CR Chondrules Research Articles

Late Pebble Accretion of Comet 81P/Wild 2 Nucleus: Evidence from a Plagioclase-bearing Chondrule Fragment, Pyxie

Comet 81P/Wild 2 is a ∼4.5 km-sized primordial object that almost has not been modified by internal heating by 26Al decay. Its nucleus could have been formed by hierarchical agglomeration or gravitational collapse of pebble swarms concentrated by streaming instability. To shed light on the cometesimal formation mechanism from laboratory sample analysis, we reexamined the 26Al–26Mg isotope systematics of the plagioclase-bearing fragment, Pyxie (from Wild 2 track 81), with significantly improved analytical precision. The revised upper limit of the initial (26Al/27Al)0 of Pyxie is ≤1.5 × 10−6, 2 times smaller than those estimated from other Wild 2 fragments. Assuming homogenous distribution of 26Al in the early solar system, the minimum crystallization age of Pyxie is estimated to be >3.6 Ma after calcium–aluminum-rich inclusions. Additional petrologic examination demonstrated that it is a chondrule fragment formed in disk environments enriched in moderately volatile elements comparable to the Si-rich rim of CR chondrules before accreting by comet Wild 2. The late accretion of the Wild 2 nucleus with most silicates likely from a common source are not favored by the hierarchical agglomeration model that considers early and continuous accretion. Instead, the results are more in line with comet formation by gentle gravitational collapse of pebbles when the 26Al abundance is extremely low (26Al/27Al ≤ 1.5 × 10−6) before gas dispersal.

Comet 81P/Wild 2 dust impactors of Stardust turnip-like tracks analogous to cluster IDPs

We measured oxygen isotope ratios of 16 silicate fragments from seven aerogel tracks (turnip-like type B tracks 77, 149, 172, 191, and 220; carrot-like type A tracks 22 and 175) of the comet 81P/Wild 2 collector from NASA’s Stardust mission using secondary ion mass spectrometry. Thirteen were prepared by ultramicrotomy; three from track 220 were prepared by sputtering resin blocks using a SIMS Kohler beam, a new procedure aiming to mine as many cometary particles encased in aerogel/resin as possible. Combining new and literature results, we recognized that most silicate fragments of individual type B tracks have diverse mineralogy but consistent mass-independent fractionation of oxygen isotopes (Δ17O = δ17O − 0.52 × δ18O). These observations suggest that their impactors are loosely bound aggregates of unequilibrated materials originating mainly from similar protoplanetary disk regions, resembling the cluster IDP U2-20-GCA. Furthermore, silicate fragments from type A track 22 have almost identical mineralogy and Δ17O values, confirming that its impactor is a single chondrule-like fragment. The terminal particle of type A track 175 is pure forsterite with Δ17O of ∼ –23 ‰.Six iron-rich fragments of this study have positive oxygen isotope ratios (Δ17O∼ +2 ‰) and ordinary chondrite chondrule-like olivine compositions. Together with five similar fragments in the literature, a unique population (Mg# ≤ 86) of Wild 2 fragments that resemble chondrules from the inner solar system (O-E-R) chondrites or the outer solar system CH-CB chondrites was identified. The remaining 16O-poor Wild 2 fragments are Mg# ≥ 79 silicates with Δ17O ∼ –2 ‰ and a small amount of Mg# ≤ 79 silicates with Δ17O ∼ 0 ‰, which are most consistent with CR chondrite chondrules. Thus, we conclude that in addition to the possible major source of CR chondrite chondrule-like materials, the inner solar system or CH-CB chondrule-like materials are a minor component of comet Wild 2, like the cluster IDP U2-20-GCA.

