Compound Chondrules Research Articles

Micro‐XCT chondrule classification for subsequent isotope analysis

AbstractChondrules are microscopic, recrystallized melt droplets found in chondritic meteorites. High‐resolution isotope analyses of minor elements require large enough element quantities which are obtained by dissolving entire chondrules. This work emphasizes the importance of X‐ray computed tomography (XCT) to detect features that can significantly affect the bulk chondrule isotope composition. It thereby expands on other works by looking into chondrules from a wide range of chondrites including CR, CV, CB, CM, L, and EL samples before turning toward complex and time‐consuming chemical processing. The features considered are metal and igneous rims, compound chondrules, matrix remnants, and metal contents. In addition to the identification of these features, computed tomography prevents the inclusion of non‐chondrule samples (pure matrix or metal) as well as samples where two different chondrule fragments with potentially different isotope compositions are held together by matrix. Matrix surrounding chondrules is also easily detected and the affected chondrules can be omitted or reprocessed. The results strongly encourage to perform XCT before dissolution of chondrules for isotope analysis as a non‐invasive method.

Geochemical constraints on the formation of chondrules: Implication from Os and Fe isotopes and HSE abundances in metals from CR chondrites

CR chondrites are suitable for understanding the genetic linkage between metals and chondrules due to the unique characteristics of the coexisting metal phases with chondrules. Metal grains are found in three different locations of CR chondrites; chondrule interior (“interior grain”), chondrule surficial shells (“margin grain”), and the matrix (“isolated grain”). Here we report the abundances of highly siderophile elements (HSEs) and major elements in three types of metals (interior, margin, and isolated grains) from three CR chondrites (NWA 801, NWA 7184, and Dhofar 1432) by using femtosecond LA-ICP-MS (fs LA-ICP-MS) and EPMA. Additionally, we report the isotopic compositions of Os and Fe in the metals by using micro-milling sampling coupled with N-TIMS and MC-ICP-MS. The CR metals have variations in 187Os/188Os and δ57Fe values ranging from 0.1193 to 0.1314 and from −1.05 to +0.25, respectively. HSE abundances, except for Pd and Au, in the three types of metals increase as the abundance of Ir increases. A possible explanation for the variations of HSE abundances within and among grains, 187Os/188Os values within each grain, and δ57Fe values among grains, is the condensation of liquid metal from a gaseous reservoir followed by fractional crystallization. Most of the CR metals have negative δ57Fe values, suggesting that Fe in metal phases might have formed by condensation prior to Fe condensation in silicate phases. The chondrules and three types of metal grains in CR chondrites are believed to have formed contemporaneously in the same region. The existence of large isolated metals in matrix and compound chondrules might be the result of collision and merging of the metal and silicate droplets.

Collisions and compositional variability in chondrule-forming events

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.

An unusual compound object in Yamato 793408 (H3.2‐an): The missing link between compound chondrules and macrochondrules?

AbstractWe found a large (~2 mm) compound object in the primitive Yamato 793408 (H3.2‐an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe‐ and high Ca, Al‐concentrations, although no 16O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s−1) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed.

Compound Chondrule Formation in Optically Thin Shock Waves

Abstract Shock-wave heating within the solar nebula is one of the leading candidates for the source of chondrule-forming events. Here we examine the possibility of compound chondrule formation via optically thin shock waves. Several features of compound chondrules indicate that they are formed via the collisions of supercooled precursors. We evaluate whether compound chondrules can be formed via the collision of supercooled chondrule precursors in the framework of the shock-wave heating model by using semi-analytical methods and discuss whether most of the crystallized chondrules can avoid destruction upon collision in the post-shock region. We find that chondrule precursors immediately turn into supercooled droplets when the shock waves are optically thin, and they can maintain supercooling until the condensation of evaporated fine dust grains. Owing to the large viscosity of supercooled melts, supercooled chondrule precursors can survive high-speed collisions on the order of 1 km s−1 when the temperature is below ∼1400 K. From the perspective of the survivability of crystallized chondrules, shock waves with a spatial scale of ∼104 km may be potent candidates for the chondrule formation mechanism. Based on our results from one-dimensional calculations, a fraction of compound chondrules can be reproduced when the chondrule-to-gas mass ratio in the pre-shock region is ∼2 × 10−3, which is approximately half of the solar metallicity.

