Ca-Al-rich Inclusions Research Articles

Atomic-scale structure and non-stoichiometry of meteoritic hibonite: A transmission electron microscope study

Abstract Hibonite (CaAl12O19) is a common refractory mineral in Ca-Al-rich inclusions (CAIs) in primitive meteorites. Transmission electron microscope (TEM) studies have identified enigmatic planar defects in different occurrences of hibonite in the Allende meteorite that give rise to strong streaking along c* in electron diffraction patterns. Atomic resolution high-angle annular dark-field (HAADF) imaging and energy-dispersive X-ray (EDX) analyses were used to determine the nature and origin of these planar features. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing, and less commonly 2.0 and 2.5 nm spacings, interspersed in stoichiometric hibonite showing 1.1 nm (002) spacing. Stoichiometric hibonite consists of alternating Ca-containing (“R”) and spinel-structured (“S”) blocks stacked in a sequence RS. In contrast, the 1.6 nm layers result from a doubled S block such that the stacking sequence is RSS, while in the widest defect observed, the stacking sequence is RSSSS. These intergrowths are epitaxial and have coherent, low-strain boundaries with the host hibonite Meteoritic hibonite shows common Ti and Mg substitution for Al in its structure. Atomic-resolution EDX maps of hibonite grains in the Allende CAI confirm the preferred site occupancy of Mg on tetragonal M3 sites in S blocks and of Ti on trigonal bipyramidal M2 and octahedral M4 sites in R blocks. Mg is highly concentrated, but Ti is absent in the planar defects where wider S blocks show Al-rich compositions compared to stoichiometric MgAl2O4 spinel. Therefore, Mg likely played the major role in the formation and metastability of planar defects in hibonite. Electron energy loss spectroscopy data from the Ti L2,3 edge show the presence of mixed Ti oxidation states with ~15–20% of Ti as Ti3+ in hibonite, suggesting a direct substitution of Ti3+ ↔ Al3+ in hibonite. The remaining ~80–85% of Ti is present as Ti4+ and corresponding EDX analyses are consistent with the well-known coupled substitution 2Al3+ ↔ Ti4+ + Mg2+ being the major mechanism for Ti and Mg substitution in hibonite. The formation of planar defects in hibonite occurred during high-temperature nebular condensation or melting/crystallization processes. The occurrence of non-stoichiometric hibonite in the Allende CAI deviates from the mineral formation sequence predicted from equilibrium condensation models. Overall, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal growth during the high-temperature formation of refractory solids in the early solar nebula.

Evidence for protosolar graphene in Allende and QUE 94366 CV3 meteorites

Nanoscale graphene morphologies are reported in the Allende and QUE 94366 CV3-type carbonaceous chondrites via Confocal Raman Imaging Spectroscopy. These morphologies are found embedded in the refractory calcium-aluminum-rich inclusion (CAI) rims in Allende and within a chondrule inclusion in QUE 94366. Earlier investigation already revealed graphite whiskers (GWs) presence in both these meteorites. Further inspection of the meteoritic sections, coupled with advancements in the knowledge of carbon materials, reveal a re-interpretation of Raman features of a subset of the reported GWs and newer analyzed features as graphene. This meteoritic graphene perhaps originated from the same protosolar carbon reservoir that synthesized the GWs. The graphene was most likely synthesized concurrent to the inclusions and CAIs, in a high-temperature zone near the proto-Sun and during the solar system's earliest eon. However, in the case of Allende we cannot totally rule out synthesis during later aqueous alteration of the original mineral CAI assemblage. • Nanoscale graphene is found embedded in refractory inclusions of two different CV3 meteorites, Allende and QUE 94366. • The detected graphene could be as old as the calcium-aluminum-rich inclusions, and perhaps synthesized in the inner protosolar nebula, dating around 4.5 billion years. • The graphene detected here could be the building block of graphite whiskers already found in the same refractory inclusion and the two could be remnants of a protosolar reservoir of carbon allotropes. • Such carbon allotropes resemble morphologies that have been theorized to thermalize ambient radiation to generate the ~3 ​K microwave background.

