Macrochondrules in chondrites—Formation by melting of mega-sized dust aggregates and/or by rapid collisions at high temperatures?

Authors


*Corresponding author. E-mail. bischoa@uni-muenster.de

Abstract

Abstract– Seventy-four macrochondrules with sizes >3 mm were studied. Considering the extraordinary size of the chondrules (occasionally achieving a mass of 1000 times (and more) the mass of a normal-sized chondrule), the conditions in the formation process must have been somewhat different compared with the conditions for the formation of the common chondrules. Macrochondrules are typically rich in olivine and texturally similar to specific chondrule types (barred, radial, porphyritic, and cryptocrystalline) of normal-sized chondrules. However, our studies show that most of the macrochondrules are fine-grained or have elongated crystals (mostly BO, RP, and C), which lead to the assumption that they were once totally molten and cooled quite rapidly. Porphyritic chondrules belong to the least abundant types of macrochondrules. This distribution of chondrule types is highly unusual and just a reverse of the distribution of chondrule types among the typical-sized chondrules in most chondrite groups except for the CH and CB chondrites. New chondrule subtypes (like radial-olivine [RO] or multi-radial [MR] chondrules) are defined to better describe the textures of certain large chondrules. Macrochondrules may have formed due to melting of huge precursor dust aggregates or due to rapid collisions of superheated melt droplets, which led to the growth of large molten spherules in regions with high dust densities and high electrostatic attraction.

Introduction

Most primitive and weakly altered chondrites consist of abundant chondrules. This characteristic makes chondrules important for the investigation of the early solar system processes. The textures and compositions of the chondrules allow conclusions to be drawn about the formation processes of chondrules and chondrites. Concerning texture and composition, different types of chondrules can be distinguished. Based on their textures, chondrules are grouped into porphyritic and nonporphyritic chondrules (e.g., Gooding and Keil 1981). Considering their chemical compositions, other classifications like Type I, Type II, or Al-rich chondrules are also used in literature (e.g., McSween 1977; Bischoff and Keil 1983a, 1984). We have carried out a textural study on the characteristics of unusually large chondrules within different classes of chondrites.

Large chondrules have been described in several papers (e.g., Binns 1967; Rubin and Keil 1984; Weisberg et al. 1988; Bridges and Hutchison 1997; Ruzicka et al. 1998, 2000; Weyrauch 2011). Weisberg et al. (1988) were the ones who established the term “macrochondrules.” They defined a macrochondrule as having more than 5 mm in maximum diameter. In their study, they report seven macrochondrules in ordinary chondrites. The biggest within the L3 chondrite Gunlock, porphyritic in texture, measures 5 cm and is more closely discussed by Prinz et al. (1988), who describe the chondrule as metal-free with a rim of metal-troilite. The other chondrules range up to 10.3 mm in diameter. According to Weisberg et al. (1988), macrochondrules are presumably more frequent than they are thought to be, as large fragments in the chondrites’ matrix may be parts of broken macrochondrules.

Bridges and Hutchison (1997) looked through all the ordinary chondrites of the collection of the Natural History Museum in London and found 36 macrochondrules in 833 chondrites. Because of the similar chemical compositions and textures of macrochondrules and normal-sized chondrules, these authors assumed a common origin for both. Ruzicka et al. (1998, 2000) studied 10 large (mega-) chondrules in Julesburg (L3), Caraweena (L3), and Homestead (L5). Furthermore, Tanaka et al. (1975) described a giant olivine-rich chondrule in Allende, which is 7 mm in diameter, and Nelson and Rubin (2002) mentioned a large (>3 mm) chondrule in Krymka (LL3). Weisberg et al. (2011) characterized a 4 mm sized barred olivine chondrule from the Sahara 97096 EH3 chondrite, which––based on oxygen isotope ratios––must have been formed in an E chondrite oxygen reservoir.

The investigation of the properties of macrochondrules can give us information about their formation processes. This information may also help support distinct models evolved for the formation of common chondrules. Although several different processes for the formation of macrochondrules are discussed in the various papers mentioned above, the formation processes of macrochondrules remain as uncertain as the theories for the formation of typical-sized chondrules. The objective of this paper is to document the distribution and properties of macrochondrules and to discuss possible mechanisms for their formation. Preliminary data and characteristics on these chondrules have been presented by Weyrauch and Bischoff (2012).

Samples and Analytical Procedures

Samples

In the course of this survey, approximately 2800 thin sections of chondritic meteorites were inspected. Moreover, about 1000 to 1500 chondrite hand specimens were checked to find chondrites with relatively large chondrules. Thin sections and hand specimens were first inspected with the naked eye. The ones that contained large objects (macrochondrules) were chosen for microscopic examination or thin section preparation. Most of the objects appear rather bright in the transmitted light as well as in the bulk chondrite hand specimens. After the preparation of new thin sections, chondrules from 64 chondrites were chosen for further investigation. The macrochondrules in this study are all larger than 3 mm in apparent diameter.

Microscopy and Size Measurements

All thin sections discussed in this work were provided by the Institut für Planetologie of the Westfälische Wilhelms-Universität Münster. They were studied with a Zeiss-Axiophot polarizing microscope in transmitted and reflected light and most were chosen for further examination with a scanning electron microscope (SEM; see below). The photos were taken with the “analySIS” software by Olympus. Photos of the large chondrules were also taken with the Keyence 3D polarizing microscope (KEYENCE VHX-500F). These are shown in Figs. 1 and 2.

