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• W1497200410 abstract "We review recent results on O- and Mg-isotope compositions of refractory grains (corundum, hibonite) and calcium, aluminum-rich inclusions (CAIs) from unequilibrated ordinary and carbonaceous chondrites. We show that these refractory objects originated in the presence of nebular gas enriched in 16O to varying degrees relative to the standard mean ocean water value: the Δ17OSMOW value ranges from approximately −16‰ to −35‰, and recorded heterogeneous distribution of 26Al in their formation region: the inferred (26Al/27Al)0 ranges from approximately 6.5 × 10−5 to <2 × 10−6. There is no correlation between O- and Mg-isotope compositions of the refractory objects: 26Al-rich and 26Al-poor refractory objects have similar O-isotope compositions. We suggest that 26Al was injected into the 26Al-poor collapsing protosolar molecular cloud core, possibly by a wind from a neighboring massive star, and was later homogenized in the protoplanetary disk by radial mixing, possibly at the canonical value of 26Al/27Al ratio (approximately 5 × 10−5). The 26Al-rich and 26Al-poor refractory grains and inclusions represent different generations of refractory objects, which formed prior to and during the injection and homogenization of 26Al. Thus, the duration of formation of refractory grains and CAIs cannot be inferred from their 26Al-26Mg systematics, and the canonical (26Al/27Al)0 does not represent the initial abundance of 26Al in the solar system; instead, it may or may not represent the average abundance of 26Al in the fully formed disk. The latter depends on the formation time of CAIs with the canonical 26Al/27Al ratio relative to the timing of complete delivery of stellar 26Al to the solar system, and the degree of its subsequent homogenization in the disk. The injection of material containing 26Al resulted in no observable changes in O-isotope composition of the solar system. Instead, the variations in O-isotope compositions between individual CAIs indicate that O-isotope composition of the CAI-forming region varied, because of coexisting of 16O-rich and 16O-poor nebular reservoirs (gaseous and/or solid) at the birth of the solar system, or because of rapid changes in the O-isotope compositions of these reservoirs with time, e.g., due to CO self-shielding in the disk. 26Al is a short-lived radionuclide that decays to 26Mg with a half-life of 0.705 ± 0.024 (1σ) Ma (Norris et al. 1983) and that was present in the early solar system (Lee et al. 1977). The short half-life of 26Al and its presence in the earliest solids formed in the inner solar system (CAIs, chondrules, chondrites, and achondrites) make it an important heating source of early accreted planetesimals, and, if 26Al was uniformly distributed in the inner solar system, for the chronology of CAI and chondrule formation, melting and differentiation of planetesimals (e.g., McKeegan and Davis 2003; Kita et al. 2005; Nyquist et al. 2009; Dauphas and Chaussidon 2011 and references therein). Both the origin (local, irradiation versus external, stellar) and distribution (homogeneous versus heterogeneous) of 26Al in the early solar system remain controversial (e.g., Lee et al. 1998; Sahijpal et al. 1998; Gounelle et al. 2001, 2006; Sahijpal and Soni 2006; Larsen et al. 2011; Makide et al. 2011; Liu et al. 2012). A local, solar energetic particle irradiation origin of 26Al received some support after discovery in CAIs of short-lived radionuclides, which could be produced only by irradiation—10Be (McKeegan et al. 2000), and, possibly, 7Be (Chaussidon et al. 2006). However, the difficulties in explaining the apparently uniform distribution of 26Al with the inferred initial 26Al/27Al ratio of approximately 5.2 × 10−5 (e.g., Jacobsen et al. 2008) and