Ultrarapid chondrite formation by hot chondrule accretion? Evidence from unequilibrated ordinary chondrites

Authors


E-mail: knut.metzler@uni-muenster.de

Abstract

Abstract– Unequilibrated ordinary chondrites (UOCs) of all groups (H, L, LL) contain unique chondrite clasts, which are characterized by a close-fit texture of deformed and indented chondrules. These clasts, termed “cluster chondrites,” occur in 41% of the investigated samples with modal abundances between 5 and 90 vol% and size variations between <1 mm and 10 cm. They show the highest chondrule abundances compared with all chondrite classes (82–92 vol%) and only low amounts of fine-grained interchondrule matrix and rims (3–9 vol%). The mean degree of chondrule deformation varies between 11% and 17%, compared to 5% in the clastic portions of their host breccias and to values of 3–5% found in UOC literature, respectively. The maximum deformation of individual chondrules is about 50%, a value which seemingly cannot be exceeded due to geometric limitations. Both viscous and brittle chondrule deformation is observed. A model for cluster chondrite formation is proposed where hot and deformable chondrules together with only small amounts of co-accreting matrix formed a planetesimal or reached the surface of an already existing body within hours to a few days after chondrule formation. They deformed in a hot stage, possibly due to collisional compression by accreting material. Later, the resulting rocks were brecciated by impact processes. Thus, cluster chondrite clasts are interpreted as relicts of primary accretionary rocks of unknown original dimensions. If correct, this places a severe constraint on chondrule-forming conditions. Cluster chondrites would document local chondrule formation, where chondrule-forming heating events and the accretion of chondritic bodies were closely linked in time and space.

Introduction

It is usually assumed that chondrules from unequilibrated ordinary chondrites (UOCs) solidified as isolated objects in the protoplanetary disk, reaching nearly spherical shapes due to surface tension (e.g., Grossman et al. 1988; Lauretta et al. 2006). In contrast to this, a significant fraction of chondrules in many UOCs deviate from this ideal shape and appear to be deformed in a viscous state by indentation of neighboring chondrules. These deformations were seemingly established during chondrule agglomeration and parent body accretion. First hints on this are given by Makowsky and Tschermak (1879), who stated that “sometimes spherules with round impressions can be found indicating viscosity of these spherules during encounter.”Gooding and Keil (1981) investigated a great number of ordinary chondrites and concluded that “many chondrules with indentations possibly formed by low-velocity collisions between viscous and solid chondrules.” Further observations on deformed chondrules have sporadically been described in the literature and interpreted as the result of hot chondrule accretion (e.g., Hutchison et al. 1979; Hutchison and Bevan 1983; Taylor et al. 1983; Holmén and Wood 1986; Sanders and Hill 1994; Hutchison 1996, 2004; Zanda 2004). Chondrule cooling rates are on the order of 5–3000 k h−1 (e.g., Hewins 1983; Desch and Connolly 2002; Ferraris et al. 2002; Hewins et al. 2005; Lauretta et al. 2006), probably less than 1000 k h−1 in most cases (Hewins, personal communication). For this reason, the concentration and accretion of deformable chondrules must have occurred within hours to a few days after chondrule formation. After this short period, chondrules should solidify to form rigid objects, which are no longer viscously deformable. Estimations in the literature for chondrule temperatures at the time of chondrule contacts vary between 800°C (Hutchison et al. 1979; Hutchison and Bevan 1983) and 1000 to 1100°C (Holmén and Wood 1986). Based upon glass composition, it is assumed that the overall solidification temperature of mesostasis occurs close to 990°C (Lauretta et al. 2006).

As the period between chondrule formation and chondrite accretion (planetesimal formation) is poorly constrained, it would be an advantage to identify and characterize chondritic rock units whose textures indicate such rapid accretion. This would be a severe constraint and could help favor or discard certain models of chondrule formation. Based on the investigation of a large number of UOCs, evidence is found for a ubiquitous occurrence of such lithologies. These results are presented in the following section.

Several authors have argued against hot chondrule accretion (e.g., Grossman et al. 1988; Skinner 1989; Rubin 1995) and their arguments and alternative models are discussed below.

The Term “Cluster Chondrite”

Deformed chondrules are not homogenously dispersed within a given UOC, but are restricted to fragments of a specific type of chondritic rock (Figs. 1–3a). This rock is characterized by close-fit textures composed of deformed and undeformed chondrules, high chondrule abundances, low abundances of distinct chondrule fragments, and low abundances of interchondrule matrix (Metzler 2010, 2011). Although textural units with deformed chondrules have sporadically been described in the literature (Zanda et al. 2002; Hutchison 2004), no term exists so far for them. It was proposed by Metzler (2010) to use the term “cluster chondrite” because it describes their main properties. First, these units represent clusters of well-developed chondrules. Secondly, a clast with a longest dimension of about 10 cm is found (Fig. 2), which proves that they are not microscopic objects, but can reach sizes of hand rock samples. For that reason, these clasts and their parent lithologies, respectively, deserve a rock-describing term, namely “chondrite.” It is important to note that cluster chondrites are not a new group of UOCs, but represent specific textural units in each of these groups (H, L, LL), obviously genetically linked to their host breccias.

Figure 1.

 Image of Krymka (LL3.2) with enlarged detail showing a cluster chondrite clast with its typical close-fit texture composed of deformed and indented chondrules (polished thin section, transmitted light).

Figure 2.

 Largest cluster chondrite clast found so far (upper part). The area below the line represents the clastic matrix of this meteorite (NWA 5205; LL3.7). Image courtesy of Marcin Cimala, Poland.

Figure 3.

 Textures of two investigated UOCs. a) Sample of NWA 5205 (LL3.7) consisting of a large cluster chondrite clast (left of white line) with adjacent clastic matrix (right of white line). b) Example for a typical UOC (NWA 4572; LL3.6) with only small amounts of (tiny) cluster chondrite clasts. Polished thin sections; transmitted light.

Cluster Chondrites Versus “Regular” Chondrites

One of the first intentions at the beginning of this study was to identify “regular” chondrites as counterexamples to cluster chondrites to compare textures and identify characteristic differences (Metzler 2011). This attempt turned out to be difficult and finally impractical. After close inspection, in many “regular” UOCs, very small clasts of cluster chondrites were found (e.g., NWA 4572; Figs. 3b and 4a), sometimes consisting of only five or fewer adhering chondrules. Even if they are missing in one thin section of a given UOC, they can occur in another. It turned out that cluster chondrites and “regular” chondrites are not counterexamples, but that cluster chondrites occur in many UOCs as lithic clasts. For this reason, the term “regular” chondrite will not be used anymore.

Figure 4.

 Clastic portions of UOCs. a) Overall texture of NWA 4572 (LL3.6) with embedded tiny cluster chondrite clasts (white outlines); b) Clastic portion between large cluster chondrite clasts in NWA 5205 (LL3.7). Both portions are texturally similar and are characterized by a high abundance of chondrule fragments, large amounts of fine-grained clastic material, and the occurrence of nearly spherical chondrules (arrows). SEM-BSE images.

Aims of This Study

Cluster chondrite textures differ strongly from UOC textures described in the literature. For this reason, a comprehensive study of these rocks is currently in progress that includes petrologic, mineralogic, and microchemical investigations, as well as measurements of oxygen and noble gas isotopes. The first results presented here are confined to the textural properties of these rocks and their components. The data in the following are mainly based on in-depth investigations of one L and five LL chondrites. For meteorite names, see Table 1. Additionally, thin sections from 52 other UOCs have been surveyed to obtain statistical data on the occurrence, frequency, and size of cluster chondrite clasts. These results are given in the appendix Table A1. H chondrites are underrepresented in this study. Some data for H chondrites (Devgaon, Dhajala; Tanezrout 028) are given in Tables 1 and A1, and some images are shown in the following. There is no doubt that cluster chondrite clasts occur frequently in H chondrites and some earlier descriptions came especially from these rocks (Makowsky and Tschermak 1879; Hutchison et al. 1979; Sanders and Hill 1994).

Table 1. Modal data for cluster chondrite clasts and clastic meteorite portions obtained by point-counting on BSE images.
MeteoriteTypeLithologyCounted area (mm2) n Chondrules
(vol%)
Interchondrule
sulfide + metal
(vol%)
Matrix (vol%)
  1. n = number of counted points; clch = cluster chondrite.

  2. aclast M-06-44-1-a (described by Metzler et al. 2011).

  3. bOfficial classification: LL3.2 (see text).

  4. cGrossman et al. (1988).

  5. dBrearley and Jones (1998).

  6. eHutchison (2004).

  7. fHuss et al. (1981).