Constraints on compound chondrule formation from laboratory high-temperature collisions

In laboratory experiments, spherical 1 mm-wide glass and basalt particles are heated, and the hot particles collide at about 1 m/s with a flat glass target that is at room temperature. When the particles are heated below 900 K, the collisions are essentially elastic with coefficients of restitution of about 0.9, but above 900 K collisions become increasingly inelastic and the coefficient of restitution decreases with increasing temperature. At 1100 K the glass particles approach sticking but, simultaneously, at the same temperature the particles melt on timescales of minutes. The basalt particles approach sticking at 1200 K. Only above 1400 K do basalt grains in contact with each other fuse together, forming compounds on timescales of hours, and at 1500 K basalt grains completely fuse together. Therefore, cooling basalt grains only have a 100 K window for compound formation, and velocities very likely have to be below 1 m/s for sticking in the first place. We predict that this puts constraints on compound chondrule formation and particle densities in the solar nebula.

Effects of Different Walking Aids on Three-Dimensional Trunk Kinematics of Patients Having Undergone First-Time, Elective Transforaminal Lumbar Interbody Fusion

Sulfide assemblages are commonly found in chondritic meteorites as small inclusions in the matrix or in association with chondrules. These assemblages are widely hypothesized to form through pre-accretionary corrosion of metal by H2S gas or through parent body processes. We report here on two unequilibrated R chondrite samples that contain large, chondrule-sized sulfide nodules in the matrix. Both samples are from Mount Prestrud (PRE) 95404. Chemical maps and spot and broad-beam electron microprobe analyses (EMPA) were used to assess the distribution, stoichiometry, and bulk composition of sulfide nodules and silicate chondrules in the clasts. Oxygen isotope data were collected via secondary ion mass spectrometry (SIMS) to assess the relationship of chondrules to other chondrite groups. Scanning electron microscopy (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM) analyses were used to assess fine-scale features and identify crystal structures in sulfide assemblages. Thermodynamic models were used to assess the temperature, sulfur fugacity (fS2), total pressure, dust-to-gas ratio, and oxygen fugacity (fO2) conditions during sulfide nodule and chondrule formation.The unequilibrated clasts include a mixture of type I and type II chondrules, as well as non-porphyritic chondrules. Chondrule oxygen isotopes overlap with ordinary-chondrite chondrules. Sulfide nodules average 200 µm in diameter, have rounded shapes, and are primarily composed of pyrrhotite, pentlandite, and magnetite. Some are deformed around chondrules in a petrologic relationship similar in appearance to compound chondrules. Both nodules and sulfides in chondrules include phosphate inclusions and Cu-rich lamellae, which suggests a genetic relationship between sulfides in chondrules and in the matrix. Ni/Co ratios for matrix and chondrule sulfides are solar, while Fe and Ni are non-solar and inversely related.We hypothesize that sulfide nodules formed via pre-accretionary melt processes. During chondrule formation, precursors composed of a mixture of silicate and sulfide material were heated to form immiscible melt droplets, which separated and cooled to form Si-rich chondrules and S-rich nodules. Sulfide melt was stabilized by a high total pressure (∼1 atm) in a dust- or ice-enriched environment. Heating of this material contributed to a high fS2 (2 × 10−3 atm at 1138 °C), and high fO2 (IW – 1 to IW – 4), in an environment with peak temperatures between 1539 °C and 1750 °C. Oxygen isotopic compositions in this region were similar to those recorded by the LL-chondrite chondrules.

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Cover: Within the carbonaceous chondrite Allende (CV3) a very complex compound chondrule of ~1 mm in size was found consisting of at least 16 sub‐chondrules. Concerning the formation of this compound chondrule calculations suggest the existence of a super‐dense chondrule‐forming region with an extremely high solid‐to‐gas mass ratio. For details see the article by Addi Bischoff et al. on p. 906. Image courtesy of A. Bischoff.

