Abstract– Reduced CV3 chondrites are relatively pristine rocks and prime candidates for studies exploring processes that predated planet formation. We closely examined the petrographic features and trace elemental composition of different CV3 constituents in the accretionary breccia Leoville. The petrographic results are presented here. Our sample (2.2 cm2) is not brecciated. The main ingredient—about 65 area%—is fine- to coarse-grained ferromagnesian type I chondrules. Minor constituents (in order of 2-D abundance) include refractory inclusions, Al-rich chondrules, and very fine-crystalline clasts of moderately volatile composition. Type II chondrules and metal nuggets occur sporadically. The chondrule–matrix ratio is approximately 3:1. Medium- and coarse-grained chondrules exhibit porphyritic textures, probably caused by incomplete melting, and frequent, partial or continuous, recrystallized dust rims. The fine-grained population most likely represents randomly sectioned dust rims. The rim material and some of the medium-grained objects are relatively troilite-rich. Iron-nickel metal is rare. In addition, almost all constituents show strikingly ragged or convoluted outlines. Only a few, rim-less components exhibit smooth contours. Evidence for incomplete melting and the formation of recrystallized or igneous rims in carbonaceous chondrites is well established, suggesting that both processes were widespread events. The observed features in Leoville support this conclusion. In addition, our findings indicate that surface abrasion in a turbulent dust-filled regime may have taken place after the consolidation of dust rims. Alternatively, the irregular, convoluted nature of at least the rimmed chondrules may have been inherent to the dust accretion event and was not erased by subsequent heating.
In CV3 meteorites, carbonaceous chondrites of the Vigarano type, chondrule proportions of approximately 30–60 vol% have been observed (Ebel et al. 2009). Two subtypes, the oxidized and the reduced CV3s, exist (McSween 1977). Samples of the oxidized subgroup show relatively high amounts of matrix, whereas the reduced population features lower matrix abundances. Based on this observation, Ebel et al. (2009) concluded that the extent of oxidative alteration correlates with matrix proportions. A different study, looking into the petrologic types of CV3 chondrites from a Raman spectroscopic point of view, principally agrees with this conclusion with one exception: the oxidized CV3 Kaba exhibits the lowest grade of thermal metamorphism (Bonal et al. 2006). Chemically, most CV3 chondrules are FeO-poor (type I; McSween 1977).
Leoville was found in the state of Kansas (USA) in 1961 or 1962. It was subsequently classified as a member of the reduced CV3 chondrite subgroup and recognized as one of its most primitive samples (Keil et al. 1969; McSween 1977; Kracher et al. 1982; Krot et al. 1995; Bonal et al. 2006). The first comprehensive report on Leoville was published by Kracher et al. (1985). They describe their aliquot as a polymict accretionary breccia containing host chondrules, Ca-Al-rich inclusions (CAIs), and abundant xenoliths of CM2 and host-like origin. The CM-like clasts exhibited different degrees of hydrous alteration. All components were set in a heterogeneous matrix.
While Kracher et al. (1985) focused on the overall composition of their sample including various xenolithic clasts, our multifaceted investigation exclusively targets the reduced CV3 parent rock, i.e., the Leoville groundmass rock including matrix, chondrules, and refractory inclusions but lacking allochthonous clasts/xenoliths. In this article, we examined the petrographic features of basic CV3 components as clues to the rock’s pre-accretionary history. Deciphering Leoville’s early evolution is a necessary foundation for our subsequent trace element study (in preparation). The sample studied was 2.2 cm2 in size and contained 163 objects >300 μm. We determined type and abundance of all of those objects and selected 32 representative constituents for further examination (Tables A1 and A2, Fig. S1).
Sample and Analytical Techniques
The polished thick section of Leoville used for this study measured approximately 10 × 22 mm and was provided by the Senckenberg Naturmuseum, Germany (J. Zipfel). We conducted a general optical investigation, backscattered electron (BSE) imaging, X-ray elemental area mapping, and electron microprobe spot analyses. At first, 43 individual BSE images covering the entire slab were taken at the University of Göttingen using a JEOL JXA 8900R electron microanalyzer (20 kV accelerating voltage, 10 nA sample current). Subsequently, X-ray elemental maps (Mg, Fe, Ca, Ti, and Al) were obtained for the entire section and for individual representative objects. Applying standard image processing software, the BSE images and elemental X-ray images were combined into a BSE and a red (Mg)-blue (Al)-green (Ca) composite map, respectively. For additional BSE images taken at the University of Knoxville, the microprobe (Cameca SX 100) was run at 15 kV accelerating voltage and 10 nA sample current.