Isotopic evidence for two chondrule generations in CR chondrites and their relationships to other carbonaceous chondrites

Among primitive meteorites, CR chondrites have peculiar isotopic compositions, the origin of which is uncertain and may have involved contributions from primordial molecular cloud material or the chondrites' formation and agglomeration late during the evolution of the protoplanetary disk. Here, we report a comprehensive textural and isotopic characterization of type I CR chondrules and provide new insights on their formation conditions. We find that two chondrule populations characterized by different sizes and oxygen isotopic compositions co-exist in CR chondrites. The typically larger, 16O-poor (Δ17O>-4‰) chondrules (type I-CR chondrules) appear to have formed late out of a CR reservoir already populated by typically smaller, 16O-rich (Δ17O<-4‰) chondrules (type I-CO chondrules). Before formation of type I-CR chondrules, the CR reservoir was likely dominated by CI-like dust, in line with the proximity of CR with CI chondrites for many isotopic ratios. The CR reservoir thus may have largely belonged to the continuum shown by other carbonaceous chondrites, although some isotopic ratios maintain some originality and suggest isotopic variation of CI-like dust in the outer disk. Combined with literature data, our data (i) demonstrates that recycling processes are responsible for the singular compositions of CR chondrites and their chondrules for isotopic systems with drastically different geochemical behaviors (O, Cr, Te) and (ii) support the homogeneous distribution of 26Al throughout the protoplanetary disk.

In-situ O-isotope analysis of relict spinel and forsterite in small (

We report high-precision secondary ion mass spectrometer triple oxygen isotope systematics (95 individual analyses) from 37 micrometeorites (MMs) collected from South Pole Water Well (SPWW), Antarctica. The study population focuses on unmelted coarse-grained (Cg) MMs (n = 23) with both multiple (n = 14) and single-mineral (n = 9) varieties investigated. We also analysed relict minerals in porphyritic cosmic spherules (n = 13) and the relict matrix in a single scoriaceous fine-grained (Fg) MM. The target minerals investigated are primarily olivine (Fo ∼ 43–99%) and spinel. Textural, chemical and isotopic data confirm that both olivine and spinel grains have retained their pre-atmospheric O-isotope compositions, allowing inferences to be drawn about their formation and parent body affinities. We separate the study population into three groups: spinel-free particles (consisting of the CgMMs and PO cosmic spherules), spinel-bearing MMs and the single FgMM.Olivine grains in spinel-free MMs vary between δ17O: −12.6‰ and +3.5‰, δ18O: −9.6‰ and +7.5‰, and Δ17O: −9.5‰ and +1.3‰ and define a slope-1 profile in δ18O–δ17O isotope space. They are most likely fragmented chondrules, with both type I and type II varieties represented. Their observed Mg#-Δ17O distribution is best explained by a mixture of CM chondrules and either CR chondrules, Tagish Lake chondrules or WILD2 cometary silicates. One of these chondrule-like MMs has an isotopically heterogeneous composition, characterised by a single olivine grain with a markedly 16O-rich composition (Δ17O: −16.3‰), suggesting it is a relict silicate fragment of AOA material that was incorporated into the chondrule precursor.We analysed 11 spinel grains in five spinel-bearing MMs. In all instances spinels are nearly pure MgAl2O4 with isotopically light (16O-rich) compositions (ranging from δ17O: −34.4‰ to −0.9‰, δ18O: −30.8‰ to +11.0‰, and Δ17O: −18.3‰ to −4.4‰). They are therefore 16O-poor relative to spinel found in unaltered CAIs, indicating a different origin. Grains with high Cr2O3 contents (>0.5 wt.%) are interpreted originating from Al-rich chondrule precursors, while low Cr2O3 spinels (<0.5 wt.%) are interpreted as CAI-derived material affected by parent body aqueous alteration.Finally, we report a single FgMM with a 16O-poor composition (Δ17O > 0‰ and δ18O > +15.0‰). This particle adds to our growing inventory of water-rich C-type asteroid samples united by their formation history which is characterised by accretion of abundant heavy water.Our work strongly supports findings from earlier in-situ O-isotope studies, concluding that small MMs overwhelmingly sample material from CC parent bodies and that CgMMs largely sample chondrules and, to a lesser extent, CAI material. The analysis of CgMMs therefore provides insights into the primitive O-isotope reservoirs that were present in the early solar system and how they interacted.