Figure 1.

 Porphyritic macrochondrules in chondrites. a) Special type of barred olivine chondrule (BO) within the Dhofar 683 (H5) ordinary chondrite. b) BO chondrule within Sahara 97062 (L5/6) ordinary chondrite showing several sets of olivine bars. c) Porphyritic olivine-pyroxene chondrule (POP) within Hammadah al Hamra 199 (L5) ordinary chondrite. Photomicrographs in transmitted light, crossed polarizers.

Figure 2.

 Nonporphyritic macrochondrules in chondrites. a) Radial pyroxene chondrule (RP) within the Parnallee (LL3) ordinary chondrite. b) Radial olivine (ROP) chondrule in Acfer 135, an H5 chondrite. c) Radial olivine-pyroxene chondrule (ROP) from the carbonaceous chondrite Acfer 187 (CR2). A detailed BSE-image is shown in Fig. 3. d) Radial pyroxene-olivine chondrule (RPO) within the Bluff (a) L5 chondrite. See Fig. 3 for a detailed image. e) Multi-radial olivine-pyroxene chondrule (MROP) in the LL3 chondrite Hammadah al Hamra 093 (compare Fig. 3 for details). f) Cryptocrystalline chondrule (C) within the chondritic breccia Acfer 102 (L3–5; compare Fig. 3d). Photomicrographs in transmitted light, crossed polarizers.

The chondrule sizes were determined by the measurement of their maximum diameter. The diameters are the actual diameters in the thin sections. For some chondrule fragments having less than 50% of their original shape, the primordial size was extrapolated (Table 1). The original size was determined by completing the outline of the chondrule on the photo with a pair of compasses.

Table 1. Samples studied and the main characteristics of 74 macrochondrules.
MeteoritePTSClassSize (mm)TypeMineralsb
  1. aFor chondrule fragments having less than 50% of their original shape, the primordial size was extrapolated as determined with a pair of compasses.

  2. bFa and Fs contents have been obtained for the core areas of olivine and pyroxene in macrochondrules from the most primitive chondrites.

  3. PTS = polished thin section; in this column, the thin section numbers of the Institut für Planetologie (PL) are given.

  4. The chondrule size in [mm] gives the maximum apparent diameter of the chondrule. The following chondrule types were found: BO = barred olivine, BOP = barred olivine-pyroxene, PO = porphyritic olivine, POP = porphyritic olivine-pyroxene, RP = radial pyroxene, RO = radial olivine, ROP = radial olivine-pyroxene, RPO = radial pyroxene-olivine, C = cryptocrystalline, and MR = multi-radial. Minerals in the chondrule are abbreviated as follows: px = low-Ca pyroxene, ol = olivine, chr = chromite, kam = kamacite, tr = troilite, Cl-ap = Cl-apatite, and cpx for Ca-rich pyroxene (additionally, all chondrules have a feldspar-normative mesostasis or plagioclase). Hammadah al Hamra 180 is an ungrouped, type 3 chondrite.