DevgaonH3.8clch clast24449 82 13  5
NWA 869L3-6clch clast (<L3.5)a7363 92  5  3
NWA 5206LL3.05clch clast31319 87  8  5
NWA 1756LL3.10clch clast9436 87  4  9
KrymkaLL3.2clch clast32444 89  3  8
NWA 5421LL3.7clch clast22390 86  9  5
NWA 5205LL3.7bclch clast #129371 91  5  4
  clch clast #218340 84 12  4
  clastic portion49432 65  8 27
NWA 4572LL3.6clastic portion49451 76  5 19
Literature data
(Ordinary chondrites)
    65–75c
 60–80d
  5–12e 10–15f

Samples and Analytical Methods

Polished thin sections from 58 UOCs were inspected by optical microscopy. A ZEISS polarizing microscope (Axiophot) was used for investigation in transmitted and reflected light. Slices and polished thin sections from six of these samples (one L and five LL chondrites) were studied in detail by scanning electron microscopy (see Table 2). Northwest Africa (NWA) 5205, which is officially classified as an LL3.2 chondrite, is treated here as type 3.7, as all of its lithologies analyzed so far reveal the latter type (percentage mean deviation [PMD] of olivine compositions: 22–30). A JEOL 840A scanning electron microscope, equipped with an energy dispersive X-ray (EDX) analysis system (INCA, Oxford Instruments) was used for mineral identification. The modal compositions of bulk meteorites were determined by point-counting techniques, using various grids with point distances between 25 and 150 μm. These results are summarized in Table 1.

Table 2. Apparent diameter and degree of deformation of chondrules in cluster chondrite clasts and clastic meteorite portions.
MeteoriteTypeLithology n Apparent chondrule diameteraDegree of chondrule deformationFraction of deformedb chondrules
Min–Max
(mm)
Mean
(mm)
Min–Max
(%)
Mean
(%)
  (%)
  1. n = number of analyzed chondrules; clch = cluster chondrite.

  2. aCalculated from measured chondrule cut faces in thin sections (see text).

  3. bDeformed: >20% deformation (by definition; see text).

  4. cGrossman et al. (1988).

  5. ddevoid of NWA 5205, clast #1.

  6. eNelson and Rubin (2002).

  7. fRubin (2010).

  8. gDescribed in Metzler et al. (2011).

NWA 869L3-6clch clast SM-06-44-1-ag670.1–1.30.521–4916  30
Literature data
(L chondrites)
    0.4f–0.8c   
NWA 5206LL3.05clch clast490.2–1.50.672–5011  10
NWA 1756LL3.10clch clast400.3–1.70.721–4917  37
KrymkaLL3.2clch clast350.2–1.10.603–3816  27
NWA 5205LL3.7clch clast #1990.4–2.81.381–5617  34
  clch clast #2580.3–1.50.901–4113  18
Mean clch clasts (LL)    0.74d 15  26
NWA 5205LL3.7clastic portion470.4–1.80.981–1450
NWA 4572LL3.6clastic portion520.2–2.40.820–1650
Mean clastic portions    0.9 50
Literature data
(LL chondrites)
    0.57f–0.9c 3–5e 

To quantify the phenomenon of chondrule deformation, the perimeter and the area of chondrule cut faces in thin sections were determined. For these measurements, SEM-BSE images were used and the software “Measure” (DatInf) was applied. From these data, the mean apparent chondrule diameters were calculated for each sample (Table 2) and for each chondrule textural type (Table 3). The apparent chondrule diameter is defined as the diameter of a circle with the area of the measured chondrule cut face. The mean, minimum, and maximum values obtained are summarized in Table 2. Furthermore, the degree of deformation was calculated for each chondrule. I define this parameter as the ratio between the perimeter of a chondrule cut face and the perimeter of an equally sized circle, which corresponds to the convolution index used by Zanda et al. (2002). Hence, the degree of deformation corresponds to the percentage of perimeter increase. The 2-sigma standard error for this method is about 3%, which was obtained by measuring a strongly deformed chondrule 10 times. This method was applied to 447 chondrules from 6 different UOCs and the results are summarized in Tables 2 and 3.

Table 3. Fraction of chondrule textural types, their mean apparent diameter, and their mean degree of deformation in cluster chondrite clasts and clastic meteorite portions.
  n POPPOPPBORPGOPC
Fraction (%)Mean appar
diam (mm)
Mean deform (%)Fraction (%)Mean appar
diam (mm)
Mean deform (%)Fraction (%)Mean appar
diam (mm)
Mean deform (%)Fraction (%)Mean appar
diam (mm)
Mean deform (%)Fraction (%)Mean appar
diam (mm)
Mean deform (%)Fraction (%)Mean appar
diam (mm)
Mean deform (%)Fraction (%)Mean appar
diam (mm)
Mean deform (%)
  1. n = number of chondrules; Mean appar diam = Mean apparent diameter (calculated from measured chondrule cut faces in thin sections; see text); Mean deform = Mean degree of deformation; clch: cluster chondrite; Chondrule textural types (Gooding and Keil 1981): POP = porphyritic olivine–pyroxene; PO = porphyritic olivine; PP = porphyritic pyroxene; BO = barred olivine; RP = radial pyroxene; GOP = granular olivine–pyroxene; C = cryptocrystalline.

  2. aGooding and Keil (1981).

  3. bData for LL3 chondrites (Nelson and Rubin 2002).

NWA 869 clch clast67 320.5313 330.5818210.461720.391850.631550.472120.3828
NWA 5206 clch clasts49 200.6416 410.7411230.571060.511121.49940.771240.649
NWA 1756clch clasts40 150.6911 400.6813200.682751.0711130.6218070.8824
Krymka clch clasts35 280.5714 280.7515350.5118060.60630.52230
NWA 5205 clch clast #199 351.4116 201.3826121.2917141.3615161.581011.73811.3017
NWA 5205 clch clast #258 310.8711 310.9715140.731620.316150.9910070.9411
Mean clch clasts  29 14 31 1819 176 1310 112 143 18
NWA 5205 clastic portion47 340.855 211.014260.96540.656111.265041.572
NWA 4572 clastic portion52 380.655 320.93780.67581.404100.77420.23320.803
Mean clastic portions  36 5 27 6 15 56 510 41 33 3
Literature data (UOCs)  47–52a 3b 15–27a 3b 9–11a 3b3–4a 3b7–9a 3b2–5a 5b3–5a 3b

The terminology concerning chondrule textural types (POP, PO, PP, BO, RP, GOP, C; type I and type II) in the following is adopted from the work of Gooding and Keil (1981) and McSween (1977), respectively.

Results

Cluster Chondrite Clasts and Their Host Breccias

Cluster chondrite clasts occur in unequilibrated H, L, and LL chondrites and are present in the most primitive UOC (Semarkona, LL3.00) as well as in more metamorphosed samples up to petrologic type 3.9 (Acfer 080, Tanezrouft 040; Table A1). This indicates that their overall appearance did not change during thermal metamorphism on the parent body. To obtain data on the occurrence, frequency, size, and textural setting of cluster chondrite clasts, 58 UOCs were surveyed (Table A1). These samples represent 24 genomict breccias (see Bischoff et al. 2006) and 34 “unbrecciated” samples, according to their current classification. The latter show evidence for mechanical modification, as well, either as obvious clastic portions or as some internal cataclasis with rearrangement of components (e.g., Fig. 4a). These observations confirm that mechanically unaltered UOCs are the exception, not the rule (e.g., Scott 1984; Romstedt and Metzler 1994).

Occurrence, Frequency, and Sizes of Cluster Chondrite Clasts

The host rocks of all cluster chondrite clasts are chondritic breccias of various kinds. Only one meteorite among the 24 genomict samples contains those clasts (NWA 869; Table A1). In contrast, in 23 of 34 “unbrecciated” UOCs, cluster chondrite clasts were found. In summary, 41% of all investigated UOCs contain cluster chondrite clasts. Obviously, they are a ubiquitous component, i.e., their parent lithology made up a considerable portion of UOC material during the first stages of accretion. It must be stated here that the data in Table A1 are very conservative and the presence of these clasts is only reported in very clear cases. Thus, these data represent the minimum values for the frequency of cluster chondrite clasts in UOCs.

There are large differences in clast frequency. Apart from meteorites that are obviously free of them, they contribute less than 10 vol% to most of the samples investigated (14 out of 23 samples; Table A1). Six samples consist of less than 50 vol%, one sample less than 90 vol%, and one sample more than 90 vol% cluster chondrite clasts. The coarse graduation of these frequency classes indicates that these data are gross estimates, as, in many cases, it is very difficult to identify the boundaries between clasts and their surroundings (see below). The size of the clasts varies enormously between the different breccias, where most are in the size range between 1 and 10 mm (Table A1; Fig. 1). On the other hand, the sample of the LL3.7 chondrite NWA 5205 (slice of about 300 g) consists almost entirely of a large clast with a size of about 10 cm (Fig. 2).

