High precision oxygen three‐isotope analyses of anhydrous chondritic interplanetary dust particles

Abstract– Oxygen three‐isotope ratios of three anhydrous chondritic interplanetary dust particles (IDPs) were analyzed using an ion microprobe with a 2 μm small beam. The three anhydrous IDPs show Δ¹⁷O values ranging from ?5‰ to +1‰, which overlap with those of ferromagnesian silicate particles from comet Wild 2 and anhydrous porous IDPs. For the first time, internal oxygen isotope heterogeneity was resolved in two IDPs at the level of a few per mil in Δ¹⁷O values. Anhydrous IDPs are loose aggregates of fine‐grained silicates (≤3 μm in this study), with only a few coarse‐grained silicates (2–20 μm in this study). On the other hand, Wild 2 particles analyzed so far show relatively coarse‐grained (≥ few μm) igneous textures. If anhydrous IDPs represent fine‐grained particles from comets, the similar Δ¹⁷O values between anhydrous IDPs and Wild 2 particles may imply that oxygen isotope ratios in cometary crystalline silicates are similar, independent of crystal sizes and their textures. The range of Δ¹⁷O values of the three anhydrous IDPs overlaps also with that of chondrules in carbonaceous chondrites, suggesting a genetic link between cometary dust particles (Wild 2 particles and most anhydrous IDPs) and carbonaceous chondrite chondrules.     

Refractory inclusions in the CH/CB‐like carbonaceous chondrite Isheyevo: I. Mineralogy and petrography

Abstract— –The CH/CB‐like chondrite Isheyevo consists of metal‐rich (70–90 vol% Fe,Ni‐metal) and metal‐poor (7–20 vol% Fe,Ni‐metal) lithologies which differ in size and relative abundance of Fe,Ni‐metal and chondrules, as well as proportions of porphyritic versus non‐porphyritic chondrules. Here, we describe the mineralogy and petrography of Ca,Al‐rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in these lithologies. Based on mineralogy, refractory inclusions can be divided into hibonite‐rich (39%), grossite‐rich (16%), melilite‐rich (19%), spinel‐rich (14%), pyroxene‐anorthite‐rich (8%), fine‐grained spinel‐rich CAIs (1%), and AOAs (4%). There are no systematic differences in the inclusion types or their relative abundances between the lithologies. About 55% of the Isheyevo CAIs are very refractory (hibonite‐rich and grossite‐rich) objects, 20–240 μm in size, which appear to have crystallized from rapidly cooling melts. These inclusions are texturally and mineralogically similar to the majority of CAIs in CH and CB chondrites. They are distinctly different from CAIs in other carbonaceous chondrite groups dominated by the spinel‐pyroxene ± melilite CAIs and AOAs. The remaining 45% of inclusions are less refractory objects (melilite‐, spinel‐ and pyroxene‐rich CAIs and AOAs), 40–300 μm in size, which are texturally and mineralogically similar to those in other chondrite groups. Both types of CAIs are found as relict objects inside porphyritic chondrules indicating recycling during chondrule formation. We infer that there are at least two populations of CAIs in Isheyevo which appear to have experienced different thermal histories. All of the Isheyevo CAIs apparently formed at an early stage, prior to chondrule formation and prior to a hypothesized planetary impact that produced magnesian cryptocrystalline and skeletal chondrules and metal grains in CB, and possibly CH chondrites. However, some of the CAIs appear to have undergone melting during chondrule formation and possibly during a major impact event. We suggest that Isheyevo, as well as CH and CB chondrites, consist of variable proportions of materials produced by different processes in different settings: 1) by evaporation, condensation, and melting of dust in the protoplanetary disk (porphyritic chondrules and refractory inclusions), 2) by melting, evaporation and condensation in an impact generated plume (magnesian cryptocrystalline and skeletal chondrules and metal grains; some igneous CAIs could have been melted during this event), and 3) by aqueous alteration of pre‐existing planetesimals (heavily hydrated lithic clasts). The Isheyevo lithologies formed by size sorting of similar components during accretion in the Isheyevo parent body; they do not represent fragments of CH and CB chondrites.     

