Chondrules In Ordinary Chondrites Research Articles

Isotopic evolution of the inner solar system revealed by size-dependent oxygen isotopic variations in chondrules

The systematic isotopic difference between refractory inclusions and chondrules, particularly for oxygen, has long indicated an isotopic evolution of the solar protoplanetary disk. However, it remains underconstrained whether such evolution continued during chondrule formation. Intrigued by past reports of the size-dependent oxygen isotopic compositions of chondrules in ordinary chondrites (OC), we analyzed type I olivine-rich chondrules of various sizes in two LL3 chondrites. Although most chondrules show positive Δ17O values comparable to bulk ordinary chondrites, a population of smaller (less than about 0.1 mm2 in cross-section), including many isolated olivine grains (sensu lato), are 16O-enriched (with Δ17O values down to −13.2 ‰). Literature data allow the same observation for R chondrites. All sub-TFL type I chondrules (i.e., Δ17O < 0) chondrules have Mg# > 97 mol% while the supra-TFL type I chondrule olivines extend to the formal boundary with type II chondrules (i.e., Mg# = 90 mol%). The sub-TFL chondrules are likely genetically related to isotopically similar aluminum-rich chondrules described in the literature. They therefore must have formed earlier than most OC and R chondrules when the inner disk was still sub-TFL. This interpretation is supported by the presence of similarly 16O-rich relict grains in supra-TFL OC and R chondrules that must be remains of this incompletely recycled precursor material. The non-carbonaceous reservoir was thus still evolving isotopically towards 16O-poorer composition when chondrule formation began, whether by mixing with outer disk material, late accretion streamers and/or an increase in the solid/gas ratio due to magnetothermal disk winds.

Barred olivine chondrules in ordinary chondrites: Constraints on chondrule formation

AbstractIn general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO‐rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO‐poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO‐poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.

High precision 26Al-26Mg chronology of chondrules in unequilibrated ordinary chondrites: Evidence for restricted formation ages

Chondrules in ordinary chondrites are considered to form in high density environments, likely related to the evolution of protoplanets and large planetesimals. In order to determine the timing of their formation at high time resolution (≤0.1 Ma), we conducted high precision Al-Mg chronology of 17 porphyritic chondrules from 6 different unequilibrated ordinary chondrites (UOCs) of low petrologic subtypes (3.00–3.05). Detailed petrology, mineralogy, and oxygen isotope ratios of individual chondrules were also obtained that include 10 additional chondrules without Al-Mg ages. Seventeen chondrules for Al-Mg chronology consist of 14 chondrules with plagioclase (An1-An87) and three chondrules with Na-rich glassy mesostasis, all of which have high 27Al/24Mg ratios (30–3000). The inferred initial (26Al/27Al)0 ratios range between (6.5 ± 0.6) × 10–6 to (9.5 ± 1.0) × 10–6, corresponding to chondrule formation ages of 1.74 ± 0.12/0.11 Ma to 2.13 ± 0.09 Ma after CAIs, which have a canonical (26Al/27Al)0 ratio of 5.25 × 10–5. Six albite-bearing chondrules (An<30) show a much more restricted ages range, spanning between 2.00 ± 0.11/0.10 Ma and 2.07 ± 0.11/0.10 Ma. Including 14 anorthite-bearing chondrules studied previously, chondrules in ordinary chondrites have a restricted range of formation ages from 1.8 Ma to 2.2. Ma after CAIs.Based on the newly acquired oxygen isotope data and previous high precision studies, chondrules in L and LL chondrites do not show systematic difference in their δ18O and δ17O signatures. Majority of plagioclase-bearing chondrules studied for Al-Mg chronology show similar oxygen isotope ratios to those of glass-bearing chondrules. There is no obvious difference in Al-Mg ages of chondrules between L and LL chondrites. Thus, chondrules in L and LL chondrites would have formed in common environments and processes, though they accreted to two separate parent bodies by 2.2 Ma after CAIs, which timing is consistent with the proposed thermal model for ordinary chondrite parent bodies. Onset of chondrule formation at 1.8 Ma after CAIs may be caused by the delay of Jupiter formation or the formation of protoplanets in ordinary chondrite chondrule forming regions if chondrules formed by large scale disk shock or impact jetting of protoplanets. Alternatively, early formed chondrules would not be preserved before 1.8 Ma if chondrules formed by the impacts of molten planetesimals.