Oxygen isotope systematics of crystalline silicates in a giant cluster IDP: A genetic link to Wild 2 particles and primitive chondrite chondrules

Anhydrous interplanetary dust particles (IDPs) collected from Earth's stratosphere are the most primitive extraterrestrial materials that likely are the remnants of major building blocks of our solar system. While they probably originated from icy outer solar system comets, this hypothesis needs to be further verified via comparison with Wild 2 (a Jupiter-family comet) particles returned by the Stardust mission. Besides, the origins of their components can be further constrained by comparing with materials in primitive chondrite. Here we investigate the petrology and oxygen isotope systematics of 20 fragments extracted from a giant cluster IDP U2-20GCA. Fifteen are monomineralic or polymineralic fragments composed of olivine and/or pyroxene. Others are plagioclase/glass bearing fragments similar to barred-olivine chondrules, enstatite-rich chondrules, and Al-rich chondrules (ARC). They show a range of Mg# [mol% Mg/(Mg+Fe)] in olivine and pyroxene (99-75) and MnO and Cr2O3 abundances in olivine that resemble Wild 2 particles. Individual fragments have relatively 16O-poor oxygen isotope ratios with δ18O and δ17O varying from −6.2±1.0‰ to 6.8±1.9‰ and −6.7±2.6‰ to 5.3±1.2‰, respectively. Most fragments show Δ17O (=δ17O-0.52×δ18O) increasing from ∼−3‰ to ∼0‰ with decreasing Mg#, similar to those observed in Wild 2 particles and CR chondrite chondrules. Four fragments (including the ARC-like fragment) show oxygen isotope signatures of ordinary (O), R, or CH-CB chondrite chondrules. The similarities among the giant cluster IDP, Wild 2 particles, and primitive chondrite chondrules demonstrate (i) a cometary origin of anhydrous IDPs; (ii) the major source of crystalline silicates in comets is likely to be CR chondrite chondrule-like materials, while minor sources include O, R, or CH-CB chondrite chondrule-like materials. The conclusions support the hypothesis that anhydrous IDPs sampled extremely heterogeneous materials formed in wide solar system regions like Wild 2 particles.

Origin of volatile element depletion among carbonaceous chondrites

Compared to the composition of CI chondrites and the Sun, all other carbonaceous chondrites are variably depleted in volatile elements. However, the origin of these depletions, and how they are related to volatile loss during high-temperature processes within the solar nebula, are unclear. To better understand the processes that caused volatile element fractionations among carbonaceous chondrites, we obtained mass-dependent Te isotopic compositions and Te concentrations for a comprehensive set of samples from the major carbonaceous chondrite groups. The chondrites exhibit well-resolved inter-group Te isotope variations towards lighter isotopic compositions for increasingly volatile-depleted samples. The Te isotopic compositions and concentrations are also correlated with the mass fraction of matrix and with nucleosynthetic ε54Cr anomalies. Combined, these correlations indicate mixing between volatile-rich, isotopically heavy, and 54Cr-rich CI-like matrix with volatile-poor, isotopically light, and 54Cr-poorer chondrules or chondrule precursors. The Te-Cr isotopic correlation suggests that all carbonaceous chondrites contain CI-like matrix, and that chondrules and this CI-like matrix formed from isotopically distinct material originating from different regions of the disk. The only samples plotting off the Te-Cr correlation are CR chondrites, indicating that CR chondrules formed from different precursor material than chondrules from other carbonaceous chondrites, either because they formed at greater heliocentric distance and/or at a later time. Plots of volatile element abundances versus matrix mass fraction reveal that chondrules/chondrule precursors display CI-chondritic ratios for volatile elements with 50% condensation temperatures below ∼750 K, with an overall abundance of ∼0.13 × CI. Mixing between these two components, therefore, naturally results in CI-like ratios for these elements in all carbonaceous chondrites, in spite of different degrees of volatile depletion. A corollary of this observation is that the CI-like ratios of volatile elements in the bulk silicate Earth may result from the accretion of volatile-depleted materials and do not require accretion of CI chondrites themselves.