the lack of correlation between 26Al/27Al ratio and 10Be/9Be ratio in CAIs (Sugiura et al. 2001; MacPherson et al. 2003; Liu et al. 2010; Wielandt et al. 2012) make the irradiation model for the origin of 26Al doubtful. Recently, Duprat and Tatischeff (2007) showed that the maximum amount of irradiation-induced 26Al can only account for a homogeneous distribution of this radionuclide over a rocky reservoir of only 2–3 Earth’s masses (not over the minimum mass solar nebula of 0.01 solar mass, Weidenschilling [1977]) and that the well-defined canonical 26Al/27Al ratio observed in CAIs is probably incompatible with an in situ production in the embedded phase of the Sun. These observations are interpreted as evidence supporting an external, stellar origin of 26Al in the early solar system (e.g., Duprat and Tatischeff 2007; Gounelle and Meynet 2011). However, the stellar source of 26Al and the manner in which it was injected into the solar system remain controversial: it could have been produced by an asymptotic giant branch star (Wasserburg et al. 2006), a type II supernova (e.g., Meyer 2005), or by a wind from a massive star prior to its explosion as a type Ib/Ic supernova (Arnould et al. 2006; Gaidos et al. 2009; Tatischeff et al. 2010; Gounelle and Meynet 2011), and injected either into the protosolar molecular cloud (Gaidos et al. 2009; Vasileiadis et al. 2011), protosolar cloud core (Sahijpal et al. 1998; Boss et al. 2008), or protoplanetary disk (Ouellette et al. 2007). Constraining a stellar source of 26Al in the solar system is one of the fundamental questions in cosmochemistry, as it provides important information for understanding an astrophysical setting of the solar system formation. Calcium–aluminum-rich inclusions from the CV carbonaceous chondrites (CV CAIs) are the oldest solar system solids dated using U-Pb absolute chronology (Amelin et al. 2002; Bouvier and Wadhwa 2010; Bouvier et al. 2011a; Connelly et al. 2011) and, therefore, provide important constraints on the origin and distribution of 26Al in the early solar system. On the aluminum–magnesium isochron diagram (26Mg/24Mg versus 27Al/24Mg), the whole-rock magnesium-isotope compositions of CAIs from the oxidized CV chondrite Allende measured by multicollector inductively coupled plasma mass-spectrometry (MC-ICP-MS) yield a well-defined model isochron with 26Al/27Al ratio of (5.23 ± 0.13) × 10−5 (Jacobsen et al. 2008), named the “canonical” ratio. Similar initial 26Al/27Al ratio, (5.23 ± 0.02) × 10−5, was later inferred from the whole-rock aluminum–magnesium isotopic measurements of CAIs and amoeboid olivine aggregates (AOAs) from the reduced CV chondrite Efremovka (Larsen et al. 2011). The canonical 26Al/27Al ratio and the reported homogeneity of this ratio to approximately 2% in the whole-rock CV CAIs have been interpreted as evidence for (1) uniform distribution of 26Al in the solar system and (2) a very short (<0.04 or <0.002 Ma) duration of CAI formation, respectively (e.g., Jacobsen et al. 2008; Davis et al. 2010; Larsen et al. 2011). There are several caveats in this interpretation: (1) CAIs appear to have originated in a hot (>1300 K), localized region of the protoplanetary disk, probably near the protoSun (McKeegan et al. 1998; Fagan et al. 2000; Guan et al. 2000a; Ciesla 2009; Krot et al. 2009), which may have had different initial 26Al/27Al ratio than the solar system or even than the rest of the protoplanetary disk. (2) The duration of CAI formation is probably longer than 0.04 or 0.002 Ma as defined on the basis of the uncertainties of the model isochrons recorded by the whole-rock CV CAIs (Jacobsen et al. 2008; Larsen et al. 2011): (i) This whole-rock model isochron recorded aluminum–magnesium fractionation by evaporation–condensation processes prior to and during formation