Ca-Al-rich Inclusions Research Articles

Calcium–aluminum-rich inclusions recycled during formation of porphyritic chondrules from CH carbonaceous chondrites

We report on the mineralogy, petrography, and O-isotope compositions of ∼60 Ca, Al-rich inclusions (CAIs) incompletely melted during formation of porphyritic chondrules from the CH metal-rich carbonaceous chondrites and Isheyevo (CH/CB). These include (i) relict polymineralic CAIs in porphyritic chondrules, (ii) CAIs surrounded by chondrule-like igneous rims, (iii) igneous pyroxene-rich and Type C-like CAIs, and (iv) plagioclase-rich chondrules with clusters of relict spinel grains. 26Al–26Mg systematics were measured in 10 relict CAIs and 11 CAI-bearing plagioclase-rich chondrules. Based on the mineralogy, the CH CAIs incompletely melted during chondrule formation can be divided into grossite-rich (n=13), hibonite-rich (n=11), spinel±melilite-rich (n=33; these include plagioclase-rich chondrules with clusters of relict spinel grains) types. Mineralogical observations indicate that these CAIs were mixed with different proportions of ferromagnesian silicates and experienced incomplete melting and gas–melt interaction during chondrule formation. These processes resulted in partial or complete destruction of the CAI Wark–Lovering rims, replacement of melilite by Na-bearing plagioclase, and dissolution and overgrowth of nearly end-member spinel by chromium- and iron-bearing spinel. Only two relict CAIs and two CAI-bearing chondrules show resolvable excess of radiogenic 26Mg; the inferred initial 26Al/27Al ratios are (1.7±1.3)×10–6, (3.7±3.1)×10–7, (1.9±0.9)×10–6 and (4.9±2.6)×10–6. There is a large range of Δ17O among the CH CAIs incompletely melted during chondrule formation, from ∼−37‰ to ∼−5‰; the unmelted minerals in individual CAIs, however, are isotopically uniform and systematically 16O-enriched relative to the host chondrules and chondrule-like igneous rims, which have Δ17O ranging from ∼−7‰ to ∼+4‰. Most of the CH CAIs incompletely melted during chondrule formation are mineralogically and isotopically similar to the CH CAIs surrounded by Wark–Lovering rims and apparently unaffected by chondrule melting. The mineralogy and O-isotope compositions of the CH CAI-bearing chondrules are similar to those of the CH porphyritic chondrules without relict CAIs.We conclude that CH porphyritic chondrules formed by incomplete melting of isotopically diverse solid precursors, including mineralogically and isotopically unique CAIs commonly observed only in CH chondrites. Therefore, the CH porphyritic chondrules must have formed in a distinct disk region, where the CH CAIs were present at the time of chondrule formation. Because most CH CAIs avoided chondrule melting, we infer that chondrule formation was highly localized. These observations preclude formation of CH porphyritic chondrules by splashing of molten planetesimals, by impact processing of differentiated planetesimals, and by large scale nebular shocks, e.g., shocks driven by disk gravitational instabilities or by X-ray flares. Instead, they are consistent with small-scale chondrule-forming mechanisms proposed in the literature, such as nebular processing of dust balls by bow shocks and by current sheets.

Searching for calcium‐aluminum‐rich inclusions in cometary particles with Rosetta/COSIMA

AbstractThe calcium‐aluminum‐rich inclusions (CAIs) found in chondritic meteorites are probably the oldest solar system solids, dating back to 4567.30 ± 0.16 million years ago. They are thought to have formed in the protosolar nebula within a few astronomical units of the Sun, and at a temperature of around 1300 K. The Stardust mission found evidence of CAI‐like material in samples recovered from comet Wild 2. The appearance of CAIs in comets, which are thought to be formed at lower temperatures and larger distances from the Sun, is only explicable if some mechanism allows the efficient transfer of such objects from the inner solar nebula to the outer solar nebula. Such mechanisms have been proposed such as an X‐wind or turbulence. In this work, particles collected from within the coma of comet 67P/Churyumov–Gerasimenko are examined for compositional evidence of the presence of CAIs. COSIMA (the Cometary Secondary Ion Mass Analyzer) uses secondary ion mass spectrometry to analyze the composition of cometary dust captured on metal targets. While CAIs can have a radius of centimeters, they are more typically a few hundred microns in size, and can be smaller than 1 μm, so it is conceivable that particles visible on COSIMA targets (ranging in size from about 10 μm to hundreds of microns) could contain CAIs. Using a peak fitting technique, the composition of a set of 13 particles was studied, looking for material rich in both calcium and aluminum. One such particle was found.

