Ca-Al-rich Inclusions Research Articles

In situ SIMS oxygen isotope analyses: Evidence for the formation of aluminum-rich chondrules from ordinary chondrites

Aluminum-rich chondrules (ARCs), which share mineralogic and chemical properties with both Ca, Al-rich inclusions (CAIs) and ferromagnesian chondrules, play an important role in revealing their temporal and petrogenetic relationships. In this work, seven ARCs were found in three ordinary chondrites GRV 022410 (H4), GRV 052722 (H3.7) and Julesburg (L3.6). They contain bulk Al2O3 ∼ 17%–33% and exhibit igneous textures composed of olivine, high- and low-Ca pyroxene, plagioclase, spinel and glass. In situ SIMS analyses show that ARCs have oxygen isotopic compositions (δ 18O=−6.1‰–7.1‰; δ 17O= −4.5‰–5.1‰) close to ferromagnesian chondrules but far more depleted in 16O than CAIs (δ 18O=−40‰; δ 17O=−40‰). Most ARCs plot close to the terrestrial mass fractionation (TF) line, and a few between the TF and carbonaceous chondrite anhydrous mixing (CCAM) lines. Plagioclase, nepheline and glass suffered O-isotopic exchanges during the metamorphism processes in the parent body. Spinel, olivine and pyroxene represent the primary O-isotopic compositions of ARCs, and define a fitted line with a slope of ∼ 0.7±0.1. Compared with the results of previous studies, shallower slope as well as more depleted 16O compositions further demonstrates that ARCs in ordinary chondrites are not a simple mixing product of ferromagnesian chondrules and CAIs. Instead, they probably experienced higher-degree oxygen isotope exchange with a 16O-poor nebular gas reservoir during multiple melting episodes.

COSIMA-Rosetta calibration for in situ characterization of 67P/Churyumov–Gerasimenko cometary inorganic compounds

COmetary Secondary Ion Mass Analyzer (COSIMA) is a time-of-flight secondary ion mass spectrometry (TOF-SIMS) instrument on board the Rosetta space mission. COSIMA has been designed to measure the composition of cometary dust particles. It has a mass resolution m/Δm of 1400 at mass 100u, thus enabling the discrimination of inorganic mass peaks from organic ones in the mass spectra. We have evaluated the identification capabilities of the reference model of COSIMA for inorganic compounds using a suite of terrestrial minerals that are relevant for cometary science. Ground calibration demonstrated that the performances of the flight model were similar to that of the reference model. The list of minerals used in this study was chosen based on the mineralogy of meteorites, interplanetary dust particles and Stardust samples. It contains anhydrous and hydrous ferromagnesian silicates, refractory silicates and oxides (present in meteoritic Ca–Al-rich inclusions), carbonates, and Fe–Ni sulfides. From the analyses of these minerals, we have calculated relative sensitivity factors for a suite of major and minor elements in order to provide a basis for element quantification for the possible identification of major mineral classes present in the cometary particles.

Oxygen isotopic composition of relict olivine grains in cosmic spherules: Links to chondrules from carbonaceous chondrites

Most olivine relict grains in cosmic spherules selected for the present study are pristine and have not been disturbed during their atmospheric entry, thereby preserving their chemical, mineralogical and isotopic compositions. In order to understand the origin of the particles, oxygen isotope compositions of relict olivine grains in twelve cosmic spherules collected from deep sea sediments of the Indian Ocean were studied using secondary ion mass spectrometry. Most of the data lie close to the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line, with Δ17O ranging from −5‰ to 0‰. The data overlap oxygen isotopic compositions of chondrules from carbonaceous chondrites such as CV, CK, CR and CM, which suggests that chondrules from carbonaceous chondrites are the source of relict grains in cosmic spherules. Chemical compositions of olivine in cosmic spherules are also very similar to chondrule olivine from carbonaceous chondrites. Several olivine relict grains in three cosmic spherules are 16O-rich (Δ17O −21.9‰ to −18.7‰), similar to oxygen isotopic compositions observed in calcium aluminum rich inclusions (CAIs), amoeboid olivine aggregates (AOAs), and some porphyritic chondrules from carbonaceous chondrites. These grains appear to have recorded the initial oxygen isotopic composition of the inner solar nebula. Three olivine grains from two cosmic spherules have δ18O values >+20‰, which could be interpreted as mixing with stratospheric oxygen during atmospheric entry.