Weak Magnetic Fields in the Outer Solar Nebula Recorded in CR Chondrites

AbstractTheoretical investigations suggest that magnetic fields may have played an important role in driving rapid stellar accretion rates and efficient planet formation in protoplanetary disks. Experimental constraints on magnetic field strengths throughout the solar nebula can test the occurrence of magnetically driven disk accretion and the effect of magnetic fields on planetary accretion. Here we conduct paleomagnetic experiments on chondrule samples from primitive CR (Renazzo type) chondrites GRA 95229 and LAP 02342, which likely originated in the outer solar system between 3 and 7 AU approximately 3.7 million years after calcium aluminum‐rich inclusion formation. By extracting and analyzing 18 chondrule subsamples that contain primary, igneous ferromagnetic minerals, we show that CR chondrules carry internally non‐unidirectional magnetization that requires formation in a nebular magnetic field of ≤8.0 ± 4.3 μT (2σ). These weak magnetic fields may be due to the secular decay of nebular magnetic fields by 3.7 million years after calcium aluminum‐rich inclusions, spatial heterogeneities in the nebular magnetic field, or a combination of both effects. The possible inferred existence of spatial variations in the nebular magnetic field would be consistent with a prominent role for disk magnetism in the formation of density structures leading to gaps and planet formation.

The role of Bells in the continuous accretion between the CM and CR chondrite reservoirs.

CM meteorites are dominant members of carbonaceous chondrites (CCs), which evidently accreted in a region separated from the terrestrial planets. These chondrites are key in determining the accretion regions of solar system materials, since in Mg and Cr isotope space, they intersect between what are identified as inner and outer solar system reservoirs. In this model, the outer reservoir is represented by metal‐rich carbonaceous chondrites (MRCCs), including CR chondrites. An important question remains whether the barrier between MRCCs and CCs was a temporal or spatial one. CM chondrites and chondrules are used here to identify the nature of the barrier as well as the timescale of chondrite parent body accretion. We find based on high precision Mg and Cr isotope data of seven CM chondrites and 12 chondrules, that accretion in the CM chondrite reservoir was continuous lasting <3 Myr and showing late accretion of MRCC‐like material reflected by the anomalous CM chondrite Bells. We further argue that although MRCCs likely accreted later than CM chondrites, CR chondrules must have initially formed from a reservoir spatially separated from CM chondrules. Finally, we hypothesize on the nature of the spatial barrier separating these reservoirs.

Extended chondrule formation intervals in distinct physicochemical environments: Evidence from Al-Mg isotope systematics of CR chondrite chondrules with unaltered plagioclase.

Al-Mg isotope systematics of twelve FeO-poor (type I) chondrules from CR chondrites Queen Alexandra Range 99177 and Meteorite Hills 00426 were investigated by secondary ion mass spectrometry (SIMS). Five chondrules with Mg#'s of 99.0 to 99.2 and Δ17O of -4.2‰ to -5.3‰ have resolvable excess 26Mg. Their inferred (26Al/27Al)0 values range from (3.5 ± 1.3) × 10‒6 to (6.0 ± 3.9) × 10‒6. This corresponds to formation times of 2.2 (-0.5/+1.1) Myr to 2.8 (‒0.3/+0.5) Myr after CAIs, using a canonical (26Al/27Al)0 of 5.23 × 10-5, and assuming homogeneously distributed 26Al that yielded a uniform initial 26Al/27Al in the Solar System. Seven chondrules lack resolvable excess 26Mg. They have lower Mg#'s (94.2 to 98.7) and generally higher Δ17O (-0.9‰ to -4.9‰) than chondrules with resolvable excess 26Mg. Their inferred (26Al/27Al)0 upper limits range from 1.3 × 10‒6 to 3.2 × 10‒6, corresponding to formation >2.9 to >3.7 Myr after CAIs. Al-Mg isochrons depend critically on chondrule plagioclase, and several characteristics indicate the chondrule plagioclase is unaltered: (1) SIMS 27Al/24Mg depth profile patterns match those from anorthite standards, and SEM/EDS of chondrule SIMS pits show no foreign inclusions; (2) transmission electron microscopy (TEM) reveals no nanometer-scale micro-inclusions and no alteration due to thermal metamorphism; (3) oxygen isotopes of chondrule plagioclase match those of coexisting olivine and pyroxene, indicating a low extent of thermal metamorphism; and (4) electron microprobe data show chondrule plagioclase is anorthite-rich, with excess structural silica and high MgO, consistent with such plagioclase from other petrologic type 3.00-3.05 chondrites. We conclude that the resolvable (26Al/27Al)0 variabilities among chondrules studied are robust, corresponding to a formation interval of at least 1.1 Myr. Using relationships between chondrule (26Al/27Al)0, Mg#, and Δ17O, we interpret spatial and temporal features of dust, gas, and H2O ice in the FeO-poor chondrule-forming environment. Mg# ≥ 99, Δ17O ~-5‰ chondrules with resolvable excess 26Mg initially formed in an environment that was relatively anhydrous, with a dust-to-gas ratio of ~100×. After these chondrules formed, we interpret a later influx of 16O-poor H2O ice into the environment, and that dust-to-gas ratios expanded (100× to 300×). This led to the later formation of more oxidized Mg# 94-99 chondrules with higher Δ17O (-5‰ to -1‰), with low (26Al/27Al)0, and hence no resolvable excess 26Mg. We refine the mean CR chondrite chondrule formation age via mass balance, by considering that Mg# ≥ 99 chondrules generally have resolved positive (26Al/27Al)0 and that Mg# < 99 chondrules generally have no resolvable excess 26Mg, implying lower (26Al/27Al)0. We obtain a mean chondrule formation age of 3.8 ± 0.3 Myr after CAIs, which is consistent with Pb-Pb and Hf-W model ages of CR chondrite chondrule aggregates. Overall, this suggests most CR chondrite chondrules formed immediately before parent body accretion.

