Oxygen isotopic and chemical composition of chromites in micrometeorites: Evidence of ordinary chondrite precursors

We identified 66 chromite grains from 42 of ~5000 micrometeorites collected from Indian Ocean deep‐sea sediments and the South Pole water well. To determine the chromite grains precursors and their contribution to the micrometeorite flux, we combined quantitative electron microprobe analyses and oxygen isotopic analyses by high‐resolution secondary ion mass spectrometry. Micrometeorite chromite grains show variable O isotopic compositions with δ¹⁸O values ranging from ?0.8 to 6.0‰, δ¹⁷O values from 0.3 to 3.6‰, and Δ¹⁷O values from ?0.9 to 1.6‰, most of them being similar to those of chromites from ordinary chondrites. The oxygen isotopic compositions of olivine, considered as a proxy of chromite in chromite‐bearing micrometeorites where chromite is too small to be measured in ion microprobe have Δ¹⁷O values suggesting a principal relationship to ordinary chondrites with some having carbonaceous chondrite precursors. Furthermore, the chemical compositions of chromites in micrometeorites are close to those reported for ordinary chondrite chromites, but some contribution from carbonaceous chondrites cannot be ruled out. Consequently, carbonaceous chondrites cannot be a major contributor of chromite‐bearing micrometeorites. Based on their oxygen isotopic and elemental compositions, we thus conclude with no ambiguity that chromite‐bearing micrometeorites are largely related to fragments of ordinary chondrites with a small fraction from carbonaceous chondrites, unlike other micrometeorites deriving largely from carbonaceous chondrites.     

Triple oxygen isotope composition of Australasian tektites

Major and trace element analyses and triple oxygen isotope measurements were performed on 11 individual specimens of Australasian tektites (AAT) with exactly known field positions from Laos. The sample set was dominated by Muong Nong‐type tektites (MNAAT), including separated layers of glass of different appearance and chemistry from four samples. This first larger set of oxygen isotope data of MNAAT revealed the δ¹⁸O range 8.7 ≤ δ¹⁸O ≤ 11.6‰ on VSMOW2 scale (12 analyses), only slightly wider than the previously reported range for splash‐form AAT. The Δ’¹⁷O values of MNAAT (?0.098 ≤ Δ’¹⁷O ≤ ?0.069‰; 12 analyses) and splash‐form AAT (?0.080 ≤ Δ’¹⁷O ≤ ?0.068‰; three analyses) are all in the range of data typical for terrestrial crustal rocks, with no mass‐independent oxygen isotope fractionation (from impactor or from exchange with atmospheric O₂) being observed.     

Oxygen isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia

Abstract— We report in situ measurements of O‐isotopic compositions of magnetite and primary and secondary olivine in the highly unequilibrated oxidized CV chondrites Kaba and Mokoia. In both meteorites, the magnetite and the secondary olivine (fayalite, Fa_90–100) have O‐isotopic compositions near the terrestrial fractionation (TF) line; the mean Δ¹⁷O (= δ¹⁷O‐0.52 × δ¹⁸O) value is about ?1%‰. In contrast, the compositions of nearby primary (chondrule), low‐FeO olivines (Fa_1–2) are well below the TF line; Δ¹⁷O values range from ?3 to ?9%‰. Krot et al. (1998) summarized evidence indicating that the secondary phases in these chondrites formed by aqueous alteration in an asteroidal setting. The compositions of magnetite and fayalite in Kaba and Mokoia imply that the O‐isotopic composition of the oxidant was near or somewhat above the TF line. In Mokoia the fayalite and magnetite differ in δ¹⁸O by ～20%‰, whereas these same materials in Kaba have virtually identical compositions. The difference between Mokoia magnetite and fayalite may indicate formation in isotopic equilibrium in a water‐rich environment at low temperatures, ～300 K. In contrast, the similar compositions of these phases in Kaba may indicate formation of the fayalite by replacement of preexisting magnetite in dry environment, with the O coming entirely from the precursor magnetite and silica. The Δ¹⁷O of the oxidant incorporated into the CV parent body (as phyllosilicates or H₂O) appears to have been much (7–8%‰) lower than that in that incorporated into the LL parent body (Choi et al, 1998), which suggests that the O‐isotopic composition of the nebular gas was spatially or temporally variable.     

