Part I - Observations of Chondrules

Summary

In this chapter, we summarize our current knowledge of the mineralogy, petrography, oxygen-isotope compositions, and trace element abundances of precursors of chondrules and igneous Ca,Al-rich inclusions (CAIs), which provide important constraints on the mechanisms of transient heating events in the protoplanetary disk. We infer that porphyritic chondrules, the dominant textural type of chondrules in most chondrite groups, largely formed by incomplete melting of isotopically diverse solid precursors, including refractory inclusions (CAIs and amoeboid olivine aggregates (AOAs)), fragments of chondrules from earlier generations, and fine-grained matrix-like material during highly-localized transient heating events in dust-rich disk regions characterized by ¹⁶O-poor average compositions of dust (Δ¹⁷O ~ ‒5‰ to +3‰). These observations preclude formation of the majority of porphyritic chondrules by splashing of differentiated planetesimals; instead, they are consistent with melting of dustballs during localized transient heating events, such as bow shocks and magnetized turbulence in the protoplanetary disk, and, possibly, by collisions between chondritic planetesimals. Like porphyritic chondrules, igneous CAIs formed by incomplete melting of isotopically diverse solid precursors during localized transient heating events. These precursors, however, consisted exclusively of refractory inclusions, and the melting occurred in an ¹⁶O-rich gas (Δ¹⁷O ~ ‒24‰) of approximately solar composition, most likely near the protosun. The U-corrected Pb–Pb absolute and Al–Mg relative chronologies of igneous CAIs in CV chondrites indicate that these melting events started contemporaneously with condensation of CAI precursors (4567.3 ± 0.16 Ma) and lasted up to 0.3 Ma, providing evidence for the earliest transient heating events capable of melting refractory dustballs in the innermost part of the disk. There is no evidence that chondrules were among the precursors of igneous CAIs, which is consistent with an age gap between CAIs and chondrules. In contrast to typical (non–metal-rich) chondrites, the CB metal-rich carbonaceous chondrites contain exclusively magnesian nonporphyritic chondrules formed during a single-stage event ~5 Ma after CV CAIs, most likely in an impact-generated gas–melt plume. Bulk chemical compositions of CB chondrules and equilibrium thermodynamic calculations suggest that at least one of the colliding bodies was differentiated. The uniformly ¹⁶O-depleted igneous CAIs in CB chondrites most likely formed by complete melting of preexisting refractory inclusions that was accompanied by gas–melt interaction in the plume. CH metal-rich carbonaceous chondrites represent a mixture of the CB-like materials (magnesian skeletal olivine and cryptocrystalline chondrules and uniformly ¹⁶O-depleted igneous CAIs) formed in an impact plume and the typical chondritic materials (magnesian, ferroan, and Al-rich porphyritic chondrules, uniformly ¹⁶O-rich CAIs, and chondritic lithic clasts) that appear to have largely predated the impact plume event. We conclude that there are multiple mechanisms of transient heating events that operated in the protoplanetary disk during its entire lifetime and resulted in formation of chondrules and igneous CAIs.