Spot analyses of individual minerals were obtained in Knoxville (microprobe operating conditions: 1 μm spot size, 15 kV accelerating voltage, 10–15 nA sample current, 20 s count time). Matrix compositions were obtained in Göttingen (defocused beam of 20 μm, 15 kV accelerating voltage and 10–15 nA sample current, count times of 15 s for Si, Na, K, Al, Mg, Ca, Fe and 30 s for Ti, Mn, Cr, Ni). Calibration based on natural and synthetic standards (wollastonite, albite, sanidine, TiO2, rhodonite, anorthite, MgO, Cr2O3, hematite, NiO) and matrix correction codes after Armstrong (1995) were applied. Only analyses with significant totals (98.0–101.5 wt%) and acceptable stoichiometry (stoichiometric cation totals ±0.1) were considered.
Our clearly foliated specimen of Leoville is dominated by deformed chondrules and fine-grained matrix (Fig. 1, left). The overall distribution of both components is relatively homogeneous. The chondrule–matrix ratio is approximately 3:1. No xenoliths, dark clasts, or any other indication of brecciation exist in our sample.
The section investigated includes 163 clasts measuring ≥300 μm (Table 1). The lower limit of size of 300 μm was chosen based on the smallest diameter of obviously intact components. Smaller objects generally appeared to be fragments. About 90% of all objects ≥300 μm are chondrules. With respect to their texture, the majority of them are fine-grained PPs (pyroxene phenocrysts in more or less mesostasis) or POPs (pyroxene and olivine phenocrysts + mesostasis). Coarse-grained POPs and POs (olivine phenocrysts in mesostasis) are the second most abundant type, followed by rimmed/composite chondrules displaying continuous recrystallized rims. Based on optical inspection, the area% of chondrule groundmass is usually distinctly lower than the fraction of crystals. However, sporadically, chondrules may contain ≥50 area% mesostasis.
Table 1. Summary of objects identified.
Notes: PP = porphyritic pyroxene; POP = porphyritic pyroxene + olivine; PO = porphyritic olivine; CAI = Ca-Al-rich inclusion; AOA = ameboid olivine aggregate. Grain size: fine = generally <25 μm.
The degree of flattening encountered for the chondrules differs from barely to severely elongate with fine-grained species typically being more affected (Fig. 2). The average aspect ratio of fine-grained chondrules is 2.0 (98 objects) and 1.8 for the medium- and coarse-grained population (49 objects; see also Cain et al. 1986). In addition to the flattening, most chondrules exhibit irregular, fringed shapes. The majority also display highly irregular (convoluted) edges. Furthermore, we observed many rimmed or mantled species including a compound chondrule featuring an attached secondary chondrule.
In principle, three different populations of chondrules can be distinguished (Table 2). Considering the restricted 2-D view of the sample, it appears likely that the fine-grained convoluted chondrules are sulfide-rich rims of larger objects above or below the slab surface (3-D investigations of carbonaceous chondrites were carried out by, e.g., Ebel et al. 2008b; Hezel and Kießwetter 2010).
Table 2. General types of chondrules encountered in Leoville.
Note: PO = porphyritic olivine; POP = porphyritic pyroxene + olivine; PP = porphyritic pyroxene.
aMaximum dimension in millimeters.
bGenerally <25 μm.
Up to 1.5
Very irregularly shaped sulfide-rich objects, ragged looking surfaces, homogeneously dispersed opaques; possibly recrystallized rim layers of coarser-grained chondrules that are not visible in 2-D
Medium- to coarse-grained, rimmed
Up to >3
Fine-grained silicates and sulfides
Smooth, medium- to coarse-grained metal-poor centers with variable amounts of mesostasis (mostly POs and POPs, some PPs), partial or continuous rims of variable thickness; larger species may also exhibit a high-sulfide peripheral layer in addition to fine-grained sulfide-rich rim material
Coarse-grained, more rounded
Up to 1.8
Mostly POs containing variable amounts of mesostasis, metal-poor to virtually metal-free; no rims
Apart from the chondrules, five CAIs and a single ameboid olivine aggregate (AOA) were detected. The AOA and most CAIs are small objects (≤1 mm in diameter) and fine- or very-fine-grained. The only large CAI present in the examined slab (>3 mm, truncated by the slab edge) is coarse-grained. Furthermore, we identified a minor number of distinct, very-fine-crystalline objects that are lath-shaped and measure up to several millimeters. Like for the chondrules, the profiles of refractory inclusions and lath-shaped clasts are generally irregular and ragged.
Overall and disregarding alteration veins, our specimen is relatively rich in opaque phases, on a level comparable to H chondrites (5–10 vol%). The vast majority of opaque minerals is contained in chondrules (either as blebs <20 μm or irregular patches of sulfide of up to 300 μm) and fine-grained chondrule rims. They still appear partially pristine. Secondary, iron-rich weathering products (Lee and Bland 2004) precipitated along an extensive network of cracks of various widths. One of the major cracks (up to 100 μm wide) contains a high-Ca deposit; in all likelihood terrestrial calcite. A single opaque nugget present in our sample is heavily weathered.