Dust changes in Sakurai’s Object: new PAHs and SiC with coagulation of submicron-sized silicate dust into 10 μm-sized melilite grains

ABSTRACT 6–14 μm Spitzer spectra obtained at 6 epochs between 2005 April and 2008 October are used to determine temporal changes in dust features associated with Sakurai’s Object (V4334 Sgr), a low mass post-AGB star that has been forming dust in an eruptive event since 1996. The obscured carbon-rich photosphere is surrounded by a 40-milliarcsec torus and 32 arcsec PN. An initially rapid mid-infrared flux decrease stalled after 2008 April 21. Optically thin emission due to nanometre-sized SiC grains reached a minimum in 2007 October, increased rapidly between 2008 April 21–30 and more slowly to 2008 October. 6.3-μm absorption due to PAHs increased throughout. 20 μm-sized SiC grains might have contributed to the 6–7 μm absorption after 2007 May. Mass estimates based on the optically thick emission agree with those in the absorption features if the large SiC grains formed before 1999 May and PAHs formed in 1999 April–June. Estimated masses of PAH and large-SiC grains in 2008 October, were 3 × 10−9 M⊙ and 10−8 M⊙, respectively. Some of the submicron-sized silicates responsible for a weak 10 μm absorption feature are probably located within the PN because the optical depth decreased between 2007 October and 2008 October. 6.9-μm absorption assigned to ∼10 μm-sized crystalline melilite silicates increased between 2005 April and 2008 October. Abundance and spectroscopic constraints are satisfied if $\lesssim$2.8 per cent of the submicron-sized silicates coagulated to form melilites. This figure is similar to the abundance of melilite-bearing calcium–aluminium-rich inclusions in chondritic meteorites.

Constraints on Chondrule Generation, Disk Dynamics, and Asteroid Accretion from the Compositions of Carbonaceous Meteorites

Abstract The elemental and isotopic compositions of meteorites are expected to reflect several key processes that occurred in the early solar system, including the migration of gas and dust throughout the protoplanetary disk, the formation of chondrules, and the accretion of the first planetary bodies. However, the specific origins of the various compositions measured among these rocks are currently poorly constrained, limiting our understanding of these processes. Here, we use previously measured elemental and isotopic compositions of chondrites and iron meteorites to identify that carbonaceous (CC) meteorites are mixtures of noncarbonaceous (NC) material, calcium–aluminum-rich inclusion (CAI) material, and CI (Ivuna-like) material, in varying proportions. These trends indicate that chondrules in CO (Ornans-like), CM (Mighei-like), CV (Vigarano-like), and TL (Tagish Lake) chondrites share near-identical average proportions of CI material, arguing that they were generated through the remelting of preexisting NC chondrules all in the same disk environment. Because this proportion likely evolved over space and time throughout the disk, this similarity argues that these chondrules originate from a restricted spatial region and time interval, favoring their generation through a localized event. Moreover, the compositions of CR (Renazzo-like) chondrites indicate that their constituents formed through mechanisms different from those in CO, CM, CV, and TL chondrites. The recovered proportions of CI material in CC iron meteorites and chondrites together also argue for evolution in either the predominant direction of dust and gas motion in the first ∼10 au of the disk or the radial distance of planetesimal accretion throughout the CC reservoir.