Textural Setting of Cluster Chondrite Clasts

Basically, two different textural settings for cluster chondrite clasts can be discerned. In the first, they occur embedded in the clastic matrix of their host breccias. In this case, the boundaries between both lithologies are sharp and the clastic matrix is characterized by a much smaller grain size (Figs. 3a and 4b). The proportions between these clasts and clastic matrix vary strongly. While NWA 5205 consists of about 90 vol% of cluster chondrite clasts, NWA 4572 contains less than 5 vol% of them (Table A1). Several samples investigated are obviously free of them. The proportions in the other meteorites are distributed between these extremes, indicating a continuous variation in the clast/matrix mixing ratio.

In the second occurrence, cluster chondrite clasts and their surroundings are quite similar (e.g., Krymka, NWA 5206), making it difficult to discern between the lithologies. The overall texture of these samples is suggestive of a conglomerate of small cluster chondrite clasts, separated by isolated chondrules with negligible amounts of clastic matrix (e.g., Krymka; Fig. 1).

Main Characteristics of Cluster Chondrites

Cluster chondrite clasts from all investigated samples share many typical characteristics, but can differ a lot in their mean chondrule size, even if they originate from the same breccia. Because NWA 5205 is the clearest example of this (Fig. 5), two cluster chondrite clasts from this meteorite with very different mean chondrule sizes (#1 and #2) were investigated in greater detail.

Figure 5.

 Differences in mean chondrule sizes between two cluster chondrite clasts from the same meteorite (NWA 5205; LL3.7). Obviously, a very effective size-sorting process, which acted prior to chondrule accretion and deformation, is needed to explain these extraordinary differences (sawn surfaces). Image courtesy of Peter Marmet, Switzerland.

The textures of these rocks indicate that they seemingly escaped secondary brecciation after they solidified from unconsolidated chondrule agglomerates. This distinguishes cluster chondrites from other textural units in UOCs.

The main common property of cluster chondrites is their close-fit texture of mutually indented chondrules, which makes these rocks so unique (Fig. 6). In the case of large chondrules (L and LL chondrites), this feature is easily visible to the naked eye. In thin sections, chondrule boundaries in cluster chondrites appear blurred, so that these units somewhat resemble chondritic clasts of petrologic type 4 or 5 (Fig. 7). Investigations by SEM prove the unequilibrated state of these samples (large variations in olivine and pyroxene chemistry) and document the intimate chondrule intergrowth very well (Fig. 8).

Figure 6.

 Texture of a cluster chondrite clast from NWA 5205 (LL3.7). It consists of coexisting deformed and undeformed chondrules with very low amounts of interchondrule matrix. Chondrules are indented and their number density exceeds that of closely packed spheres. (Sawn surface; width of field is 1 cm).

Figure 7.

 Examples for the appearance of cluster chondrite clasts in UOCs of different groups: a) H chondrite Devgaon (H3.8); b) L chondrite HaH 163 (L3.8); c) LL chondrite NWA 2461 (LL3). Polished thin sections; transmitted light.

Figure 8.

 Textures of cluster chondrite clasts in H, L, and LL chondrites. a) Devgaon (H3.8); b) a type 3 clast in NWA 869 (L3-6); c) Semarkona (LL3.00). SEM-BSE images.

The number density of chondrules in cluster chondrites exceeds that of closely packed spheres, thus chondrules appear to be transformed from spheroidal into polygonal or even amoeboid shapes, eliminating interchondrule voids. Examples for extremely deformed chondrules are shown in Figs. 9 and 10. These chondrules are squeezed between their neighboring chondrules and obviously rearranged their outer shape to fill formerly empty interchondrule space.

Figure 9.

 Examples for deformed chondrules in cluster chondrite clasts from different UOCs (a, c: NWA 5205, LL3.7; b, e, f: NWA 1756, LL3.10; d: NWA 5421, LL3.7). a) strongly deformed porphyritic pyroxene (PP) chondrule (type I; center) in intimate contact with an undeformed cryptocrystalline (C) chondrule (right center); b) Porphyritic pyroxene chondrule (type II; center, white outline), squeezed between two undeformed neighboring chondrules. c) Radial pyroxene (RP) chondrule (type I; center); d) Radial pyroxene (RP) chondrule (type II; center, white outline), impinged by an undeformed porphyritic olivine (PO) chondrule (type I; right); e) Porphyritic olivine-pyroxene (POP) chondrule (type I; center); f) Porphyritic olivine (PO) chondrule (type II; center). SEM-BSE images.

Figure 10.

 Extremely deformed chondrule (center; PO II) in the LL3.7 chondrite NWA 5205. The chondrule is squeezed between its neighboring chondrules and obviously rearranged its outer shape to fill formerly empty interchondrule space. SEM-BSE image.

Both spherical and deformed chondrules can be observed, where most chondrules are intact (Figs. 6–10) and only few distinct chondrule fragments are found. Many chondrules show sulfide-rich rims and in some cases, fine-grained chondrule rims occur (Fig. 11). Another characteristic is the very low abundance of interchondrule matrix compared with other UOC lithologies (Table 1).

Figure 11.

 Cluster chondrite clast in NWA 1756 (LL3.10). Deformed and undeformed chondrules are in intimate contact. Some of them are mantled by fine-grained dust (arrows). Porphyritic olivine (PO) chondrule (type II) with dust mantle (left); porphyritic pyroxene (PP) chondrule (type I; top center); porphyritic olivine–pyroxene (POP) chondrule (type II; right); cryptocrystalline (C) chondrule (bottom center). SEM-BSE image.

Based on these observations and the details given below, a definition for cluster chondrites can be given as follows: (1) close fit-texture of interlocking and mutually indenting chondrules; (2) content of fine-grained matrix (incl. fine-grained chondrule rims) less than 10 vol%; (3) chondrule content larger than 80 vol%; (4) mean degree of chondrule deformation larger than 5%; (5) frequent occurrence of highly deformed chondrules (degree of deformation of individual chondrules larger than 20%; and (6) absence of clastic matrix.

Components of Cluster Chondrites

All components of cluster chondrites are similar to those in other UOC lithologies, but distinct differences in modal abundances can be observed. The modal data for eight clasts from seven different UOCs are listed in Table 1. It is found that the amount of chondrules in cluster chondrites is distinctively higher (82–92 vol%) compared with literature data for other UOCs (65–75 vol%, Grossman et al. 1988; 60–80 vol%; Brearley and Jones 1998). All major textural types of ferromagnesian silicate chondrules (POP, PO, PP, BO, RP, GOP, C; type I and type II) are observed (Figs. 9–11). As in other UOCs, rare chondrule types also occur, including Al-rich chondrules, barred pyroxene chondrules, and agglomeratic chondrules (e.g., Weisberg and Prinz 1996), but are not part of the following statistics. Igneous chondrule rims are occasionally found, but seem to be restricted to undeformed chondrules. Bleaching of C and RP type chondrules, attributed to aqueous activity on the parent body (e.g., Grossman et al. 2000), is frequently found. Adhering compound chondrules are discernable and are mostly of type C.

The modal amount of sulfide + metal in cluster chondrites varies between 3 and 12 vol% (Table 1), comparable to literature data for other UOCs (5–12 vol%; Hutchison 2004). Metal mostly occurs interstitially to chondrules, and most of the sulfide is concentrated in chondrule rims. Within a given cluster chondrite clast subunits (chondrule grouplets) with high- and low-sulfide contents, respectively, occur directly adjacent to each other. One metal chondrule with an apparent diameter of 1.3 mm was found in NWA 5205.

Fine-grained matrix is clearly underrepresented in cluster chondrites, compared with occurrences in other UOCs. While literature data give values of 10–15 vol% (Huss et al. 1981), it is found that cluster chondrites contain only between 3 and 9 vol% matrix, with a mean of about 5 vol% (Table 1). There are two occurrences of fine-grained matrix. First, it is present in the interstices between chondrules. Second, fine-grained rims were found in some cases, indicating that some chondrules were dust-coated prior to their incorporation into cluster chondrites (Fig. 11). The properties of both matrix components have yet to be studied in detail, but seem to be similar to matrix descriptions from other UOCs (e.g., Huss et al. 1981; Alexander et al. 1989).

