,200 K in a dust-enriched nebula. nearby matrix occurrences are probably pieces of rims or aggregates of interstitial material. However, heavily altered matrix- 1.07.5.8 Matrix Material rich lumps called dark clasts with distinctive probably have a different origin Matrix material is best defined as the opti- (Section 1.07.5.8.10). In CH and CB chond- cally opaque mixture of mineral grains 10 nm rites, all the matrix material is present as dark 44 Chondrites and Their Components Table 5 Mineralogy of chondrite matrices. Group Phases Reference Carbonaceous CI Serpentine, saponite, ferrihydrite, , Ca–Mg 1 carbonate, pyrrhotite CM Serpentine, tochilinite, pyrrhotite, amorphous phase, 1 calcite CR2 Olivine, serpentine, saponite, minor magnetite, FeS, 2 , pyrrhotite, calcite CO3.0: ALHA77307 Amorphous silicate, Fo30–98, low-calcium pyroxene, 3 Fe,Ni metal, magnetite, sulfides CO3.1–3.6 Fayalitic olivine, minor phyllosilicates, and ferric oxide 4 CV3 reduced Fayalitic olivine, low-calcium pyroxene, low-nickel 5 metal, FeS CV3 oxidized: 6 Bali-like Fayalitic olivine, phyllosilicate, fayalite, Ca–Fe pyroxene, pentlandite, magnetite Allende-like Fayalitic olivine, Ca–Fe pyroxene, nepheline, 7 pentlandite Ungrouped C: 8 Acfer 094 Amorphous silicate, forsterite, enstatite, pyrrhotite, ferrihydrite, traces of phyllosilicate Adelaide Fayalitic olivine, amorphous silicate, enstatite, 9 pentlandite, and magnetite Ordinary LL3.0: Semarkona Smectite, fayalitic olivine, forsterite, enstatite, calcite, 10 magnetite H, L, LL 3.1–3.6 Fayalitic olivine, amorphous material, pyroxene, albite, 11 Fe,Ni metal Other K3: Kakangari Enstatite, forsterite, albite, ferrihydrite, troilite, Fe,Ni 12 metal

References: 1, Zolensky et al. (1993); 2, Endress et al. (1994); 3, Brearley (1993); 4, Brearley and Jones (1998); 5, Lee et al., (1996); 6, Keller et al., (1994); 7, Scott et al. (1988); 8, Greshake (1997); 9, Brearley (1991); 10, Alexander et al. (1989a); 11, Alexander et al. (1989b); 12, Brearley (1989). clasts: there are no matrix rims or interstitial chondrule fragments and experienced aqueous material. alteration and metamorphism (e.g., Scott et al., According to Anders and Kerridge (1988), 1988). However, nebular mechanisms for for- ‘‘controversy permeates every aspect of the ming magnetite and phyllosilicates are still study of matrix material, from definition of being studied (Hong and Fegley, 1998; Ciesla matrix to theories of its origin.’’ Although et al., 2003). Matrix investigations are handi- controversy remains, major progress has been capped by the extremely fine-grained nature of made in identifying matrix components and the constituents and the difficulty of distingui- understanding their origin. Since matrix is shing primary and secondary mineralogical commonly richer in volatiles than other com- features (Brearley, 1996). The fine grain size ponents, it is commonly referred to as the ‘‘low- (commonly as small as 50–100 nm), high po- temperature component’’ of chondrites. But rosity, and permeability of matrix ensure that it this is a misnomer as it is composed of diverse is more susceptible to alteration by aqueous materials that formed under different condi- fluids and metamorphism in asteroids than tions. Prior to detailed TEM studies and the other chondritic components. acquisition of mineralogical and isotopic Most of the matrix data that we review are evidence for aqueous alteration on asteroids, chemical and mineralogical as there is little all matrix minerals in carbonaceous chond- isotopic information about individual matrix rites including magnetite, carbonates, and grains. Bulk oxygen-isotopic compositions for phyllosilicates were thought to have condensed matrix samples commonly differ from those in the solar nebula (see McSween, 1979; see of associated chondrules (e.g., Scott et al., Chapter 1.09). However, matrix minerals in 1988), but lack of oxygen-isotope data for most chondrites are now thought to be a com- samples of key chondrites and specific matrix plex mixture of presolar materials and ne- components severely limits inferences about bular condensates that were mixed with fine matrix origins. Chondritic Components 45 valuable clues to the thermal and alteration history of specific matrix samples.

