Oxygen-isotopic compositions of low-FeO relicts in high-FeO host chondrules in Acfer 094, a type 3.0 carbonaceous chondrite closely related to CM

With one exception, the low-FeO relict olivine grains within high-FeO porphyritic chondrules in the type 3.0 Acfer 094 carbonaceous chondrite have Δ¹⁷O (= δ¹⁷O − 0.52 × δ¹⁸O) values that are substantially more negative than those of the high-FeO olivine host materials. These results are similar to observations made earlier on chondrules in CO3.0 chondrites and are consistent with two independent models: (1) Nebular solids evolved from low-FeO, low-Δ¹⁷O compositions towards high-FeO, more positive Δ¹⁷O compositions; and (2) the range of compositions resulted from the mixing of two independently formed components. The two models predict different trajectories on a Δ¹⁷O vs. log Fe/Mg (olivine) diagram, but our sample set has too few values at intermediate Fe/Mg ratios to yield a definitive answer.Published data showing that Acfer 094 has higher volatile contents than CO chondrites suggest a closer link to CM chondrites. This is consistent with the high modal matrix abundance in Acfer 094 (49 vol.%). Acfer 094 may be an unaltered CM chondrite or an exceptionally matrix-rich CO chondrite. Chondrules in Acfer 094 and in CO and CM carbonaceous chondrites appear to sample the same population. Textural differences between Acfer 094 and CM chondrites are largely attributable to the high degree of hydrothermal alteration that the CM chondrites experienced in an asteroidal setting.     

Ubiquitous low-FeO relict grains in type II chondrules and limited overgrowths on phenocrysts following the final melting event

Type II porphyritic chondrules commonly contain several large (>40 μm) olivine phenocrysts; furnace-based cooling rates based on the assumption that these phenocrysts grew in a single-stage melting-cooling event yield chondrule cooling-rate estimates of 0.01-1 K s⁻¹. Because other evidence indicates much higher cooling rates, we examined type II chondrules in the CO3.0 chondrites that have experienced only minimal parent-body alteration. We discovered three kinds of evidence indicating that only minor (4-10 μm) olivine growth occurred after the final melting event: (1) Nearly all (>90%) type II chondrules in CO3.0 chondrites contain low-FeO relict grains; overgrowths on these relicts are narrow, in the range of 2-12 μm. (2) Most type II chondrules contain some FeO-rich olivine grains with decurved surfaces and acute angles between faces indicating that the grains are fragments from an earlier generation of chondrules; the limited overgrowth thicknesses following the last melting event are too thin to disguise the shard-like nature of these grains. (3) Most type II chondrules contain many small (<20 μm) euhedral or subhedral phenocrysts with central compositions that are much more ferroan than the centers of the large phenocrysts; their small sizes document the small amount of growth that occurred after the final melting event. If overgrowth thicknesses were small (4-10 μm) after the final melting event, it follows that large fractions of coarse (>40 μm) high-FeO phenocrysts are relicts from earlier generations of chondrules, and that cooling rates after the last melting event were much more rapid than indicated by models based on a single melting event. These observations are thus inconsistent with the “classic” igneous model of formation of type II porphyritic chondrules by near-total melting of a precursor mix followed by olivine nucleation on a very limited number of nuclei (say, ≤10) and by growth to produce the large phenocrysts during a period of monotonic (and roughly linear) cooling. Our observations that recycled chondrule materials constitute a large component of the phenocrysts of type II chondrules also imply that this kind of chondrule formed relatively late during the chondrule-forming period.     

Non-spherical lobate chondrules in CO3.0 Y-81020: General implications for the formation of low-FeO porphyritic chondrules in CO chondrites