The effect of site geometry, Ti content and Ti oxidation state on the Ti K-edge XANES spectrum of synthetic hibonite

The Al-rich oxide hibonite (CaAl12O19) is modeled to be the second mineral to condense from a gas of solar composition and is found within calcium–aluminum-rich inclusions and the matrix of chondritic meteorites. Both Ti3+ and Ti4+ are reported in meteoritic hibonite, so hibonite has been proposed as a single mineral oxybarometer that could be used to elucidate conditions within the first 0.2Myrs of the Solar System.Synthetic hibonites with Ti3+/(Ti3++Ti4+) (hereafter Ti3+/ΣTi) ranging between 0 and 1 were prepared as matrix-matched standards for meteoritic hibonite. The largest yield of both Ti-free and Ti-bearing hibonite at ∼1300 and ∼1400°C was obtained by a single sinter under reducing conditions.In situ micro-beam Ti K-edge X-ray absorption near edge structure (XANES) spectra were recorded from the synthetic hibonites, as well as from terrestrial hibonite. Spectral features in the post-crest region were shown to correlate with the Ti4+ content. Furthermore, Ti4+ on the M2 trigonal bipyramidal and the adjoining M4 octahedral sites appears to cause variability in the post-crest region as a function of orientation. For this suite of synthetic hibonites it was observed that the pre-edge peak region is not influenced by orientation, but is controlled by Ti3+/ΣTi, site geometry and/or Ti concentration. In particular, the pre-edge peak intensities reflect Ti coordination environment and distortion of the M4 octahedral site. Therefore, although pre-edge peak intensities have previously been used to determine Ti3+/ΣTi in meteoritic minerals, we excluded use of the pre-edge peak intensities for quantifying Ti valence states in hibonite.The energy of the absorption edge at a normalized intensity of 0.8 (E0.8) and the energy of the minimum between the pre-edge region and the absorption edge (Em1) were found to vary systematically with Ti3+/ΣTi. Ti3+/ΣTi in hibonite as a function of Em1 was modeled by a quadratic function that may be used to quantify Ti3+/ΣTi in meteoritic hibonite when the synthetic hibonite standards are crystal-chemically matched to the natural samples and are measured during the same analytical session as the meteoritic hibonites.

Corundum–hibonite inclusions and the environments of high temperature processing in the early Solar System

Corundum-bearing Ca–Al-rich inclusions (CAIs) are a rare class of high-temperature condensates from the inner regions of the protoplanetary disk. Their mineralogy is intermediate between isolated corundum grains and CAIs where corundum has been replaced by lower-temperature phases. These inclusions sample a critical transitional period of the inner nebula where both the Sun and protoplanetary disk were rapidly evolving. We conducted O isotopic, Al–Mg chronological, petrographic, and crystallographic studies of four corundum-bearing inclusions in the Murchison CM2 and ALHA 77307 CO3.0 carbonaceous chondrites. Within each inclusion, corundum, hibonite, and spinel have indistinguishable 16O-rich compositions. The O isotopic compositions from all inclusions fall within a narrow range of Δ17O=−22.8±3.6‰ that matches values of most previously studied micrometer-sized corundum grains and mineralogically pristine CAIs. These data indicate that, with few exceptions, the most refractory inclusions in carbonaceous chondrites formed from the same O isotopic reservoir. One CAI from ALHA 77307, ALH-61, exhibits a continuous corundum mantle overlying a hibonite core, opposite the equilibrium condensation sequence at typical nebular pressures and dust/gas ratios. Transmission electron microscopy examination of the hibonite–corundum interface suggests that the corundum condensed on the hibonite and was itself then partially overlain with spinel. Additionally, high dust/gas ratios are interpreted from the W- and Mo-depleted composition of a refractory metal nugget within a second corundum-bearing CAI, ALH-160. Together, these observations show that the primary formation conditions of some corundum-bearing CAIs involved non-equilibrium condensation in environments with elevated dust-gas ratios.The corundum-bearing CAIs studied here have inferred initial 26Al/27Al ratios that fall within the roughly bimodal distribution of values observed in most CAIs. ALH-160 retains no resolvable excess 26Mg while ALH-61 has a well-resolved initial 26Al/27Al ratio of 4.2±0.4×10−5. The presence or absence of live 26Al at the time of CAI formation may record distinct chronology if 26Al was initially homogeneously distributed in the early Solar System. Alternatively, variations in 26Al/27Al ratios may reflect late injection and/or heterogeneous distribution of 26Al. Regardless of which model for 26Al distribution is correct, the data presented here indicate that formation of corundum-bearing CAIs was repeated during multiple heating and non-equilibrium condensation events throughout early Solar System history and within a single oxygen isotopic reservoir.