No cosmic‐ray precompaction exposure of chondrules in CR2.7 MIL 090657

AbstractCosmogenic nuclides are useful for reconstructing the cosmic‐ray precompaction exposure history of the chondritic components in the early solar system. Here, we present He and Ne isotope data for 22 chondrules and 4 matrix samples from Miller Range (MIL) 090657, which is the third most pristine Renazzo‐type (CR) carbonaceous chondrite (CR2.7). The studied samples contain variable mixtures of galactic cosmic ray (GCR)‐produced cosmogenic noble gases and trapped noble gases of presolar origin. Remarkably, all chondrules and matrix samples have within analytical uncertainty almost identical nominal 21Ne cosmic‐ray exposure (CRE) ages in the range 3.45–6.20 Ma. The nominal 3He CRE ages are mostly discordant, possibly due to partial 3H and 3He losses and/or an inaccurate correction for trapped 3He. The absence of precompaction exposure effects longer than a few Ma in MIL 090657 is at variance with data for other CR chondrites. We conclude that most CR chondrules experienced the same precompaction exposure history—although the nature of the precompaction exposure differs between the chondrites—in regions of the disk that were either exposed to or shielded from GCRs but in which exposure effects due to solar wind and/or solar cosmic rays (SCRs) were negligible. The new data do not support the X‐wind model of Shu and coworkers.

Magnesium and 54Cr isotope compositions of carbonaceous chondrite chondrules – Insights into early disk processes

We report on the petrology, magnesium isotopes and mass-independent 54Cr/52Cr compositions (μ54Cr) of 42 chondrules from CV (Vigarano and NWA 3118) and CR (NWA 6043, NWA 801 and LAP 02342) chondrites. All sampled chondrules are classified as type IA or type IAB, have low 27Al/24Mg ratios (0.04–0.27) and display little or no evidence for secondary alteration processes. The CV and CR chondrules show variable 25Mg/24Mg and 26Mg/24Mg values corresponding to a range of mass-dependent fractionation of ∼500ppm (parts per million) per atomic mass unit. This mass-dependent Mg isotope fractionation is interpreted as reflecting Mg isotope heterogeneity of the chondrule precursors and not the result of secondary alteration or volatility-controlled processes during chondrule formation. The CV and CR chondrule populations studied here are characterized by systematic deficits in the mass-independent component of 26Mg (μ26Mg∗) relative to the solar value defined by CI chondrites, which we interpret as reflecting formation from precursor material with a reduced initial abundance of 26Al compared to the canonical 26Al/27Al of ∼5×10−5. Model initial 26Al/27Al values of CV and CR chondrules vary from (1.5±4.0)×10−6 to (2.2±0.4)×10−5. The CV chondrules display significant μ54Cr variability, defining a range of compositions that is comparable to that observed for inner Solar System primitive and differentiated meteorites. In contrast, CR chondrites are characterized by a narrower range of μ54Cr values restricted to compositions typically observed for bulk carbonaceous chondrites. Collectively, these observations suggest that the CV chondrules formed from precursors that originated in various regions of the protoplanetary disk and were then transported to the accretion region of the CV parent asteroid whereas CR chondrule predominantly formed from precursor with carbonaceous chondrite-like μ54Cr signatures. The observed μ54Cr variability in chondrules from CV and CR chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir. The coupled μ26Mg∗ and μ54Cr systematics of CR chondrules establishes that these objects formed from a thermally unprocessed and 26Al-poor source reservoir distinct from most inner Solar System asteroids and planetary bodies, possibly located beyond the orbits of the gas giants. In contrast, a large fraction of the CV chondrules plot on the inner Solar System correlation line, indicating that these objects predominantly formed from thermally-processed, 26Al-bearing precursor material akin to that of inner Solar System solids, asteroids and planets.

