High precision SIMS oxygen three isotope study of chondrules in LL3 chondrites: Role of ambient gas during chondrule formation

We report high precision SIMS oxygen three isotope analyses of 36 chondrules from some of the least equilibrated LL3 chondrites, and find systematic variations in oxygen isotope ratios with chondrule types. FeO-poor (type I) chondrules generally plot along a mass dependent fractionation line (Δ¹⁷O ∼ 0.7‰), with δ¹⁸O values lower in olivine-rich (IA) than pyroxene-rich (IB) chondrules. Data from FeO-rich (type II) chondrules show a limited range of δ¹⁸O and δ¹⁷O values at δ¹⁸O = 4.5‰, δ¹⁷O = 2.9‰, and Δ¹⁷O = 0.5‰, which is slightly ¹⁶O-enriched relative to bulk LL chondrites (Δ¹⁷O ∼ 1.3‰). Data from four chondrules show ¹⁶O-rich oxygen isotope ratios that plot near the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Glass analyses in selected chondrules are systematically higher than co-existing minerals in both δ¹⁸O and Δ¹⁷O values, whereas high-Ca pyroxene data in the same chondrule are similar to those in olivine and pyroxene phenocrysts.Our results suggest that the LL chondrite chondrule-forming region contained two kinds of solid precursors, (1) ¹⁶O-poor precursors with Δ¹⁷O > 1.6‰ and (2) ¹⁶O-rich solid precursors derived from the same oxygen isotope reservoir as carbonaceous chondrites. Oxygen isotopes exhibited open system behavior during chondrule formation, and the interaction between the solid and ambient gas might occur as described in the following model. Significant evaporation and recondensation of solid precursors caused a large mass-dependent fractionation due to either kinetic or equilibrium isotope exchange between gas and solid to form type IA chondrules with higher bulk Mg/Si ratios. Type II chondrules formed under elevated dust/gas ratios and with water ice in the precursors, in which the ambient H₂O gas homogenized chondrule melts by isotope exchange. Low temperature oxygen isotope exchange may have occurred between chondrule glasses and aqueous fluids with high Δ¹⁷O (∼5‰) in LL the parent body. According to our model, oxygen isotope ratios of chondrules were strongly influenced by the local solid precursors in the proto-planetary disk and the ambient gas during chondrule melting events.     

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.     

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.     

Microscale oxygen isotopic exchange and magnetite formation in the Ningqiang anomalous carbonaceous chondrite

We report in situ measurements of O-isotopic compositions of magnetite, olivine and pyroxene in chondrules of the Ningqiang anomalous carbonaceous chondrite. The petrographic setting of Ningqiang magnetite is similar to those in oxidized-CV chondrites such as Allende, where magnetite is found together with Ni-rich metal and sulfide in opaque assemblages in chondrules. Both magnetite and silicate oxygen data fall close to the carbonaceous-chondrite-anhydrous-mineral line with relatively large ranges in δ¹⁸O in magnetite (−4.9 to +4.2‰) and in silicates (−15.2 to −4.5‰). Magnetite and silicates are not in O-isotopic equilibrium: the weighted average Δ¹⁷O (=δ¹⁷O − 0.52 × δ¹⁸O) values of magnetite are 1.7 to 3.6‰ higher than those of the silicates in the same chondrules. The petrological characteristics and O-isotopic disequilibrium between magnetite and silicates suggest the formation of Ningqiang magnetite by the oxidation of preexisting metal grains by an aqueous fluid during parent body alteration. The weighted average Δ¹⁷O of −3.3 ± 0.3‰ is the lowest magnetite value measured in unequilibrated chondrites and there is a positive correlation between Δ¹⁷O values of magnetite and silicates in each chondrule. These observations indicate that, during aqueous alteration in the Ningqiang parent asteroid, the water/rock ratio was relatively low and O-isotopic exchange between the fluid and chondrule silicates occurred on the scale of individual chondrules.     

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.     