Ca–Al-rich inclusions in carbonaceous chondrites: the oldest solar system objects

This paper presents a review of recent available data on the first solid condensates of the Solar System, which include refractory CAIs (Ca–Al-rich Inclusions) mostly composed of Ca, Al, Mg, and Ti minerals. A theoretical condensation sequence calculated from thermodynamic data confirmed that CAIs formed as fine-grained aggregates in the protoplanetary disk from an 16О-rich gas of solar composition at temperatures >1300° K and pressures <10–4 bar. Based on the diversity of CAI types, their mineralogical, bulk chemical, and isotopic compositions, it can be concluded that CAIs experienced melting and evaporation, possibly by shock waves, which may have occurred in the protoplanetary disk within a brief time interval. Some CAIs may have experienced multiple events such as melting, evaporation, and recycling back to the disk by means of a bipolar outflow. The CAIs having an absolute age of 4567.30 ± 0.16 Myr are the oldest objects in the Solar System. The study of CAIs revealed two distinct oxygen isotope reservoirs (16О-rich and 16О-poor) and established a chronology of the sequence of processes forming individual CAI components using Mg–Al, Cr–Mn and Pb–Pb isotopic systematics.

Precise titanium isotope compositions of refractory inclusions in the Allende CV3 chondrite by LA-MC-ICPMS

We present here analyses of mass-independent effects in the Ti isotope ratios of 17 coarse-grained (compact Type A and Type B) calcium–aluminum-rich inclusions (CAIs) from the Allende CV3 chondrite utilizing a method developed for rapid measurement of Ti isotopes by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Based on the analyses of the synthetic CMAS glasses doped with varying amounts of Ca, Cr and V, an empirical scheme for correction of potential isobaric interferences and matrix effects was developed. Using this scheme, mass-independent variations of the Ti isotope ratios 46Ti/47Ti, 48Ti/47Ti and 50Ti/47Ti were measured with external reproducibilities (2SD) of ±0.4, ±0.5, and ±1.8 for ε46Ti, ε48Ti, and ε50Ti, respectively, based on repeat measurements of standard glasses. The Ti isotope compositions of the 17 Allende refractory inclusions analyzed here show that most of these CAIs are “normal” with limited variation in their mass-independent Ti isotope composition. One CAI (designated as CMS-1) has a significantly larger mass-independent ε50Ti anomaly and is identified as having Fractionation and Unidentified Nuclear (FUN) effects. The limited range in the Ti isotope compositions of normal CAIs (ε50Ti excesses ranging from 2.9 to 11.4) suggests that they originated from a nebular reservoir that was relatively well-mixed, although not completely homogenized (at the level of precision of the analyses reported here), in its isotopic composition. The distinctive isotopic composition of the FUN CAI CMS-1 indicates that it formed from a reservoir in the protoplanetary disk that was spatially or temporally distinct from that from which the normal CAIs were formed.

A note on cement in asteroids

Cement mineral tobermorite was formed in hydrothermal experiments on alternation of calcium–aluminum-rich inclusions (CAIs) in carbonaceous chondrite meteorites. Unidentified bands at 14 µm were measured for CAIs and the matrix of the Allende meteorite sample, as well as for Hektor and Agamemnon asteroids. The presence of cement nanoparticles may explain the feature at 14 µm.