Fine-grained chondrule rims, when present, vary in thickness and consist of ferromagnesian silicates and homogeneously distributed sulfides (Fig. 3). Generally, the fayalite content of olivine in the rim material (average of 5.4 ± 2.6%) is slightly higher than that of olivine in the chondrule center (average of 1.8 ± 1.2%; Fig. 4). Objects 1, 11, 25, and 29 (two ferromagnesian and two Al-rich chondrules [ARCs], respectively) exhibit particularly sulfide-rich rims: Object 1 has a sulfide-rich superficial film that is rather thin and almost continuous followed by a fine-grained mantle (see Fig. 6d). Object 11 shows large irregular patches of troilite in its periphery that is otherwise made up of fine-grained rim material (Fig. 3). Object 25 is characterized by a continuous irregular high-sulfide rim layer grading into a recrystallized rim (see Fig. 5a). Finally, Object 29 displays a thin, porous looking sulfide film on its surface but only very little fine-grained rim material (see Fig. 5b).
Backscattered electron and combined elemental maps of Leoville (Fig. 1) illustrate the variety of objects in terms of type and size. All chondrules are of type I (reduced, FeO-poor), with one exception: a single, smooth kidney-shaped chondrule is of type II (oxidized, FeO-rich). In terms of modal mineralogy, type A (olivine-rich), type B (pyroxene-rich), and ARCs were identified. Most of the larger species belong to type A.
Three spherules (objects 25, 29, and 31; Fig. 5) qualify as ARCs (Bischoff and Keil 1984; MacPherson and Huss 2005). Their predominant phase is Ca-rich plagioclase forming substantial lath (up to 500 μm long and 120 μm wide) in objects 25 and 29. It is accompanied by blocky forsterite (object 25) or low-Ca pigeonite (object 29) and interstitial Ca-rich pyroxenes. Accessory phases in object 25 include fassaite and enstatite. Object 31 features blocky forsterite heterogeneously dispersed in fine-grained dendritic Ca-rich plagioclase and pyroxene.
Among the refractory inclusions, prevalent mineral phases are melilite, spinel, and Ca-Al-rich pyroxenes. The large mixed-grained inclusion is a classical type B1 CAI containing long (up to 300 μm) laths of anorthite set in fine-grained melilite + fassaite + spinel. A small core area of this CAI is more spinel-rich.
Upon investigation of the opaque phases, almost all grains turned out to be troilite. Only accessory kamacite (Ni concentrations range from 3.35 to 6.71 wt%) and rare pyrrhotite have been detected. We acknowledge that opaque grains were analyzed randomly and the presence of small Ni-rich metal and/or Ni-rich sulfides is not excluded. However, they appear to account for an insignificant portion of opaque minerals, if at all. Chemical data of the relatively large opaque nugget (see Fig. 1) suggest that its primary phase was Fe-Ni metal.
Cain et al. (1986) pointed out the scarcity of xenolithic clasts in Leoville (about two to five occurrences per 100 cm2 sample area). Considering the relatively small size of our sample (2.2 cm2), the lack of evidence for brecciation is not unexpected. Ebel et al. (2009), who performed a numerical study of the abundance and size distribution of major components in CV3 chondrites (for technical details see Ebel et al. 2008a), report a significantly lower clast–matrix ratio for their Leoville sample (1.64 cm2). This is based on a larger amount of matrix (Table 1). AOAs, on the other hand, are stated to occur considerably more often than in our aliquot. While the latter discrepancy may be fully attributable to sampling effects, the difference in clast–matrix ratios is probably due to both sample heterogeneity and the higher estimated uncertainty of our results (10 rel.%). Despite the variation and in agreement with the general definition of reduced CV3 meteorites (Ebel et al. 2009), we can confirm that Leoville is distinctly matrix-poor.
Chondrules and Very-Fine-Crystalline Clasts
The difference in appearance of Leoville chondrules can be explained in analog to the findings of Zanda et al. (2002) who investigated the CR chondrite Renazzo. Their study was designed to illuminate the relationship between metal and chondrules in CR chondrites. Our sample of Leoville visually resembles Renazzo in that the abundance and distribution of opaque phases and the presence of very irregularly contoured (convoluted) chondrules are similar.
According to Zanda et al. (2002), the fine-grained convoluted chondrules represent incompletely melted material (see also McCoy et al. 1999). With increasing degree of heating, chondrule outlines smoothened and finely dispersed metal coalesced into bigger grains. The metal phase then started migrating to the chondrule surface forming metal films (see also Wasson and Rubin 2010). Finally, substantial coarsening of silicate grains and complete loss of metal from the chondrule interior occurred.