Chronological constraints on the thermal evolution of ordinary chondrite parent bodies from the 53Mn-53Cr system

The 53Mn-53Cr isotope systematics in ordinary chondrites constrains the accretion and thermal history of their parent bodies. Mineralogical observations and olivine-spinel geothermometry suggest that chromite in ordinary chondrites formed during prograde thermal metamorphism with the amount of chromite increasing with petrologic grades in type 3 to type 6 ordinary chondrites. Assuming a chondritic evolution of the respective parent bodies, 53Cr/52Cr model ages for chromite range from 3.99-0.79+0.93 to 11.1-2.8+6.0 Ma after the formation of calcium-aluminium-rich inclusions (CAIs). Chromite and silicate-metal-sulphide isochrons define an age range from 2.78-0.50+0.55 to 15.4-1.6+2.4 Ma. Both chromite model ages and isochron ages correlate with the petrological grade of the samples, which is consistent with an onion-shell structure of the chondrite parent bodies. The study shows that unlike the isochron ages, which are prone to impact-related disturbances or partial re-equilibration during cooling from high temperatures, the chromite model ages are not easily affected by thermal metamorphism or later events and yield robust mineral growth ages. The results are consistent with a homogenous distribution of 53Mn and an initial canonical 53Mn/55Mn = 6.28 × 10−6. The estimated closure temperatures for the Mn-Cr system in chromites range from ~760 °C for type 6 to ~540–620 °C for type 3 ordinary chondrites. The high closure temperatures estimated for type 3 and type 6 ordinary chondrites imply that the chromite ages correspond to the peak metamorphic temperature reached during the thermal history of the chondrite parent bodies. The oldest chromite model age obtained for type 3 samples along with the established Al-Mg chondrule formation ages constrain the accretion of the parent bodies to >2.1 Ma after CAI formation, implying that planetesimal accretion immediately followed chondrule 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.

Shape and porosity of refractory inclusions in CV3 chondrites: A micro‐computed tomography (µCT) study

AbstractRefractory calcium‐aluminum‐rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in chondritic meteorites are the earliest solids of our solar system, bearing the information of nebular condensation as well as accretion and asteroidal shock and metasomatism processes. While the compositions of refractory inclusions have been intensely studied for ~50 years, their physical properties such as shape and porosity are poorly constrained. Here, we present a microcomputed tomography (µCT) study on 16 refractory inclusions of condensate origin in five CV3 chondrites. We find that they are prolate or triaxial in shape with very rough morphologies. The CAIs have nodular textures and are thought to form by agglomerating individual nodules via collision‐induced bouncing and/or fragmentation, where the nodules were grown by gas–solid reactions during condensation. On the parent body, refractory inclusions from the CVR meteorite Leoville experienced intense shocks that led to the flattening of their shapes and lowering of their porosities. High‐temperature metasomatism in CVOxA meteorites and low‐temperature metasomatism in CVOxB meteorites do not seem to have large effects on the porosities of their refractory inclusions, which have similar ranges and pore‐size distributions. Instead, we infer that their pores are mostly inherited from the gas–solid condensation and subsequent agglomeration processes. The porosities of CAIs are higher than those of AOAs, which is mainly due to the high‐temperature sintering process of AOAs.

The Thermal Evolution of Planetesimals During Accretion and Differentiation: Consequences for Dynamo Generation by Thermally‐Driven Convection

AbstractThe meteorite paleomagnetic record indicates that differentiated (and potentially, partially differentiated) planetesimals generated dynamo fields in the first 5–40 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs). This early period of dynamo activity has been attributed to thermal convection in the liquid cores of these planetesimals during an early period of magma ocean convection. To better understand the controls on thermal dynamo generation in planetesimals, we have developed a one dimensional model of the thermal evolution of planetesimals from accretion through to the shutdown of convection in their silicate magma oceans for a variety of accretionary scenarios. The heat source of these bodies is the short‐lived radiogenic isotope 26Al. During differentiation, 26Al partitions into the silicate portion of these bodies, causing their magma oceans to heat up and introducing stable thermal stratification to the top of their cores, which inhibits dynamo generation. In “instantaneously” accreting bodies, this effect causes a delay on the order of >10 Myr to whole core convection and dynamo generation while this stratification is eroded. However, gradual core formation in bodies that accrete over >0.1 Myr can minimize the development of this stratification, allowing dynamo generation from ∼4 Myr after CAI formation. Our model also predicts partially differentiated planetesimals with a core and mantle overlain by a chondritic crust for accretion timescales >1.2 Myr, although none of these bodies generate a thermal dynamo field. We compare our results from thousands of model runs to the meteorite paleomagnetic record to constrain the physical properties of their parent bodies.