New constraints on the relationship between 26Al and oxygen, calcium, and titanium isotopic variation in the early Solar System from a multielement isotopic study of spinel-hibonite inclusions

We report oxygen, calcium, titanium and 26Al–26Mg isotope systematics for spinel-hibonite inclusions (SHIBs), a class of calcium–aluminum-rich inclusions (CAI) common in CM chondrites. In contrast to previous studies, our analyses of 33 SHIBs and four SHIB-related objects obtained with high spatial resolution demonstrate that these CAIs have a uniform Δ17O value of approximately −23‰, similar to many other mineralogically pristine CAIs from unmetamorphosed chondrites (e.g., CR, CV, and Acfer 094). Five SHIBs studied for calcium and titanium isotopes have no resolvable anomalies beyond 3σ uncertainties. This suggests that nucleosynthetic anomalies in the refractory elements had been significantly diluted in the environment where SHIBs with uniform Δ17O formed. We established internal 26Al–26Mg isochrons for eight SHIBs and found that seven of these formed with uniformly high levels of 26Al (a multi-CAI mineral isochron yields an initial 26Al/27Al ratio of ∼4.8×10−5), but one SHIB has a smaller initial 26Al/27Al of∼2.5×10−5, indicating variation in 26Al/27Al ratios when SHIBs formed. The uniform calcium, titanium and oxygen isotopic characteristics found in SHIBs with both high and low initial 26Al/27Al ratios allow for two interpretations. (1) If subcanonical initial 26Al/27Al ratios in SHIBs are due to early formation, as suggested by Liu et al. (2012), our data would indicate that the CAI formation region had achieved a high degree of isotopic homogeneity in oxygen and refractory elements before a homogeneous distribution of 26Al was achieved. (2) Alternatively, if subcanonical ratios were the result of 26Al–26Mg system resetting, the clustering of SHIBs at a Δ17O value of ∼−23‰ would imply that a 16O-rich gaseous reservoir existed in the nebula until at least ∼0.7Ma after the formation of the majority of CAIs.

U–Pb and Al–Mg systematics of the ungrouped achondrite Northwest Africa 7325

Northwest Africa (NWA) 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet Mercury. In order to understand the origin of this meteorite and the nature of its parent body, we have determined its crystallisation age using the long-lived U–Pb and short-lived Al–Mg chronometers. An internal Pb–Pb isochron defined by six acid leached pyroxene fractions yields an age of 4563.4±2.6Ma, assuming that the 238U/235U ratio for NWA 7325 is identical to the bulk Earth and Solar System value of 137.794. The Al–Mg isotope analyses of seven fractions (four plagioclase, one pyroxene, one olivine and one whole rock) define a regression line corresponding to 26Al/27Al0=(3.03±0.14)×10−7 and an initial δ26Mg∗ of 0.093±0.004‰. When anchored to the D’Orbigny angrite, this initial 26Al/27Al yields an age of 4563.09±0.26Ma. The Pb–Pb age of 4563.4±2.6Ma and Al–Mg age of 4563.09±0.26Ma are in complete agreement, but the low U concentrations of NWA 7325 resulted in a relatively low precision Pb–Pb age. The observed excess in initial δ26Mg∗ can be explained by 27Al/24Mg fractionation and subsequent Mg isotopic evolution after planetary differentiation. Furthermore, the parental magma of NWA 7325 most likely formed within 1.72Ma after calcium-aluminium rich inclusion (CAI) formation. NWA 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at ∼4563Ma, suggesting that a multitude of planetary bodies had formed and differentiated by ∼4–5Myr after CAI formation. This ancient age may be interpreted as an argument against NWA 7325 originating from Mercury, however it does not completely rule it out.