A temporal shift of chondrule generation from the inner to outer Solar System inferred from oxygen isotopes and Al-Mg chronology of chondrules from primitive CM and CO chondrites

Deciphering the spatial and temporal evolution of chondrules allows for a better understanding of how asteroidal seeds formed, migrated, and eventually accreted into parent asteroids. Here we report high precision Al-Mg ages and oxygen three-isotope ratios of fifteen FeO-poor chondrules from the least metamorphosed Mighei-like (CM) and Ornans-like (CO) carbonaceous chondrites, Asuka 12236 (CM2.9), Dominion Range 08006 (CO3.01), and Yamato 81020 (CO3.05). This is the first report of Al-Mg ages of chondrules from the CM chondrite group. All but one of the fifteen chondrules exhibit a restricted range of inferred initial 26Al/27Al ratios, and all ratios are ≤ 6.0 × 10−6, which is systematically lower than those of the majority of ordinary chondrite (OC) chondrules. These observations indicate that the majority of chondrules in the outer Solar System were produced ≥2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), which postdates OC chondrule formation in the inner Solar System (≤2.2 Ma after CAI formation). We propose that the discrete chondrule-forming events in different disk regions reflect a time difference in growth and orbital evolution of planetesimals within the first 4 Ma of the Solar System.One chondrule from Asuka 12236 has an age of 1.9 Ma after CAI formation and is therefore significantly older than the other fourteen chondrules, meaning this chondrule formed contemporaneously with the majority of OC chondrules. This old chondrule also exhibits 16O-depleted oxygen isotope characteristics compared to the other chondrules, suggesting a distinct formation region, probably inside the disk region relative to where the majority of CM and CO chondrules formed. Our results indicate that this old chondrule has migrated from the inner to the outer part of the protoplanetary disk within ∼1 Ma and then accreted into the CM parent asteroid >3 Ma after CAI formation, although its formation exterior to the accretion region of the CM parent asteroid and subsequent inward migration cannot be ruled out completely.

Oxygen isotope systematics of chondrules in Rumuruti chondrites: Formation conditions and genetic link with ordinary chondrites

AbstractRumurutiites (R chondrites) are rare, highly oxidized chondrites belonging to the noncarbonaceous superclan and characterized by low chondrule abundances. Although textural and chemical features of Rumurutiite chondrules resemble those of ordinary chondrites (OCs), their formation conditions and potential genetic link remain debated. Here, we report high‐resolution elemental X‐ray mapping analyses and in situ O isotopic measurements of olivine grains from five chondrules and eight isolated olivine grains (IOGs) in the NWA 12482 R3 chondrite. The chondrules show chemical zonings similar to their counterparts in ordinary and carbonaceous chondrites (CCs), implying that gas–melt interaction processes between chondrule precursors and SiO‐ and Mg‐rich gas were operative throughout the circumsolar disk. Our isotopic data show that R chondrules are isotopically similar to ordinary chondrules, although differences in their abundances of relict olivine grains and chondrule textural characteristics suggest different formation environments, with R chondrules being formed from 16O‐poorer precursors. As with chondrules in OCs, the O isotopic characteristics of R chondrules and IOGs suggest limited transport between CC and noncarbonaceous reservoirs.

