Carbonaceous Chondrite Anhydrous Mineral Line Research Articles

Infiltration metasomatism of the Allende coarse-grained calcium-aluminum-rich inclusions

We report on the mineralogy, petrography, and O and Al-Mg isotopic systematics of secondary mineralization in the metasomatically altered igneous Ca,Al-rich inclusions (CAIs) [compact type A (CTA), B1, B2, forsterite-bearing B (FoB), and C] from the CV3 carbonaceous chondrite Allende. This alteration affected mainly melilite, and to a lesser degree anorthite, and resulted in the formation of a variety of secondary minerals, including adrianite, Al-diopside, andradite, anorthite, calcite, celsian, clintonite, corundum, dmisteinbergite, ferroan olivine, ferroan monticellite, ferroan Al-diopside, forsterite, grossular, heazlewoodite, hedenbergite, hutcheonite, kushiroite, margarite, monticellite, Na-melilite, nepheline, pentlandite, pyrrhotite, sodalite, spinel, tilleyite, wadalite, and wollastonite. The secondary mineral assemblages are mainly defined by chemical compositions of the primary melilite replaced and elements introduced by an aqueous fluid. Gehlenitic melilite (Åk<35) in CTAs and mantles of B1s is mainly replaced by anorthite + grossular; clintonite, corundum, spinel, and Al-diopside are minor. Åkermanitic melilite (Åk35-90) in type B2s, FoBs, and cores of B1s is replaced by the grossular + monticellite + wollastonite, grossular + monticellite, and grossular + Al-diopside assemblages; forsterite, spinel, clintonite, and Na-melilite are minor. In type Cs, lacy melilite (åkermanitic melilite with rounded inclusions of anorthite) is pseudomorphically replaced by the grossular + forsterite + monticellite and grossular + Al-diopside assemblages; Na-melilite is minor. Primary and secondary anorthites in the peripheral portions of CAIs are replaced by nepheline, sodalite, and ferromagnesian olivine. Some CAIs contain voids and cracks filled by andradite, hedenbergite, wollastonite, ±sodalite, ±grossular, ±monticellite, ±tilleyite, and ±calcite. All CAIs studied are surrounded by Wark-Lovering rims, fine-grained matrix-like rims composed of lath-shaped ferroan olivine and abundant nepheline grains, and a layer of salite-hedenbergite pyroxenes + andradite + wollastonite. Grossular associating with monticellite, Al-diopside, and forsterite and replacing åkermanitic melilite (27Al/24Mg ~ 2) has high 27Al/24Mg ratios (30−100) and shows no resolvable excess of radiogenic 26Mg (26Mg*). The 27Al/24Mg ratios (7−10) and 26Mg* (2−3‰) in the nearly monomineralic grossular veins crosscutting gehlenitic melilite are similar to those of the host melilite and plot along a regression line with 26Al/27Al ratio of ~5×10−5. Oxygen isotopic compositions of secondary minerals in the most Type Bs measured in situ with the UH Cameca ims-1280 and matrix-matched standards plot along mass-dependent fractionation line with ∆17O of ~ −3±2‰ with δ18O ranging from ~0 to ~10‰. Primary melilite and anorthite in the host CAIs are similarly 16O-depleted, whereas spinel, forsterite, and most Al,Ti-diopside grains have 16O-rich compositions (∆17O ~ −25±2‰). Secondary grossular and forsterite in type Cs and type B1 CAI TS-34 show a range of ∆17O, from ~ −15 to ~ −1‰; the 16O-enriched compositions of grossular and forsterite plot along the carbonaceous chondrite anhydrous mineral line. The similar ranges of ∆17O and positions on the three-isotope oxygen diagram are observed for primary anorthite; melilite is generally 16O-depleted compared to anorthite (∆17O ~ −5 to −1±2‰); spinel and fassaite are 16O-rich (except very Ti-rich fassaite in TS-34 and CTA CAIs). We conclude that Allende CAIs experienced an open-system in situ metasomatic alteration at relatively high temperatures (200-250 °C) in the presence of CO2- and H2O-bearing fluid with ∆17O of ~ −3±2‰ followed by thermal metamorphism at ~ 500 °C on the CV chondrite parent asteroid. During the alteration, most elements were mobile: Si, Na, Cl, K, Fe, S, and Ni were introduced; Al, Ti, Mg, and Ba were locally mobilized; Ca and some Mg and Al were lost from the host inclusions. The alteration occurred after nearly complete decay of 26Al, >3 Ma after crystallization of CAIs with the canonical (26Al/27Al)0 of (5.25±0.02)×10-5; 26Mg* in grossular was inherited from the primary melilite and provide no chronological significance. Oxygen isotopic heterogeneity of primary minerals in the Allende CAIs at least partly is due to isotopic exchange with an aqueous fluid that largely affected melilite, anorthite, perovskite, Zr- and Sc-rich oxides and silicates, and possibly very Ti-rich fassaite.