Major and trace element geochemistry of S‐type cosmic spherules

AbstractMicrometeorites that pass through the Earth's atmosphere undergo changes in their chemical compositions, thereby making it difficult to understand if they are sourced from the matrix, chondrules, or calcium–aluminum‐rich inclusions (CAIs). These components have the potential to provide evidence toward the understanding of the early solar nebular evolution. The variations in the major element and trace element compositions of 155 different type (scoriaceous, relict bearing, porphyritic, barred, cryptocrystalline, and glass) of S‐type cosmic spherules are investigated with the intent to decipher the parent sources using electron microprobe and laser ablation inductively coupled plasma‐mass spectrometry. The S‐type cosmic spherules appear to show a systematic depletion in volatile element contents, but have preserved their refractory trace elements. The trends in their chemical compositions suggest that the S‐type spherules comprise of components from similar parent bodies, that is, carbonaceous chondrites. Large fosteritic relict grains observed in this investigation appear to be related to the fragments of chondrules from carbonaceous chondrites. Furthermore, four spherules (two of these spherules enclose spinels and one comprised entirely of a Ca‐Al‐rich plagioclase) show enhanced trace element enrichment patterns that are drastically different from all the other 151 cosmic spherules. The information on the chemical composition and rare earth elements (REEs) on cosmic spherules suggest that the partially to fully melted ones can preserve evidences related to their parent bodies. The Ce, Eu, and Tm anomalies found in the cosmic spherules have similar behavior as that of chondrites. Distinct correlations observed between differentREEs and types of cosmic spherules reflect the inherited properties of the precursors.

Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI

A major unanswered question in solar system formation is the origin of the oxygen isotopic dichotomy between the Sun and the planets. Individual Calcium–Aluminum-rich inclusions (CAIs) from CV chondrites exhibit almost the full isotopic range, but how their composition evolved is still unclear, which prevents robust astrochemical conclusions. A key issue is notably the yet unsolved origin of the 16O-rich isotopic composition of pyroxene in type B CAIs. Here, I report an in-situ oxygen isotope study of the archetypal type B CAI USNM-3529-Z from Allende with emphasis on the isotopic composition of pyroxene and its isotopic and petrographic relationships with other major minerals. The O isotopic composition of pyroxene is correlated with indicators of magmatic growth, indicating that the pyroxene evolved from a 16O-poor composition and became progressively enriched in 16O during its crystallization, contrary to the long held assumption that pyroxene was initially 16O-rich. This variation is well explained by isotopic exchange between a 16O-poor partial melt having the isotopic composition of melilite and a 16O-rich gas having the isotopic composition of spinel, during pyroxene crystallization.The isotopic evolution of 3529-Z is consistent with formation in an initially 16O-rich environment where spinel and gehlenitic melilite crystallized, followed by a 16O-depletion associated with melilite partial melting and recrystallization and finally a return to the initial 16O-rich environment before pyroxene crystallization. This strongly suggests that the environment of CAI formation was globally 16O-rich, with local 16O-depletions systematically associated with high temperature events. The Al/Mg isotopic systematics of 3529-Z further indicates that this suite of isotopic changes occurred in the first 150 000 yr of the solar system, during the main CAI formation period. A new astrophysical setting is proposed, where the 16O-depletion occurs in an optically thin surface layer of the disk and may have originated by evaporation of 16O-poor interstellar dust or non-mass-dependant isotopic fractionation.

Chronological study of oxygen isotope composition for the solar protoplanetary disk recorded in a fluffy Type A CAI from Vigarano

Fluffy Type A Ca–Al-rich inclusions (CAIs) containing reversely zoned melilite crystals are suggested to be aggregates of direct condensates from solar nebular gas. We conducted an investigation of 26Al−26Mg systematics of a fluffy Type A CAI from Vigarano, named V2-01, with known oxygen isotopic distributions of reversely zoned melilite crystals; we also conducted oxygen isotope measurements of coexisting minerals. Two of six reversely zoned melilite crystals show continuous variations in magnesium isotopic composition, with δ25Mg decreasing along the inferred direction of crystal growth, which supports the idea that they originated through condensation. Petrography suggests that the constituent minerals of V2-01 formed in the following order: first spinel and fassaite enclosed by melilite, then reversely zoned melilite crystals, and spinel and diopside in the Wark–Lovering rim. The spinel enclosed by melilite has 16O-rich compositions (Δ17O∼−24‰) and on an Al−Mg evolutionary diagram plots along model isochron with an initial value of (26Al/27Al)0=(5.6±0.2)×10−5. The fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions (Δ17O∼−12‰ and −17‰) and plots on an isochron with (26Al/27Al)0=(5.6±0.2)×10−5. The oxygen isotopic compositions of reversely zoned melilite showed continuous variations in Δ17O along the inferred direction of crystal growth, suggesting that surrounding nebular gas, during the formation of the reversely zoned melilite, changed from 16O-poor (Δ17O values larger than −10‰) to 16O-rich (Δ17O∼−25‰). The six reversely zoned melilite crystals show indistinguishable initial 26Al/27Al values with an average (26Al/27Al)0 of (4.7±0.3)×10−5, which is clearly distinguishable from the value of enclosed spinel and fassaite, indicating a younger formation age than the enclosed spinel and fassaite. The spinel and diopside from the Wark–Lovering rim show 16O-rich compositions (Δ17O∼−23‰) with (26Al/27Al)0=(4.5±0.4)×10−5. The values of (26Al/27Al)0 are consistent with the formation sequence inferred from petrography. The formation period for the V2-01 CAI is estimated to be 0.18±0.07Myr from the difference in initial 26Al/27Al values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from 16O-rich to 16O-poor and back to 16O-rich during the first ∼0.2Myr of Solar System formation.