Hammadah al Hamra 180PL 96219ung; type 36.7RPOpx (Fs11–13), ol (Fa14–16), kam, il
GujbaPL 09091CBa7.8RPcpx, px (Fs2–3), ol
Dar al Gani 289PL 97246CO33.0BOol
Acfer 139PL 01251CR26.6aROol (Fa1–2)
Acfer 187PL 91173CR27.0ROPol (Fa1–3), cpx,
El Djouf 001PL 90359CR24.2RPpx (Fs2–3)
NWA 5747PL 09006CV33.4BOol (Fa18–21)
Acfer 005PL 11023H 3.9/45.7POol
Hammadah al Hamra 335PL 11032H33.0Col (Fa20–24)
Ilafegh 013PL 90341H3.53.0BOPol, px (Fs24–31)
Acfer 028PL 91093H3.84.1POol
Acfer 111PL 94056H3-63.8BOol (Fa16–18), px
Plateau du Tademait 001PL 92169H46.4Col, cpx, kam
Acfer 135PL 93058H53.9ROPol, px
Dhofar 683PL 01391H54.6BOol, chr
Hammadah al Hamra 143PL 95177H56.0BOol, px
Sahara 98034PL 11050H53.0BOol
Hammadah al Hamra 214PL 11031H64.4BOol
Dhofar 1023PL 03097L(LL)36.8aRPOpx, ol (Fa27–29), kam, chr
Adrar 90003 (1)PL 11026L/LL3.15.3RPOol (Fa17–20), px, chr
Adrar 90003 (2)PL 11026L/LL3.14.8RPOpx, ol (Fa23–25)
Sahara 97210PL 11049L/LL3.23.8BOol (Fa8–10), px (Fs5–9)
MoorabiePL 11037L3.86.7BOol
Acfer 080PL 93192L3.93.4BOPol, px
Acfer 102PL 90298L3–53.8Cpx (Fs5–13)
Dar al Gani 323PL 98149L44.3BOPol, px
Dhofar 1495PL 09189L44.9BOol
NWA 055PL 00025L43.1BOol
Acfer 122PL 11025L4/54.3RPcpx, ol
Dar al Gani 020PL 95235L4/56.9BOol
Hammadah al Hamra 128 (1)PL 95163L4/53.5BOol
Lahmada 005PL 11034L4/58.8aBOol, px, Cl-ap, chr
Lahmada 005PL 11034L4/55.5POPpx, ol, kam
Pampa (c)PL 04033L4/54.3RPOpx, ol
Bluff (a)PL 01007L57.1RPOpx, ol,
Hammadah al Hamra 157PL 95330L53.4BOPol, px
Hammadah al Hamra 199PL 97131L53.8POPol, px
Ilafegh 89010 (1)PL 11043L53.6ROPol, px, chr
Ilafegh 89010 (2)PL 11043L56.0ROPpx
Ilafegh 89010 (3)PL 11043L53.1BOol, chr
New AlmeloPL 11038L54.6RPOpx, ol, kam
Sahara 97062PL 11045L5/66.0BOol
Dar al Gani 210PL 97125L63.8ROol
Forrest 002PL 11029L63.6ROol
Hammadah al Hamra 066PL 95035L63.1ROPol, px, Cl-ap, chr
Hammadah al Hamra 067 (1)PL 95036L63.5RPol, px
Hammadah al Hamra 067 (2)PL95036L63.8BOol
Hammadah al Hamra 136PL 95170L63.5BOol
Hammadah al Hamra 177PL 96113L68.4aBOol
Hammadah al Hamra 206PL 97137L64.4BOol
Hammadah al Hamra 224PL 98080L64.9RPpx, chr, kam
Johnson CityPL 11033L64.6BOol
Majdul (1)PL 11035L64.6POol
Majdul (2)PL 11035L65.7POol
MonzePL 11036L65.0BOol
NWA 1488PL 02180L63.9BOol
NWA 2155PL 11039L66.5BOol
NWA 4213PL 06077L65.1BOol
Safsaf (1)PL 99024L63.3RPOpx, ol, chr
Safsaf (2)PL 99024L63.6BOPol, cpx, chr
Sahara 97134PL 11047L64.7BOol
Sahara 97199PL 11048L69.8POol
NWA 5072 (1)PL 07216LL(L)33.3BOol (Fa1–23)
NWA 5072 (2)PL 07216LL(L)34.0BOol (Fa15–21)
Dar al Gani 288PL 97083LL35.9BOol (Fa25–33), px (Fs21–26), tr
NWA 4572 (1)PL 07014LL33.2ROPol, px, tr
NWA 4572 (2)PL 07014LL33.2RPpx (Fs7–10; outer cpx), tr
ParnalleePL 89527LL3.63.5RPOpx, ol
Hammadah al Hamra 093PL 95090LL3.94.6MROPol, px
NWA 6400PL 10103LL3-67.0ROPol (Fa8–11), px
Hammadah al Hamra 123 (2)PL 11030LL48.0MRPOpx, ol, tr
Acfer 120 (1)PL 11024LL66.6POol
Acfer 120 (2)PL 11024LL63.4BOol
DhurmsalaPL 11028LL610.0aROPol, px, chr

Electron Microscopy

To study the mineralogy and the texture of the fine-grained chondrules in more detail, the JEOL 840A and JEOL JSM-6010 LV scanning electron microscopes (SEM) of ICEM (Interdisciplinary Center for Electron Microscopy und Microanalysis) at the Westfälische Wilhelms-Universität Münster were used (Fig. 3). Both have a conventional detector system for backscattered electrons (BSE) and secondary electrons (SE). For the semi-quantitative analysis of the mineralogy, the SEMs are equipped with the INCA EDX-systems from Oxford Instruments.

Figure 3.

 Photomicrographs of details from macrochondrules in chondrites. a) Acfer 187 (CR2): Skeletal laths of olivine (ol; gray), partly covered by Ca-rich pyroxene (cpx; light gray) are embedded within a feldspar-normative mesostasis (Mes), which is variable in composition (gray - dark gray). The white particles are predominantly Fe-sulfides; compare Fig. 2c. b) Bluff (a) (L5): details of the radial-textured interior of the RPO chondrule from Fig. 2d showing abundant needles of olivine (ol; light gray) and pyroxene (px; gray). Feldspars and pores are black; the light particles are predominantly chromites (chr). c) Hammadah al Hamra 093 (LL3): fine-grained texture of the multi-radial olivine-pyroxene chondrule shown in Fig. 2e. ol = olivine, px = low-Ca pyroxene. d) Acfer 102 (L3–5): the cryptocrystalline chondrule consists of a fine-grained intergrowth low-Ca pyroxene (px), Ca-pyroxene (cpx), and an interstitial plagioclase-normative mesostasis (mes). Some chromites (chr) are also present. Back-scattered electron (BSE) images.

Results

Foreword: Chondrule Types and Their Abundances

Chondrule classification is mainly based on texture and mineralogy. Based on textural characteristics, porphyritic and nonporphyritic chondrules can be distinguished. Four different major types of porphyritic chondrules exist (PO, PP, POP, and BO) as well as four types of nonporphyritic chondrules: RP, GOP, C, and M (e.g., Gooding and Keil 1981).

