Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 431

Petrology and Origin of Ferromagnesian Silicate Chondrules

Dante S. Lauretta University of Arizona

Hiroko Nagahara University of Tokyo

Conel M. O’D. Alexander Carnegie Institution of Washington

Ferromagnesian silicate chondrules are major components of most primitive meteorites. The shapes, textures, and mineral compositions of these chondrules are consistent with crystalliza- tion of a molten droplet that was floating freely in space in the presence of a gas. The texture and mineralogy of a chondrule reflects the nature and composition of its precursor material as well as its thermal history. There is an enduring debate about the degree to which chondrules interacted with the ambient gas during formation. In particular, it is uncertain whether or not chondrules experienced evaporation during heating and recondensation during cooling. The extent to which these processes took place in chondrule melts varied as a function of the du- ration of heating as well as the environmental conditions such as pressure, temperature, com- position, and size. Thus, locked in chondrule bulk compositions, mineralogy, and textures are clues to the ambient conditions of the solar nebula and the nature of material processing in the inner solar system at the early stages of planet formation. Here we survey what is known about the properties of ferromagnesian chondrules in primitive meteorites and use this information to place constraints on these important parameters. The topics covered in this chapter are listed in Table 1.

TABLE 1. Contents of this chapter.

Section Topic 1 Introduction 2 Properties of Ferromagnesian Chondrules 3 Precursor Chemistry and Mineralogy 4 Thermal Histories and Crystallization of Chondrules 5 Metallogenesis and Redox Reactions in the Melt 6 Elemental Evaporation and Recondensation 7 Volatile Recondensation and Formation of Chondrule Rims 8 Constraints on Models of Chondrule Formation

1. INTRODUCTION

The most primitive meteorites, chondrites, have textures that suggest formation by accumulation of material that was freely floating in the solar nebula (Fig. 1). In a sense, they can be viewed as sedimentary rocks that swept up material in specific regions of the early solar system. While part of their parent asteroids, chondrites experienced varying de- grees of aqueous alteration, thermal metamorphism, and Fig. 1. Plane-polarized transmitted light image of the Semarkona impact shock. Mineral compositions within and among the (LL3.0) primitive unequilibrated ordinary chondrite (from Lau- least-altered chondrites are highly variable. This wide chem- retta and Killgore, 2005). Chondrules make up ~70 vol% of this ical variation indicates that the different components have meteorite. The remainder is composed of dark, opaque, fine- not been in chemical communication long enough to homog- grained matrix. FOV = 1.5 cm.

431 432 Meteorites and the Early Solar System II

TABLE 2. Representative bulk compositions drule diameter), fairly uniformly sized crystals set in a fine- for major chondrule types (in wt%). grained or glassy mesostasis. Porphyritic chondrules that are dominated by olivine (>10/1 volume ratio) are referred to IA* IAB† IB† IIA‡ IIAB§ IIB§ as porphyritic olivine (PO) chondrules, while those that con-

SiO2 44.8 56.4 57.9 45.1 53.8 58.7 tain abundant pyroxene (>10/1 volume ratio) are porphy- TiO2 0.19 0.11 0.11 0.1 0.16 0.18 ritic pyroxene chondrules (PP). Chondrules with silicate Al2O3 3.9 2.4 2.4 2.68 4.8 4.6 mineral abundances between these two extremes are called Cr2O3 0.44 0.58 0.59 0.51 0.5 0.59 porphyritic olivine-pyroxene (POP) chondrules. Granular FeO 1.18 3.4 2.8 14.9 8.8 7.8 (G) chondrules contain many small grains of uniform size MnO 0.12 0.36 0.28 0.39 0.6 0.54 (<10 µm) with poorly defined crystal outlines. Granular MgO 40.7 33.9 33.4 31.3 25.1 21.4 chondrules are also subdivided into GO, GOP, and GP CaO 3.5 1.7 1.7 1.89 2.7 3.3 Na O 0.52 0.56 0.46 1.65 2.1 1.9 groups. Barred-olivine chondrules (BO) contain olivine crys- 2 tals that have one dimension that is markedly longer than K2O 0.07 0.06 0.05 0.17 0.28 0.29 the other two when viewed in thin section. Olivine grains in P2O5 — — — 0.34 — — barred chondrules either all have the same crystallographic FeS 0.2 0.3 0.09 0.94 0.28 0.13 orientation or contain several sets of such oriented grains. FeNi 3.76 0.32 0.28 0.51 — 0.08 Radial-pyroxene chondrules (RP) consist of fan-like arrays *Jones and Scott (1989). of low-Ca pyroxene, which radiate from one or more points † Jones (1994). near their surface. Cryptocrystalline chondrules are objects ‡ Jones (1990). that do not exhibit any recognizable crystal structure un- § Jones (1996). der an optical microscope. They have a high abundance of glassy material and may contain submicrometer dendrites. Metallic chondrules consist of Fe,Ni metal, some troilite (FeS), and minor amounts of accessory phases such as enize. These meteorites, referred to as unequilibrated chon- schreibersite and metallic Cu (La Blue and Lauretta, 2004). drites, preserve a number of chemical, mineralogical, and Neither chondrule size nor shape is strongly correlated with structural signatures inherited from the protoplanetary disk. textural type. Since the study of Gooding and Keil (1981), The major components (50–80 vol%) of most chondrites two other chondrule classification schemes have been de- are ~1-mm-diameter igneous spheres, called chondrules. veloped. The most commonly used scheme divides chon- Representative bulk compositions of natural chondrules drules into two broad categories: type-I and type-II, based are summarized in Table 2, and illustrate the range in their on bulk FeO contents (McSween, 1977; Scott and Taylor, bulk chemistry. In this chapter, we focus on the most com- 1983). Type-I chondrules are characterized by the presence mon types of chondrules, which are primarily composed of FeO-poor olivine and pyroxene (Fo and En > 90). Type-II of the ferromagnesian minerals olivine [(Mg,Fe)2SiO4], chondrules contain FeO-rich olivine and pyroxene (Fo and low-Ca pyroxene [(Mg,Fe)SiO3], and high-Ca pyroxene En < 90). The textural characteristics of both the type-I and [Ca(Mg,Fe)Si2O6]. These objects also contain a silicate- type-II series are gradational. These two main categories are glass mesostasis rich in Na, Al, Si, Ca, and K, relative to further subdivided into subtypes A and B based on their chondrite bulk composition. Iron-based alloys, sulfides, and abundances of olivine and pyroxene. Type-IA and IIA chon- oxides are present in varying abundances. drules contain abundant olivine (>80 vol%), type-IB and IIB In unequilibrated chondrites, chondrules are often sur- chondrules contain abundant pyroxene (>80 vol%), and chon- rounded by rims of fine-grained material. Chondrules and drules with intermediate abundances of olivine and pyrox- rims are themselves cemented together by a fine-grained ene are classified as type-IAB or IIAB. This approach along material mineral matrix. Like the chondrules, the rims and with the Gooding and Keil (1981) classification scheme are matrix are dominated by silicates, metals, and sulfides. How- often used in conjunction with one another. ever, they are compositionally and mineralogically distinct In addition to the textural and chemical classification from chondrules. schemes presented above, another, independent chondrule classification system has been proposed by Sears et al. 2. PROPERTIES OF FERROMAGNESIAN (1992) and DeHart et al. (1992). This system does not rely CHONDRULES on textural properties or modal mineralogy. Instead, chon- drules are classified based on their cathodoluminescence 2.1. Chondrule Classification properties and mineral compositions. Cathodoluminescence refers to the emission of visible light from a solid that is Gooding and Keil (1981) performed a comprehensive excited by interaction with an electron beam. Cathodolu- petrographic survey of chondrules in unequilibrated ordi- minescence is highly dependent on trace-element abun- nary chondrites. They classified chondrules based on their dances and crystal lattice defects. Chondrules that luminesce textures (see Fig. 2 and Table 3). Porphyritic chondrules brightly are classified as group-A chondrules, while those contain abundant relatively large (up to one-half the chon- that show little or no luminescence are group-B chondrules. Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 433

(a) (b)

(c) (d)

(e) (f)

Fig. 2. Optical microscopy images illustrating the variety of chondrule textures. FOV = 1.35 mm in all cases except (h) (FOV = 2.7 mm). (a) PO chondrule from QUE 97008 (L). (b) Reflected image of the same chondrule shown in (a). Note the rounded droplets of metal concentrated near the chondrule boundary. (c) Another PO chondrule from Clovis (H) illustrating a larger grain size distribu- tion compared to the chondrule in (a). (d) A PO-RP chondrule pair from EET 90066 (L). (e) A PP chondrule from ALH 78119 (large chondrule). Note also the small cryptocrystalline chondrule in the upper left. (f) A POP chondrule from Bishunpur (LL3.1). Note the olivine crystals concentrated near the center and the pyroxene toward the outer edge.

These two groups are further subdivided based on the mean the other groups represent alteration of primitive chondrules FeO and CaO concentration in olivine and the composition on meteorite parent asteroids. Cathodoluminescence is a less of the chondrule mesostasis. Sears et al. (1992) identified widely available technique, and as a result the Sears et al. four groups of chondrules as primitive and suggested that classification is not commonly used. 434 Meteorites and the Early Solar System II

(g) (h)

(i) (j)

(k) (l)

Fig. 2. (continued). (g) A coarse-grained porphyritic pyroxene chondrule from EET 90066 (L). (h) A barred-olivine chondrule from Bishunpur (LL3.1). (i) A barred-olivine chondrule from Saratov (L/LL4) showing multiple barred units. Note the presence of some por- phyritic olivine crystals. (j) A radiating pyroxene chondrule from ALH 78119 (L). (k) A granular-olivine chondrule from QUE 97008 (L). (l) A pair of crypto-crystalline chondrules from ALH 78119 (L).

2.2. Chondrule Petrology remember that the sizes and relative abundances of chon- drules vary significantly among chondrite classes. The most detailed studies of chondrule petrology have Type-IA porphyritic chondrules from the Semarkona been conducted on Semarkona (LL3.0), one of the least- (LL3.0) chondrite have been described in detail (Jones and metamorphosed ordinary chondrites. The results of this Scott, 1989). These objects contain metallic Fe,Ni and abun- work are summarized below. However, it is important to dant small olivine crystals in a glassy to microcrystalline Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 435

TABLE 3. Chondrule classification systems.

Textural Classification Abbreviation Abundance in OCs (%) Texture POP 47–52 Porphyritic, both olivine and pyroxene PO 15–27 Porphyritic, dominated by olivine (>10/1 by volume) PP 9–11 Porphyritic, abundant pyroxene (>10/1 by volume) RP 7–9 Radial pyroxene BO 3–4 Barred olivine CC 3–5 Cryptocrystalline GOP 2–5 Granular olivine-pyroxene M <1 Metallic

Compositional Classification Silicate Type Composition Subtype Silicate Abundances I FeO-poor A Abundant olivine (>80 vol%) (Fo, En > 90) AB Intermediate abundances of olivine and pyroxene II FeO-rich (Fo, En < 90) B Abundant pyroxene (>80 vol%)