Oxygen isotope characteristics of chondrules from the Yamato‐82094 ungrouped carbonaceous chondrite: Further evidence for common O‐isotope environments sampled among carbonaceous chondrites

High‐precision secondary ion mass spectrometry (SIMS) was employed to investigate oxygen three isotopes of phenocrysts in 35 chondrules from the Yamato (Y) 82094 ungrouped 3.2 carbonaceous chondrite. Twenty‐one of 21 chondrules have multiple homogeneous pyroxene data (?¹⁷O 3SD analytical uncertainty: 0.7‰); 17 of 17 chondrules have multiple homogeneous pyroxene and plagioclase data. Twenty‐one of 25 chondrules have one or more olivine data matching coexisting pyroxene data. Such homogeneous phenocrysts (1) are interpreted to have crystallized from the final chondrule melt, defining host O‐isotope ratios; and (2) suggest efficient O‐isotope exchange between ambient gas and chondrule melt during formation. Host values plot within 0.7‰ of the primitive chondrule mineral (PCM) line. Seventeen chondrules have relict olivine and/or spinel, with some δ¹⁷O and δ¹⁸O values approaching ?40‰, similar to CAI or AOA‐like precursors. Regarding host chondrule data, 22 of 34 have Mg#s of 98.8–99.5 and ?¹⁷O of ?3.9‰ to ?6.1‰, consistent with most Acfer 094, CO, CR, and CV chondrite chondrules, and suggesting a common reduced O‐isotope reservoir devoid of ¹⁶O‐poor H₂O. Six Y‐82094 chondrules have ?¹⁷O near ?2.5‰, with Mg#s of 64–97, consistent with lower Mg# chondrules from Acfer 094, CO, CR, and CV chondrites; their signatures suggest precursors consisting of those forming Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules plus ¹⁶O‐poor H₂O, at high dust enrichments. Three type II chondrules plot slightly above the PCM line, near the terrestrial fractionation line (?¹⁷O: ~+0.1‰). Their O‐isotopes and olivine chemistry are like LL3 type II chondrules, suggesting they sampled ordinary chondrite‐like chondrule precursors. Finally, three Mg# >99 chondrules have ?¹⁷O of ?6.7‰ to ?8.1‰, potentially due to ¹⁶O‐rich refractory precursor components. The predominance of Mg# ~99, ?¹⁷O: ?5‰ ± 1‰ chondrules and a high chondrule‐to‐matrix ratio suggests bulk Y‐82094 characteristics are closely related to anhydrous dust sampled by most carbonaceous chondrite chondrules.     

Petrology and oxygen isotopes of NWA 5492, a new metal‐rich chondrite

Abstract– Northwest Africa 5492 is a new metal‐rich chondrite breccia that may represent a new oxygen reservoir and new chondrite parent body. It has some textural similarities to CB and CH chondrites, but silicates are more reduced, sulfides are more common and not associated with metal, and metal compositions differ from CB and CH chondrites. Oxygen isotope ratios indicate that Northwest Africa (NWA) 5492 components (chondrules and lithic fragments) formed in at least two different oxygen reservoirs. The more common, and presumably host, component plots in a region above the terrestrial fractionation line, below ordinary chondrite compositions, and just above enstatite chondrites in 3‐oxygen space. The only other chondritic materials that plot in this region are chondrules from the Grosvenor Mountains (GRO) 95551 ungrouped metal‐rich chondrite. The other rare component plots near the CR, CB, and CH chondrites. Based on petrologic characteristics and oxygen isotopic compositions, NWA 5492 appears to be related to the ungrouped metal‐rich GRO 95551 chondrite.     

Mineralogy,petrology, and oxygen isotopic compositions of chondritic and achondritic lithologies in the anomalous CB carbonaceous chondrites Sierra Gorda 013 and Fountain Hills