Genetic relationship between Na-rich chondrules and Ca,Al-rich inclusions? – Formation of Na-rich chondrules by melting of refractory and volatile precursors in the solar nebula

Al-rich objects (Ca,Al-rich inclusions (CAIs), Al-rich chondrules, Al-rich fragments) occur in all chondrite classes. These objects can be centimeter-sized in CV3 carbonaceous chondrites, but they are generally much smaller in other chondrite groups and classes. Within the ordinary chondrites, most Al-rich objects are chondrules that vary from Ca- to Na-rich.Here, we have investigated the mineralogy and major element chemistry of 32 Na-rich chondrules and 3 Na-rich fragments from 15 different chondrites. Most objects (chondrules and chondrule fragments) are from ordinary chondrites (petrologic types 3.2–3.8), but two of the chondrules are from two CO3 chondrites, and three of the chondrules are from one Rumuruti (R)-chondrite. We found that these Na-rich objects have bulk Na2O-concentrations between 4.3 and 15.2wt%. Texturally, they typically consist of euhedral to subhedral (often skeletal) mafic minerals (olivine and pyroxenes) embedded within a nepheline-normative, glassy mesostasis, which is brownish in transmitted light. In addition, some chondrules contain euhedral to subhedral spinel. Bulk chondrule compositions show group II, group III, and ultrarefractory rare earth element (REE) patterns similar to those found in CAIs. These results clearly demonstrate that the Na-rich chondrules must have been formed by melting of precursors containing an (ultra-)refractory element-rich component and Na-rich constituents. The Na-rich chondrules showed Sm and Eu anomalies, indicating that they must have formed at low oxygen fugacities. Based on the chemical composition of the Na-rich objects, we can rule out that they were formed as a result of planetary formation due to metasomatic processes or processes related to collisions between molten planetesimals.

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.

147,146Sm–143,142Nd, 176Lu–176Hf, and 87Rb–87Sr systematics in the angrites: Implications for chronology and processes on the angrite parent body

Angrites are a group of basaltic achondrites with distinctive mineralogic and geochemical characteristics that have the potential to provide insights into processes occurring on planetesimals in the early Solar System. These achondrites have been used as anchors linking the relative age information obtained from short-lived, extinct chronometers (e.g., Al–Mg, Hf–W, and Mn–Cr) with absolute chronometers (e.g., U–Pb). Angrites provide excellent examples of early differentiation processes, such as core formation and silicate differentiation, on protoplanetary bodies. The significant increase in the number of known angrite samples in recent years has offered the opportunity to compare several short- and long-lived isotopic systems in samples with different petrogenetic histories that formed on the same parent body. To this end, the 147Sm–143Nd, 146Sm–142Nd, 176Lu–176Hf, and 87Rb–87Sr isotope systematics have been investigated in a suite of plutonic, coarse-grained (NWA 4590, NWA 4801, and NWA 2999) and quenched, fine-grained (D’Orbigny) angrites. The coupled 147,146Sm–143,142Nd systematics indicate possible isotopic disturbances in two angrites (D’Orbigny and NWA 2999) resulting from post-crystallization processes. The internal 146Sm–142Nd isochrons of two coarse-grained angrites (NWA 4590 and NWA 4801) provide an updated best estimate of the initial Solar System 146Sm/144Sm ratio (i.e., at 4568Ma) of 0.0084±0.0003. The 176Lu–176Hf isotope systematics in these angrites do not provide evidence of a previously proposed intense irradiation event in the early Solar System. The internal 176Lu–176Hf isochrons for the NWA 4590 and D’Orbigny angrites provide an estimate for the Solar System initial 176Hf/177Hf ratio of 0.279775±0.000031 (2σ) that agrees within uncertainty with the value of average chondrites reported by Bouvier et al. (2008). Finally, the calculated initial 87Sr/86Sr ratios based on the measured Sr-isotopic composition of plagioclase in these angrites yield an estimated initial 87Sr/86Sr ratio of 0.698980±0.000011 for the angrite parent body. This is indistinguishable from a recently determined value for the Solar System initial 87Sr/86Sr based on values measured in calcium–aluminum-rich inclusions (CAIs) after correcting for nucleosynthetic effects in the CAIs. The low initial 87Sr/86Sr of the angrite parent body implies that it acquired its volatile element depleted characteristic within 1.8Ma of Solar System formation, likely because it accreted from volatile depleted planetesimals that formed in the hot inner nebula. These integrated isotopic systematics suggest a complex history for the angrite parent body not previously inferred from short-lived chronometers and provide new estimates for the initial isotopic composition of the early Solar System.