1.07.5.8.1 CI1 chondrites CI1 chondrites contain neither chondrules nor refractory inclusions and have generally CAI been considered as virtually pure matrix mate- rial. They are composed almost entirely of phyllosilicates, oxides, sulfides, carbonates, and other minerals (Table 5) that were once widely 0 − 1610 100 JSM 5300 thought to be nebular condensates but are now (a) known to be products of aqueous alteration (B&J, 1998, p. 192). CI1 chondrites contain 65 wt.% of serpentine–saponite intergrowths, which are too fine-scale for accurate analysis, chd 10% magnetite, 7% pyrrhotite, 5% poorly crystalline ferrihydrite (5Fe2O3 9H2O), and smaller amounts of pentlandite and other AOA phases (Bland et al., 2002; B&J, 1998, p. 192). Carbonates constitute B5% of the meteorites: the largest have been dated by 53Mn–53Cr isotope systematics at B6–20 Ma after CAI formation (Endress et al., 1996). CI1 chondrites also contain a small fraction 100 JSM-5900 (b) of isolated olivine and pyroxene grains up to 400 mm in size with chemical, oxygen-isotopic Figure 28 Backscattered electron images showing compositions, and rounded inclusions of metal- fine-grained matrix rims on (a) Ca–Al-rich inclusion lic Fe,Ni and trapped melt indicating that they (CAI), and (b) an amoeboid–olivine aggregate were derived from chondrules (Leshin et al., (AOA) in ALHA77307. The light-gray rims contain 1997). Brearley and Jones (1998) estimate FeO-rich silicate material and are crossed by dark cracks; arrows mark the outer edge of the rims. the abundance of olivine and pyroxene at o1 vol.%, but X-ray diffraction studies by Bland et al. (2002) indicate 7 wt.% olivine. The difference probably reflects a high abundance of Other reviews of chondrite matrices have olivine crystallites embedded in phyllosilicates addressed aqueous alteration in the matrices of (P. A. Bland, private communication). CI1 carbonaceous chondrites (Buseck and Hua, chondrites also contain rare refractory grains 1993; see also Chapter 1.09), matrix miner- with oxygen-isotopic compositions comparable alogies and origins (Scott et al., 1988; Huss to those in CAIs (see Scott and Krot, 2001). et al., 2005), possible relations between matrix Why do the most highly altered chondrites and chondrules (Brearley, 1996), the diverse provide the best match with the composition of mineralogy of matrices in different chondrite the solar photosphere? We have yet to answer groups (B&J, 1998, pp. 191–244) and relations this question satisfactorily, but many factors between matrices of primitive chondrites and are probably responsible. First, chondrules that cometary silicates (Nuth et al., 2005). Here we are deficient in volatiles relative to matrix ma- focus on geochemical and mineralogical terial were scarce in the CI precursor material. features in type 1–3 chondrites that offer the Second, the rocks appear to have been affected best insights into the origin of the components by closed-system alteration. The prominent of matrix and chondrites and the relationship sulfate veins in CI1 chondrites appear to have between matrix and chondrules. Unfortunately, formed in fractures on Earth (Gounelle and well-characterized matrix material from E3 and Zolensky, 2001) and not in the parent body, R3 chondrites is lacking. so it is likely that aqueous solutions did not Some important constituents of matrix percolate through the rock. Third, CI1 chond- material are discussed elsewhere: presolar rites are unique fine-grained breccias as they grains (see Chapter 1.02), carbon and organic are entirely composed of diverse, altered lithic phases (see Chapter 1.10), and noble gases (see fragments less than a few hundred micrometers Chapter 1.14). The abundances and nature of in size (Morlok et al., 2002). They appear to be these constituents in chondrite matrices provide very representative samples of one or more 46 Chondrites and Their Components asteroids that were thoroughly scrambled. metallic Fe,Ni (Lauretta et al., 2000). Chizma- Fourth, they probably formed further from dia and Brearley (2003) found more abundant the Sun than other chondrite groups and there- amorphous material in the CM2 chondrite, fore acquired higher concentrations of carbon- Y 791198. (Origins of matrix components are aceous, volatile-rich material. reviewed in Section 1.07.5.8.11.) Rim thickness appears to be correlated with object size (Metzler et al., 1992). However, rims 1.07.5.8.2 CM2 chondrite matrices around grains of calcite, tochilinite, and other alteration products may be artifacts of altera- In CM2 chondrites, the interstitial matrix tion rather than preaccretionary features. material and the matrix rims around chond- Trigo-Rodriguez et al. (2006) infer that the rules are very similar mineralogically (Zolensky ‘‘primary accretionary rocks’’ of Metzler et al. et al., 1993). Matrices are dominated by phyllo- (1992) did not form in the nebula but rather in silicates, which are also present in chondrules asteroidal regoliths. Hua et al. (2002) compared and CAIs, and formed as a result of aqueous the concentrations of trace and minor elements alteration, but the location is controversial in matrix rims around different types of chond- (see Chapter 1.09). Phyllosilicates are mostly rules and other objects in two CM chondrites 10–100 nm in size and are largely serpentines and found no relationship between bulk com- with diverse morphologies, degrees of crystal- positions of rims and enclosed objects. Abun- linity, and compositions (Barber, 1981; Zolen- dances of moderately volatile elements in CM sky et al., 1993). Crystals of the Fe3 þ -rich matrices are depleted relative to CI chondrites, serpentine, cronstedite, reach up to 1 10 mm but enriched relative to bulk CM chondrites in size and may be intergrown with tochilinite, (Bland et al., 2005; Figure 29). (Fe,Mg)(OH)2 (Fe,Ni)S (see B&J, 1998, p. 202). Metzler et al. (1992), Bischoff (1998), and Lauretta et al. (2000) argue that alteration 1.07.5.8.3 CR2 chondrite matrices occurred mainly in small precursor planet- CR2 chondrite matrices are predomi- esimals prior to accretion of the CM body. nantly composed of olivine with grain sizes Hanowski and Brearley (2000, 2001) disagree of 0.2–0.3 mm accompanied by intergrowths of pointing to the occurrence of cross-cutting veins serpentine and saponite with dimensions of of alteration minerals, loss of calcium from 20–300 nm and minor sulfide and magnetite chondrule mesostases and deposition as carbon- grains 0.1–25 mm in size (Zolensky et al., 1993; ate in the surrounding matrix, and other Endress et al., 1994). CR2 chondrites also con- features favoring in situ alteration on asteroids. tain B3 vol.% of more altered matrix lumps CM2 matrices also contain small olivine mostly 0.1–1 mm in size, which are similar in grains down to 0.1 mm in size, which are close bulk composition to the other matrix material to Fo in composition (Barber, 1981). These 100 (Endress et al., 1994). However, phyllosilicates, forsterites lack defects, are sometimes euhedral, magnetite, and chondrule fragments are more and appear to have formed before the phyllo- abundant in the matrix lumps than in other silicates (Barber, 1981; B&J, 1998, p. 216). matrix material, and sulfides are richer in nickel Euhedral Fa olivines are also embedded in 40–50 suggesting that the lumps are chondritic clasts phyllosilicate (Lauretta et al., 2000). Pyroxenes that formed in a totally different environment. are rarer and are dominantly magnesium-rich, Fso2 (Brearley, 1995). Some olivines and pyroxenes are especially enriched in mangan- 1.07.5.8.4 CO3 chondrite matrices ese (1–2 wt.% MnO) and are comparable in composition to the low iron, high manganese The matrix of the type 3.0 CO chondrite, olivines and pyroxenes present in IDPs (Klo¨ ck ALHA77307, is very different mineralogic- et al., 1989). These were called LIME silicates ally from matrices in CO3.1–3.6 chondrites (low-iron, manganese enriched) by Klo¨ ck et al., (Table 5). It is largely composed of amorphous who argued that they are nebular condensates. silicate material (Figure 30), which forms dis- As much as 15% of CM matrices may be crete regions 1–5 mm in size, and 30–40 vol.% of amorphous silicate, which is associated with heterogeneously distributed tiny olivine crystals comparable volumes of very poorly crystalline and crystal aggregates (Brearley, 1993). Matrix phyllosilicates (Barber, 1981). In ALH81002, rims around chondrules and refractory inclu- 5 vol.% of the matrix rims are composed of sions are fairly homogeneous (Figure 31), min- amorphous regions a few micrometers across eralogically and chemically indistinguishable, that are rich in silicon, aluminum, magnesium, and nearly isochemical with interstitial material iron, and presumably oxygen, and contain (Brearley et al., 1995; Chizmadia et al.,2005). 5–40 nm grains of iron-rich olivines and Rims are slightly lower in manganese and Chondritic Components 47

2.0 CM chondrites

1.0 Abundance/CI/Yb Bulk (Kallemeyn and Wasson, 1981) Matrix (Bland et al., 2005) 0.1 Lu Ni Cr Mn Li As Au Cu K Ag Sb Ga Ge Rb Cs Bi Pb Zn Se In 2.0 CV chondrites

1.0 Abundance/CI/Yb Bulk (Kallemeyn and Wasson, 1981) Matrix (Bland et al., 2005) 0.1 Lu Ni Cr Mn Li As Au Cu K Ag Sb Ga Ge Rb Cs Bi Pb Zn Se In 2.0 CR chondrites

1.0

Abundance/CI/Yb Bulk (Kallemeyn et al., 1994) Renazzo matrix Al Rais matrix 0.1 Lu Ni Cr Mn Li As Au Cu K Ag Sb Ga Ge Rb Cs Bi Pb Zn Se In 2.0 CO chondrites

1.0 Abundance/CI/Yb Bulk (Kallemeyn and Wasson, 1981) Matrix (Bland et al., 2005) 0.1 Lu Ni Cr Mn Li As Au Cu K Ag Sb Ga Ge Rb Cs Bi Pb Zn Se In Elements in order of volatility Figure 29 Elemental abundances in matrices of CM, CV, CR, and CO chondrites normalized to bulk CI chondrites using the refractory element Yb; data from Bland et al. (2005); figure from Huss et al. (2005). Matrices, like bulk chondrites, show increasing depletions with increasing elemental volatility. However, matrices are less depleted than bulk chondrites. Siderophile and chalcophile elements in CM, CO, and CV chondrites tend to have higher abundances than adjacent lithophiles, which are marked by gray bands. Reproduced by permission of Astronomical Society of the Pacific from Huss et al. (2005). 48 Chondrites and Their Components calcium than interstitial matrix (Table 6). As in other chondrites, matrix material is broadly FeS chondritic in composition but aluminum, nick- el, and potassium are enriched relative to sil- icon and CI chondrites, and calcium and titanium are depleted. Moderately volatile ele- OL ments are typically enriched relative to bulk CO chondrite by factors of 2–3 (Figure 31; see also Bland et al., 2005). The amorphous phase in the ALHA77307 Kam matrix is heterogeneous on the submicrometer scale but homogeneous on a 10 mm scale (Table 6). It is mostly composed of silicon, iron, magnesium, and oxygen with relatively high concentrations of aluminum, nickel, sulfur, and Pyx OL phosphorus. Its Ca/Al ratio is much lower than those in chondrule mesostases, which are closer to chondritic values, precluding an origin from chondrule glass (Brearley, 1993). Brearley’s µ 0.2 m chemical and textural studies also suggest that the amorphous component was not formed by shock melting of matrix material, as Bunch Figure 30 Transmission electron micrograph of the et al. (1991) proposed for the matrix rims. In matrix in ALHA77307 (CO3.0) showing that it is some areas, the amorphous phase appears to largely composed of amorphous material with em- show incipient formation of phyllosilicates. bedded crystallites of kamacite (Kam), pyrrhotite Three types of olivine occur in the amorphous (FeS), olivine (OL), and low-calcium pyroxene phase of ALHA77307 (Brearley, 1993): (1) (Pyx). Matrices of all carbonaceous chondrites abundant irregularly shaped and poorly crys- probably resembled this matrix material prior to as- talline olivines o300 nm in size with Fa10–70; teroidal alteration and metamorphism. Reproduced (2) well-crystallized, subhedral, larger forsterite by permission of Elsevier from Brearley (1993). crystals 200 nm to 44 mminsize(Fo95–98); and