Oxygen Isotopic Compositions in a Plagioclase‐Olivine Inclusion from Ningqiang Similar to Those in Al‐rich Chondrules

AbstractWe report the petrology and oxygen isotopic composition, using a Cameca NanoSIMS 50L ion microprobe, of a plagioclase–olivine inclusion, C#1, found in the Ningqiang carbonaceous chondrite. In addition to major phases (plagioclase, spinel and olivine), C#1 is also surrounded by a pyroxene rim (64 vol % Ca‐rich and 36 vol % Ca‐poor pyroxenes). On a three‐isotope oxygen diagram, δ17O vs. δ18O, the compositions of individual minerals analyzed in C#1 fall along the carbonaceous chondrite anhydrous mineral line (CCAM), and oxygen isotopic compositions in C#1 show significant variability in δ18O and δ17O. The oxygen isotopic compositions of the pyroxene rim minerals are similar to those of the other host minerals, which suggests that the rim likely formed from the same melting process as the host. The rim is considered to have formed as a result of interaction between an 16O‐poor gas and a melt. Some spinel grains are typically 16O‐rich and likely of relict origin, which is similar to 16O‐rich Ca‐, Al‐rich inclusions, which are probably a precursor of C#1. The inclusion then likely melted in an 16O‐poor region where chondrules form, accompanied by oxygen isotope exchange with an 16O‐poor gas. Some anorthite, pyroxene and spinel might have undergone fluid‐assisted thermal metamorphism on the Ningqiang chondrite parent body. The oxygen isotope data and evolution of the C#1 plagioclase–olivine inclusion are similar with those of Al‐chondrules in chondrites.

Oxygen and Al‐Mg isotopic constraints on cooling rate and age of partial melting of an Allende Type B CAI, Golfball

AbstractCoarse‐grained, igneous Ca‐Al‐rich inclusions (CAIs) in CV chondrites formed through multiple melting events. We conducted in situ O‐isotope analysis and Al‐Mg systematics by secondary ion mass spectrometry of relict and overgrown minerals from a partial melting event in an Allende Type B CAI, Golfball. Golfball has a Type B CAI bulk composition and a unique structure: a fassaite‐rich mantle enclosing a melilite‐rich core. Many of the blocky melilite crystals in the core have irregularly shaped, Al‐rich (Åk5–15) cores enclosed in strongly zoned (Åk30–70) overgrowths. Since the Al‐rich melilite grains could not have formed from a melt of Golfball, they are interpreted as relict grains that survived later melting events. The O‐isotopic compositions of the blocky melilite crystals plot along the carbonaceous chondrite anhydrous mineral line, ranging between Δ17O ~ −14‰ and −5‰. The Al‐rich relict melilite grains and their overgrowths exhibit the same O‐isotopic compositions, while the O‐isotopic compositions are varied spatially among melilites. We found that the O‐isotopic compositions steeply change across several melilite crystals within few tens of micrometers, indicating the O‐isotopic compositions of the melt could not have been homogenized during the partial melting in that scale. According to the time scale of O self‐diffusivity in the melt, the cooling rate of the partial melting event is calculated to be &gt;6 × 104 K h−1. Al‐Mg isotope data for core minerals plot on a straight line on an Al‐Mg evolution diagram. A mineral isochron for Golfball gives initial 26Al/27Al of (4.42 ± 0.20) × 10–5 and initial δ26Mg* of −0.035 ± 0.050‰. The chemical and O‐isotopic compositions of melilite and those initial values imply that its precursor consisted of fluffy Type A and/or fine‐grained CAIs. The partial melting event for Golfball may have occurred in very short order after the precursor formation.