Hf‐W chronology of a macrochondrule from the L5/6 chondrite Northwest Africa 8192

AbstractA large, igneous‐textured, and 2 cm‐sized spherical object from the L5/6 chondrite NWA 8192 was investigated for its chemical composition, petrography, O isotopic composition, and Hf‐W chronology. The petrography and chemical data indicate that this object closely resembles commonly found chondrules in ordinary chondrites and is therefore classified as a “macrochondrule.* As a result of metal loss during its formation, the macrochondrule exhibits elevated Hf/W, which makes it possible to date this object using the short‐lived 182Hf‐182W system. The Hf‐W data provide a two‐stage model age for metal–silicate fractionation of 1.4 ± 0.6 Ma after Ca‐Al‐rich inclusion (CAI) formation, indicating that the macrochondrule formed coevally to normal‐sized chondrules from ordinary chondrites. By contrast, Hf‐W data for metal from the host chondrite yield a younger model age of ~11 Ma after CAIs. This younger age agrees with Hf‐W ages of other type 5–6 ordinary chondrites, and corresponds to the time of cooling below the Hf‐W closure temperature during thermal metamorphism on the parent body. The Hf‐W model age difference between the macrochondrule and the host metal demonstrates that the Hf‐W systematics of the bulk macrochondrule were not disturbed during thermal metamorphism, and therefore, that the formation age of such objects can still be determined even in strongly metamorphosed samples. Collectively, this study illustrates that chondrule formation was not limited to mm‐size objects, implying that the rarity of macrochondrules reflects either that this process was very inefficient, that subsequent nebular size‐sorting decimated large chondrules, or that large precursors were rare.

Various Size-sorting Processes for Millimeter-sized Particles in the Sun’s Protoplanetary Disk? Evidence from Chondrules in Ordinary Chondrites

Abstract Chondrules are approximately millimeter-sized beads of crystallized silicate melt. They formed mainly in the first ∼3 Ma of the Sun’s protoplanetary disk and are the main constituents of chondritic asteroids. Here we report on the size–frequency distributions (2D and 3D) of chondrules in the brecciated ordinary chondrite (OC) Northwest Africa (NWA) 5205. We investigated three large (centimeter- to decimeter-sized) chondritic lithic clasts of a particular textural type (“cluster chondrite”) with eye-catching different chondrule sizes. One clast shows the largest mean chondrule size (∼1.5 mm) ever measured in a chondrite. As in the other OCs, we find a positive correlation between the minimum and mean chondrule size, which we consider as an argument for chondrule size sorting. Chondrule size–frequency distributions in the clasts are distinctly more symmetric than the about log-normal distributions in other OCs. Furthermore, we find a co-enrichment of chondrule types with a priori small mean sizes (type I, porphyritic) in clasts with overall small mean chondrule sizes. We consider this as the fingerprint of an additional/second size-sorting process, which acted later on these chondrule populations. This process possibly subdivided a typical LL-type chondrule population into several subpopulations with different mean chondrule sizes. We speculate that this second sorting occurred in a unidirectional gas stream or headwind, e.g., by settling of chondrules through an asteroidal atmosphere or interaction with an expanding impact plume. Possibly, fine-grained matrix was almost completely removed by this, and the size-sorted chondrule subpopulations accreted in a hot state separately in different regions of the asteroid.

