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).     

Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions

Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa_<2), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (Åk_<15). ∼10% of AOAs contain low-Ca pyroxene (Fs_1-3Wo_1-5) that is in corrosion relationship with forsterite and is found in three major textural occurrences: (i) thin (<15 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5-10-μm-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 μm), commonly with abundant tiny (3-5 μm) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%-20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally ¹⁶O-rich isotopic compositions (Δ¹⁷O < −20‰). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are ¹⁶O-enriched (Δ¹⁷O < −20‰), whereas those of (iii) are ¹⁶O-depleted (Δ¹⁷O = −6‰ to −4‰). One of the extensively melted (>50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is ¹⁶O-rich (Δ¹⁷O ∼ −22‰), whereas the neighboring plagioclase mesostasis is ¹⁶O-depleted (Δ¹⁷O ∼ −11‰).We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAIs composed of Al-diopside, anorthite, spinel, and ±melilite) formed in an ¹⁶O-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted ¹⁶O-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into chondrules. The original ¹⁶O-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains.     

Petrography and oxygen isotopic compositions in refractory inclusions from CO chondrites

Fine-grained Ca-Al-rich inclusions (FGIs) in Yamato-81020 (CO3.0) and Kainsaz (CO3.1-CO3.2) chondrites have been studied by secondary ion mass spectrometry. The FGIs from Yamato-81020 consist of aggregates of hibonite, spinel, melilite, anorthite, diopside and olivine grains with no petrographic evidence of alteration. In contrast, the FGIs from Kainsaz commonly contain alteration products such as nepheline. From replacement textures and chemical compositions of altered and unaltered FGIs, we conclude that the alteration products formed by decomposition of melilite and anorthite. All phases in the Yamato-81020 FGIs are enriched in ¹⁶O, with δ^{17, 18}O = ∼−40‰ except for one FGI that experienced melting. Oxygen isotopic compositions of melilite, anorthite, some spinel and diopside in Kainsaz FGIs changed from δ^{17, 18}O = ∼−40‰ toward 0‰ by aqueous alteration. Alteration products in FGIs are depleted in ¹⁶O relative to primary phases, with δ^{17, 18}O = ∼0‰. These results show that FGIs in CO chondrites commonly had ¹⁶O-rich compositions in the solar nebula. The original ¹⁶O-rich FGIs were modified to ¹⁶O-poor compositions during aqueous alteration in the parent body.     

Oxygen isotopic evolution of amoeboid olivine aggregates in the reduced CV3 chondrites Efremovka, Vigarano, and Leoville

Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Vigarano, and Leoville consist of forsteritic olivine, FeNi-metal and a refractory component composed of spinel, Al-diopside, ±anorthite. Secondary ferrous olivine and alkali-rich minerals (nepheline and sodalite), commonly observed in the oxidized CVs, are rare. Mineralogy and chemical compositions of AOAs are similar to those predicted by equilibrium thermodynamic condensation models, suggesting that AOAs formed primarily by gas-solid condensation over a narrow temperature range, slightly below the temperatures over which most Ca-Al-rich inclusions (CAIs) formed. AOAs in the reduced CVs preserve a 1^st-generation ¹⁶O-rich signal (δ^17,18O ∼ −40‰) similar to that observed in many CAIs, suggesting that these refractory objects originated from a common source in the solar nebula. In fact AOAs and many fine-grained CAIs may have formed by the same processes, but at slightly different temperatures, and can be considered a single class of refractory objects.Alteration of the AOAs is manifested by differing extents of ¹⁶O-depletion in original AOA minerals, FeO-enrichment in olivine, and formation of interstitial very fine grained Na-bearing phases. From the six AOAs and one fine-grained, melilite-pyroxene-rich CAI examined in this study, five distinct patterns of alteration were identified. (1) One unaltered AOA from Vigarano is characterized by ¹⁶O-rich forsterite without FeO-rich rims and interstitial Na-bearing phases. (2) Weak alteration in the melilite-pyroxene-rich CAI is characterized by incomplete ¹⁶O-depletion in some melilite and precipitation of Na-bearing phases near the CAI rim. (3) Oxygen isotopic composition and mineralogy are correlated in two AOAs from Leoville with ¹⁶O-rich olivine, ¹⁶O-poor anorthite and a range of intermediate compositions in Al-diopside. This pattern is consistent with model diffusion between original grains and a ¹⁶O-poor reservoir during a relatively short-term (<60 yr), high-temperature (900-1100°C) event. (4) Original forsterite has been enriched in FeO, but remained ¹⁶O-rich in one AOA from Vigarano. This result is consistent with the slower rate of diffusion of O than Fe and Mg in olivine. At least some interstitial phases are ¹⁶O-rich, and Na-bearing phases are abundant in this AOA. (5) In contrast, oxygen isotopic composition and Fo-content are correlated in two AOAs from Efremovka. The olivine in these AOAs tends to have forsteritic ¹⁶O-rich cores and FeO-rich ¹⁶O-depleted rims. The general correlation between oxygen isotopic composition and Fo-content is difficult to model by diffusion, and may have formed instead by aqueous dissolution and precipitation along the margins of preexisting olivine grains.Independent evidence for aqueous alteration of the Efremovka AOAs is provided by OH-rich signals detected during ion beam sputtering of some of the ¹⁶O-poor olivine. Elevated ¹⁶OH-count rates and order of magnitude increases in ¹⁶OH detected during single analyses reflect trapping of an aqueous phase in ¹⁶O-depleted olivine. An elevated ¹⁶OH signal was also detected in one analysis of relatively ¹⁶O-poor melilite in the melilite-pyroxene CAI from Vigarano, suggesting that this object also was altered by aqueous fluid.     

Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CB_b chondrite Hammadah al Hamra 237 and none were observed in the CB_a chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa_<2), Fe, Ni-metal (5-12 wt% Ni), and Ca, Al-rich inclusions (CAIs) composed of Al-diopside, spinel, anorthite, and very rare melilite. Melilite is typically replaced by a fine-grained mixture of spinel, Al-diopside, and ±anorthite; spinel is replaced by anorthite. About 10% of AOAs contain low-Ca pyroxene replacing forsterite. Forsterite and spinel are always ¹⁶O-rich (δ^17,18O∼−40‰ to −50‰), whereas melilite, anorthite, and diopside could be either similarly ¹⁶O-rich or ¹⁶O-depleted to varying degrees; the latter is common in AOAs from altered and metamorphosed carbonaceous chondrites such as some CVs and COs. Low-Ca pyroxene is either ¹⁶O-rich (δ^17,18O∼−40‰) or ¹⁶O-poor (δ^17,18O∼0‰). Most AOAs in CV chondrites have unfractionated (∼2-10×CI) rare-earth element patterns. AOAs have similar textures, mineralogy and oxygen isotopic compositions to those of forsterite-rich accretionary rims surrounding different types of CAIs (compact and fluffy Type A, Type B, and fine-grained, spinel-rich) in CV and CR chondrites. AOAs in primitive carbonaceous chondrites show no evidence for alteration and thermal metamorphism. Secondary minerals in AOAs from CR, CM, and CO, and CV chondrites are similar to those in chondrules, CAIs, and matrices of their host meteorites and include phyllosilicates, magnetite, carbonates, nepheline, sodalite, grossular, wollastonite, hedenbergite, andradite, and ferrous olivine.Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in ¹⁶O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in ¹⁶O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with ¹⁶O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in ¹⁶O-poor gaseous reservoirs or coated by ¹⁶O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids.     

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.     

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.     

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.     