Non-spherical chondrules (arbitrarily defined as having aspect ratios ≥1.20) in CO3.0 chondrites comprise multi-lobate, distended, and highly irregular objects with rounded margins; they constitute ∼70% of the type-I (low-FeO) porphyritic chondrules in Y-81020, ∼75% of such chondrules in ALHA77307, and ∼60% of those in Colony. Although the proportion of non-spherical type-I chondrules in LL3.0 Semarkona is comparable (∼60%), multi-lobate OC porphyritic chondrules (with lobe heights equivalent to a significant fraction of the mean chondrule diameter) are rare. If the non-spherical type-I chondrules in CO chondrites had formed from totally molten droplets, calculations indicate that they would have collapsed into spheres within ∼10⁻³ s, too little time for their 20-μm-size olivine phenocrysts to have grown from the melt. These olivine grains must therefore be relicts from an earlier chondrule generation; the final heating episode experienced by the non-spherical chondrules involved only minor amounts of melting and crystallization. The immediate precursors of the individual non-spherical chondrules may have been irregularly shaped chondrule fragments whose fracture surfaces were rounded during melting. Because non-spherical chondrules and “circular” chondrules form a continuum in shape and have similar grain sizes, mineral and mesostasis compositions, and modal abundances of non-opaque phases, they must have formed by related processes. We conclude that a large majority of low-FeO chondrules in CO3 chondrites experienced a late, low-degree melting event. Previous studies have shown that essentially all type-II (high-FeO) porphyritic chondrules in Y-81020 formed by repeated episodes of low-degree melting. It thus appears that the type-I and type-II porphyritic chondrules in Y-81020 (and, presumably, all CO3 chondrites) experienced analogous formation histories. Because these two types constitute ∼95% of all CO chondrules, it is clear that chondrule recycling was the rule in the CO chondrule-formation region and that most melting events produced only low degrees of melting. The rarity of significantly non-spherical, multi-lobate chondrules in Semarkona may reflect more-intense heating of chondrule precursors in the ordinary-chondrite region of the solar nebula.     

Oxygen isotope systematics of chondrules in the Allende CV3 chondrite: High precision ion microprobe studies

The oxygen three-isotope systematics of 36 chondrules from the Allende CV3 chondrite are reported using high precision secondary ion mass spectrometer (CAMECA IMS-1280). Twenty-six chondrules have shown internally homogenous Δ¹⁷O values among olivine, pyroxene, and spinel within a single chondrule. The average Δ¹⁷O values of 19 FeO-poor chondrules (13 porphyritic chondrules, 2 barred olivine chondrules, and 4 chondrule fragments) show a peak at −5.3 ± 0.6‰ (2SD). Another 5 porphyritic chondrules including both FeO-poor and FeO-rich ones show average Δ¹⁷O values between −3‰ and −2‰, and 2 other FeO-poor barred olivine chondrules show average Δ¹⁷O values of −3.6‰ and 0‰. These results are similar to those for Acfer 094 chondrules, showing bimodal Δ¹⁷O values at −5‰ and −2‰. Nine porphyritic chondrules contain olivine grains with heterogeneous Δ¹⁷O values as low as −18‰, indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions. However, most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules. The Δ¹⁷O values of four barred olivine chondrules range from −5‰ to 0‰, indicating that not all BO chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses. Based on these data, we suggest the presence of multiple oxygen isotope reservoirs in local dust-rich protoplanetary disk, from which the CV3 parent asteroid formed.A compilation of 225 olivine and low-Ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between carbonaceous chondrite anhydrous mineral (CCAM) and Young and Russell lines. These data define a correlation line of δ¹⁷O = (0.982 ± 0.019) × δ¹⁸O − (2.91 ± 0.10), which is similar to those defined by chondrules in CV3 chondrites and Acfer 094 in previous studies. Plagioclase analyses in two chondrules plot slightly below the CCAM line with Δ¹⁷O values of −2.6‰, which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the CV parent body.     

Oxygen isotope heterogeneity in chondrules from the Mokoia CV3 carbonaceous chondrite

We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by δ¹⁷O ∼ −2.5‰) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have δ¹⁸O values around 0‰, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher δ¹⁸O values than olivines in the same chondrules. Our bulk chondrule data spread to lower δ¹⁸O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk δ¹⁸O values (−7.5‰ and −11.7‰) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to δ¹⁷O, δ¹⁸O around -50‰ in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an ¹⁶O-rich precursor and an ¹⁶O-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had δ¹⁷O and δ¹⁸O values around -50‰, similar to those observed in refractory inclusions.     