Simultaneous determination of mass-dependent Mg isotopic variations and radiogenic 26Mg by laser ablation-MC-ICP-MS and implications for the formation of chondrules

Improvements in our understanding of the formation of chondrules requires a better knowledge of the thermal histories and the nature of their solid precursors. We present an in situ nanosecond laser ablation multi-collector inductively-coupled-plasma mass-spectrometry (LA-MC-ICP-MS) technique to measure simultaneously mass-dependent Mg isotopic fractionations and radiogenic 26Mg in chondritic components, thus allowing us to investigate within a chronological framework the thermal processes redistributing Mg in chondrules and their precursors. The internal 26Al-26Mg isochrons provide initial 26Al/27Al ratios from 5.46 (±0.38) × 10−5 to 6.14 (±0.92) × 10−5 for amoeboid olivine aggregates (AOAs) and Ca-, Al-rich inclusions (CAIs), and from 0.16 (±0.08) × 10−5 to 1.87 (±0.92) × 10−5 for chondrules from Allende and Leoville chondrites, which are consistent with the previously reported values. The combination of these values with up to 2.5‰ variation of the 25Mg/24Mg ratio within the studied chondrules shows that: (i) AOAs and the precursors of chondrules were likely formed via condensation of rapid-cooling gas reservoirs, and (ii) Mg stable isotopes are probably at disequilibrium between olivines and mesostases in some chondrules, likely due to Mg loss by vaporization during chondrule formation. We use these new observations to propose that Mg isotopes can likely serve as a tracer for the thermal histories of chondrules. We present here a scenario taking into account Mg loss by vaporization from chondrule melt and Mg gain into the melt by olivine dissolution. The existing Mg isotopic observations in chondrule melts and olivines can be explained in a scenario with a homogeneous distribution of Mg isotopes and initial 26Al in the accretion disk, provided that chondrule precursors have been heated up to sufficiently high peak temperatures (up to 2123 K) and stayed above 1800 K for several tens of minutes to allow for significant Mg evaporation. These conditions are most consistent with a shock wave model for the origin of chondrules.

Heating events in the nascent solar system recorded by rare earth element isotopic fractionation in refractory inclusions.

Equilibrium condensation of solar gas is often invoked to explain the abundance of refractory elements in planets and meteorites. This is partly motivated, by the observation that the depletions in both the least and most refractory rare earth elements (REEs) in meteoritic group II calcium-aluminum-rich inclusions (CAIs) can be reproduced by thermodynamic models of solar nebula condensation. We measured the isotopic compositions of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb in eight CAIs to test this scenario. Contrary to expectation for equilibrium condensation, we find light isotope enrichment for the most refractory REEs and more subdued isotopic variations for the least refractory REEs. This suggests that group II CAIs formed by a two-stage process involving fast evaporation of preexisting materials, followed by near-equilibrium recondensation. The calculated time scales are consistent with heating in events akin to FU Orionis- or EX Lupi-type outbursts of eruptive pre-main-sequence stars.