Oxygen isotopic SIMS analysis in Allende CAI: details of the very early thermal history of the solar system

The oxygen isotopic micro-distributions within and among minerals in a coarse-grained Ca, Al-rich inclusion (CAI), 7R-19-1 from the Allende meteorite, were measured by in situ using secondary ion mass spectrometry (SIMS). All values of O isotopic ratios in 7R-19-1 minerals fall along the carbonaceous chondrite anhydrous mineral mixing (CCAM) line on a δ¹⁷O_SMOW vs. δ¹⁸O_SMOW plot. Major refractory minerals (spinel, fassaite and melilite) in 7R-19-1 showed large negative anomalies of Δ¹⁷O in the order, spinel (−21‰) > ¹⁶O-rich melilite (∼−18‰) > fassaite (−15 to +1‰) > ¹⁶O-poor melilite (−8 to +2‰). However, the lower limit values of Δ¹⁷O are similar at about −21‰, a value commonly observed in CAIs. The similarity in the extreme values of the isotope anomaly anomalies suggests that crystallization of all CAIs started from an ¹⁶O enrichment of 21‰ (Δ¹⁷O) relative to terrestrial values. The order of the O isotopic anomalies observed for 7R-19-1, except for ¹⁶O-poor melilite, is parallel to the crystallization sequence determined by experiment from CAI liquid (Stolper, 1982), indicating that the O isotopic exchange in 7R-19-1 occurred between CAI melt and surrounding gas while 7R-19-1 was crystallizing from the ¹⁶O enriched CAI liquid (∼−21‰ in Δ¹⁷O) in the ¹⁶O-poor solar nebula. However, the a single crystallization sequence during the cooling stage cannot explain the existence of ¹⁶O-poor melilite. The presence of ¹⁶O-poor melilite suggests that multiple heating events occurred during CAI formation. The sharp contact between ¹⁶O-rich and ¹⁶O-poor melilite crystals and within ¹⁶O-rich melilite indicates that these multiple heatings occurred quickly. Based on the O isotopic and chemical compositions, fassaite crystals were aggregates of relic crystals formed from CAI melt whichthat have had various O isotopic compositions from the remelting processes. The results of intra-mineral distributions of O isotopes also support multiple heating events during CAI formation.     

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.     

Two oxygen isotopic components with extra-selenial origins observed among lunar metallic grains - In search for the solar wind component

Oxygen isotopic analyses were performed in the surface layers of lunar metallic grains from lunar regolith samples 71501 and 79035, presumably exposed at the Moon surface at different times. We were able to reproduce the two extreme O components previously found [Hashizume K. and Chaussidon M. (2005) A non-terrestrial ¹⁶O-rich isotopic composition for the protosolar nebula. Nature434, 619-622; Ireland T. R., Holden P., Norman M. D. and Clarke J. (2006) Isotopic enhancements of ¹⁷O and ¹⁸O from solar wind particles in the lunar regolith. Nature440, 776-778], with a range observed of −12 ± 5 < Δ¹⁷O < +33 ± 3‰ (1σ). The relatively minor ¹⁶O-rich component corresponding to an end-member Δ¹⁷O value lower than −20‰ is likely the solar component. This comes from the fact that its concentration roughly agrees with the maximum solar wind abundance expected among the grains from the two samples. At variance the ¹⁶O-poor component is 5-10 times more abundant and thus likely non-solar. The δ¹⁸O range found for the ¹⁶O-poor component may reflect various processes such as isotope exchange reaction during oxidation of metallic iron and/or isotope fractionation by evaporation/condensation at the surface of the Moon or during implantation at depth in the lunar metallic grains. The present study suggests that planetary solid materials in bulk are systematically depleted in ¹⁶O relative to the solar isotopic composition, suggesting the existence of non-mass-dependent isotopic fractionations associated to the formation of solids in the accretion disk.     