Hf-W chronology of ordinary chondrites

Abstract Fifteen H, L, and LL ordinary chondrites of petrologic types 4–6 have been analyzed for Hf-W isotope systematics to constrain the chronology, internal structure, and thermal history of their parent bodies. For most samples coarse-grained metals plot below the isochrons defined by silicate-dominated fractions which consist of variable mixtures of silicate minerals with tiny metal inclusions. This offset results from an earlier Hf-W closure in the large metal grains and provides a new means for simultaneously determining cooling rates and Hf-W closure ages for individual samples. For most type 5 and 6 samples, cooling rates and Hf-W ages are inversely correlated, indicating that these samples derive from concentrically zoned bodies in which more strongly metamorphosed samples derive from greater depth. These data, therefore, provide strong evidence for a common ‘onion shell’ structure for the H, L, and LL chondrite parent bodies. The cooling rates and Hf-W ages of some type 5 and 6 chondrites overlap, indicating that the Hf-W systematics provide a more robust measure of the thermal history and burial depth of a given sample than the simple petrographic distinction between types 5 and 6. Two type 6 samples deviate from the correlation between cooling rates and Hf-W ages and cooled much faster than expected for their Hf-W age. These samples likely were excavated by impacts that occurred during high-temperature metamorphism and prior to complete closure of the Hf-W system at ∼10 Ma after CAI formation. As these impacts would have disturbed the asteroid’s cooling history, these samples likely derive from different bodies than samples with undisturbed cooling histories, implying that ordinary chondrites derive from more than just three parent bodies. The Hf-W data reveal that metal-silicate fractionation among the H, L, and LL groups occurred between ∼2 and ∼2.7 Ma after CAI formation and, hence, was about coeval to chondrule formation. As both metal-silicate fractionation and chondrule formation occurred prior to chondrite parent body accretion, there should be no ordinary chondrite chondrules that are younger than ∼2.7 Ma. Finally, ordinary chondrite precursors had lower Hf/W ratios than carbonaceous chondrites, suggesting that inner and outer solar system materials, respectively, were chemically distinct even for refractory elements.

Mineralogically zoned chondrules in ordinary chondrites as evidence for open system chondrule behaviour

Abstract Chondrules are a major component of chondritic meteorites. Understanding their formation conditions provides fundamental insights about how the early solar system formed and evolved. We studied the textures of ∼650 chondrules from all three groups (H, L, LL) of ordinary chondrites, in 2-dimensional (2D) sections through the meteorites. About 40% of the chondrules are mineralogically zoned. They consist of an olivine-rich core, which is surrounded by a low-Ca pyroxene-rich rim. Chondrules sectioned through their low-Ca pyroxene rim do not appear as zoned chondrules, hence, considering such sectioning effects, their true fraction might be as high as ∼50%. Mineralogical zonation is, therefore, a typical chondrule texture in basically all ordinary chondrites, and records a fundamental process during chondrule formation. Chondrules were open systems, and initially olivine-rich chondrules reacted with their surrounding gas to form low-Ca pyroxene rims. Zoned and unzoned chondrules have the same range of bulk compositions, thus ordinary chondrite chondrules were likely affected by two sequential episodes: in the first episode, gaseous SiO was added to all chondrules, thereby forming low-Ca pyroxene rims around all chondrules. In the second episode, only a portion of the chondrules were reheated, thereby remelting and homogenizing their initial pyroxene rims, but retaining their bulk compositions. It is therefore likely that all chondrules in ordinary chondrites were affected by gas-melt interactions during their formation. Open system exchange is consistent with previous studies of chondrule formation and can explain many chondrule textures and bulk chondrule compositional variations in single meteorites. Hence, the open system behaviour recorded in zoned chondrules provides a pivotal constraint on chondrule formation conditions.

Ti isotopic evidence for a non-CAI refractory component in the inner Solar System

Understanding the relationships between and among chondritic components of various chondrite groups is of prime importance for deciphering the dynamics of material transport and planetary accretion in the early Solar System. Here we obtain insights into these processes and the reservoirs present by investigating the nucleosynthetic Ti isotopic signatures of individual Ca,Al-rich inclusions (CAIs) and Na–Al-rich chondrules from ordinary and CO chondrites. This specific type of chondrule is of interest as it is thought to have incorporated refractory, CAI-like material as precursors. Our data show that CAIs from ordinary and CO chondrites exhibit 50Ti excesses that are indistinguishable from CV CAIs, and thus indicate a common source reservoir for refractory inclusions in ordinary, CO, and CV chondrites. Na–Al-rich chondrules from CO chondrites also show 50Ti excesses, indicating the presence of CAIs from this reservoir in the precursor materials of CO chondrules. In contrast, Na–Al-rich chondrules from ordinary chondrites show no 50Ti excesses and are indistinguishable from the bulk values for ordinary chondrites. Thus, known CAIs cannot have been the refractory precursor of the Na–Al-rich chondrules in ordinary chondrites. Consequently, within the accretion region of the ordinary chondrites, two different types of refractory components must have existed: (1) a 50Ti-enriched refractory component that is present as CAIs and either arrived at the accretion region of the ordinary chondrites after chondrule formation, or was only present in insignificant amounts, and (2) another type of refractory material without a 50Ti excess, which was involved as precursor in the chondrule formation process. Our data thus imply that refractory components with condensation signatures must have formed in at least two isotopically distinct nebular regions. These may be related to non-carbonaceous and carbonaceous source regions, that is, the inner and outer Solar System, divided by the early formation of Jupiter.

