The antiquity of iron meteorites and the inferred early intense heating by the decay of 26Al suggest that many planetesimals were molten beneath a thin insulating cap at the same time as chondrules were being made. As those planetesimals were colliding and merging, it seems inevitable that impact plumes of droplets from their liquid interiors would have been launched into space and cooled to form chondrules. We call the process splashing; it is quite distinct from making droplets by jetting during hypervelocity impacts. Evidence both for the existence of molten planetesimals, and for the cooling of chondrules within a plume setting, is strong and growing. Detailed petrographic and isotopic features of chondrules, particularly in carbonaceous chondrites (that probably formed beyond the orbit of Jupiter), suggest that the chondrule plume would have been ‘dirty’ and the otherwise uniform droplets would have been contaminated with earlier-formed dust and larger grains from a variety of sources. The contamination possibly accounts for relict grains, for the spread of oxygen isotopes along the primitive chondrule mineral (PCM) line in carbonaceous chondrites, and for the newly recognized nucleosynthetic isotopic complementarity between chondrules and matrix in Allende.

Chondrule formation and meteorite parent body assembly link the historically geochemically- and petrologically-oriented field of meteoritics to the observationally- and theoretically-oriented field of astrophysics. Laboratory measurements’ high precisions and resolutions constrain planet formation on scales and in parameter spaces inaccessible to even the most powerful telescopes. The dynamic and cosmochemical canvas confronting theoretical and numerical studies of protoplanetary disks and planet formation is too daunting to face without meteoritic signposts. Conversely, with only ancient solid evidence in hand, meteoriticists need astrophysical observations and protoplanetary disk models to give context to their witch’s brew of chemical, isotopic, and petrologic constraints. The ever-increasing wealth of protoplanetary disk observations, along with major advances such as the (re)discovery of the Magneto-Rotational Instability (or MRI), numerical simulations of disk dynamics, and laboratory investigations of dust coagulation have expanded our understanding of protoplanetary disks, leaving us well positioned to review proposed chondrule formation scenarios both from the perspective of astrophysical plausibility (i.e., could they occur and produce melted grains at a meaningful rate), and from a cosmochemical and petrologic perspective (i.e., would the melted grains look like chondrules and be incorporated into chondrite-like planetesimals). In this chapter, we evaluate several chondrule formation scenarios for our own solar system, but one inherent truth in astrophysics is that the universe is large enough for almost any conceivable process to find a home. Potential chondrule-formation mechanisms that fail to make the cut here may yet be important elsewhere.

We review silicate chondrules and metal-sulfide nodules in unequilibrated enstatite chondrites (EH3 and EL3). Their unique mineral assemblages, with a wide diversity of opaque phases, nitrides, and nearly FeO-free enstatite, testify to exceptionally reduced conditions. While those have long been ascribed to a condensation sequence at supersolar C/O ratios, with the oldhamite-rich nodules among the earliest condensates, evidence for relatively oxidized local precursors suggests that their peculiarities may have been acquired during the chondrule-forming process itself. Silicate phases may have been then sulfidized in an O-poor and S-rich environment; whereas metal-sulfide nodules in EH3 chondrites could have originated in the silicate chondrules, those in EL3 may be impact products. The astrophysical setting (nebular or planetary) where such conditions were achieved, whether by depletion in water or enrichment in dry organics-silicate mixtures, is uncertain, but was most likely sited inside the snow line, consistent with the Earth-like oxygen isotopic signature of most EC silicates, with little data constraining its epoch yet.

In this chapter, we review the history of chondrule research and introduce some of the basic concepts in the study of chondrules, including the classification of chondrites and the nomenclature of chondrule types.