Ca-, Al-Rich Inclusions in Two New Carbonaceous Chondrites from Grove Mountains, Antarctica

Two new carbonaceous chondrites, GRV 023155 and GRV 050179, collected from the Grove Mountains (GRV), Antarctica, have been classified as the oxidized CV3 and CM2 chondrites, respectively. A total of 9 Ca-, Al-rich inclusions (CAIs) were found in the two meteorites. Most of the inclusions are extensively altered, with phyllosilicates commonly found in the alteration assemblages of CAIs, chondrules and matrix in the GRV 050179 CM2 chondrites, suggesting that aqueous alteration occurred on the host meteorite parent body. In contrast, feldspathoids and hedenbergite were identified in the CAIs from GRV 023155. The FeO-rich phases in the CAIs from GRV 023155 indicate alteration of these CAIs happened under high oxygen fugacity. All 9 inclusions can be classified as Type A or spinel-pyroxene rich inclusions, and they probably represent a continuum of solar nebular condensation. The survey of Ca-, Al-rich inclusions in GRV 023155 (CV3) and 050179 (CM2) suggests that Type A and spinel-pyroxene inclusions are common in these two meteorites.

Mineralogical anatomy and implications of a Ti–Sc-rich ultrarefractory inclusion from Sayh al Uhaymir 290 CH3 chondrite

Titanium-rich minerals are common in Ca–Al-rich inclusions from primitive chondrites. They are important not only for testing the condensation models for a gas with a solar composition, but also for constraining the redox conditions of the early solar nebula. In this study, we report the detailed mineralogical features and its oxygen isotope compositions of a Ti–Sc-rich ultrarefractory inclusion A0031 from a CH3 chondrite Sayh al Uhaymir 290. The A0031 inclusion has a compact and layered texture with the interior consisting of panguite, Sc-rich anosovite, Ti-rich davisite, and anorthite. A few hexaferrum, perovskite, and spinel crystals are present as inclusions in these minerals. Outside of Ti-rich davisite are a layer of Al–Ti-rich diopside and two grains of enstatite. This texture strongly suggests that A0031 has a condensation origin. Panguite is its third occurrence in nature and similar in composition to the type panguite from the Allende meteorite. Sc-rich anosovite in A0031 has a chemical formula of (Ti4+,Ti3+,Mg,Sc,Al)3O5 with the pseudobrookite structure. This is the second report of Ti3O5 in nature, but is the first description of anosovite formed in the solar nebula as an ultrarefractory phase. The discovery of Sc-rich anosovite in A0031 reveals the stability of Ti3O5 in the early solar nebula and supports the prediction of previous equilibrium condensation calculations. The panguite, Sc-rich anosovite, and Ti-rich davisite in A0031 show a large variation in Ti3+/Titot. The primitive nature of A0031 implies that the variations in Ti3+/Titot among different Ti-rich minerals are primary features. We propose that the distribution of Ti3+ and Ti4+ could be controlled mainly by their various competition abilities of incorporating into these Ti–Sc–Al-rich minerals. Similarity of Ti3+/Titot value between Ti-rich davisite from A0031 and those in other carbonaceous chondrites indicates that most refractory inclusions might have formed in highly reducing nebular settings. The 16O-depleted isotope compositions of A0031 confirm the existence of diverse oxygen reservoirs for CH Ca–Al-rich inclusions in the solar nebula.