Oxygen isotopic compositions of chondrules from the metal-rich chondrites Isheyevo (CH/CBb), MAC 02675 (CBb) and QUE 94627 (CBb)

It has been recently suggested that (1) CH chondrites and the CB_b/CH-like chondrite Isheyevo contain two populations of chondrules formed by different processes: (i) magnesian non-porphyritic (cryptocrystalline and barred) chondrules, which are similar to those in the CB chondrites and formed in an impact-generated plume of melt and gas resulted from large-scale asteroidal collision, and (ii) porphyritic chondrules formed by melting of solid precursors in the solar nebula. (2) Porphyritic chondrules in Isheyevo and CH chondrites are different from porphyritic chondrules in other carbonaceous chondrites ( [Krot et al., 2005], [Krot et al., 2008a] and [Krot et al., 2008b]). In order to test these hypotheses, we measured in situ oxygen isotopic compositions of porphyritic (magnesian, Type I and ferroan, Type II) and non-porphyritic (magnesian and ferroan cryptocrystalline) chondrules from Isheyevo and CB_b chondrites MAC 02675 and QUE 94627, paired with QUE 94611, using a Cameca ims-1280 ion microprobe.On a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), compositions of chondrules measured follow approximately slope-1 line. Data for 19 magnesian cryptocrystalline chondrules from Isheyevo, 24 magnesian cryptocrystalline chondrules and 6 magnesian cryptocrystalline silicate inclusions inside chemically-zoned Fe,Ni-metal condensates from CB_b chondrites have nearly identical compositions: Δ¹⁷O = −2.2 ± 0.9‰, −2.3 ± 0.6‰ and −2.2 ± 1.0‰ (2σ), respectively. These observations and isotopically light magnesium compositions of cryptocrystalline magnesian chondrules in CB_b chondrites (Gounelle et al., 2007) are consistent with their single-stage origin, possibly as gas-melt condensates in an impact-generated plume. In contrast, Δ¹⁷O values for 11 Type I and 9 Type II chondrules from Isheyevo range from −5‰ to +4‰ and from −17‰ to +3‰, respectively. In contrast to typical chondrules from carbonaceous chondrites, seven out of 11 Type I chondrules from Isheyevo plot above the terrestrial fractionation line. We conclude that (i) porphyritic chondrules in Isheyevo belong to a unique population of objects, suggesting formation either in a different nebular region or at a different time than chondrules from other carbonaceous chondrites; (ii) Isheyevo, CB and CH chondrites are genetically related meteorites: they contain non-porphyritic chondrules produced during the same highly-energetic event, probably large-scale asteroidal collision; (iii) the differences in mineralogy, petrography, chemical and whole-rock oxygen isotopic compositions between CH and CB chondrites are due to various proportions of the nebular and the impact-produced materials.     

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.     

Chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in CV chondrites: Constraints on the evolution of nebular dust

A correlation of petrography, mineral chemistry and in situ oxygen isotopic compositions of fine-grained olivine from the matrix and of fine- and coarse-grained olivine from accretionary rims around Ca-Al-rich inclusions (CAIs) and chondrules in CV chondrites is used here to constrain the processes that occurred in the solar nebula and on the CV parent asteroid. The accretionary rims around Leoville, Vigarano, and Allende CAIs exhibit a layered structure: the inner layer consists of coarse-grained, forsteritic and ¹⁶O-rich olivine (Fa_1-40 and Δ¹⁷O = −24‰ to −5‰; the higher values are always found in the outer part of the layer and only in the most porous meteorites), whereas the middle and the outer layers contain finer-grained olivines that are more fayalitic and ¹⁶O-depleted (Fa_15-50 and Δ¹⁷O = −18‰ to +1‰). The CV matrices and accretionary rims around chondrules have olivine grains of textures, chemical and isotopic compositions similar to those in the outer layers of accretionary rims around CAIs. There is a correlation between local sample porosity and olivine chemical and isotopic compositions: the more compact regions (the inner accretionary rim layer) have the most MgO- and ¹⁶O-rich compositions, whereas the more porous regions (outer rim layers around CAIs, accretionary rims around chondrules, and matrices) have the most MgO- and ¹⁶O-poor compositions. In addition, there is a negative correlation of olivine grain size with fayalite contents and Δ¹⁷O values. However, not all fine-grained olivines are FeO-rich and ¹⁶O-poor; some small (<1 μm in Leoville and 5-10 μm in Vigarano and Allende) ferrous (Fa_>20) olivine grains in the outer layers of the CAI accretionary rims and in the matrix show significant enrichments in ¹⁶O (Δ¹⁷O = −20‰ to −10‰). We infer that the inner layer of the accretionary rims around CAIs and, at least, some olivine grains in the finer portions of accretionary rims and CV matrices formed in an ¹⁶O-rich gaseous reservoir, probably in the CAI-forming region. Grains in the outer layers of the CAI accretionary rims and in the rims around chondrules as well as matrix may have also originated as ¹⁶O-rich olivine. However, these olivines must have exchanged O isotopes to variable extents in the presence of an ¹⁶O-poor reservoir, possibly the nebular gas in the chondrule-forming region(s) and/or fluids in the parent body. The observed trend in isotopic compositions may arise from mixtures of ¹⁶O-rich forsterites with grain overgrowths or newly formed grains of ¹⁶O-poor fayalitic olivines formed during parent body metamorphism. However, the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of ¹⁶O-rich forsterite and subsequently exchanged Fe-Mg and O isotopes in situ in the presence of aqueous solutions (i.e., fluid-assisted thermal metamorphism).     