mesostasis. Olivine grain sizes are typically 15–40 µm. This fectly spherical (Jones, 1990). They contain euhedral oli- small grain size is a diagnostic property of type-IA porphy- vine (10–250 µm in diameter averaging 50 µm). Olivine ritic chondrules. The olivine grains are typically euhedral grains have average compositions of Fo85. They are chemi- with well-formed crystal faces in contact with the meso- cally zoned with FeO increasing toward grain edges. Troi- stasis. Some olivines contain inclusions of mesostasis. It is lite occurs as small (<10 µm) rounded blebs distributed common for the low-Ca pyroxene in these chondrules to be throughout the phenocrysts and mesostasis. Small (<1 µm) concentrated near the chondrule margins and poikilitically euhedral grains of chromite (FeCr2O4) also occur in the enclose olivine grains. Rims of Ca-rich pyroxene occur on mesostasis. Low-calcium pyroxene and Fe,Ni metal are rare. the low-Ca pyroxene grains. Rounded grains of Fe,Ni metal In Semarkona, type-IIB chondrules pyroxene morpholo- and associated minerals are evenly distributed throughout gies vary from euhedral crystals to sets of subparallel bars the chondrules or concentrated near their margins. (Jones, 1990). The pyroxene cores consist of enstatite or In type-IAB porphyritic chondrules in Semarkona, euhe- clinoenstatite and are overgrown with narrow rims of Ca- dral equant olivine crystals are up to 150 µm in diameter rich pyroxene, typically successive overgrowth of augite on (Jones, 1994). Olivine (averaging 25 µm across) also occurs pigeonite. Augite occurs as prisms on the ends of clino- in large poikilitic low-Ca pyroxene crystals. Crystals of low- enstatite laths. Mesostasis consists of glass with abundant Ca pyroxene are twinned, generally tabular, and contain microcrystallites of augite. Small rounded grains of metal lamellar, compositional zoning. Calcium-rich pyroxene oc- and sulfide are present. curs as thin (<20 µm), discontinuous overgrowth rims on low-Ca pyroxene. Contacts between low-Ca and high-Ca 3. PRECURSOR CHEMISTRY pyroxene are sharp. Chemical concentration gradients are AND MINERALOGY common in these Ca-rich pyroxene rims from the inner edge in contact with the low-Ca pyroxene to the outer edge next In order to understand the chemical evolution of chon- to the mesostasis. Grains of Fe,Ni metal are also common. drule melts, it is necessary to place some constraints on the Mesostasis is glassy and, in some cases, contains needle- composition and mineralogy of their precursor material. like crystallites of Ca-rich pyroxene. Based on our current models of the early solar system, there Type-IB porphyritic chondrules in Semarkona are FeO- are several possible sources of this precursor material, in- poor. They are distinguished from type-IAB chondrules cluding presolar material, nebular condensates, fragments of by their high proportion of pyroxene and low abundance earlier generations of chondrules, and fragments from plan- (<20%) of olivine (Jones, 1994). However, olivine is present etesimals. in even the most pyroxene-rich type-IB chondrules and Presolar material is primordial dust inherited from the commonly occurs as relict grains (see section 3.2.2 below). protosolar molecular cloud, some of which still survives in Type-IIA porphyritic chondrules in Semarkona have an the chondrule rims and matrices of chondrites, including overall rounded morphology but are not, in general, per- circumstellar grains and interstellar organic matter (Huss et 436 Meteorites and the Early Solar System II al., 1981). A significant proportion of material in the inner olivine-rich condensates formed above the condensation solar system may have been vaporized in the high-tempera- temperature of enstatite, while the nonrefractory component ture environment near the proto-Sun. As the nebula cooled, (FeO-rich pyroxene, moderately volatile lithophiles, etc.) the most refractory material condensed as solid grains. may have formed from fine-grained materials that were able These nebular condensates may have then been reheated to equilibrate at lower nebular temperatures (Grossman and during the event (or events) responsible for chondrule for- Wasson, 1983b). Type-IIA chondrules from Semarkona mation. Some models suggest that the vapor pressure of the (LL3.0) have (Na + K)/Al atomic ratios that approach but major rock-forming elements was high enough in the inner do not exceed unity, which has been taken as evidence that solar system for chondrule melts to condense directly from the principle alkali-bearing precursor was albite (Hewins, the gas phase. If there were multiple heating events, then 1991). The fact that chondrules from different chondrite chondrules could have been fragmented and remelted many groups exhibit different element-mineral correlations would times, becoming their own precursors. Finally, it is possible require that the condensate precursors for each chondrite that chondrules formed after larger planetesimals underwent group formed under somewhat different conditions. chemical differentiation. In this case, chondrule precursors There are two potential problems with this interpretation. could be material that was ejected by collisions between By the time olivine condenses there are several predicted planetesimals or during volcanic eruptions on asteroid sur- condensate minerals that would be the hosts for the refrac- faces. In order to constrain the various models for chondrule tory elements: corundum (Al2O3), hibonite (CaAl12O19), spi- precursors, we look to their mineralogy, bulk chemistry, and nel (MgAl2O4), perovskite (CaTiO3), melilite [Ca2(Mg,Al) isotopic compositions, as well as to the information pre- (Si,Al)2O7], anorthite (CaAl2Si2O8), and diopside (CaMgSi2O6). served in the relict grains that occur in many chondrules. The correlation between the refractory lithophiles would re- quire little fractionation between these minerals, despite the 3.1. Constraints on Chondrule Precursors fact that some fractionation between condensates is required from Bulk Compositions to produce the less-refractory lithophile component(s). Per- haps more importantly, none of these minerals have been Studies of chondrule bulk compositions have been used to found as relict grains in ordinary chondrite chondrules (see infer the nature of chondrule precursors. In a series of de- section 3.2.2). Nor have they been found in significant tailed investigations, the bulk compositions, accompanied by amounts in the rims or matrix of the least-altered chondrites. petrographic control, were determined for ordinary (Gross- Yet rims and matrix do preserve primitive materials, includ- man and Wasson, 1982, 1983a; Swindle et al., 1991a,b), ing presolar materials. enstatite (Grossman et al., 1985), and carbonaceous chon- An alternative explanation for the interelement correla- drite (Rubin and Wasson, 1987, 1988) chondrules. Detailed tions is that they were produced by some combination of reviews of these studies are given in Grossman and Wasson evaporation, metal/sulfide loss, FeO reduction, and/or ran- (1983b), Grossman et al. (1988), and Grossman (1996). dom sampling of recycled chondrule material (Sears et al., Elemental correlations are present in chondrules that 1996; Alexander, 1994, 1996). Starting with a homogenous appear to be related to their volatility and chemical affin- population of precursors, variable degrees of evaporation, ity, although there are some variations between chondrite FeO reduction, and metal/sulfide loss would automatically groups. The three most universal correlations occur within induce a correlation between refractory lithophile elements the refractory lithophiles (Al, Ca, Ti, Sc, and REEs), com- by enriching them in the residual chondrules. As discussed mon siderophiles and some chalcophiles (Fe, Co, Ni, Se), in section 3.2.2, recycled chondrule material was an impor- and the alkalis (Na and K). The behavior of elements like tant component of chondrule precursors. All the refractory Mg, Mn, and Cr are more complex, and this is reflected in lithophiles behave incompatibly in chondrule melts, so they how the compositions of olivine and pyroxene correlate with are concentrated in the glass. Thus, a correlation between the the different elemental groups. For instance, the refractory refractory lithophiles could be produced by variable amounts lithophiles correlate with the presence of FeO-poor olivine of glass from an earlier generation of chondrules in the pre- in the ordinary and enstatite chondrites, with FeO-poor py- cursors. Metal/silicate fractionation during chondrule for- roxene in the CM and CO chondrites, and with neither in the mation would create siderophile-element correlations. The CV chondrites. As would be expected, the siderophile and upper limit of 1 for (Na + K)/Al ratios in type-II Semarkona chalcophile elements are generally associated with metal chondrules may simply reflect the fact that for the alkalis and troilite, although the behavior of Au, As, and Ga varies to be easily accommodated in a silicate melt or glass re- between the chondrite groups and the refractory siderophiles quires the coupled substitution (Na,K)AlO2 — SiO2 (Alex- Ir, Os, and Ru appear to have affinities with the common ander, 1994). siderophiles and refractory lithophiles. Ultimately, the relative importance of precursors and These elemental correlations are thought to reflect vari- chondrule formation conditions depends on the degree to able abundances of different nebular condensates in their which individual chondrules were open or closed chemical precursors. For example, the refractory component in ordi- systems during formation (see section 6). However, bulk- nary chondrite chondrules has been interpreted as being chondrule refractory-element fractionations are unlikely to Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 437

be produced during evaporation and recondensation asso- 1997) and “fine-grained lumps” (Rubin, 1984). A compre- ciated with ferromagnesian chondrule formation. Therefore, hensive description of these objects is given by Weisberg if they are present they must reflect variations in precursor and Prinz (1996). They are comparable in size to true drop- compositions. let chondrules. They are typically composed of a coarser- The rare earth elements (REEs) and other refractory grained core surrounded by material primarily comprised of lithophiles are very useful probes of geochemical and cos- fine-grained (<5 µm) olivine, which constitutes 57–94 vol%. mochemical processes. The REEs can be strongly fraction- Larger grains of olivine, up to 400 µm, can also be present. ated from one another during planetary differentiation, They also contain lesser amounts of pyroxene and felds- during high-temperature condensation, and during extreme pathic glass (<20 vol%). Minor components include chrom- evaporation. Most chondrules from ordinary (Grossman and ite, metal, and sulfide (<5 vol%). Some agglomeratic olivine Wasson, 1983a; Kurat et al., 1984; Alexander, 1994), ensta- chondrules contain fragments of prior generations of chon- tite (Grossman and Wasson, 1985), and CH (Krot et al., drules and refractory inclusions. 2001a) chondrites have nearly flat CI-normalized REE pat- Olivine compositions in a single agglomeratic chondrule terns, although some have small anomalies in Eu and pos- span a wide range of FeO contents, indicating formation sibly Yb. There is no evidence in the REE patterns of these in diverse environments with little subsequent equilibration. chondrules that their precursors had experienced planetary Many of these objects are layered and exhibit distinct tex- differentiation, evaporation, or condensation. tural, mineralogical, compositional, and O-isotopic differ- Some chondrules from CV and CO carbonaceous chon- ences between core and rim. Their textures are consistent drites exhibit REE patterns that resemble patterns in some with a thermal history in which they were briefly (minutes CAIs (Misawa and Nakamura, 1988a,b, 1996). The abun- to hours) heated up to temperatures high enough to result dance of CAIs in CVs and COs makes the inclusion of CAI in sintering, but not enough to produce significant melting. material in some chondrule precursors very likely, unless Based on the characteristics of agglomeratic chondrules, we CAIs and chondrules were mixed together after chondrule can infer that chondrule precursor materials were dominated formation. The preservation of a few CAIs inside chon- by olivine crystals 2–5 µm in size with varying amounts of drules (Itoh and Yurimoto, 2003) suggests that they were pyroxene, metal, troilite, chromite, and alkali feldspar or mixed together before chondrule formation. The reason that feldspathic glass. more CAIs are not found inside chondrules is probably Metal is a common constituent of type-I and agglomer- because their liquidus temperatures are lower than those of atic chondrules and was likely a component of chondrule most chondrules. precursors. Additional metal may have formed as a result There are very few CAIs in ordinary or enstatite chon- of in situ reduction either by carbonaceous material in the drites (Guan et al., 2000; Fagan et al., 2001). Thus, it is chondrule precursors or reaction with the nebula gas. Car- perhaps not surprising that most chondrules in these mete- bonaceous material occurs in chondrule interiors as graphite orites have unfractionated REE patterns. However, Pack et inclusions in metal (Mostefaoui and Perron, 1994), as fine- al. (2004) have found two chondrules in two ordinary chon- grained carbonaceous material associated with metal and drites with large negative anomalies in Sm, Eu, and Yb. The sulfide (Lauretta and Buseck, 2003), and as poorly graphi- pattern of anomalies are consistent with predictions of con- tized C included in chondrule silicates (Fries and Steele, densation under very reducing conditions, but have not been 2005). observed even in CAIs from the highly reduced enstatite In many chondrules sulfide occurs as droplets and as chondrites. The two chondrules themselves are not very eutectic mixes when associated with metal. These textures reduced, but reduced condensates could have been minor are consistent with crystallization of a sulfide melt. In the components of their precursors. If this is the case, it is sur- finest-grained, least-heated chondrules, a troilite-pentland- prising that similar patterns have not been found in enstatite ite assemblage occurs associated with minor amounts of chondrite chondrules. kamacite. The co-occurrence of these two sulfide phases is consistent with experimental studies of kamacite sulfuriza- 3.2. Mineralogical Constraints on tion under solar nebula conditions (Lauretta et al., 1997, Chondrule Precursors 1998) and suggests that sulfides were an important compo- nent of some chondrule precursors (Yu et al., 1996). 3.2.1. Primitive chondrules. Chondrules exhibit a wide Some researchers have suggested that chondrules could variety of textures and grain sizes. Since many chondrules have been produced by volcanism on planetesimals (Hutchi- were heated enough to melt, some chondrules must have son and Graham, 1975; Kennedy et al., 1992) or by im- experienced temperatures between ambient and melting. pacts between partially molten planetesimals (Hutchison et The finest-grained chondrules are considered to be the least al., 1988). Chondrules formed in this way would contain altered and therefore best preserve precursor mineralogy distinct geochemical or isotopic signatures associated with and bulk compositions. Foremost among such objects are the partial melting and differentiation of their planetary bod- agglomeratic olivine chondrules, also referred to in the lit- ies. Igneous fragments occur in primitive ordinary chon- erature as “dark zoned chondrules” (Dodd, 1971; Hewins, drites that contain just such signatures (Graham et al., 1976; 438 Meteorites and the Early Solar System II