Titanium isotope fractionation in solar system materials

New methods to determine the titanium (Ti) mass-dependent isotope fractionation of solar system materials to high precision were developed by combining internally normalised Ti isotope data with double-spike analyses utilising a 47Ti-49Ti double spike. The procedure includes a three-stage ion-exchange separation procedure to isolate Ti from the sample matrix that provides high-purity Ti fractions that are necessary for high-precision Ti isotope analyses. Analyses of sample aliquots that were spiked before and after the ion-exchange separation procedure demonstrate that Ti isotope fractionation can be induced by the separation procedure. This outcome requires the addition of the double spike before the ion exchange separation procedure in order to accurately determine the natural mass-dependent Ti isotope fractionation of samples. Multiple double spike analyses of an Alfa Aesar Ti standard performed over eight months yielded a reproducibility (2σ standard deviation) of 0.033‰ for δ49/47Ti (differences in 49Ti/47Ti relative to the OL-Ti standard). Terrestrial sample analyses display a 2σ reproducibility of 0.018 to 0.031‰ for δ49/47Ti. Titanium isotope results for three terrestrial USGS magmatic reference samples (AGV-2, BHVO-2 and BCR-2) agree well with literature data and therefore demonstrate the accuracy and precision of the presented methodologies. Achondritic meteorites display an overall range of 0.75‰ for δ49/47Ti. The ungrouped achondrite NWA 7325 has a more positive composition by 0.64‰ for δ49/47Ti compared to all other investigated samples likely reflecting Ti isotope fractionation induced by magmatic differentiation associated with highly reducing conditions and potentially associated with oxide and plagioclase formation. In contrast, eucrites with δ49/47Ti of -0.020 ± 0.070 and -0.003 ± 0.033 and the first mass-dependent Ti isotope data for an acapulcoite (Dhofar 125; δ49/47Ti = 0.094 ± 0.033) show only limited magmatic Ti isotope fractionation. Chondrites also display a relatively restricted range of 0.085‰ for δ49/47Ti, including one calcium‑aluminum rich inclusion (CAI) from Allende and the first mass-dependent Ti isotope data for two Rumuruti chondrites (NWA 753 and NWA 755). Furthermore, the mass-dependent Ti isotope composition of chondrites overlaps with that of eucrites and the acapulcoite Dhofar 125 indicating that nebular processes induce only limited Ti isotope fractionation. Additionally, the Ti isotope data indicate that thermal metamorphism also produced marginal Ti isotope fractionation at the bulk sample scale for chondrites. Small mass-dependent Ti isotope variations between different bulk meteorite samples are also evident, which might reflect sample heterogeneity. Importantly, the mass-dependent Ti isotope composition of the Earth and Moon overlap with the composition of the investigated chondrites, eucrites and acapulcoites within the 2 standard deviation uncertainties.

Arrival and magnetization of carbonaceous chondrites in the asteroid belt before 4562 million years ago

Meteorite magnetizations can provide rare insight into early Solar System evolution. Such data take on new importance with recognition of the isotopic dichotomy between non-carbonaceous and carbonaceous meteorites, representing distinct inner and outer disk reservoirs, and the likelihood that parent body asteroids were once separated by Jupiter and subsequently mixed. The arrival time of these parent bodies into the main asteroid belt, however, has heretofore been unknown. Herein, we show that weak CV (Vigarano type) and CM (Mighei type) carbonaceous chondrite remanent magnetizations indicate acquisition by the solar wind 4.2 to 4.8 million years after Ca-Al-rich inclusion (CAI) formation at heliocentric distances of ~2–4 AU. These data thus indicate that the CV and CM parent asteroids had arrived near, or within, the orbital range of the present-day asteroid belt from the outer disk isotopic reservoir within the first 5 million years of Solar System history.

Timing Solar System formation

Cosmochemistry The oldest solids that formed in the Solar System are calcium-aluminium–rich inclusions (CAIs), small metallic droplets that were later incorporated into meteorites. The ages of CAIs are conventionally taken as the age of the Solar System, but which exact moment in star formation they correspond to has been unclear. Brennecka et al. measured molybdenum isotope ratios in CAIs and found a wide range of origins in both the inner and outer Solar System. They propose that CAIs formed from heterogeneous material accreting from the presolar nebula and that the ages of CAIs coincide with the Sun's transition from a protostar to a pre–main sequence star. Science , this issue p. [837][1] [1]: /lookup/doi/10.1126/science.aaz8482

Astronomical context of Solar System formation from molybdenum isotopes in meteorite inclusions.