The CB (Bencubbin-like) metal-rich carbonaceous chondrites are subdivided into the CB_a and CB_b subgroups. The CB_a chondrites are composed predominantly of ~cm-sized skeletal olivine chondrules and unzoned Fe,Ni-metal ± troilite nodules. The CB_b chondrites are finer grained than the CB_as and consist of chemically zoned and unzoned Fe,Ni-metal grains, Fe,Ni-metal ± troilite nodules, cryptocrystalline and skeletal olivine chondrules, and rare refractory inclusions. Both subgroups contain exceptionally rare porphyritic chondrules and no interchondrule fine-grained matrix, and are interpreted as the products of a gas–melt impact plume formed by a high-velocity collision between differentiated planetesimals about 4562 Ma. The anomalous metal-rich carbonaceous chondrites, Fountain Hills and Sierra Gorda 013 (SG 013), have bulk oxygen isotopic compositions similar to those of other CBs but contain coarse-grained igneous clasts/porphyritic chondrule-like objects composed of olivine, low-Ca-pyroxene, and minor plagioclase and high-Ca pyroxene as well as barred olivine and skeletal olivine chondrules. Cryptocrystalline chondrules, zoned Fe,Ni-metal grains, and interchondrule fine-grained matrix are absent. In SG 013, Fe,Ni-metal (~80 vol%) occurs as several mm-sized nodules; magnesiochromite (Mg-chromite) is accessory; daubréelite and schreibersite are minor; troilite is absent. In Fountain Hills, Fe,Ni-metal (~25 vol%) is dispersed between chondrules and silicate clasts; chromite and sulfides are absent. In addition to a dominant chondritic lithology, SG 013 contains a chondrule-free lithology composed of Fe,Ni-metal nodules (~25 vol%), coarse-grained olivine and low-Ca pyroxene, interstitial high-Ca pyroxene and anorthitic plagioclase, and Mg-chromite. Here, we report on oxygen isotopic compositions of olivine, low-Ca pyroxene, and ±Mg-chromite in Fountain Hills and both lithologies of SG 013 measured in situ using an ion microprobe. Oxygen isotope compositions of olivine, low-Ca pyroxene, and Mg-chromite in these meteorites are similar to those of magnesian non-porphyritic chondrules in CB_a and CB_b chondrites: on a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O), they plot close to a slope-1 (primitive chondrule mineral) line and have a very narrow range of Δ¹⁷O (=δ¹⁷O–0.52 × δ¹⁸O) values, −2.5 ± 0.9‰ (avr ± 2SD). No isotopically distinct relict grains have been identified in porphyritic chondrule-like objects. We suggest that magnesian non-porphyritic (barred olivine, skeletal olivine, cryptocrystalline) chondrules in the CB_as, CB_bs, and porphyritic chondrule-like objects in SG 013 and Fountain Hills formed in different zones of the CB impact plume characterized by variable pressure, temperature, cooling rates, and redox conditions. The achondritic lithology in SG 013 represents fragments of one of the colliding bodies and therefore one of the CB chondrule precursors. Fountain Hills was subsequently modified by impact melting; Fe,Ni-metal and sulfides were partially lost during this process.     

Oxygen isotope and 26Al‐26Mg systematics of aluminum‐rich chondrules from unequilibrated enstatite chondrites

Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for ²⁶Al‐²⁶Mg systematics, only one aluminum‐rich chondrule contains excess ²⁶Mg from the in situ decay of ²⁶Al; the inferred initial ratio (²⁶Al/²⁷Al)_o = (6.8 ± 2.4) × 10^?6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ?0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more ¹⁶O‐rich compositions with a range in δ¹⁸O of about ?12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess ²⁶Mg from the in situ decay of ²⁶Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of ²⁶Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that ²⁶Al was at least approximately homogeneously distributed across the chondrite‐forming region.     

The Isheyevo meteorite: Mineralogy,petrology, bulk chemistry,oxygen, nitrogen,carbon isotopic compositions,and 40Ar‐39Ar ages

Abstract— Isheyevo is a metal‐rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7–90 vol%). The metal‐rich lithologies with 50–60 vol% of Fe,Ni metal are dominant. The metal‐rich and metal‐poor lithologies are most similar to the CB_b and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components—Fe,Ni metal, chondrules, refractory inclusions (Ca, Al‐rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts—but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non‐porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in ¹⁵N (δ¹⁵N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non‐magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CB_b chondrites, ～20% of Fe,Ni‐metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni‐rich (>20 wt% Ni). In contrast to CB_b and CH chondrites, most metal grains are thermally decomposed into Ni‐rich and Ni‐poor phases. Similar to CH chondrites, chondrules have porphyritic and non‐porphyritic textures and ferromagnesian (type I and II), silica‐rich, and aluminum‐rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite‐, grossite‐, and melilite‐rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico‐chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine‐grained dust. Such a scenario is consistent with the presence of solar wind—implanted noble gases in Isheyevo and with its comparatively old K‐Ar age. We cannot exclude that the K‐Ar system was affected by a later collisional event. The cosmic‐ray exposure (CRE) age of Isheyevo determined by cosmogenic ³⁸Ar is ～34 Ma, similar to that of the Bencubbin (CB_a) meteorite.     