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.     

The Cu isotopic composition of iron meteorites

Abstract– High‐precision Cu isotopic compositions have been measured for the metal phase of 29 iron meteorites from various groups and for four terrestrial standards. The data are reported as the δ⁶⁵Cu permil deviation of the ⁶⁵Cu/⁶³Cu ratio relative to the NIST SRM 976 standard. Terrestrial mantle rocks have a very narrow range of variations and scatter around zero. In contrast, iron meteorites show δ⁶⁵Cu approximately 2.3‰ variations. Different groups of iron meteorites have distinct δ⁶⁵Cu values. Nonmagmatic IAB‐IIICD iron meteorites have similar δ⁶⁵Cu (0.03 ± 0.08 and 0.12 ± 0.10, respectively), close to terrestrial values (approximately 0). The other group of nonmagmatic irons, IIE, is isotopically distinct (?0.69 ± 0.15). IVB is the iron meteorite group with the strongest elemental depletion in Cu and samples in this group are enriched in the lighter isotope (δ⁶⁵Cu down to ?2.26‰). Evaporation should have produced an enrichment in ⁶⁵Cu over ⁶³Cu (δ⁶⁵Cu >0) and can therefore be ruled out as a mechanism for volatile loss in IVB meteorites. In silicate‐bearing iron meteorites, Δ¹⁷O correlates with δ⁶⁵Cu. This correlation between nonmass‐dependent and mass‐dependent parameters suggests that the Cu isotopic composition of iron meteorites has not been modified by planetary differentiation to a large extent. Therefore, Cu isotopic ratios can be used to confirm genetic links. Cu isotopes thus confirm genetic relationships between groups of iron meteorites (e.g., IAB and IIICD; IIIE and IIIAB); and between iron meteorites and chondrites (e.g., IIE and H chondrites). Several genetic connections between iron meteorites groups are confirmed by Cu isotopes, (e.g., IAB and IIICD; IIIE and IIIAB); and between iron meteorites and chondrites (e.g., IIE and H 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.     

Refractory inclusions in carbonaceous chondrites: Records of early solar system processes

Chondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [CAIs] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAIs in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk (PPD) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAIs formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAIs show large variations in the initial ²⁶Al/²⁷Al ratios, from <5 × 10^–6 to ~5.25 × 10^–5. These variations and the inferred low initial abundance of ⁶⁰Fe in the PPD suggest late injection of ²⁶Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the ²⁶Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using ²⁶Al‐²⁶Mg systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAIs surrounded by Wark–Lovering rims have uniform Δ¹⁷O (= δ¹⁷O?0.52 × δ¹⁸O) of ~ ?24‰; however, there is a large range of Δ¹⁷O (from ~?40 to ~ ?5‰) among them, suggesting the coexistence of ¹⁶O‐rich (low Δ¹⁷O) and ¹⁶O‐poor (high Δ¹⁷O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ¹⁷O of CAIs may be explained if three major O‐bearing species in the solar system (CO, H₂O, and silicate dust) had different O‐isotope compositions, with H₂O and possibly silicate dust being ¹⁶O‐depleted relative to both the Genesis solar wind Δ¹⁷O of ?28.4 ± 3.6‰ and even more ¹⁶O‐enriched CO. Oxygen isotopic compositions of CO and H₂O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of ¹⁶O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially ¹⁶O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD. To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD, samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO3 and CV3 chondrites, most CAIs exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are ¹⁶O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAIs with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with ¹⁶O‐poor (Δ¹⁷O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO3.1 and CV3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAIs. In some coarse‐grained igneous CV CAIs, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAIs could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.     

The abundance and isotopic composition of water in eucrites

Volatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of eucrites. Here, the measurements of water (reported as equivalent H₂O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic eucrites and one cumulate eucrite are reported. Apatite H₂O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of ?34 ± 67‰. No systematic variations or correlations are observed in H₂O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for eucrites and confirm the striking homogeneity in the H‐isotopic composition of water in eucrites, which is consistent with a common source for water in the inner solar system.     

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.     