10.00

1.00

ALHA77307 rims Rim 1 0.10 Rim 2 Element/CO chondrite Rim 3 Rim 4 Rim 5 Ornans matrix

0.01 Al Ca Ti Mg Si Cr Mn Na K Ni Fe Cu Ga Ge Se Zn S Figure 31 Elemental abundances in matrix material in CO3 chondrites normalized to bulk chondrite: data for rims on chondrules in ALHA77307 (type 3.0) obtained by synchrotron X-ray fluorescence microprobe analysis (Brearley et al., 1995) and for bulk matrix in Ornans (type 3.3) analyzed by instrumental neutron activation (data from Rubin and Wasson, 1988). Chondrule rims in ALHA77307 are uniform in composition but deviate significantly from the bulk chondrite composition. Matrix in Ornans is relatively unfractionated. Reproduced by permission of Elsevier from Brearley et al. (1995). Chondritic Components 49 Table 6 Composition of interstitial matrix and matrix rims on chondrules in the CO3.0 chondrite, ALHA77307, and amorphous matrix phases in ALHA77307 and Acfer 094. Weight% Interstitial matrix 3 Matrix rims 8 Amorphous phase Amorphous phase areas ALHA77307 chondrules ALHA77307 Acfer 094 matrix ALHA77307 matrix (CO3.0) (ungrouped) No. anal. 36 106 8 6 SiO2 33.6 (0.9) 33.3 (0.7) 53 (9) 47 (4) TiO2 0.07 (0.02) 0.06 (0.01) o0.04 NA Al2O3 4.4 (0.3) 4.7 (0.3) 5.2 (1.6) 3.8 (0.9) Cr2O3 0.43 (0.04) 0.36 (0.02) 0.44 (0.18) 0.7 (0.2) FeO 36.2 (2.5) 37.3 (2.1) 20 (6) 26 (3) MnO 0.27 (0.04) 0.16 (0.05) 0.24 (0.20) 0.6 (B0.2) MgO 16.7 (1.5) 15.8 (0.8) 8.3 (1.7) 10.1 (3) CaO 1.1 (0.2) 0.76 (0.12) 0.41 (0.38) 1.4 (0.4) Na2O 0.32 (0.06) 0.25 (0.06) NA B0.7 K2O 0.11 (0.02) 0.14 (0.05) NA NA NiO 3.4 (0.4) 3.9 (0.7) 4.7 (2.6) 5.2 (2.2) P2O5 0.32 (0.09) 0.25 (0.09) 3.6 (2.1) NA SO3 3.4 (0.8) 3.0 (0.6) 4.5 (2.8) 4.0 (2.0) Total 100 100 100 100

Sources: Brearley (1993) and Greshake (1997). Note: Matrix analyzed by electron microprobe: amorphous phase by analytical transmission electron microscopy. Totals are normalized to 100%. Figures in parentheses are 1s of means of analyzed regions for matrix and 1s of analyses for amorphous phase. NA, not analyzed.

(3) micrometer-sized, isolated crystals and crystalline forsterites crystallized at higher similar-sized aggregates of 100 nm grains of temperatures. Brearley (1993) argued against well-crystallized manganese-rich forsterites, a condensation origin for the iron-poor, with up to 2 wt.% MnO. Pyroxene, which is manganese-rich silicates as proposed by Klo¨ ck not as abundant as olivine, is mostly poor in et al. (1989), and suggested that they formed by calcium and iron and also occurs in several annealing of amorphous material. varieties. Orthopyroxene forms angular crystals Matrices in the more metamorphosed CO3.1– 1–1.5 mm in size and is intergrown with clinopyr- 3.6 chondrites are unlike the ALHA77307 oxene in aggregates of 0.2–0.3 mm crystals. matrix as they are dominated by submicrometer High-resolution TEM images of an intergrow- grains of FeO-rich olivine Fo40–60 with minor th indicate cooling from the protopyroxene pyroxene, and metallic Fe,Ni (see B&J, stability field above 1,000 1CatB1,000– 1998, pp. 217–220). In Lance´ and Ornans, the 5,000 1Ch1 (Jones and Brearley, 1988). matrix, like the mesostasis in chondrules, also Manganese-rich enstatite crystals are B0.1 mm contains minor phyllosilicates and other alter- in size, largely clinoenstatite, indicative of rapid ation products. Lance´ matrix contains magne- cooling from above 1,000 1Cat41,000 1Ch1, sium-rich serpentine and poorly crystalline and are associated with manganese-rich olivines material between FeO-rich olivines (Keller in micrometer-sized aggregates. and Buseck, 1990). Hydrous ferric oxides are Some amorphous regions in ALHA77307 also present on grain boundaries in Kainsaz contain rounded grains of kamacite with and Warrenton indicating incipient alteration. 4–5% Ni and nickel-bearing sulfides, which Matrix olivines in type Z3.4 chondrites have are both o200 nm in size. Elsewhere, magnetite more equilibrated compositions than those in occurs as crystals as small as 50 nm and types 3.1–3.3 (B&J, 1998, p. 220). Ornans aggregates o10 mm in size. The close proxim- matrix is close to bulk CO composition (Rubin ity of magnetite-rich and metal-sulfide-rich and Wasson, 1988; Figure 31). regions suggests that they formed separately The evidence that CO3 chondrites form a under very different conditions in the nebula metamorphic sequence (Scott and Jones, 1990; (Brearley, 1993). From a weak correlation B&J, 1998, p. 38) suggests that they all origin- between the Mg/(Mg þ Fe) ratios of the oli- ally contained matrix material like that in vines and the adjacent amorphous material, ALHA77307. The amorphous material in Brearley (1993) proposed that the olivines had ALHA77307, which shows only incipient for- crystallized from the amorphous phase during mation of fayalitic olivine and phyllosilicates, low-temperature annealing. However, the very could have been readily replaced by fayalitic different properties of the magnesian and fer- olivine by further fluid-assisted metamorphism roan olivines suggest they formed under differ- to produce the type 3.1–3.6 matrices. Additional ent conditions. Presumably, the larger, highly studies are needed to see if the minor feldspathic 50 Chondrites and Their Components component between FeO-rich olivines in types KABA 3.1–3.6 (B&J, 1998, p. 217) is related to the amorphous component in ALHA77307. fa

fa 1.07.5.8.5 CK3 chondrite matrices Matrices of CK4–6 chondrites are composed largely of olivine, low-calcium pyroxene and Ca−Fe px plagioclase grains that are mostly 20–100 mmin size with minor magnetite and sulfide (B&J, 1998, p. 239). CK3 chondrite matrices appear to resemble those in CV3 chondrites, which are discussed below. Matrix textures in the Kobe 50 µm CK4 chondrite were interpreted by Tomeoka (a) et al. (2001) to result from partial melting ALHA81258 during shock metamorphism, although Kobe itself is only classified as a shock stage S2.