Hutcheon and Hutchison, 1989; Hutchison and Graham, ment embedded in a CAI from the Yamato 81020 (CO3.0) 1975; Hutchison et al., 1988; Kennedy et al., 1992). How- chondrite, providing evidence that either chondrules con- ever, such igneous fragments are relatively rare and do not tributed some material to CAI precursors, or that remelt- appear to have been the primary source of chondrule pre- ing of CAIs took place during or after chondrule formation. cursors, nor do chondrules show the major- or trace-element Initial 26Al/27Al ratios in Allende chondrules and CAIs also fractionations that are characteristic of partial melting and support the idea that chondrule formation began contem- differentiation. poraneously with the formation of CAIs (Bizzaro et al., 3.2.2. Relict grains in chondrules. Some olivine and 2004). orthopyroxene grains have compositions, particularly FeO Based on major- and minor-element compositions, most contents, that are distinctly different from those of the other relict grains formed in earlier generations of chondrules grains in their host chondrules (Nagahara, 1981; Rambaldi, (Jones, 1996; Nagahara, 1983); relict forsterite and enstatite 1981). These so-called relict grains often display composi- grains found in FeO-rich chondrules were derived from FeO- tional zoning that is inconsistent with in situ crystallization poor chondrules, and dusty olivine grains found in FeO- from their host chondrule melt (Rambaldi, 1981). They ap- poor chondrules formed in FeO-rich chondrules (Jones, pear to be precursor material that survived chondrule forma- 1996). Oxygen-isotopic data also support the view that the tion without melting or dissolving. Therefore, they provide low-FeO relict grains formed in a previous generation of a record of the compositions and mineralogies of chondrule low-FeO porphyritic chondrules that were subsequently precursors, although this record is biased toward those grains fragmented (Kunihiro et al., 2004). Relict grains, chondrule with higher melting temperatures (and therefore lower dis- fragments, and isolated olivine and pyroxene grains in ma- solution rates) than the peak temperatures experienced by trix all show that fragmentation and recycling of chondrule the chondrules. Thus, it is likely that there were many more material was an important process in the chondrule-forming phases in the precursors that did not survive. region (Alexander et al., 1989a; Jones, 1992). Furthermore, Rambaldi (1981) estimated that approximately half of all based on the abundances of nonspherical chondrules (arbi- porphyritic chondrules in unequilibrated ordinary chondrites trarily defined as having aspect ratios >1.20) in CO3.0 chon- contain relict grains, while Jones (1996) estimated that 15% drites, Rubin and Wasson (2005) suggest that chondrule of all chondrules contain relict grains. Jones (1996) found recycling was the rule in the CO chondrule-formation re- forsteritic relict grains in 3 out of 11 type-II Semarkona gion and that most melting events produced only low de- (LL3.0) chondrules and in 5 out of 6 such chondrules in grees of melting. They also suggest that the rarity of sig- ALHA 77307 (CO3.0). However, it is important to keep in nificantly nonspherical, multilobate chondrules in ordinary mind that there is no a priori reason to assume that all relict chondrites may reflect more-intense heating of chondrule grains will have anomalous compositions. Such grains are precursors in the region of the solar nebula in which they simply the easiest to identify. It is likely that there are many formed. more relict grains that are not readily identified because Weinbruch et al. (2000) suggest that some refractory, their compositions are not disparate. relict forsterite grains cannot have formed by crystallization Dusty relict olivines, which contain numerous submicro- from chondrule melts. They favor an origin by condensa- meter to 10-µm inclusions of Fe-metal, are probably also tion from an oxidized nebular gas. Rambaldi et al. (1983) relict and are observed in ~10% of all chondrules in ordinary noted that igneous olivine within Qingzhen (EH3) chon- chondrites (Nagahara, 1983; Grossman et al., 1988). Iron- drules always contain detectable amounts of CaO, while metal grains in dusty olivines are severely depleted in Ni, relict olivines are essentially CaO-free. They also suggest relative to CI abundances and likely formed when the chon- that the relict olivines did not have an igneous origin and drule melted in the presence of a reducing agent, such as might represent condensates from the solar nebula. Regard- H2 in the surrounding gas or solid carbonaceous material less of their origin, the sporadic occurrence of large re- (Rambaldi, 1981). Dusty olivine grains are less common lict grains in such chondrules suggests a mix of occasional in CM and CV chondrites (Jones, 1996). Olivine crystals large grains among finer material in chondrule precursors that are completely enclosed in pyroxene grains in porphy- (Hewins, 1997). ritic chondrules may also be relict grains (Nagahara, 1983). However, Lofgren and Russell (1986) showed that similar 3.3. Experimental Constraints on textures can be produced during chondrule crystallization. Chondrule Precursors Nearly pure forsterite exhibiting blue cathodoluminescence occurs inside chondrules of carbonaceous and ordinary There have only been a few experiments that constrain chondrites and is thought to represent another population the nature of chondrule precursors. In a series of experi- of relict grains (Steele, 1986). ments, it was demonstrated that the grain size of chondrule Misawa and Fujita (1994) report the occurrence of an precursors has a significant effect on the resulting chondrule intact CAI fragment in a ferromagnesian chondrule from textures (Radomsky and Hewins, 1990; Connolly and Hew- Allende, which confirms that some CAIs contributed to ins, 1995, 1996; Connolly et al., 1998; Hewins and Fox, chondrule precursor material. Furthermore, Itoh and Yuri- 2004). These experiments suggest that granular chondrule moto (2003) discovered a ferromagnesian chondrule frag- textures (grain size <10 µm) result from the incomplete Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 439

melting of fine-grained starting materials, less than 20 µm in The Allende chondrules are mass fractionated by up to diameter. Furthermore, Hewins and Fox (2004) show that it 1.4‰/amu in the lighter Sr isotopes. Patchett (1980) sug- is very difficult to generate porphyritic chondrules by melt- gests that these chondrules derived their fractionated Sr ing such fine-grained precursors. These textures require pre- from a region of the nebula made isotopically light by par- cursor grain sizes >40 µm. tial kinetic mass separation of elements in the vapor phase. Experimental simulations of chondrule-like heating Later, the solid objects may have moved to a more isotopi- events using hydrous silicates as precursors always yield cally normal region, where the Allende matrix accreted. high abundances of vesicles. The fact that vesicles are ex- However, these signatures could also be due to condensa- ceedingly rare in chondrules is given as evidence that chon- tion or metamorphism. Allende chondrule mesostasis is drules were generated from anhydrous precursor minerals almost always recrystallized, chemically zoned (as are the (Maharaj and Hewins, 1994, 1998). However, if chondrule olivines), and can contain halogens, sodalite, etc. melts were heated to within 50°–100° of their liquidus tem- perature, vesicles could have migrated out of the spheres. 3.5. Direct Condensation Deloule et al. (1998) report high concentrations of water in from a Vapor Phase chondrule silicates from the ordinary chondrites Semarkona and Bishunpur, which they interpret as being due to the Several researchers have suggested that chondrule melts presence of hydrous silicates in their precursors. On the condensed directly from the vapor phase (e.g., Blander and other hand, both chondrites experienced parent-body aque- Katz, 1967; Blander et al., 1976, 2001, 2004). Nelson et ous alteration (Alexander et al., 1989a), and Grossman et al. al. (1972) suggested that chondrule textures can result from (2002) concluded that the zoning of water and halogens in rapid crystallization of relatively slowly cooling molten the Semarkona chondrules was due to this alteration. droplets and suggested that this history is consistent with condensation from a nebula of solar composition and solidi- 3.4. Constraints on Chondrule fication in an ambient medium at high temperature. Precursors from Isotopic Studies Ebel and Grossman (2000) performed detailed equilib- rium calculations of the sequence of condensation of the Chondrules in some carbonaceous chondrites show evi- elements from gases with various enrichments in a dust of dence of 16O-rich components. This signature is often in- CI-chondrite composition. They determined that the FeO terpreted as evidence for the survival or limited processing contents typical of type-IIA chondrules can only form at of a presolar component in the chondrule precursors (Wood, high total pressures (>10–3 bar) and high dust enrichments 1981; Clayton et al., 1991; Yu et al., 1995). However, the (100–1000× solar) at temperatures high enough for dust- absence of correlated anomalies in any other element, ex- gas equilibration to occur (~900°C). Even under these con- 16 cept perhaps Cu (Luck et al., 2003), and the rarity of O- ditions, the condensates contain much more CaO and Al2O3 rich presolar grains (Zinner, 1998) strongly argue against (relative to MgO and SiO2) than most ordinary-chondrite the presolar precursor explanation. Heterogeneity of O-iso- chondrules. Furthermore, most chondrule compositions are topic ratios within these chondrules may be the result of too Na-rich compared to predictions of condensate bulk incorporation of relict grains from objects such as amoe- compositions. However, the higher chondrule Na contents boid olivine aggregates, followed by solid-state chemical could reflect fractionation prior to chondrule formation or diffusion without concomitant O equilibration (Jones et al., alkali reintroduction at lower temperatures. 2004). More recently, it was argued that the mass-indepen- A condensation origin has been proposed for chondrules dent fractionations observed in chondrule O isotopes result in the Hammadah al Hamra 237 and QUE 94411 bencub- from chemical processes in the gas phase that accompanied binite chondrites, as well as the recently recognized CH their formation (Thiemens, 1996; Clayton, 2004; Lyons and group of chondrites (Krot et al., 2001a,b). Chondrules in Young, 2003; Jones et al, 2004). Even if the diversity of these meteorites generally have barred-olivine and cryptoc- O-isotopic compositions are due to non-mass-dependent rystalline textures and are essentially metal-free. Zoned isotopic fractionation, the relationships between the O-iso- metallic chondrules are also abundant in these meteorites. topic compositions and different chondrite components are The presence of inclusions with chondrule-like textures in still unknown. the zoned metal grains suggests that the two components Isotopic anomalies of Ti are present in chondrules from formed in the same nebular region, with silicate chondrules Allende (CV3) (Niemeyer, 1988). There is no known way forming at temperatures above those of metal condensation of generating such anomalies during or after chondrule (1773°C). formation, requiring that the Ti-isotopic diversity in chon- Two formation scenarios for these chondrules have been drules is a feature that they inherited from their precursors. considered. Either these chondrules formed by direct gas- The chondrule with the largest measured Ti-isotopic anom- liquid condensation or by prolonged heating of chondrule aly is also Al-rich, suggesting that these anomalies may re- precursors above their liquidus temperatures, which de- flect the presence of CAIs in their precursors. stroyed all solid nuclei. The complementary behavior of The isotopic composition of Sr was obtained for eight refractory lithophile elements (Ca, Al, Ti, REEs) in barred- olivine chondrules from Allende (CV3) (Patchett, 1980). olivine and cryptocrystalline chondrules suggests that these 440 Meteorites and the Early Solar System II chondrules formed in a chemically closed system (Krot et radial-pyroxene chondrules (Lofgren and Lanier, 1990; al., 2001a). Furthermore, high Cr2O3 (0.4–0.7 wt%) and Radomsky and Hewins, 1990). FeO (1–4 wt%) contents in the chondrules and complemen- Porphyritic and granular textures form when abundant tary depletions in the FeNi-metal grains suggests formation nuclei are present during cooling. The simplest interpreta- in a closed system under relatively oxidizing conditions. tion of this result is that chondrule peak temperatures were These compositional trends are consistent with formation slightly below their liquidus temperatures (Lofgren and in a dust-enhanced (10–50× solar) region of the solar nebula Russell, 1986), which enabled the survival of nuclei. These (Krot et al., 2001b; Petaev et al., 2001). The inferred en- unmelted relicts provided abundant nucleation sites that hanced initial dust/gas ratios supports the theory that the generated the characteristic porphyritic textures on cooling chondrules in these meteorites formed by direct gas-liquid (Lofgren, 1989; Hewins and Radomsky, 1990). Chondrule condensation (Ebel and Grossman, 2000). liquidus temperatures therefore provide important con- One of the primary arguments against direct condensa- straints on the peak temperatures experienced by chondrule tion is the extreme conditions required to produce the FeO melt droplets. Droplets that were completely molten (e.g., contents of type-II chondrules. However, Blander et al. those that crystallized as barred, excentroradial, and cryp- (2001, 2004) suggest that the condensation of metallic Fe tocrystalline chondrules) have liquidus temperatures that was suppressed because of nucleation constraints. The in- range from 1200°C to over 1900°C and are highly depen- hibition of Fe condensation results in supersaturation of Fe dent on bulk composition (Herzberg, 1979; Hewins and in the gas phase. As a result, higher levels of FeO entered Radomsky, 1990). Given this wide range of temperatures, it silicate melts by condensing directly from the vapor. This is clear that bulk composition is at least as important as ini- model also explains the general absence of metal in type-II tial temperature in controlling chondrule textures (Connolly chondrules. and Hewins, 1996). Based on the abundances, compositions, The strongest evidence that at least some chondrules and textures of incompletely melted vs. completely melted formed through melting of preexisting materials and did not chondrules in carbonaceous and ordinary chondrites, it has condense from a gas is the presence of large, composition- been estimated that few chondrules with liquidus tempera- ally distinct relict grains (see section 3.2.2 above). These tures over 1750°C were completely melted, and few with grains clearly survived the chondrule formation process liquidus temperatures under 1400°C were incompletely without melting. In addition, porphyritic chondrules, the melted. Therefore, it was concluded that most chondrules most abundant type of chondrule, require the preservation reached peak temperatures of 1500°–1550°C (Radomsky of numerous crystal nuclei (see section 4.1), implying that and Hewins, 1990). their precursors were also at least partially crystalline. Further constraints of chondrule thermal histories have been obtained by experimental simulations that accurately 4. THERMAL HISTORIES AND reproduce chondrule grain sizes and textures (Hewins and CRYSTALLIZATION OF CHONDRULES Radomsky, 1990; Lofgren, 1996; Connolly et al., 1998; Tsu- chiyama et al., 2004). These studies show that it is impor- It has long been established that many of the features tant to understand fully not only the peak temperatures that seen in chondrules are the result of crystallization from a chondrules experienced, but also the duration of the heat- melt. In the previous section, we describe the constraints ing event. This dual consideration is important because, for that can be placed on the precursor material. Here we iden- kinetically driven processes, the effect of heating to a high tify the constraints that can be placed on the peak tempera- peak temperature for a short time can be equivalent to that tures and duration of heating from experimental simulations of heating to a lower temperature for longer times (Jones of chondrule formation, and considerations of melting ki- et al., 2000). The total amount of energy, as opposed to the netics of relict grains. We also describe how chondrule tex- peak temperatures, is therefore constrained to be below that tures and disequilibrium features, such as compositional required both to raise the precursor material to the liquidus zoning in minerals, can be used to constrain the cooling his- and to supply the necessary latent heat of fusion to melt all tories of chondrules. solid precursor material. Connolly et al. (1998) showed that a short heat pulse, with a peak temperature well above the 4.1. Peak Temperatures and liquidus, can also produce porphyritic chondrule textures. Preservation of Nucleation Sites Nuclei survive because the timescales are shorter than the melting and dissolution kinetics. Thus, the maximum tem- A prerequisite for the crystallization of a melt is the perature achieved during chondrule formation could have presence of crystal nuclei, and nuclei abundance is a major been approximately 2100°C, 200°C higher than the liquidus factor in determining chondrule textures (Lofgren, 1996). temperatures of even the most refractory chondrules. The On the timescales of chondrule formation, spontaneous distinctive olivine rims on barred-olivine chondrules also formation of nuclei in the melt is a relatively slow process. provide information about peak temperatures. The forma- As a result, nuclei-free chondrule melts can cool well below tion of compact olivine rims on these chondrules requires a their liquidus temperatures and become supercooled. When heating temperature about 100°C or more above the liquidus a nucleus forms there is a burst of rapid crystal growth, to promote heterogeneous nucleation of olivine on the sur- producing textures similar to those of barred-olivine and face of the melt droplet (Tsuchiyama et al., 2004). Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 441

