Collisions and compositional variability in chondrule-forming events

Abstract

Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a "nebular" setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies .

50 pages, 12 figures

References in corpus (14)

• Global Simulations of the Inner Regions of Protoplanetary Disks with Comprehensive Disk Microphysics
• Impact splash chondrule formation during planetesimal recycling
• Mixing and transport of dust in the early solar nebula as inferred from titanium isotope variations among chondrules
• Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect
• Magmatic sulfides in the porphyritic chondrules of EH enstatite chondrites
• Northwest Africa 5958: a weakly altered CM-related ungrouped chondrite, not a CI3
• Origin of isolated olivine grains in carbonaceous chondrites
• Trace element geochemistry of ordinary chondrite chondrules: the type I/type II chondrule dichotomy
• Trace element geochemistry of CR chondrite metal
• Chondrule trace element geochemistry at the mineral scale
• Harvesting the decay energy of ${}^{26}$Al to drive lightning discharge in protoplanetary discs
• Chondrule destruction in nebular shocks
• Compound Chondrules fused Cold
• Compound chondrule formation in optically thin shock waves

Cited by in corpus (2)

• Constraints on planetesimal accretion inferred from particle-size distribution in CO chondrites
• Meteorite petrology versus genetics: Toward a unified binominal classification