Porphyritic Chondrules

The acronym PO stands for porphyritic olivine, PP for porphyritic pyroxene, and POP for porphyritic olivine-pyroxene. Most of these chondrules are relatively coarse-grained. According to Gooding and Keil (1981), the phenocryst size may range from less than 0.5 of the chondrule maximum diameter downward to nearly that of the larger microlites, which are distinctive components of the mesostasis. The portion of mesostasis in porphyritic chondrules makes up to 30–50 vol%. Porphyritic chondrules are called PP or PO, when either pyroxene or olivine is the main phenocryst having at least 90 vol% (Gooding and Keil 1981). POP chondrules are chondrules that consist of at least 10 vol% olivine and pyroxene phenocrysts. The related subgroup PPO (porphyritic pyroxene-olivine) was defined by Wasson et al. (1995) considering greater pyroxene abundance than that of olivine. According to Gooding and Keil (1981), barred olivine chondrules (BO) might also be considered as a special type of porphyritic chondrules. Optically, the bars within these chondrules show the same orientation indicated by the same interference color. Thus, all bars must belong to a huge skeletal single crystal in three-dimensional perspective. The mesostasis is similar in texture and composition to that found in typical porphyritic chondrules (Gooding and Keil 1981). Barred olivine-pyroxene (BOP), barred pyroxene-olivine (BPO) and barred pyroxene (BP) chondrules have been characterized as related chondrule types (Weisberg et al. 1988; Wasson et al. 1995; Rubin et al. 2003). Also, all chondrite groups contain a small abundance of Al-rich chondrules (e.g., Bischoff and Keil 1983, 1984; Bischoff et al. 1985, 1989, 1994, 2011; Rout and Bischoff 2008; Rout et al. 2010) that are mostly porphyritic in texture and often contain abundant An-rich plagioclase, fassaite, and/or spinel as phenocrysts. Also, SiO2-rich chondrules exist having a SiO2-polymorph as phenocrysts (e.g., Brigham et al. 1986; Murty et al. 2004; Hezel et al. 2006).

Nonporphyritic Chondrules

The radial pyroxene chondrules (RP), which are the most abundant nonporphyritic chondrules in ordinary chondrites, mainly consist of pyroxene. The crystals are elongated and fine-grained and all appear to have one starting point of crystallization. In a cut orthogonal to the view in the thin sections, it can clearly be seen that the crystals forming the fan-like structure are thin plates or blades. Furthermore, the gaps between the pyroxene blades are filled with feldspar-normative mesostasis, which is similar to the one in porphyritic chondrules, but less abundant. Moreover, these chondrules may contain granular areas and small amounts of olivine (all according to Gooding and Keil 1981). Wasson et al. (1995) defined the related subtypes ROP (radial olivine-pyroxene) and BP (barred pyroxene). Furthermore, cryptocrystalline chondrules (C) are another nonporphyritic chondrule type. These chondrules are so fine-grained that it is not possible to clearly define their mineral constituents with a polarizing microscope. Grossman et al. (1988) define C chondrules as having grain sizes of ≤2 μm. According to Gooding and Keil (1981), this type of chondrule has a transitional textural relationship with fine-grained radial pyroxene chondrules. They assume this because the electron microprobe analysis showed that cryptocrystalline chondrules have “pyroxene-like bulk compositions which overlap those of RP chondrules” (Gooding and Keil 1981). Granular olivine-pyroxene chondrules (GOP) are barely described in detail. Gooding and Keil (1981) describe that these chondrules have abundant olivine as the sole or dominant crystalline phase and that they “are distinguished by their uniformly small grain size and generally anhedral crystals.” They may contain abundant unmelted components (Wasson et al. 1995). In addition, Gooding and Keil (1981) also considered metallic (M) chondrules as a very rare type of nonporphyritic chondrules.

Abundances of Textural Types Among Common Chondrules

Gooding and Keil (1981) studied >1600 chondrules within ordinary chondrites and found that the abundances of the different chondrule types are very similar in all chondrites studied. Most abundant are the porphyritic olivine-pyroxene (POP; approximately 47–52%) chondrules followed by porphyritic olivine (PO; 15–27%), porphyritic pyroxene (PP; 9–11%), and barred olivine (BO; 3–4%) chondrules. The nonporphyritic chondrules are less abundant (RP = 7–9%; GOP = 2–5%, C = 3–5; M = ≤1%; Gooding and Keil 1981). Similarly, proportions of chondrule textures for LL3 chondrites were determined by Nelson and Rubin (2002), who found higher abundances of PP chondrules than Gooding and Keil (1981). As it will be shown in this study, the distribution of various chondrule types among the macrochondrules is very different compared with the type distribution of normal-sized chondrules in ordinary and most classes of carbonaceous chondrites (Fig. 4).

Figure 4.

 Bar chart of the distribution of the textural types among chondrules. The three bars on the left-hand side show literature data about the distributions of chondrule types among CV3-, CO3-, and ordinary chondrites (OC) (e.g., Simon and Haggerty 1979; Gooding and Keil 1981; Rubin and Wasson 1987; Grossman et al. 1988). The bars on the right-hand side show the distribution among macrochondrules observed in this study class-divided by a) size and b) petrologic types. The number of chondrules (n) is given on the upper x-axis. The last bar combines own and literature data on macrochondrules, which are texturally described (Rubin and Keil 1984; Weisberg et al. 1988, 2011; Bridges and Hutchison 1997; Ruzicka et al. 1998; Nelson and Rubin 2002; Lee and Choi 2007; Weyrauch 2011).

