Complementary chemical and isotopic relationships between chondrules and matrix have the potential to distinguish between categories of chondrule forming mechanisms, e.g., exclude all mechanisms that require different reservoirs for chondrules and matrix. The complementarity argument is, however, often misunderstood. Complementarity requires different average compositions of an element or isotope ratio in each of the two major chondrite components chondrules and matrix, and a solar or CI chondritic bulk chondrite ratio of the considered elements or isotopes. For example, chondrules in carbonaceous chondrites typically have superchondritic Mg/Si ratios, while the matrix is subchondritic. Another example would be the Hf/W ratio, which is superchondritic in chondrules and subchondritic in matrix. We regard these ratios to be complementary in chondrules and matrix, because the bulk chondrite has solar Mg/Si and Hf/W ratios. In contrast, Al/Na ratios are also different in chondrules and matrix, but the bulk is not solar; therefore, Al/Na does not have a complementary relationship. A number of publications over the past decade have reported complementary relationships for many element pairs in different types of chondrites. Recently, isotopic complementarities have also been reported. A related, though different, argument can be made for volatile depletion patterns in chondrules and matrix, which can then also be considered as being complementary. The various models for chondrule formation require either that chondrules and matrix formed from a single (i.e., common) parental reservoir, or that chondrules and matrix formed in separate regions of the protoplanetary disk and were later mixed together. As chondrules and matrix have different compositions, mixing of these two components would result in a random bulk chondrite composition. The observation of complementary chondrule–matrix relationships together with a CI chondritic, element or isotope ratio is unlikely to be the result of a random mix of chondrules and matrix. It seems much more likely that chondrules and matrix formed in a single reservoir with initially CI chondritic element or isotope ratios. Incorporation of different minerals in chondrules and matrix together with volatile element depletion of the entire reservoir then resulted in chondrule-matrix complementarities and bulk chondrite volatile depletion. This excludes any chondrule formation mechanism that requires separate parental reservoirs for these components. Any chondrule forming mechanism must explain complementarity. Chondrules and matrix must have formed from a common reservoir.

The 26Al–26Mg systematics of chondrules from ordinary and carbonaceous chondrites and their implications are reviewed. The initial 26Al/27Al ratios [(26Al/27Al)0] based on in situ analyses of chondrules from the least metamorphosed chondrites range from unresolved from zero to ~1.2 × 10‒5 and thus no chondrules have ${\left({}_{}{}^{26}\text{A}\text{l}/{}_{}{}^{27}\text{A}\text{l}\right)}_0^{\mathit{\text{Internal}}}$ ratios corresponding to the canonical level (~5.2 × 10‒5) recorded by CAIs. Assuming homogeneous distribution of 26Al in the protoplanetary disk at the canonical level, these observations suggest chondrule formation started ~1.5 million years after CAIs and lasted over a few million years. The 26Al–26Mg systematics of bulk chondrules could have recorded ${\left({}_{}{}^{26}\text{A}\text{l}/{}_{}{}^{27}\text{A}\text{l}\right)}_0^{\mathit{\text{Bulk}}}$ ratios of chondrule precursors and may suggest that Al–Mg fractionation recorded by chondrule precursors started contemporaneously with CAIs and lasted over ~1.5 million years. The comparisons of formation ages of different meteorites and their components have been made with 26Al–26Mg, 182Hf–182W, and 206Pb–207Pb systematics. While the ages determined by 26Al–26Mg and 182Hf–182W systematics are generally consistent, those determined by 26Al–26Mg and 206Pb–207Pb systematics are largely inconsistent. The homogeneous versus heterogeneous distribution of 26Al in the protoplanetary disk remains controversial.

Nebular shock heating is one of the most fully developed and rigorous models for chondrule formation, and is also the most consistent with the meteoritic record. In this review, we compare the results of current shock modeling to the wealth of meteoritic observations, to highlight where there is agreement and where there is potential failure of the models. The discussion is focused on gravitational disk instability-driven, large-scale shocks and on local bow shocks, with attention to the astrophysical setting for both. We suggest that more than one shock driver may be physically motivated and necessary to explain the variety of chondrules.