Formation of unequilibrated R chondrite chondrules and opaque phases

Sulfide assemblages are commonly found in chondritic meteorites as small inclusions in the matrix or in association with chondrules. These assemblages are widely hypothesized to form through pre-accretionary corrosion of metal by H2S gas or through parent body processes. We report here on two unequilibrated R chondrite samples that contain large, chondrule-sized sulfide nodules in the matrix. Both samples are from Mount Prestrud (PRE) 95404. Chemical maps and spot and broad-beam electron microprobe analyses (EMPA) were used to assess the distribution, stoichiometry, and bulk composition of sulfide nodules and silicate chondrules in the clasts. Oxygen isotope data were collected via secondary ion mass spectrometry (SIMS) to assess the relationship of chondrules to other chondrite groups. Scanning electron microscopy (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM) analyses were used to assess fine-scale features and identify crystal structures in sulfide assemblages. Thermodynamic models were used to assess the temperature, sulfur fugacity (fS2), total pressure, dust-to-gas ratio, and oxygen fugacity (fO2) conditions during sulfide nodule and chondrule formation.The unequilibrated clasts include a mixture of type I and type II chondrules, as well as non-porphyritic chondrules. Chondrule oxygen isotopes overlap with ordinary-chondrite chondrules. Sulfide nodules average 200µm in diameter, have rounded shapes, and are primarily composed of pyrrhotite, pentlandite, and magnetite. Some are deformed around chondrules in a petrologic relationship similar in appearance to compound chondrules. Both nodules and sulfides in chondrules include phosphate inclusions and Cu-rich lamellae, which suggests a genetic relationship between sulfides in chondrules and in the matrix. Ni/Co ratios for matrix and chondrule sulfides are solar, while Fe and Ni are non-solar and inversely related.We hypothesize that sulfide nodules formed via pre-accretionary melt processes. During chondrule formation, precursors composed of a mixture of silicate and sulfide material were heated to form immiscible melt droplets, which separated and cooled to form Si-rich chondrules and S-rich nodules. Sulfide melt was stabilized by a high total pressure (∼1atm) in a dust- or ice-enriched environment. Heating of this material contributed to a high fS2 (2×10−3atm at 1138°C), and high fO2 (IW – 1 to IW – 4), in an environment with peak temperatures between 1539°C and 1750°C. Oxygen isotopic compositions in this region were similar to those recorded by the LL-chondrite chondrules.

The Allende multicompound chondrule (ACC)—Chondrule formation in a local super‐dense region of the early solar system

AbstractIn Allende, a very complex compound chondrule (Allende compound chondrule; ACC) was found consisting of at least 16 subchondrules (14 siblings and 2 independents). Its overall texture can roughly be described as a barred olivine object (BO). The BO texture is similar in all siblings, but does not exist in the two independents, which appear as relatively compact olivine‐rich units. Because of secondary alteration of pristine Allende components and the ACC in particular, only limited predictions can be made concerning the original compositions of the colliding melt droplets. Based on textural and mineralogical characteristics, the siblings must have been formed on a very short time scale in a dense, local environment. This is also supported by oxygen isotope systematics showing similar compositions for all 16 subchondrules. Furthermore, the ACC subchondrules are isotopically distinct from typical Allende chondrules, indicating formation in or reaction with a more 16O‐poor reservoir. We modeled constraints on the particle density required at the ACC formation location, using textural, mineral‐chemical, and isotopic observations on this multicompound chondrule to define melt droplet collision conditions. In this context, we discuss the possible relationship between the formation of complex chondrules and the formation of macrochondrules and cluster chondrites. While macrochondrules may have formed under similar or related conditions as complex chondrules, cluster chondrites certainly require different formation conditions. Cluster chondrites represent a mixture of viscously deformed, seemingly young chondrules of different chemical and textural types and a population of older chondrules. Concerning the formation of ACC calculations suggest the existence of very local, kilometer‐sized, and super‐dense chondrule‐forming regions with extremely high solid‐to‐gas mass ratios of 1000 or more.

Relationship between CAIs and chondrules: A case study of a compound chondrule from the Allende (CV3) meteorite

We report the mineralogy, petrology, major, minor and trace element geochemistry, O and Si isotopes of a complex compound chondrule from the Allende meteorite. The chondrule contains zones of refractory (Ca, Al-rich) regions along with regions more similar to ferromagnesian chondrules. The bulk silicon isotopic composition of the object is δ30Si = –0.71 ± 0.03‰. The oxygen isotopic composition of the different phases within the compound chondrules fall along the Allende chondrule line and range from Δ17O – 12.5 to –2‰. Rare earth element abundances are enriched compared to chondritic levels by up to 15× CI, and show a nebular condensate signature with depletions in Eu and Yb. Our data show that the object evolved in oxygen isotopes in a nebular environment, most likely due to formation from a mixture of diverse components combined with interaction with nebular gas. In addition, differences in olivine composition across the inclusion suggest isotopically distinct oxygen regions also existed in dust in the protoplanetary disk. This object and other compound objects demonstrate that CAIs were present in the chondrule forming region, however they are not found in the majority of chondrules. We speculate that the bulk of the CAIs may have been added into the CV parent body after initial accretion of the body.