Oxygen isotope compositions of chondrules in CR chondrites

We report in situ ion microprobe analyses of oxygen isotopic compositions of olivine, low-Ca pyroxene, high-Ca pyroxene, anorthitic plagioclase, glassy mesostasis, and spinel in five aluminum-rich chondrules and nine ferromagnesian chondrules from the CR carbonaceous chondrites EET92042, GRA95229, and MAC87320. Ferromagnesian chondrules are isotopically homogeneous within ±2‰ in Δ¹⁷O; the interchondrule variations in Δ¹⁷O range from 0 to −5‰. Small oxygen isotopic heterogeneities found in two ferromagnesian chondrules are due to the presence of relict olivine grains. In contrast, two out of five aluminum-rich chondrules are isotopically heterogeneous with Δ¹⁷O values ranging from −6 to −15‰ and from −2 to −11‰, respectively. This isotopic heterogeneity is due to the presence of ¹⁶O-enriched spinel and anorthite (Δ¹⁷O = −10 to −15‰), which are relict phases of Ca,Al-rich inclusions (CAIs) incorporated into chondrule precursors and incompletely melted during chondrule formation. These observations and the high abundance of relict CAIs in the aluminum-rich chondrules suggest a close genetic relationship between these objects: aluminum-rich chondrules formed by melting of spinel-anorthite-pyroxene CAIs mixed with ferromagnesian precursors compositionally similar to magnesium-rich (Type I) chondrules. The aluminum-rich chondrules without relict CAIs have oxygen isotopic compositions (Δ¹⁷O = −2 to −8‰) similar to those of ferromagnesian chondrules. In contrast to the aluminum-rich chondrules from ordinary chondrites, those from CRs plot on a three-oxygen isotope diagram along the carbonaceous chondrite anhydrous mineral line and form a continuum with amoeboid olivine aggregates and CAIs from CRs. We conclude that oxygen isotope compositions of chondrules resulted from two processes: homogenization of isotopically heterogeneous materials during chondrule melting and oxygen isotopic exchange between chondrule melt and ¹⁶O-poor nebular gas.     

Formation processes of compound chondrules in CV3 carbonaceous chondrites: Constraints from oxygen isotope ratios and major element concentrations

We found thirty compound chondrules in two CV3 carbonaceous chondrites. The abundance in each meteorite relative to single chondrules is 29/1846 (1.6%) in Allende and 1/230 (0.4%) in Axtell. We examined petrologic features, major element concentrations and oxygen isotopic compositions. Textural, compositional and isotopic evidence suggests that multiple, different mechanisms are responsible for the formation of compound chondrules.Seven compound chondrules are composed of two conjoined porphyritic chondrules with a blurred boundary. At the boundary region of this type of compounds, a poikilitic texture is commonly observed. This suggests that the two chondrules were melted when they came to be in contact. On the other hand, seventeen compound chondrules consist of two conjoined chondrules with a discrete boundary. The preservation of spherical boundary planes of an earlier-formed chondrule of this type implies that it already solidified before fusing with a later-formed chondrule that was still melted. Six samples out of 17 compound chondrules of this type are composed of two BO chondrules. The BO-BO compound chondrules have a unique textural feature in common: the directions of the barred olivines are mostly parallel between two chondrules. This cannot be explained by a simple collision process and forces another mechanism to be taken into consideration.The remaining six compound chondrules differ from the others; they consist of an earlier-formed chondrule enclosed by a later-formed chondrule. A large FeO enrichment was observed in the later-formed chondrules and the enrichment was much greater than that in the later-formed chondrules of other types of compounds. This is consistent with the relict chondrule model, which envisages that the later-formed chondrule was made by a flash melting of a porous FeO-rich dust clump on an earlier-formed chondrule. The textural evidence of this type of compound shows that the earlier-formed chondrule has melted again to varying degrees at the second heating event. This implies that FeO concentrations in bulk chondrules increases during the second heating event if an earlier-formed chondrule was totally melted together with the FeO-rich dust aggregates.Silicate minerals such as olivine and low-Ca pyroxene in compound chondrules have oxygen isotope compositions similar to those in single chondrules from CV3 chondrites. The oxygen isotope composition of each part of the compound chondrule is basically similar to their chondrule pair, but silicates in some chondrules show varying degrees of ¹⁶O-enrichment down to −15‰ in δ¹⁸O, while those in their partners have ¹⁶O-poor invariable compositions near 0 ‰ in δ¹⁸O. This implies that the two chondrules in individual compounds formed in the same environments before they became conjoined and the heterogeneous oxygen isotope compositions in some chondrules resulted from incomplete exchange of oxygen atoms between ¹⁶O-rich chondrule melts and ¹⁶O-poor nebular gas.     