Calcium-aluminum-rich inclusions (CAIs) in meteorites are the first solids to have formed in the Solar System, defining the epoch of its birth on an absolute time scale. This provides a link between astronomical observations of star formation and cosmochemical studies of Solar System formation. We show that the distinct molybdenum isotopic compositions of CAIs cover almost the entire compositional range of material that formed in the protoplanetary disk. We propose that CAIs formed while the Sun was in transition from the protostellar to pre-main sequence (T Tauri) phase of star formation, placing Solar System formation within an astronomical context. Our results imply that the bulk of the material that formed the Sun and Solar System accreted within the CAI-forming epoch, which lasted less than 200,000 years.

Microporosity and parent body of the rubble-pile NEA (162173) Ryugu

Both observations of C-type near-Earth asteroids and laboratory investigations of carbonaceous chondritic meteorites provide strong evidence for a high microporosity of C-type asteroids. Boulder microporosity values derived from in-situ measurements at the surface of the rubble-pile NEA (162173) Ryugu are as high as 55 %, which is substantially higher than for water-rich carbonaceous chondrite samples and could indicate distinct evolution paths for the parent body of Ryugu and parent bodies of carbonaceous chondrites, despite spectral similarities. In the present study, we calculate the evolution of the temperature and porosity for early solar system's planetesimals in order to constrain the range of parameters that result in microporosities compatible with Ryugu's high-porosity material and likely burial depths for the boulders observed at the surface. By varying key properties of the parent body, such as accretion time t0 and radius R that have strong influence on temperature and porosity and by comparing the interior porosity distribution with the measured boulder microporosity, hydration, and partial dehydration of the material, we constrain a field within the (R,t0)-diagram appropriate for bodies that are likely to have produced such material. Our calculations indicate a parent body size of only a few km and its early accretion within ≲2 − 3 Myr after the formation of Ca-Al-rich inclusions (CAIs). A gradual final porosity profile of best-fit bodies indicates production of both low- and high-density boulders from the parent body material. By contrast, parent body properties for CI and CM chondrites obtained by fitting carbonate formation data indicate a radius of ≈20 − 25 km and an accretion time of ≈3.75 Myr after CAIs. These results imply a population of km-sized early accreting highly porous planetesimals as parent bodies of the rubble-pile NEA Ryugu (and, potentially, other NEAs) and a population of larger and late accreting less porous planetesimals as parent bodies of water-rich carbonaceous chondrites.

Irradiation origin of 10Be in the solar nebula: Evidence from Li-Be-B and Al-Mg isotope systematics, and REE abundances of CAIs from Yamato-81020 CO3.05 chondrite

We have performed in situ analyses of Li-Be-B and Al-Mg isotope systematics, and abundances of rare earth elements (REEs) in two Ca-Al-rich inclusions (CAIs) from the Ornans-like carbonaceous chondrite Yamato-81020 (CO3.05). The present CO CAIs are depleted in ultra-refractory heavy REEs (group II REE pattern), suggesting condensation of these CAIs or their precursors from the solar nebula. Initial 26Al/27Al ratios, (26Al/27Al)0, of these CO CAIs are found to be (4.8 ± 0.5) × 10–5 and (4.9 ± 0.3) × 10–5 (95% confidence), indicating their contemporaneous formation with a majority of CAIs from Vigarano-like carbonaceous (CV) chondrites. Melilite grains in the present CO CAIs show clear excesses in 10B, ranging from ∼370 to ∼4300‰ relative to the chondritic B isotopic composition, which are correlated well with 9Be/11B ratios. The correlation indicates in situ decay of 10Be in the present CO CAIs and yields initial 10Be/9Be ratios, (10Be/9Be)0, for the individual CAIs of (2.9 ± 0.6) × 10–3 and (2.2 ± 1.0) × 10–3 (95% confidence), which are significantly greater than the average (10Be/9Be)0 = ∼0.7 × 10–3 recorded in CAIs from CV chondrites. The apparent variation in (10Be/9Be)0 between the CO and CV CAIs, despite having indistinguishable (26Al/27Al)0 of ∼5 × 10–5, provides evidence for heterogeneous distribution of 10Be in the CAI forming-regions at the very beginning of the Solar System. The elevated (10Be/9Be)0 and group II REE patterns in the CO CAIs may reflect that compared with the CV CAIs having unfractionated REEs the present CO CAIs have formed closer to the Sun where 10Be was produced more efficiently through solar cosmic ray irradiation caused by solar flares. Alternatively, if the present CO CAIs and CV CAIs formed in the same region, and 26Al was distributed homogeneously at the CAI-forming region, our results indicate that solar cosmic ray fluxes at the forming region have fluctuated by a factor of six within a short duration (∼0.2 million years) inferred from the Al-Mg chronology.