It is also possible that chondrule formation occurred in dercooling, and abundance of crystallization nuclei. As the a dusty environment, which provided abundant nucleation degree of undercooling increases, forsterite morphology sites. Experimental simulations that seeded molten droplets evolves from tabular to hopper. At the most rapid cooling with small grains of dust have reproduced a wide variety rates, dendritic crystals form. of chondrule textures (Connolly and Hewins, 1995). Tex- Euhedral olivine crystals develop in silicate melts with tures vary as a function of the temperature at which dust is abundant nuclei. No supercooling occurs and crystallization introduced to the melt, the number density of dust grains, begins as soon as the temperature drops below the liquidus. and their mineralogy. The resulting textures are uniformly sized olivine phenoc- rysts in a glassy mesostasis (Lofgren, 1989; Radomsky and 4.2. Kinetics of Melting and Hewins, 1990). The cooling rates required to produce por- Survival of Relict Grains phyritic textures in experimental chondule melts are less than 1000°C/h with those on the order of 100°C/h best re- Another independent constraint on the duration of chon- producing the textures in porphyritic chondrules (Lofgren, drule melting is provided by the presence of relict grains 1989; Radomsky and Hewins, 1990). that were not in contact with melt for a long enough time Barred-olivine chondrules contain olivine crystals with to dissolve. Early studies used thermal diffusion calculations single-plate dendritic texture. These textures are the result to determine the relationship between the duration of the of crystallization of nuclei-free melt droplets. A significant chondrule heating event and the maximum attainable tem- amount of undercooling may have occurred in these melt perature inside chondrules (Fujii and Miyamoto, 1983). droplets. Comparison of the olivine morphology in barred These researchers determined that the duration of each chondrules with analog textures produced in experiments heating event had to be less than 0.01 s for chondrules with indicates that these chondrules cooled at rates of 500°– radii of 0.3 mm for the survival of relict grains. 2300°C/h (Lofgren and Lanier, 1990; Tsuchiyama et al., More recently, Greenwood and Hess (1996) used a model 2004). for the kinetics of congruent melting to place constraints Pyroxene crystallization textures also vary as a function on the duration and maximum temperature experienced by of melt composition and cooling history. Porphyritic pyrox- the interiors of relict-bearing chondrules. They argue that ene textures are produced only in experiments that do not relict forsteritic olivine grains in chondrules were not heated exceed the liquidus temperature or exceed it only for a brief above 1900°C, approximately the melting point of forsterite. period of time (Lofgren and Russell, 1986). These textures Their temperature limit for chondrules with Mg-rich py- form when cooling rates are well in excess of 100°C/h. roxene relicts is 1557°C, the incongruent melting point of However, at cooling rates greater than 2000°C/h, the crys- enstatite. In both cases, the input of energy could not have tal sizes are significantly below those seen in most chon- lasted for more than a few seconds. At temperatures below drules of this type. their melting points, relict grains will dissolve if they are Crystals in radial pyroxene chondrules are generally not in equilibrium with the melt. Dissolution is slower and highly acicular, radial, and often in the form of spherulites. quantitatively less well understood than melting, but ulti- These morphologies are indicative of growth from a melt mately relict grains may be used to better constrain the ther- that experienced very high degrees of undercooling. Nucle- mal histories of a wide range of chondrules. ation sites, which are initially absent in the melt, form from crystal embryos that exceed some critical radius. Once nu- 4.3. Chondrule Cooling Rates cleated, growth of the pyroxene crystals is very rapid. The large degree of supercooling stabilizes the highly non-equi- 4.3.1. Chondrule textures. Chondrule textures are con- librium crystal shapes. The radial or spherulitic textures can trolled by nucleation density, degree of undercooling, and develop even at quite slow cooling rates (5°C/h) in any melt cooling rate. The density of nucleation sites is a function where olivine is not a liquidus or near-liquidus phase (Lof- of the duration and peak temperature of the heating event gren and Russell, 1986). as well as the abundance of dust particles in the chondrule 4.3.2. Zoning profiles. Experimental studies suggest formation region (section 4.2). The abundance of nucleation that chondrule textures largely reflect the abundance of nu- sites in the melt also controls the degree of undercooling cleation sites in the melt, which directly influences the de- that can occur. These factors together with the subsequent gree of undercooling that a droplet can experience. Even cooling rate have direct effects on the development of both chondrules that clearly crystallized rapidly, e.g., radial py- crystal morphology and the elemental distributions within roxene chondrules, can form at relatively slow cooling rates. the resulting chondrule grains. Thus, chondrule texture appears to be controlled predomi- Dynamic crystallization experiments reveal that forsterite nantly by the intensity and duration of the heating event. crystallizes with one of four distinct morphologies: euhe- However, olivine crystals in many chondrules are hetero- dral (well-defined crystal faces), tabular (preferential devel- geneous and are chemically zoned from core to rim. These opment in two directions), hopper (hourglass shape with zoning profiles indicate that the chondrules cooled too hexagonal outline), and dendritic (intricate branching struc- quickly for the olivines to maintain equilibrium with the tures) (Faure et al., 2003). The morphology of forsterite crys- remaining melts and for diffusion to homogenize the oli- tals varies as a function of the cooling rate, degree of un- vines. Thus, detailed characterization of the chemical varia- 442 Meteorites and the Early Solar System II tion in chondrule olivines can provide additional informa- In FeO-rich porphyritic pyroxene chondrules in Semar- tion about the cooling rates that these objects experienced. kona it appears that the order of crystallization of olivine Olivine zonation is particularly prominent in type-IIA and pyroxene is variable (Jones, 1996). Intergrown pyrox- porphyritic olivine chondrules in the least-equilibrated or- ene and olivine phenocrysts indicate that both minerals were dinary chondrites, such as Semarkona (Jones, 1990). Zona- crystallizing simultaneously as the chondrule cooled. In tion occurs in FeO (9–27 wt%), Cr2O3 (0.2–0.6 wt%), MnO several chondrules olivine is dominant and appears to have (0.2–0.7 wt%), CaO (0.1–0.4 wt%), and P2O5 (0–0.5 wt%), preceded growth of pyroxene. In the coarse-barred chon- with abundances increasing from cores to rims. Both the drules, olivine is sometimes present as small rounded in- olivine textures and the normal zoning profiles are consis- clusions, indicating that it crystallized before pyroxene. tent with an igneous origin. The zonation likely results from Textural relationships between the various pyroxene fractional crystallization of the chondrule melts. phases in Semarkona chondrules indicate that the crystalli- The zoning profiles in porphyritic olivine chondrules zation sequence was one of progressive enrichment in wol- have been compared to those in experimentally produced lastonite (Wo) content. In these objects, low-Ca pyroxenes analogs (Jones and Lofgren, 1993; Weinbruch et al., 1998). are overgrown with pigeonite followed by augite, which In Semarkona chondrules, the shapes of the zoning profiles, only occurs as rims on low-Ca pyroxene phenocrysts (Jones, as well as the absolute values of FeO and minor-element 1996). Pigeonitic rims form at temperatures above 1000°C. contents, of the natural and synthetic olivine crystals are By subsolidus exsolution, high-pigeonite inverts to twinned similar at cooling rates as low as 5°C/h. Olivine zoning low-pigeonite between 935° and 845°C. Augite crystallizes is much more restricted at low cooling rates in bulk com- as euhedral grains hosted within the glassy matrix, or as positions, which permit pyroxene nucleation and growth skeletal crystals, most probably just after the pigeonitic rim (Radomsky and Hewins, 1990). growth between 1150° and 1000°C. Pigeonitic and augitic 4.3.3. Exsolution microstructures. Another constraint rims form augitic and enstatitic sigmoids, respectively, by on cooling rate is exsolution microstructure (Carpenter, exsolution between 1000° and 640°C. Based upon glass 1979; Kitamura et al., 1983; Muller et al., 1995; Watanabe composition, the overall solidification temperature of the et al., 1985; Weinbruch and Muller, 1995; Weinbruch et al., mesostasis occurs close to 990°C, the minimum melt tem- 2001). Lamellar exsolution has been observed in some chon- perature in the quartz-albite-orthoclase system (Hamilton drule pyroxenes. In particular, pigeonite/diopside exsolution and Mackenzie, 1965). lamellae occurs in granular-olivine-pyroxene chondrules from the Allende meteorite (Weinbruch and Muller, 1995). 5. METALLOGENESIS AND REDOX It is suggested that these textures are produced when cooling REACTIONS IN THE MELT rates are on the order of 5°–25°C/h in the temperature range of 1200°–1350°C. These exsolution textures have also been 5.1. Metal in Chondrule Interiors produced in synthetic analogs of chondrules (Weinbruch et al., 2001) under cooling rates of 10°–50°C/h between 1000° Metal-bearing type-I chondrules have highly reduced and 1455°C. In these experiments, higher cooling rates (50°– mineral assemblages, such as silicates with low FeO con- 450°C/h) yield only modulated structures that are unlike tents (Jones and Scott, 1989) and low-Ni metal grains that those seen in chondrule pyroxenes. contain variable amounts of P, Cr, and Si (Lauretta and Buseck, 2003; Rambaldi and Wasson, 1981; Zanda et al., 4.4. Mineral Crystallization Sequence 1994). Despite much interest, the origin of the metal in these chondrules is still the subject of debate. Some researchers The sequence in which minerals crystallize in chondrule propose that metal grains are derived from metallic nebu- melts is highly dependent on the bulk composition as well lar condensates that avoided oxidation even during chon- as the thermal history. In many chondrules, olivine is the drule formation (Grossman and Wasson, 1985; Rambaldi earliest phase nucleating from the melt, forming between and Wasson, 1981). Other studies suggest that the metal 1600°C and 1400°C (Ferraris et al., 2002). This temperature formed by in situ reduction of preexisting sulfides, oxides, range is consistent with temperatures derived from chon- and FeO-rich silicates in the melt (Connolly et al., 1994; drule chromite-olivine pairs, which have Fe-Mg partitioning Lauretta et al., 2001; Lauretta and Buseck, 2003). In this characteristic of phases that crystallized from silicate melts section, we examine the evidence for the origin of metal in at temperatures >1400°C (Johnson and Prinz, 1991). silicate chondrules. Liquid Fe-based alloys containing less than 5.2 atom% Ni will preciptitate metal crystals with a body-centered 5.2. Formation of Metal by Condensation cubic structure (δ-Fe) that have slightly lower Ni contents (<4.1 atom%) between 1536° and 1513°C. Nickel-rich met- Several researchers have suggested that metal grains in als (>5.2 atom% Ni) will crystallize with the face-centered chondrule interiors formed by melting of metal and sulfide cubic structure. Crystallization temperatures decrease with nebular condensates. It is suggested that these grains main- increasing Ni content, reaching a minimum of 1425°C be- tained their chemical identity during the melting process, tween 60 and 70 atom% Ni. presumably by surviving as immiscible droplets that did not Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 443 exchange with the surrounding silicate melt. Evidence for have been established by nebular condensation, but are the this idea comes from the abundances of siderophile and result of metal-silicate equilibration within chondrules. chalcophile elements in Semarkona (LL3.0) chondrules A link between chondrule-derived