Size‐frequency distributions of chondrules and chondrule fragments in LL3 chondrites: Implications for parent‐body fragmentation of chondrules

Abstract— We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 φ units or 570 μm, much smaller than the previous rough estimate of ～900 μm. Chondrule fragments in the five LL3 chondrites have a mean apparent cross‐section of 1.60 φ units or 330 μm. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/ chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine‐pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet‐textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent‐body chondrule‐fragmentation events and not to chondrule‐formation processes in the solar nebula.     

Oxygen isotope systematics of chondrule olivine,pyroxene, and plagioclase in one of the most pristine CV3Red chondrites (Northwest Africa 8613)

We performed in situ oxygen three‐isotope measurements of chondrule olivine, pyroxenes, and plagioclase from the newly described CV_Red chondrite NWA 8613. Additionally, oxygen isotope ratios of plagioclase in chondrules from the Kaba CV3_OxB chondrite were determined to enable comparisons of isotope ratios and degree of alteration of chondrules in both CV lithologies. NWA 8613 was affected by only mild thermal metamorphism. The majority of oxygen isotope ratios of olivine and pyroxenes plot along a slope‐1 line in the oxygen three‐isotope diagram, except for a type II and a remolten barred olivine chondrule. When isotopic relict olivine is excluded, olivine, and low‐ and high‐Ca pyroxenes are indistinguishable regarding Δ¹⁷O values. Conversely, plagioclase in chondrules from NWA 8613 and Kaba plot along mass‐dependent fractionation lines. Oxygen isotopic disequilibrium between phenocrysts and plagioclase was caused probably by exchange of plagioclase with ¹⁶O‐poor fluids on the CV parent body. Based on an existing oxygen isotope mass balance model, possible dust enrichment and ice enhancement factors were estimated. Type I chondrules from NWA 8613 possibly formed at moderately high dust enrichment factors (50× to 150× CI dust relative to solar abundances); estimates for water ice in the chondrule precursors range from 0.2× to 0.6× the nominal amount of ice in dust of CI composition. Findings agree with results from an earlier study on oxygen isotopes in chondrules of the Kaba CV chondrite, providing further evidence for a relatively dry and only moderately high dust‐enriched disk in the CV chondrule‐forming region.     

Anorthite‐rich chondrules in CR and CH carbonaceous chondrites: Genetic link between calcium‐aluminum‐rich inclusions and ferromagnesian chondrules

Abstract— Anorthite‐rich chondrules in CR and CH carbonaceous chondrites consist of magnesian low‐Ca pyroxene and forsterite phenocrysts, FeNi‐metal nodules, interstitial anorthite, Al‐Ti‐Cr‐rich low‐Ca and high‐Ca pyroxenes, and crystalline mesostasis composed of silica, anorthite and high‐Ca pyroxene. Three anorthite‐rich chondrules contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, ±Al‐diopside, and ± forsterite. A few chondrules contain regions which are texturally and mineralogically similar to magnesian (type I) chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Anorthite‐rich chondrules in CR and CH chondrites are mineralogically similar to those in CV and CO carbonaceous chondrites, but contain no secondary nepheline, sodalite or ferrosilite. Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the anorthite‐rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that anorthite‐rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors (olivine, pyroxenes, FeNi‐metal) mixed with the refractory materials, including relic CAIs, composed of anorthite, spinel, high‐Ca pyroxene and forsterite. The observed mineralogical and textural similarities of the anorthite‐rich chondrules in several carbonaceous chondrite groups (CV, CO, CH, CR) may indicate that these chondrules formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated. This may explain the relative enrichment of anorthite‐rich chondrules in ¹⁶O compared to typical ferromagnesian chondrules (Russell et al., 2000).     

Refractory calcium‐aluminum‐rich inclusions and aluminum‐diopside‐rich chondrules in the metal‐rich chondrites Hammadah al Hamra 237 and Queen Alexandra Range 94411