Correlations between oxygen-isotopic composition and petrologic setting in a coarse-grained Ca, Al-rich inclusion

Oxygen-isotopic compositions in conjunction with petrologic investigation have been determined for a coarse-grained type B2 Ca, Al-rich inclusion (CAI) from the reduced CV3 Vigarano using secondary ion mass spectrometry. The oxygen-isotopic compositions of minerals are distributed along the carbonaceous chondrite anhydrous mineral line indicating mixing between ¹⁶O-rich and ¹⁶O-poor nebular components. The O-isotopic heterogeneities among and within minerals in the CAI interior indicate that CAI formation started in an ¹⁶O-rich nebula and subsequently continued in an ¹⁶O-poor nebula. ¹⁶O-rich signatures of melilite and fassaite in the Wark-Lovering rim and of olivine of the accretionary rim indicate that the nebular environment during formation of the CAI returned to an ¹⁶O-rich composition after processing in an ¹⁶O-poor nebula. These O-isotopic variations in the CAI support multiple heating events in the solar nebula and indicate that the nebular environments fluctuated from ¹⁶O-rich to ¹⁶O-poor and back to ¹⁶O-rich compositions during the formation of a single CAI.     

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.     

Amoeboid olivine aggregates in the Allende meteorite

Greyish-brown, irregularly-shaped aggregates composed predominantly of olivine make up ~2% of the Allende meteorite by volume. Many of the aggregates are constructed of subspherical lumps of micron-sized crystals of olivine, pyroxene, nepheline and sodalite surrounded by coarsergrained olivine. Rarely, anorthite, spinel and perovskite are also present. The olivine ranges in composition from Fo64 to Fo99. Pyroxenes range from aluminous diopside to hedenbergite to very Al-rich and Ti-Al-rich varieties. The nepheline contains 1.6–2.4% K₂O and 1.6–5.2% CaO but the sodalite is significantly poorer in these elements. The spinel contains 2.1–13.4% FeO. Textural information and oxygen isotopic data suggest that the aggregates are composed of primary, solid condensates from the solar nebula. The perovskite. spinel and Ti-Al-rich pyroxenes are the remains of high-temperature condensates but the olivine compositions and the presence of feldspathoids indicate that some of the grains continued to react with the solar nebular vapor in the temperature range 500–900°K.     

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.     

Oxygen- and magnesium-isotope compositions of calcium-aluminum-rich inclusions from Rumuruti (R) chondrites

We report oxygen- and magnesium-isotope compositions of Ca,Al-rich inclusions (CAIs) from several Rumuruti (R) chondrites measured in situ using a Cameca ims-1280 ion microprobe. On a three-isotope oxygen diagram, δ¹⁷O vs. δ¹⁸O, compositions of individual minerals in most R CAIs analyzed fall along a slope-1 line. Based on the variations of Δ¹⁷O values (Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O) within individual inclusions, the R CAIs are divided into (i) ¹⁶O-rich (Δ¹⁷O ∼ −23-26‰), (ii) uniformly ¹⁶O-depleted (Δ¹⁷O ∼ −2‰), and (iii) isotopically heterogeneous (Δ¹⁷O ranges from −25‰ to +5‰). One of the hibonite-rich CAIs, H030/L, has an intermediate Δ¹⁷O value of −12‰ and a highly fractionated composition (δ¹⁸O ∼ +47‰). We infer that like most CAIs in other chondrite groups, the R CAIs formed in an ¹⁶O-rich gaseous reservoir. The uniformly ¹⁶O-depleted and isotopically heterogeneous CAIs subsequently experienced oxygen-isotope exchange during remelting in an ¹⁶O-depleted nebular gas, possibly during R chondrite chondrule formation, and/or during fluid-assisted thermal metamorphism on the R chondrite parent asteroid.Three hibonite-bearing CAIs and one spinel-plagioclase-rich inclusion were analyzed for magnesium-isotope compositions. The CAI with the highly fractionated oxygen isotopes, H030/L, shows a resolvable excess of ²⁶Mg (²⁶Mg^∗) corresponding to an initial ²⁶Al/²⁷Al ratio of ∼7 × 10⁻⁷. Three other CAIs show no resolvable excess of ²⁶Mg (²⁶Mg^∗). The absence of ²⁶Mg^∗ in the spinel-plagioclase-rich CAI from a metamorphosed R chondrite NWA 753 (R3.9) could have resulted from metamorphic resetting. Two other hibonite-bearing CAIs occur in the R chondrites (NWA 1476 and NWA 2446), which appear to have experienced only minor degrees of thermal metamorphism. These inclusions could have formed from precursors with lower than canonical ²⁶Al/²⁷Al ratio.     