Compound chondrule formation via collision of supercooled droplets

We present a novel model showing that compound chondrules are formed by collisions of supercooled droplets. This model reproduces two prominent observed features of compound chondrules: the nonporphyritic texture and the size ratio between two components.

Compound chondrules fused cold

About 4–5% of chondrules are compound: two separate chondrules stuck together. This is commonly believed to be the result of the two component chondrules having collided shortly after forming, while still molten. This allows high velocity impacts to result in sticking. However, at T∼1100K, the temperature below which chondrules collide as solids (and hence usually bounce), coalescence times for droplets of appropriate composition are measured in tens of seconds. Even at 1025K, at which temperature theory predicts that the chondrules must have collided extremely slowly to have stuck together, the coalescence time scale is still less than an hour. These coalescence time scales are too short for the collision of molten chondrules to explain the observed frequency of compound chondrules. We suggest instead a scenario where chondrules stuck together in slow collisions while fully solid; and the resulting chondrule pair was subsequently briefly heated to a temperature in the range of 900–1025K. In that temperature window the coalescence time is finite but long, covering a span of hours to a decade. This is particularly interesting because those temperatures are precisely the critical window for thermally ionized MRI activity, so compound chondrules provide a possible probe into that vital regime.

The quasi-universality of chondrule size as a constraint for chondrule formation models

Primitive meteorites are dominated by millimeter-size silicate spherules called chondrules. The nature of the high-temperature events that produced them in the early Solar System remains enigmatic. Beside their thermal history, one important clue is provided by their size which shows remarkably little variation (less than a factor of 6 for the mean chondrule radius of most chondrites) despite the extensive range of ages and heliocentric distances sampled. It is however unclear whether chondrule size is due to the chondrule melting process itself, or has been simply inherited from the precursor material, or yet results from some sorting process. I examine these different possibilities in terms of their analytical size predictions. Unless the chondrule-forming “window” was very narrow, radial sorting can be excluded as a size-determining process because of the large variations it would predict. Molten planetesimal collision or impact melting models, which derive chondrules from the fragmentation of larger melt bodies, would likewise predict too much size variability by themselves; more generally any size modification during chondrule formation is limited in extent by evidence from compound chondrules and the considerable compositional variability of chondrules. Turbulent concentration would predict a low size variability but lack of evidence of any accretion bias in carbonaceous chondrites may be difficult to reconcile with any form of local sorting upon agglomeration. Growth by sticking (especially if bouncing-limited) of aggregates as chondrule precursors would yield limited variations of their final radius in space and time, and would be consistent with the relatively similar size of other chondrite components such as refractory inclusions. This suggests that the chondrule-melting process(es) simply melted such nebular aggregates with little modification of mass.

Petrography and mineralogy of the ungrouped type 3 carbonaceous chondrite Dar al Gani 978

AbstractDar al Gani (DaG) 978 is an ungrouped type 3 carbonaceous chondrite. In this study, we report the petrography and mineralogy of Ca,Al‐rich inclusions (CAI), amoeboid olivine aggregates (AOAs), chondrules, mineral fragments, and the matrix in DaG 978. Twenty‐seven CAIs were found: 13 spinel‐diopside‐rich inclusions, 2 anorthite‐rich inclusions, 11 spinel‐troilite‐rich inclusions, and 1 spinel‐melilite‐rich inclusion. Most CAIs have a layered texture that indicates a condensation origin and are most similar to those in R chondrites. Compound chondrules represent a high proportion (approximately 8%) of chondrules in DaG 978, which indicates a local dusty chondrule‐forming region and multiple heating events. Most spinel and olivine in DaG 978 are highly Fe‐rich, which corresponds to a petrologic type of >3.5 and a maximum metamorphic temperature of approximately 850–950 K. This conclusion is also supported by other observations in DaG 978: the presence of coarse inclusions of silicate and phosphate in Fe‐Ni metal, restricted Ni‐Co distributions in kamacite and taenite, and low S concentrations in the matrix. Mineralogic records of iron‐alkali‐halogen metasomatism, such as platy and porous olivine, magnetite, hedenbergite, nepheline, Na‐rich in CAIs, and chlorapatite, are present, but relatively limited, in DaG 978. The fine‐grained, intergrowth texture of spinel‐troilite‐rich inclusions was probably formed by reaction between pre‐existing Al‐rich silicates and shock‐induced, high‐temperature S‐rich gas on the surface of the parent body of DaG 978. A shock‐induced vein is present in the matrix of DaG 978, which indicates that the parent body of DaG 978 at least experienced a shock event with a shock stage up to S3.