Distribution of Chondrule Textural Types

Chondrules in Cluster Chondrites

The textural types of 348 chondrules in six cluster chondrite clasts from five different UOCs were determined (Table 3). The fractions of BO, RP, GOP, and C type chondrules are similar to literature data for UOCs (Gooding and Keil 1981; Nelson and Rubin 2002). On the other hand, there are major discrepancies in the abundances of porphyritic chondrule types. The POP fraction of chondrules varies from 15% to 35%, compared to a range of 47–52% for other UOCs; i.e., there is no overlap in their abundances. The PO fraction varies between 20% and 41%, compared to 15–27% in UOCs, while the PP fraction of chondrules is between 12% and 34%, compared to 9–11%. It is found that the sum of the porphyritic chondrule types (POP + PO + PP) is 79 vol% (Table 3), nearly identical to the value given in the literature (81 vol%; Gooding and Keil 1981).

Chondrules in Clastic Meteorite Portions

The textural types of 99 chondrules in the clastic portions of NWA 4572 and NWA 5205 were determined as well (Table 3). Both samples contain cluster chondrite clasts in very different modal amounts (<5 vol% versus >90 vol%; Table A1). The fractions of BO, RP, GOP, and C type chondrules are similar to that in cluster chondrites and comparable to UOC literature data (Gooding and Keil 1981; Nelson and Rubin 2002). As in the case for cluster chondrites, there is a major discrepancy for porphyritic chondrules. The mean percentages of POP, PO, and PP chondrules in the clastic portions of NWA 5205 and in NWA 4572 (data in parentheses) are 34(38), 21(32), and 26(8) compared to 50, 21, and 10 as given in the literature. Again, the sums for porphyritic chondrule types (POP+PO+PP) given in the literature (81%; Gooding and Keil 1981) and obtained here for the clastic meteorite portions (81% for NWA 5205; 78% for NWA 4572) are nearly identical (see Table 3). Altogether, the data for the clastic portions are somewhat closer to UOC literature data than those for cluster chondrites clasts.

Apparent Chondrule Sizes

The mean apparent chondrule diameters show considerable variations between the investigated cluster chondrite clasts (Table 2). The smallest mean diameter (0.52 mm) was found in the clast SM-06-44-1-a (described in Metzler et al. 2011) from the L chondrite NWA 869. This size is in the range of values given for L chondrites in the literature (0.4–0.8 mm; Grossman et al. 1988; Rubin 2010). The largest mean size (1.38 mm) was measured in clast #1 from NWA 5205, an extraordinary example shown in Fig. 2. The largest apparent diameter of an individual chondrule (2.8 mm) was also found in this sample. The mean apparent chondrule sizes in the other cluster chondrite samples vary between 0.60 mm (Krymka) and 0.90 mm (clast #2 in NWA 5205). The mean apparent chondrule size in clasts from LL chondrites, excluding the data for the extraordinary clast #2, is 0.74 mm. Again, this value fits to the literature data for the corresponding group (LL chondrites; 0.57–0.9 mm; Grossman et al. 1988; Rubin 2010). The mean apparent chondrule diameter in the clastic portions of two UOCs is 0.90 mm.

Degree of Chondrule Deformation

The extraordinary chondrule deformation features are the main characteristic of cluster chondrites (Figs. 6–11). Sometimes, these deformations are so extreme that chondrules are transformed into amoeboid shapes (e.g., Figs. 9 and 10). To quantify this phenomenon and to compare cluster chondrite clasts with other UOC lithologies, the degree of chondrule deformation in cluster chondrite clasts and clastic portions was calculated. For details of this method, see above. The results are summarized in Table 2. Here, the mean values for both parameters are listed, together with the variability, i.e., the minimum and maximum values for individual chondrules. Additionally, the textural type of each chondrule was determined and the obtained data sets for 7 different chondrule textural types are listed in Table 3. To visualize these data and to find possible correlations, the apparent size (measured chondrule cut face in mm2) of each chondrule is plotted against its degree of deformation and shown separately for each sample in Fig. 12.

Figure 12.

 Apparent sizes (cut faces; mm2) of chondrules in thin sections versus their degree of deformation in six cluster chondrite clasts (a–f) from five different UOCs. The maximum degree of deformation is about 50% and small cut faces appear more strongly deformed than larger ones. Chondrules in the clastic portions of NWA 5205 (g) and NWA 4572 (h) are similar to each other, i.e., they are only weakly deformed and do not show a correlation between both parameters.

Chondrule Deformation in Cluster Chondrite Clasts

The data shown in Fig. 12 were obtained from 348 chondrules in 6 cluster chondrite clasts from 5 UOCs. The mean degree of chondrule deformation varies between 11% (NWA 5206) and 17% (clast #2, NWA 5205), with an overall mean of 15% (Table 2). The data spread for the deformation of individual chondrules is remarkably similar between all clasts and a maximum value of about 50% seems to exist, which is not exceeded. The values range between 1% and 56% deformation, where Krymka has the lowest maximum value (38%), and clast #1 from NWA 5205 has the highest (56%). These data are shown in Figs. 12a–f, where the similar spread of deformation values for all samples is clearly visible. Clast #1 from NWA 5205 (Fig. 12a) is peculiar concerning its unusually large chondrules. In all plots, a typical feature arises, namely an apparent relationship between measured chondrule size and chondrule deformation. Small chondrule cut faces tend to be more deformed than larger ones, which is most prominent in both clasts from NWA 5205 (Figs. 12a and 12b).

Chondrule Deformation in Clastic Portions

For comparison, the degree of chondrule deformation was measured on 99 chondrules from the clastic matrix of NWA 4572 and NWA 5205 (Figs. 12g and 12h). In these cases, the mean degree of chondrule deformation is consistently low, i.e., on the order of 5% (Table 2). Similar numbers (3–5%) were found by Nelson and Rubin (2002) (Table 2) who investigated chondrules in various (brecciated) LL3 chondrites. The maximum degree of deformation for individual chondrules is similar for both clastic portions, i.e., 14% for NWA 5205 and 16% in the case of NWA 4572.

Fractions of “Deformed” Chondrules

The values for chondrule deformation in the clastic portions are distinctly different from those for cluster chondrite clasts. To clarify this point, I define two chondrule classes, namely chondrules with deformations <20% and those with deformations >20%. Chondrules with deformations >20% are defined as “deformed”; i.e., they were influenced by the process that formed cluster chondrites. This boundary value was chosen because the largest chondrule deformation observed in clastic portions is 16%. This could be the maximum value of primary chondrule deformation, attributed to processes that influenced chondrules as isolated objects prior to accumulation. The fractions of deformed chondrules in the investigated clasts are listed in Table 2 and shown in Fig. 13. It turns out that NWA 1756 contains the highest fraction of deformed chondrules (37%), while NWA 5206 contains the fewest (10%), with a mean of 26% for all measured clasts. As shown above, the clastic portions are free of those chondrules.

Figure 13.

 Frequency of chondrules belonging to the deformation classes <20% and >20% (for definition see text). The data are obtained from six cluster chondrite clasts in five different UOCs and the clastic portions of NWA 5205 and NWA 4572. The ratios of chondrules from both deformation classes differ between the investigated clasts. All chondrules from the clastic portions show deformations below 20%. clch: cluster chondrite.

Deformation of Chondrules of Various Textural Types

Chondrules in cluster chondrites. To check whether the various textural chondrule types show differences in their physical properties, the maximum, minimum, and mean values for the apparent diameter and the degree of deformation were determined (Table 3). At first sight, there seem to be no systematic differences; the data for clast #1 from NWA 5205 are shown as an example (Fig. 14). All chondrule types that are represented by a sufficiently high number of chondrules (POP, PO, PP, BO, RP) exhibit similar ranges in the degree of deformation. Nevertheless, there seem to be subtle differences as indicated by Fig. 15a. In this graph, the mean degree of deformation is represented by a filled diamond and the spread of maximum and minimum values for individual chondrules is shown for each textural type. The spread of deformation values is similar for all chondrule types, starting at about 1% and ending up with values between 40% (RP) and 56% (POP). The statistics show that RP chondrules have a mean deformation of only 11%, which is lower than that for other textural types (13% to 18%); RP chondrules also show the lowest maximum values for individual chondrules.

Figure 14.

 Apparent size versus degree of deformation for chondrules of various textural types in cluster chondrite clast #1 from NWA 5205. This clast has a size of about 10 cm and is characterized by very large chondrules. All major chondrule textural types occur in deformed and undeformed shapes. The data were obtained from thin section measurements.

Figure 15.

 Mean values (filled diamonds) and data spread for the degree of deformation for 348 chondrules of various textural types in cluster chondrite clasts (a), for 47 chondrules in the clastic portions of NWA 5205 (b), and for 52 chondrules in the clastic portions of NWA 4572 (c). The number of chondrules for each textural type is shown in parentheses. For chondrule type abbreviations, see Table 3. The data in a) are obtained from cluster chondrite clasts in 5 different UOCs (Table 3). Chondrules in these clasts show high mean degrees of deformation (11% to 18%) and a large spread of data for individual chondrules (1% to 56%). In contrast to this, chondrules from the clastic portions of NWA 5205 (b) and NWA 4572 (c) show low mean degrees of deformation (2–7%) and a very restricted data spread for individual chondrules (0–16%).