The porphyritic textures of medium- and coarse-grained chondrules in Leoville reveal that melting was incomplete (e.g., Nettles et al. 2006). Nettles et al. (2006) confirmed the hypothesis that “the transition from granular textures that form at very low degrees of partial melting to more traditional porphyritic textures is simply a function of the degree of partial melting and the subsequent cooling rate.” Based on their work and the fact that zoning of porphyritic grains in Leoville’s medium- and coarse-grained chondrules was not observed, we suggest that cooling rates of <10 °C h−1 were achieved. The recognition of incompletely melted chondrules in carbonaceous chondrites is not new (e.g., Weisberg and Prinz 1996; Zanda et al. 2002; Jones et al. 2005; Nettles et al. 2006; Scott 2007; Rubin 2010). For Leoville’s chondrules, the attribute of incomplete melting appears to be the rule.
While generally incompletely melted, chondrules also represent different stages of thermal overprinting (Fig. 6). In addition to the sequence described above, a handful of smaller coarse-grained chondrules seem to have lost their metal phase altogether (Fig. 6b). Alternatively, they may have sampled metal-poor precursor material and, hence, were metal-poor in the first place (e.g., Grossman and Wasson 1985). A recent investigation into the kinematic aspects of metal segregation in chondrules considered the loss of metal from a chondrule’s surface difficult (Uesugi et al. 2008), thus strongly supporting the latter explanation. Yet, it cannot be ruled out that the metal phase is heterogeneously distributed within the chondrule and stays unrevealed due to the restricted 2-D view (see Ebel et al. 2009).
The presence of fine-grained rims partially or continuously enveloping many chondrules and the occurrence of one compound chondrule (in sensu stricto: object 11; Fig. 3) testify to a multistage accretional history. Layered chondrules found in CR chondrites have repeatedly been interpreted in the context of progressive accretion (e.g., Prinz et al. 1985; Weisberg et al. 1992; Downen and Ebel 2010; Rubin 2010). Apparently, the fine-grained sulfide-rich material predominant in our sample was mixed with medium- and coarse-grained chondrules and formed rim layers. Its density and the character of intergrain boundaries suggest that it may have been sulfide-rich silicate dust that was partially melted after adhering to chondrules (“igneous rims,” e.g., Rubin 2010). The fine-grained convoluted chondrules in Leoville, that texturally and compositionally resemble igneous rim material, are probably recrystallized rims of primary objects that remain unexposed on the examined 2-D slab surface (rather than representing a separate population of incompletely melted chondrules). The accretion of dust rims by chondrule-sized particles in a turbulent protoplanetary nebula has been discussed, for instance, by Cuzzi (2004). Other chondrite classes like CM chondrites also contain prominent evidence of ubiquitous pre-accretionary dust rims (e.g., Metzler et al. 1992; see also Rubin 2010). Some smaller metal-poor chondrules, the highly flattened component, and refractory inclusions appear to have escaped mantling/dust accretion. Most likely, they were introduced to the accretionary region at a later point (see also Pack et al. 2004; Russell et al. 2005; Hezel and Palme 2007).
Aluminum-rich chondrules account for approximately 3 area% of the sample. Their predominant constituent is plagioclase (“plagioclase-phyric” subgroup as defined by MacPherson and Huss 2005). The plagioclase-phyric subgroup exhibits the highest abundances of Al and Ca among ARCs. MacPherson and Huss (2005) documented a clear genetic link between the Al-rich and ferromagnesian chondrules on one hand, and ARCs and CAIs on the other (see also Krot et al. 2002). This link is based on the degree of bulk compositional volatility, with CAIs as the most refractory, ARCs as intermediate, and ferromagnesian chondrules as the most volatile constituents. All three types of objects define a continuous, although nonlinear, trend consistent with equilibrium condensation at low pressure. Their compositions are mainly the result of volatility-controlled gas–solid reactions (e.g., MacPherson and Huss 2005).
Textural aspects of the plagioclase-rich chondrules in Leoville (objects 25 and 29; Figs. 5a and 5b) suggest that plagioclase laths crystallized prior to blocky forsterite/pigeonite and interstitial augite. This is consistent with the findings of MacPherson and Huss (2005). The prevalent dendritic pattern of Ca-rich plagioclase and pyroxene found in object 31, however, implies very rapid crystal growth. In this case, the blocky forsterite may have formed first. Cooling histories among “plagioclase-phyric” ARCs apparently differed.