Origin of minerals in åkermanite-rich patch texture and oxygen isotopic evolution of compact Type A Ca-Al-rich inclusions from the Northwest Africa 7865 CV chondrite

We report the in-situ oxygen isotopic distributions corresponding to the petrographic-mineralogical observation on a compact Type A (CTA) Ca-Al-rich inclusion (CAI), KU-N-02, from a reduced CV3 chondrite, Northwest Africa 7865. The CTA has an igneous texture and mainly consists of spinel, melilite, and Al-Ti-rich clinopyroxene (fassaite). Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are poikilitically enclosed in the melilite and fassaite and are uniformly 16O-rich (Δ17O = approximately − 23‰). The fassaite is texturally classified into two types: blocky fassaite and intergranular fassaite. The blocky fassaite crystals exhibit growth zoning as they change from Ti-rich to Ti-poor along the inferred directions of crystal growth from core to rim, while the oxygen isotopic compositions change from 16O-poor (Δ17O = approximately − 6‰) to 16O-rich (Δ17O = approximately − 23‰) with crystal growth. The intergranular fassaite crystals exist between the melilite crystals and exhibit variable Ti abundance and oxygen isotopic compositions. Additionally, their relationships between Ti contents and oxygen isotopic composition are similar to those of the blocky fassaite. The melilite grains are homogeneously 16O-poor (Δ17O = approximately − 2‰), irrespective of their åkermanite (Åk) content. Each melilite grain generally exhibits growth zoning with increasing Åk contents from core to rim, although the melilite contains Åk-rich patches within single crystal. Åk-rich patches often include two types of fassaite: small blebby crystals attached to spinel crystals and round crystals. The oxygen isotopic compositions of the Åk-rich patch and blebby fassaite are 16O-poor (Δ17O = approximately − 2‰), similar to that of the host melilite. On the other hand, the round fassaite exhibits significant variation in oxygen isotopic compositions ranging from Δ17O = −23‰ to − 4‰, which are different from those of the host melilite. These petrographic textures and oxygen isotopic variations indicate the presence of a solid precursor with variable oxygen isotopic compositions for the CTA. The spinel and round fassaite grains are relicts of the precursor that melted in the 16O-poor nebular gas, resulting in the crystallization of the host melilite from the 16O-poor melt. The Åk-rich patches and blebby fassaite crystallized from melts trapped by the growing host melilite crystals. The blocky and intergranular fassaite crystallized after the melilite did, and the oxygen isotopic composition of the melt changed to 16O-rich during the crystallization process, suggesting that the oxygen isotopic composition of the surrounding nebular gas could be varied. The inferred oxygen isotopic evolution for CTA is consistent with those inferred for Type B CAIs, suggesting that coarse-grained igneous CAIs formed in a similar nebular environment regardless of precursor chemistry.

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

AbstractPrecise 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 &gt;150 °C and &lt;600 °C.

Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite

TS34 is a Type B1 Ca-Al-rich inclusion (CAI) from the Allende CV3 chondrite, consisting of spinel, melilite, Ti-Al-rich clinopyroxene (fassaite) and minor anorthite in an igneous texture. Oxygen and magnesium isotopic compositions were measured by secondary ion mass spectrometry in spots of known chemical composition in all major minerals in TS34. Using the sequence of formation from dynamic crystallization experiments and from chemical compositions of melilite and fassaite, the oxygen isotopic evolution of the CAI melt was established. Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are uniformly 16O-rich (Δ17O=−22.7±1.7‰, 2SD), while the melilite grains are uniformly 16O-poor (Δ17O=−2.8±1.8‰) irrespective of their åkermanite content and thus their relative time of crystallization. The fassaite crystals exhibit growth zoning overprinting poorly-developed sector zoning; they generally grow from Ti-rich to Ti-poor compositions. The fassaite crystals also show continuous variations in Δ17O along the inferred directions of crystal growth, from 16O-poor (Δ17O∼−3‰) to 16O-rich (Δ17O∼−23‰), covering the full range of oxygen isotopic compositions observed in TS34. The early-crystallized 16O-poor fassaite and the melilite are in oxygen isotope equilibrium and formed simultaneously. The correlation of oxygen isotopic compositions with Ti content in the fassaite imply that the oxygen isotopic composition of the CAI melt evolved from 16O-poor to 16O-rich during fassaite crystallization, presumably due to oxygen isotope exchange with a surrounding 16O-rich nebular gas. Formation of spinel, the liquidus phase in melts of this composition, predates crystallization of all other phases, so its 16O-rich composition is a relic of an earlier stage. Anorthite exhibits oxygen isotopic compositions between Δ17O∼−2‰ and −9‰, within the range of those of fassaite, indicating co-crystallization of these two minerals during the earliest to intermediate stage of fassaite growth. The melilite and fassaite yield an 26Al–26Mg mineral isochron with an initial value of (26Al/27Al)0=(5.003±0.075)×10−5, corresponding to a relative age of 0.05±0.02Myr from the canonical Al–Mg age of CAIs. These data demonstrate that both 16O-rich and 16O-poor reservoirs existed in the solar nebula at least ∼0.05Myr after the birth of the Solar System.