Chondrules in clastic meteorite portions. In the same manner, the data for chondrules in the clastic portions of NWA 5205 and NWA 4572 were determined (Figs. 15b and 15c; Table 3). The data for both meteorites are quite similar and differ distinctly from those of cluster chondrites. The mean values for all chondrule types vary between 2% and 7%. The data spread for individual chondrules is very restricted and varies between 1% and 16%.

Three-Dimensional Visualization of Cluster Chondrite Textures

To visualize the texture of a typical cluster chondrite in 3D space, two different attempts were made. First of all, a μ-CT scan was performed on a sample of NWA 5205 and the resulting stack of images was transformed into a movie (Fig. S1; courtesy by D. Hezel, University of Cologne). Here, type I and type II chondrules are easily discernible by their darker and lighter appearance, respectively, and both types can be recognized in deformed and undeformed shapes. Furthermore, chondrules mantled by sulfide-rich layers (white outlines) occur beside chondrules without those layers.

As a second attempt, a square column (1 × 1 × 1.8 cm) of NWA 5205 was prepared and successively ground down in 120 steps of 80 μm each (Fig. S2). Every step was photographed and the stack of images was also transformed into a movie (Fig. S3). Again, the typical close-fit texture of interlocking chondrules and the coexistence of deformed and undeformed chondrules are clearly visible.

Discussion

Formation of Cluster Chondrite Clasts

All investigated meteorites (58 UOCs; Table A1) are brecciated rocks, irrespective of the presence or absence of cluster chondrite clasts. The latter unambiguously formed from pre-existing larger rock units of unknown original dimensions. Based on the overall textures, two different modes of breccia formation can be discerned.

Genomict (Fragmental and Regolith) Breccias

All genomict breccias and several of the “unbrecciated” samples from Table A1 are fragmental or regolith breccias and the product of hypervelocity impacts into consolidated or unconsolidated chondritic lithologies on parent body surfaces. This is proven by the existence of clasts of coherent chondritic rocks, embedded in fine-grained clastic material. While one of these breccias (NWA 869; L3-6 regolith breccia; Metzler et al. 2011) contains cluster chondrite clasts, all others (23 samples) are apparently free of them. It is striking that these clasts seem to be so rare in this breccia type. Either only small amounts of cluster chondrite lithologies were involved in their formation or impact processes have destroyed most of them later on. A hint to the latter could come from the observation that a large number of clasts in these breccias are not rock fragments, but isolated abraded chondrules and chondrule fragments. In many cases, isolated chondrules with nearly spherical shapes are admixed (Figs. 3a, 4a, and 4b), and were obviously never part of consolidated rock until now. Comparable observations were made on the CM chondritic regolith breccias Murchison and Murray (Metzler et al. 1992).

Accretionary Breccias

Two of the investigated UOCs (Krymka, NWA 5206) and several samples from Table A1 show textures that indicate only low degrees of brecciation and mixing. They consist of an assortment of small cluster chondrite units and isolated spherical chondrules with only negligible amounts of clastic matrix. These rocks probably represent accretionary breccias (see Keil 1982; Scott and Taylor 1982; Bischoff et al. 2006). In this scenario, fragments of cluster chondrites formed either by in situ comminution of larger source rocks inside parent bodies or by disruption and reassembly of the latter (Scott and Taylor 1982 and references therein).

Uniqueness of Cluster Chondrites

The properties of cluster chondrites differ distinctly from typical UOCs in three important aspects. First, the occurrence of deformed chondrules with >16% deformation is clearly restricted to cluster chondrite clasts (Table 2; Fig. 12). Second, cluster chondrites show the highest modal chondrule abundances (82–92 vol%; Table 1) among all known chondrites (see Grossman et al. 1988; Brearley and Jones 1998). Third, as an effect of the high chondrule abundances, the amounts of matrix are by far lower than usually observed in UOCs (3–9 vol% versus 10–15%; Table 1). To visualize these differences, the mean degrees of chondrule deformation for 6 cluster chondrite clasts and two clastic portions are plotted against their modal amount of matrix (clastic and nonclastic) in Fig. 16. In this graph, the literature data on UOCs are shown as a shaded field. The data for this field were obtained as follows: the axial ratios of 719 intact chondrules from five LL3 chondrites were measured by Nelson and Rubin (2002). It turned out that the mean axial ratio in these samples is 1.2 ± 0.18, which corresponds to a mean degree of deformation of about 3%. Furthermore, Martin and Mills (1976, 1978) inspected a great number of isolated chondrules from Bjurböle (L/LL4), Chainpur (LL3.4), and Allegan (H4), and found that chondrules depart from sphericity by only small amounts. This is also indicated by the chondrule outlines in thin section maps of these meteorites (Hughes 1978). The mean degree of chondrule deformation in Chainpur, obtained by my own measurement of chondrule outlines in the corresponding thin section map in Hughes (1978), is only 5% (60 chondrules measured). These data (3–5% chondrule deformation) are combined with the data on modal amounts of matrix in UOCs (10–15 vol%; Huss et al. 1981) to construct the shaded field in Fig. 16. The obtained data for Chainpur (filled square) are also shown for comparison. It becomes evident from this figure that cluster chondrites (shaded triangles) plot apart from the literature field. The Chainpur data are close to literature values and the clastic portion of NWA 4572 (filled circle) also plots not far away. The clastic portion of NWA 5205 is separated due to its higher modal amount of matrix. A formation of such lithologies and those with even higher amounts of matrix by increasing brecciation is conceivable (e.g., Romstedt and Metzler 1994).

Figure 16.

 Modal amount of matrix (clastic and nonclastic; see Table 1) versus mean degree of chondrule deformation (see Table 2) in six cluster chondrite clasts from five UOCs (for samples see Table 2). Additionally, the data for the clastic portions of NWA 5205 and NWA 4572 are shown. The shaded field represents the spread of literature data for typical UOCs (matrix data: Huss et al. 1981; data on chondrule deformation: Nelson and Rubin 2002; Hughes 1978). Cluster chondrites plot apart from other UOCs and clastic meteorite portions. The mean degree of chondrule deformation is different for each clast. The properties of clastic portions can be explained by increasing brecciation.

Possible Differences in the Abundances of Chondrule Textural Types

The modal data on chondrule textural types differ between cluster chondrites (Table 3) and literature data on UOCs. While the fractions of BO, RP, GOP, and C type chondrules are in gross accordance with the literature (Gooding and Keil 1981), those for the porphyritic chondrule types are not (see above). On the other hand, the fact that the sum of porphyritic chondrule types (POP+PO+PP) from the literature (81%) and those obtained from cluster chondrites (79%) is nearly identical (see Table 3) could be taken as a hint that the samples are not that different as the discrete data might indicate. An explanation for this could be the differences in analytical methods of investigation. While the data by Gooding and Keil (1981) were obtained by optical microscopy, the data presented here are obtained exclusively by SEM techniques (BSE images).

The data in Table 3 show considerable variations in chondrule type fractions between various cluster chondrite clasts. This is especially evident in the case of the clasts #1 and #2 from NWA 5205. While the fraction of BO chondrules is 14% and 2%, respectively, the fraction of C chondrules is 1% and 7%, respectively. In my view, these data, obtained from 157 chondrules, reflect true variations between these clasts.

Mean Apparent Chondrule Sizes

The mean apparent chondrule sizes in cluster chondrite clasts from various UOCs differ considerably (Table 2), namely from 0.52 mm (NWA 869) to 1.38 mm (clast #1 in NWA 5205). This even holds for clasts from the same meteorite as other clasts from the latter meteorite have mean chondrule sizes of 0.90 mm (clast #2; Table 2) and 0.60 mm (left clast in Fig. 5).

It can be concluded that mean chondrule sizes in UOCs seem to be more diverse than previously thought. Literature data probably describe mixtures of chondrules from cluster chondrites and clastic portions of these meteorites. Obviously there is a need to establish new data sets for this parameter, separately for petrologically well-defined lithologies.

Chondrule Size Sorting and Possible Stratification of Parent Bodies

A very efficient size-sorting process, possibly caused by chondrule–gas interaction in the disk, was active between chondrule formation and accretion. This can be deduced from the observation that cluster chondrite clasts with very different mean chondrule sizes can be found (e.g., clasts #1 and #2 from NWA 5205; Table 2; Fig. 5). It is astonishing that rocks with these differences, but otherwise similar properties, finally ended up in the same breccia. It seems possible that these clasts originate from different units or layers of the same parent body and were subsequently mixed into the present breccia by impact processes. For instance, the grain size of infalling chondrule material could have varied with time and produced subsequent layers with different mean chondrule sizes.