Microscopic oxygen isotopic homogeneity/heterogeneity in the matrix of the Vigarano CV3 chondrite

Two-dimensional ¹⁸O/¹⁶O isotopic analysis of the Vigarano matrix was conducted by secondary ion-imaging using a novel two-dimensional ion-imager. Quantitative oxygen-isotope images (isotopographs) of the Vigarano matrix show that ¹⁶O-rich micrograins are scattered within ¹⁶O-poor matrix. This heterogeneous O-isotopic distribution indicates that matrix is composed of different O-isotopic components that formed in different locations and/or at different times. However, the O-isotopic composition of groundmass in the matrix is the same as the bulk isotopic composition of the matrix within ±5‰ uncertainty. The spatial resolution and isotopic precision of our technique should allow submicron-size objects (>0.2 μm) with extreme O-isotopic anomalous characteristics (δ¹⁸O_SMOW ∼250‰) to be detectable in isotopographs. Because the mean grain size of the matrix is ∼0.2 μm, the inability to detect such O-isotopic anomalous objects indicates that isotopically anomalous micrograins (e.g., presolar grains) are extremely rare in the Vigarano matrix and that most objects in the matrix were formed in the solar nebula or in the parent body.     

Oxygen- and magnesium-isotope compositions of calcium-aluminum-rich inclusions from CR2 carbonaceous chondrites

We report both oxygen- and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates - most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have ¹⁶O-rich (Δ¹⁷O = −23.3 ± 1.9‰, 2σ error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, in 15 of 16 ¹⁶O-rich CAIs measured are consistent with the canonical value of (4.5-5) × 10⁻⁵ and a short duration (<0.5 My) of CAI formation. These data do not support the “supra-canonical” values of (²⁶Al/²⁷Al)₀ [(5.85-7) × 10⁻⁵] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite-grossite-rich CAI El Djouf 001 MK #5 has uniformly ¹⁶O-rich (Δ¹⁷O = −23.0 ± 1.7‰) composition, but shows a deficit of ²⁶Mg and no evidence for ²⁶Al. Because this inclusion is ¹⁶O-rich, like CAIs with the canonical (²⁶Al/²⁷Al)₀, we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical ²⁶Al abundance. Another ¹⁶O-enriched (Δ¹⁷O = −20.3 ± 1.2‰) inclusion, a spinel-melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen- and magnesium-isotope compositions (∼11 and 23‰/amu, respectively), a deficit of ²⁶Mg, and a relatively low (²⁶Al/²⁷Al)₀ = (2.0 ± 1.7) × 10⁻⁵. This could be the first FUN (Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly ¹⁶O-depleted compared to most CR2 CAIs and has lower than the canonical (²⁶Al/²⁷Al)₀, it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over  My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (Δ¹⁷O ranges from −23.5‰ to −1.7‰), suggesting dilution by ¹⁶O-depleted chondrule material and possibly exchange with an ¹⁶O-poor (Δ¹⁷O > −5‰) nebular gas. The low inferred (²⁶Al/²⁷Al)₀ ratios of these CAIs (<0.7 × 10⁻⁵) indicate melting >2 My after crystallization of CAIs with the canonical (²⁶Al/²⁷Al)₀ and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale. Because CAIs in CR2 and CV chondrites appear to have originated in a similarly ¹⁶O-rich reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an ¹⁶O-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid-solid isotope exchange on the CV asteroidal body rather than gas-melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those in the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.     

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.     

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.