Abstract— The metal‐rich chondrites Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627, contain relatively rare (<1 vol%) calcium‐aluminum‐rich inclusions (CAIs) and Al‐diopside‐rich chondrules. Forty CAIs and CAI fragments and seven Al‐diopside‐rich chondrules were identified in HH 237 and QUE 94411/94627. The CAIs, ～50–400 μm in apparent diameter, include (a) 22 (56%) pyroxene‐spinel ± melilite (+forsterite rim), (b) 11 (28%) forsterite‐bearing, pyroxene‐spinel ± melilite ± anorthite (+forsterite rim) (c) 2 (5%) grossite‐rich (+spinel‐melilite‐pyroxene rim), (d) 2 (5%) hibonite‐melilite (+spinel‐pyroxene ± forsterite rim), (e) 1 (2%) hibonite‐bearing, spinel‐perovskite (+melilite‐pyroxene rim), (f) 1 (2%) spinel‐melilite‐pyroxene‐anorthite, and (g) 1 (2%) amoeboid olivine aggregate. Each type of CAI is known to exist in other chondrite groups, but the high abundance of pyroxene‐spinel ± melilite CAIs with igneous textures and surrounded by a forsterite rim are unique features of HH 237 and QUE 94411/94627. Additionally, oxygen isotopes consistently show relatively heavy compositions with Δ¹⁷O ranging from ?6%0 to ?10%0 (1σ = 1.3%0) for all analyzed CAI minerals (grossite, hibonite, melilite, pyroxene, spinel). This suggests that the CAIs formed in a reservoir isotopically distinct from the reservoir(s) where “normal”, ¹⁶O‐rich (Δ¹⁷O < ?20%0) CAIs in most other chondritic meteorites formed. The Al‐diopside‐rich chondrules, which have previously been observed in CH chondrites and the unique carbonaceous chondrite Adelaide, contain Al‐diopside grains enclosing oriented inclusions of forsterite, and interstitial anorthitic mesostasis and Al‐rich, Ca‐poor pyroxene, occasionally enclosing spinel and forsterite. These chondrules are mineralogically similar to the Al‐rich barred‐olivine chondrules in HH 237 and QUE 94411/94627, but have lower Cr concentrations than the latter, indicating that they may have formed during the same chondrule‐forming event, but at slightly different ambient nebular temperatures. Aluminum‐diopside grains from two Al‐diopside‐rich chondrules have O‐isotopic compositions (Δ¹⁷O ? ?7 ± 1.1 %0) similar to CAI minerals, suggesting that they formed from an isotopically similar reservoir. The oxygen‐isotopic composition of one Ca, Al‐poor cryptocrystalline chondrule in QUE 94411/94627 was analyzed and found to have Δ¹⁷O ? ?3 ± 1.4%0. The characteristics of the CAIs in HH 237 and QUE 94411/94627 are inconsistent with an impact origin of these metal‐rich meteorites. Instead they suggest that the components in CB chondrites are pristine products of large‐scale, high‐temperature processes in the solar nebula and should be considered bona fide chondrites.     

26Al‐26Mg systematics of Ca‐Al‐rich inclusions,amoeboid olivine aggregates,and chondrules from the ungrouped carbonaceous chondrite Acfer 094

Abstract— We report in situ magnesium isotope measurements of 7 porphyritic magnesium‐rich (type I) chondrules, 1 aluminum‐rich chondrule, and 16 refractory inclusions (14 Ca‐Al‐rich inclusions [CAIs] and 2 amoeboid olivine aggregates [AOAs]) from the ungrouped carbonaceous chondrite Acfer 094 using a Cameca IMS 6f ion microprobe. Both AOAs and 9 CAIs show radiogenic ²⁶Mg excesses corresponding to initial ²⁶Al/²⁷Al ratios between ～5 × 10^?5 ～7 × 10^?5 suggesting that formation of the Acfer 094 CAIs may have lasted for ～300,000 years. Four CAIs show no evidence for radiogenic ²⁶Mg; three of these inclusions (a corundum‐rich, a grossite‐rich, and a pyroxene‐hibonite spherule CAI) are very refractory objects and show deficits in ²⁶Mg, suggesting that they probably never contained ²⁶Al. The fourth object without evidence for radiogenic ²⁶Mg is an anorthite‐rich, igneous (type C) CAI that could have experienced late‐stage melting that reset its Al‐Mg systematics. Significant excesses in ²⁶Mg were observed in two chondrules. The inferred ²⁶Al/²⁷Al ratios in these two chondrules are (10.3 ± 7.4) × 10^?6 (6.0 ± 3.8) × 10^?6 (errors are 2σ), suggesting formation 1.6^+1.2_‐0.6 and 2.2^+0.4_‐0.3 Myr after CAIs with the canonical ²⁶Al/²⁷Al ratio of 5 × 10^?5. These age differences are consistent with the inferred age differences between CAIs and chondrules in primitive ordinary (LL3.0–LL3.1) and carbonaceous (CO3.0) chondrites.     