Aspects of Chondrule Deformation

All chondrule textural types occur with a similar spread of deformation (Table 3; Fig. 15a), i.e., the deformation process affected all of them to about the same extent. RP chondrules with their low mean deformation (11%; Table 3; Fig. 15a) are exceptional, and also BO and POP chondrules have somewhat lower mean values than PO, PP, and GOP chondrules. Due to the large number of chondrules (e.g., 36 RP, 101 POP, and 109 PO), this finding appears to be real.

The fractions of deformed chondrules (>20% deformation by definition; see above) vary between 10% and 37% between the various cluster chondrite clasts (Table 2; Fig. 13). This means that these clasts are remnants of parental rocks whose chondrules were affected by the deformation process to varying degrees. Low amounts of deformed chondrules either mean that the parental rock formed from less deformable (cooler?) chondrules or the pressure was less intensive during this stage. It is not clear whether the chondrule deformation and compaction occurred early while the material was in the shape of small-sized (<10 cm) chondrule clusters in space or later at the time of planetesimal accretion (see formation model below).

Limit of chondrule deformation. From Fig. 12 and Table 2, it is evident that the degree of chondrule deformation in cluster chondrites is limited. The strongest deformation measured for an individual chondrule is 56%, while many others reach roughly 50%. Obviously, geometric limitations hindered more extreme deformations, i.e., a given chondrule had only limited space to deform in. It obviously could protrude only into the immediately adjacent space between its neighboring chondrules. Further deformation obviously stopped, when the deforming material of one chondrule encountered rigid material or deforming material of another, until the entire available space was filled. Such a behavior would be expected in the case of hot pressing.

Low Matrix Abundances as the Central Boundary Condition for Cluster Chondrite Formation

Irrespective of the exact formation process for cluster chondrites, the in situ deformation of spherical chondrules requires a very low abundance of co-accreted fine-grained matrix as a central boundary condition. A more or less empty interchondrule space is clearly needed to accommodate the protruding parts of deforming chondrules (see Fig. 10). An addition of fine-grained matrix results in the reduction of interchondrule space and complicates chondrule deformation. In the case of densely packed, equally sized spherical chondrules, this space is about 26 vol% and gets reduced by mixing of chondrules of various sizes.

Provided that cluster chondrite textures are actually the result of hot chondrule accretion and compaction, the question arises whether this process would be recognizable in the case of larger amounts of matrix. I speculate that comparable chondrule deformations cannot be expected in chondrites with high amounts of co-accreted matrix (e.g., some carbonaceous chondrites, R chondrites), even if the P- and T-conditions are comparable to those during cluster chondrite formation. In these cases, all interstitial space is a priori consumed by matrix that would hinder the expansion of deformable chondrules into interchondrule voids.

Mechanisms for Chondrule Deformation

Petrologic observations reveal that the deformation of chondrules was obviously caused by more than a single mechanism. It is found that several chondrules were seemingly deformed in a viscous state, indicated by the absence of internal brecciation (Fig. 17a). Others are obviously affected by brittle deformation, deduced from strong internal cataclasis and reorientation of fragments (Fig. 17b). It is conceivable that the agglomerates of deformable chondrules cooled down quickly during the compaction process. In this case, partly deformed chondrules that cooled below the glass transition point of their mesostasis were no longer viscously deformable and reacted in a brittle fashion to any subsequent stresses from then on.

Figure 17.

 Viscous versus brittle chondrule deformation. Two examples of PO II chondrules from a) Krymka (LL3.2) and b) NWA 1756 (LL3.10) are shown. In the case of Krymka, the chondrule obviously deformed in a viscous state without internal brecciation (black cracks are preparation artifacts). The chondrule in NWA 1756 (white outline) shows brittle deformation with strong internal cataclasis and reorientation of fragments. SEM-BSE images.

Interestingly, in all cluster chondrite clasts, there are nearly spherical chondrules of many textural types that obviously escaped the deformation process (e.g., Figs. 7, 8c, 9a, and 9d). Intuitively, one might argue that those chondrules were cold and rigid at the time of compaction, but it is difficult to imagine those temperature variations on an mm scale. An argument against considerable temperature differences between deformed and undeformed chondrules at the time of encounter is the absence of quench textures at their contact faces.

Another explanation for a different mechanical behavior could be differences in the internal strength between partly molten chondrules of different textural types. At a given temperature, semiliquid BO and RP chondrules may react more or less rigidly due to their internal framework of interlocking olivine and pyroxene dendrites, while chemically identical porphyritic chondrules still may be viscously deformable due to slidable isometric crystals floating in the liquid mesostasis. This would explain the lower mean degree of deformation for BO and RP chondrules (see above).

The nature of the compression force for the compaction of cluster chondrites and the deformation of chondrules remains to be identified. Was the overburden pressure by gentle gravitational settling of chondrules strong enough to cause compaction and chondrule deformation? According to Weidenschilling and Cuzzi (2006), the pressure in the center of a body with a radius of 100 km is only 10 bar. In my view, the pressure resulted from continuously accreting material with fairly high relative velocities, leading to collisional compression and compaction. On the other hand, no indication of unidirectional pressure has been found in this study.

Close Linkage Between Chondrule Formation and Chondrite Accretion?

From the modal data in Table A1, a first-order estimate can be made that UOCs as a whole consist of about 5–10 vol% cluster chondrite material. This low number could mean that in most cases, chondrite accretion occurred only after the chondrules solidified, i.e., more than a few days after chondrule formation. On the other hand, much of the originally existing cluster chondrite material could have been destroyed during asteroidal processing. Irrespective of the cluster chondrite frequency, the more important point is the existence of these lithologies. Provided that the interpretations given here are right, this proves that a process of hot chondrule accretion basically took place. In this case, we have a severe constraint on chondrule-forming conditions as the results would point to a close linkage between chondrule formation and chondrite accretion. This would favor models of local chondrule formation, where only restricted parts of the protoplanetary disk were involved.

There are several other lines of evidence for local chondrule formation and rapid accretion. One observation is that the chemical compositions of chondrules and fine-grained matrix in carbonaceous chondrites are complementary in many cases. Although both components are chemically very different, taken together in a given meteorite, a near solar composition is found (e.g., Wood 1987; Huss 1988; Palme et al. 1993; Hezel and Palme 2008, 2010). Furthermore, a high Na partial pressure in the ambient gas phase obviously prevented Na-loss during chondrule heating, implying a high density of chondrules in space prior to accretion (Alexander et al. 2008; Hewins and Zanda 2012; Hewins et al. 2012). The latter conclusion is also drawn by Hezel et al. (2010) based on the limited fractionations of Fe isotopes in chondrules from CV chondrites. Altogether, these observations point to the fact that chondrites accreted before chondrule reservoirs were homogenized by turbulence and radial mixing (e.g., Jones 2010) and that chondrule formation and chondrite accretion could have been closely linked (Alexander 2005a; Alexander et al. 2008; Alexander and Ebel 2012).

Sanders and Scott (2012; this volume) state that the formation of cluster chondrites would fit to their scenario of chondrule formation by low-velocity collisions between molten planetesimals, which is modelled in detail by Asphaug et al. (2011).

Proposed Model for Cluster Chondrite Formation

Based on the data presented here, the following model for cluster chondrite formation is proposed, that involves four evolutionary stages (Fig. 18).

Figure 18.

 Proposed model for cluster chondrite formation. A population of free-floating hot chondrules (a) possibly formed first units of chondrule clusters in the protoplanetary disk (b) and accreted hot to form a planetesimal (c). During compaction by ongoing accretion, many chondrules deformed extensively, resulting in massive bedrock with cluster chondrite texture. This rock was later fragmented by secondary impact brecciation where clasts of cluster chondrites formed (d). During this stage, spherical chondrules were admixed to the resulting breccias.

Stage A

A high-temperature event produced a population of molten, free-floating chondrules in the protoplanetary disk. The observed variations between different cluster chondrite clasts (see Fig. 5) document that chondrule populations with very different mean chondrule sizes existed. If these differences were not produced by the high-temperature events themselves, chondrules were size-sorted on their way from the formation region to the accreting planetesimal, possibly by chondrule–gas interaction.

Stage B

As the chondrule density must have been extremely high in the region of chondrule accretion, it is conceivable that chondrules encountered and interacted with each other to form small first cluster chondrite units shortly before they got trapped into the accreting planetesimal. A clue to whether this happened could come from the observation that different subunits (chondrule grouplets) with high- and low-sulfide contents (mostly sulfide chondrule rims), respectively, occur directly adjacent to one another in a given cluster chondrite clast.