1.07.5.8.6 CV3 chondrite matrices Matrix material in CV3 chondrites occurs as Ca−Fe px rims on chondrules and other components, as well as interstitial material, which is coarser grained. The volume of a matrix rim is approximately equal to the volume of the enclosed chondrule (Paque and Cuzzi, 1997). µ Matrices are mineralogically diverse but are (b) 50 m dominated by iron-rich olivine, Fa30–60, which is coarser than in other chondrite groups. It Figure 32 Backscattered electron images showing forms elongate grains up to 20 mm in length and matrix material in the Bali-like oxidized CV3 chond- rite, Kaba (a) and the Allende-like, oxidized CV3 subhedral finer grains o100 nm to 3 mminthe chondrite, ALHA81258 (b). Kaba matrix contains interstices (B&J, 1998, p. 220; Watt et al., fayalite (fa), Ca–Fe pyroxene, fine-grained, iron-rich 2006). Olivines contain planar defects and olivine, pentlandite, magnetite, and phyllosilicates. small inclusions of sulfide, metal or magnetite, The ALHA81258 matrix lacks phyllosilicates and and poorly graphitized carbon (Tomeoka and contains coarse-grained laths of fayalitic olivine Buseck, 1990; Keller et al., 1994; Brearley, (light-gray) and patches of Ca–Fe pyroxene plus ne- 1999). Reduced CV3 chondrites appear to be pheline and magnetite. Both matrices lack the amor- the least altered as they have abundant metallic phous silicate present in ALHA77307 and appear to Fe,Ni rather than magnetite: minor matrix be highly altered. minerals include low-calcium pyroxene, metal- lic, Fe,Ni, and FeS. In the Bali-like oxidized CV3 chondrites, matrices lack metal and alteration minerals like nepheline in Allende contain abundant phyllosilicates, fayalite formed at B600–800 1C whereas estimates (Fa495), calcium-rich pyroxene, pentlandite, from zoning in olivine point to maximum and magnetite in addition to iron-rich olivine temperatures of B400–550 1C. They, therefore, (Figure 32a). In the Allende-like oxidized CV3s, conclude that the anhydrous alteration hydrated silicates and fayalite are rare or absent, occurred prior to accretion of the chondritic but Ca–Fe-rich pyroxene, nepheline, and pent- components. Brenker et al. (2000) who studied landite are abundant, and magnetite is present aggregates of andradite and hedenbergite (Krot et al., 1995, 1998a, b, c, 2001f; Brearley, 20–50 mm in size in the Allende matrix, infer 1999; B&J, 1998, pp. 220–225)(Figure 32b). that the Ca–Fe-rich pyroxene cooled rapidly Proponents of a nebular origin for the sec- from 41,050 1Cat410 1Ch1. They argue ondary minerals in oxidized CV3 chondrites that a localized nebular environment is more argue that fayalitic olivine condensed in the compatible with this thermal history than an solar nebula at high fO2 following evaporation asteroidal environment. of chondritic dust, and coated preexisting Evidence favoring progressive alteration in chondrules (Nagahara et al., 1988; Palme and asteroids includes fayalite veins that cross-cut Fegley, 1990; Weinbruch et al., 1990). Kimura chondrules and matrices, replacement of and Ikeda (1998) infer that anhydrous chondrules by fayalite-rich matrix material in Chondritic Components 51 dark inclusions, and comparable alteration The highly unusual combination of amor- minerals in chondrules and matrices (Krot phous material and forsterite in Acfer 094 et al., 1995, 1998a, b, c, 2001f; Kimura and matches that in the CO3.0 chondrite, Ikeda, 1998). In Bali-like CV3s, saponite re- ALH77307. The amorphous phases in the places calcium-rich minerals in chondrules and two chondrites are compositionally similar CAIs; magnetite is replaced by fayalite and (Table 6) and both contain submicrometer calcium-rich pyroxene. In Allende-like CV3s, and forsterites with high concentra- fayalitic olivine replaces low-calcium pyroxene; tions of MnO (0.6–2 wt.%; Greshake, 1997). magnetite–sulfide grains in matrices and These concentrations are higher than in chondrules are replaced by fayalitic olivine chondrules and comparable to those in high- and calcium-rich pyroxenes; nepheline and cal- manganese, low-iron silicates reported in cium-rich pyroxene form veins in and around IDPs by Klo¨ ck et al. (1989). Acfer 094, like CAIs and dark inclusions. Isotopic data also ALH77307, contains o1% phyllosilicate in favor asteroidal sites for alteration. Coexisting the amorphous material and 100–300 nm magnetite and fayalite in Mokoia show large sulfide grains (pyrrhotite and minor pent- mass fractionation in oxygen isotopes, consist- landite) as in ALHA77307, but it lacks ent with aqueous alteration (Choi et al., 2000). matrix metal grains. Since ferrihydrite is Nepheline lacks evidence for 26Mg excesses in- present in the amorphous material in Acfer dicating formation 43–5 Myr after CAI for- 094, metal may once have been present. mation (MacPherson et al., 1995). 53Mn–53Cr Greshake (1997) proposed that the amorphous isotope systematics suggest that fayalite formed material was a presolar or solar condensate B7–16 Myr after CAIs (Hutcheon et al., 1998). and that the FeO-poor silicates formed in the The mineralogy of CV3 chondrite matrices solar nebula by condensation or annealing of prior to alteration is not clear. The two reduced amorphous material. CV3 chondrites that have been studied by TEM Adelaide, which also has links to CO and are not ideal samples. In Leoville, which is mod- CM groups, has a matrix that lacks phyllosil- erately shocked, Nakamura et al. (1992) found icates and is dominated by fayalitic olivine aggregates of rounded, 10–100 nm olivine grains Fa31–84 occurring as aggregates of 100–200 nm with interstitial amorphous material, but attrib- crystals and platy and corroded single crystals uted this to shock. In Vigarano, which is a brec- (Brearley, 1991). Amorphous material, which cia of altered and unaltered material (Lee et al., is rich in iron and sulfur, is present but is 1996), oxygen-isotopic mapping of the matrix less abundant than in ALHA77307. High- revealed 16O-rich forsterite grains 2 mminsize manganese enstatite forms 1–1.5 mm clusters that may be fragments of AOA or related ob- of 100–500 nm crystals but forsterites are absent. jects, as well as similar-sized 16O-poor fayalite Pentlandite forms aggregates of 10–30 nm and enstatite grains (Kunihiro et al., 2005b). crystals and magnetite occurs as 400–6,500 nm Clearly this technique is essential for unraveling clusters of 100 nm grains. the origins of matrix components.