Early cosmic ray irradiation of chondrules and prolonged accretion of primitive meteorites

Chondrules, together with Ca-Al-rich inclusions (CAIs) and matrix, are the major constituents of primitive meteorites. It is clear that chondrules formed as molten objects and the conditions under which this happened seem well constrained. Partially overlapping in age, but mostly ∼2–3 million years younger than the CAIs, they appear to have formed over an extended period of time (e.g., Kita et al., 2013). We have analyzed chondrules in two highly primitive CR3 meteorites, QUE 99177 and MET 00426, and find that they contain highly variable amounts of noble gases produced by irradiation with cosmic rays. The lack of implanted solar wind and the composition of the cosmogenic component in QUE 99177 chondrules argue against irradiation in a parent body regolith, which leaves irradiation in the early solar system as the most likely explanation. The cosmogenic composition also points to irradiation primarily by galactic cosmic rays (GCR), not solar cosmic rays (SCR), i.e. not by an active early sun. To allow effective production of cosmogenic isotopes by GCR, but not SCR, this should have happened rather “late” in a largely, but not completely, dust-free environment. Our results support the suggestion that chondrules formed as free-floating objects in the solar nebula; also consistent with the noble gas data is pre-irradiation in small (∼dm-size) aggregates that broke up before or during accretion to the CR parent body. In both cases, chondrules spent an extended period of time before incorporation into the most primitive meteorite parent bodies, which puts constraints on accretion time scales.

Synthesis of refractory metal nuggets and constraints on the thermal histories of nugget‐bearing Ca, Al‐rich inclusions

AbstractTiny refractory metal nuggets are mainly observed inside Ca, Al‐rich inclusions (CAIs) from chondritic meteorites and are commonly assumed to be condensates from a solar composition gas. However, recent detailed studies of metal nugget compositions and their comparison with predictions from condensation show that the observed abundance patterns are extremely difficult to achieve in this way. As a test for the proposed alternative, precipitation from a silicate liquid, we conducted melting experiments, in which nine different refractory metals (nugget components) were equilibrated with each other along with a CAI‐like liquid at reducing conditions. When quenched, minerals similar to those in CAIs formed from such liquids including refractory metal nuggets exhibiting compositions and appearances similar to those of the meteoritic nuggets. The run products and their comparison with a meteoritic nugget‐bearing CAI is evidence for formation of refractory metal nuggets during cooling of Ca, Al‐rich liquids at rates about 1000°/40 s (in the interval from 1900 to 900 °C). To achieve the formation of refractory metal nuggets and the textures observed in the host inclusions, during cooling the rate probably changed. Refractory metal nuggets apparently formed during quenching before spinel crystallized.

Early accretion of protoplanets inferred from a reduced inner solar system 26Al inventory

The mechanisms and timescales of accretion of 10–1000 km sized planetesimals, the building blocks of planets, are not yet well understood. With planetesimal melting predominantly driven by the decay of the short-lived radionuclide 26Al (26Al→26Mg; t1/2=0.73 Ma), its initial abundance determines the permissible timeframe of planetesimal-scale melting and its subsequent cooling history. Currently, precise knowledge about the initial 26Al abundance [(26Al/27Al)0] exists only for the oldest known solids, calcium aluminum-rich inclusions (CAIs) – the so-called canonical value. We have determined the 26Al/27Al of three angrite meteorites, D'Orbigny, Sahara 99555 and NWA 1670, at their time of crystallization, which corresponds to (3.98±0.15)×10−7, (3.64±0.18)×10−7, and (5.92±0.59)×10−7, respectively. Combined with a newly determined absolute U-corrected Pb–Pb age for NWA 1670 of 4564.39±0.24 Ma and published U-corrected Pb–Pb ages for the other two angrites, this allows us to calculate an initial (26Al/27Al)0 of (1.33−0.18+0.21)×10−5 for the angrite parent body (APB) precursor material at the time of CAI formation, a value four times lower than the accepted canonical value of 5.25×10−5. Based on their similar 54Cr/52Cr ratios, most inner solar system materials likely accreted from material containing a similar 26Al/27Al ratio as the APB precursor at the time of CAI formation. To satisfy the abundant evidence for widespread planetesimal differentiation, the subcanonical 26Al budget requires that differentiated planetesimals, and hence protoplanets, accreted rapidly within 0.25±0.15 Ma of the formation of canonical CAIs.