In terms of compositional differences, no forsterite was detected in object 29. Instead, the blocky mineral in this chondrule is low-Ca pigeonite (Wo6–11). Furthermore, the primary cores of objects 25 and 29 are virtually opaque-free. In contrast to this observation, object 31 contains isolated areas with tiny troilite blebs. The sulfide appears to always be associated with blocky forsterite. In addition, troilite also occurs as a very thin, irregular, discontinuous film on the chondrule’s surface. Object 29 displays a more substantial, continuous superficial film of sulfide. The highest content of peripheral troilite, however, is to be found in object 25. This ARC exhibits a porous, irregular high-troilite layer grading into troilite-rich recrystallized rim material. Textural attributes suggest that the sulfide phase sintered and moved toward the chondrule upon partial melting of the rim layer. As to objects 29 and 31 (the two smaller ones), some or most of the rim material was possibly stripped off later. The troilite blebs within chondrule 31 may have been incorporated into a possibly fluffy or porous object along with forsterite prior to accretion to the Leoville parent body.
In two ferromagnesian chondrules, we detected isolated grains of low-Cr spinel (40 and 18 μm, objects 23 and 32, respectively; Table A1). Spinel in CV chondrules is rare (in particular and inherently, in 2-D samples) but has been reported before (e.g., Kimura and Ikeda 1998; Maruyama et al. 1999; Simon et al. 2000). Its origin has been discussed as either condensates (and related to CAIs) or as products of chondrule crystallization (Maruyama et al. 1999; Simon et al. 2000; Maruyama and Yurimoto 2003). CAI-related spinel has been described as characteristically 16O-rich and serrated (Maruyama and Yurimoto 2003). While the oxygen isotopic composition of the spinel grains found here was not determined, the exterior texture of both grains is smooth. In one case (40 μm, object 23), the spinel was associated with mesostasis. In the other (18 μm, object 32), it was enclosed in olivine. Compositions were relatively Cr-poor (max. 3 wt% Cr2O3) and FeO-rich (3.5 and 7.0 wt%, respectively). Interestingly, the small spinel enclosed in olivine yields a higher FeO content than its host (mg# 93 versus 97–100, respectively). It may therefore be a relict grain, whereas the other occurrence (object 23) appears to be a late-stage chondrule phase (see also Simon et al. 2000).
The very-fine-crystalline, roughly lath-shaped clasts in Leoville (e.g., object 14; Fig. 7) are difficult to classify. Their shape is most likely due to severe flattening of relatively weak material yielding most readily to impact deformation. The vermicular texture is similar to that of some very-fine-grained CAIs (objects 9 and 33). Detected mineral compositions, however, equal those of ultramafic and mafic silicates (olivine [Fo94–98], orthopyroxene [En86, Fs14], Al-rich augite [En75, Fs3, Wo21]) and Ca-rich plagioclase (An99). No refractory phases were found at the given analytical limit of approximately 10 μm. High-resolution analyses are necessary to further constrain the mineralogical properties of these clasts.
One of the most comprehensive studies of fine-grained spinel-rich inclusions in reduced CV3 chondrites (Efremovka and Leoville) was published by Krot et al. (2004). Those authors concluded that fine-grained spinel-rich CAIs formed as gas–solid condensates in the early Solar nebula and subsequently underwent low-temperature nebular alteration. Later, triggered by a re-heating event, they lost SiO and Mg while melilite was added to their mineralogical inventory.
Our sample of Leoville includes a small but diverse group of CAIs (Table A2). Object 9 (Fig. 8) resembles the unzoned spinel-rich CAIs listed by Krot et al. (2004). Another fine-grained CAI (object 22) we found is bimodal and consists of two halves (Mg-rich [spinel] and Ca-rich [Al-diopside], respectively). It may be a fragment of a larger zoned object. The large coarse-grained type B1 CAI (object 12) is also zoned. According to Krot et al. (2004), macroscale zoning and the generation of melilite-rich mantles could be achieved by evaporation-controlled loss of Si and Mg (see also Grossman et al. 2000; Richter et al. 2002).
Some of the fine-grained spinel-rich inclusions listed by Krot et al. (2004) display forsterite-rich accretionary rims. In addition to forsterite, the rims are described as containing fine-grained Al-diopside, Fe-Ni metal, and refractory nodules in variable amounts. While their composition clearly differs from that of the recrystallized rims in Leoville, their paths of formation may have been similar, only sampling compositionally distinct nebular regions. All CAIs identified here are rimless.
McSween (1977) listed metal and troilite as the predominant opaque minerals of reduced CV3s. Kracher et al. (1985) found coexisting kamacite and taenite in Leoville host material but did not specify the absolute amounts or proportions of sulfides and Fe-Ni metal. Similarly, Zanda et al. (1990) reported the occurrence of “abundant Fe-Ni grains” in chondrules and matrix of Leoville, without providing further compositional data. As opposed to the studies above, our aliquot is almost devoid of pristine Fe-Ni phases. The predominant opaque mineral is troilite.