Hf‐W chronology of a macrochondrule from the L5/6 chondrite Northwest Africa 8192

AbstractA large, igneous‐textured, and 2 cm‐sized spherical object from the L5/6 chondrite NWA 8192 was investigated for its chemical composition, petrography, O isotopic composition, and Hf‐W chronology. The petrography and chemical data indicate that this object closely resembles commonly found chondrules in ordinary chondrites and is therefore classified as a “macrochondrule.* As a result of metal loss during its formation, the macrochondrule exhibits elevated Hf/W, which makes it possible to date this object using the short‐lived 182Hf‐182W system. The Hf‐W data provide a two‐stage model age for metal–silicate fractionation of 1.4 ± 0.6 Ma after Ca‐Al‐rich inclusion (CAI) formation, indicating that the macrochondrule formed coevally to normal‐sized chondrules from ordinary chondrites. By contrast, Hf‐W data for metal from the host chondrite yield a younger model age of ~11 Ma after CAIs. This younger age agrees with Hf‐W ages of other type 5–6 ordinary chondrites, and corresponds to the time of cooling below the Hf‐W closure temperature during thermal metamorphism on the parent body. The Hf‐W model age difference between the macrochondrule and the host metal demonstrates that the Hf‐W systematics of the bulk macrochondrule were not disturbed during thermal metamorphism, and therefore, that the formation age of such objects can still be determined even in strongly metamorphosed samples. Collectively, this study illustrates that chondrule formation was not limited to mm‐size objects, implying that the rarity of macrochondrules reflects either that this process was very inefficient, that subsequent nebular size‐sorting decimated large chondrules, or that large precursors were rare.

Did a Complex Carbon Cycle Operate in the Inner Solar System?

Solids in the interstellar medium consist of an intimate mixture of silicate and carbonaceous grains. Because 99% of silicates in meteorites were reprocessed at high temperatures in the inner regions of the Solar Nebula, we propose that similar levels of heating of carbonaceous materials in the oxygen-rich Solar Nebula would have converted nearly all carbon in dust and grain coatings to CO. We discuss catalytic experiments on a variety of grain surfaces that not only produce gas phase species such as CH4, C2H6, C6H6, C6H5OH, or CH3CN, but also produce carbonaceous solids and fibers that would be much more readily incorporated into growing planetesimals. CH4 and other more volatile products of these surface-mediated reactions were likely transported outwards along with chondrule fragments and small Calcium Aluminum-rich Inclusions (CAIs) to enhance the organic content in the outer regions of the nebula where comets formed. Carbonaceous fibers formed on the surfaces of refractory oxides may have significantly improved the aggregation efficiency of chondrules and CAIs. Carbonaceous fibers incorporated into chondritic parent bodies might have served as the carbon source for the generation of more complex organic species during thermal or hydrous metamorphic processes on the evolving asteroid.