MINERAL PROCESSING BY SHORT CIRCUITS IN PROTOPLANETARY DISKS

Meteoritic chondrules were formed in the early solar system by brief heating of silicate dust to melting temperatures. Some highly refractory grains (Type B calcium-aluminum-rich inclusions, CAIs) also show signs of transient heating. A similar process may occur in other protoplanetary disks, as evidenced by observations of spectra characteristic of crystalline silicates. One possible environment for this process is the turbulent magnetohydrodynamic flow thought to drive accretion in these disks. Such flows generally form thin current sheets, which are sites of magnetic reconnection, and dissipate the magnetic fields amplified by a disk dynamo. We suggest that it is possible to heat precursor grains for chondrules and other high-temperature minerals in current sheets that have been concentrated by our recently described short-circuit instability. We extend our work on this process by including the effects of radiative cooling, taking into account the temperature dependence of the opacity; and by examining current sheet geometry in three-dimensional, global models of magnetorotational instability. We find that temperatures above 1600 K can be reached for favorable parameters that match the ideal global models. This mechanism could provide an efficient means of tapping the gravitational potential energy of the protoplanetary disk to heat grains strongly enough to form high-temperature minerals. The volume-filling nature of turbulent magnetic reconnection is compatible with constraints from chondrule-matrix complementarity, chondrule-chondrule complementarity, the occurrence of igneous rims, and compound chondrules. The same short-circuit mechanism may perform other high-temperature mineral processing in protoplanetary disks such as the production of crystalline silicates and CAIs.

A new model for the origin of Type‐B and Fluffy Type‐A CAIs: Analogies to remelted compound chondrules

Abstract– In the scenario developed here, most types of calcium‐aluminum‐rich inclusions (CAIs) formed near the Sun where they developed Wark‐Lovering rims before being transported by aerodynamic forces throughout the nebula. The amount of ambient dust in the nebula varied with heliocentric distance, peaking in the CV–CK formation location. Literature data show that accretionary rims (which occur outside the Wark‐Lovering rims) around CAIs contain substantial 16O‐rich forsterite, suggesting that, at this time, the ambient dust in the nebula consisted largely of 16O‐rich forsterite. Individual sub‐millimeter‐size Compact Type‐A CAIs (each surrounded by a Wark‐Lovering rim) collided in the CV–CK region and stuck together (in a manner similar to that of sibling compound chondrules); the CTAs were mixed with small amounts of 16O‐rich mafic dust and formed centimeter‐size compound objects (large Fluffy Type‐A CAIs) after experiencing minor melting. In contrast to other types of CAIs, centimeter‐size Type‐B CAIs formed directly in the CV–CK region after gehlenite‐rich Compact Type‐A CAIs collided and stuck together, incorporated significant amounts of 16O‐rich forsteritic dust (on the order of 10–15%) and probably some anorthite, and experienced extensive melting and partial evaporation. (Enveloping compound chondrules formed in an analogous manner.) In those cases where appreciably higher amounts of 16O‐rich forsterite (on the order of 25%) (and perhaps minor anorthite and pyroxene) were incorporated into compound Type‐A objects prior to melting, centimeter‐size forsterite‐bearing Type‐B CAIs (B3 inclusions) were produced. Type‐B1 inclusions formed from B2 inclusions that collided with and stuck to melilite‐rich Compact Type‐A CAIs and experienced high‐temperature processing.

Visualisation and quantification of CV chondrite petrography using micro-tomography

Micro-computed tomography is a non-destructive technique that allows the study of 3D meteorite petrography. The technique produces a unique and instructive visualisation of the meteorite for quantifying its components. We studied the overall petrography of the two CV chondrites Allende and Mokoia to constrain their formation histories. A set of movies and stereographic images detail the 3D petrography. Component modal abundances agree with previous reports and modal abundance differences between Allende and Mokoia support the chondrule–matrix complementarity and that chondrules and matrix formed from the same chemical reservoir. We identified two types of chondrules, a normal type and one where a normal type I or II chondrule is almost completely encapsulated by an opaque-rich layer. This layer was probably acquired during a late stage condensation process. The appearance of opaques in chondrules and matrix is different, not supporting a genetic relationships between these. Low abundances of compound chondrules (1.75vol% in Allende and 2.50vol% in Mokoia) indicate low chondrule densities and/or low relative component velocities in chondrule formation regions. Porosities on a scale <10–20μm allowed for only local aqueous alteration processes on the meteorite parent bodies.