Fine,nickel-poor Fe-Ni grains in the olivine of unequilibrated ordinary chondrites

Fine (?2 μm), Ni-poor (? 10 mg/g) Fe-Ni grains are common inclusions in the olivine in porphyritic chondrules in unequilibrated ordinary chondrites. The olivine grains appear to be relicts that survived chondrule formation without melting. The most common occurrence of this “dusty” metal is in the core of olivine grains having clear Fe-poor rims and surrounded either by small euhedral clear olivine grains zoned with FeO increasing toward the border of the grains or by large elongated Fe-poor orthopyroxenes oriented parallel to the chondrule surface and enclosing small round olivine grains. Various amounts of Ca, Al-rich glass are always present. The dusty metal is occasionally found in the rims of olivine grains either isolated in the matrix or included in chondrules. A rare occurrence is as bands in highly deformed olivines.This dusty metal appears to be the product of in situ reduction of FeO from the host olivine. Among the possible reductants H₂ or carbonaceous matter (CH₂)_n seem the most likely. Hydrogen may have been implanted by solar-wind or solar-flare irradiation, but this requires that dissipation of nebular gas occurred before the end of the chondrule formation process. Carbonaceous matter may have been implanted by shock. Less likely reductants are nebular CO or C dissolved in the olivine lattice. The large relict olivine grains may be nebular condensates or, more likely, fragments broken off earlier generations of chondrules.     

Relationships among intrinsic properties of ordinary chondrites: Oxidation state, bulk chemistry, oxygen-isotopic composition, petrologic type, and chondrule size

The properties of ordinary chondrites (OC) reflect both nebular and asteroidal processes. OC are modeled here as having acquired nebular water, probably contained within phyllosilicates, during agglomeration. This component had high Δ¹⁷O and acted like an oxidizing agent during thermal metamorphism. The nebular origin of this component is consistent with negative correlations in H, L, and LL chondrites between oxidation state (represented by olivine Fa) and bulk concentration ratios of elements involved in the metal-silicate fractionation (e.g., Ni/Si, Ir/Si, Ir/Mn, Ir/Cr, Ir/Mg, Ni/Mg, As/Mg, Ga/Mg). LL chondrites acquired the greatest abundance of phyllosilicates with high Δ¹⁷O among OC (and thus became the most oxidized group and the one with the heaviest O isotopes); H chondrites acquired the lowest abundance, becoming the most reduced OC group with the lightest O isotopes.Chondrule precursors may have grown larger and more ferroan with time in each OC agglomeration zone. Nebular turbulence may have controlled the sizes of chondrule precursors. H-chondrite chondrules (which are the smallest among OC) formed from the smallest precursors. In each OC region, low-FeO chondrules formed before high-FeO chondrules during repeated episodes of chondrule formation.During thermal metamorphism, phyllosilicates were dehydrated; the liberated water oxidized metallic Fe-Ni. This caused correlated changes with petrologic type including decreases in the modal abundance of metal, increases in olivine Fa and low-Ca pyroxene Fs, increases in the olivine/pyroxene ratio, and increases in the kamacite Co and Ni contents. As water (with its heavy O isotopes) was lost during metamorphism, inverse correlations between bulk δ¹⁸O and bulk δ¹⁷O with petrologic type were produced.The H5 chondrites that were ejected from their parent body ∼7.5 Ma ago during a major impact event probably had been within a few kilometers of each other since they accreted ∼4.5 Ga ago. There are significant differences in the olivine compositional distributions among these rocks; these reflect stochastic nebular sampling of the oxidant (i.e., phyllosilicates with high Δ¹⁷O) on a 0.1-1 km scale during agglomeration.     

Oxygen isotopic compositions of chondrules: Implications for evolution of oxygen isotopic reservoirs in the inner solar nebula