1.07.5.8.8 H–L–LL3 chondrite matrices 1.07.5.8.7 Matrix of ungrouped C chondrites, Acfer 094, and Adelaide Matrix material in type 3 ordinary chond- rites forms rims on chondrules, metal grains, These two chondrites are among the least and other components and occurs as lumps altered and metamorphosed chondrites and like (B&J, 1998, pp. 231–237). In the LL3.0 chond- ALHA77307, their matrices contain presolar rite, Semarkona, matrix consists largely of silicates (Nagashima et al.,2004; Kobayashi phyllosilicates formed by aqueous alteration et al.,2005). Acfer 094, which is closely linked to with fine-grained olivine Fa5–38 and low-calcium the CO and CM groups, has abundant matrix pyroxene, calcite, and magnetite (Alexander material and higher concentrations of presolar et al., 1989a). Magnetite grains show large mass SiC than any other chondrite (Newton et al., fractionation of oxygen isotopes suggesting that 1995). Its matrix is composed of 30 vol.% a limited supply of aqueous fluid was largely forsterites 10–300 nm in size and Fa0–2 in consumed during asteroidal alteration (Choi composition, 40% amorphous material, and et al.,1998). Klo¨ ck et al. (1989) identified an 20% enstatite with Fs0–3 and grain sizes of additional population of forsterites Fao1 and 200–300 nm (Greshake, 1997). The enstatites enstatites Fso2 having MnO concentrations of show intergrowths of ortho- and clinoenstatite 0–1.4 wt.% that are correlated with FeO. The indicating rapid cooling at B1,000 1Ch1. Minor high-manganese silicates are comparable in components included 5% sulfides, o1% composition to the iron-poor, manganese-rich phyllosilicates, and o2% ferrihydrite. silicates identified in IDPs (Klo¨ ck et al.,1989). 52 Chondrites and Their Components Matrices in type 3.1–3.6 chondrites are troilite (Brearley, 1989). At least three kinds of largely composed of FeO-rich olivines, which polycrystalline clusters appear to have formed may have skeletal or elongate morphologies, separately: (1) enstatite-rich units with forsterite and an amorphous feldspathic phase (Nagahara, inclusions, (2) coarser versions with albite and 1984; B&J, 1998, pp. 231–237). The origins of olivine inclusions, and (3) forsterite–anorthite these phases are uncertain. Alexander et al. units with no enstatite. Enstatite crystals are (1989b) argued that interchondrule matrix and intergrowths of ortho- and clinopyroxene and matrix rims in ordinary chondrites were derived their microstructures indicate cooling from from broken chondrules, contrary to Brearley 41,000 1CatB1,000 1Ch1. Brearley (1989) et al. (1989), who suggested an origin by anneal- suggests that the Kakangari matrix formed from ing of amorphous material. Ikeda et al. (1981) amorphous or partly crystalline particles identified two types of matrix in ordinary chond- o10 mm in size that were annealed at 1,100– rites with abundant amorphous material, one 1,200 1C or possibly higher, and then rapidly contained Fe,Ni droplets 10–100 nm in size. cooled in an hour. The chondrules in Kakangari They speculated that the amorphous material could have formed from similar material that formed by shock. Fayalitic olivine (Fa54–94) was heated to higher temperatures, partly occurs in the matrices of Krymka (LL3.1) and melted, and quenched at comparable rates, Bishunpur where it rims metal grains (Weisberg provided that the chondrules acquired lower et al.,1997; Lauretta et al.,2001). These authors 16O concentrations when molten. argued that it formed in the solar nebula, not The bulk composition of Kakangari suggests during aqueous or hydrothermal alteration. that the hypothesized precursor material for Strong evidence for asteroidal alteration is the chondrules and matrix had near-CI levels of present in Tieschitz (H3.6), which contains ne- moderately volatile elements that were not lost pheline and albite in veins between chondrules during high-temperature processing. Reduction and in chondrule mesostases (Hutchison et al., probably occurred during this processing. 1998). This so-called white matrix formed when aqueous fluids attacked chondrule mesostases forming voids. 1.07.5.8.10 Heavily altered, matrix-rich lithic clasts