Chondrules and matrices make up most of the bulk of chondrites. Chondrites have elemental compositions close to that of the Sun except for volatility related depletions and iron fractionation relative to silicon. Being igneous, chondrules are volatile depleted, while their host matrices are considered to have assembled mostly from low-temperature (volatile-rich) material. There is an outstanding controversy as to whether (i) chondrules and matrices formed independently and subsequent mixing of these components explains the departure of chondrite compositions from solar abundances for major elements, or whether (ii) their complementary patterns imply that the two dissimilar components were formed together from a reservoir with a solar composition and accreted without being separated to maintain this solar composition. The question around which this controversy centers has important implications for our understanding of the working of the protoplanetary disk, as complementarity between chondrules and matrices would rule out some chondrite formation models, such as those that imply an origin of these components from widely separated parts of the disk. In the present paper, we discuss literature data for chondrules, matrices, and bulk carbonaceous chondrites. We point out that, except for those of the CI group, all chondrites are fractionated with respect to the Sun for all major and minor elements across the condensation temperature range. We show that bulk chondrite compositions are better reproduced by adding a generic chondrule composition to the proper amount of CI-composition matrix, rather than by combining the in situ measured compositions of their chondrules and matrices. These results indicate that chondrule and matrix compositions in any given chondrite are not genetically related to one another. We also discuss the case of moderately volatile elements, for which a similarity of patterns between chondrules and matrices has been reported, and argue that this similarity was established as a result of exchange during alteration on the chondrite parent body and does not reflect a common nebular reservoir. Last, we contend that the recently discovered tungsten and molybdenum isotopic differences between chondrules and matrices argue in favor of distinct isotopic reservoirs for these chondritic components and hence against a genetic relationship between chondrules and their host matrices. A time interval between the formation of chondrules and of matrices and long-range relative transport of these components in the disk are, therefore, not ruled out by existing chemical and isotopic constraints.

Chondrules are the millimeter-scale previously molten droplets found in chondritic meteorites. These pervasive yet enigmatic particles hint at energetic processes at work in the nascent solar system. Chondrules and chondrites are well studied and many of the details about their compositions, ages, and thermal histories are well known. Without the proper context of a formation mechanism, however, we can only imagine what chondrules may reveal about the processes at work in the early solar system. In this chapter, we explore the hypothesis that chondrules were formed by impacts between growing planetary embryos. Specifically, we focus on shock heating associated with accretionary impacts as a means for melting chondrule precursor material. Although we discuss previous work on impact origin for chondrules, much of this chapter focuses on a new incarnation of this old idea, the impact jetting model. We explore the predictions of this model and its implications for our understanding of early solar system history and meteoritics. Throughout the chapter, we discuss potential issues and uncertainties with the model while identifying avenues for further development and testing of the impact origin hypothesis.

Chondrules are the millimeter-scale previously molten droplets found in chondritic meteorites. These pervasive yet enigmatic particles hint at energetic processes at work in the nascent solar system. Chondrules and chondrites are well studied and many of the details about their compositions, ages, and thermal histories are well known. Without the proper context of a formation mechanism, however, we can only imagine what chondrules may reveal about the processes at work in the early solar system. In this chapter, we explore the hypothesis that chondrules were formed by impacts between growing planetary embryos. Specifically, we focus on shock heating associated with accretionary impacts as a means for melting chondrule precursor material. Although we discuss previous work on impact origin for chondrules, much of this chapter focuses on a new incarnation of this old idea, the impact jetting model. We explore the predictions of this model and its implications for our understanding of early solar system history and meteoritics. Throughout the chapter, we discuss potential issues and uncertainties with the model while identifying avenues for further development and testing of the impact origin hypothesis.

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.

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.