We review the oxygen isotopic compositions of minerals in chondrules and compound objects composed of a chondrule and a refractory inclusion, and bulk oxygen isotopic compositions of chondrules in unequilibrated ordinary, carbonaceous, enstatite, and Kakangari-like chondrites, focusing on data acquired using secondary ion mass-spectrometry and laser fluorination coupled with mass-spectrometry over the last decade. Most ferromagnesian chondrules from primitive (unmetamorphosed) chondrites are isotopically uniform (within 3–4‰ in Δ¹⁷O) and depleted in ¹⁶O (Δ¹⁷O>−7‰) relative to amoeboid olivine aggregates (AOAs) and most calcium–aluminum-rich inclusions (CAIs) (Δ¹⁷O<−20‰), suggesting that these classes of objects formed in isotopically distinct gaseous reservoirs, ¹⁶O-poor and ¹⁶O-rich, respectively. Chondrules uniformly enriched in ¹⁶O (Δ¹⁷O<−15‰) are exceptionally rare and have been reported only in CH chondrites. Oxygen isotopic heterogeneity in chondrules is mainly due to the presence of relict grains. These appear to consist of chondrules of earlier generations and rare refractory inclusions; with rare exceptions, the relict grains are ¹⁶O-enriched relative to chondrule phenocrysts and mesostasis. Within a chondrite group, the magnesium-rich (Type I) chondrules tend to be ¹⁶O-enriched relative to the ferrous (Type II) chondrules. Aluminum-rich chondrules in ordinary, enstatite, CR, and CV chondrites are generally ¹⁶O-enriched relative to ferromagnesian chondrules. No systematic differences in oxygen isotopic compositions have been found among these chondrule types in CB chondrites. Aluminum-rich chondrules in carbonaceous chondrites often contain relict refractory inclusions. Aluminum-rich chondrules with relict CAIs have heterogeneous oxygen isotopic compositions (Δ¹⁷O ranges from −20‰ to 0‰). Aluminum-rich chondrules without relict CAIs are isotopically uniform and have oxygen isotopic compositions similar to, or approaching, those of ferromagnesian chondrules. Phenocrysts and mesostases of the CAI-bearing chondrules show no clear evidence for ¹⁶O-enrichment compared to the CAI-free chondrules. Spinel, hibonite, and forsterite of the relict refractory inclusions largely retained their original oxygen isotopic compositions. In contrast, plagioclase and melilite of the relict CAIs experienced melting and ¹⁶O-depletion to various degrees, probably due to isotopic exchange with an ¹⁶O-poor nebular gas. Several igneous CAIs experienced isotopic exchange with an ¹⁶O-poor nebular gas during late-stage remelting in the chondrule-forming region. On a three-isotope diagram, bulk oxygen isotopic compositions of most chondrules in ordinary, enstatite, and carbonaceous chondrites plot above, along, and below the terrestrial fractionation line, respectively. Bulk oxygen isotopic compositions of chondrules in altered and/or metamorphosed chondrites show evidence for mass-dependent fractionation, reflecting either interaction with a gaseous/fluid reservoir on parent asteroids or open-system thermal metamorphism. Bulk oxygen isotopic compositions of chondrules and oxygen isotopic compositions of individual minerals in chondrules and refractory inclusions from primitive chondrites plot along a common line of slope of 1, suggesting that only two major reservoirs (gas and solids) are needed to explain the observed variations. However, there is no requirement that each had a permanently fixed isotopic composition. The absolute (²⁰⁷Pb–²⁰⁶Pb) and relative (²⁷Al–²⁶Mg) chronologies of CAIs and chondrules and the differences in oxygen isotopic compositions of most chondrules (¹⁶O-poor) and most refractory inclusions (¹⁶O-rich) can be interpreted in terms of isotopic self-shielding during UV photolysis of CO in the initially ¹⁶O-rich (Δ¹⁷O−25‰) parent molecular cloud or protoplanetary disk. According to these models, the UV photolysis preferentially dissociates C¹⁷O and C¹⁸O in the parent molecular cloud and in the peripheral zones of the protoplanetary disk. If this process occurs in the stability field of water ice, the released atomic ¹⁷O and ¹⁸O are incorporated into water ice, while the residual CO gas becomes enriched in ¹⁶O. During the earliest stages of evolution of the protoplanetary disk, the inner solar nebula had a solar H₂O/CO ratio and was ¹⁶O-rich. During this time, AOAs and the ¹⁶O-rich CAIs and chondrules formed. Subsequently, the inner solar nebula became H₂O- and ¹⁶O-depleted, because ice-rich dust particles, which were depleted in ¹⁶O, agglomerated outside the snowline (5 AU), drifted rapidly towards the Sun and evaporated. During this time, which may have lasted for 3 Myr, most chondrules and the ¹⁶O-depleted igneous CAIs formed. We infer that most chondrules formed from isotopically heterogeneous, but ¹⁶O-depleted precursors, and experienced isotopic exchange with an ¹⁶O-poor nebular gas during melting. Although the relative roles of the chondrule precursor materials and gas–melt isotopic exchange in establishing oxygen isotopic compositions of chondrules have not been quantified yet, mineralogical, chemical, and isotopic evidence indicate that Type I chondrules may have formed in chemical and isotopic equilibrium with nebular gas of variable isotopic composition. Whether these variations were spatial or temporal are not known yet.     