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.

Mixing and Transport of Dust in the Early Solar Nebula as Inferred from Titanium Isotope Variations among Chondrules

Abstract Chondrules formed by the melting of dust aggregates in the solar protoplanetary disk and as such provide unique insights into how solid material was transported and mixed within the disk. Here, we show that chondrules from enstatite and ordinary chondrites show only small 50Ti variations and scatter closely around the 50Ti composition of their host chondrites. By contrast, chondrules from carbonaceous chondrites have highly variable 50Ti compositions, which, relative to the terrestrial standard, range from the small 50Ti deficits measured for enstatite and ordinary chondrite chondrules to the large 50Ti excesses known from Ca–Al-rich inclusions (CAIs). These 50Ti variations can be attributed to the addition of isotopically heterogeneous CAI-like material to enstatite and ordinary chondrite-like chondrule precursors. The new Ti isotopic data demonstrate that isotopic variations among carbonaceous chondrite chondrules do not require formation over a wide range of orbital distances, but can instead be fully accounted for by the incorporation of isotopically anomalous “nuggets” into chondrule precursors. As such, these data obviate the need for disk-wide transport of chondrules prior to chondrite parent body accretion and are consistent with formation of chondrules from a given chondrite group in localized regions of the disk. Finally, the ubiquitous presence of 50Ti-enriched material in carbonaceous chondrites and the lack of this material in the non-carbonaceous chondrites support the idea that these two meteorite groups derive from areas of the disk that remained isolated from each other, probably through the formation of Jupiter.

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 formation conditions of enstatite chondrites: Insights from trace element geochemistry of olivine‐bearing chondrules in Sahara 97096 (EH3)

AbstractWe report in situ LA‐ICP‐MS trace element analyses of silicate phases in olivine‐bearing chondrules in the Sahara 97096 (EH3) enstatite chondrite. Most olivine and enstatite present rare earth element (REE) patterns comparable to their counterparts in type I chondrules in ordinary chondrites. They thus likely share a similar igneous origin, likely under similar redox conditions. The mesostasis however frequently shows negative Eu and/or Yb (and more rarely Sm) anomalies, evidently out of equilibrium with olivine and enstatite. We suggest that this reflects crystallization of oldhamite during a sulfidation event, already inferred by others, during which the mesostasis was molten, where the complementary positive Eu and Yb anomalies exhibited by oldhamite would have possibly arisen due to a divalent state of these elements. Much of this igneous oldhamite would have been expelled from the chondrules, presumably by inertial acceleration or surface tension effects, and would have contributed to the high abundance of opaque nodules found outside them in EH chondrites. In two chondrules, olivine and enstatite exhibit negatively sloped REE patterns, which may be an extreme manifestation of a general phenomenon (possibly linked to near‐liquidus partitioning) underlying the overabundance of light REE observed in most chondrule silicates relative to equilibrium predictions. The silicate phases in one of these two chondrules show complementary Eu, Yb, and Sm anomalies providing direct evidence for the postulated occurrence of the divalent state for these elements at some stage in the formation reservoir of enstatite chondrites. Our work supports the idea that the peculiarities of enstatite chondrites may not require a condensation sequence at high C/O ratios as has long been believed.