Like the difference in fractions of matrix and AOAs, the “metal discrepancy” may be related to sample heterogeneities on a centimeter-scale. However, the observed high volume of troilite in absence of any significant Fe-Ni metal is conspicuous. Hypothetically, sulfurization may lead to the observed predominance of troilite. Lauretta et al. (2002) found that some dust-rich environments where rimmed chondrules formed must have also shown S fugacities high enough to effectively convert kamacite into troilite. In view of the ubiquitous dust rims, many Leoville clasts likely formed in a dust-rich region. Other products of sulfurization like the Ni-rich mineral pentlandite and a P-bearing phase (Lauretta et al. 1998), however, were not identified in the present sample.
Another factor possibly responsible for the low abundance of detectable Fe-Ni metal is terrestrial alteration. In a hot desert environment (versus Antarctic environment; Lee and Bland 2004), troilite weathers at approximately the same rate as Fe-Ni metal. Leoville was exposed to a semi-arid steppe climate and is clearly weathered. It potentially lost most of its original metal contingent to this process. However, judging from the predominance of pristine troilite, the original fraction of Fe-Ni metal must have been very small to begin with. Therefore, the seemingly very local (centimeter-scale) scarceness of Fe-Ni metal remains puzzling.
In some chondrules, rim troilite occurs not only as fine-grained material but coalesced into larger patches or a superficial film, suggesting variable degrees of reheating. The fine-grained sulfide-rich chondrules and most rims of that type of material (dust rims) were reheated without significant melting (now displaying microporphyritic textures). On the other hand, rims of other constituents that show high-sulfide peripheral layers or large patches of troilite (e.g., objects 1, 11, 25, and 29) may have undergone partial melting. The first minerals thermally mobilized in a sulfide–silicate mixture would be sulfides. The prevailing S fugacity was high enough, however, for troilite to not evaporate. Instead, troilite coalesced into bigger patches and migrated toward the primary chondrule surface (see also Uesugi et al. 2008). The distinctly troilite-rich compound chondrule with the integrated secondary chondrule (object 11; Table A1) appears to have undergone both coarsening in conjunction with metal segregation (Zanda et al. 2002; Uesugi et al. 2008) and partial melting of the fine-grained rim material.
In place of or in addition to the chondrule maturation process described above, sulfide–silicate separation could have taken place before or during chondrule formation. Ebel et al. (2008a, 2008b) considered accumulative growth of chondrules by sequentially adding layers of preseparated material (e.g., sulfides).
Closing Thoughts on the Irregular Appearance of Leoville Constituents
The majority of constituents in Leoville display irregular shapes and also ragged looking surfaces. Exceptions include a very few (coarse-grained) ferromagnesian chondrules, two of the ARCs, the metal nugget, and the single type II chondrule. The degree of roughness varies. As a general rule, the finer the grain size the more convoluted the object is. A series of immediate, yet unresolved questions arise from this observation: Was this textural detail established prior to or after accretion? If it is a postaccretionary feature: Is it related to the impact deformation of the Leoville parent rock? Why are there exceptions? A common denominator of the exceptions listed above is their lack of recrystallized dust mantles. But CAIs and the highly flattened component are devoid of those rims as well. And yet, they show more or less rugged outlines, thus arguing against the postaccretionary option. Assuming a pre-accretionary event, it must have taken place after the mantling period (rim–primary chondrule borders are smooth).
For the fine-grained sulfide-rich chondrules and the rimmed constituents, surface roughening may have been produced by the irregular adhesion of silicate–sulfide dust and incomplete melting thereof. This would also explain the smoothness of most rimless constituents. Alternatively, or additionally, some form of grain size- and hardness-controlled erosion may have taken place prior to accretion, e.g., wind-induced abrasion in a turbulent, dust-filled regime (see also Cuzzi 2004). This type of process could have affected the rimmed and highly flattened clasts and CAIs. The ARCs with superficial sulfide films (objects 29 and 31) appear to have lost all or most of their dust rims due to those erosive forces. In contrast to them and based on their smooth outlines as well as the assumption of a separate origin (in space and/or time), the single type II chondrule and the metal nugget were not exposed to erosion or mantling (Fig. 9).
Rubin and Wasson (2005) recently presented a study on nonspherical lobate chondrules in CO3 chondrites. Some of the objects they investigated closely resemble chondrules sighted in Leoville. In the particular case of fig. 1d in Rubin and Wasson (2005), the authors proposed that the roughness of the chondrule contour was generated by fracture surfaces. Subsequent low-degree melting of chondrules in CO3 chondrites failed to erase the feature. For Leoville, fracture surfaces may play a minor role. The full scope of circumstances leading to the generation of irregular, i.e., convoluted, ameboid profiles, however, remains to be explored.