metal and opaque (Grossman and Wasson, 1985), which are presumably hosted assemblages in the matrix of primitive chondrites has also by the metal and sulfide. The abundances of these elements been established, based on the chemistry of Cr, Si, and P in chondrules are highly variable. The nonrefractory sidero- (Lauretta et al., 2001; Lauretta and Buseck, 2003). It is phile abundance patterns in Ni-rich chondrules are smooth likely that the metal melted in and equilibrated with a sili- functions of volatility, which have been interpreted as nebu- cate liquid under high-temperature, reducing conditions. In lar signatures. However, the volatile siderophile fractiona- this environment the molten alloys incorporated varied tions may also reflect evaporation during chondrule melting. amounts of Si, Ni, P, Cr, and Co, depending on the oxygen Metal from chondrule interiors in Renazzo, Al Rais, and fugacity and temperature of the melt. As the system cooled, the related chondrite, MacAlpine Hills 87320 (Lee et al., some metal oxidation occurred in the chondrule interior, 1992), shows generally high Ni and Co, flat Ni and Co producing metal-associated phosphate, chromite, and silica. profiles, and moderately large grain-to-grain compositional Metal that migrated to chondrule boundaries experienced variations (even within chondrules). Metal from chondrule extensive corrosion as a result of exposure to an external, margins adjacent to matrix has convex (inverted U-shaped) oxidizing atmosphere. Ni and Co zoning profiles with the highest Ni and Co con- Trace-element data also support a chondrule-melt origin centrations at grain centers. These profiles are similar to for metal grains in ordinary chondrites (Kong and Ebihara, those observed in the CH chondrites, which have been in- 1996, 1997). Characteristics retained in the bulk metals of terpreted as resulting from condensation in the solar nebula unequilibrated H, L, and LL chondrites (abundant C; high (Meibom et al., 1999) or a postimpact vapor plume (Camp- contents of Cr, V, Mn and low contents of W, Mo, and Ga bell et al., 2002). However, Lee et al. (1992) interpreted compared to metals of equilibrated chondrites; less enrich- these patterns to reflect dilution by Fe produced by FeO ment of W than Mo; and fractionation of Co from Ni) dem- reduction, which occurred during a brief period of parent- onstrate that chondrite metals are not nebular condensates. body metamorphism. All these characteristics are consistent with melting. Fur- thermore, these data suggest that chondrite metals are not 5.3. Metallogenesis by Reduction in melting remnants of previously condensed metals. Rather, Chondrule Melts they have been produced by reduction of CI- or CM-like material during the melting process. Fine (~2 µm), Ni-poor (~1 wt%) metal grains are com- Volatile-siderophile-element abundance patterns of me- mon inclusions in the olivine in porphyritic chondrules in tallic and nonmagnetic fractions of CR chondrites are com- unequilibrated ordinary chondrites (Rambaldi and Wasson, plementary and volatility-dependent (Kong et al., 1999). In 1981, 1982). The most common occurrence of this “dusty” Renazzo, fine-grained metal is located inside chondrules or metal is in the core of olivine grains having clear Fe-poor in chondrule rims. Matrix contains mostly coarse metal rims. The original compositions of dusty grains, which have grains. Both types of metal, fine and coarse, exhibit simi- been determined by assuming that all Fe metal was origi- lar chemical signatures, comparatively high in Cr and low nally present as FeO (Jones and Danielson, 1997), closely in Ni, suggesting a genetic relationship (Kong and Palme, match those of olivines in chondrules from unequilibrated 1999) in which Renazzo chondrules and metal were formed chondrites. These assemblages likely reflect high tempera- by reduction of oxidized precursors compositionally simi- ture in situ reduction of FeO from olivine (Rambaldi and lar to CI chondrites. The differences in siderophile-element Wasson, 1981; Jones and Danielson, 1997). pattern between chondrules, chondrule metal, and matrix The evidence that dusty olivine grains formed by in situ indicate evaporation and recondensation of volatile elements reduction suggests that similar processes may have affected during chondrule formation. the compositions of other chondrule metal phases. Chon- Connolly et al. (2001) performed a detailed study of drule kamacite in Krymka and Chainpur has Ni concentra- metal in CR chondrites from chondrule interiors, chondrule tions (3–4 wt%) that are significantly below that expected rims, and in the matrix. They found that the refractory plati- for nebular condensates (~5 wt%). This kamacite may have num group elements Os, Ir, and Pt behave coherently in been diluted by Fe reduced from the silicates during chon- metal. They are all uniformly enriched, depleted, or unfrac- drule formation by either H2 gas or carbonaceous precursor tionated with respect to Fe. Metal with solar ratios of Os/ matter (Rambaldi and Wasson, 1984). Fe, Ir/Fe, and Pt/Fe occur primarily in chondrule interiors. The elements Cr, Si, and P are common in metal grains All metal grains have essentially CI ratios of Ni/Fe and Co/ in the least-metamorphosed ordinary and carbonaceous Fe. Almost all metal grains have lower-than-CI ratios of the chondrites and provide additional information on redox re- volatile siderophile elements, Au and P. Furthermore, the actions in type-I chondrules (Zanda et al., 1994). The con- bulk compositions of chondrules are generally unfraction- centrations of these elements are inversely correlated with ated relative to CI chondrites for elements more refractory the FeO concentrations of their host chondrule olivines. This than Cr, but are depleted in the more volatile elements. The relation suggests that Cr, Si, and P concentrations could not abundances of siderophile elements correlate strongly with 444 Meteorites and the Early Solar System II the metal abundance in the chondrules, which implies that elle et al., 2001). A reaction layer composed of Fe-Ni pre- the siderophile depletions are due to expulsion of metal cipitates and FeO-depleted olivine develops between the from the chondrule melts. These data are interpreted to sug- surface and an inner reaction front at 1350°C (Lemelle et gest that most metal formed during melting via reduction al., 2000). The reaction layer contains two reaction fronts of oxides by C that was part of the chondrule precursor. (inner and outer) corresponding to the beginning of metal Nickel has a lower volatility than Fe, and Humayun et precipitation and to the total depletion of Fe2+ from oliv- al. (2002) found that Ni/Fe ratios in metal in CR chondrule ine. The propagation of the two reaction fronts follows a interiors tend to be higher than those in the rims and ma- parabolic law and is limited by the interdiffusion of Fe/Ni trix. In addition, Pd has nearly the same volatility as Fe, and Mg in the olivine lattice. In both natural and experi- and Pd/Fe ratios are nearly the same in metal in chondrules, mental samples, micrometer-sized metal blebs observed in rims and matrix. Copper is more volatile than Fe, and Cu/ the dusty region often show preferential alignments along Fe ratios are much lower in chondrule metal relative to that crystallographic directions of the olivine grains, have low in rims and matrix. These observations can be explained by Ni contents (typically <2 wt%), and are frequently sur- evaporation of volatile siderophile elements, including Fe, rounded by a silica-rich glass layer (Leroux et al., 2003). and subsequent recondensation mostly onto chondrule rims. These features suggest that dusty olivines are formed by a In very FeO-poor ordinary chondrite chondrules, there is a subsolidus reduction of initially fayalitic olivines. Compari- correlated decrease in Fe, Mn, and Cr (Alexander, 1994). son with experimentally produced dusty olivines suggests Both Fe and Cr are siderophile under reducing conditions that timescales on the order of minutes usually inferred for but Mn is not, suggesting that these correlated depletions chondrule formation are also adequate for the formation of are due to evaporation. Rims with large correlated enrich- dusty olivines. These observations are consistent with the ments of FeO, Si, Mn, and Cu have also been reported in hypothesis that at least part of the metal phase in chondrites ordinary chondrites, and may result from recondensation of originated from reduction during chondrule formation. evaporated material (Alexander, 1995). There have also been recent experiments on the nature of chemical reduction in silicate melts in a reducing atmos- 5.4. Experimental Studies of Chondrule phere (Everman and Cooper, 2003). Melt samples were Melt Redox Reactions heated to 1300°–1400°C under a flowing gas mixture of 1-atm CO/CO ( f = QIF – 2). The melt compositions are 2 O2 Experimental studies have added to our understanding enriched in Al2O3 and depleted in MgO and CaO relative of the role of in situ reduction in producing type-I chon- to ferromagnesian chondrules. The melt experienced an in- drule textures and mineral compositions. Experiments in ternal reaction in which a dispersion of nanometer-scale which C, in the form of graphite or diamond, is flash-heated Fe-metal precipitates formed at an internal interface. The dis- together with magnesian silicates reproduce many minera- tance between this interface and the melt surface increased logical features characteristic of type-I chondrules, including throughout the experiment. Oxygen was lost from the melt Mg-rich and dusty olivines, Cr and Si dissolved in metal, at the free surface via conversion of CO to CO2 resulting in and silica inclusions inside metal (Connolly et al., 1994). an increase in the cation-to-oxygen ratio and net reduction These results provide strong evidence that C in chondrule of the melt. melts is an effective reducing agent and that chondrule melts Thus, while it seems likely that nebular condensate metal were internally buffered. Furthermore, they suggest that C was incorporated into chondrule precursors, it is also likely played a key role in chondrule formation by briefly creating that this metal experienced significant interaction with the a reducing environment within chondrule melts that was de- silicate melt. In chondrule melts that incorporated a signifi- coupled from the nebula gas. cant amount of carbonaceous material or that interacted Further experiments have provided more detail on the with a highly reduced vapor phase, additional metal was nature of the reduction reaction. Cohen and Hewins (2004) created by reduction of Fe and associated elements from performed an experimental study of metallic Fe formation silicate and other oxidized phases. in chondrule-like melts. They showed that troilite and Fe- Subsequent petrologic work supports the model that metal are extremely volatile under canonical chondrule- some chondrules incorporated C-rich material in their pre- formation conditions. Conversely, at higher total pressures cursors (e.g., Lauretta et al., 2001; Lauretta and Buseck, (P H2 = 1 atm) Fe metal is less volatile and remains in the 2003; Fries and Steele, 2005). Small (<1–20 µm) C inclu- melt for longer times. These experimental results suggest sions are common in the metal of type-3 ordinary chondrites that metal-bearing chondrules formed in environments with (Lauretta et al., 2001; Mostefaoui et al., 2000) and fine- elevated pressures relative to the canonical nebula. Alter- grained carbonaceous inclusions occur in some chondrule natively, unlike in the experiments, the chondrule melt and interiors (Lauretta and Buseck, 2003; Scott et al., 1984). It the vapor phase may have reached equilibrium in the solar is likely that these inclusions are the remains of carbona- nebula, in which case the metal may have stabilized. ceous matter that escaped oxidation during chondrule for- Additional studies have investigated the transformations mation. There are claims of high C contents (several weight of olivine single crystals in contact with graphite powder. percent) in chondrule olivines from the primitive chondrites At 1150°C precipitates of Fe,Ni and a thin amorphous layer Allende (CV3), Semarkona (LL3.0), and Bishunpur (LL3.1) of silica formed on the surface of the olivine crystals (Lem- (Hanon et al., 1998). These researchers report that mean Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 445