Fe and O isotope composition of meteorite fusion crusts: Possible natural analogues to chondrule formation?

Meteorite fusion crust formation is a brief event in a high‐temperature (2000–12,000 K) and high‐pressure (2–5 MPa) regime. We studied fusion crusts and bulk samples of 10 ordinary chondrite falls and 10 ordinary chondrite finds. The fusion crusts show a typical layering and most contain vesicles. All fusion crusts are enriched in heavy Fe isotopes, with δ⁵⁶Fe values up to +0.35‰ relative to the solar system mean. On average, the δ⁵⁶Fe of fusion crusts from finds is +0.23‰, which is 0.08‰ higher than the average from falls (+0.15‰). Higher δ⁵⁶Fe in fusion crusts of finds correlate with bulk chondrite enrichments in mobile elements such as Ba and Sr. The δ⁵⁶Fe signature of meteorite fusion crusts was produced by two processes (1) evaporation during atmospheric entry and (2) terrestrial weathering. Fusion crusts have either the same or higher δ¹⁸O (0.9–1.5‰) than their host chondrites, and the same is true for Δ¹⁷O. The differences in bulk chondrite and fusion crust oxygen isotope composition are explained by exchange of oxygen between the molten surface of the meteorites with the atmosphere and weathering. Meteorite fusion crust formation is qualitatively similar to conditions of chondrule formation. Therefore, fusion crusts may, at least to some extent, serve as a natural analogue to chondrule formation processes. Meteorite fusion crust and chondrules exhibit a similar extent of Fe isotope fractionation, supporting the idea that the Fe isotope signature of chondrules was established in a high‐pressure environment that prevented large isotope fractionations. The exchange of O between a chondrule melt and an ¹⁶O‐poor nebula as the cause for the observed nonmass dependent O isotope compositions in chondrules is supported by the same process, although to a much lower extent, in meteorite fusion crusts.     

A new metal‐rich chondrite grouplet

Abstract— A new grouplet of primitive, metal‐rich chondrites, here called the CB (C, carbonaceous; B, bencubbinite) chondrites, has been recognized. It includes Bencubbin, Weatherford, Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH chondrites, all of which are members of the more inclusive CR clan. CB chondrites have much greater metal/silicate ratios than any other chondrite group, widely increasing the range of metal/silicate fractionation recorded in solar nebular processes. They also have the greatest moderately volatile lithophile element depletions of any chondritic materials. Metal has compositional trends and zoning patterns that suggest a primitive condensation origin, in contrast with metal from other chondrite groups. CB chondrites, as well as other CR clan chondrites, have much heavier nitrogen (higher ¹⁵N/¹⁴N) than that in other chondrite groups. The primitive characteristics of the CB chondrites suggest that they contain one of the best records of early nebular processes. Another chondrite, Grosvenor Mountains 95551, is petrographically similar to the CB chondrites, but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir.     

Oxygen isotopic compositions and origins of calcium‐aluminum‐rich inclusions and chondrules

Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ¹⁸O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the ¹⁶O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted ¹⁶O‐rich presolar dust that exchanged oxygen with ¹⁶O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with ¹⁶O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in ¹⁶O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated ¹⁶O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.     

Chemistry and oxygen isotopic composition of cluster chondrite clasts and their components in LL3 chondrites

Cluster chondrites are characterized by close‐fit textures of deformed and indented chondrules, taken as evidence for hot chondrule accretion (Metzler 2012 ). We investigated seven cluster chondrite clasts from six brecciated LL3 chondrites and measured their bulk oxygen isotopic and chemical composition, including REE, Zr, and Hf. The same parameters were measured in situ on 93 chondrules and 4 interchondrule matrix areas. The CI‐normalized REE patterns of the clasts are flat, showing LL‐chondritic concentrations. The mean chemical compositions of chondrules in clasts and other LL chondrites are indistinguishable and we conclude that cluster chondrite chondrules are representative of the normal LL chondrule population. Type II chondrules are depleted in MgO, Al₂O₃ and refractory lithophiles (REE, Zr, Hf) by factors between 0.65 and 0.79 compared to type I chondrules. The chondrule REE patterns are basically flat with slight LREE < HREE fractionations. Many chondrules exhibit negative Eu anomalies while matrix shows a complementary pattern. Chondrules scatter along a correlation line with a slope of 0.63 in the oxygen 3‐isotope diagram, interpreted as the result of O‐isotope exchange between chondrule melts and ¹⁸O‐rich nebular components. In one clast, a distinct anticorrelation between chondrule size and δ¹⁸O is found, which may indicate a more intense oxygen isotope exchange by smaller chondrules. In some clasts the δ¹⁸O values of type I chondrules are correlated with concentrations of SiO₂ and MnO and anticorrelated with MgO, possibly due to the admixture of a SiO₂‐ and MnO‐rich component to chondrule melts during oxygen isotope exchange. Two chondrules with negative anomalies in Sm, Eu, and Yb were found and may relate their precursors to refractory material known from group III CAIs. Furthermore, three chondrules with strong LREE > HREE and Zr/Hf fractionations were detected, whose formation history remains to be explained.     