Macrochondrules in Chondrites

In this study, 74 macrochondrules from different chondrite classes will be described in detail (Table 1). From these, 11 are from H, 42 from L, 14 from LL, and 6 are from carbonaceous chondrites. Furthermore, one macrochondrule is from the ungrouped meteorite Hammadah al Hamra 180. Concerning the carbonaceous chondrites, three objects are within CR chondrites and one macrochondrule each in a CV, CO, and CB chondrite. The largest chondrule has a diameter of approximately 1cm and is present within the LL chondrite Dhurmsala.

Based on their textures, the studied macrochondrules can be subdivided into (1) porphyritic (8 chondrules), (2) barred (35), (3) radial (28), and (4) cryptocrystalline chondrules (3). Considering the mineralogy of these large chondrules, in this study, the major chondrule types had to be meaningfully subdivided, resulting in 10 different chondrule types and subtypes (Table 1): porphyritic (PO, POP), barred (BO, BOP), radial (RP, RO, RPO, ROP, MR), and cryptocrystalline (C). In addition, some chondrules are compound. Compound chondrules consist of two or more chondrules that fused together, while one of them was already solid enough to retain its shape. It is also possible that these chondrules formed due to collisions among chondrules which were still molten (Wasson et al. 1995). The compound chondrules found in the course of this survey have similar textures in the primary and the secondary chondrule.

Porphyritic Macrochondrules

From all studied macrochondrules, only eight are porphyritic in texture. Most of these dominantly consist of olivine (PO) and two chondrules also contain considerable abundances of pyroxene (POP). A typical POP chondrule from Hammadah al Hamra 199 (L5) is shown in Fig. 1c. One of the porphyritic chondrules in Acfer 028 (H3) is very fine-grained and looks like a barred olivine chondrule at the first moment. But since the olivine grains are not parallel oriented, the chondrule is assigned to the porphyritic olivine chondrules.

Barred Macrochondrules

The most abundant chondrules are the barred chondrules. Thirty-five chondrules were detected within the samples, and two from Dhofar 683 (H5) and Sahara 97062 (L5/6) are shown in Fig. 1. Most of them are BO (30) and five are BOP chondrules. The barred olivine-pyroxene chondrules are chondrules with subparallel oriented grains, which are mainly olivine and pyroxene. The order of the letters written in the abbreviation (BOP) shows that olivine is more abundant than pyroxene. In this study, there is no barred chondrule with two main minerals in which pyroxene is the dominant phase and no BP chondrule as described in Parnallee (LL3) by Weisberg et al. (1988).

Radial Macrochondrules

The radial chondrules account for 28 objects, which make them only slightly less abundant than the barred chondrules. There are only six radial chondrules, which consist of 90 vol% of pyroxene, besides mesostasis (RP). One example from Parnallee (LL3) is shown in Fig. 2a. Most of the radial chondrules consist of two main minerals (17). In nine of them, pyroxene is the most abundant (RPO; Fig. 2d); in eight, olivine is the dominating mineral (ROP). Typical examples of ROP chondrules are the macrochondrules from Acfer 187 (CR2) and Acfer 135 (H5) shown in Figs. 2b and 2c. Three of the 28 radial chondrules are radial olivine chondrules (RO). These chondrules mainly consist of olivine. The amount of the mineral olivine (except mesostasis) makes up at least 90 vol% of the bulk chondrule. Pyroxene is only a minor constituent of these chondrules. This chondrule type is not described in the papers of Gooding and Keil (1981) and Wasson et al. (1995). However, King and King (1978) mentioned “excentroradial” olivine-rich chondrules, but without naming this type specifically. Like the radial pyroxene chondrules, the RO chondrules have interstitial feldspar-normative mesostasis. In this study, two macrochondrules are classified as multi-radial chondrules (MR). This kind of chondrules has not been described before, at least not under this name. The chondrules are very fine-grained. The grains are strongly elongated and narrow (Fig. 2e). The orientation of the major minerals is different from that of the simple radial chondrules. In the multiradial chondrules, several common starting points of crystallization occur (Fig. 2e). Subparallel to fan-like textures are built up by the crystals starting at these nucleation points. The fan-like structures often intersect or grow into each other. The space between the pyroxene or olivine blades is also filled with feldspar-normative mesostasis.

Cryptocrystalline Macrochondrules

The cryptocrystalline chondrules (C) account for three objects. These chondrules are the finest-grained ones (Fig. 2f). The mineral constituents of these chondrules are so fine that it is not possible to identify the phases with the polarizing microscope. SEM studies show that two of the C chondrules within Plateau du Tademait 001 (H4) and Acfer 102 (L3–5) have abundant pyroxene (Fig. 3d). Only the chondrule in Hammadah al Hamra 335 (H3) consists of approximately 90 vol% olivine (excluding mesostasis).

Discussion

Distribution and Abundance of Macrochondrules in Chondrites

Apparently, most of the macrochondrules appear to be in L chondrites. They are rare, but in some L and LL chondrites, several macrochondrules were found in one thin section. A good example is the chondrite Ilafegh 010 (L5), which contains three macrochondrules within one thin section (Table 1). On the other hand, there is no thin section from H chondrites or carbonaceous chondrites that contains more than one macrochondrule.