&lt;sup&gt;26&lt;/sup&gt;Al in chondrules from CR2 chondrites

anoa, 0 ) in these chondrules range from ~1 × 10 -6 to ~6 × 10 -6 . The other eight chondrules show no detectable 26 Mg excesses and have upper limits of ( 26 Al/ 27 Al)0 ~(2-3) × 10 -6 . Most CR chondrules have ( 26 Al/ 27 Al)0 significantly lower than those in the other least metamorphosed primitive chondrites, LL3.0, CO3.0, and Acfer 094 (ungrouped). Assuming uniform distri- bution of 26 Al in the protoplanetary disk, these observations suggest that majority of CR chondrules formed >1 Myr later than those in LL3.0, CO3.0, and Acfer 094.

Oxygen isotope ratios of FeO-poor chondrules in CR3 chondrites: Influence of dust enrichment and H2O during chondrule formation

We present detailed electron microprobe analyses and oxygen three-isotope measurements by high precision secondary ion mass spectrometry on 45 type I (FeO-poor) chondrules/fragments and 3 type II (FeO-rich) chondrule fragments from Meteorite Hills 00426 and Queen Alexandra Range 99177, two of the most primitive CR3 chondrites. Type I chondrules/fragments have Mg#’s (defined as the Mg# of constituent olivine and/or low-Ca pyroxene) ranging from 94.2 to 99.2; type II chondrule fragments have Mg#’s of 53–63. Oxygen three-isotope measurements plot on the slope ∼1 primitive chondrule mineral (PCM) line. Within chondrules, Δ17O (=δ17O–0.52×δ18O) values of coexisting olivine, pyroxene, and plagioclase are homogeneous, with propagated uncertainties of 0.3‰. This indicates each phase crystallized from the final chondrule melt, and that efficient oxygen isotope exchange occurred between ambient gas and chondrule melt. Among type I chondrules there is a well-defined increase in Δ17O, from –5.9‰ to ∼−1‰, as Mg#’s decrease from 99.2 to ∼96; type II chondrule fragments are comparatively 16O-poor (Δ17O: ∼0.2–0.6‰). The relationship between Mg# and Δ17O among type I chondrules confirms that addition of a 16O-poor oxidizing agent to the highest Mg# chondrule precursors resulted in forming lower Mg# CR chondrules. Using aspects of existing equilibrium condensation models and a mass balance we estimate that type I CR chondrules formed at dust enrichments of 100–200×, from dusts with 0–0.8 times the atomic abundance of ice, relative to CI dust. The type II chondrule fragments are predicted to have formed at CI dust enrichments near 2500×.