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 δ¹⁸O and δ¹⁷O values of pyroxene and olivine appear to be strongly correlated for all the studied CR and CV chondrules according to:

     

Chondrule reheating experiments and relict olivine

Chondrules contain foreign objects, including some olivine grains that obviously did not crystallize from their silicate melt. The term recycling is usually applied to chondrules with relict grains, implying that the precursor contained relicts of a previous generation of chondrules. This has given rise to the idea that the pervasive melt droplet formation that affected the early solar system involved repeated events in which chondrules or chondrule debris were reheated. We conducted experiments in which synthetic chondrules generated from fine-grained mineral aggregates were heated and cooled a second time to see what the textural consequences of this reheating would be. Charges were heated to peak temperatures for 1 min and were cooled to near-solidus temperatures over 35 min, for both thermal cycles. We first made microporphyritic olivine charges and on reheating and second cooling observed coarser grain sizes and disappearance of relict grains, if the second peak temperature was the same as or higher than the first (but insufficient for destroying all nuclei). The coarsening was due to the dissolution of the smallest first generation crystals and additional growth on the relicts during cooling. Reheated barred olivine spheres generated barred olivine spheres again, no matter how low the peak temperature. This is because the number of remaining olivine grains or nuclei that acted as sites for regrowth was constant. Generating the observed distribution of chondrule textures, dominantly porphyritic, directly from a fine-grained precursor such as nebular or presolar condensates is impossible with a single event. With reheating of chondrules, generating the texture distribution is possible provided that subsequent heating events have higher peak temperatures than the first, so that total dissolution of the smallest grains occurs, with consequent coarsening. For our thermal history and a reasonable distribution of peak temperatures, multiple recycling events might be needed to make most chondrules porphyritic. Alternatively, the predominance of porphyritic textures in chondrules could be explained by heating times hours long for a fine-grained precursor or by heating of a coarse-grained precursor.The presence of relict grains derived from older chondrules or other material suggests that an aggregate has been heated for the first time, because recycling brings an approach to equilibrium. There appears to be no reliable way to use textures to tell just how many chondrules have been heated more than once. The relict grains simply indicate the nature of the precursors, which were at least in part derived from earlier chondrules, and of the peak temperatures too low for total melting and heating times too short for total dissolution. Rim thicknesses on relict grains depend on number density of crystals and melt composition, and are not a reliable guide to the chondrule cooling rate.     

Mineralogy,petrology, and oxygen isotopic composition of Northwest Africa 12379, metal-rich chondrite with affinity to ordinary chondrites

Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa_18.1–28.3, average Fa_24.9±3.2, PMD = 12.8; Cr₂O₃ = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs_3.2–18.7Wo_0.2–4.5; average Fs_14.7±3.7Wo_1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ¹⁷O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa₂₇) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ¹⁷O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ¹⁸O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.     

Heating duration of igneous rim formation on a chondrule in the Northwest Africa 3118 CV3oxA carbonaceous chondrite inferred from micro-scale migration of the oxygen isotopes

Due to their common occurrence in various types of chondrites, igneous rims formed on pre-existing chondrules throughout chondrule-forming regions of the solar nebula. Although the peak temperatures are thought to reach similar values to those achieved during chondrule formation events, the heating duration in chondrule rim formation has not been well defined. We determined the two-dimensional chemical and oxygen isotopic distributions in an igneous rim of a chondrule within the Northwest Africa 3118 CV3_oxA chondrite with sub-micrometer resolution using secondary ion mass spectrometry and scanning electron microscopy. The igneous rim experienced aqueous alteration on the CV parent body. The aqueous alteration resulted in precipitation of the secondary FeO-rich olivine (Fa_40–49) and slightly disturbed the Fe-Mg distribution in the MgO-rich olivine phenocrysts (Fa_11–22) at about a 1 μm scale. However, no oxygen isotopic disturbances were observed at a scale greater than 100 nm. The MgO-rich olivine, a primary phase of igneous rim formation, has δ¹⁷O = −6 ± 3‰ and δ¹⁸O = −1 ± 4‰, and some grains contain extreme ¹⁶O-rich areas (δ¹⁷O, δ¹⁸O = ∼−30‰) nearly 10 μm across. We detected oxygen isotopic migration of approximately 1 μm at the boundaries of the extreme ¹⁶O-rich areas. Using oxygen self-diffusivity in olivine, the heating time of the igneous rim formation could have continued from several hours to several days at near liquidus temperatures (∼2000 K) in the solar nebula suggesting that the rim formed by a similar flash heating event that formed the chondrules.     