The occurrence of macrochondrules does not depend on the petrologic type of the chondrite. Unusually large chondrules were found among all petrologic types (3–6) and they are quite equally distributed among all petrological types (Table 1; Fig. 4). One could imagine that porphyritic macrochondrules could be much less recognizable in chondrites of higher petrologic type than barred or nonporphyritic chondrules, but this appears not to be the case as shown in Fig. 4.

Also, there is no apparent relationship between the chondrule type and the chondrite class. Considering the barred olivine chondrules, 5 objects were found in H chondrites, 23 in L chondrites, 4 in LL chondrites, and 2 in carbonaceous chondrites. Similarly, most of the radial chondrules (13) exist in L chondrites, nine radial chondrules occur in LL chondrites, and one within an H chondrite. In addition, one radial macrochondrule was found in the Hammadah al Hamra 180 chondrite, which is ungrouped. Moreover, four radial chondrules occur in carbonaceous chondrites (Table 1).

To our knowledge, a total of 144 macrochondrules have been mentioned in the literature until today (Binns 1967 [1 chondrule]; Tanaka et al. 1975 [1]; Rubin and Keil 1984 [8]; Weisberg et al. 1988 [7]; Bridges and Hutchison 1997 [36]; Ruzicka et al. 1998 [9]; Nelson and Rubin 2002 [1]; Lee and Choi 2007 [6]; Weisberg et al. 2011 [1]; Weyrauch 2011 [74]), and from 128 textural details have been reported (compare Fig. 4). A total of 134 chondrules are from ordinary type and 10 from other chondrite types.

Based on the database of the Meteoritical Society (http://www.lpi.usra.edu/meteor/about.php; January 19, 2012), 16348 H, 14723 L, and 5422 LL chondrites are registered. Thus, the relative proportions are about 3:3:1. For about 3800 individual meteorite samples of the collection of the Institut für Planetologie at the Westfälische Wilhelms-Universität Münster, similar proportions of H, L, and LL chondrites can be assumed. Considering the ordinary chondrites, 20 of 134 chondrules were found in H, 88 in L, 26 in LL chondrites. Based on the mean chondrule size of the different classes of ordinary chondrite, one would expect more macrochondrules within LL and L chondrites, as their mean chondrule sizes are significantly larger (900 and 700 μm, respectively; Weisberg et al. 2006) than those of H chondrites (300 μm; Weisberg et al. 2006). This is definitely the case. Macrochondrules in H chondrites are about 4 times less abundant than in L chondrites. The reason for macrochondrules being more often found in L than in LL chondrites is only due to the fact that there are about 3 times as many L chondrites as LL chondrites (see above). Macrochondrules are more often found in L than in LL chondrites because there are about three times more L chondrites than LL chondrites.

Mineralogy and Petrographic Types of Macrochondrules

Apparently, most of the chondrules contain high olivine abundances. There are only a few among the 74 studied macrochondrules that contain very low olivine abundances besides mesostasis (e.g., six RP chondrules and some C chondrules). In almost 60 macrochondrules, olivine is the dominant mineral and in many cases, pyroxene is only a minor constituent (excluding mesostasis; Table 1). This leads to the assumption that the precursor materials, from which the macrochondrules formed, consisted of material that allowed the favored precipitation of olivine upon melting and crystallization. This was also assumed by Tanaka et al. (1975), who studied a chondrule of approximately 7 mm having a chemical composition of almost pure olivine (Fo79).

Considering all reported 144 macrochondrules, it has to be clearly stated that the porphyritic types are quite rare. One even has to discuss, if all porphyritic macrochondrules described in the literature are true chondrules. Already Binns (1967) mentioned that the large macrochondrule found in Parnallee may be a fragment of a larger body being disrupted and mixed together with typical chondrules as also suggested by Dodd (1969). This porphyritic object may also be a rounded fragment of an impact melt rock. Similar objects have been described from various chondrites (e.g., Bischoff et al. 2006, 2011).

Certainly, the porphyritic chondrules belong to the least abundant chondrule types in this study and account for only about 10% of the observed macrochondrules (compare Fig. 4). On the other hand, barred chondrules usually belong to the least abundant chondrule types among normal-sized chondrules, but are with about 50% the most abundant chondrule types among the macrochondrules. This distribution of chondrule types within the ordinary chondrite classes is highly unusual, as this seems to be just a reverse of the distribution of typical-sized chondrules in ordinary chondrites (Fig. 4). Considering the abundances of various chondrule types within the Bencubbin-like carbonaceous chondrites, the situation is completely different (see below). Rubin et al. (2003) discuss that large silicate globules in Gujba are typically very fine-grained or have elongated crystals and suggest that the typical chondrule types (C, RP, BP, and BO) in this meteorite formed from completely molten droplets. Thus, chondrule textures in CB chondrites are very similar to the textures of macrochondrules in other chondrite classes.

Large Chondrules and Fragments in CR, CB, and CH Chondrites

As pointed out before, chondrules in CB chondrites are typically fine-grained with elongated crystals. The macrochondrules in the studied CR chondrites are texturally similar to the many macrochondrules (or chondrule-like objects) described from the CB chondrite Gujba (Rubin et al. 2003). In both cases, the olivine is Fa-poor (Fa<3) similar to the Fa-contents of chondrule olivines in other CB chondrites (Weisberg et al. 2001); thus probably, also the compositions of these chondrules are quite similar. This indicates that macrochondrules within CR and CB chondrites are essentially poor in FeO (see also Table 1 concerning the compositions of olivine and pyroxene). Thus, they have to be chemically classified as FeO-poor chondrules. Due to their composition, they should have been formed at similar high temperatures as most FeO-poor, Type I chondrules in other chondrite groups.