Thermal evolution model for the H chondrite asteroid-instantaneous formation versus protracted accretion

We present a model of the thermal evolution of asteroids. Assuming an onion shell model for the H chondrite parent body we obtain constraints for the H chondrite asteroid parameters by fitting empirical H chondrite cooling ages of Estacado, Guareña, Kernouvé, Mt. Browne, Richardton, Allegan, Nadiabondi, Ste. Marguerite, and Forest Vale by using a genetic algorithm for parameter optimisation. The model improves previous calculations on the thermal history calculated in the instantaneous accretion approximation considering sintering and porosity dependent heat conduction. The model is extended to include a finite growth time of the parent body to study whether the meteoritic record constrains the duration of the growth phase of the parent body where it assembles most of its mass. It is found that only short accretion times of up to 0.1Ma are compatible with the empirical data on H chondrite cooling histories. Best fit models yield excellent agreement with the cooling age data. Particularly, they indicate that (i) 26Al was the major heat source driving metamorphism, while 60Fe contributed rather marginally, (ii) maximum temperatures remained below partial melting temperatures throughout the body, indicating that no partial differentiation occurred on the H chondrite parent asteroid, (iii) the H chondrite asteroid formed 2Ma after CAIs, briefly after most ordinary chondrite chondrules formed (if 26Al abundance defines a chronological sequence).

Chondrules: Precursors and interactions with the nebular gas

Abstract– Chondrule compositions suggest either ferroan precursors and evaporation, or magnesian precursors and condensation. Type I chondrule precursors include granoblastic olivine aggregates (planetary or nebular) and fine‐grained (dustball) precursors. In carbonaceous chondrites, type I chondrule precursors were S‐free, while type II chondrules have higher Fe/Mn than in ordinary chondrites. Many type II chondrules contain diverse forsteritic relicts, consistent with polymict dustball precursors. The relationship between finer and coarser grained type I chondrules in ordinary chondrites suggests more evaporation from more highly melted chondrules. Fe metal in type I, and Na and S in type II chondrules indicate high partial pressures in ambient gas, as they are rapidly evaporated at canonical conditions. The occurrence of metal, sulfide, or low‐Ca pyroxene on chondrule rims suggests (re)condensation. In Semarkona type II chondrules, Na‐rich olivine cores, Na‐poor melt inclusions, and Na‐rich mesostases suggest evaporation followed by recondensation. Type II chondrules have correlated FeO and MnO, consistent with condensation onto forsteritic precursors, but with different ratios in carbonaceous chondrites and ordinary chondrites, indicating different redox history. The high partial pressures of lithophile elements require large dense clouds, either clumps in the protoplanetary disk, impact plumes, or bow shocks around protoplanets. In ordinary chondrites, clusters of type I and type II chondrules indicate high number densities and their similar oxygen isotopic compositions suggest recycling together. In carbonaceous chondrites, the much less abundant type II chondrules were probably added late to batches of type I chondrules from different O isotopic reservoirs.

Agglomeratic olivine (AO) objects and Type II chondrules in ordinary chondrites: Accretion and melting of dust to form ferroan chondrules