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:

     

Oxygen-isotopic compositions of relict and host grains in chondrules in the Yamato 81020 CO3.0 chondrite

We report the oxygen-isotope compositions of relict and host olivine grains in six high-FeO porphyritic olivine chondrules in one of the most primitive carbonaceous chondrites, CO3.0 Yamato 81020. Because the relict grains predate the host phenocrysts, microscale in situ analyses of O-isotope compositions can help assess the degree of heterogeneity among chondrule precursors and constrain the nebular processes that caused these isotopic differences. In five of six chondrules studied, the Δ¹⁷O (=δ¹⁷O −0.52 · δ¹⁸O) compositions of host phenocrysts are higher than those in low-FeO relict grains; the one exception is for a chondrule with a moderately high-FeO relict. Both the fayalite compositions as well as the O-isotope data support the view that the low-FeO relict grains formed in a previous generation of low-FeO porphyritic chondrules that were subsequently fragmented. It appears that most low-FeO porphyritic chondrules formed earlier than most high-FeO porphyritic chondrules, although there were probably some low-FeO chondrules that formed during the period when most high-FeO chondrules were forming.     

The oxygen-isotope composition of chondrules and isolated forsterite and olivine grains from the Tagish Lake carbonaceous chondrite

The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ¹⁷O = 0.95 * δ¹⁸O − 3.24; R² = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo_99.6-99.8; δ¹⁸O = −7.2‰ to −5.5‰; δ¹⁷O = −9.6‰ to −8.2‰) are more ¹⁶O-rich than the isolated olivine grains (Fo_39.6-86.8; δ¹⁸O = 3.1‰ to 5.1‰; δ¹⁷O = −0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo_97.2-99.8) with oxygen-isotope compositions (δ¹⁸O = −5.2‰ to 5.9‰; δ¹⁷O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ¹⁷O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with ¹⁶O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R² = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM_slope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with ¹⁶O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.     

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.     

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.     

Mass-independent fractionation of oxygen isotopes in the mesostasis of a chondrule from the Semarkona LL3.0 ordinary chondrite

A large chondrule from Semarkona, the most primitive ordinary chondrite known, has been discovered to contain a record of mass transport during its formation. In most respects, it is a normal Type I, group A1, low-FeO chondrule that was produced by reduction and mass-loss during the unidentified flash-heating event that produced the chondrules, the most abundant structural component in primitive meteorites. We have previously measured elemental abundances and abundance profiles in this chondrule. We here report oxygen isotope ratio abundances and ratio abundance profiles. We have found that the mesostasis is zoned in oxygen isotope ratio, with the center of the chondrule containing isotopically heavier oxygen than the outer regions, the outer regions being volatile rich from the diffusion of volatiles into the chondrule during cooling. The δ¹⁷O values range from −2.0‰ to 9.9‰, while δ¹⁸O range from −1.9‰ to 9.6‰. More importantly, a plot of δ¹⁷O against δ¹⁸O has a slope of 1.1 ± 0.2 (1σ) and 0.88 ± 0.10 (1σ) when measured by two independent methods. Co-variation of δ¹⁷O with δ¹⁸O that does not follow mass-dependent fractionation has often been seen in primitive solar system materials and is usually ascribed to the mixing of different oxygen reservoirs. We argue that petrographic and compositional data indicate that this chondrule was completely melted at the time of its formation so that relic grains could not have survived. Furthermore, there is petrographic and compositional evidence that there was no aqueous alteration of this chondrule subsequent to its formation. Although it is possible to formulate a series of exchanges between the chondrule and external ¹⁶O-rich and ¹⁶O-poor reservoirs that may explain the detailed oxygen isotope systematics of this chondrule, such a sequence of events looks very contrived. We therefore hypothesize that reduction, devolatilization, and crystallization of the chondrule melt may have produced ¹⁶O-rich olivines and ¹⁶O-poor mesostasis plotting on a slope-one line as part of the chondrule-forming process in an analogous fashion to known chemical mass-independent isotopic fractionation mechanisms. During cooling, volatiles and oxygen near the terrestrial line in oxygen isotope composition produced the outer zone of volatile rich and ¹⁶O-rich mesostasis. The chondrule therefore not only retains a record of considerable mass transport accompanying formation, but also may indicate that the isotopes of oxygen underwent mass-independent fractionation during the process.     