Evidence in CO3.0 chondrules for a drift in the O isotopic composition of the solar nebula

Abstract— Several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher Δ¹⁷O (=δ¹⁷O ? 0.52 × δ¹⁸O) values than those recorded in unaltered chondrules. Other recent studies have shown that, in sets of chondrules from carbonaceous chondrites, Δ¹⁷O tends to increase as the FeO contents of the silicates increase. We report a comparison of the O isotopic composition of olivine phenocrysts in low‐FeO (≤Fa₁) type I and high‐FeO (≥Fa₁₅) type II porphyritic chondrules in the highly primitive CO3.0 chondrite Yamato‐81020. In agreement with a similar study of chondrules in CO3.0 ALH A77307 by Jones et al. (2000), Δ¹⁷O tends to increase with increasing FeO. We find that Δ¹⁷O values are resolved (but only marginally) between the two sets of olivine phenocrysts. In two of the high‐FeO chondrules, the difference between Δ¹⁷O of the late‐formed, high‐FeO phenocryst olivine and those in the low‐FeO cores of relict grains is well‐resolved (although one of the relicts is interpreted to be a partly melted amoeboid olivine inclusion by Yurimoto and Wasson [2002]). It appears that, during much of the chondrule‐forming period, there was a small upward drift in the Δ¹⁷O of nebular solids and that relict cores preserve the record of a different (and earlier) nebular environment.     

δ18O and chemical composition of Libyan Desert Glass,country rocks,and sands: New considerations on target material

Oxygen isotope and chemical measurements were carried out on 25 samples of Libyan Desert Glass (LDG), 21 samples of sandstone, and 3 of sand from the same area. The δ¹⁸O of LDG samples range from 9.0‰ to 11.9‰ (Vienna Standard Mean Ocean Water [VSMOW]); some correlations between isotope data and typological features of the LDG samples are pointed out. The initial δ¹⁸O of a bulk parent material may be slightly increased by fusion due to the loss of isotopically light pore water with no isotope exchange with oxygen containing minerals. Accordingly, the δ¹⁸O of the bulk parent material of LDG may have been about 9.0 ± 1‰ (VSMOW). The measured bulk sandstone and sand samples have δ¹⁸O values ranging from 12.6‰ to 19.5‰ and are consequently ruled out as parent materials, matching the results of previous studies. However, separated quartz fractions have δ¹⁸O values compatible with the LDG values suggesting that the modern surface sand inherited quartz from the target material. This hypothesis fits previous findings of lechatelierite and baddeleyite in these materials. As the age of the parent material reported in previous studies is Pan‐African, we measured the δ¹⁸O values of bulk rock and quartz from intrusives of Pan‐African age and the results obtained were compatible with the LDG values. The main element abundances (Fe, Mg, Ca, K, Na) in our LDG samples conform to previous estimates; Fe, Mg, and K tend to be higher in heterogeneous samples with dark layers. The hypothesis of a low‐altitude airburst involving silica‐rich surface materials deriving from weathered intrusives of Pan‐African age, partially melted and blown over a huge surface by supersonic winds matches the results obtained.     

Isotopic composition of carbonates in the SNC meteorites Allan Hills 84001 and Nakhla

Abstract— We have measured the ¹³C/¹²C and ¹⁴C/¹²C ratios in CO₂ released by acid etching of the carbonate-bearing SNC meteorites Allan Hills 84001 and Nakhla. Most of the C released is strongly enriched in ¹³C. In 10 out of 12 samples, 15‰ <δ¹³C < 55‰. Terrestrial values of carbonateδ¹³C from weathering products are generally between ?10 and +10‰. Two leachate samples especially rich in ¹³C, ALH 84001,27 and Nakhla 25, have elemental Si/Mg ratios much lower than those of the bulk meteorites and ¹⁴C activities that are much lower than the values expected for terrestrial carbonates. The former observation indicates that these leachates consist primarily of carbonates and, less likely, phosphates. The latter observation implies that heavy C was introduced not by terrestrial weathering but by extraterrestrial processes. For ALH 84001,121 (sample 27) and Nakhla (BM 1913,26) δ¹³C = +41‰ and +35‰, respectively. The measured ¹⁸O/¹⁶O ratios in the leaches are similar: δ¹⁸O ～ 15 ± 5‰, contrasting with 4.2‰ in the bulk silicates. We infer that the C in the carbonates retains an extraterrestrial isotopic signature, but probably not O, due to its ease of isotopic exchange (Cole and Ohmoto, 1986).     