of the CV CAIs. In contrast, crystallization of igneous CAIs from CV chondrites (e.g., Compact type A, type B, and type C), resulted from melting of condensates or evaporative residues, is recorded by internal isochrons, which are based on the aluminum–magnesium isotopic measurements of individual co-crystallizing minerals in a CAI. The internal isochrons (if not disturbed) show a range of the inferred initial 26Al/27Al ratios from the canonical to approximately 4 × 10−5, indicating that melting of CAIs could have lasted approximately 0.3 Ma or even longer (MacPherson et al. 2010a, 2012; Kita et al. 2012). (ii) There are multiple generations of CAIs in different chondrite groups (e.g., Simon et al. 2002; Krot et al. 2008a), but their whole-rock magnesium-isotope compositions have not been measured yet. Calcium–aluminum-rich inclusions and AOAs are believed to have formed in a gas of approximately solar composition, consistent with their initial 16O-rich compositions (e.g., Itoh et al. 2004; Yurimoto et al. 2008; Makide et al. 2009a) similar to that of the solar wind returned by the GENESIS spacecraft (Δ17O = −28 ± 2‰, 2 standard errors; McKeegan et al. 2011; where Δ17O = δ17O − 0.52 × δ18O; δ17,18O = ((17,18O/16O)sample/(17,18O/16O)SMOW – 1) × 1000, and SMOW is standard mean ocean water), and with the high Ti3+/Ti4+ ratios in CAI pyroxenes and hibonite (Simon et al. 2007, 2011; Doyle et al. 2011). In contrast, most chondrules and matrix grains formed in dusty nebular regions with nonsolar composition (e.g., Alexander et al. 2008; Alexander and Cuzzi 2011) and in the presence of 16O-poor gas (Δ17O from approximately −5‰ to approximately +3‰) (e.g., Kita et al. 2005, 2010; Krot et al. 2006). The uranium–lead absolute chronology (Amelin et al. 2002; Amelin and Krot 2007; Bouvier and Wadhwa 2010; Bouvier et al. 2011a) and aluminum–magnesium relative chronology (Kita et al. 2005, 2012b, and references therein) of CAI and chondrule formation indicate an apparent age gap between CAIs and chondrules of approximately 1 Ma and prolonged, for approximately 2–3 Ma, duration of chondrule formation. The presence of relict CAIs inside chondrules (Krot et al. 2005a; Russell et al. 2005, and references therein) and the observed chondrule-like igneous rims around some CAIs (Krot et al. 2005b) support formation of some chondrules after CAIs. It has been previously shown that melting of CAIs during chondrule formation resulted in oxygen-isotope exchange between the CAIs and 16O-poor nebular gas and resetting of 26Al–26Mg systematics of the melted CAIs (Krot et al. 2005b, 2011; MacPherson et al. 2010b, 2012). Therefore, the combined oxygen and magnesium isotopic measurements and petrographic observations can be used for distinguishing CAIs remelted during chondrule formation from CAIs, which avoided such melting. Finally, the internal aluminum–magnesium isochrons of CAIs in metamorphosed CV chondrites are often disturbed and show a range of the inferred (26Al/27Al)0, from subcanonical to supracanonical values (e.g., Young et al. 2005). Among the CAI minerals, melilite and anorthite on one side and hibonite and Al,Ti-diopside on another appear to be the most and the least affected by the disturbance. Whether these variations are due to closed-system (diffusive exchange of magnesium among CAI minerals after decay of 26Al) or open-system behavior of magnesium isotopes remain an open question (e.g., Connolly et al. 2009, 2010, 2011; Simon and Young 2011), which is beyond the scope of this study (for details see Kita et al. 2012b). Here, we summarize recent results on the mineralogy, petrography, and combined oxygen- and magnesium-isotope measurements of refractory objects (isolated corundum and hibonite grains and CAIs) in unequilibrated ordinary (UOCs) and carbonaceous chondrites (CCs). Oxygen isotopes are used for distinguishing solar from presolar grains (the latter are characterized by isotopically anomalous compositions, e.g., Nittler 2003), as well as for identifying refractory grains and inclusions that experienced remelting in an 16O-poor nebular reservoir, possibly during chondrule formation, which could have re-set their 26Al-26Mg systematics. The 26Al/27Al ratios in most of the refractory objects discussed in the paper are inferred from internal isochrons. Model isochrons are used only for the refractory objects showing limited variations in 27Al/24Mg ratios (e.g., in platy hibonite crystals from CM chondrites). CAIs from metamorphosed chondrites often show disturbed 26Al-26Mg isochrons and are not discussed in the paper; FUN (fractionation and unidentified nucleosynthetic effects) CAIs, which are mostly found in CV chondrites, which experienced thermal metamorphism, are the only exception. In the Corundum Grains from Acid-Resistant Residues and Matrices of Unequilibrated Ordinary and Carbonaceous Chondrites and Corundum-Bearing Inclusions from Carbonaceous Chondrites sections of the paper, we review recent data on the mineralogy, oxygen- and 26Al-26Mg systematics of isolated corundum grains and corundum-bearing CAIs from UOCs and CCs. In the Refractory Inclusions in CM, CR, CO, CV, Rumuruti-Like, Ordinary, and Enstatite Chondrites section, the mineralogy, oxygen- and 26Al-26Mg systematics of less refractory CAIs from unequilibrated ordinary, enstatite, and carbonaceous chondrites are reviewed. In the CAIs in CB and CH Metal-Rich Carbonaceous Chondrites section, we discuss oxygen- and 26Al-26Mg systematics of CAIs from metal-rich (CB and CH) carbonaceous chondrites. The components in these meteorites appear to have experienced late-stage reprocessing in an impact-generated plume that could have reset oxygen- and 26Al-26Mg systematics of the CH and CB CAIs. In the FUN and FUN-like CAIs section, we summarize the existing oxygen- and magnesium-isotope data on FUN CAIs. The Refractory Inclusions in a Comet 81P/Wild 2 section describes 26Al-26Mg systematics of CAIs from a comet 81P/Wild 2. The results in the following sections, Corundum Grains From Acid-Resistant Residues and Matrices of Unequilibrated Ordinary and Carbonaceous Chondrites, Corundum-bearing Inclusions from Carbonaceous Chondrites, Refractory Inclusions in CM, CR, CO, CV, Rumuruti-like, Ordinary, and Enstatite Chondrites, Hibonite-Rich Inclusions in CM Chondrites, 26Al-Poor and 26Al-Rich CAIs in CO Chondrites, 26Al-Poor and 26Al-Rich CAIs in Acfer 094 (Ungrouped) Carbonaceous Chondrite, 26Al-Poor and 26Al-Rich CAIs in CR Chondrites, 26Al-Poor and 26Al-Rich Hibonite-Rich CAIs in Ningqiang, 26Al-Poor and 26Al-Rich CAIs in Enstatite Chondrites, 26Al-Poor and 26Al-Rich CAIs in Ordinary Chondrites, CAIs in CB and CH Metal-Rich Carbonaceous Chondrites, FUN and FUN-like CAIs, Refractory Inclusions in a Comet 81P/Wild 2 show that two major groups of refractory grains and inclusions occur in all chondrites studied—those with high inferred initial 26Al/27Al ratios, approximately (4–5) × 10−5, and those with low ratios, <5 × 10−6. Both groups have similar, 16O-rich compositions. We attribute the observed heterogeneity in distribution of 26Al/27Al among refractory grains and inclusions to 26Al heterogeneity in the solar system during an epoch of CAI formation. In the following sections, Summary of Observations, Variations in (26Al/27Al)0 Among the Earliest solar system Solids: Implication For Understanding Stellar Sources of 26Al, Viscous Disk Evolution and Redistribution of Refractory Grains and Inclusions, we discuss the implication of this conclusion for understanding stellar sources of 26Al in the early solar system, redistribution of refractory objects in the