Calcium‐aluminum‐rich inclusions in enstatite chondrites (II): Oxygen isotopes

Abstract— In situ io n microprobe analyses of spinel in refractory calcium‐aluminium‐rich inclusions (CAIs) from type 3 EH chondrites yield ¹⁶O‐rich compositions (δ ¹⁸O and δ ¹⁷O about‐40‰). Spinel and feldspar in a CAI from an EL3 chondrite have significantly heavier isotopic compositions (δ ¹⁸O and δ ¹⁷O about ?5‰). A regression through the data results in a line with slope 1.0 on a three‐isotope plot, similar to isotopic results from unaltered minerals in CAIs from carbonaceous chondrites. The existence of CAIs with ¹⁶O‐rich and ¹⁶O‐poor compositions in carbonaceous as well as enstatite chondrites indicates that CAIs formed in at least two temporally or spatially distinct oxygen reservoirs. General similarities in oxygen isotopic compositions of CAIs from enstatite, carbonaceous, and ordinary chondrites indicate a common nebular mechanism or locale for the production of most CAIs.     

Magnesium isotopic fractionation in chondrules from the Murchison and Murray CM2 carbonaceous chondrites

We present high‐precision measurements of the Mg isotopic compositions of a suite of types I and II chondrules separated from the Murchison and Murray CM2 carbonaceous chondrites. These chondrules are olivine‐ and pyroxene‐rich and have low ²⁷Al/²⁴Mg ratios (0.012–0.316). The Mg isotopic compositions of Murray chondrules are on average lighter (δ²⁶Mg ranging from ?0.95‰ to ?0.15‰ relative to the DSM‐3 standard) than those of Murchison (δ²⁶Mg ranging from ?1.27‰ to +0.77‰). Taken together, the CM2 chondrules exhibit a narrower range of Mg isotopic compositions than those from CV and CB chondrites studied previously. The least‐altered CM2 chondrules are on average lighter (average δ²⁶Mg = ?0.39 ± 0.30‰, 2SE) than the moderately to heavily altered CM2 chondrules (average δ²⁶Mg = ?0.11 ± 0.21‰, 2SE). The compositions of CM2 chondrules are consistent with isotopic fractionation toward heavy Mg being associated with the formation of secondary silicate phases on the CM2 parent body, but were also probably affected by volatilization and recondensation processes involved in their original formation. The low‐Al CM2 chondrules analyzed here do not exhibit any mass‐independent variations in ²⁶Mg from the decay of ²⁶Al, with the exception of two chondrules that show only small variations just outside of the analytical error. In the case of the chondrule with the highest Al/Mg ratio (a type IAB chondrule from Murchison), the lack of resolvable ²⁶Mg excess suggests that it either formed >1 Ma after calcium‐aluminum‐rich inclusions, or that its Al‐Mg isotope systematics were reset by secondary alteration processes on the CM2 chondrite parent body after the decay of ²⁶Al.     

Oxygen isotope signatures in bulk chondrules: Implications for the aqueous alteration and thermal metamorphism on the Allende CV3 parent body