C concentrations are 3× higher in type-I chondrules than in type-II chondrules. However, nuclear reaction analyses show that chondrule olivines have C contents of less than 120 ppm (Varela and Metrich, 2000).

6. ELEMENTAL EVAPORATION AND RECONDENSATION

6.1. Chemical and Isotopic Fractionations Resulting from Evaporation

The origin of the wide range of compositions of ferro- magnesian chondrules remains controversial. Figure 3 sum- marizes the variation and the average compositions of Se- markona (Jones and Scott, 1989; Jones, 1990, 1994, 1996; Huang et al., 1996; Tachibana et al., 2003) and Renazzo (Kong and Palme, 1999) chondrules. Type-I chondrules in particular exhibit significant chemical fractionations, with depletions of elements like Cr, Mn, Na, K, and Fe, and en- richments of elements such as Al, Ca, and Ti, relative to Mg. These compositional variations suggest that elemental volatility is the primary factor controlling chondrule bulk compositions. The relative volatility of elements can be esti- mated in two ways: (1) from the calculated equilibrium tem- perature at which 50% of an element has condensed from –4 a gas of solar composition typically at Ptot = 10 bar (e.g., Lodders, 2003), and (2) from evaporation experiments of chondritic melts (e.g., Hashimoto, 1983; Floss et al., 1996; Wang et al., 2001). Both approaches suggest that the order of decreasing volatility is roughly S, Na-K, Fe-Mn-Cr, Si, Mg, and Ca-Ti-Al. This order is qualitatively consistent with the variations in chondrule compositions. However, the de- gree to which these variations were inherited from chon- drule precursors or generated during chondrule formation is Fig. 3. Major-element abundances, normalized to CI chondrites unknown (e.g., Gooding et al., 1980; Grossman and Was- for (a) type-I and (b) type-II chondrules. son, 1983b; Jones, 1990; Connolly et al., 2001; Huang et al., 1996; Sears et al., 1996; Alexander, 1994, 1996, Hewins et al., 1997). The bulk chemistry of chondrules alone is in- ration rates (e.g., Richter et al., 2002), as will any back reac- sufficient to resolve this debate. Two other approaches have tion with the evaporated gas. been used: petrologic evidence for volatile loss and recon- Whether or not volatile-element fractionation is accom- densation in chondrule rims and matrix (see section 7) and panied by isotopic fractionation depends on the conditions isotopic signatures of evaporation and recondensation. under which the volatile loss took place. Under Rayleigh Both evaporation and condensation are generally accom- conditions, large isotopic fractionations will correlate with panied by mass-dependent isotopic fractionation. Evapora- elemental fractionation. Such correlated elemental and iso- tion can result in enrichment in the heavy isotopes in the topic fractionations are seen in low-pressure evaporation residue. Condensation enhances the abundances of light experiments (section 6.3) and in cosmic spherules that are isotopes in the condensates, relative to the gas. The largest severely heated during atmospheric entry heating (Alex- isotopic fractionations occur during Rayleigh fractionation ander et al., 2002). If volatile loss occurs far from the Ray- and the isotopic fractionations correlate with the extent of leigh condition, there is little associated isotopic fractiona- volatile loss or gain. For Rayleigh fractionation to occur, tion. In this case, it is not possible to tell whether volatile the evaporating residue or condensing gas must remain iso- loss was inherited from chondrule precursors or produced topically uniform and the evaporation or condensation must by chondrule formation. be unidirectional (i.e., there is no back reaction). At the other Since there is this potential ambiguity in the elemental extreme, if diffusion or convection occurs in the evaporat- and isotopic data, considerable effort has been put into ing material on timescales that are much slower than evapo- developing a theoretical understanding of the kinetics of ration, mass loss and elemental fractionation can occur with evaporation and condensation associated with chondrule little or no isotopic effects. Similarly, high ambient gas pres- formation. The ultimate aim of these efforts is to be able sures suppress isotopic fractionation and absolute evapo- to predict the volatile loss, and accompanied isotopic frac- 446 Meteorites and the Early Solar System II tionations, that would be expected under a given set of Kong and Palme (1999) showed that chondrules and ma- chondrule formation conditions. By comparing these pre- trix in Renazzo are complementary in the elemental abun- dictions to observations of chondrule compositions, it is dances; chondrules are depleted in refractory and volatile hoped the conditions of chondrule formation can be fur- elements. There are also observations that suggest that Na ther constrained. Here we review the elemental, isotopic, and K may have recondensed into chondrules after their for- and petrologic studies of chondrules and the experimental mation. Ikeda (1982) was the first to report an increase in and theoretical progress that has been made in understand- Na, K, and Fe near the edge of a Ca-Al-rich chondrule in ing evaporation and condensation, in an attempt to place ALH 77003. Matsunami et al. (1993) described a Semar- limits on the conditions of chondrule formation. kona chondrule, in which Na, Mn, and Si increase outward, that they suggest resulted from recondensation after crys- 6.2. Evidence for Evaporation and tallization of forsterite. Libourel et al. (2003) reported a Recondensation positive correlation between SiO2 and Na2O in mesostasis and enrichment of Si in the outer portion of some FeO-poor 6.2.1. Sulfur. Sulfur is the most volatile major element chondrules in CR2 chondrites and Semarkona. They inter- in chondrites. Consequently, it is expected to rapidly evapo- pret these observations as resulting from recondensation of rate from chondrules and recondense on metal in rims and alkalis and SiO2. It is, however, not clear whether the zon- matrix. Despite it being a potentially good indicator for ing pattern is an indication of condensation or later meta- evaporation, our knowledge of the petrology and cosmo- somatic exchange of Na and Ca between chondrules and chemistry of S in chondrules is fairly limited. Troilite is matrix (Grossman et al., 2002). uncommon in FeO-poor chondrules in Semarkona. Hewins Sodium is monoisotopic but K has two stable isotopes. et al. (1997) showed that the abundance of S in Semarkona The isotopic composition of K has been measured in whole FeO-poor (type-I) chondrules decreases with increasing size Allende (CV3) chondrules (Humayun and Clayton, 1995) of olivine phenocrysts (Fig. 4). They argue that increasing and chondrule mesostasis and melt inclusions in Semarkona grain size indicates more severe heating and, therefore, (LL3.0) and Bishunpur (LL3.1) (Alexander et al., 2000; fewer crystal nuclei surviving to develop into phenocrysts on Alexander and Grossman, 2005). Most K-isotopic analy- cooling. Tachibana and Huss (2005) measured S isotopes ses are normal within error with no evidence of systematic of small troilite grains included in Fe-Ni metal spherules isotopic fractionation with K abundance. These results rule in six Bishunpur FeO-poor chondrules and four large troi- out volatile loss of K under Rayleigh conditions. lite grains in the matrix. All the troilite grains have the same 6.2.3. Iron, nickel, manganese, and chromium. The re- S-isotopic compositions within analytical errors. Since evap- duced state of Fe is the defining criterion for type-I chon- oration of Fe-S melt under Rayleigh conditions does result drules, and its abundance variation among chondrules is the in isotopic mass fractionation (McEwing et al., 1980), the most significant of all major elements (Fig. 3). The complex large elemental fractionation without accompanying isoto- behavior of Fe as metal, oxide, or sulfide has made identi- pic mass fractionation in chondrules indicates either vola- fying the origin(s) of the depletions difficult. Nevertheless, tilization of S under non-Rayleigh conditions or subsequent evidence exists for evaporative loss of some siderophile ele- isotopic reequilibration. ments in CR2 chondrules (see section 5.3). 6.2.2. Sodium and potassium. As with S, Na and K Alexander and Wang (2001) found that olivines (Fo99.7 abundances in type-I chondrules decrease with increasing to Fo78) in nine Chainpur (LL3.4) chondrules are Fe-isoto- grain size of olivine phenocrysts (Fig. 4), suggesting pro- pically unfractionated within analytical errors (2σ < 1–2‰/ gressive alkali loss with the increasing severity of heating. amu). Bulk Tieschitz (H3.4) chondrules show very small fractionations of <–0.25‰/amu (Kehm et al., 2003). The small light-isotope enrichments may be due either to con- densation or parent-body processes. Four Allende chon- drules show mass-dependent isotopic fractionation of up to 1.1‰/amu (Mullane et al., 2003). However, Allende chondrules are chemically and isotopically zoned, including in Fe, and this zonation is likely a metamorphic overprint (Zhu et al., 2001). The isotopic mass fractionation of other siderophile elements and Cr in chondrules have not been measured. The textures of barred-olivine chondrules may also be an indication of evaporation during chondrule formation (Tsuchiyama et al., 2004). Partial evaporation (mainly FeO) enhances the selective nucleation by cooling the surface, relative to the interior. This process produces the compact, coarse-grained rims that are characterisitic of these chon- Fig. 4. Graph illustrating the variation in volatile-element abun- drules. The occurrence of such rims on barred olivine chon- dances as a function of nominal grain size in chondrules. drules suggests chondrule formation in an open system. Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 447

6.2.4. Silicon and magnesium. If type-I chondrules have been made about how evaporation from chondrules formed from type-II- or CI-like precursors, there must be could be achieved without corresponding isotopic fraction- loss of both FeO and SiO2 during chondrule formation ations. Subsolidus evaporation of S- and K-bearing chon- (Jones, 1990; Sears et al., 1996, Cohen et al., 2004). This drule precursors (Young, 2000; Tachibana and Huss, 2005) scenario is consistent with the FeO- and SiO2-rich rims that is one possibility. High partial pressures of H could lead to are often present on type-IA chondrules in ordinary chon- non-Rayleigh evaporation of melts (e.g., Nagahara and drites. There is petrologic evidence for recondensation of Ozawa, 1996c; Galy et al., 2000). Diffusion of evaporat- SiO2 back into chondrules (Matsunami et al., 1993; Libourel ing species through the ambient gas can also suppress iso- et al., 2003). In the most highly reducing melts, it is pos- topic fractionation (Richter et al., 2002). Alternatively, sible that a significant amount of Si was incorporated into chondrules may have exchanged with the ambient gas dur- the Fe-based metallic melt component. Expulsion of some ing chondrule formation (Alexander et al., 2000; Alexander, of this material is another mechanism for simultaneous Fe 2004). and Si loss from bulk chondrule compositions (Lauretta and 6.3.1. Types of evaporation experiments. Experimental Buseck, 2003). studies of silicate-melt evaporation are summarized in Table 4. The Si-isotopic compositions of 34 individual chondrules In earlier experiments, continuous or stepwise heating ex- from type-3 ordinary chondrites analyzed by Clayton et al. periments were employed to see how chondritic materials (1991) exhibit a range of ±0.25‰/amu about the bulk chon- suffered from “evaporation metamorphism” (Hashimoto et drite value. If type-I chondrules were part of the analyzed al., 1979) or to see how alkali elements were fractionated suite of chondrules, the observed range of Si-isotopic com- during evaporation from lunar materials (Gibson and Hub- positions is less than would be expected if Si was lost via bard, 1972; Gibson et al., 1973). These experiments focused Rayleigh evaporation. on single condensed phases. Hashimoto (1983) performed Evaporative loss should be faster for smaller objects be- systematic isothermal experiments on multicomponent sili- cause of the large surface area/volume ratio. Clayton et al. cate melts, which provide an excellent dataset for model- (1991) analyzed chondrules of varying size from type-3 ing chemical fractionation (e.g., Alexander, 2001, 2002). ordinary chondrites. For Dhajala (H3.8), smaller chondrules Since then, a large number of experiments have been per- are isotopically heavier. However, the total change in Si- formed to understand the kinetics of gas-silicate melt reac- isotopic fractionation is only ~0.5‰/amu. For Chainpur tions aimed at understanding the chemical and isotopic (LL3.4) there was no systematic isotopic variation with chon- fractionations during chondrule formation. drule size. Free evaporation experiments have been performed at Magnesium should not experience much evaporative loss pressures of less than ~10–4 bar for various starting com- from ferromagnesian chondrules, although significant Mg positions, such as chondrites (Murchison and Allende, etc.) loss may have occured during formation of Al-rich chon- (Floss et al., 1996; Hashimoto et al., 1979), a synthetic CI drules. Galy et al. (2000) measured Mg-isotopic fraction- composition (Hashimoto, 1983; Wang et al., 1994, 2001), ations of up to ~1.5‰/amu in eight Allende chondrules. the CMAS system with a type-B CAI composition (Rich- They also found that the degree of fractionation is inversely ter et al., 2002; Richter et al., 1999) or near-chondritic correlated with the bulk Mg/Al ratios of the chondrules. composition (Nagahara and Ozawa, 2003), and a simple However, the isotopic fractionations are much smaller than Mg2SiO4-SiO2 system (Nagahara, 1996; Nagahara and expected for Mg evaporation under Rayleigh conditions. Ozawa, 1996a,b). Experiments that provide unequivocal Young et al. (2002) measured Mg-isotopic compositions information on evaporation must be isothermal and in the in four Allende chondrules. One barred olivine chondrule absence of crystalline phases that complicate evaporation is isotopically zoned; the center has δ25Mg = 1.9–1.7‰ processes by inhibiting direct evaporation from the silicate (relative to a standard SRM980), whereas that of the mar- melt (Nagahara, 1996; Ozawa and Nagahara, 1997). Free gin is 0.3‰. An interesting result is that the Mg-isotopic evaporation experiments provide evaporation coefficients, variation is correlated with δ17O. The authors suggest that the extent of inhibition of evaporation reactions, and un- mixing of prechondrule materials with distinct Mg- and O- equivocal proof that chemical and isotopic fractionations isotopic compositions and later alteration may be respon- are generally coupled. sible for this pattern. Experiments performed in the presence of ambient gas are grouped into four categories depending on the type of 6.3. Experimental Evaporation of the ambient gas: (1) air (or residual gas) at pressures less Chondrule Melts than 1 atm (Notsu et al., 1978; Shimaoka and Nakamura, 1991), (2) 1-atm experiments with specified oxygen fugac- Kinetic parameters for gas-condensed phase reactions ity controlled by flowing either H2/CO2 or CO/CO2 mixture along with thermodynamic data can be applied to models (Richter and Davis, 1997; Richter et al., 1999, 2002; Shirai of reaction processes relevant to chondrule formation. To et al., 2000; Tsuchiyama et al., 1981), (3) H2 or He gas rep- date, the range of isotopic fractionations in S, K, Fe, Si, resenting solar-nebula conditions that may enhance evapo- and Mg measured in chondrules are much smaller than ex- ration (Richter et al., 2002; Richter et al., 1999; Wang et al., pected for elemental fractionation resulting from evapora- 1999; Yu et al., 2003), and (4) gas with species consisting tion under Rayleigh conditions. A number of suggestions of evaporating condensed phases that may inhibit evapora- 448 Meteorites and the Early Solar System II

TABLE 4. Experimental studies of gas-liquid reactions related to ferromagnesian chondrule formation.

