Shapes of chondrules determined from the petrofabric of the CR2 chondrite NWA 801

Abstract

The approximately spherical shapes of chondrules has long been attributed to surface tension acting on ~1 mm melt droplets that formed and cooled in the microgravity field of the solar nebula. However, chondrule shapes commonly depart significantly from spherical. In this study, 109 chondrules in a sample of CR2 chondrite NWA 801 were imaged by X‐ray computed tomography and best‐fitted to ellipsoids. The analysis confirms that many chondrules are indeed not spherical, and also that the chondrules’ collective shape fabric records a definite 13% compaction in the host meteorite. Dehydration of phyllosilicates within chondrules may account for that strain. However, retro‐deforming all chondrules shows that a large majority were already far from spherical prior to accretion. Possible models for these initial shapes include prior deformation of individual chondrules in earlier hosts, and, as suggested by previous authors, rotation of chondrules as they were solidifying, and/or “streaming” of molten chondrules by their differential velocities with their gaseous hosts after melting. More in situ 3‐D work such as this study on a variety of unequilibrated chondrites, combined with detailed structural petrography, should help further constrain these models and refine our understanding of chondrite formation.

Size‐frequency distributions and physical properties of chondrules from x‐ray computed microtomography and digital data extraction

“…tion of chondrules seems to have a greater diversity of shape than those of the NWA 801 CR chondrite investigated byCharles et al (2018).…”

“…It is commonly thought that chondrules are nominally spherical in shape, although deeper inspection indicates nonspherical, ellipsoidal shapes due to both pre-accretionary (Tsuchiyama et al, 2003;Weisberg & Prinz, 1996) and post-accretionary or parent body processes acting on the chondrules (Cain et al, 1986;Charles, Robin, Davis, and McCausland (2018)). To quantify the shapes of chondrules and to investigate if chondrules within our datasets define a petrofabric within a chondrite, we used a procedure like that found in Friedrich et al (2008).…”

“…Chondrule shapes have been used to infer the impact histories of the rocks that contain them (Cain et al, 1986; Hanna et al, 2015; Lindgren et al, 2015; Ruzicka et al, 2020), and it has been shown that impact is a plausible reason for a petrofabric within a chondrite (Cain et al, 1986; Dodd, 1965; Friedrich et al, 2008; Friedrich et al, 2017; Hanna et al, 2015; Lindgren et al, 2015; Ruzicka et al, 2020). However, there is also evidence for nonspherical chondrule shapes prior to accretion into chondrites (Charles et al, 2018; Tsuchiyama et al, 2003).…”

Collisions and compositional variability in chondrule-forming events

“…Of course, 2D measurements underestimate the departure from sphericity of chondrules. After correcting for postaccretional deformation, Charles et al (2018) still found that 100 out of 109 chondrules of CR2 chondrite NWA 801 examined in X-ray tomography had aspect ratios above 1.1, and 52 above 1.3. Twelve of the 20 Allende (CV3) chondrules studied by Tsuchiyama et al (2003) with the same technique had aspect ratios exceeding 1.1.…”

“…They should further characterize the possibility of postaccretional deformation, e.g. shear or lineation (Charles et al 2018). The CO chondrites may e.g.…”