1.07.5.8.9 K3 chondrite matrix (Kakangari) Heavily altered, matrix-rich chondritic clasts are very different from the most common lithic Kakangari has many unusual properties as clasts in chondrites, which are fragments of chondrules and matrix both contain enstatite chondrules and CAIs or lithic fragments and forsterite, and matrix silicates are more resembling the host rock. They are also unlike magnesian than those in chondrules (Weisberg rare foreign chondritic clasts that are shocked et al., 1996; B&J, 1998, p. 82). Matrix and and appear to have impacted the parent body chondrules have similar bulk chemical compo- at high speed after accretion, e.g., an LL sitions though matrix is richer in 16O(Weisberg chondrite clast in an H chondrite (Lipschutz et al., 1996). et al., 1989). Kakangari matrix contains mostly euhedral Altered, matrix-rich chondritic clasts are to subhedral crystals with grain sizes of 200 nm similar in size to chondrules and CAIs in the to 1.5 mm, which occur as individual crystals and host chondrite, unshocked, and appear to have polycrystalline units or clusters 2–8 mminsize, been gently incorporated into the host material some of which have well-annealed textures. It at an early stage. Prior to incorporation, they is composed of 50 vol.% enstatite, Fs2–5, B20 were aqueously altered in a different location. vol.% forsterite, Fa0–3, 10 vol.% albite crystals They occupy a few volume% of CR2 (see 250–1,000 nm in size, Oro3,ando1vol.% Section 1.07.5.7.3), CV3, CO3, CH, and CB anorthite up to 500 nm in size. Minor phases chondrites. In CV3s they tend to have fewer include chromium-spinel, troilite grains 1–5 mm and smaller chondrules than the host chond- in size and B10 vol.% ferrihydrite, which forms rites and some are rimmed by matrix material acicular aggregates up to 2 mminlengthin (Kracher et al., 1985; Johnson et al., 1990). pores. The lower FeO concentrations in olivine Although Palme et al. (1989) and Weisberg and relative to coexisting pyroxene in matrix and Prinz (1998) argued that some are unaltered chondrules as in EH3 chondrites, suggest that aggregates of nebular condensates, there is now reduction occurred during cooling as olivine abundant evidence favoring formation by diffusion rates are faster than those in pyroxene asteroidal alteration (Kojima et al., 1993; (Scott and Taylor, 1983; B&J, 1998, pp. 72–74). Kojima and Tomeoka, 1996; Krot et al., Olivine occurs mainly as inclusions in ensta- 1997, 1998a, b, c, 2001e). I–Xe ages of some tite and is also found in albite, spinel, and CV3 clasts confirm that they were altered Chondritic Components 53 before their hosts (Pravdivtseva et al.,2003). grains. Micrometer-sized corundum, spinel, Dark clasts in CO3 chondrites are smaller than and hibonite grains of solar-system origin are in CV3 chondrites (50–200 mm; cf. millimeter to much more common than presolar grains (e. g., centimeter in CV3s) (Itoh and Tomeoka, 2003). Virag et al., 1991; Strebel et al., 2000; Nittler In CH3 chondrites and CB3 chondrites, matrix- et al., 1994) and probably formed as dust in the rich chondritic clasts are the only matrix mate- CAI-forming region (Scott and Krot, 2005a), rial and they are heavily altered to type 1 or or elsewhere when CAIs were forming. 2 levels, unlike the chondrules and metal grains, Not enough is known about unaltered which are pristine (Greshake et al.,2002). In CH matrices in ordinary and enstatite chondrites chondrites these clasts are mostly 5–200 mmin to infer their detailed mineralogy. However, size and have a size distribution comparable to enstatite chondrite matrices probably resemble that of the chondrules (Scott, 1988), suggesting the matrix of Kakangari in having enstatite, that sizes of dark clasts are correlated with metallic Fe,Ni, and sulfide as important con- chondrule size. stituents in addition to parts per million levels Some authors infer that matrix-rich clasts are of presolar grains (Ebata et al., 2006). merely pieces of the body that supplied the Amorphous ferromagnesian silicate is best host chondrite (e.g., Itoh and Tomeoka, 2003). preserved in the matrices of ALHA77307, However, their unusual mineralogy, limited Acfer 094, and Adelaide, though it is absent size ranges, which are correlated with chond- in Kakangari. It is more fractionated than bulk rule size, and matrix rims on some clasts favor matrix with higher Al/Ca, Ni/Si, and P/Si than a separate origin. The rims suggest they were CI chondrites. Metallic Fe,Ni is abundant only lofted after alteration into the dusty environ- in the ALHA77307 matrix but it may have ment that formed rims on chondrules and other been altered in the other chondrites. Related components. Most matrix-rich clasts may be amorphous material may be present in fragments of bodies that formed before the CM, CO, and ordinary chondrites. The alumi- parent body of the host chondrite, and were num-rich nature of amorphous material may then accreted with another batch of chondrules account for the high Al/Ca ratio of some and CAIs to form a later generation of matrix materials (Figure 27), the depletion of chondritic asteroids. Such planetesimals have refractory elements with respect to aluminum been invoked by Metzler et al. (1992) and in matrix materials in ordinary chondrites Bischoff (1998). Early formed, volatile-rich (Alexander, 1995), and the low Ca/Al ratio of planetesimals may have been disrupted by exp- certain chondrites like Adelaide and Kakangari losive volatile release (Wilson et al., 1999)or (Brearley, 1991). The cause of the enrichment impact fragmentation and dispersal. of aluminum, nickel, and phosphorus in amor- The unique chondrite, Kaidun, appears to phous matrix is uncertain as these elements be a breccia composed almost entirely of milli- have different geochemical properties. This meter- and submillimeter-sized fragments of amorphous material did not form from diverse kinds of chondrites (Zolensky and quenched melt from chondrule mesostases, by Ivanov, 2003). Kaidun may be a clast-rich, impact in situ or elsewhere, or by dehydration chondrule-poor chondrite that formed in a of phyllosilicates. It appears to have accreted nebula region where chondrules were much into chondrule rims as small particles (1–5 mm rarer than clasts (Scott, 2002). CI chondrites in size in ALHA77307). and the Tagish Lake chondrite also appear to Forsterites Fao5 and enstatites Fao5, which have formed from submillimeter particles of are 10–300 nm in size, are found in the matrices altered chondritic material (Nakamura et al., of CM chondrites, ALHA77307, Acfer 094, 2003). Thus disagregation of early-formed and probably also in Semarkona, for which planetesimals may have been widespread. grain sizes were not reported. Enstatites and forsterite grains with higher FeO concentra- tions tend to be MnO-rich (0.6–2 wt.%). 1.07.5.8.11 Origins of matrix phases Intergrowths of ortho- and clinoenstatite and forsterite–enstatite associations indicate that Unaltered carbonaceous chondrite matrices all magnesian silicates cooled from 41,300 K are composed largely of amorphous silicate and at B103 to 104 1Ch1 as micrometer- and sub- submicrometer forsterites and enstatites and micrometer-sized grains prior to accretion. micrometer-sized aggregates of such crystals Oxygen-isotopic mapping of micrometer-sized (Scott and Krot, 2005a; Nuth et al.,2005). In grains shows that primitive matrices contain addition, they contain up to a few volume per- B10–100 ppm of presolar silicates (Nagashima cent of carbonaceous material, sulfides, parts et al., 2004). Therefore, the vast majority of per million levels of presolar silicate and oxide iron-poor matrix silicates must have formed in grains (see Chapter 1.02), and solar refractory the solar nebula. 54 Chondrites and Their Components Primitive chondrite matrices are surprisingly shocks. We infer that magnesian silicates were similar to porous chondritic aggregate IDPs ubiquitous in the solar nebula at 2–10 AU as a in that both contain amorphous material, result of high-temperature processing and FeO-poor olivines, and pyroxenes, some of quenching in hours or days (Scott and Krot, which are MnO-rich (Klo¨ ck et al., 1989), re- 2005a). The similarity of the cooling rates in- fractory oxides, and presolar silicate and oxide ferred for the magnesian silicates in the matrix grains (Scott and Krot, 2005a; Nuth et al.,2005). and for chondrules suggests that matrix silicates Oort cloud and Jupiter-family comets also and chondrules may have been formed in related contain amorphous and crystalline FeO-poor events. olivines and pyroxenes (Hanner, 1999; Wooden Submicrometer fayalitic olivines are hetero- et al., 2005; Harker et al., 2005), and the same geneously distributed in the matrices of prim- minerals are observed in disks around proto- itive chondrites. In Acfer 094 they are absent stars (see Nuth et al., 2005). 16O-rich refractory but the ALHA77307 matrix contains 30–40 micrograins are present in vol.