Starting Composition Temperature (°C) Pressure (Pa) References Evaporation in vacuum Lunar basalts, breccias, and soil 950–1400 ~2.5 × 10–4 Gibson and Hubbard (1972), Gibson et al. (1973) Murchison (CM) meteorite 1300, 1900 <10–3 Hashimoto et al. (1979) Synthetic chondritic Si-Al-Fe-Mg-Ca 1700–2000 <10–5 Hashimoto (1983) Etter (L5) chondrite 1150–1450 ~10–3 Shimaoka and Nakamura (1989) Allende (CV) meterorite peak: 1500–2100 ~10–4 Floss et al. (1996) –4 Fe0.947O 1450–1750 ~10 Wang et al. (1994) Oxide mixture with CI composition peak: 1500–2000 ~10–4 Wang et al. (1994, 2001) Fe-Mg-Si-Ca-Al-Ti-REE –4 Mg2SiO4–SiO2 600, 1700 <10 Nagahara and Ozawa (1996a,b), Nagahara (1996) Synthetic Type B CAI-like Ca-Al-Si-Mg 1700, 1800 <10–4 Richter et al. (1999, 2002) Synthetic chondrule-like CMAS 1500, 1600 <10–4 Nagahara and Ozawa (2003)

Evaporation in gas Allende (CV), JB-1 basalt ~2000 or 1300–2000 105 or ~1 Notsu et al. (1978) Etter (L5) chondrite 1300 10–10–3 Shimaoka and Nakamura (1991) Synthetic Type IIAB chondrule 1450, 1485 1–140 Yu et al. (2003), Wang et al. (1999) Mixture of silicates from H5 chondrite 1450–1600 105 Tsuchiyama et al. (1981) Knippa basalt 1300, 1325, 1350 105 Lewis et al. (1993) Synthetic Type IIAB and Type I peak: 1470–1750 105 Yu et al. (1996) and IA chondrules + sulfide Mineral mix with chondrule-like peak: 1470–1750 105 Yu and Hewins (1998) composition Synthetic Type B CAI-like Ca-Al-Si-Mg-Ti 1400 105 Richter and Davis (1997), Richter et al. (1999, 2002) Synthetic Type IIAB chondrule 1450 105 Yu et al. (2003), Wang et al. (1999) 5 SiO2-Na2O system 1300–1400 10 Shirai et al. (2000) Synthetic Type B CAI-like Ca-Al-Si-Mg-Ti 1500 ~20 Richter et al. (1999, 2002) Synthetic Type IIAB chondrule 1450, 1485 7–103 Yu et al. (2003), Wang et al. (1999) Mineral mixture with CI composition 1580 13 Cohen et al. (2000)

Jilin (H5) meteorite 1300, 1400 PNa = 0 ~ 1.0, Nagahara and Ozawa (2000), PK = 0 ~ 0.07 Nagahara (2000) Synthetic chondrule-like CMAS 1500, 1600 NA Nagahara and Ozawa (2003)

Transpiration (condensation?) Knippa basalt 1330 105 Lewis et al. (1993) Allende (CV), Ornans (CO) (±quartz), 1400–1500 105 Yu et al. (1995) Eagle Station pallasite Allende (CV), Ornans (CO) chondrites + quartz 1500–1000 105 Yu et al. (1995)

SiO2 + C source, CMAS, or target: 1330–1480 8–30 Tissandier et al. (2000; 2002) Murchison (CM) melt target 5 K2CO3 + C source, CMAS ± K target source: 1000, 10 Georges et al. (1999) target: 1400

Dust melt collision Synthetic Type IIA, Type IIAB chondrules melt: 1564–1600, 105 Connolly and Hewins (1995) dust: 1100–1560

tion (Nagahara, 2000; Nagahara and Ozawa, 2000, 2003). type-B CAI-like compositions have also been studied Evaporation is suppressed at high partial pressures of melt- (Hashimoto, 1983; Richter et al., 2002; Wang et al., 2001). component species or high total pressures due to signifi- In both systems, Fe is the most volatile, followed by Si, Mg, cant back reaction. Evaporation is enhanced at low oxygen Ti, Ca, and Al. Silicon and Mg have fairly similar evapora- fugacity and high H pressures. tion behavior; however, Si evaporates more rapidly at earlier 6.3.2. Major-element fractionations during evaporation. stages and Mg at later stages of evaporation. Aluminum, Ca, Compositional changes during free evaporation of CI- and and Ti do not evaporate until the final stages of mass loss. Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 449

6.3.3. Trace-element fractionations during evaporation. tal transport in the melt through diffusion or convection; Rare earth elements experience little evaporation from sili- (3) the incongruent nature of evaporation and condensation; cate melts in vacuum. An exception is Ce, which evapo- and (4) transportation of gas species in the ambient gas. rates at a rate similar to Fe (Floss et al., 1996; Wang et al., Surface reaction kinetics have been treated by applying 2001). This behavior is thought to result from oxidation of the Hertz-Knudsen equation Ce2O3 to the more volatile form CeO2 in the residual melt. P − P The REE abundance, relative to CI, along with the degree = eq Jmax (1) of Ce depletion can thus be used as an indicator for evapo- 2πmkT ration of CI-composition precursors under oxidizing con- ditions. to multicomponent systems, which is, in the strictest sense, 6.3.4. Isotopic fractionation due to evaporation. Experi- good only for metallic phases in the presence of monatomic mental evaporation of chondritic materials causes mass-de- gas molecules. The maximum possible evaporation rate pendent isotopic fractionation, which results in heavy iso- (Jmax) is proportional to the difference between the equi- tope enrichment in Mg, Si, Ca, Ti, K, and O. The heavy librium partial pressure (Peq) and the actual pressure (P) and isotope enrichments follow the Rayleigh law provided that the average velocity of gas molecules. Kinetic barriers are diffusion within the experimental melts is not a rate-limit- incorporated into the above equation by multiplying P and α ing process. The degree of mass fractionation is smaller than Peq by evaporation and condensation coefficients ( evap and α theoretical expectations based on thermodynamic consid- cond), respectively, to give the actual evaporation rate J as erations (Floss et al., 1996; Wang et al., 2001; Richter et al., 2002). However, the relative degree of isotopic fraction- α P ⋅ α P J = evap eq cond (2) ation decreases with increasing atomic number (Wang et al., 2πmkT 2001). Kinetic processes at the surface of an evaporating melt may suppress isotopic fractionation. The reason why this simple equation cannot be extended to more complex oxide systems without ambiguity is that 6.4. Theoretical Modeling of the presence of other components involved in the reaction, Chondrule Melt Evaporation such as O or H, cannot be properly treated by the equation. In spite of this fact, the ease of use of this approximation 6.4.1. Stability of silicate melt in equilibrium conditions. and the absence of well-established alternatives means that Although it is well known that chondrule formation in- most researchers use it in their modeling. One approach volved kinetic processes, it is important to know the envi- combines thermodynamic calculations for a given gas com- ronmental conditions required to stabilize silicate melts. position to obtain the driving force and the evaporation α Wood and Hashimoto (1993) suggested that dust enrichment coefficient, evap, is estimated from the results of free evapo- greater than 1000× solar is required to stabilize silicate melt ration experiments (Alexander, 2002; Ebel, 2002; Grossman in the solar nebula, assuming ideal mixing of the multicom- et al., 2002; Richter et al., 1999). Another method expresses ponent silicate melt. Nagahara and Ozawa (1995) and the term for a specific evaporation (or condensation) reac- Ozawa and Nagahara (1997) examined melt stability over tion as a function of the partial pressure of related gas spe- a wide range of dust-enrichment factors, and delineated the cies in the ambient gas, the activity of the component in stability field as a function of total pressure vs. dust enrich- the melt multiplied by evaporation coefficient, and the equi- ment at a given temperature, and determined that the total librium constant for the reaction (Alexander, 2001, 2002; pressure of 10–6 bar is the lower limit of melt stability at Shirai et al., 2000; Tsuchiyama and Tachibana, 1999). The 1627°C. Yoneda and Grossman (1995) incorporated non- equilibrium constant is assumed to be extendable to kinetic ideality in CMAS melts using the model of Berman (1983) domains according to the principle of detailed balancing and examined the stability of this melt over a wide range of (Lasaga, 1998). A third approach uses equation (2) and temperatures, pressures, and dust enrichments. They found combines kinetic effects and nonideality of silicate melts that CMAS melts are stable at a much lower dust enrich- into the evaporation coefficient (Ozawa and Nagahara, ments than suggested by Wood and Hashimoto (1993). Ebel 2001). This approach is applicable to the situation where and Grossman (2000) examined the stability of ferromag- O is much more abundant than metallic gas species. nesian melts in the solar nebula and suggested that dust enrichments of 20× solar stabilizes ferromagnesian melts 6.5. Isotopic Fractionation or Lack Thereof at 10–3 bar and 1127°–1527°C. 6.4.2. Kinetic models. Equilibrium may have been The fundamental discrepancy between chemical and iso- achieved between gas and condensed phases at high tem- topic fractionations in chondrules is that they are depleted peratures in the early solar nebula. However, petrographic in volatile elements but not isotopically fractionated. This observations show that chondrule formation involved ki- problem was first addressed by Humayun and Clayton netic processes, such as rapid heating and cooling. Kinetic (1995) and discussed intensively by many later workers. aspects that must be considered in modeling chondrule gas- This decoupling between chemical and isotopic fraction- melt reactions are (1) surface reaction kinetics, which is the ations is a major constraint on the origin of chondrules. major barrier for evaporation and condensation; (2) elemen- There are several possible explanations. Highly volatile 450 Meteorites and the Early Solar System II elements can be fractionated with negligible fractionation bulk compositions, and all three components may have been if the evaporation Peclet number (Pe) is very large (Ozawa derived from a common reservoir (Klerner and Palme, and Nagahara, 1998; Galy et al., 2000). The Peclet num- 1999). It should be noted that in many studies no distinc- ber reflects the relative rates of diffusion and evaporation tion is made between chondrule rims and interchondrule and is defined as matrix, and they are collectively referred to as matrix. How- ever, rims partially or completely surrounding chondrules J ⋅ r are texturally and often chemically distinct from the inter- P = (3) e D chondrule matrix (Ashworth, 1977; Allen et al., 1980; Scott et al., 1988). where J is the elemental evaporation rate, r is the radius of Chondrule rims can be broadly divided into either ac- the condensed grain, and D is the elemental diffusion rate. cretionary or igneous rims. As the name implies, accretion- Alternatively, volatile elements can be fractionated with ary rims formed by the accretion of fine-grained dust onto negligible isotopic fractionation if there is extensive back the chondrules. Accretionary rims exhibit a range of com- reaction during evaporation (Ozawa and Nagahara, 1998). positions and textures from fine-grained, FeO-rich rims with Finally, the presence of a gaseous atmosphere inhibits the few discernable grains even in secondary electron images, escape of evaporated gas species and decreases the isotopic to clastic rims that chemically, texturally, and mineralogi- fractionation factor to a value close to unity, thus suppress- cally resemble the interchondrule matrix (Ashworth, 1977; ing isotopic fractionation while preserving notable chemi- Alexander et al., 1989b). Igneous rims, on the other hand, cal fractionation (Richter et al., 2002). have relatively coarse-grained igneous textures indicative of Diffusion is not expected to play a limiting role in chon- partial melting (Rubin, 1984; Rubin and Wasson, 1987; Krot drule melt droplets less than a few millimeters in size. Back and Wasson, 1995). reaction with the gas is essential if equilibrium between gas Perhaps the best-developed accretionary rims occur in and silicate melt is approached. However, it is questionable CM chondrites (Metzler et al., 1992; Lauretta et al., 2000; if total equilibrium was achieved in the solar nebula. Even Zega and Buseck, 2003). In the unbrecciated areas of CM if chondrule melts could not attain equilibrium with the sur- chondrites (so-called primary rock), rims surround almost rounding vapors, isotopic fractionation of volatile to mod- all objects, including chondrules, chondrule fragments, and erately volatile elements would have been suppressed while mineral fragments. The most abundant phase in rims around still achieving elemental fractionations if the chondrule chondrules in CM chondrites is Mg-rich serpentine. Other melts experienced a limited range of cooling rates (Ozawa minerals embedded within the rims include cronstedtite, and Nagahara, 1998). The slow escape rate of evaporating tochilinite, chlorite, pentlandite, gypsum, olivine, kamacite, gas species due to the presence of H is only effective if H taenite, and chromite. In these rims, regions containing hy- pressure is higher than 10–3 bar (Richter et al., 2002). drated and anhydrous minerals are in direct contact with Combining the evidence for evaporation and recondensa- each other, which has been interpreted as rim material ac- tion recorded in natural chondrules, experiments, and mod- creting directly from multiple nebular reservoirs. It has also eling, the most important conclusions are summarized as been suggested that aqueous alteration and metamorphism follows: (1) Chondrule precursors were heated to lose mod- on the CM chondrite parent body has significantly modified erately volatile components by evaporation. The degree of the primary mineralogy of the accretionary rims (Brearley evaporative loss varied depending on the melt composition, and Geiger, 1993; Chizmadia and Brearley, 2003). There grain size, rate of heating, total heating time, and the physi- is a clear relationship between the radius of the core ob- cal conditions such as pressure and temperature. (2) The ject and the thickness of its rim. A similar relationship has evaporated gas was not lost from the system. Rather, signi- been reported for the CV chondrites (Paque and Cuzzi, ficant recondensation took place during cooling. Lithophile 1997) and may or may not be present in the ordinary chon- elements reentered crystallizing chondrules to various de- drites (Matsunami, 1984; Huang et al., 1993). grees, but siderophile and chalcophile elements condensed In a few meteorites, such as Bishunpur (LL3.1) and as new phases, which are now in chondrule rims and/or ma- ALH 77307 (CO3.0), there is an abundance of amorphous trix. (3) Isotopic fractionation should have been significant material in the rims and interchondrule matrix (Alexander at the early stages of evaporation; however, isotopic reequili- et al., 1989a,b; Brearley, 1993). Amorphous material is very bration ocurred during subsequent cooling, which resulted susceptible to alteration and recrystallization, and its pres- in little isotopic fractionation preserved in chondrules. ervation suggests that some of the primary material has survived. Brearley and Geiger (1993) and Chizmadia and 7. VOLATILE RECONDENSATION AND Brearley (2003) argue from the lack of phyllosilicate pseu- FORMATION OF CHONDRULE RIMS domorphs that CM rims were also largely amorphous prior to alteration. Rims around chondrules are an additional source of in- There are likely to have been several sources of amor- formation about chondrule formation conditions, as well as phous material. In Bishunpur, chondrule mineral fragments their precursors. In addition, rims and interchondrule matrix are abundant in the interchondrule matrix and rims. These are important complementary components to chondrules must be accompanied by amorphous, Al-, Ca-, and Si-rich that are required to make up the various chondrite group chondrule mesostasis. Indeed, the finest-grained rim and Lauretta et al.: Petrology and Origin of Ferromagnesian Silicate Chondrules 451