The earlier suggested relationship between the CB, CR, and CH chondrites (Bischoff 1992; Weisberg et al. 1995) is also supported by the observation that within the CH chondrites, many large fragments occur (Fig. 2a in Bischoff et al. 1993b) similar in texture to the textures observed for the CR macro chondrules (Fig. 2c) and the silicate globules in CB chondrites (e.g., Rubin et al. 2003). Recently, CB-like fragments were also observed as fragments of the Almahata Sitta meteorite shower (Bischoff et al. 2012; Horstmann et al. 2012), whose chondrules and chondrule-like constituents have similar textures. Concerning the formation of chondrules in CB chondrites and, perhaps, similarly textured objects in CR and CH chondrites, it has to be considered that all these chondrules probably formed late, perhaps from a vapor-melt plume produced by a giant impact (Krot et al. 2005).

Formation of Macrochondrules

Macro chondrules are described as being “petrologically similar to average chondrules” (Weisberg et al. 1988). Therefore, these authors assume that macrochondrules were also formed in the solar nebula. Furthermore, they explain the unusually large size of the macrochondrules by suspecting their origin “in nebular regions with greater dust densities, differing dust/gas ratios, and/or higher electrostatic attraction between particles.” The largest mean diameters of chondrules are on the order of 1 mm for specific chondrite types (e.g., CV, CR, CBa). Considering a typical size distribution, one would expect chondrules that are slightly larger or smaller than the mean chondrule size of a chondrite class. However, as shown and discussed here, macrochondrules exist that are 10 times (or more) larger than typical chondrules. The largest, earlier described chondrule from the Gunlock (L3) chondrite has an apparent diameter of 5 cm (Prinz et al. 1988).

These findings and our data lead to the question: how may these huge chondrules have formed? Considering the density of typical chondrule silicates (approximately 3 g cm−3), the masses for chondrules of different sizes can be calculated. Taking the mean chondrule sizes for H, L, and LL ordinary chondrites from Weisberg et al. (2006; 300, 700, and 900 μm, respectively) and Rubin (2010; 300, 400, and 570 μm, respectively) into account, the masses for average chondrules in the ordinary chondrites would be approximately 0.04 mg for H, approximately 0.54 mg (approximately 0.10 mg based on Rubin) for L, and approximately 1.1 mg (approximately 0.29 mg based on Rubin) for LL chondrites. For a chondrule with a diameter of 5 mm, the mass would be 196 mg and for a chondrule with a diameter of 9 mm, which is close to the largest chondrule found in this study (Table 1), the mass would be 1.1 g. Thus, the mass of this chondrule is 1000 times or more the mass of the common chondrules in LL chondrites. Comparing this mass with typical masses of H chondrite chondrules, it is >25,000 times heavier.

Because of their texture, the common chondrules and the macrochondrules are expected to have formed due to melting of precursor materials. In the chondrule-forming process, the precursors were heated rapidly until complete or partial melting was reached, and cooled rapidly afterward. Considering the extraordinary size of macrochondrules, one may discuss the possibility that the precursor aggregates were much bigger than those available during formation of the common-sized chondrules. This may lead to the questions: What are the differences in the formation processes of macro- and normal-sized chondrules? Are higher temperatures required to melt larger precursor aggregates than to melt smaller sized aggregates for the formation of chondrules of typical size?

Weisberg et al. (1988) assumed that macrochondrules have formed in a region of higher dust densities, where the electrostatic attraction was very high. Considering areas with a higher density, possibly, (1) more material was available which could be melted and (2) the probability of collisions among molten droplets was much higher than in nebula locations with low-dust densities. If––in addition––frictional forces were part of the heating process, these forces would have influenced the increase in temperature more significantly in areas of higher density than in areas with lower densities. The higher temperature in the denser area would have more easily led to superheating of the melt spherules. Thus, the superheated melt droplet could stay in a molten state significantly longer than typical-sized chondrules. Here, it is suggested that macrochondrules stayed longer in a molten state than chondrules of normal size. Thus, for a longer time, chondrule growth was possible due to heavy collision activities between abundant molten droplets. Wasson et al. (1995) write that it is a “widely accepted view that the porphyritic types (PO, POP, and PP) as well as the granular olivine-pyroxene (GOP) chondrules were incompletely melted during chondrule formation, whereas the other three types (BO, RP, and C) were completely melted” (see also Wasson 1993). This clearly indicates that the temperatures for the formation of the latter were higher than those of the porphyritic ones. Considering the macrochondrules from the ordinary chondrites, it is difficult to make assumptions concerning the original melt temperature of the macrochondrules based on the composition of the main mineral constituents, olivine and pyroxene. No estimates about the original composition can be made for chondrules within chondrites that have been significantly metamorphosed (Types: >∼3.6–6). In these cases, equilibration processes have significantly or completely changed (equilibrated) the olivine and pyroxene compositions during metamorphism. For the chondrules from more primitive chondrites (e.g., Adrar 003 or Sahara 97210), the situation is also ambiguous. Some of the macrochondrules have olivines and pyroxenes with low Fa and Fs contents, respectively, and others just show the opposite (Table 1). Thus, it cannot be clearly found out if the original melt compositions were FeO-poor or FeO-rich. In the latter case, of course, lower temperature would have been required to completely melt the macrochondrules than in the case of having FeO-poor melt compositions.