protoplanetary disk during its evolution, and abundance and distribution of 26Al in the solar system and in the fully formed protoplanetary disk. The Initial 26Al/27Al Ratio in the Fully Formed Protoplanetary Disk section contains the summary and conclusions. At total pressure <10−3 bar, corundum (Al2O3) is the first mineral predicted to condense from a gas of solar composition at approximately 1770 K; at higher total pressure, the first condensate is hibonite (CaAl12O19) (Fig. 1). Zi,Sc,Y,Hf-rich oxides have even higher condensation temperatures (Lodders 2003); however, because 26Al/27Al ratios in these oxides have not been measured yet, they are not discussed in the paper. At lower temperatures, corundum reacts with the nebular gas to form hibonite followed by perovskite (CaTiO3), grossite (CaAl4O7), and melilite (Ca2MgSi2O7–Ca2Al2SiO7 solid solution) (Yoneda and Grossman 1995; Ebel and Grossman 2000; Petaev and Wood 2005; Ebel 2006; Petaev 2009). Therefore, corundum and hibonite condensates can potentially provide important constraints on the origin of 26Al and degree of its heterogeneity in the early solar system. Note that the condensation temperatures of minerals and, therefore, the calculated condensation sequence, depend on the thermodynamic properties of minerals used in calculations (e.g., Petaev 2009). Equilibrium condensation curves and mineral phase stability fields in a gas of solar composition at different total pressure (from Petaev and Wood 2005). At total pressure <10−3 bar, corundum condenses first. At lower temperatures, it reacts with the nebular gas to form hibonite. The condensation sequence is reversed at total pressure >10−3 bar. en = enstatite; FeNi = Fe,Ni-metal; fo = forsterite; hib = hibonite; mel = melilite; per = perovskite; sp = spinel. The pioneering work on solar corundum grains was reported by Virag et al. (1991) who studied 26 individual 3–20 μm corundum grains from acid-resistant residues of the Murchison (CM) carbonaceous chondrite. On the basis of their oxygen- and magnesium-isotope compositions, the grains were divided into three groups: group 1 (n = 17) and group 2 (n = 5) grains show radiogenic 26Mg excesses (δ26Mg*) corresponding to (26Al/27Al)0 of 5 × 10−5 and 5 × 10−6, respectively. Group 3 grains (n = 4) show no resolvable δ26Mg* excesses; 2σ upper limit on (26Al/27Al)0 is <3 × 10−7. On a three-isotope oxygen diagram, δ17O versus δ18O, most corundum grains measured plot along approximately slope-1 line, called the carbonaceous chondrite anhydrous mineral (CCAM) line. All but one of the group 1 grains fall in the main cluster at δ17,18O ≈ −50‰, whereas four of five group 2 grains have highly fractionated 16O-rich compositions, resembling those of FUN inclusions (e.g., Lee et al. 1980); group 3 grains scatter widely (see Fig. 2 in Virag et al. 1991). Backscattered electron (BSE) images of corundum grains from (a) acid-resistant residue of Semarkona and (b, c) in CM matrices (from Makide et al. 2009a, forthcoming). Recently, Makide et al. (2009b, 2011) reported measurements of oxygen- and magnesium-isotope compositions of micron-sized corundum grains (Fig. 2a) from acid-resistant residues of UOCs (Semarkona, LL3.0, Bishunpur, LL3.1, and Roosevelt County 075, H3.1) and unmetamorphosed CCs (Orgueil, CI1, Murray, CM2, Renazzo, CR2, and Allan Hills [ALH] A77307, CO3.0) by secondary ion mass-spectrometry (SIMS or ion microprobe). The results of these studies are summarized in 3-5. The vast majority of the corundum grains measured (n = 108) have 16O-rich compositions (Δ17Oavr ≈ −23 ± 7‰, 2σ), which, within uncertainty of the measurements, are similar to the composition of the solar wind (McKeegan et al. 2011). Three corundum grains have isotopically anomalous oxygen-isotope compositions and are probably presolar in origin. In