The oxygen isotope compositions of olivine in main group (MG) pallasites: New measurements by adopting an improved laser fluorination approach

Oxygen isotope measurements of olivine in main group (MG) pallasites by traditional laser fluorination method are associated with some uncertainties including terrestrial weathering, incomplete olivine reaction, and sample state. We improved our laser fluorination approach by pretreating olivine grains with acid to remove terrestrial weathering products and by modifying the sample holder for an efficient and complete laser reaction. Our experiments on Brahin olivine demonstrate that acid-washing successfully removes the terrestrial weathering with <0.1‰ variation in δ¹⁸O value and, at the same time, improving the ∆¹⁷O value significantly. We also achieved a complete olivine fluorination by employing a custom-designed sample holder with “V”-shaped profile having rounded bottom because incomplete/partial reaction of olivine gives comparatively lighter δ¹⁸O values. Using these new techniques, we present precise triple oxygen isotope data (N = 72) of 25 olivine samples separated from main group pallasites. The data are, on average, ~0.5‰ heavier in δ¹⁸O relative to the values published in the literature for the same samples. Critically, the ∆¹⁷O values of MG pallasites and to some extent their Fo-contents suggest that there are at least two populations of olivine. Based on our improved data set, we propose that MG pallasites potentially have high-∆¹⁷O- and low-∆¹⁷O-bearing subgroups that are statistically distinct. The subgroups present average ∆¹⁷O values of −0.166 ± 0.003 (2SE; N = 16) and −0.220 ± 0.003 (2SE; N = 9), respectively. Furthermore, the high-∆¹⁷O-bearing subgroup samples trend toward lower Fo-contents compared to the other subgroup. Taken together, our data provide evidence that argues against a single parent body origin for MG pallasites.     

Oxygen isotopic composition of the solar nebula gas inferred from high‐precision isotope imaging of melilite crystals in an Allende CAI

Abstract– High‐precision isotope imaging analyses of reversely zoned melilite crystals in the gehlenitic mantle of Type A CAI ON01 of the Allende carbonaceous chondrite reveal that there are four types of oxygen isotopic distributions within melilite single crystals: (1) uniform depletion of ¹⁶O (δ¹⁸O ≈ ?10‰), (2) uniform enrichment of ¹⁶O (δ¹⁸O ≈ ?40‰), (3) variations in isotopic composition from ¹⁶O‐poor core to ¹⁶O‐rich rim (δ¹⁸O ≈ ?10‰ to ?30‰, ?20‰ to ?45‰, and ?10‰ to ?35‰) with decreasing åkermanite content, and (4) ¹⁶O‐poor composition (δ¹⁸O ≥ ?10‰) along the crystal rim. Hibonite, spinel, and perovskite grains are ¹⁶O‐rich (δ¹⁸O ≈ ?45‰), and adjoin ¹⁶O‐poor melilites. Gas‐solid or gas‐melt isotope exchange in the nebula is inconsistent with both the distinct oxygen isotopic compositions among the minerals and the reverse zoning of melilite. Fluid‐rock interaction on the parent body resulted in ¹⁶O‐poor compositions of limited areas near holes, cracks, or secondary phases, such as anorthite or grossular. We conclude that reversely zoned melilites mostly preserve the primary oxygen isotopic composition of either ¹⁶O‐enriched or ¹⁶O‐depleted gas from which they were condensed. The correlation between oxygen isotopic composition and åkermanite content may indicate that oxygen isotopes of the solar nebula gas changed from ¹⁶O‐poor to ¹⁶O‐rich during melilite crystal growth. We suggest that the radial excursions of the inner edge of the protoplanetary disk gas simultaneously resulted in both the reverse zoning and oxygen isotopic variation of melilite, due to mixing of ¹⁶O‐poor disk gas and ¹⁶O‐rich coronal gas. Gas condensates aggregated to form the gehlenite mantle of the Type A CAI ON01.     