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 (Fs0.6–8.8Wo0.7–9.3) has much higher Al2O3 and TiO2 contents (up to 12.5 and 2.3 wt%, respectively) than that in ferromagnesian chondrules. High-Ca pyroxene (Fs0.3–2.0Wo33–54) contains less Al2O3 and TiO2 than that in Ca,Al-rich inclusions (CAIs). Plagioclase (An77–99Ab1–23) 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.

Oxygen isotope and chemical compositions of magnetite and olivine in the anomalous CK3 Watson 002 and ungrouped Asuka‐881595 carbonaceous chondrites: Effects of parent body metamorphism

AbstractWe report in situ O isotope and chemical compositions of magnetite and olivine in chondrules of the carbonaceous chondrites Watson‐002 (anomalous CK3) and Asuka (A)‐881595 (ungrouped C3). Magnetite in Watson‐002 occurs as inclusion‐free subhedral grains and rounded inclusion‐bearing porous grains replacing Fe,Ni‐metal. In A‐881595, magnetite is almost entirely inclusion‐free and coexists with Ni‐rich sulfide and less abundant Ni‐poor metal. Oxygen isotope compositions of chondrule olivine in both meteorites plot along carbonaceous chondrite anhydrous mineral (CCAM) line with a slope of approximately 1 and show a range of Δ17O values (from approximately −3 to −6‰). One chondrule from each sample was found to contain O isotopically heterogeneous olivine, probably relict grains. Oxygen isotope compositions of magnetite in A‐881595 plot along a mass‐dependent fractionation line with a slope of 0.5 and show a range of Δ17O values from −2.4‰ to −1.1‰. Oxygen isotope compositions of magnetite in Watson‐002 cluster near the CCAM line and a Δ17O value of −4.0‰ to −2.9‰. These observations indicate that magnetite and chondrule olivine are in O isotope disequilibrium, and, therefore, not cogenetic. We infer that magnetite in CK chondrites formed by the oxidation of pre‐existing metal grains by an aqueous fluid during parent body alteration, in agreement with previous studies. The differences in Δ17O values of magnetite between Watson‐002 and A‐881595 can be attributed to their different thermal histories: the former experienced a higher degree of thermal metamorphism that led to the O isotope exchange between magnetite and adjacent silicates.

Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca‐Al‐rich inclusion of Efremovka CV3 chondrite

Abstract– Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca‐Al‐rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core‐mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous 16O‐poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ18O = 5–10‰, which suggests that the domain was formed in a 16O‐poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from 16O‐rich (δ18O = −40‰) to 16O‐poor (δ18O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more 16O‐rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI.

Heavily metamorphosed clasts from the CV chondrite breccias Mokoia and Yamato‐86009

Abstract–  Metamorphosed clasts in the CV carbonaceous chondrite breccias Mokoia and Yamato‐86009 (Y‐86009) are coarse‐grained, granular, polymineralic rocks composed of Ca‐bearing (up to 0.6 wt% CaO) ferroan olivine (Fa34–39), ferroan Al‐diopside (Fs9–13Wo47–50, approximately 2–7 wt% Al2O3), plagioclase (An37–84Ab63–17), Cr‐spinel (Cr/(Cr + Al) =  0.19–0.45, Fe/(Fe + Mg) = 0.60–0.79), nepheline, pyrrhotite, pentlandite, Ca‐phosphate, and rare grains of Ni‐rich taenite; low‐Ca pyroxene is absent. Most clasts have triple junctions between silicate grains, indicative of prolonged thermal annealing. Based on the olivine‐spinel and pyroxene thermometry, the estimated metamorphic temperature recorded by the clasts is approximately 1100 K. Few clasts experienced thermal metamorphism to a lower degree and preserved chondrule‐like textures. The Mokoia and Y‐86009 clasts are mineralogically unique and different from metamorphosed chondrites of known groups (H, L, LL, R, EH, EL, CO, CK) and primitive achondrites (acapulcoites, brachinites, lodranites). On a three‐isotope oxygen diagram, compositions of olivine in the clasts plot along carbonaceous chondrite anhydrous mineral line and the Allende mass‐fractionation line, and overlap with those of the CV chondrule olivines; the Δ17O values of the clasts range from about −4.3‰ to −3.0‰. We suggest that the clasts represent fragments of the CV‐like material that experienced metasomatic alteration, high‐temperature metamorphism, and possibly melting in the interior of the CV parent asteroid. The lack of low‐Ca pyroxene in the clasts could be due to its replacement by ferroan olivine during iron‐alkali metasomatic alteration or by high‐Ca ferroan pyroxene during melting under oxidizing conditions.