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.     

Extremely rapid cooling of a carbonaceous-chondrite chondrule containing very O-rich olivine and a Mg-excess

We describe a phenocryst in a CO-chondrite type-II chondrule that we infer to have formed by melting an amoeboid olivine aggregate (AOA). This magnesian olivine phenocryst has an extremely ¹⁶O-rich composition Δ¹⁷O (=δ¹⁷O - 0.52 · δ¹⁸O) = −23‰. It is present in one of the most pristine carbonaceous chondrites, the CO3.0 chondrite Yamato 81020. The bulk of the chondrule has a very different Δ¹⁷O of −1‰, thus the Δ¹⁷O range within this single chondrule is 22‰, the largest range encountered in a chondrule. We interpret the O isotopic and Fe-Mg distributions to indicate that a fine-grained AOA assemblage was incompletely melted during the flash melting that formed the chondrule. Some Fe-Mg exchange but negligible O-isotopic exchange occurred between its core and the remainder of the chondrule. A diffusional model to account for the observed Fe-Mg and O-isotopic exchange yields a cooling rate of 10⁵ to 10⁶ K hr⁻¹. This estimate is much higher than the cooling rates of 10¹ to 10³ K hr⁻¹ inferred from furnace simulations of type-II chondrule textures (e.g. Lofgren, 1996); however, our cooling-rate applies to higher temperatures (near 1900 K) than are modeled by the crystal-growth based cooling rates. We observed a low ²⁶Al/²⁷Al initial ratio ((4.6 ± 3.0) · 10⁻⁶) in the chondrule mesostasis, a value similar to those in ordinary chondrites (Kita et al., 2000). If the ²⁶Al/²⁷Al system is a good chronometer, then chondrule I formed about 2 Ma after the formation of refractory inclusions.     

Chemistry, petrology and bulk oxygen isotope compositions of chondrules from the Mokoia CV3 carbonaceous chondrite

We report bulk chemical compositions and physical properties for a suite of 94 objects, mostly chondrules, separated from the Mokoia CV3_ox carbonaceous chondrite. We also describe mineralogical and petrologic information for a selected subset of the same suite of chondrules. The data are used to examine the range of chondrule bulk compositions, and to investigate the relationships between chondrule mineralogy, texture and bulk compositions, as well as oxygen isotopic properties that we reported previously. Most of the chondrules show minimal metamorphism, corresponding to petrologic subtype <3.2. In general, elemental fractionations observed in chondrule bulk compositions are reflected in the compositions of constituent minerals. For chondrules, mean bulk compositions and compositional ranges are very similar for large (>2 mg) and small (<2 mg) size fractions. Two of the objects studied are described as matrix-rich clasts. These have similar bulk compositions to the chondrule mean, and are potential chondrule precursors. One of these clasts has a similar bulk oxygen isotopic composition to Mokoia chondrules, but the other has an anomalously high value of Δ¹⁷O (+3.60‰).Chondrules are diverse in bulk chemical composition, with factor of 10 variations in most major element abundances that cannot be attributed to secondary processes. The chondrules examined show evidence for extensive secondary oxidation, and possible sulfidization, as expected for an oxidized CV chondrite, but minimal aqueous alteration. Some of the bulk chondrule compositional variation might be the result of chemical (e.g. volatilization or condensation) or physical (e.g. metal loss) processes during chondrule formation. However, we suggest that it is mainly the result of significant variations in the assembly of particles that constituted chondrule precursors. Precursor material likely included a refractory component, possibly inherited from disaggregated CAIs, an FeO-poor ferromagnesian component such as olivine or pyroxene, an oxidized ferromagnesian component, and a metal component. Bulk oxygen isotope ratios of chondrules can be explained if refractory and ferromagnesian precursor materials initially shared similar oxygen isotopic compositions of δ¹⁷O, δ¹⁸O around −50‰, and then significant exchange occurred between the chondrule and surrounding ¹⁶O-poor gas during melting.     