Agglomeratic olivine objects (AO objects) and possibly related chondrules in three ordinary chondrites (NWA 4910 [LL3.1], NWA 3127 [LL3.1], Sahara 98175 [LL3.5]) were studied using petrographic and microanalytical techniques to evaluate the origins of these materials. AO objects are mixtures of fine-grained (⩽5–10 μm-diameter) ferroan (Fa 12–35) olivine, troilite that is often concentrated towards the periphery of objects, pyroxene, feldspathic material, relict magnesian olivine and pyroxene grains, and relict chondrules. One micro-CAI with a grossite core was also found. AO objects commonly rim chondrules. AO objects show transitional variations in texture and chemistry with Type II chondrules, ranging from AO objects that are finer grained and show no evidence of melting (AO-U objects), to weakly melted and more melted AO objects (AO-WM and AO-M objects, respectively), to fine-grained Type II chondrules (olivine grain size ∼5–60 μm), to coarse-grained Type II chondrules (olivine grain size ∼10–250 μm across); S contents and Na/Al values are typically higher in AO objects than in Type II chondrules. The properties of AO objects and Type II chondrules are interpreted to reflect progressive heating of dust of quasi-chondritic composition, accompanied by grain coarsening during melting, partial loss of the most volatile elements (chiefly S, also Na) during evaporative melting, and back-reaction with gas, to form troilite-rimmed AO objects. Data-model comparisons suggest that progressive heating of chondritic dust to form AO objects and Type II chondrules could have occurred in a dusty environment to yield a transient, oxidizing gas of high pressure (∼10 −3 bar), with gas derived from vaporized dust being much (>500–1000× or even up to 10 4–10 5×) more abundant than ambient solar composition gas. AO objects are protochondrules, but are themselves composed of chondrule debris of different types, suggesting that they represent one step of a chondrule recycling process that also included chondrule disaggregation and additional chemical processing. Our data appear to be compatible with the nebular shock wave model for chondrule formation.

Kosmochloric Ca‐rich pyroxenes and FeO‐rich olivines (Kool grains) and associated phases in Stardust tracks and chondritic porous interplanetary dust particles: Possible precursors to FeO‐rich type II chondrules in ordinary chondrites

Abstract— Terminal particles and mineral fragments from comet 81P/Wild 2 were studied in 16 aerogel tracks by transmission and secondary electron microscopy. In eight tracks clinopyroxenes with correlated Na2O and Cr2O3 contents as high as 6.0 wt% and 13.0 wt%, respectively, were found. Kosmochloric (Ko) clinopyroxenes were also observed in 4 chondritic interplanetary dust particles (IDPs). The Ko‐clinopyroxenes were often associated with FeO‐rich olivine ± Cr‐rich spinel ± aluminosilicate glass or albitic feldspar, assemblages referred to as Kool grains (Ko = kosmochloric Ca‐rich pyroxene, ol = olivine). Fine‐grained (submicron) Kool fragments have textures suggestive of crystallization from melts while coarse‐grained (&gt;1 μm) Kool fragments are often glass‐free and may have formed by thermal metamorphism in the nebula. Average major and minor element distributions between clinopyroxenes and coexisting FeO‐rich olivines are consistent with these phases forming at or near equilibrium. In glass‐bearing fine‐grained Kool fragments, high concentrations of Na in the clinopyroxenes are inconsistent with existing experimentally determined partition coefficients at equilibrium. We speculate that the availability of Cr in the melt increased the clinopyroxene Na partition coefficient via a coupled substitution thereby enhancing this phase with the kosmochlor component. The high temperature minerals, fine‐grain sizes, bulk compositions and common occurrence in the SD tracks and IDPs support the idea that Kool grains could have been precursors to type II chondrules in ordinary chondrites. These grains, however, have not been observed in these meteorites suggesting that they were destroyed during chondrule formation and recycling or were not present in the nebula at the time and location where meteoritic chondrules formed.

Relict olivine grains, chondrule recycling, and implications for the chemical, thermal, and mechanical processing of nebular materials