Structure of type A CAI-like melts: A view from multi-nuclear NMR study of melilite (Ca2Al2SiO7-Ca2MgSi2O7) glasses

Exploring the polymerization and structural disorder of Ca-Al-rich inclusion (CAI)-like melts is a key question in cosmochemistry due to strong implications for macroscopic properties of melts (i.e., viscosity and diffusivity). Here, we report experimental results showing the effect of composition on the structure of melilite glasses and melts [åkermanite (Åk, Ca2MgSi2O7) - gehlenite (Gh, Ca2Al2SiO7)] in type A CAIs with varying composition using high-resolution solid-state nuclear magnetic resonance (NMR). The 27Al magic angle spinning (MAS) and 3Q (triple quantum) MAS NMR spectra of melilite glasses show predominant [4]Al. A non-negligible fraction of [5]Al is observed in Åk50Gh50 and Åk72Gh28 glasses and it slightly increases with increasing åkermanite content. The 17O 3QMAS NMR spectra of melilite glasses show that bridging oxygens (BOs, Si–O–Si, Al–O–Al, and Si–O–Al) and non-bridging oxygens (NBOs, Ca–O-Si, Ca-O-Al, and mixed {Ca, Mg}–NBO) are partially resolved. Despite the strong preference of Si over Al for NBOs, for the first time, Ca-O-Al is observed in natural melts (i.e., gehlenite and Åk25Gh75 glasses and melts). The results show that Al-NBO (~150 ppm in MAS dimension) can be distinguished from Si-NBO (~110 ppm) in melilite glasses and melts. The fraction of NBO increases with increasing åkermanite content. The experimental results suggest that composition-induced structural changes should be considered to interpret the melt viscosity, diffusivity, and oxygen isotopic composition of CAI-like melts in the early Solar System.

Genetics, age and crystallization history of group IIC iron meteorites

The eight iron meteorites currently classified as belonging to the IIC group were characterized with respect to the compositions of 21 siderophile elements. Several of these meteorites were also characterized for mass independent isotopic compositions of Mo, Ru and W. Chemical and isotopic data for one, Wiley, indicate that it is not a IIC iron meteorite and should be reclassified as ungrouped. The remaining seven IIC iron meteorites exhibit broadly similar bulk chemical and isotopic characteristics, consistent with an origin from a common parent body. Variations in highly siderophile element (HSE) abundances among the members of the group can be well accounted for by a fractional crystallization model with all the meteorites crystallizing between ∼10 and ∼26% of the original melt, assuming initial S and P concentrations of 8 wt.% and 2 wt.%, respectively. Abundances of HSE estimated for the parental melt suggest a composition with chondritic relative abundances of HSE ∼6 times higher than in bulk carbonaceous chondrites, consistent with the IIC irons sampling a parent body core comprising ∼17% of the mass of the body.Radiogenic 182W abundances of two group IIC irons, corrected for a nucleosynthetic component, indicate a metal-silicate segregation age of 3.2 ± 0.5 Myr subsequent to the formation of Calcium-Aluminum-rich Inclusions (CAI). When this age is coupled with thermal modeling, and assumptions about the Hf/W of precursor materials, a parent body accretion age of 1.4 ± 0.5 Myr (post-CAI) is obtained.The IIC irons and Wiley have 100Ru mass independent “genetic” isotopic compositions that are identical to other irons with so-called carbonaceous chondrite (CC) type genetic affinities, but enrichments in 94,95,97Mo and 183W that indicate greater s-process deficits relative to most known CC iron meteorites. If the IIC irons and Wiley are of the CC type, this indicates variable s-process deficits within the CC reservoir, similar to the s-process variability within the NC reservoir observed for iron meteorites. Nucleosynthetic models indicate that Mo and 183W s-process variability should correlate with Ru isotopic variability, which is not observed. This may indicate the IIC irons and Wiley experienced selective thermal processing of nucleosynthetic carriers, or are genetically distinct from the CC and NC precursor materials.