TITANIUM ISOTOPE SOURCE RELATIONS AND THE EXTENT OF MIXING IN THE PROTO-SOLAR NEBULA EXAMINED BY INDEPENDENT COMPONENT ANALYSIS

The Ti isotope variations observed in hibonites represent some of the largest isotope anomalies observed in the solar system. Titanium isotope compositions have previously been reported for a wide variety of different early solar system materials, including calcium, aluminum rich inclusions (CAIs) and CM hibonite grains, some of the earliest materials to form in the solar system, and bulk meteorites which formed later. These data have the potential to allow mixing of material to be traced between many different regions of the early solar system. We have used independent component analysis to examine the mixing end-members required to produce the compositions observed in the different data sets. The independent component analysis yields results identical to a linear regression for the bulk meteorites. The components identified for hibonite suggest that most of the grains are consistent with binary mixing from one of three highly anomalous nucleosynthetic sources. Comparison of these end-members show that the sources which dominate the variation of compositions in the meteorite parent body forming regions was not present in the region in which the hibonites formed. This suggests that the source which dominates variation in Ti isotope anomalies between the bulk meteorites was not present when the hibonite grains were forming. One explanation is that the bulk meteorite source may not be a primary nucleosynthetic source but was created by mixing two or more of the hibonite sources. Alternatively, the hibonite sources may have been diluted during subsequent nebula processing and are not a dominant solar system signatures.

Formation and transformations of Fe-rich serpentines by asteroidal aqueous alteration processes: A nanoscale study of the Murray chondrite

Fe-rich serpentines are an abundant product of the early aqueous alteration events that affected the parent bodies of CM carbonaceous chondrites. Alteration assemblages in these meteorites show a large chemical variability and although water–rock interactions occurred under anoxic conditions, serpentines contain high amounts of ferric iron. To date very few studies have documented Fe valence variations in alteration assemblages of carbonaceous chondrites, limiting the understanding of the oxidation mechanisms. Here, we report results from a nanoscale study of a calcium–aluminum-rich inclusion (CAI) from the Murray chondrite, in which alteration resulted in Fe import and Ca export by the fluid phase and in massive Fe-rich serpentines formation. We combined scanning and transmission electron microscopies and scanning transmission X-ray microscopy for characterizing the crystal chemistry of Fe-serpentines. We used reference minerals with known crystallographic orientations to quantify the Fe valence state in Fe-rich serpentines using X-ray absorption spectroscopy at the Fe L2,3-edges, yielding a robust methodology that would prove valuable for studying oxidation processes in other terrestrial or extra-terrestrial cases of serpentinization. We suggest that aqueous Fe2+ was transported to the initially Fe-depleted CAI, where local changes in pH conditions, and possibly mineral catalysis by spinel promoted the partial oxidation of Fe2+ into Fe3+ by water and the formation of Fe-rich serpentines close to the cronstedtite endmember. Such mechanisms produce H2, which opens interesting perspectives as hydrogen may have reacted with carbon species, or escaped and yield increasingly oxidizing conditions in the parent asteroid. From the results of this nanoscale study, we also propose transformations of the initial cronstedtite, destabilized by later input of Al- and Mg-rich solutions, leading to Fe2+ leaching from serpentines, as well as to random serpentine-chlorite interstratifications. Such transformations towards polysomatic assemblages that are un-equilibrated from the structural, chemical and redox point of views are probably controlled by the various rates of alteration of primary minerals, but also by porosity gradients, as in terrestrial hydrothermal systems. We suggest that the proposed mechanisms may have played a role in the early formation of (Fe2+,Fe3+)-rich serpentines documented in CM chondrites, as well as in their transformation with on-going alteration towards Fe-poorer compositions inferred from previous petrologic, mineralogical and magnetic studies of CM chondrites.