The parent nuclides 238U and 235U decay to 206Pb and 207Pb, respectively, with half-lives that makes this system uniquely suited to define the temporal framework of the solar protoplanetary disk, including the timing and duration of chondrule formation. Lead isotope data for 22 individual nebular chondrules indicate that the oldest chondrules formed contemporaneously with CAIs and that chondrules were recycled for ~4 Myr within the protoplanetary disk. Integrating the initial Pb isotopic compositions and ages of these individually-dated chondrules reveals that they appear to have formed in two distinct epochs. A primary phase of chondrule production occurred within 1 Myr of the formation of the Sun during the most energetic phase of the protoplanetary disk when mass accretion rates were highest. This epoch of primary chondrule production transitioned into a phase dominated by the reworking of existing chondrules, which lasted for the remainder of the protoplanetary disk’s lifetime. Such a model is consistent with a transition from heating by shock waves related to gravitational instabilities during the more energetic first 1 Myr to heating by bow shocks around early formed planetesimals and planetary embyros. The age of chondrules from the CB meteorite Gujba formed from a vapor–melt plume caused by impacting planetary embyros indicates that the solar protoplanetary disk had dissipated within 4.5 Myr. The Pb–Pb ages require that any appearance of chemical or isotopic complementarity between matrix and chondrules does not imply rapid chondrule formation and accretion or that matrix and chondrules in a single chondrite group have a strict cogenetic relationship. In this view, inferences about the range of ages for chondrule formation based on a 182Hf–182W decay method and the assumption of cogenetically-formed matrix and chondrules cannot be meaningful. Finally, the preponderance of chondrules (>50%) having formed in the first 1 Myr of the protoplanetary disk lifetime is consistent with models of early, efficient growth of planetary embryos by pebble accretion.

Chondrules contain ferromagnetic minerals that may retain a record of the magnetic field environments in which they cooled. Paleomagnetic experiments on separated chondrules can potentially reveal the presence of remanent magnetization from the time of chondrule formation. The existence of such a magnetization places quantitative bounds on the frequency of interchondrule collisions, while the intensity of magnetization may be used to infer the strength of nebular magnetic fields and thereby constrain the mechanism of chondrule formation. Recent advances in laboratory instrumentation and techniques have permitted the isolation of nebular remanent magnetization in chondrules, providing the potential basis to probe the formation environments of chondrules from a range of chondrite classes.

The bulk volatile contents of chondritic meteorites provide clues to their origins. Matrix and chondrules carry differing abundances of moderately volatile elements, with chondrules carrying a refractory signature. At the high temperatures of chondrule formation and the low pressures of the solar nebula, many elements, including Na and Fe, should have been volatile. Yet the evidence is that even at peak temperatures, at or near the liquidus, Na and Fe (as FeO and Fe-metal) were present in about their current abundances in molten chondrules. This seems to require very high solid densities during chondrule formation to prevent significant evaporation. Evaporation should also be accompanied by isotopic mass fractionation. Evidence from a wide range of isotopic systems indicates only slight isotopic mass fractionations of moderately volatile elements, further supporting high solid densities. However, olivine-rich, FeO-poor chondrules commonly have pyroxene-dominated outer zones that have been interpreted as the products of late condensation of SiO2 into chondrule melts. Late condensation of more refractory SiO2 is inconsistent with the apparent abundances of more volatile Na, FeO and Fe-metal in many chondrules. Despite significant recent experimental work bearing on this problem, the conditions under which chondrules behaved as open systems remain enigmatic.

We review recent chondrule oxygen isotope studies by secondary ion mass spectrometry (SIMS). We discuss primary O-isotope fractionation characteristics of chondrule phases, and how they are used to garner information related to the physicochemical environment from which they formed. This includes high temperature gas–melt interactions, sampling of common precursors among different chondrite types, and how precursor compositions influenced redox states during chondrule formation. We also explore how primary O-isotope ratios of chondrule phases are disturbed by secondary alteration.