addition, Krot et al. (2012a) reported in situ oxygen-isotope measurements of several corundum grains in matrices of CM chondrites. The matrix corundum grains are typically euhedral and found as either isolated grains or their aggregates (Figs. 2b and 2c). Oxygen-isotope compositions of the matrix corundum grains are similar to those of corundum grains from acid-resistant residues of UOCs and CCs (Fig. 3b). These observations are generally consistent with a condensation origin of the corundum grains from a gas of solar composition, but do not preclude an evaporation origin for at least some of the grains measured by Makide et al. (2009b): Formation of corundum grains by evaporation of dust or melted dust is expected to result in mass-dependent fractionation of oxygen isotopes. No significant mass-dependent fractionation of oxygen isotopes was found for matrix corundum grains measured in situ by Krot et al. (2012a). However, oxygen-isotope data of isolated, unpolished corundum grains from acid-resistant residues measured by Makide et al. (2009b) are difficult to correct for instrumental mass fractionation, and, therefore, their formation by evaporation of 16O-rich precursors cannot be excluded. (a) Three-isotope oxygen diagram of the corundum-bearing CAIs and matrix corundum grains (data from Fahey et al. 1987; Simon et al. 2002; Liu et al. 2009; Makide et al. forthcoming). (b) Δ17O values of the corundum-bearing CAIs, matrix corundum grains, GENESIS solar wind (data from McKeegan et al. 2011), and corundum grains from acid-resistant residues of UOCs and CCs (data from Makide et al. 2011). Error bars for CAIs and corundum grains are 2 standard deviations; error bars for solar wind are 2 standard errors). There are no differences in O-isotope compositions between 26Al-rich and 26Al-poor corundum-bearing objects. a) Δ17O values versus initial 26Al/27Al ratios in corundum grains from acid-resistant residues of UOCs and CCs. The canonical 26Al/27Al ratio and oxygen-isotope composition of the solar wind are from Jacobsen et al. (2008) and McKeegan et al. (2011), respectively. b) Al–Mg evolution diagram of the corundum grains #02-01 and #01-10 from the CI carbonaceous chondrite Orgueil (data from Makide et al. 2011). Red and blue symbols represent individual Al–Mg isotope measurements of the grains. Consecutive measurements of a corundum grain show decrease in 27Al/24Mg ratio and δ26Mg* due to decrease of its size by sputtering and increase of contamination by adhering grains (typically spinel) having low 27Al/24Mg ratio and δ26Mg*. Probability density plots of (26Al/27Al)0 in solar corundum grains from acid-resistant residues of UOCs (a), CCs (b), UOCs + CCs (c) (data from Makide et al. 2011) and corundum-bearing CAIs from CCs (d) (data from Bar-Matthews et al. 1982; Hinton and Bischoff 1984; Fahey et al. 1987b; Simon et al. 2002; Sugiura and Krot 2007; Liu et al. 2009; and Makide et al. forthcoming). Magnesium-isotope compositions of the solar corundum grains (only grains from acid-resistant residues have been measured so far) indicate large variations of the inferred (26Al/27Al)0 (4, 5): 52% of grains have no resolvable δ26Mg* excesses: 2σ upper limit on (26Al/27Al)0 is approximately 2 × 10−6; 40% of grains have high (26Al/27Al)0 ranging from approximately 3.0 × 10−5 to approximately 6.5 × 10−5; 8% of grains have intermediate values of (26Al/27Al)0, (1–2) × 10−5 (Makide et al. 2011). There is no difference in distribution of (26Al/27Al)0 in corundum grains from UOCs and CCs (3, 5). There is also no correlation between oxygen- and magnesium-isotope compositions of the corundum grains (3, 4). The coexistence of the 26Al-rich and 26Al-poor corundum grains having similar 16O-rich compositions in the same primitive meteorite precludes late-stage, i.e., after