Metal in CR chondrites

In section many low-FeO CR chondrules are surrounded by rings of metal; this metal-cladding seems to have formed during chondrule melting events as films of metal that wetted the surface. Electron microprobe studies show that in each ring the metal is very uniform in composition, consistent with efficient mixing during formation of the metal film. In contrast the mean Ni contents of 13 different rings vary by up to a factor of 2. There is no FeS associated with ring metal. Ring metal Co is positively correlated with Ni but the Co/Ni ratio seems to decrease with increasing Ni. We observed a weak negative correlation between ring metal Ni and the fayalite content of the host olivine. Coarse interior metal has higher Ni contents than that in the surrounding rings. At any specific chondrule location, smaller grains tend to have lower Ni contents than larger grains. These trends in Ni seem to reflect two processes: (1) The mean Ni content of metal (and easily reduced sulfides or oxides) in chondrule precursor materials seems to have decreased with the passage of time; on average, the metal in earlier-formed chondrules had higher Ni contents than the metal in later-formed chondrules. (2) Some oxidized Fe was reduced during chondrule formation leading to lower Ni contents in small grains compared to large grains; prior to reduction the Fe was in FeS or in FeO in accessible (fine-grained) sites. We suggest that the compositional evolution of nebular solids was responsible for the interchondrule variations whereas reduction of minor amounts of FeS or FeO was responsible for the size-related small variations in Ni content. We suggest that, during chondrule formation events, CR chondrules experienced relatively long thermal pulses that were responsible for the thorough loss of FeS and the common granoblastic texture observed in low-FeO chondrules. The preservation of the structures of internal rings shows, however, that even though high temperatures occurred in the secondary chondrule, temperatures in the centers of large (>20 μm) metal and silicate grains in the primary chondrule did not get high enough to cause appreciable melting.

Oxygen isotopic constraints on the origin of magnesian chondrules and on the gaseous reservoirs in the early Solar System

We report in situ ion microprobe analyses of the oxygen isotopic composition of the major silicate phases (olivine, low-Ca pyroxene, silica, and mesostasis) of 37 magnesian porphyritic (type I) chondrules from CV (Vigarano USNM 477-2, Vigarano UH5, Mokoia, and Efremovka) and CR (EET 92042, EET 92147, EET 87770, El Djouf 001, MAC 87320, and GRA 95229) carbonaceous chondrites. In spite of significant variations of the modal proportions of major mineral phases in CR and CV chondrules, the same isotopic characteristics are observed: (i) olivines are isotopically homogeneous at the ‰ level within a chondrule although they may vary significantly from one chondrule to another, (ii) low-Ca pyroxenes are also isotopically homogeneous but systematically 16O-depleted relative to olivines of the same chondrule, and (iii) all chondrule minerals analyzed show 16O-enrichments relative to the terrestrial mass fractionation line, enrichments that decrease from olivine (±spinel) to low-Ca pyroxene and to silica and mesostasis. The observation that, in most of the type I chondrules studied, the coexisting olivine and pyroxene crystals and glassy mesostasis have different oxygen isotopic compositions implies that the olivine and pyroxene grains are not co-magmatic and that the glassy mesostasis is not the parent liquid of the olivine. The δ 18O and δ 17O values of pyroxene and olivine appear to be strongly correlated for all the studied CR and CV chondrules according to: δ 18 O (pyroxene) = 0.72 - 0.16 + 0.13 × δ 18 O (olivine) + 1.21 - 0.54 + 0.79 δ 17 O (pyroxene) = 0.81 - 0.15 + 0.13 × δ 17 O (olivine) + 0.60 - 1.00 + 0.95 These correlations are consistent with mass balance of oxygen isotopes calculated in the case of chondrule formation according to the following gas–melt–mineral reactions: Mg 2 SiO 4(olivine) + SiO 2(melt) = Mg 2 Si 2 O 6(pyroxene) SiO 2(melt) = SiO (gas) + 1 / 2 × O 2(gas) These equations imply that 2/3 of the oxygen incorporated by low-Ca pyroxene during its crystallization in the melt is derived from the dissolution of precursor olivine and 1/3 from the addition of chemical components such as SiO from the nebular gas. The composition of the nebular gaseous reservoir in which type I CR and CV chondrules formed is determined from the present data set to be δ 18O (gas) ∼ 3.6 ± 1‰ and δ 17O (gas) ∼ 1.8 ± 1‰. This isotopic composition is in excellent agreement with that of the gaseous reservoir previously inferred from the oxygen isotope systematics of bulk chondrules from various chondrite groups (i.e., CV, CR, unequilibrated ordinary, and EH). Thus, the oxygen isotopic systematic strongly supports the idea that most of the olivines in type I chondrules are relict grains and that gas–melt interaction played a major role in the formation of type I chondrules.