Carbonate abundances and isotopic compositions in chondrites

We report the bulk C abundances, and C and O isotopic compositions of carbonates in 64 CM chondrites, 14 CR chondrites, 2 CI chondrites, LEW 85332 (C2), Kaba (CV3), and Semarkona (LL3.0). For the unheated CMs, the total ranges of carbonate isotopic compositions are δ¹³C ≈ 25–75‰ and δ¹⁸O ≈ 15–35‰, and bulk carbonate C contents range from 0.03 to 0.60 wt%. There is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. Unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the CM parent body, probably prior to or at the start of aqueous alteration. Most of the ranges in CM carbonate isotopic compositions can be explained by their formation at different temperatures (0–130 °C) from a single fluid in which the carbonate O isotopes were controlled by equilibrium with water (δ¹⁸O ≈ 5‰) and the C isotopes were controlled by equilibrium with CO and/or CH₄ (δ¹³C ≈ ?33‰ or ?20‰ for CO‐ or CH₄‐dominated systems, respectively). However, carbonate formation would have to have been inefficient, otherwise carbonate compositions would have resembled those of the starting fluid. A quite similar fluid composition (δ¹⁸O ≈ ?5.5‰, and δ¹³C ≈ ?31‰ or ?17‰ for CO‐ or CH₄‐dominated systems, respectively) can explain the carbonate compositions of the CIs, although the formation temperatures would have been lower (~10–40 °C) and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the CMs. The CR carbonates exhibit a similar range of O isotopes, but an almost bimodal distribution of C isotopes between more (δ¹³C ≈ 65–80‰) and less altered samples (δ¹³C ≈ 30–40‰). This bimodality can still be explained by precipitation from fluids with the same isotopic composition (δ¹⁸O ≈ ?9.25‰, and δ¹³C ≈ ?21‰ or ?8‰ for CO‐ or CH₄‐dominated systems, respectively) if the less altered CRs had higher mole fractions of CO₂ in their fluids. Semarkona and Kaba carbonates have some of the lightest C isotopic compositions of the meteorites studied here, probably because they formed at higher temperatures and/or from more CO₂‐rich fluids. The fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices. By analogy with cometary ices, CO₂ and/or CO would have dominated the trapped volatile species in the ices. The chondrites studied are too oxidized for CO‐dominated fluids to have formed in their parent bodies. If CH₄ was the dominant C species in the fluids during carbonate formation, it would have to have been generated in the parent bodies from CO and/or CO₂ when oxidation of metal by water created high partial pressures of H₂. The fact that the chondrite carbonate C/H₂O mole ratios are of the order predicted for CO/CO₂‐H₂O ices that experienced temperatures of >50–100 K suggests that the chondrites formed at radial distances of <4–15 AU.     

New triple oxygen isotope data of bulk and separated fractions from SNC meteorites: Evidence for mantle homogeneity of Mars

We report precise triple oxygen isotope data of bulk materials and separated fractions of several Shergotty–Nakhla–Chassigny (SNC) meteorites using enhanced laser‐assisted fluorination technique. This study shows that SNCs have remarkably identical Δ¹⁷O and a narrow range in δ¹⁸O values suggesting that these meteorites have assimilated negligibly small surface materials (<5%), which is undetectable in the oxygen isotope compositions reported here. Also, fractionation factors in coexisting silicate mineral pairs (px‐ol and mask‐ol) further demonstrate isotopic equilibrium at magmatic temperatures. We present a mass‐dependent fractionation line for bulk materials with a slope of 0.526 ± 0.016 (1SE) comparable to the slope obtained in an earlier study (0.526 ± 0.013; Franchi et al. 1999). We also present a new Martian fractionation line for SNCs constructed from separated fractions (i.e., pyroxene, olivine, and maskelynite) with a slope of 0.532 ± 0.009 (1SE). The identical fractionation lines run above and parallel to our terrestrial fractionation line with Δ¹⁷O = 0.318 ± 0.016‰ (SD) for bulk materials and 0.316 ± 0.009‰ (SD) for separated fractions. The conformity in slopes and Δ¹⁷O between bulk materials and separated fractions confirm oxygen isotope homogeneity in the Martian mantle though recent studies suggest that the Martian lithosphere may potentially have multiple oxygen isotope reservoirs.     