Stage C

During this stage, hot chondrules formed a planetesimal or reached the surface of an already existing body. This obviously took place in a region of the protoplanetary disk where the density of fine-grained dust was remarkably low, as only about 5 vol% of this material accreted into the forming rocks as fine-grained matrix and rims. The hot chondrules were subsequently deformed in a hot stage. According to the most plausible cooling rates (1–1000 k h−1; Hewins, personal communication), chondrule accretion occurred only hours to a few days after their formation. During this stage, cluster chondrites with their typical textures formed as large-scale lithologies.

Stage D

Finally, an increase in the number and relative velocities of impacting bodies led to impact brecciation and the formation of typical chondritic breccias. Clasts of cluster chondrites in these breccias are relicts of former large-scale bedrocks, namely the rocks formed during stage C. During stage D, further chondritic components are admixed to the breccias which is documented, e.g., by the existence of undeformed chondrules in the clastic matrix of NWA 5205 (Fig. 4b). Additionally, the statistics on this meteorite indicate significant addition of PP chondrules during brecciation. While cluster chondrite clasts in this meteorite contain 12–14% PP chondrules, they make up 26% of all chondrules in its clastic matrix (Table 3).

It is not concluded here that the majority or even all of UOC matter passed through these evolutionary stages. Many chondrules in UOCs that reside outside of cluster chondrite clasts show clear indications for undisturbed crystallization as isolated objects in space, representing nearly ideally shaped spheres. Those chondrules dominate most UOCs, which could be the reason that the deformed chondrules and their host lithologies did not attract much attention to date.

Alternative Scenarios

From the present state of research, the above model seems to be the most suitable. Nevertheless, several alternative formation scenarios have been presented in the literature and are discussed below.

Coincidental fitting. The simplest explanation for cluster chondrite formation is coincidental fitting of irregularly shaped chondrules, as argued by Rubin and Brearley (1996). There are at least two arguments against this hypothesis. First, close inspections reveal that in most cases, the chondrule fitting is by far too exact as to be explained by such a process (see Fig. 6). Unavoidable distance variations between two neighboring chondrules are seldom observed. Second, the origin of myriads of chondrules with enormous degrees of deformation has to be explained, as there is no need for a droplet of silicate liquid to deform in this manner when it crystallizes independently in space. For these reasons, I rule out coincidental fitting as the main formation process for cluster chondrites.

Jostling and wedging during chondrite compaction. It is suggested by Rubin and Brearley (1996) that the close-fit textures formed by jostling and wedging during chondrite compaction. Although these processes certainly occurred and contributed to the densification of chondrule agglomerates to some extent, they do not have the ability to explain the extraordinary exact fitting between chondrules. The unusual chondrule deformations cannot be explained by this scenario, as well.

Shock metamorphism. It is proposed by Rubin and Brearley (1996) that deformed and indented chondrules are the product of uniaxial deformation during shock. They argue that Sneyd et al. (1988) found a correlation between shock facies and percent flattening of chondrules in various ordinary chondrites. A special case is described by Rubin and Swindle (2011) who infer that strongly flattened chondrules in the LL5 chondrite LAP 04581 are the result of an oblique impact. In this case, the aspect ratio of deformed chondrules range from 1.4 to 3.0, which corresponds to a maximum degree of deformation of about 21%. This deformation is much less than observed for many chondrules in cluster chondrites (see Fig. 12) and no indications for irregular chondrule deformations and chondrule indentations are described. None of the investigated cluster chondrite clasts show chondrule flattening, which clearly discriminates them from shock-deformed chondrites.

Short period of extensive thermal metamorphism.Grossman et al. (1988) propose that chondrules were reheated by an unknown heat source on the parent body, where the corresponding processes and the textural setting of the rocks in question are not addressed. In this scenario, chondrules cooled down, solidified, concentrated, and accreted in the shape of cold spheres. In a second step, the temperature of this layer or unit rose dramatically to exceed the glass transition points of chondrule mesostasis glass. During this process, the pressure must have been high enough to transform the chondrule assemblages into the highly compacted state that is typical for cluster chondrites. In a third step, the material has to cool down fast to keep up the very low petrologic type of the processed material (e.g., LL3.00 for Semarkona; LL3.10 for NWA 1756) and to preserve presolar silicates (see Metzler 2011b). An underlying bedrock, internally heated by the decay of 26Al, seems highly improbable as the period of reheating would be by far too long and should have ended up with OCs of higher petrologic type. Reheating by contact with impact melt lithologies would be a more plausible explanation, but would not explain the compact nature of these rocks.

Pressure induced diffusion. Close-fit textures in ordinary chondrites have been ascribed to a process called pressure-induced diffusion (PID), “a form of diffusive mass transport analogous to pressure solution” (Skinner 1989). Intensive investigations revealed no indication for such a process. As is evident from Fig. 11, many chondrules in cluster chondrite clasts are still isolated from each other by fine-grained dust rims and sulfide-rich rims, which should disappear during this process. Similar arguments against chondrule deformation by PID were already given by Hutchison (1996b). From the observed textures, the formation of cluster chondrites by this process can be ruled out.

Misinterpretation as compound chondrules. Some authors argued that cluster chondrite textures are misinterpreted compound chondrules (e.g., Rubin 1995). These occur with abundances of 2.5 to >5% in chondrites (e.g., Gooding and Keil 1981; Wasson et al. 1995; Ciesla et al. 2004) and usually consist of a rigid central chondrule with an adhering or enveloping secondary chondrule. Compound chondrules formed when isolated objects encountered each other in space and fused together due to the molten state of at least one chondrule. On the contrary, cluster chondrites obviously formed by concentration of huge amounts of hot chondrules during chondrite accretion. Although both cluster chondrites and compound chondrules consist of an assortment of interacted chondrules, there are cardinal differences between them.

  • 1 In cluster chondrites, sulfide-rich rims and fine-grained matrix rims separate most chondrules from each other. Surface-fused chondrule contacts, which are typical for compound chondrules, are seldom observed.
  • 2 Many compound chondrules are made of chondrules of the same textural type (e.g., Wasson et al. 1995). This is never the case for cluster chondrites, which consist of more or less typical mixtures of chondrule textural types (e.g., Fig. 14; Table 3).
  • 3 The most important difference is that compound chondrules consist of only a few adhering chondrules (2–16; Bischoff and Weyrauch 2012; this volume), while cluster chondrite clasts with about a million chondrules are found. This estimation comes from the largest clast in NWA 5205 (Fig. 2) that has a diameter of 10 cm.

Furthermore, compound chondrules were unambiguously identified as usual ingredients of cluster chondrites, i.e., they are discernable from cluster chondrite textures due to their typical characteristics. On the other hand, a significant number of pretended compound chondrules appear in cluster chondrites as the result of cutting effects, due to intimate chondrule intergrowths (e.g., Fig. 6). Thus, it is possible that a certain fraction of compound chondrules described in the literature (e.g., Lauretta et al. 2002) in reality represents deformed chondrules from cluster chondrite clasts.

Cluster Chondrites: Primary Accretionary Rocks with Unaltered Textures

One aim in UOC research is the identification of primitive units in these rocks to learn about chondrule-forming processes. In my view, cluster chondrites belong to these primitive units. They could represent relicts of the earliest rock generations, possibly formed only hours to a few days after chondrule-forming events. Even if it turns out that an alternative model explains the observed chondrule deformations better than hot chondrule accretion, the internal texture of cluster chondrites points to their primitive, i.e., mechanically unaltered state. These rocks seem to have preserved an earlier state of chondrite accretion than many other UOCs and probably represent primary accretionary rocks with unaltered textures.

Open Questions

Volatiles, Organics, and Presolar Grains in Fine-Grained Matrix and Rims

Fine-grained matrix and rims are present in cluster chondrites, i.e., they survived the accretion process. In the case of the hot accretion model, these components would have been trapped between the semiliquid chondrules during cluster chondrite formation. Fine-grained matrix and rims are the carriers for volatiles (e.g., Alexander 2005b), organics (e.g., Alexander et al. 2007), and presolar grains (e.g., Huss 1990) in UOCs. It will be important to investigate these components in cluster chondrites to obtain information on their possible interaction with hot chondrule material. Those data on volatiles and organics are missing to date, but presolar silicate and oxide grains have been already identified in fine-grained rims in a cluster chondrite clast from NWA 1756 (LL3.10; Metzler 2011b). Cluster chondrites must have cooled fairly fast to preserve their inventory of presolar silicates and their low petrologic type (e.g., Semarkona; LL 3.00; Fig. 8c) (Zanda et al. 2002).