Isotopic mass fractionation laws for magnesium and their effects on 26Al–26Mg systematics in solar system materials

Magnesium isotope ratios are known to vary in solar system objects due to the effects of 26Al decay to 26Mg and mass-dependent fractionation, but anomalies of nucleosynthetic origin must also be considered. In order to infer the amount of enhancement of 26Mg/24Mg due to 26Al decay or to resolve small nucleogenetic anomalies, the exact relationship between 26Mg/24Mg and 25Mg/24Mg ratios due to mass-dependent fractionation, the mass-fractionation “law”, must be accurately known so that the 25Mg/24Mg ratio can be used to correct the 26Mg/24Mg ratio for mass fractionation. Mass-dependent fractionation in mass spectrometers is reasonably well characterized, but not necessarily fully understood. It follows a simple power fractionation law, sometimes referred to as the “exponential law”. In contrast, mass fractionation in nature, in particular that due to high temperature evaporation that likely caused the relatively large effects observed in calcium-, aluminum-rich inclusions (CAIs), is reasonably well understood, but mass-fractionation laws for magnesium have not been explored in detail. The magnesium isotopic compositions of CAI-like evaporation residues produced in a vacuum furnace indicate that the slope on a log 25Mg/24Mg vs. log 26Mg/24Mg plot is ∼0.5128, and different from those predicted by any of the commonly used mass-fractionation laws. Evaporation experiments on forsterite-rich bulk compositions give exactly the same slope, indicating that the measured mass-fractionation law for evaporation of magnesium is applicable to a wide range of bulk compositions. We discuss mass-fractionation laws and the implications of the measured fractionation behavior of magnesium isotopes for 26Al–26Mg chronology.

Growth of calcium–aluminum-rich inclusions by coagulation and fragmentation in a turbulent protoplanetary disk: Observations and simulations

Whereas it is generally accepted that calcium–aluminum-rich inclusions (CAIs) from chondritic meteorites formed in a hot environment in the solar protoplanetary disk, the conditions of their formation remain debated. Recent laboratory studies of CAIs have provided new kind of data: their size distributions. We report that size distributions of CAIs measured in laboratory from sections of carbonaceous chondrites have a power law size distribution with cumulative size exponent between −1.7 and −1.9, which translates into cumulative size exponent between −2.5 and −2.8 after correction for sectioning. To explain these observations, numerical simulations were run to explore the growth of CAIs from micrometer to centimeter sizes, in a hot and turbulent protoplanetary disk through the competition of coagulation and fragmentation. We show that the size distributions obtained in growth simulations are in agreement with CAIs size distributions in meteorites. We explain the CAI sharp cut-off of their size distribution at centimeter sizes as the direct result from the famous fragmentation barrier, provided that CAI fragment for impact velocities larger than 10m/s. The growth/destruction timescales of millimeter- and centimeter-sized CAIs is inversely proportional to the local dust/gas ratio and is about 10years at 1300K and up to 104years at 1670K. This implies that the most refractory CAIs are expected to be smaller in size owing to their long growth timescale compared to less refractory CAIs. Conversely, the least refractory CAIs could have been recycled many times during the CAI production era which may have profound consequences for their radiometric age.

OUTWARD MOTION OF POROUS DUST AGGREGATES BY STELLAR RADIATION PRESSURE IN PROTOPLANETARY DISKS

We study the dust motion at the surface layer of protoplanetary disks. Dust grains in surface layer migrate outward due to angular momentum transport via gas-drag force induced by the stellar radiation pressure. In this study, we calculate mass flux of the outward motion of compact grains and porous dust aggregates by the radiation pressure. The radiation pressure force for porous dust aggregates is calculated using the T-Matrix Method for the Clusters of Spheres. First, we confirm that porous dust aggregates are forced by strong radiation pressure even if they grow to be larger aggregates in contrast to homogeneous and spherical compact grains to which efficiency of radiation pressure becomes lower when their sizes increase. In addition, we find that the outward mass flux of porous dust aggregates with monomer size of 0.1 $\mu$m is larger than that of compact grains by an order of magnitude at the disk radius of 1 AU, when their sizes are several microns. This implies that large compact grains like calcium-aluminum rich inclusions (CAIs) are hardly transported to outer region by stellar radiation pressure, whereas porous dust aggregates like chondritic-porous interplanetary dust particles (CP-IDPs) are efficiently transported to comet formation region. Crystalline silicates are possibly transported in porous dust aggregates by stellar radiation pressure from inner hot region to outer cold cometary region in the protosolar nebula.