A single asteroidal source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis

We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments (∼470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average Δ¹⁷O (‰-deviation from the terrestrial mass-fractionation line, measured in situ from 15 μm spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 ± 0.09‰ (2σ) and 1.25 ± 0.16‰, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 ± 0.09‰, and 1.11 ± 0.12‰, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average Δ¹⁷O = 1.07 ± 0.18‰, but also with the LL chondrite group average Δ¹⁷O = 1.26 ± 0.24‰ (Clayton et al., 1991). We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their Δ¹⁷O values coincide with averages of Δ¹⁷O of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments.     

An Fe isotope study of ordinary chondrites

The Fe isotope composition of ordinary chondrites and their constituent chondrules, metal and sulphide grains have been systematically investigated. Bulk chondrites fall within a restricted isotopic range of <0.2‰ δ⁵⁶Fe, and chondrules define a larger range of >1‰ (−0.84‰ to 0.21‰ relative to the IRMM-14 Fe standard). Fe isotope compositions do not vary systematically with the very large differences in total Fe concentration, or oxidation state, of the H, L, and LL chondrite classes. Similarly, the Fe isotope compositions of chondrules do not appear to be determined by the H, L or LL classification of their host chondrite. This may support an origin of the three ordinary chondrite groups from variable accretion of identical Fe-bearing precursors.A close relationship between isotopic composition and redistribution of Fe during metamorphism on ordinary chondrite parent bodies was identified; the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types. The clear link between element redistribution and isotopic composition has implications for many other non-traditional isotope systems (e.g. Mg, Si, Ca, Cr). Isotopic compositions of chondrules may also be determined by their melting history; porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean. Very large chondrules preserve the greatest heterogeneity of Fe isotopes.The mean Fe isotope composition of bulk ordinary chondrites was found to be −0.06‰ (±0.12‰ 2 SD); this is isotopically lighter than the terrestrial mean composition and all other published non-chondritic meteorite suites e.g. lunar and Martian samples, eucrites, pallasites, and irons. Ordinary chondrites, though the most common meteorites found on Earth today, were not the sole building blocks of the terrestrial planets.     

The origin and history of ordinary chondrites: A study by iron isotope measurements of metal grains from ordinary chondrites

Chondrules and chondrites provide unique insights into early solar system origin and history, and iron plays a critical role in defining the properties of these objects. In order to understand the processes that formed chondrules and chondrites, and introduced isotopic fractionation of iron isotopes, we measured stable iron isotope ratios ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe in metal grains separated from 18 ordinary chondrites, of classes H, L and LL, ranging from petrographic types 3-6 using multi-collector inductively coupled plasma mass spectrometry. The δ⁵⁶Fe values range from −0.06 ± 0.01 to +0.30 ± 0.04‰ and δ⁵⁷Fe values are −0.09 ± 0.02 to +0.55 ± 0.05‰ (relative to IRMM-014 iron isotope standard). Where comparisons are possible, these data are in good agreement with published data. We found no systematic difference between falls and finds, suggesting that terrestrial weathering effects are not important in controlling the isotopic fractionations in our samples. We did find a trend in the ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe isotopic ratios along the series H, L and LL, with LL being isotopically heavier than H chondrites by ∼0.3‰ suggesting that redox processes are fractionating the isotopes. The ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe ratios also increase with increasing petrologic type, which again could reflect redox changes during metamorphism and also a temperature dependant fractionation as meteorites cooled. Metal separated from chondrites is isotopically heavier by ∼0.31‰ in δ⁵⁶Fe than chondrules from the same class, while bulk and matrix samples plot between chondrules and metal. Thus, as with so many chondrite properties, the bulk values appear to reflect the proportion of chondrules (more precisely the proportion of certain types of chondrule) to metal, whereas chondrule properties are largely determined by the redox conditions during chondrule formation. The chondrite assemblages we now observe were, therefore, formed as a closed system.