Evolution of Oxygen Isotopic Composition in the Inner Solar Nebula

Changes in the chemical and isotopic composition of the solar nebula with time are reflected in the properties of different constituents that are preserved in chondritic meteorites. CR-group carbonaceous chondrites are among the most primitive of all chondrite types and must have preserved solar nebula records largely unchanged. We have analyzed the oxygen and magnesium isotopes in a range of the CR constituents of different formation temperatures and ages, including refractory inclusions and chondrules of various types. The results provide new constraints on the time variation of the oxygen isotopic composition of the inner (<5 AU) solar nebula—the region where refractory inclusions and chondrules most likely formed. A chronology based on the decay of short-lived 26 Al (t1=2 � 0:73 Myr) indicates that the inner solar nebula gas was 16 O-rich when refractory inclusions formed, but less than 0.8 Myr later, gas in the inner solar nebula became 16 O-poor, and this state persisted at least until CR chondrules formed � 1‐2 Myr later. We suggest that the inner solar nebula became 16 O-poor because meter-sized icy bodies, which were enriched in 17 Oa nd 18 O as a result of isotopic self-shielding during the ultraviolet photodissociation of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated outside the snow line, drifted rapidly toward the Sun, and evaporated at the snow line. This led to significant enrichment in 16 O-depleted water,whichthenspreadthroughtheinnersolarsystem.Astronomicalstudiesofthespatialandtemporalvariations of water abundance in protoplanetary disks may clarify these processes. Subject headingg meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — planetary systems: formation — solar system: formation

Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions.

The lead-lead isochron age of chondrules in the CR chondrite Acfer 059 is 4564.7 +/- 0.6 million years ago (Ma), whereas the lead isotopic age of calcium-aluminum-rich inclusions (CAIs) in the CV chondrite Efremovka is 4567.2 +/- 0.6 Ma. This gives an interval of 2.5 +/- 1.2 million years (My) between formation of the CV CAIs and the CR chondrules and indicates that CAI- and chondrule-forming events lasted for at least 1.3 My. This time interval is consistent with a 2- to 3-My age difference between CR CAIs and chondrules inferred from the differences in their initial 26Al/27Al ratios and supports the chronological significance of the 26Al-26Mg systematics.

Distribution of siderophile elements in CR chondrites: evidence for evaporation and recondensation during chondrule formation

New data on the chemical composition of bulk samples, and of metallic and nonmagnetic fractions of six CR chondrites (Renazzo, Y793495, PCA91082, EET92042, Acfer 209 and El Djouf 001) are reported. It is shown that volatile siderophile element abundance patterns of metallic and nonmagnetic fractions of CR chondrites are complementary and volatility-dependent. In the metallic fraction CI-normalized abundances of Au, As, Sb and Ga decrease with increasing volatility, whereas in the nonmagnetic fraction abundances increase in the same sequence as in the metallic fraction. We argue that the siderophile element patterns of the metallic and nonmagnetic fractions reflect those of the chondrule and matrix fractions of CR chondrites, respectively, based on: (1) CR metal is mostly located inside chondrules and those metal grains outside chondrules probably were also derived from chondrules; (2) element partitioning within chondrules has been reset during chondrule formation; and (3) resetting of element distribution within chondrules occurred at sufficiently reducing conditions to allow partitioning of Au, As, Sb and Ga into CR metal.The complementary siderophile element patterns of CR chondrules and matrix are difficult to explain by gradual gas loss during condensation. The CI proportions of highly volatile elements in the bulk CR chondrites further argue against the possibility of loss of solids during condensation. Thus, the fractionation of volatile elements in CR chondrites is unlikely to be the result of gas-solid fractionation during condensation.The fractionation of volatile siderophile elements between CR chondrules and matrix requires evaporation of volatile elements during chondrule formation. The matrix pattern indicates recondensation of evaporated volatile elements. It appears, based on the composition of CR matrix, that the depletion and fractionation of moderately volatile elements in the bulk CR chondrites is due to formation of CR chondrites before complete recondensation of volatile elements which were evaporated during chondrule formation. This implies that agglomeration of CR chondrites proceeded simultaneously with chondrule formation.