matrix material tends to be enriched in these elements. of presolar material in rim/matrix in the least-metamor- However, in ALH 77307 clastic mineral fragments are much phosed members of the chondrite groups are very similar to less abundant, leading Brearley (1993) to suggest that much that of CI chondrites (Huss and Lewis, 1995; Alexander et of the amorphous material is a condensate. Numerous au- al., 1998). The CI chondrites are thought to have contained thors have suggested that rim and interchondrule matrix ma- few if any chondrules or CAIs. They most likely escaped terial is composed of nebular condensates (e.g., Larimer and high-temperature processes prior to accretion and their pre- Anders, 1967; Huss et al., 1981; Nagahara, 1984). solar abundances should most closely resemble those of the An alternative source of condensates is material evap- primordial dust. The implication of the presolar abundances, orated during chondrule formation. The shorter timescale particularly since rims make up the bulk of the fine-grained of chondrule formation would also be more conducive to material in many chondrites, is that the major component formation of amorphous material. There is some direct evi- of chondrule rims and matrix resembled CI chondrites in dence of recondensation during chondrule formation. Iron, composition and did not experience the high temperatures Si, and Mn are among the most volatile of the major and associated with nebula-wide condensation or with chondrule minor elements, and they are depleted in type-I chondrules. formation. The CI chondrites are almost completely aque- Alexander (1995) described compositionally and texturally ously altered, but perhaps the most likely analogs for them zoned rims around some type-I Bishunpur chondrules, with prior to alteration are the anhydrous interplanetary dust par- large FeO, Si, and Mn enrichments at the rim-chondrule ticles. They are very fine-grained and can contain abundant interface that often grade into matrix-like compositions and amorphous material. textures at the rims’ outer edges. Transmission electron Intuitively, one might expect accretionary rims that microscopy of rims with similar compositions shows this formed in a dusty nebula to resemble loosely packed dust Fe-,Si-rich material to be amorphous. There is also evidence balls. Yet, accretionary rims have surprisingly low porosity of partial to complete reequilibration of clastic olivine and (<20%). They were also robust enough to survive accre- pyroxene grains with the FeO-rich material, as well as re- tion onto the chondrite parent bodies, as well as any regolith action with protuberances from the chondrules. processing that occurred prior to final burial and compac- Petrologic evidence shows that accretion of rims began tion. Recondensation of vaporized material from chondrules before expulsion of metal and sulfide from chondrules was would help cement a rim, as well as fill in porosity. Mod- complete, and that the metal/sulfide that was trapped in the els and experiments (e.g., Dominik and Tielens, 1997; Blum rims reacted with the nebular gases as the system cooled and Wurm, 2000) find that sticking of micrometer-sized dust (Allen et al., 1980; Alexander et al., 1989b; Lauretta and is most efficient when collisions occur at less than 10–20 m/s. Buseck, 2003). In the CR chondrites, at least, there is evi- At speeds of up to several meters per second, the collisions dence of both evaporation of metal from chondrules and actually tend to compact fluffy aggregates. Electrostatic its recondensation onto rims (Kong and Palme, 1999; Con- attraction between grains may enhance sticking efficiencies, nolly et al., 2001; Humayun et al., 2002; Zanda et al.,2002). as well as increase the range of relative speeds over which Finally, rims and matrix-like material are sometimes trapped sticking and compaction occurs (Marshall and Cuzzi, 2001). between plastically deformed or compound chondrules, The most detailed modeling of rim formation has been done indicating that the chondrules were still hot when they col- in the context of the turbulent concentration mechanism lided (e.g., Hutchison and Bevan, 1983; Scott et al., 1984; (Cuzzi, 2004). In a weakly turbulent nebula, the timescales Holmen and Wood, 1986; Lauretta and Buseck, 2003). All for making rims by this mechanism are likely to be 10– these features suggest that the rims began accreting while 1000 yr. Such timescales are possible for rims that accreted the chondrules were still hot. onto cold chondrules, but are clearly inconsistent with rims While Fe and Si are relatively volatile, the alkalis are that show evidence of accretion while the chondrules were much more so. If there was some evaporation of Fe and Si still hot. during chondrule formation, there should have been con- Igneous rims appear to have formed by partial melting siderable loss of the alkalis from chondrules, and enrich- of preexisting fine-grained rims (Rubin, 1984; Rubin and ment of alkalis in rims and interchondrule matrix. Evidence Wasson, 1987; Krot and Wasson, 1995). They probably form for recondensation of alkalis in rims is hampered by their a continuum with dark-zoned or microporphyritic chon- mobility during even very mild aqueous alteration or meta- drules, and with so-called enveloping compound chondrules morphism. Rims and interchondrule matrix are generally (Wasson, 1993). Their fine grain size and their limited in- enriched in alkalis relative to chondrules in the least altered/ teraction with their host chondrules suggest that igneous metamorphosed chondrites. Of the OCs, alkali enrichments rims formed at lower peak temperatures and from material are greatest in Semarkona (LL3.0), but even in this mete- with lower solidus temperatures than their host chondrules. orite there is ample evidence of reentry of alkalis into chon- More intense heating would presumably lead to the mutual drules (Grossman et al., 2002). assimilation of rim and host, producing a new chondrule. While there is evidence that some rims accreted while Thus, it is likely that fine-grained rim and matrix material their host chondrules were still hot, this cannot be true for was one component of the chondrule precursors. some or, perhaps, even most rims. The rims and matrix of In summary, the presence of fine-grained rims around chondrites are the host of presolar material: circumstellar chondrules suggests that they formed in relatively dusty en- grains and interstellar organic matter. Indeed, the abundances vironments, consistent with chondrule cooling rates. In at 452 Meteorites and the Early Solar System II least some instances, rims began accreting while the chon- Chondrules in some carbonaceous chondrites show evi- drules were still hot and acted as sites for recondensation dence of 16O-rich components. These signatures may be of material evaporated from the chondrules. The dust may evidence of the survival or limited processing of a presolar also have provided nuclei for completely melted chondrules component in the chondrule precursors. Alternatively, they (see section 4.1). Whether this early accreted material sur- may result from chemical processes in the gas phase that vived chondrule formation or was mixed into the chondrule- occurred prior to or during chondrule formation forming region soon after it began to cool is unclear. While Chondrule textures result from a complex interplay be- recondensation may have aided rim accretion at this stage, tween the peak temperature, duration of heating, and pres- it is likely that by the time rims began to accrete the rela- ervation of nucleation sites, either due to incomplete melt- tive velocities of chondrules and micrometer-sized dust ing of chondrule precursors or the presence of fine-grained were <10–20 m/s, placing some constraints on models that dust in the chondrule-formation region. Experimental stud- involve gas drag for heating and/or sorting of chondrules. ies place a firm limit of 2100°C on the maximum tempera- Later, mixing of cold chondrules and unheated material must ture of chondrule formation. Few chondrules with liquidus have occurred. The CI-like abundances of presolar material temperatures over 1750°C were completely melted, and few in rim/matrix requires that most rim formation occurred after with liquidus temperatures under 1400°C were incompletely chondrules had cooled. Finally, the presence of igneous rims melted. Most chondrules reached peaked temperatures of on chondrules point to multiple episodes of chondrule and 1500°–1550°C. rim formation, requiring a relatively dynamic environment, Simulation experiments and chondrule textures have also and suggests that fine-grained rim/matrix material was one placed constraints on chondrule cooling rates. The cooling component of the chondrule precursors. rates required to produce porphyritic olivine textures are less than 1000°C/h with rates on the order of 100°C/h providing 8. CONSTRAINTS ON MODELS OF the best experimental analogs. Porphyritic pyroxene textures CHONDRULE FORMATION develop at slow cooling rates, on the order of 2°–50°C/h, regardless of the degree of undercooling. Cooling rates as The primary purpose of this chapter is to summarize the low as 5°C/h are also consistent with the zoning observed petrologic history of those chondrules dominated by ferro- in Semarkona chondrules. Barred-olivine chondrules cooled magnesian silicates. It is not the intent of this chapter to at rates of 500° to 2300°C/h. Radial pyroxene chondrules discuss various models for chondrule formation, which grew from a melt that experienced very high degrees of un- place this history in the larger context of environments and dercooling at quite slow cooling rates (5°C/hr). Pigeonite/ processes in the early solar system (see Connolly et al., diopside exsolution lamellae suggest that these textures are 2006). However, the information presented here is essential produced when cooling rates are on the order of 5°–25°C/ for constraining models of chondrule formation. Any such h in the temperature range of 1200°–1350°C. model must account for all the features present in chon- An important constraint on models of chondrule forma- drules, summarized below. tion is the variation in the oxidation states of their compo- Chondrule precursors were composed of anhydrous min- nents. Type-I chondrules experienced in situ reduction either erals, predominantly fine-grained olivine (<5 µm). However, by incorporating reducing material in their precursors or larger grains of olivine, up to 400 µm, were also incorpo- interacting with a reducing gas phase. Type-I chondrules rated. Lesser amounts of pyroxene and feldspathic glass, also exhibit significant chemical fractionations, with deple- chromite, metal, and sulfide were likely also present. The tions of elements like Cr, Mn, Na, K, and Fe, and enrich- presence of sulfide places some constraints on the minimum ments of elements, such as Al, Ca, and Ti, relative to Mg. temperatures of chondrule precursors. In a canonical solar- The order of these depletions and enrichments suggest vola- composition system, this temperature is ~427°C, where sul- tility has played a role. fide condensation begins (Lauretta et al., 1996). However, Chondrule precursors were heated and lost moderately in systems with enhanced S/H ratios, relative to solar, this volatile components by evaporation. However, significant temperature could be as high as 1000°C for total gas pres- recondensation took place during cooling. Lithophile ele- sures <10–4 bar (Lauretta et al., 2001). Carbonaceous mate- ments recondensed into crystallizing chondrules to various rial was included in some chondrule melts and likely played degrees during cooling, but siderophile and chalcophile an important role in controlling the oxidation state of the elements condensed as new phases or recondensed into resulting mineralogy. metal expelled from chondrules that did not completely At least some chondrules formed through melting of pre- evaporate, which are now in chondrule rims and matrix. existing materials, indicated by the presence of large, com- Isotopic fractionation should have been significant at the positionally distinct grains. These relict grains were part of early stage of evaporation; however, isotopic reequilibration the precursor material that survived chondrule formation during subsequent cooling must have been almost complete, without melting or dissolving. 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