Evidence for an early nitrogen isotopic evolution in the solar nebula from volatile analyses of a CAI from the CV3 chondrite NWA 8616

Nitrogen and noble gas (Ne–Ar) abundances and isotope ratios, determined by CO2 laser extraction static mass spectrometry analysis, as well as Al–Mg and O isotope data from secondary ion mass spectrometry (SIMS) analyses, are reported for a type B calcium–aluminum-rich inclusion (CAI) from the CV3 chondrite NWA 8616. The high (26Al/27Al)i ratio of (5.06±0.50)×10−5 dates the last melting event of the CAI at 39-99+109ka after “time zero”, limiting the period during which high-temperature exchanges between the CAI and the nebular gas could have occurred to a very short time interval. Partial isotopic exchange with a 16O-poor reservoir resulted in Δ17O>−5‰ for melilite and anorthite, whereas spinel and Al–Ti-pyroxene retain the inferred original 16O-rich signature of the solar nebula (Δ17O⩽−20‰). The low 20Ne/22Ne (⩽0.83) and 36Ar/38Ar (⩽0.75) ratios of the CAI rule out the presence of any trapped planetary or solar noble gases. Cosmogenic 21Ne and 38Ar abundances are consistent with a cosmic ray exposure (CRE) age of ∼14 to 20Ma, assuming CR fluxes similar to modern ones, without any evidence for pre-irradiation of the CAI before incorporation into the meteorite parent body. Strikingly, the CAI contains 1.4–3.4ppmN with a δ15N value of +8‰ to +30‰. Even after correcting the measured δ15N values for cosmogenic 15N produced in situ, the CAI is highly enriched in 15N compared to the protosolar nebula (δ15NPSN=−383±8‰; Marty et al., 2011), implying that the CAI-forming region was contaminated by 15N-rich material within the first 0.15Ma of Solar System history, or, alternatively, that the CAI was ejected into the outer Solar System where it interacted with a 15N-rich reservoir.

Origin and history of ureilitic material in the solar system: The view from asteroid 2008 TC3 and the Almahata Sitta meteorite

AbstractAsteroid 2008 TC3 (approximately 4 m diameter) was tracked and studied in space for approximately 19 h before it impacted Earth's atmosphere, shattering at 44–36 km altitude. The recovered samples (>680 individual rocks) comprise the meteorite Almahata Sitta (AhS). Approximately 50–70% of these are ureilites (ultramafic achondrites). The rest are chondrites, mainly enstatite, ordinary, and Rumuruti types. The goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid. Almahata Sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts. However, we calculate that the mass of fallen material was ≤0.1% of the pre‐atmospheric mass of the asteroid. Based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples, we infer that the lost material was mostly ureilitic. Therefore, 2008 TC3 probably contained only a few percent nonureilitic materials, similar to other polymict ureilites except less well consolidated. From available data for the AhS meteorite fragments, we conclude that 2008 TC3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites. We therefore suggest that the immediate parent of 2008 TC3 was the immediate parent of all ureilitic material sampled on Earth. We trace critical stages in the evolution of that material through solar system history. Based on various types of new modeling and re‐evaluation of published data, we propose the following scenario. (1) The ureilite parent body (UPB) accreted 0.5–0.6 Ma after formation of calcium‐aluminum‐rich inclusions (CAI), beyond the ice line (outer asteroid belt). Differentiation began approximately 1 Ma after CAI. (2) The UPB was catastrophically disrupted by a major impact approximately 5 Ma after CAI, with selective subsets of the fragments reassembling into daughter bodies. (3) Either the UPB (before breakup), or one of its daughters (after breakup), migrated to the inner belt due to scattering by massive embryos. (4) One daughter (after forming in or migrating to the inner belt) became the parent of 2008 TC3. It developed a regolith, mostly ≥3.8 Ga ago. Clasts of enstatite, ordinary, and Rumuruti‐type chondrites were implanted by low‐velocity collisions. (5) Recently, the daughter was disrupted. Fragments were injected or drifted into Earth‐crossing orbits. 2008 TC3 comes from outer layers of regolith, other polymict ureilites from deeper regolith, and main group ureilites from the interior of this body. In contrast to other models that have been proposed, this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 TC3 and other polymict ureilites.