Questions, questions: Can the contradictions between the petrologic, isotopic, thermodynamic, and astrophysical constraints on chondrule formation be resolved?

Abstract– Here, we show that several geochemical indicators point to number densities during chondrule formation that were far higher than can be accounted for by known nebula processes. The number densities implied by compound chondrules and nonspherical chondrules are shown to be significantly higher than estimated in previous studies. At the implied chondrule number densities, if a chondrule formation region survived a formation event it would have been gravitationally bound and would have collapsed quite rapidly to form an asteroidal‐sized body. The diversity of chondrule compositions and textures in a chondrite group could have formed in a single event in subvolumes of a formation region that were chemically isolated from one another because of slow diffusion in the gas. Within these subvolumes, equilibration between chondrules with different compositions would have been fairly rapid, although small isotopic mass fractionations in elements like Fe, Si, Mg, and O may persist. This could explain the existence of the small isotopic mass fractionations in these elements that have been observed in chondrules. However, the evidence for recycling of chondrules requires that there was more than one chondrule formation event prior to formation of a parent asteroid. Finally, we argue that OC and CO chondrule Mg‐Al systematics are both consistent with single ages or narrow ranges of ages, and that the CO, and possibly the OC, ages date parent body alteration. This would resolve the conundrum of needing to preserve in a turbulent nebula physically and chemically distinct CO and OC chondrule populations for 1–2 Myr.

Evaporation and recondensation of sodium in Semarkona Type II chondrules

We have investigated the Na distributions in Semarkona Type II chondrules by electron microprobe, analyzing olivine and melt inclusions in it, mesostasis and bulk chondrule, to see whether they indicate interactions with an ambient gas during chondrule formation. Sodium concentrations of bulk chondrule liquids, melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine ± pyroxene. The most primitive olivine cores in each chondrule are mostly between Fa 8 and Fa 13, with 0.0022–0.0069 ± 0.0013 wt.% Na 2O. Type IIA chondrule olivines have consistently higher Na contents than olivines in Type IIAB chondrules. We used the dependence of olivine–liquid Na partitioning on FeO in olivine as a measure of equilibration. Extreme olivine rim compositions are ∼Fa 35 and 0.03 wt.% Na 2O and are close to being in equilibrium with the mesostasis glass. Olivine cores compared with the bulk chondrule compositions, particularly in IIA chondrules, show very high apparent D Na, indicating disequilibrium and suggesting that chondrule initial melts were more Na-rich than present chondrule bulk compositions. The apparent D Na values correlate with the Na concentrations of the olivine, but not with concentrations in the bulk melt. We use equilibrium D Na to find the Na content of the true parent liquid and estimate that Type IIA chondrules lost more than half their Na and recondensation was incomplete, whereas Type IIAB chondrules recovered most of theirs in their mesostases . Glass inclusions in olivine have lower Na than expected from fractionation of bulk composition liquids, and mesostases have higher Na than expected in calculated daughter liquids formed by fractional crystallization alone. These observations also require open system behavior of chondrules, specifically evaporation of Na before formation of melt inclusions followed by recondensation of Na in mesostases. Within this record of evaporation followed by recondensation, there is no indication of a stage with zero Na in the chondrules, which is predicted by models for shock wave cooling at canonical nebular pressures, suggesting high P T. The high Na concentrations in olivine and mesostases indicate very high P Na while chondrules were molten. This may be explained by local, very high particle densities where Type II chondrules formed. The high P T, P Na and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle-rich clumps in the disk. Encounters of partially molten chondrules should have been frequent in these dense swarms. However, in many ordinary chondrites like Semarkona, “cluster chondrites”, compound chondrules are not abundant but instead chondrules aggregated into clusters. Chondrule melting, cooling and clustering in dense swarms contributed to rapid accretion, possibly after collision, by fallback on the grandparent body and by reaccretion as a new body downrange.