Chondrules and isolated forsterites in five low-subtype ordinary chondrites [NWA 3127 (LL3.1), Sahara 97210 (LL3.2), Wells (LL3.3), Chainpur (LL3.4), and Sahara 98175 (LL3.5)] were studied using petrographic, EMPA, and SIMS techniques to better constrain the origin of chondrules and the olivine grains within them. Our results imply that igneous crystallization, vapor fractionation, redox effects, and open-system behavior were important processes. All olivine grains, including normal, relict, and isolated forsterite grains, show evidence for igneous fractionation under disequilibrium conditions, with olivine crystallizing during rapid cooling (closer to 2000 °C/h than to 100 °C/h). Vapor fractionation is manifested by anti-correlated abundances between refractory elements (Al, Sc, Y, Ti, Ca, V) and volatile elements (Cr, Mn, P, Rb, Fe) in olivine. Redox effects are evidenced in various ways, and imply that Fe, Co, Ni, and P were partitioned more into metal, and V was partitioned more into olivine, under reducing conditions in the most FeO-poor melts. There is no obvious evidence for systematic variations in olivine composition according to meteorite subtype, but shock melting in Sahara 97210 resulted in the injection of glass-derived melt into olivine, resulting in artificially high abundances of Ba, Sr, Na, Ti, and some other incompatible elements in olivine. Terrestrial weathering in a hot desert environment may have mobilized Ba and Sr in some glasses. Our data suggest that chondrules in ordinary chondrites experienced repeated thermal, chemical, and mechanical processing during a “recycling” process over an extended time period, which involved multiple episodes of melting under fluctuating redox and heating conditions, and multiple episodes of chondrule break-up in some cases. Forsterite grains, including normal grains in forsterite-bearing type I chondrules, the cores of isolated forsterites, and relict forsterite in type II chondrules, all crystallized from similar, refractory melts under reducing conditions; relict Mg-olivine and isolated forsterite grains were thus derived from type I chondrules. Olivine in type II chondrules, including normal grains and ferroan overgrowths on relict Mg-olivine, crystallized from more volatile-rich, oxidized, and relatively unfractionated melts. Relict dusty olivine grains in type I chondrules were derived from type II chondrules during incomplete melting episodes involving reduction and some vaporization, with clear (non-dusty) grains in dusty olivine-bearing chondrules crystallizing from the reduced and partly vaporized melts. Melt compositions parental to normal olivine grains in type I and II chondrules are systematically enriched in refractory elements compared to bulk chondrule compositions, implying that chondrules often experienced open-system exchange with more volatile-rich surroundings after some olivine had crystallized, possibly while the chondrules were still partly molten. Type II chondrules could have been derived from type I chondrules by the addition of relatively volatile-rich material, followed by re-melting and little evaporation under oxidizing conditions. In contrast, type I chondrules could have been derived from type II chondrules by re-melting involving more-or-less evaporation under reducing conditions. Chemical, oxygen isotope, and petrographic data are best accommodated by a model in which there were several (>2–3, sometimes ⩾4–5) melting episodes for most chondrules in ordinary chondrites.

Cooling rate of chondrules in ordinary chondrites revisited by a new geospeedometer based on the compensation rule

For several decades efforts to constrain chondrite cooling rates from diffusion zoning in olivine gave rise to a range of values from 5 to 8400 K/h (Desch, S.J., Connolly Jr., H.C., 2002. A model for the thermal processing of particles in solar nebula shocks: application to cooling rates of chondrules. Meteorit. Planet. Sci. 37, 183–208; Greeney, S., Ruzicka, A., 2004. Relict forsterite in chondrules: implications for cooling rates. Lunar Planet. Sci. XXXV, abstract # 1246.). Such large uncertainties directly reflect the variability of diffusion data. Alternatively, from this variability results a compensation rule, log D 0 = a + bE (diffusion coefficients are written D = D 0 exp(− E/ RT)). We test a new geospeemetry approach, based on this rule, on cooling of chondrules in chondrites, Sahara-97210 LL 3.2 and Wells LL 3.3. Greeney and Ruzicka (2004) matched Fe–Mg diffusion profiles in olivine from these chondrites with cooling rates between 200 and 6000 K/h. In our geospeedometry model, the use of the compensation rule greatly reduces the uncertainties by avoiding the choice of one diffusion coefficient among many. The cooling rates we found are between 700 and 3600 K/h for Sahara and 700–1600 K/h for Wells. Finally, we discuss the influence of our analytical model parameters on our cooling rate estimates.