CHONDRULES AND THE PROTOPLANETARY DISK PDF

Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: AbstractMajor advances in deciphering the record of nebula processes in chondrites can be attributed to analytical improvements that allow coordinated isotopic and mineralogical studies of components in chondrites and to a wealth of new meteorites from hot and cold deserts. View PDF.

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Chondrite meteorites are samples of primitive asteroidal bodies that have escaped melting and differentiation. These solids formed by transient heating events during the lifetime of the solar protoplanetary disk. Collectively, CAIs and chondrules provide time-sequenced samples allowing us to probe the composition of the disk material that accreted to form planetesimals and planets. Here, we showcase the current state-of-the-art data with respect to the chronology and stable isotopic compositions of individual chondrules from various chondrite groups and discuss how these data can be used to provide novel insights into the thermal and chemical evolution of the solar protoplanetary disk, including mass transport processes.

Protoplanetary disks are flattened, rotating structures consisting of cool dust and gas surrounding most young low-mass stars, and are a consequence of the requirement to conserve angular momentum during the gravitational collapse of a prestellar core. Some material, however, coalesces into centimetre-sized particles that accrete to form larger asteroidal bodies, which represent the building blocks of planetary systems.

Thus, the study of these accreting protoplanetary disks provides direct insights into the initial conditions for planet formation. The majority of chondrules have porphyritic textures see Fig. Backscattered electron images of a — d porphyritic and e , f non-porphyritic chondrules in CO, CR, and CH carbonaceous chondrites. The porphyritic chondrules consist of ferromagnesian olivine and pyroxenes, glassy or crystalline mesostasis, Fe,Ni-metal and sulfides.

Porphyritic chondrules commonly contain relict grains a , b , indicative of incomplete melting of chondrule precursors. Some chondrules are surrounded by finer-grained igneous rims, indicative of repeatable melting events experienced by these chondrules c , d. The non-porphyritic chondrules contain neither relict grains nor igneous rims, suggesting that they experienced crystallisation from complete melts. CAIs represent the oldest Solar System dated solids and, thus, define its age at Some CAIs were subsequently melted, most in the same disk region.

Following their formation, CAIs were transported to large radial distances where they accreted into chondritic and cometary parent bodies. Therefore, CAIs and chondrules provide time-sequenced samples allowing us to probe the composition of the disk material that accreted to form planetesimals and planets.

The majority of chondrules formed as melt droplets in high-density regions of the protoplanetary disk and accumulated in the disk mid-plane together with other chondritic components. In this model, it is proposed that chondrules and matrix are genetically related and formed in highly localised regions of the protoplanetary disk. The chronology of chondrule formation is typically based on the short-lived 26 Al to 26 Mg decay system [ 26 Al decays to 26 Mg with a half-life of 0.

In this view, chondrule formation is restricted to the inner regions of the solar protoplanetary disk. However, a number of recent studies investigating the absolute chronology of chondrule formation as well as the isotopic systematics of individual chondrules from various chondritic meteorites require a reassessment of current thinking with respect to the formation history of chondrules as well as the parent asteroids of chondrite meteorites. These data are at odds with the traditional view of a short formation history for chondrule population from individual chondrites, the basic concept of chondrule-matrix complementarity as well as the timescales and style of chondrite parent body accretion.

In this contribution, we review the current state-of-the-art data with respect to the chronology and stable isotopic compositions of individual chondrules from various chondrite groups and discuss how these data can be used to provide novel insights into the thermal and chemical evolution of the solar protoplanetary disk , including mass transport processes.

In typical i. The refractory inclusions and chondrules are isotopically uniform, but have different oxygen-isotope compositions, suggesting formation in isotopically distinct reservoirs.

Porphyritic chondrules commonly contain relict grains that did not crystallise from a host chondrule melt, and, therefore, provide constraints on the nature of chondrule precursor materials and chondrule-forming mechanism s. Relict grains in chondrules can be distinguished based on their textures, mineralogy, chemical and oxygen isotopic compositions.

Most relict ferromagnesian olivine and pyroxene grains have oxygen-isotope compositions that differ from those of the host chondrule phenocrysts and mesostasis. Oxidation states of igneous rims Fa or Fs contents in their olivines and pyroxenes are generally similar to those of the host chondrules, suggesting formation under similar redox conditions.