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 Δ 17O values among olivine, pyroxene, and spinel within a single chondrule. The average Δ 17O 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 Δ 17O values between −3‰ and −2‰, and 2 other FeO-poor barred olivine chondrules show average Δ 17O values of −3.6‰ and 0‰. These results are similar to those for Acfer 094 chondrules, showing bimodal Δ 17O values at −5‰ and −2‰. Nine porphyritic chondrules contain olivine grains with heterogeneous Δ 17O 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 Δ 17O 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 δ 17O = (0.982 ± 0.019) × δ 18O − (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 Δ 17O values of −2.6‰, which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the CV parent body.

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 (δ 17O = 0.95 * δ 18O − 3.24; R 2 = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo 99.6–99.8; δ 18O = −7.2‰ to −5.5‰; δ 17O = −9.6‰ to −8.2‰) are more 16O-rich than the isolated olivine grains (Fo 39.6–86.8; δ 18O = 3.1‰ to 5.1‰; δ 17O = −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 (δ 18O = −5.2‰ to 5.9‰; δ 17O = −8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ 17O 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, R 2 = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM slope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with 16O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.

Oxygen 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 Δ 17O; the interchondrule variations in Δ 17O 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 Δ 17O 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 (Δ 17O = −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 (Δ 17O = −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.

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.

Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

Secondary ion mass spectrometer (SIMS) oxygen isotope analyses were performed on 24 clasts, representing 9 clast types, in the Dar al Gani (DaG) 319 polymict ureilite with precisions better than 1‰. Olivine-rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Other igneous clasts, including plagioclase-bearing clasts, also plot along the CCAM line, indicating that they were derived from the ureilite parent body (UPB). Thus, we suggest that some of the plagioclase-bearing clasts in the polymict ureilites represent the “missing basaltic component” produced by partial melting on the UPB. Trace element concentrations (Mg, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr and Ba) in ureilitic plagioclase and glass from 13 clasts were obtained by using the SIMS high mass resolution method. The trace element contents of the plagioclase generally show monotonic variations with anorthite content (mol%) that are consistent with partial melting and fractional crystallization. Incompatible trace element concentrations (K, Ti, and Ba) are low and variable for plagioclase with An > 40, indicating that the parental magmas for some clasts were derived from a depleted source. We performed partial melt modeling for CI and CM precursor compositions and compared the results to the observed trace element (K, Ba, and Sr) abundances in the plagioclase. Our results indicate that (1) the UPB evolved from a alkali-rich carbonaceous chondritic precursor, (2) parent melts of porphyritic clasts could have formed by 5–20% equilibrium partial melting and subsequent fractional crystallization, and (3) parent melts of the incompatible trace element-depleted clasts could be derived from fractional melting, where low degree (<10%) partial melts were repeatedly extracted from their solid sources. Thus, both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in DaG-319 suggest that the UPB underwent localized low degree-partial melting events. The partial melts could have been repeatedly extracted from the precursor, resulting in the formation of the olivine-pigeonite monomict ureilites as the final residue.

UV laser ablation and irm-GCMS microanalysis of 18O/ 16O and 17O/ 16O with application to a calcium-aluminium-rich inclusion from the Allende meteorite

Analyses of 18O/ 16O and 17O/ 16O in silicate and oxide minerals by UV laser ablation of 100 × 80 × 50 μm sample pits combined with irm-GCMS yield precision and accuracy similar to that of conventional methods. This represents a 100-fold reduction in minimum size relative to other fluorination methods based on gas-source mass spectrometry and enables high-precision in-situ intracrystalline analysis of silicate minerals. Analyses of almandine, forsterite, and schorl of known isotopic compositions indicate an analytical precision of ±0.3‰ (1σ) in δ 18O and ±0.4 in δ 17O with an accuracy of similar magnitude. Application to meteoritic samples is demonstrated by in-situ analysis of pyroxene and melilite from a type B CAI inclusion from the Allende meteorite. The CAI data adhere to the carbonaceous chondrite anhydrous mineral line defined by conventional macroscopic fluorination methods and demonstrate that non-mass dependent differences of 1‰ amu −1 are discernible. The unique combination of analytical and spatial resolution afforded by the new UV laser microprobe will allow high-precision mapping of the distribution of anomalous oxygen in minerals from calcium-aluminum-rich inclusions on a previously unattainable scale.