The Leoville sample investigated for this study only contains highly reduced CV3 host material. This host material consists of ferromagnesian chondrules (most of which are relatively troilite-rich) and a small number of minor objects including refractory mineral assemblages (CAIs and AOAs), ARCs, and a very-fine-crystalline lath-shaped constituent. All components are set in an opaque-poor matrix. Fragments of chondrules and CAIs are also present. Parent body alteration of the host rock was minor. Shock-induced deformation, on the other hand, was substantial, leading to foliation and elongation. Terrestrial weathering took place as well.
In the slab examined here, the main ingredient is ferromagnesian chondrules. Chemically, they belong to type I (Fe-poor) with a single exception (type II, i.e., Fe-rich). Textural types are limited to PPs, POs, and POPs with variable amounts of groundmass. Petrographically, two categories can be distinguished: fine-grained sulfide-rich convoluted species (this type of material is also present as rim layers) and medium- to coarse-grained chondrules containing variable amounts of troilite. The latter category often displays recrystallized rims. Generally, the fine-grained sulfide-rich rim material contains low- and high Ca-pyroxenes, forsteritic olivine, and some Ca-rich plagioclase. Olivine compositions are slightly more oxidized in the rims than in the primary chondrule, suggesting a slight change of redox status in the nebula or parent body alteration.
Refractory inclusions are a minor ingredient in our section of Leoville (five CAIs, one AOA). They assume about 4 area% of the rock and are mostly small fine-grained unzoned species. Typical refractory phases include spinel, melilite, anorthite, Al-diopside, and fassaite. The largest (>3 mm) member containing substantial anorthitic laths classifies as a typical type B CAI. All refractory inclusions are rimless.
Among the unexpected, meaning thus far unrecognized, ingredients of Leoville, we have identified a single type II chondrule, a single metal nugget, a low number of ARCs, a very-fine-crystalline, highly flattened constituent, and two ferromagnesian chondrules containing spinel. In addition, our sample turned out to be relatively troilite-rich while Fe-Ni metal was surprisingly scarce. The ARCs make up about 3 area% of the rock. Only the plagioclase-phyric variety was detected. All of them were rimmed (although to a varying extent, possibly recording the partial loss of rim material or less efficient rim acquisition for two of them). Furthermore, their cooling histories were somewhat different. The very-fine-crystalline lathy component yielded a composition enriched in moderately volatile elements (ultramafic and mafic silicates, Ca-rich plagioclase), yet displayed textural affinities to some CAIs.
The majority of events molding and affecting individual constituents appear to have taken place prior to the final accretion of Leoville’s parent body. Most notably, many objects acquired recrystallized rims, i.e., troilite-rich dust mantles that were reheated and, in several instances, partially melted. The lack of Fe-Ni metal on a centimeter-scale may indicate localized pre-accretionary sulfurization of kamacite that possibly took place even before chondrule rim formation. In addition, almost all constituents were potentially exposed to an abrasive process leaving their contours looking ragged, convoluted, and ameboid. The nature of this process remains uncertain. Some raggedness may be caused by fracture surfaces and/or was inherent to the dust accretion event. Another conceivable mechanism is erosive abrasion in a turbulent dust-filled regime.
Acknowledgments–– The Leoville sample was kindly provided by Jutta Zipfel (Senckenberg Naturmuseum, Germany). Thanks also go to Timothy Jull (Editor in Chief) as well as Eric Palmer and anonymous referees. Thoughtful comments and thorough reviews of T. J. and E. P. in particular significantly contributed to improving the original manuscript. This work was mainly funded through DFG (German Research Foundation) grants PA1970/1-1 (Research Fellowship, Andrea Patzer) as well as PA909/2-1 and PA909/2-2 (both Emmy-Noether-Program, Andreas Pack).
Editorial Handling–– Dr. Timothy Jull
Table A1. Chondrules and other clasts >300 μm selected from Leoville.
Notes: PO = porphyritic olivine; BO = barred olivine; RP = radial pyroxene; POP = porphyritic olivine + pyroxene; G = granular; C = cryptocrystalline; Fo = forsterite; Fa = fayalite; En = enstatite; Fs = ferrosilite; Wo = wollastonite; An = anorthite; Ab = albite; Ge = gehlenite. Plagioclase/groundmass compositions contain ≤1 mole% orthoclase; fine-grained = generally <25 μm; very-fine-grained = generally <10 μm.
aMaximum dimension in millimeters.