%, which probably formed in situ by low- and chondritic aggregate IDPs suggesting ad- temperature annealing of amorphous material ditional links between chondrite matrices and (Brearley, 1993). Submicrometer-sized fayalitic particles from the outer solar system (Greshake olivines are also present in matrices of CO3.1– et al., 1996; Engrand and Maurette, 1998; 3.6, type 3 ordinary chondrites and the inter- Engrand et al., 1999). The amorphous aggre- stices of larger olivines in CV3 chondrites. An gates in ALHA77307 that are rich in metal and origin for these olivines by annealing of amor- sulfide may be related to the glass with embed- phous material also appears plausible but the ded metal and sulfide (GEMS) grains that are a evidence is weak. In some instances (e.g., in major constituent of porous chondritic IDPs ALH77307) annealing and crystallization of and resemble amorphous interstellar grains olivines may have started prior to accretion, as (Bradley et al.,1999; Rietmeijer, 1998, 2002). Kakangari matrix grains were extensively However GEMS are richer in SiO2 and much annealed before chondrules and matrix accreted. smaller: 100–500 nm in size with kamacite and However, for the coarse, abundant fayalitic sulfide grains that are o5 nm in size, cf. grains and pure fayalite in CV3 chondrites, an o200 nm in ALH77307. GEMS may have asteroidal origin is indicated as their formation formed by amorphization of interstellar grains required an aqueous fluid (Krot et al., 2004f). by cosmic-ray irradiation (like amorphous rims Although fayalitic olivine has been inferred to on some lunar grains) or by shocks (Bradley, have been a major constituent of the solar neb- 1994; Bradley et al.,1999; Chapter 1.26). ula (e.g., Nagahara et al.,1994; Ozawa and However, nearly all GEMS have solar-like Nagahara, 2001), we find no firm evidence from oxygen-isotopic compositions and are more chondrite matrices that any ‘‘crystalline,’’ FeO- likely to be solar-system products (Messenger rich silicate formed in the solar nebula. et al., 2003). Amorphous silicate material is Chemical differences between matrix and known to condense around evolved stars (Hill chondrules suggest that nearly all chondrules et al., 2001), and the amorphous matrix mate- could not have formed from matrix material. rial may be a solar-system analog. For example, matrix material is typically richer Two high-temperature processes have been in FeO than chondrules, and mean refractory proposed for magnesian silicates in matrices: elemental abundances deviate further from CI disequilibrium condensation (Klo¨ ck et al.,1989; levels than for chondrules. Some authors argue Nagahara et al.,1988; Greshake, 1997)and from chemical differences that chondrules and annealing of amorphous condensate particles matrices are chemically complementary and (Brearley, 1993; Greshake, 1997). Annealing therefore formed in the same location. For experiments suggest that magnesium-rich sili- example, Fe/Si ratios in chondrules and matrix cates can crystallize from amorphous conden- are lower and higher, respectively, than CI sates in a few hours at 1,050 K, or a year or ratios whereas the bulk compositions of CM, more below 1,000 K (Hallenbeck et al.,2000; CO, CV, and other chondrites are close to solar Rietmeijer et al.,2002). If the magnesian sili- implying that chondrules and matrix were cates formed close to the protosun by annealing chemically fractionated from one another or condensation, they could have been lofted (Wood, 1996a). The case for Fe/Si is not very across the disk by outflows powered by recon- convincing, however, as there is a wide range of necting magnetic field lines, turbulence, or radi- CI-normalized Fe/Si ratios from 0.4 in LL ation pressure (Liffman and Brown, 1996b; Shu chondrites to Z2 in CH and CB chondrites. et al., 1996, 2001; Nuth et al.,2002). Alter- However, for other elements, a stronger case can natively, Harker and Desch (2002) suggest that be made. CI-normalized, mean Ti/Al ratios for magnesian silicates in comets formed at matrix and chondrules in the Renazzo CR2 5–10 AU by annealing caused by nebular chondrite are 0.5 and 1.5, respectively, whereas Heating Mechanisms in the Early Solar System 55 bulk Ti/Al ratios for CR and other chondrites this preference for complexity is provided by are within 10% of CI levels (Klerner and Palme, the amoeboid olivine inclusions: all AOAs 2000). Palme and Klerner (2000) infer that could have formed by the same basic process: chondrules and matrix formed from a single nebular condensation with only minor subse- reservoir that behaved as a closed system during quent thermal processing. Aluminum-rich condensation and processing of refractory chondrules may provide a second exception, elements. Bland et al. (2005) infer that matrices at least within carbonaceous chondrites. and chondrules have complementary enrich- Each chondrite component appears to have ments and depletions of volatile siderophile been manufactured predominantly by brief and chacolphile elements due to exchange in high-temperature processes. Chondrules and the nebular region where chondrules formed. igneous CAIs have textures indicating cooling in minutes or hours. Many enstatite grains in matrices contain ortho–clino intergrowths like- those in chondrules, and cooled in less than a 1.07.6 FORMATION AND ACCRETION few hours. Metal grains in CH chondrites OF CHONDRITIC COMPONENTS cooled from high temperatures in days, at most. However, the generation of group II REE Several common themes have emerged from pattern characteristic of many fine-grained CAIs this review of chondritic components. Type 3 is one process that required longer timescales of chondrites, for example, are more affected by many years (Palme and Boynton, 1993). alteration than generally realized. Only a hand- The high-temperature processes that formed ful of chondrites meet the minimal requirements chondritic components typically involved gas– for pristine chondrites, and none of these es- solid or gas–liquid exchange over part of the caped the imprint of asteroidal modification. temperature range 1,200–2,000 K. Rapid cool- Whether this alteration occurred in parent bod- ing in the formation environment or removal to ies, as we infer, or in the nebula, the alteration cooler localities ensured an extraordinary diver- environment was quite different from that at the sity of mineralogical and chemical compositions location where each component formed. among chondritic components. Chemical Mean sizes of chondrules, CAIs, amoeboid fractionations during brief high-temperature olivine inclusions, dark clasts, and possibly processing were so ubiquitous and extensive metallic Fe,Ni grains tend to be correlated that they may be largely responsible for at least among the chondrite groups. Thus, in groups four of the major chemical variations shown by with relatively small chondrules, all compo- chondrites: variations in refractory and moder- nents tend to be relatively small. This sorting ately volatile elements, metal–silicate ratios, and process occurred after all components formed, forsterite–enstatite variations. possibly during turbulent accretion triggered The search for nebular condensates among by chondrule formation (Cuzzi et al., 2001). chondrites has generated several interesting Aerodynamic sorting should cause metal parti- but seemingly false leads including wollasto- cles to accrete with larger chondrules, as particles nite needles (e.g., Kerridge, 1993) and blue- are sorted according to the product of their luminescing enstatite rims (Weisberg et al., radius and density (Kuebler et al., 1999). 1994; Hsu and Crozaz, 1998). FeO-rich matrix However, Akridge and Sears (1999) have also silicates are also unlikely to be nebular con- argued for aerodynamic sorting but they pro- densates. They appear to have formed predom- posed that components were sorted in an inantly by asteroidal processes, though asteroidal regolith during degassing. some may have formed by nebular annealing Many different processes were involved in of FeO-rich amorphous material. making each chondritic component. Unaltered In the final section, we review the heat sources chondrite matrices may contain at least five dif- that may have operated in the early solar system ferent types of micrometer-to-nanometer-sized to produce such diverse chondritic components. components, which formed in diverse environm- ents: amorphous FeO-rich silicate, forsterite and enstatite crystals, refractory grains, presolar grains, and carbonaceous material. Chondrules 1.07.7 HEATING MECHANISMS IN THE formed by several nebular processes (closed- EARLY SOLAR SYSTEM system melting, condensation, and possibly evaporation) and at least one asteroidal proc- A wide variety of components in chondrites ess (impact melting in regoliths). CAIs are and chondritic IDPs were processed at high largely fine-grained condensates or coarse- temperatures in the solar nebula over a period grained igneous objects that probably formed of several million years, and several heating from fine-grained precursors. An exception to mechanisms were probably involved. Possible 56 Chondrites and Their Components heating mechanisms for making chondrules Below we review three models that are cur- have been reviewed by Cassen (1996), Boss rently popular: nebular shocks, jets and outflows (1996), Rubin (2000), and Ciesla (2005), and near the protosun, and impacts on planetesimals. for CAIs and chondrules by Jones et al. (2000a). Most heating models have focused 1.07.7.1 Nebular Shocks on chondrule origins, but Shu et al. (1996, 2001) address both CAI and chondrule forma- Nebular shock waves are discontinuities tion close to the protosun; Hood (1998) and between hot compressed gas moving faster Desch and Connolly (2002) consider mecha- 5 thanpffiffiffiffiffiffiffiffiffiffiffiffiffiffi the local sound speed ¼ 1:2 10 nisms for separately melting ferromagnesian 1 and refractory aggregates to make chondrules 300=T cm s Þ and cooler, less dense, slowly and igneous CAIs. Richter et al. (2006) infer moving gas (Cassen, 1996). Particles overtaken that igneous CAIs may have experienced by shocks are suddenly enveloped in a blast of nebula conditions like those calculated for wind moving at several kilometers per second, chondrule-forming shocks. Russell et al. which subjects the particles to frictional heating. (2005) conclude that refractory inclusions Particles are also heated by radiation from the formed at the inner edge of the accretion disk shock front before they are overtaken and by whereas chondrules formed close to the site conduction from hot gas. Desch and Connolly (2002) find that cooling rates in the range 10– where chondrites accreted. 