Inverse Pb-Pb diagram. The so-called nebular chondrules are taken here as representing chondrules formed within the protoplanetary disk before dust and gas dissipation.

A second group of chondrules apparently formed later by planetary collisions , namely chondrules from the metal- rich CB chondrites. Initial attempts to date nebular chondrules pooled a number of objects that resulted in ages that were These ages can only reflect the average age of the chondrules pooled for these studies.

Large chondrules from the Gujba CB chondrite, believed to have formed by a collision between two planetesimals , have been dated as individual inclusions with an average age of U-corrected Pb-Pb ages of individual chondrules and CAIs from various carbonaceous and ordinary chondrites.

The lower initial is interpreted as reflecting a reduced initial abundance of 26 Al in the precursor material of chondrules relative to the CAI-forming region. AC, Acfer This conclusion, however, is based on the assumption of 26 Al homogeneity, which has not been demonstrated by any study and is not constrained by their data.

Thus, it is unlikely that the array defines a meaningful isochron. As such, higher precision data is required to better understand the significance of this array. In this interpretation, the bulk Allende chondrule isochron may represent the timing of formation of chondrule precursors contemporaneously with CAI formation.

Given the mounting evidence for initial 26 Al heterogeneity, it appears unlikely that the 26 Al- 26 Mg system can provide an accurate chronology of the early Solar System. If correct, this supports the view that the Hf nuclide was homogeneously distributed in the protoplanetary disk at the time of formation of canonical CAIs.

However, given the low abundance of W in chondritic components such as chondrules, it is not possible to date individual objects thereby necessitating the pooling of a significant amount of chondrules to obtain sufficient mounts of W.

They hypothesised that the Pb-Pb ages reflect late stage parent body alteration without providing any specific mechanism to explain isochrons as old as CAIs. Instead, they used the short-lived Hf- W decay system applied to s or s of chondrules as well as matrix and bulk samples in an attempt to obtain the true age range of chondrules.

However, using matrix, bulk samples and bulk chondrules to define Hf- W isochrons requires independent evidence for a single, closed-system Hf-W fractionation event affecting the matrix and chondrules to fulfil the basic requirement of a meaningful isochron. Invoking this model to infer coeval fractionation of Hf and W between chondrules and matrix, these authors used these entities and mixtures of them to define a Hf-W array that corresponds to an age of 2. Furthermore, linearity of large multi-chondrule fractions in Hf-W space is predicted if the age distributions of the various populations are similar, even if they are not coeval.

This may indicate that both systems remained closed in each chondrule after their respective final nebular heating event and, therefore, both are capable of returning primary age information about chondrules formation, even in an aqueously altered and metamorphosed meteorite like Allende.

However, only the Pb-Pb system is capable of dating individual chondrules to determine the true age range of nebular chondrule formation. The short time interval inferred for the formation of chondrules within individual chondrite groups based on the 26 Al- 26 Mg system can be used to argue for a rapid accretion of chondrite parent bodies. This implies that chondrule formation and asteroidal accretion are intrinsically linked processes. Carbonaceous chondrites formed from material that is distinct from the bulk of the material thought to have accreted in the terrestrial planet region.

EC , enstatite chondrites. OC , ordinary chondrites. Uncertainties for ureilite data are smaller than symbols. A thermal processing event occurs at T 1 , which leads to enrichments in 26 Al and 54 Cr in a hypothetical gas and depletion in 26 Al and 54 Cr in the residual disk solids. The green box represents the predicted composition of the 26 Al-free and thermally unprocessed molecular cloud material. This clearly emphasises that chondrules formed by distinct mechanisms may occur in individual chondrite groups.

In this case, however, the two formation mechanisms result in easily identifiable distinct petrologic features for chondrules. Accepting that shock waves are the dominant heat source for producing chondrules in most chondrite groups, this raises the possibility that the spectrum of petrologic features observed in chondrules is the expression of the numerous potential sources of shock waves that were active during the lifetime of the protoplanetary disk.

For example, the energy required for the thermal processing of dust in the outer Solar System may result from planetary embryos bow shocks or, alternatively, impacts. This emphasises that the energy source required to melt dust resulting in the production of chondrules may be variable in both space and time during the evolution of the protoplanetary disk.