Groundmass-rich (An80); coarse olivine phenocrysts (Fo99–100), small olivines in groundmass (Fo90)
Compact; coarse olivine phenocrysts (Fo99), little groundmass (An75); accessory Al-rich augite (En42–48, Fs3–16, Wo36–55), low-Fe pigeonite, enstatite, and spinel (1 grain of 40 μm, mg# 97) associated with groundmass
Groundmass- and metal-rich (metal patches heavily weathered); coarse olivine phenocrysts (Fo97–99), small accessory high-Ca enstatite (Wo5) + spinel (1 grain of 18 μm [mg# 93], enclosed in olivine [mg# 97–100])
Groundmass-rich (An78); coarse-grained olivine (Fo97) + enstatite phenocrysts, minor small low-Fe augite (En58–64, Wo35–40) + olivine (Fo94–95) grains; sulfide-rich recrystallized rim
I, POP, rimmed
Coarse-grained; mostly olivine (Fo97–99), some enstatite (Wo5), anorthitic groundmass (An86–78), minor low-Fe augite (Wo37); fine-grained sulfide-rich recrystallized rim
I, POP, rimmed
Medium-grained olivine (Fo89–95) + enstatite, minor groundmass (An85–87); extensive fine-grained sulfide-rich recrystallized rim
Fine- to medium-grained; enstatite (En98), minor olivine (Fo91–94) + low-Fe augite (En60, Fs3, Wo37), anorthitic groundmass (An80–84)
Groundmass-rich (An80–88); coarse enstatite phenocrysts, minor small low-Fe augite grains (Wo39)
Groundmass-rich (An80), coarse Ca-rich enstatite (Wo5–6), minor low-Fe augite
I, PP, rimmed
Compact, coarse-grained; isolated forsteritic fragments (Fo88, Fo93, Fo97), minor interstitial low-Fe augite (Wo33–37) + plagioclase (An57–62); sulfide-rich recrystallized rim w/enstatite + low-Fe augite (Wo40), minor olivine + plagioclase
I, PP, rimmed
Coarse-grained enstatite phenocrysts; isolated olivine fragments (Fo97–98); large patches of groundmass w/high-Al augite (Fs2–4, Wo42–45); fine-grained sulfide-rich recrystallized rim
Fine-grained granular or microporphyritic, sulfide-rich chondrule or recrystallized rim; we detected low-Fe augite (En70, Fs4, Wo26), pyroxene (En93, Fs6, Wo1), plagioclase (An72)
Microcrystalline opaque-free center w/very fine-grained metal-rich edge containing olivine (Fo94–98) and Al-rich augite (En75, Fs3, Wo21); zoned with Al + Ca enriched in the center
Microcrystalline to very-fine-grained; detected phases include (but are possibly not limited to) olivine (Fo90–99), pyroxene (En86, Fs14), anorthite
Fine laths of plagioclase (An81–85), fine-grained low-Fe augite (Wo33–40) + low-Fe pigeonite (Wo6–11); thin continuous high-sulfide superficial layer
Fine- to very-fine-grained plagioclase (An83–85) and augite (En53–69, Fs1–8, Wo29–45) w/up to 11 wt% Al2O3, minor blocky olivine (Fo95–98) and low-Fe pigeonite (Wo7), some troilite in peripheral areas and on object’s surface
Large laths (up to 500 μm long and 120 μm wide) of plagioclase (An85–90) w/occasional herringbone texture, fine-grained olivine (Fo98–99) + low-Fe augite + accessory fassaite (8 wt% TiO2) and enstatite (Wo5); high-sulfide peripheral layer grading into a fine-grained sulfide-rich recrystallized rim
CAI, zoned; viscous looking shape, bimodal composition: [a] Ca-rich half w/Al-diopside (Wo53–63, 14–20 wt% Al2O3), minor melilite, and spinel; [b] Al-rich half contains spinel and Al-rich augite (En53, Fs8, Wo39, ∼42 wt% Al2O3); string of tiny Ti-rich beads (perovskite?) along the outer edge of Ca-rich half
CAI, unzoned, Ca-rich; vermicular texture: very irregularly shaped shreds of spinel and olivine (Fo94–97) surrounded by augite (En51–58, Fs1–10, Wo39–43) + Al-diopside (Wo49), minor anorthite (An100), Mg-rich melilite (Ge1), and enstatite
CAI, unzoned, Al-rich; vermicular texture: very irregularly shaped shreds of spinel surrounded by Al-rich melilite (Ge79–94) and Al-diopside (En36, Fs1, Wo63); melilite grows more Ca-rich away from spinel toward diopside; accessory Ti-rich phase (perovskite?)
CAI, zoned, Ca-rich mantle, Al-rich center; “porphyritic” texture with long laths (up to 300 μm) of anorthite embedded in abundant fine-grained Al-rich melilite (Ge59–88), fassaite, and spinel (spinel concentrated in the center)
AOA, unzoned; porous appearance, very irregular patches of olivine (Fo99–100), associated with Al-diopside (Wo54), and enstatite