Chondrules and matrix from carbonaceous chondrites exhibit complementary nucleosynthetic W isotope anomalies that result from the depletion of a metallic s-process carrier in the chondrules, and the enrichment of this carrier in the matrix. The complementarity is difficult to reconcile with an origin of chondrules in protoplanetary impacts and also with models in which chondrules and matrix formed independently of each other in distinct regions of the disk. Instead, the complementarity indicates that chondrules formed by localized melting of dust aggregates in the solar nebula. The Hf–W ages for metal-silicate fractionation in CV and CR chondrites are 2.2 ± 0.8 Ma and 3.6 ± 0.6 Ma after formation of Ca-Al-rich inclusions, and are indistinguishable from Al–Mg ages for CV and CR chondrules. The good agreement between these ages strongly suggests that 26Al was homogeneously distributed in the solar protoplanetary disk and that therefore Al–Mg ages are chronologically meaningful. The concordant Al–Mg and Hf–W ages reveal that chondrule formation (as dated by Al–Mg) was associated with metal-silicate fractionation (as dated by Hf–W), both within a given chondrite but also among the different subgroups of ordinary chondrites. These age data indicate that chondrules from a given chondrite group formed in a narrow time interval of <1 Ma, and that chondrule formation and chondrite accretion were closely linked in time and space. The rapid accretion of chondrules into a chondrite parent body is consistent with the isotopic complementarity, which requires that neither chondrules nor matrix were lost prior to chondrite accretion. Combined, these observations suggest that chondrule formation was an important step in the accretion of planetesimals.

Here we review the conclusions from the book chapters and outline some ideas for directions of future research. We discuss what we know and do not know about chondrule precursors, the chronology of chondrule formation, physical (temperature, dust/gas ratio, total pressure) and chemical conditions (fO2, partial pressure of Na, S, SiO, …) in chondrule-forming regions during chondrule formation, chondrule thermal histories, and chondrule formation models.

Thermal histories of chondrules can be deduced by studying the petrology and mineral chemistry of natural chondrules and their experimental analogs. Dynamic crystallization experiments have successfully reproduced chondrule textures, and provide general but broad constraints on peak temperatures and cooling rates. Porphyritic textures result when a chondrule is heated to a maximum temperature close to, but below, its liquidus, and cooled at initial rates between about 10 and 1,000 °C/h. Typical liquidus temperatures for chondrules range from about 1,400–1,700 °C. Nonporphyritic chondrules are produced when peak temperatures exceed the liquidus slightly (for barred/dendritic textures) and significantly (radiating textures) and chondrules cool at rates around 500–3,000 °C/h. More quantitative constraints on cooling rates can be determined by considering growth and diffusion-related zoning in chondrule minerals. Results of such modeling are consistent with dynamic crystallization experiments. Rapid dissolution rates for relict olivine grains also indicate a limited time at high temperatures, and indicate fast cooling rates of hundreds to thousands of °C/h, close to peak temperatures. Other cooling rate indicators include disequilibrium partition coefficients between minerals and chondrule glass, and consideration of chemical and isotopic diffusion between relict grains and their overgrowths. Interpretation of both these features is currently ambiguous. Several lines of evidence suggest that cooling rates decreased at lower temperatures, as the chondrule approached the solidus, to <50 °C/h. These include slow cooling required to nucleate plagioclase, cooling rates inferred from trace element diffusion profiles in metal grains, and exsolution microstructures in clinopyroxene. In contrast, clinoenstatite microstructures, the presence of chondrule glass, and dislocation densities in chondrule olivine appear to argue for rapid cooling (103–104 °C/h) through the lower temperature regime, and textures in opaque (metal/sulfide) assemblages indicate cooling rates of hundreds of degrees per hour at subsolidus temperatures. Overall, thermal histories of chondrules can provide fundamental constraints for chondrule formation models. While high-temperature thermal histories are reasonably well constrained, there are currently some open questions about the nature of the cooling curve at lower temperatures. A better understanding of chondrule cooling rates at lower temperatures would help to discriminate between chondrule formation models that make quantitative predictions for thermal histories. Within a single chondrite, cooling rates may vary widely. It is also possible that the nature of cooling histories varies within a given population of chondrules. A statistical treatment of chondrule populations in which individual chondrules show distinct thermal histories would help to make predictions about chondrule formation environments, and the diversity of processes that might be represented in a single chondrule-forming region.