26Al–26Mg chronology and oxygen isotope distributions of multiple melting for a Type C CAI from Allende

Disequilibrium oxygen isotopic distributions of Ca–Al-rich inclusions (CAIs) correspond to multiple melting events in the solar nebula. 26Al–26Mg systematics may be applicable for age differences among such melting events. We have carried out a coordinated study of detailed petrographic observations and in-situ oxygen and magnesium isotope measurements for a Type C CAI, EK1-04-2, from the Allende CV3 meteorite to determine the melting events and their ages. The CAI consists mainly of spinel, anorthite, olivine, and pyroxene, and has a core and mantle structure. Petrography of the core suggests that the crystallization sequence of the core minerals is from spinel, anorthite, olivine, and to pyroxene. The mantle has the same mineral assemblage as the core, and shows incomplete melting and solidification textures. Oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral (CCAM) line (δ18O=−44‰ to +9‰), which indicates to preserve a chemical disequilibrium status in the CAI. Spinel shows a 16O-rich signature (δ18O∼−43‰), while anorthite is 16O-poor (δ18O ∼+8‰). Olivine and pyroxene in the core have the same oxygen isotopic composition (δ18O∼−15‰), which indicates their equilibrium. Olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16O (δ18O=−13‰ to −4‰) compared with the same minerals in the core. The 26Al–26Mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes. Spinel is plotted on a line of (26Al/27Al)0=(3.5±0.2)×10−5, anorthite is plotted on a line of (−1±5)×10−7, and olivine and pyroxene in the core are plotted on a line of (−1±7)×10−6. Plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core. This study indicates that the EK1-04-2 Type C CAI underwent multiple heating events after the formation of its CAI precursor. The precursor CAI was formed ∼0.4Myr after the formation of the Solar System defined by canonical CAI formation. At least 1.6Myr after the precursor CAI formation, the CAI was partially melted and the melt exchanged oxygen isotopes with surrounding 16O-poor nebular gas. 16O-poor olivine and pyroxene in the core crystallized from the melt. Subsequently, Al-rich chondrules accreted onto the CAI, and the CAI experienced partial melting again and recrystallized to form the mantle. The oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the Allende parent body. Our study reveals that the CAI had been retained in the solar nebula for at least 1.6Myr and underwent multiple melting events in the nebula, and oxygen and 26Al–26Mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body.

Barium stable isotope composition of the Earth, meteorites, and calcium–aluminum-rich inclusions

High-precision stable Ba isotope ratios are reported in a variety of terrestrial samples, undifferentiated primitive meteorites, and calcium–aluminum-rich inclusions (CAIs) from the Allende chondrite. All whole-rock terrestrial and meteorite samples are isotopically indistinguishable at a 50 parts per million (ppm) level per atomic mass unit (amu). Three CAIs are isotopically light, with δ138/137Ba (permil deviation of the 138Ba/137Ba ratio from a terrestrial standard) values down to −0.6‰ compared to whole-rock meteorites, whereas the matrix is enriched in heavy isotopes (δ138/137Ba: +0.2‰). Similar light isotope enrichments in CAIs have been previously observed for Eu, Sr, and Ca, while for most other elements CAIs are enriched in the heavier isotopes (e.g. Mg, Fe). Kinetic isotopic fractionation is a possible explanation for the enrichment in the lightest isotopes, either by condensation from a vapor phase enriched in light isotopes by kinetic effects or by kinetic fractionation during non-equilibrium condensation of an undercooled gas as suggested for Ca isotopes. However, the common property of Ba, Eu, and Sr is that they all have a low first ionization potential. We suggest that electromagnetic sorting of ionized species in the early Solar System is a possible alternative mechanism to explain the depletion in heavy isotopes observed in refractory inclusions for those elements.