It is apparent that most of the macrochondrules are rather fine-grained, barred (skeletal), or radial in texture. As clearly shown in Fig. 4, the textural distribution of macrochondrules is completely different from that of the normal-sized chondrules. The porphyritic chondrules are the least abundant chondrule types in this study and account for only about 10% of the observed macrochondrules (compare Fig. 4). This finding is also in excellent accordance with the suggestion of Weisberg and Prinz (1996) following Wasson (1993) that radial pyroxene, cryptocrystalline, and barred olivine chondrules seem to have formed through the crystallization of completely molten droplets. This also would explain why barred and radial chondrules are so frequent among the macrochondrules. Considering the porphyritic chondrules, the lower degree of melting and the lower temperature are responsible for the finding that they are so rare among the macrochondrules. Another aspect should be mentioned in this discussion. Nelson and Rubin (2002) suggested that large porphyritic chondrules would be fragmented preferentially. In their studies, they found a relatively low percentage of intact porphyritic chondrules within the set of the largest chondrules. If brecciation really is an important aspect, then, this process should be considered to explain the relatively low abundance of porphyritic chondrules among the macrochondrules.

In summary, in our opinion, typical chondrules and macrochondrules in ordinary and carbonaceous chondrites (perhaps except CBs) were formed by a similar heating process. Based on many arguments, a nebular rather than a planetary process is considered (e.g., Rubin 2000). It is likely that macrochondrules were in a molten state significantly longer than common chondrules and formed due to frequent collisions among melted spherules. The spherules must have been completely molten, as growth due to collisions would not have been possible, if they were already solid. We know that molten spherules sometimes heavily collide with each other from the existence of compound chondrules (Fig. 5). These formed due to the collision of chondrules the impactor or target of which was already (at least partially) solid enough to stay in its original shape (e.g., Lux et al. 1981; Wasson 1993; Wasson et al. 1995).

Figure 5.

 Compound chondrule within the Allende (CV3) carbonaceous chondrite consisting of at least 16 individuals that rapidly collided. Photomicrograph in transmitted light.

Concluding Remarks

Macrochondrules are relatively rare in chondrites. H group ordinary chondrites have about 4 times lower abundance of chondrules larger than 3 mm than L or LL chondrites. This is probably because chondrules in H chondrites are typically about 3–4 times smaller than in L and LL chondrites. If normal-sized chondrules and macrochondrules formed by the same process, then the grain-size sorting process was incomplete. In the past, it was suggested that size differences of the chondrite groups may be due to aerodynamic size sorting (e.g., Clayton 1980; Cuzzi et al. 2001; Liffman 2005), but this idea was challenged by Rubin (2010), who suggested based on mineralogical criteria that the number of chondrule remelting events in different areas of the solar nebula may have been the most important factor to control the chondrule size. Rubin (2010) also suggests that if there were lots of dust in local environments, large chondrules would tend to be produced, and states further that CV, CK, and CR having on average the largest chondrules also have the lowest abundance of RP and C chondrules. He suggests that the remelting process to enlarge the chondrule size would be responsible for reducing the proportion of RP and C chondrules relative to the porphyritic chondrules at a distinct chondrule formation location. As we can see from Fig. 4, the distribution of chondrule types among the macrochondrules completely shows the opposite. Clearly, we do not know if the macrochondrules were formed in the same environment as normal-sized chondrules or not.

On the other hand, the macrochondrules could also be regarded as “foreign” objects and could have entered the parent body during early accretion or at a later stage as projectiles, similar to many other xenolithic fragments in chondrites like CI-like inclusions in CR, CH, and ordinary chondrites (e.g., Bischoff et al. 1993, 2006; Endreß et al. 1994; Funk et al. 2011) or metamorphosed/achondritic fragments in primitive, unequilibrated chondrites (e.g., Bischoff et al. 2006; Libourel and Krot 2007; Sokol et al. 2007).

From the existence of cluster chondrites that formed by rapid accretion of chondrules (Metzler 2011, 2012), we know that at least a significant number of chondrules were hot during accretion. Similarly, the existence of multi-compound chondrules (Fig. 5) points toward collisions of hot chondrules. Thus, there are strong indications that hot or superheated melt droplets may have collided early after their formation to grow significantly to form macrochondrules.

Acknowledgments— We thank Frank Bartschat for photographic work and Ulla Heitmann for thin section preparation. We appreciate the useful comments and help of Dr. Knut Metzler and Marian Horstmann. We thank the reviewers Alan Rubin and Mike Weisberg and the Associate Editor Sasha Krot for their useful comments and suggestions. This work is part of the Bachelor Thesis of M. W. at the Institut für Planetologie of the Westfälische Wilhelms-Universität Münster (Germany) and was partly supported by the German Research Foundation (DFG) within the Priority Program “The first 10 Million Years—a Planetary Materials Approach” (SPP 1385).

Editorial Handling— Dr. Alexander Krot

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