decay of 26Al, resetting of 26Al-26Mg systematics of the 26Al-poor corundum grains during thermal metamorphism on a host chondrite parent body. Makide et al. (2011) concluded that 16O-rich corundum grains with low (26Al/27Al)0 never contained the canonical abundance of 26Al, i.e., the lack of δ26Mg* excess in about half of 16O-rich corundum grains measured is a primary characteristic of either their formation region (if they are condensates) and/or their precursors (if they are evaporation residues). Corundum-bearing CAIs are very rare objects: only sixteen inclusions of this kind have been described so far; all are in carbonaceous chondrites—Murchison (CM), Murray (CM), Acfer 094 (ungrouped), and Adelaide (ungrouped) (Bar-Matthews et al. 1982; Hinton and Bischoff 1984; MacPherson et al. 1984; Fahey et al. 1987; Hinton et al. 1988; Simon et al. 2002; Sugiura and Krot 2007; Liu et al. 2009; Krot et al. 2012). The corundum-bearing CAIs typically consist of corundum, hibonite, and perovskite; other minerals, including grossite, Ca-monoaluminate (CaAl2O4), spinel, refractory metal nuggets, and Zr,Y,Sc-rich oxides are minor or accessory (Fig. 6). Because all corundum-bearing CAIs are found in meteorite finds or aqueously altered CM chondrites, some of the primary minerals, e.g., melilite, could have been replaced by products of terrestrial weathering or by phyllosilicates. BSE images (a, d, e) and combined elemental maps in Mg (red), Ca (green), and Al Kα (blue) x-rays (b, d, f) of the corundum-bearing CAIs 1344-50 from Adelaide (ungrouped), 7-9-1 and 3-9-1 from Murchison (CM). Textural relationships between hibonite and corundum in 3-9-1 (hibonite is surrounded by corundum) are inconsistent with an equilibrium condensation sequence from a gas of solar composition at total pressure <10−3 bar, suggesting either evaporation origin of corundum or condensation origin at higher total pressure. The inferred initial 26Al/27Al ratios are indicated. Both 26Al-poor CAIs are surrounded by the Wark-Lovering rims. Abbreviations: carb = carbonate; CMA = Ca-monoaluminate (CaAl2O4); cor = corundum; grs = grossite; hib = hibonite; pv = perovskite; px = Al-diopside; sp = spinel; w = weathering products (from Makide et al. forthcoming). Twelve of thirteen corundum-bearing CAIs measured for oxygen isotopes are uniformly 16O-rich with an average Δ17O value of −23 ± 5‰ (2σ), similar in composition to corundum grains from acid-resistant residues of UOCs and CCs (Δ17Oavr ≈ −23 ± 7‰, 2σ) (Makide et al. 2011) and to the solar wind (McKeegan et al. 2011) (Fig. 2). There is, however, a range of Δ17O values (from −17 ± 3‰ to −27 ± 3‰, 2σ) between individual corundum-bearing CAIs (Fig. 3b). Based on these observations, as well as the mineralogy and petrology of the corundum-bearing CAIs, it is inferred that most of them formed in an 16O-rich reservoir of approximately solar composition and avoided subsequent thermal processing in the 16O-poor gaseous reservoir. The observed range in Δ17O values of the corundum-bearing CAIs suggests that oxygen-isotope composition of the CAI-forming region was variable, which may reflect the coexistence of 16O-rich and 16O-poor nebular reservoirs (gaseous and/or solid) at the birth of the solar system (e.g., Krot et al. 2002a, 2010a; Yurimoto and Kuramoto 2004) or rapid evolution of oxygen-isotope compositions of these reservoirs with time, e.g., due to CO self-shielding in the protoplanetary disk (e.g., Lyons and Young 2005). One of the corundum-bearing CAIs, 1344-50 from Adelaide (Figs. 6a and 6b), is isotopically heterogeneous: grossite and Ca-monoaluminate in its mantle are 16O-depleted (Δ17O ≈ −2‰) compared with the uniformly 16O-rich corundum-hibonite core (Δ17O ≈ −21‰) and the 16O-rich Wark-Lovering rim layers of spinel and Al-diopside. The observed