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.
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.
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.
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).
|Meteorite||Type||Lithology||Counted area (mm2)||n||Chondrules |
sulfide + metal
|NWA 869||L3-6||clch clast (<L3.5)a||7||363||92||5||3|
|NWA 5206||LL3.05||clch clast||31||319||87||8||5|
|NWA 1756||LL3.10||clch clast||9||436||87||4||9|
|NWA 5421||LL3.7||clch clast||22||390||86||9||5|
|NWA 5205||LL3.7b||clch clast #1||29||371||91||5||4|
|clch clast #2||18||340||84||12||4|
|NWA 4572||LL3.6||clastic portion||49||451||76||5||19|
|Literature data |