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.     

Maribo—A new CM fall from Denmark

Abstract– Maribo is a new Danish CM chondrite, which fell on January 17, 2009, at 19:08:28 CET. The fall was observed by many eye witnesses and recorded by a surveillance camera, an all sky camera, a few seismic stations, and by meteor radar observatories in Germany. A single fragment of Maribo with a dry weight of 25.8 g was found on March 4, 2009. The coarse‐grained components in Maribo include chondrules, fine‐grained olivine aggregates, large isolated lithic clasts, metals, and mineral fragments (often olivine), and rare Ca,Al‐rich inclusions. The components are typically rimmed by fine‐grained dust mantles. The matrix includes abundant dust rimmed fragments of tochilinite with a layered, fishbone‐like texture, tochilinite–cronstedtite intergrowths, sulfides, metals, and carbonates often intergrown with tochilinite. The oxygen isotopic composition: (δ¹⁷O = ?1.27‰; δ¹⁸O = 4.96‰; Δ¹⁷O = ?3.85‰) plots at the edge of the CM field, close to the CCAM line. The very low Δ¹⁷O and the presence of unaltered components suggest that Maribo is among the least altered CM chondrites. The bulk chemistry of Maribo is typical of CM chondrites. Trapped noble gases are similar in abundance and isotopic composition to other CM chondrites, stepwise heating data indicating the presence of gas components hosted by presolar diamond and silicon carbide. The organics in Maribo include components also seen in Murchison as well as nitrogen‐rich components unique to Maribo.     

Evidence for high‐temperature fractionation of lithium isotopes during differentiation of the Moon

Lithium isotope and abundance data are reported for Apollo 15 and 17 mare basalts and the LaPaz low‐Ti mare basalt meteorites, along with lithium isotope data for carbonaceous, ordinary, and enstatite chondrites, and chondrules from the Allende CV3 meteorite. Apollo 15 low‐Ti mare basalts have lower Li contents and lower δ⁷Li (3.8 ± 1.2‰; all uncertainties are 2 standard deviations) than Apollo 17 high‐Ti mare basalts (δ⁷Li = 5.2 ± 1.2‰), with evolved LaPaz mare basalts having high Li contents, but similar low δ⁷Li (3.7 ± 0.5‰) to Apollo 15 mare basalts. In low‐Ti mare basalt 15555, the highest concentrations of Li occur in late‐stage tridymite (>20 ppm) and plagioclase (11 ± 3 ppm), with olivine (6.1 ± 3.8 ppm), pyroxene (4.2 ± 1.6 ppm), and ilmenite (0.8 ± 0.7 ppm) having lower Li concentrations. Values of δ⁷Li in low‐ and high‐Ti mare basalt sources broadly correlate negatively with ¹⁸O/¹⁶O and positively with ⁵⁶Fe/⁵⁴Fe (low‐Ti: δ⁷Li ≤4‰; δ⁵⁶Fe ≤0.04‰; δ¹⁸O ≥5.7‰; high‐Ti: δ⁷Li >6‰; δ⁵⁶Fe >0.18‰; δ¹⁸O <5.4‰). Lithium does not appear to have acted as a volatile element during planetary formation, with subequal Li contents in mare basalts compared with terrestrial, martian, or vestan basaltic rocks. Observed Li isotopic fractionations in mare basalts can potentially be explained through large‐degree, high‐temperature igneous differentiation of their source regions. Progressive magma ocean crystallization led to enrichment in Li and δ⁷Li in late‐stage liquids, probably as a consequence of preferential retention of ⁷Li and Li in the melt relative to crystallizing solids. Lithium isotopic fractionation has not been observed during extensive differentiation in terrestrial magmatic systems and may only be recognizable during extensive planetary magmatic differentiation under volatile‐poor conditions, as expected for the lunar magma ocean. Our new analyses of chondrites show that they have δ⁷Li ranging between ?2.5‰ and 4‰. The higher δ⁷Li in planetary basalts than in the compilation of chondrites (2.1 ± 1.3‰) demonstrates that differentiated planetary basalts are, on average, isotopically heavier than most chondrites.     

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.