oxygen-isotope heterogeneity may reflect fluctuations in oxygen-isotope composition of the nebular gas during condensation of the CAI and/or partial melting of the originally uniformly 16O-rich CAI in 16O-depleted nebular gas as commonly inferred for coarse-grained, igneous CAIs in CV chondrites (e.g., Yurimoto et al. 1998; Aléon et al. 2005; Yurimoto et al. [2008] and references therein; Simon et al. 2010; Yurimoto and Nagashima 2011). Alternatively, this heterogeneity could have resulted from gas-solid exchange postdating crystallization of the CAI and formation of the Wark-Lovering rim sequence around it (e.g., Dyl et al. 2008). The lack of knowledge of oxygen-isotope self-diffusion in Al2O3, CaAl12O19, CaAl2O4, and CaAl4O7 does not allow distinguishing between these mechanisms. Twelve corundum-bearing CAIs measured for magnesium-isotope composition (Bar-Matthews et al. 1982; Hinton and Bischoff 1984; Fahey et al. 1987; Hinton et al. 1988; Simon et al. 2002; Sugiura and Krot 2007; Liu et al. 2009; Krot et al. 2012a) show a large spread of the inferred (26Al/27Al)0 (5, 7). Seven corundum-bearing CAIs (BB5, GR-1, M98-8, and P10 from CM chondrites, 1344-50 and 1312-32 from Adelaide, and s2 from Acfer 094) show no resolvable δ26Mg* excesses (Fig. 7a). The Murchison CAI 7-9-1 (Figs. 6c and 6d) shows small δ26Mg* excess corresponding to (26Al/27Al)0 of (4.0 ± 2.0) × 10−6 (Fig. 7b). Four corundum-bearing CAIs from CM chondrites (F5, 1769-9-1, 3-9-1, and UH68-1) have large δ26Mg* excesses corresponding to (26Al/27Al)0 of (4.1 ± 0.2) × 10−5, (4.4 ± 0.2) × 10−5, (4.1 ± 0.3) × 10−5, and (3.9 ± 0.4) × 10−5, respectively (Fig. 7c) (Fahey et al. 1987b; Krot et al. 2012a). These values are lower than the canonical ratio of (5.23 ± 0.13) × 10−5 (Jacobsen et al. 2008). Aluminum–magnesium evolution diagrams of the corundum-bearing CAIs 1344-50 from Adelaide (ungrouped), 7-9-1 and 3-9-1 from Murchison (CM) (see Fig. 5). Data from Makide et al. (forthcoming). A bi-modal distribution of (26Al/27Al)0 in the corundum-bearing CAIs is similar to those in solar corundum grains from acid-resistant residues of UOCs and CCs (Figs. 5c and 5d), suggesting that some of these grains could have resulted from disintegrated corundum-bearing CAIs. Based on the mineralogy and petrography, hibonite-rich inclusions in CM chondrites are divided into several groups: spinel-hibonite CAIs (SHIBs), platy hibonite crystals (PLACs), blue aggregates of hibonite crystals (BAGs), and pyroxene-hibonite spherules (Fig. 8). All groups of hibonite-rich inclusions have 16O-rich compositions: Δ17O value ranges from −14‰ ± 3‰ to −27 ± 3‰ (2σ) (Fig. 9). PLACs and pyroxene-hibonite spherules typically show isotopic anomalies in 48Ca and 50Ti, and lack resolvable δ26Mg* excesses (many show δ26Mg* deficits); 2σ upper limit on (26Al/27Al)0 is approximately 5 × 10−6 (Fahey et al. 1987a; Ireland 1988; Ireland et al. 1991; Liu et al. 2009, 2012) (Fig. 10). SHIBs typically show no isotopic anomalies in 48Ca and 50Ti and have resolvable δ26Mg* excesses corresponding to the inferred (26Al/27Al)0 ranging from (5.5 ± 2.4) × 10−6 to (7.4 ± 1.2) × 10−5 (Fahey et al. 1987a; Ireland 1988; Ireland et al. 1991). BSE images (a, b, c, e) and combined elemental maps in Ti (red), Ca (green), and Al Kα (blue) x-r" @default.
• W1497200410 created "2016-06-24" @default.
• W1497200410 creator A5000247939 @default.
• W1497200410 creator A5017077950 @default.
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• W1497200410 creator A5084966635 @default.
• W1497200410 date "2012-11-26" @default.
• W1497200410 modified "2023-10-13" @default.
• W1497200410 title "Heterogeneous distribution of²⁶Al at the birth of the solar system: Evidence from refractory grains and inclusions" @default.
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