1 Three kinds of processes have been invoked 1,000 1Ch can be produced without invoking shocks of unreasonable speeds. They studied a for forming chondrules: melting of dust aggre- 1 gates (generally considered to be the standard range of shock speeds (5–10 km s ) and gas model), condensation of melts, or melts and densities and found conditions under which crystals (Ebel and Grossman, 2000; Krot et al., chondrules would partially melt, conditions that 2001d; Varela et al., 2005), and impacts would cause dust to evaporate, and at the higher into solid or partly molten bodies (e.g., Symes shock speeds, conditions that would evaporate et al., 1998; Sanders, 1996). As discussed above, chondrules. The shock model appears to be ca- we believe that all three processes formed pable of forming droplets of the correct chondrules, and that the standard model size (Connolly and Love, 1998; Susa and cannot account for olivine- and pyroxene-rich Nakamoto, 2002; Miura and Nakamoto, chondrules. 2005), it seems consistent with many petrologic Rubin (2000) has reviewed models for constraints from archetypal chondrules (Table forming chondrules and their consistency 7), and predicts a correlation between cooling with various petrologic constraints (Table 7): rate and the concentration of chondrules, which condensation, exothermic chemical reactions, appears consistent with compound chondrule jetting during nebular collisions between parti- statistics (Desch and Connolly, 2002). Shock cles, ablation in protoplanetary , heating models have also been investigated by nebular lightning (Desch and Cuzzi, 2000), sup- Iida et al. (2001) and Ciesla and Hood (2002) ernova shock waves, aerodynamic drag heating and are reviewed by Desch et al. (2005). Con- at the accretion edge of the disk, magnetic re- straints on shock models from the presence of connection flares, gas dynamic shock waves iron–sufide inclusions in chondrules are dis- (Section 1.07.7.1), bow shocks from early cussed by Uesugi et al. (2005). formed planetesimals (Hood, 1998; Weidens- A major problem for the shock model has chilling et al., 1998; Ciesla et al., 2004a), FU been to find a source of powerful, pervasive, and Orionis outbursts, bipolar outflows (Section repeatable shocks. Several have been proposed: 1.07.7.2), gamma-ray bursts (Duggan et al., clumpy material falling into the nebula (Tanaka 2003), radiative heating (Eisenhour and Buseck, et al.,1998), bow shocks from planetesimals 1995), and impacts into asteroidal regoliths scattered by Jupiter (Hood, 1998; Weidenschil- and molten planetesimals (Section 1.07.7.3). ling et al.,1998), spiral-arm instabilities in the Additional mechanisms include shocks in the solar nebula (Wood, 1996b), X-ray flares (Naka- envelopes of giant protoplanets (Nelson and moto et al.,2005), and close encounters with Ruffert, 2005), current sheets in magnetically sibling protostars (Bally et al.,2005). Boss and active nebula regions (Joung et al.,2004), and Durisen (2005) infer that clumps and spiral arms B 1 ablation of 1–500 m-sized planetesimals by could generate 10 km s shocks in the aster- shock (Genge, 2000). Note that the constraints oid belt (Chapter 1.04). listed in Table 7 apply to what we have called, ‘‘archetypal chondrules,’’ which are most closely 1.07.7.2 Jets and Outflows represented by type II chondrules. Chronologi- cal and magnetic constraints are not included in Two types of jet or outflow models have been Table 7. proposed. Liffman and Brown (1996a, b) Table 7 Review by Rubin (2000) of chondrule-formation models and their consistency with petrologic and geochemical constraints. Oxygen Incomplete Evaporation Relict Rapid Chondrule Chondrule Multiple Micro- Primary isotopes melting during long- grains and cooling abundance sizea heating chondrules troilite term heating moderate in rims volatiles Condensation ?/?þ þ Exothermic chemical reactions þ þ þ Jetting ? þþþþþ Meteor ablation ? þþþþþ Nebular lightning þþ þ þþþþþþþ shock waves þþ þ þþþ Aerodynamic drag heating þ þ þþ? þ Magnetic reconnection flares þþ þ þþþþþþþ Gas dynamic shock waves þþ þ þþþþþþþ Planetesimal bow shocks ? þþþþþ? þ ? þ FU Orionis outbursts ? Bipolar outflows þ Gamma-ray bursts þþ þþþ ? þ Radiative heating þþ þ þþþþþþþ Planetesimal collision þ Impact-melting in parent-body þ þ þ? þ regolith a Chondrule size distribution within a group is much narrower than the total variation shown by all chondrites. Note: þ , consistent; , inconsistent; ?, possibly consistent after ad hoc modifications; /, not applicable. 58 Chondrites and Their Components suggested that chondrules formed in a bipolar (Scott, 2002). (See Sears, 1998 for a contrary outflow by ablation of planetesimals and were view.) However, the chromite-rich chondrules then injected into the asteroid belt. Size sorting present in brecciated type 4–6 ordinary chond- of metal grains and chondrules can occur in jet- rites (Krot and Rubin, 1993) and perhaps a few flow models (Liffman, 2005), and the observed percent of other chondrules in H, L, and LL correlation between rim thickness and chond- chondrites probably did form this way after the rule size may have arisen when chondrules chondritic bodies had accreted. Since impact reentered the dusty nebula at hypersonic speeds melt will be ejected at speeds of several kilo- (Liffman and Toscano, 2000). meters per second, much faster than the escape Shu et al. (1996, 2001) proposed that chond- velocity of the asteroid (less than a few hundred rules and CAIs formed at the inner edge of the meters per second), most impact melt will not protostellar disk (see Chapter 1.04). About re-accrete to the target asteroid. Impact melt 0.05 AU from the protosun, the rotation rate of should be rarer on the surface of asteroids than the disk matches that of the protostar and the on the Moon (Taylor et al., 1983). disk is terminated. Inflowing gas is ionized and Any asteroid more than B15 km in radius either whipped into the protostar by the star’s that accreted o1–1.5 Myr after CAIs formed magnetic field, or ejected in the bipolar out- would have melted o3 Myr later, given the flows, while rocks spiral inward until they evidence that 26Al was relatively homogeneou- evaporate or are launched by the winds. CAIs sly distributed with an 26Al/27Al ratio of are thought to form during quiescent condi- B5 105 when CAIs formed (e.g., Yoshino tions, whereas chondrules form when the et al.,2003; Hevey and Sanders, 2006). The position of the inner edge of the disk fluctu- parent asteroids of differentiated asteroids ates due to intense flares from the protosun. appear to have formed at this stage before the Shu and colleagues suggest that CAIs and chondrite parent bodies, and several suffered chondrules are hurled outward by the so-called catastrophic impacts during or soon after igne- X-wind, which they infer powers the bipolar ous differentiation (Keil et al., 1994). Sanders outflows. CAIs and chondrules fall back onto (1996) argues that chondrules formed in major the disk at several astronomical units or beyond impacts on molten planetesimals with thin and are available for accretion into asteroids crusts, and that convection was vigorous and planets or recycling back to the inner edge enough to prevent gravitational segregation of the disk. Smaller particles fall back at greater of melted Fe,Ni droplets to the core (see also distances from the protosun. Detailed thermal Yoshino et al., 2003; Sanders and Taylor, histories for chondrules and CAIs have not been 2005). This model envisages that molten calculated for this model. metal–silicate droplets were dispersed into the solar nebula and that they accreted long after they had cooled and mixed with impact drop- 1.07.7.3 Impacts on Planetesimals lets from other asteroidal impacts, nebula dust, CAIs, and material from the cool unmelted Four kinds of impacts have been proposed crust of the target asteroids. The advantages of to form chondrules: hypervelocity impacts this model are that abundant chondrules are between solid bodies that melt regolith mate- generated, the model readily accommodates rial (e.g., Symes et al., 1998; Sears, 2005; the several Ma interval between CAI and Bridges et al., 1998), hypervelocity impacts on chondrule formation, and, provided gas loss Vesta-sized and larger, partly molten bodies occurred, it could explain why many chond- (Hutchison et al., 2005), impacts onto already rules lost volatiles (Lugmair and Shukolyukov, molten planetesimals at lower speeds that 2001). Issues that require further study are puncture the planet spewing forth molten drops discussed by Sanders and Taylor (2005). of silicate and metal (Sanders, 1996; Sanders and Taylor, 2005), and specifically for CB chondrites, an impact between undifferentiated ACKNOWLEDGMENTS Moon-to-Mars-sized bodies that generated a plume of melt and vapor (Rubin et al., 2003; This research has made use of NASA’s Krot et al., 2005a). Astrophysics Data System and was supported Many factors suggest that the vast majority in part by NASA grants NAG5-11591 and of chondrules could not form by impact in NAG5-11591 (K. Keil, P. I.) and NAG5-10610 regoliths (Taylor et al., 1983). Molten droplets and NNG04GH02G (A. Krot, P. I.). We thank are rare on the regolith of Vesta, the presumed A. Davis and an anonymous reviewer for many parent of the , and meteorite evidence helpful comments on the original chapter. 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