Chondrules in H3 chondrites: textures,compositions and origins

Major and minor element bulk compositions of 90 individual chondrules and 16 compound chondrule sets in unequilibrated (type 3) H-group chondrites were determined in polished thin sections by broad beam electron probe analysis and the chondrules were classified petrographically into six textural types (barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene, fine-grained). Although analyses of individual chondrules scatter widely, the mean composition of each textural type (except barred pyroxene) is rather distinct, as verified by discriminant function analysis. Al₂O₃, TiO₂ and Na₃O are correlated in chondrules, but Al₂O₃ and CaO do not correlate. Compound chondrule sets were found to consist almost entirely of chondrules or partial chondrules of similar texture and composition.The data suggest that composition played a conspicuous role in producing the observed textures of chondrules, though other factors such as cooling rates and degrees of supercooling prior to nucleation were also important. If compound chondrules formed and joined when they were still molten or plastic, then the data suggest that chondrules of each textural type could have formed together in space or time. The correlation of Al₂O₃ and TiO₂ with Na₂O and not with CaO appears to rule out formation of chondrules by direct equilibrium condensation from the nebula. We conclude that the most reasonable model for formation of the majority of chondrules is that they originated from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space. A small percentage of chondrules might have formed by impacts in meteorite parent-body regoliths.     

Petrology and oxygen isotope compositions of chondrules in E3 chondrites

Chondrules in E3 chondrites differ from those in other chondrite groups. Many contain near-pure endmember enstatite (Fs_<1). Some contain Si-bearing FeNi metal, Cr-bearing troilite, and, in some cases Mg, Mn- and Ca-sulfides. Olivine and more FeO-rich pyroxene grains are present but much less common than in ordinary or carbonaceous chondrite chondrules. In some cases, the FeO-rich grains contain dusty inclusions of metal. The oxygen three-isotope ratios (δ¹⁸O, δ¹⁷O) of olivine and pyroxene in chondrules from E3 chondrites, which are measured using a multi-collection SIMS, show a wide range of values. Most enstatite data plots on the terrestrial fractionation (TF) line near whole rock values and some plot near the ordinary chondrite region on the 3-isotope diagram. Pyroxene with higher FeO contents (∼2-10 wt.% FeO) generally plots on the TF line similar to enstatite, suggesting it formed locally in the EC (enstatite chondrite) region and that oxidation/reduction conditions varied within the E3 chondrite chondrule-forming region. Olivine shows a wide range of correlated δ¹⁸O and δ¹⁷O values and data from two olivine-bearing chondrules form a slope ∼1 mixing line, which is approximately parallel to but distinct from the CCAM (carbonaceous chondrite anhydrous mixing) line. We refer to this as the ECM (enstatite chondrite mixing) line but it also may coincide with a line defined by chondrules from Acfer 094 referred to as the PCM (Primitive Chondrite Mineral) line (Ushikubo et al., 2011). The range of O isotope compositions and mixing behavior in E3 chondrules is similar to that in O and C chondrite groups, indicating similar chondrule-forming processes, solid-gas mixing and possibly similar ¹⁶O-rich precursors solids. However, E3 chondrules formed in a distinct oxygen reservoir.Internal oxygen isotope heterogeneity was found among minerals from some of the chondrules in E3 chondrites suggesting incomplete melting of the chondrules, survival of minerals from previous generations of chondrules, and chondrule recycling. Olivine, possibly a relict grain, in one chondrule has an R chondrite-like oxygen isotope composition and may indicate limited mixing of materials from other reservoirs. Calcium-aluminum-rich inclusions (CAIs) in E3 chondrites have petrologic characteristics and oxygen isotope ratios similar to those in other chondrite groups. However, chondrules from E3 chondrites differ markedly from those in other chondrite groups. From this we conclude that chondrule formation was a local event but CAIs may have all formed in one distinct place and time and were later redistributed to the various chondrule-forming and parent body accretion regions. This also implies that transport mechanisms were less active at the time of and following chondrule formation.     

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.     

Relationship among O, Mg isotopes and the petrography of two spinel-bearing compound chondrules