Precise triple oxygen isotope compositions of 32 Allende bulk chondrules (ABCs) are determined using laser‐assisted fluorination mass spectrometry. Various chemically characterized chondrule types show ranges in δ¹⁸O that vary from ?4.80‰ to +1.10‰ (porphyritic olivine; PO, N = 15), ?3.10‰ to +1.50‰ (porphyritic olivine pyroxene; POP, N = 9), ?3.40‰ to +2.60‰ (barred olivine; BO, N = 4), and ?3.60‰ to +1.30‰ (porphyritic pyroxene; PP, N = 3). Oxygen isotope data of these chondrules yield a regression line referred to as the Allende bulk chondrule line (ABC line, slope = 0.86 ± 0.02). Most of our data fall closer to the primitive chondrule minerals line (PCM line, slope = 0.987 ± 0.013) and the carbonaceous chondrite anhydrous mineral line (CCAM line, slope = 0.94 ± 0.02) than the Allende anhydrous mineral line (AAML, slope = 1.00 ± 0.01) with a maximum δ¹⁸O value (+2.60‰) observed in a BO chondrule and a minimum δ¹⁸O value (?4.80‰) shown by a PO chondrule. Similarly, these chondrules depict variable ?¹⁷O values that range from ?5.65‰ to ?3.25‰ (PO), ?4.60‰ to ?2.80‰ (POP), ?4.95‰ to ?3.00‰ (BO), ?5.30‰ to ?3.20‰ (PP), and ?4.90‰ (CC). A simple model is proposed for the Allende CV3 chondrite with reference to the AAML and PCM line to illustrate the isotopic variations occurred due to the aqueous alteration processes. The estimated temperature ranging from 10 to 130 °C (mean ~60 °C) implies that the secondary mineralization in Allende happened in a warmer and relatively dry environment compared to Murchison. We further propose that thermal metamorphism could have dehydrated the Allende matrix at temperatures between >150 °C and <600 °C.     

Petrology,mineralogy, and oxygen isotope compositions of aluminum‐rich chondrules from CV3 chondrites

Bulk major element composition, petrography, mineralogy, and oxygen isotope compositions of twenty Al‐rich chondrules (ARCs) from five CV3 chondrites (Northwest Africa [NWA] 989, NWA 2086, NWA 2140, NWA 2697, NWA 3118) and the Ningqiang carbonaceous chondrite were studied and compared with those of ferromagnesian chondrules and refractory inclusions. Most ARCs are marginally Al‐richer than ferromagnesian chondrules with bulk Al₂O₃ of 10–15 wt%. ARCs are texturally similar to ferromagnesian chondrules, composed primarily of olivine, pyroxene, plagioclase, spinel, Al‐rich glass, and metallic phases. Minerals in ARCs have intermediate compositions. Low‐Ca pyroxene (Fs_0.6–8.8Wo_0.7–9.3) has much higher Al₂O₃ and TiO₂ contents (up to 12.5 and 2.3 wt%, respectively) than that in ferromagnesian chondrules. High‐Ca pyroxene (Fs_0.3–2.0Wo_33–54) contains less Al₂O₃ and TiO₂ than that in Ca,Al‐rich inclusions (CAIs). Plagioclase (An_77–99Ab_1–23) is much more sodic than that in CAIs. Spinel is enriched in moderately volatile element Cr (up to 6.7 wt%) compared to that in CAIs. Al‐rich enstatite coexists with anorthite and spinel in a glass‐free chondrule, implying that the formation of Al‐enstatite was not due to kinetic reasons but is likely due to the high Al₂O₃/CaO ratio (7.4) of the bulk chondrule. Three ARCs contain relict CAIs. Oxygen isotope compositions of ARCs are also intermediate between those of ferromagnesian chondrules and CAIs. They vary from ?39.4‰ to 13.9‰ in δ¹⁸O and yield a best fit line (slope = 0.88) close to the carbonaceous chondrite anhydrous mineral (CCAM) line. Chondrules with 5–10 wt% bulk Al₂O₃ have a slightly more narrow range in δ¹⁸O (?32.5 to 5.9‰) along the CCAM line. Except for the ARCs with relict phases, however, most ARCs have oxygen isotope compositions (>?20‰ in δ¹⁸O) similar to those of typical ferromagnesian chondrules. ARCs are genetically related to both ferromagnesian chondrules and CAIs, but the relationship between ARCs and ferromagnesian chondrules is closer. Most ARCs were formed during flash heating and rapid cooling processes like normal chondrules, only from chemically evolved precursors. ARCs extremely enriched in Al and those with relict phases could have had a hybrid origin (Krot et al. 2002) which incorporated refractory inclusions as part of the precursors in addition to ferromagnesian materials. The occurrence of melilite in ARCs indicates that melilite‐rich CAIs might be present in the precursor materials of ARCs. The absence of melilite in most ARCs is possibly due to high‐temperature interactions between a chondrule melt and the solar nebula.