Missing Indications for Preferred Chondrule Orientation

In this study, no distinct preferred orientations of chondrules were found, although this should be expected when viscously deformable chondrules encounter each other or accrete to a planetesimal surface with fairly high velocities. Either those textures are absent or not very obvious. There is a need for quantitative stereometric investigations to clarify this point. One hint on the existence of those textures is given by Sanders and Hill (1994) who report a parallel orientation of flattened molded chondrules in the Bovedy (L3) chondrite.

Summary and Conclusions

  • 1 A specific chondritic lithology with a unique texture has been identified in a large number of UOCs and is termed “cluster chondrite.” It occurs in the shape of lithic clasts in unequilibrated H, L, and LL chondrites and was found in 41% of the 58 investigated UOCs. The modal abundances of cluster chondrite clasts in the samples vary from 0 to >90 vol% and their sizes range from <1 mm to about 10 cm. From the modal data, the very gross but conservative estimation can be made that UOCs as a whole consist of 5 to 10 vol% cluster chondrite material. Cluster chondrites share the following characteristics:
  •  Close-fit textures, where many chondrules mutually indent each other
  •  Coexistence of deformed and undeformed chondrules
  •  Rareness of distinct chondrule fragments
  •  Low modal abundance of matrix (3–9 vol%)
  •  High modal abundance of chondrules (82–92 vol%)
  •  Absence of clastic matrix
  • 2 The internal textures of cluster chondrites indicate that they escaped secondary brecciation after they solidified from unconsolidated chondrule agglomerates. This distinguishes cluster chondrites from the majority of UOCs as the latter represent mechanically reworked rocks.
  • 3 The components of cluster chondrites are basically similar to those in other UOCs, but distinct differences in modal abundances can be observed. These rocks show the highest modal chondrule abundances among all known chondrite classes.
  • 4 The mean apparent chondrule diameters in cluster chondrites in L and LL chondrites are between 0.52 and 1.38 mm. Three clasts with very different mean chondrule sizes were found in the same breccia (NWA 5205). These clasts possibly represent different units or layers of the same parent body, i.e., the grain size of infalling chondrule material could have varied with time and produced subsequent layers with different mean chondrule sizes.
  • 5 Chondrule deformations result from accretion and compaction, not from postaccretional processes, as advocated by Rubin and Brearley (1996). The mean degree of chondrule deformation in cluster chondrites varies between 11% and 17%, with an overall mean of 15%. It turned out that small chondrule cut faces tend to be more deformed than larger ones, revealing a kind of negative correlation between apparent chondrule sizes and their degree of deformation. The spread of data for the deformation of individual chondrules is remarkably similar between all clasts and a maximum value of about 56% seems to exist which is not exceeded. Obviously, geometric limitations hindered more extreme deformations, i.e., a given deforming chondrule had only limited space for expansion. All chondrule textural types occur in both deformed and undeformed shapes. Radial pyroxene (RP) chondrules have a mean deformation of only 11%, which is lower than that for others (13% to 18%). In the case of chondrules from the clastic meteorite portions, the mean degree of chondrule deformation is much lower (about 5%), and the maximum degree of deformation for individual chondrules is only 16%. This value is taken as the maximum value for chondrule deformation caused by events that influenced chondrules during their isolated existence in space.
  • 6 It is concluded that the in situ deformation of spherical chondrules requires a very low abundance of co-accreted fine-grained matrix as a central boundary condition. A more or less empty interchondrule space is needed to accommodate the protruding parts of deforming chondrules.
  • 7 Chondrule deformation was caused by more than a single mechanism. It is found that several chondrules were seemingly deformed in a viscous state, while others are obviously affected by brittle deformation.
  • 8 A model for cluster chondrite formation is proposed, which involves four evolutionary stages. First, a high-T event of unknown nature produced a population of molten chondrules, free floating in the protoplanetary disk. In a second (optional) step, chondrules possibly formed small cluster chondrite units in space, shortly before they got trapped into the accreting planetesimal. In a third stage, hot chondrules with only small amounts of co-accreting matrix formed a planetesimal or reached the surface of an already existing body. The hot chondrules were subsequently deformed in a hot stage. In my view, the pressure was caused by continuously accreting material with fairly high relative velocities, leading to collisional compression and compaction. According to the accepted cooling rates, these processes occurred only hours to a few days after chondrule formation. During the final stage, impacting bodies led to brecciation and the formation of typical chondritic breccias with admixed clasts of cluster chondrites.
  • 9 Cluster chondrites are remnants of primary accretionary rocks of unknown original dimensions. They possibly represent relicts of the earliest rock generations of chondritic planetesimals. Their ubiquitous occurrence could mean that hot chondrule accretion was a widespread process in the protoplanetary disk. The components of a given cluster chondrite clast have the potential to be genetically much more closely linked to each other than those in any other textural setting. If this is really the case, cluster chondrites could be treated as samples with a less complex formation history than the majority of UOCs. In the best case, they formed by a single chondrule-forming event, instantly followed by planetesimal accretion. Then we would get insight into very early rock-forming processes of short duration and limited spatial extension. Hence, selecting those rocks and their components for further investigations of any sort could enhance our understanding of chondrule formation and planetesimal accretion.
  • 10 If it turns out that chondritic rocks actually formed from chondrules that were still hot and deformable, i.e., presumably not older than hours to a few days, we would have a severe constraint on chondrule formation conditions. In this case, the results indicate that chondrule-forming heating events and the accretion of chondritic bodies were closely linked in time and space. This would favor models of local chondrule formation, where only restricted parts of the protoplanetary disk were involved. This could help to favor or discard existing models on chondrule formation and planetary accretion.

Acknowledgments— I am grateful to Dieter Stöffler, Falko Langenhorst, and Herbert Palme for very helpful discussions, and to Ursula Heitmann for her thorough technical assistance. I would like to thank Roger Hewins and Conel Alexander for their very constructive reviews, which improved the manuscript. This work was financed by the German Research Foundation (DFG) within the Priority Program “The First 10 Million Years of the Solar System—a Planetary Materials Approach” (SPP 1385; ME 1115/8-1).

Editorial Handling— Dr. Alexander Krot

Appendix

Table A1. Statistics on the occurrence, maximum size, and modal abundance of cluster chondrite clasts in 58 UOCs.
MeteoriteGroup/typeGenomict brecciaClch clasts presentMaximum size of clch clasts [mm]Modal abundance of clch clasts [vol%]
  1. clch = Cluster chondrite; n.d. = not determined due to the small size of the thin section.

DevgaonH3.8 X   4 5–10
DhajalaH3.8 X   3 5–10
Tanezrouft 028H3    
Acfer 039L3.8    
Acfer 080L3.9 X   2 5–10
Acfer 133L3-5X   
Acfer 248L3.9/4 X   2 5–10
Aguemour 009L3.8 X   3 5–10
CenicerosL3.7 X   2 5–10
DaG 085L3-4X   
DaG 149L3-5X   
DaG 216L3    
HaH 163L3.8 X   610–50
NWA 869L3-6XX   8 5–10
NWA 900L3-6X   
NWA 2441L3-6X   
NWA 2453L3-6X   
NWA 2461LL3 X   710–50
NWA 2470L3-4X   
NWA 2542L3-6X   
NWA 2545L3-6X   
NWA 2547L3-5X   
NWA 3345L3-6X   
NWA 3347L3-5X   
NWA 3361L3-6X   
NWA 3367L3-6X   
NWA 4572LL3    
NWA 4691L3-6X   
NWA 6016L3.7 X   6 5–10
Sahara 98660L3-6X   
Sahara 98683L3    
Tanezrouft 030L3-5X   
Tanezrouft 038L3.7    
Tanezrouft 040L3.9    
Tanezrouft 041L3.8    
Tanezrouft 042L3.7    
Adrar 003L/LL3.1 X   5 5–10
DaG 264LL(L)3-6X   
DaG 277L(LL)3-5X   
NWA 5075L(LL)3    
Sahara 98035LL(L)3 X   2 5–10
Sahara 98316LL(L)3 X   3 5–10
Sarir Qattusah 001LL(L)3    
Acfer 160LL3.8-6X   
Adzhi-BogdoLL3-6X   
DaG 022LL3-6X   
DaG 180LL3.9 X   510–50
KrymkaLL3.1 X   4 5–10
NWA 1756LL3.10 X   510–50
NWA 2459LL3-5X   
NWA 2461LL3 X   3 5–10
NWA 4522LL3.5 X   13 5–10
NWA 4572LL3.6 X   1<5
NWA 5205LL3.2 X   100>90
NWA 5206LL3.05 X   810–50
NWA 5421LL3.7 X   3250–90
NWA 6212LL3 X   710–50
SemarkonaLL3.00 X   4n.d.

Ancillary