Similar to bulk asteroidal and planetary material , the stable isotopic compositions of individual chondrules allow us to determine the formation regions of their precursor material. At face value, these results indicate that chondrules formed from isotopically heterogeneous precursor material in different regions of the protoplanetary disk and were then transported to accretion regions of their respective parent bodies.

The non-carbonaceous group consists of ordinary and enstatite chondrites, angrites, aubrites, eucrites, diogenites, mesosiderites, acapulcoites and ureilites as well as Martian meteorites. The carbonaceous chondrites are thought to have accreted in a water-rich reservoir beyond the snow line whereas the non-carbonaceous chondrites and achondrites are believed to have formed Sunward of the snow line. The scale of the x -axis is the same for both panels.

This is consistent with the low abundance of CAIs in CR chondrites thereby supporting the view of limited transport of inner Solar System solids to their accretion region. The lack of evidence for admixing of appreciable amounts of thermally unprocessed primordial molecular cloud material in the precursors of CV chondrules suggest that their accretion region s was spatially isolated from that of metal-rich chondrites.

In other words, inward transport of outer Solar System millimetre-sized solids to the accretion region of CV chondrites appears to have been limited for a significant period of the disk lifetime. Two classes of models have been invoked to explain how high temperature refractory material was redistributed throughout the disk to be incorporated into primitive bodies.

One class of models are disk models, which explore how the inward transport of mass and angular momentum may result in outward transport in the early evolution of protoplanetary disks. In these models, the transport of material is apparently most efficient in highly turbulent disks, which may limit the efficiency of this mechanism to the earliest stages. Thus, a consequence of outward diffusion via the disk midplane is the incorporation of refractory material within early formed asteroidal and planetary embryos.

The second type of outward transport models is based on the magnetically driven outflows characteristic of young stellar objects. These outflows provide an efficient mechanism for releasing the angular momentum inherited from the accretion process. The bulk chemistry of chondrites is defined by the two major components, chondrules and matrix. These studies have concluded that the average compositions of chondrules and matrix are typically different for a number of elements in an individual chondrite whereas the bulk composition, which reflects a mixtures of chondrules and matrix, has approximately a solar elemental abundance.

This so-called chondrule-matrix complementary has been used to argue for a genetic link between these two components and, therefore, formation from a single reservoir. Thus, the chondrule-matrix complementarity requires that chondrule formation and asteroidal accretion are intimately linked.

At face value, these data appear inconsistent with the concept of chondrule-matrix complementary as originally envisaged, namely that all chondrules from an individual chondrites are all genetically related to the coexisting matrix. In these models, various chondrule populations remained in complementarity such that the bulk contribution from each source is chemically solar and, thus, so is the final mixture. However, these experiments assume that the main transport mechanism of chondrules occurs through outward diffusion via the disk midplane.

CAIs and chondrules is not expected to be efficiently coupled to the gas and, thus, it is unclear how complementary can be preserved.

A possibility is the observed chondrule-matrix complementarity is an expression of the generic process of chondrule formation and does not reflect a genetic link.

This does not require that the matrix is genetically linked to the chondrules in an individual chondrite but merely that some of it has experienced earlier chondrule formation events. In this view, fractions of the matrix in a particular chondrite may be complementary to chondrule populations in other chondritic meteorites. Time scales of solid formation and disk evolution. Therefore, the thermal regime required for CAI condensation may only have existed during the earliest stages of disk evolution typified by high mass accretion rates onto the central star.

In contrast, recurrent chondrule formation occurred throughout the lifetime of the protoplanetary disk. The metal-rich chondrites, namely the CR, CH and CB chondrites, formed from precursor material that has largely escaped the thermal processing recorded by the inner Solar System bodies.

The metal-rich chondrites and their chondrules appear to have incorporated appreciable amounts of thermally unprocessed primordial molecular cloud material, suggesting formation in the outer part of the Solar System, beyond the orbits the gas giant planets.

Thus, thermal processing of solids, including chondrule formation was not restricted to the inner disk regions but also occurred in the outer Solar System. However, the mechanism and efficiency of the thermal processing of solids at large orbital distances are poorly understood. The stable isotope compositions of the inner and outer Solar System materials are distinct, implying limited mixing of these two reservoirs.

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