The petrological properties, and O and Al-Mg isotopic compositions of two spinel-bearing chondrules from the Allende CV chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry. A coarse spinel grain in a barred-olivine (BO) chondrule is less enriched in ¹⁶O (Δ¹⁷O ∼ −5‰; Δ¹⁷O = δ¹⁷O - 0.52 δ¹⁸O), whereas smaller spinel grains in a plagioclase-rich chondrule member of a compound chondrule are extremely ¹⁶O-rich (Δ¹⁷O ∼ −17‰) and the spinels have a strongly serrated character. The petrological features and ¹⁶O-enrichments of the spinels in the plagioclase-rich chondrule indicate that spinels originating in coarse-grained Ca-Al-rich Inclusions (CAIs) were incorporated into chondrule precursors and survived the chondrule-forming event. The degree of ¹⁶O-excesses among minerals within each chondrule is correlated to the crystallization sequences. This evidence suggests that the O isotopic variation among minerals may have resulted from incomplete exchange of O isotopes between ¹⁶O-rich chondrule melt and ¹⁶O-poor nebular gas. Aqueous alteration also has changed the O-isotope compositions in the mesostasis. The feldspathic mesostasis in the BO chondrule shows a disturbed Mg-Al isochron indicating that the BO chondrule experienced secondary alteration. While plagioclase in the plagioclase-rich chondrule member of the compound chondrule shows slight ²⁶Mg-excesses corresponding to (²⁶Al/²⁷Al)₀ = [4.6±4.0(2σ)] × 10⁻⁶, nepheline formed by secondary alteration shows no detectable excess. The Al-Mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the Allende parent body.     

Spinel group minerals in LL3.00-6 chondrites: Indicators of nebular and parent body processes

We carried out a systematic study of spinel group minerals in LL3.00-3.9 and LL4-6 chondrites. With increasing petrologic type, the size and abundance of spinel increase. The compositions of spinel group minerals in type 3 chondrites depend on the occurrence; Mg-Al-rich spinel occurs mainly in chondrules. Some chromite occurs in chondrules and matrix, and nearly pure chromite is exclusively encountered in the matrix. The occurrence of nearly pure chromite and the wide compositional variations distinguish spinel group minerals in types 3.00-3.3 from those in the other types. Spinel group minerals in types 3.5-3.9 show a narrower range of compositions, and those in types 4-6 are homogeneous. The changes in composition and abundance of spinel in type 3 chondrites are most likely due to thermal metamorphism. Therefore, the chemistry of spinel group minerals could be used as a sensitive indicator of metamorphic conditions, not only for type 3-6, but also 3.00-3.9. They can be applied to identify the most primitive (least metamorphosed) chondrites. The bulk compositions of spinel-bearing chondrules and the textural setting of the spinel indicate that most spinel group minerals crystallized directly from chondrule melts. However, some spinel grains, especially those enclosed in olivine phenocrysts, can not be explained by in situ crystallization in the chondrule. We interpret these spinel grains to be relic phases that survived chondrule melting. This is supported by the oxygen isotopic composition of a spinel grain, which has significantly lighter oxygen than the coexisting olivine. The oxygen isotopic composition of this spinel is similar to those of Al-rich chondrules. Our discovery of relic spinel in chondrules is an indication of the complexities in the early solar nebular processes that ranged from formation of refractory inclusion, through Al-rich chondrule, to ferromagnesian chondrules, and attests to the recycling of earlier formed materials into the precursors of later formed materials. The characteristic features of spinel group minerals are not only sensitive to thermal metamorphism, but also shed light on chondrule formation processes.     

Oxygen, silicon, and Mn-Cr isotopes of fayalite in the Kaba oxidized CV3 chondrite: Constraints for its formation history

The iron-rich olivine end-member, fayalite, occurs in the matrix, chondrules, Ca-Al-rich inclusions (CAIs), silicate aggregates, and dark inclusions in the Kaba and Mokoia oxidized CV3 chondrites. In most occurrences, fayalite is associated with magnetite and troilite. To help constrain the origin of the fayalite (Fa_98-100), we measured oxygen and silicon isotopic compositions and Mn-Cr systematics in fayalite from two petrographic settings of the Kaba meteorite. One setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine-grained magnetite and sulfide, while the other setting consists of small fayalite-magnetite-sulfide assemblages within or at the surface of Type I barred or porphyritic olivine chondrules. Oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line, and is distinct from that in chondrule forsterites, which are enriched in ¹⁶O (Δ¹⁷O = ∼−4‰). Oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites. Silicon isotopes suggest that forsterite is ∼1‰/amu heavier than adjacent fayalite within Kaba chondrules. However, we were unable to confirm large silicon isotopic differences among fayalites reported previously. The Mn-Cr data for big Kaba fayalites give an initial ⁵³Mn/⁵⁵Mn ratio of (2.07 ± 0.17) × 10⁻⁶, consistent with literature results on Mokoia chondrule fayalites. The combined data suggest that fayalites in both petrographic settings formed at about the same time, ∼9.7 Ma after the formation of CAIs. Our data indicate that those fayalite-magnetite-troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body. The formation site and mechanism for the big fayalite laths is less clear, but the petrographic setting indicates that they did not form in situ. None of the models that have been suggested for formation of these fayalites is entirely satisfactory.