Formation of refractory metal nuggets and their link to the history of CAIs

Ca, Al-rich inclusions (CAIs) often contain numerous refractory metal nuggets (RMNs), consisting of elements like Os, Ir, Mo, Pt and Ru. The nuggets are usually thought to have formed by equilibrium condensation from a gas of solar composition, simultaneously with or prior to oxide and silicate minerals. However, the exact mechanisms responsible for their extremely variable compositions, small sizes and associations with CAI minerals remain puzzling. Expanding on previous work on chemically separated RMNs, we have studied a large number of RMNs within their host CAIs from three different meteorite types, i.e., the highly primitive chondrite Acfer 094 (C2-ungrouped), Allende (CV3ox) and Murchison (CM2). Our results show several inconsistencies between the observed features and a direct condensation origin, including a lack of correlated abundance variations in the refractory metals that are expected from variations in condensation temperature. Instead, we show that most RMN features are consistent with RMN formation by precipitation from a CAI liquid enriched in refractory metals. This scenario is additionally supported by the common occurrence of RMNs in CAIs with clear melt crystallization textures as well as the occurrence of synthetic RMNs with highly variable compositions in run products from Schwander et al. (2015). In some cases, the sizes of meteoritic RMNs correlate with the sizes of their host minerals in CAIs, which indicates common cooling rates.

Early aqueous activity on the ordinary and carbonaceous chondrite parent bodies recorded by fayalite.

Chronology of aqueous activity on chondrite parent bodies constrains their accretion times and thermal histories. Radiometric (53)Mn-(53)Cr dating has been successfully applied to aqueously formed carbonates in CM carbonaceous chondrites. Owing to the absence of carbonates in ordinary (H, L and LL), and CV and CO carbonaceous chondrites, and the lack of proper standards, there are no reliable ages of aqueous activity on their parent bodies. Here we report the first (53)Mn-(53)Cr ages of aqueously formed fayalite in the L3 chondrite Elephant Moraine 90161 as Myr after calcium-aluminium-rich inclusions (CAIs), the oldest Solar System solids. In addition, measurements using our synthesized fayalite standard show that fayalite in the CV3 chondrite Asuka 881317 and CO3-like chondrite MacAlpine Hills 88107 formed and Myr after CAIs, respectively. Thermal modelling, combined with the inferred conditions (temperature and water/rock ratio) and (53)Mn-(53)Cr ages of aqueous alteration, suggests accretion of the L, CV and CO parent bodies ∼1.8-2.5 Myr after CAIs.

CORRELATION OF 48 Ca, 50 Ti, AND 138 La HETEROGENEITY IN THE ALLENDE REFRACTORY INCLUSIONS

Precise determinations of 48Ca anomalies in Allende calcium–aluminum-rich inclusions (CAIs) are reported in this work. There are endemic positive 48Ca/44Ca anomalies in all analyzed CAIs after normalization to 42Ca/44Ca, and it is clearly shown that there is no simple correlation between 48Ca/44Ca and 50Ti/48Ti anomalies, in agreement with Jungck et al. Compared to the 48Ca/44Ca versus 50Ti/48Ti correlation line defined by differentiated meteorites, reported by Chen et al., the CAIs plot to elevated 50Ti/48Ti. Assuming the 48Ca/44Ca anomalies of both CAIs and differentiated meteorites came from the same source, excess 50Ti anomalies in CAIs can be calculated by subtracting the part associated with 48Ca/44Ca. These excesses show a linear correlation with 138La anomalies, a neutrino-process nuclide. According to current stellar nucleosynthetic models, we therefore suggest that the solar system 48Ca, 50Ti, and 138La isotopic variations are made of mixtures between grains condensed from ejecta of neutron-rich accretion-induced SNe Ia and the O/Ne–O/C zone of core-collapse SNe II.