Late formation of a comet Wild 2 crystalline silicate particle, Pyxie, inferred from Al–Mg chronology of plagioclase

We examined the Al–Mg isotope systematics of plagioclase in a FeO-poor ferromagnesian Wild 2 particle (C2092,7,81,1,0; named Pyxie) using a ∼2 μm spot. Three analyses show average 27Al/24Mg ratio of ∼65 and excess δ26Mg⁎ value of +0.1±4.5‰ (2σ), indicating no resolvable 26Mg excess in the particle. The inferred initial (26Al/27Al)0 ratio of plagioclase in Pyxie is estimated as (−0.6±4.5)×10−6 with an upper limit of 4×10−6. The result is very similar to that of the FeO-rich ferromagnesian particle “Iris” (Ogliore et al., 2012). Assuming homogeneous distribution of 26Al in the early solar system, Pyxie formed at least 2.6 Ma after the oldest Ca–Al-rich inclusions. This minimum formation age is marginally younger than formation ages of most chondrules in type ∼3.0 chondrites but comparable with those of Mg#<98 chondrules in CR3 chondrites. Considered in conjunction with similar oxygen isotope ratios between Pyxie (and Iris) and Mg#<98 chondrules in CR3 chondrites, it is inferred that the ferromagnesian Wild 2 particles and Mg#<98 chondrules in CR3 chondrites formed late in local disk environments that had similar oxygen isotope ratios and redox states.

LIGHT-ELEMENT NUCLEOSYNTHESIS IN A MOLECULAR CLOUD INTERACTING WITH A SUPERNOVA REMNANT AND THE ORIGIN OF BERYLLIUM-10 IN THE PROTOSOLAR NEBULA

The presence of short-lived radionuclides in the early solar system provides important information about the astrophysical environment in which the solar system formed. The discovery of now extinct $^{10}$Be in calcium-aluminum-rich inclusions (CAIs) with Fractionation and Unidentified Nuclear isotope anomalies (FUN-CAIs) suggests that a baseline concentration of $^{10}$Be in the early solar system was inherited from the protosolar molecular cloud. In this paper, we first show that the $^{10}$Be recorded in FUN-CAIs cannot have been produced in situ by cosmic-ray (CR) irradiation of the FUN-CAIs themselves. We then show that trapping of Galactic CRs (GCRs) in the collapsing presolar cloud core induced a negligible $^{10}$Be contamination of the protosolar nebula. Irradiation of the presolar molecular cloud by background GCRs produced a steady-state $^{10}$Be/$^9$Be ratio ~2.3 times lower than the ratio recorded in FUN-CAIs, which suggests that the presolar cloud was irradiated by an additional source of CRs. Considering a detailed model for CR acceleration in a supernova remnant (SNR), we find that the $^{10}$Be abundance recorded in FUN-CAIs can be explained within two alternative scenarios: (i) the irradiation of a giant molecular cloud by CRs produced by >50 supernovae exploding in a superbubble of hot gas generated by a large star cluster of at least 20,000 members and (ii) the irradiation of the presolar molecular cloud by freshly accelerated CRs escaped from an isolated SNR at the end of the Sedov-Taylor phase. The second model naturally provides an explanation for the injection of other short-lived radionuclides of stellar origin into the cold presolar molecular cloud ($^{26}$Al, $^{41}$Ca and $^{36}$Cl) and is in agreement with the solar system originating from the collapse of a molecular cloud shocked by a supernova blast wave.

The Petrology and Mineralogy Analysis of Noble Metal Alloys in the Inclusions of a Chondrite: An Implication on the Evolution of the Solar Nebula

The calcium-aluminium-rich inclusions (CAIs) in chondrites are believed to be the most early solids formed in the solar system, and retain the original information of the early solar nebula. However, in-depth researches revealed that most inclusions had experienced a complex history of evolution, including partial melting and secondary alteration. Focused on the refractory and chemically stable noble metal particles in the CAI of a CV meteorite (NWA 2140), a study of astrochemistry is made in this paper. The petrology and mineralogy analysis as well as the composition determination have been made on the noble metal particles. Based on the result of composition analysis, the thermodynamic process experienced by the CAI is inferred, and two kinds of noble metal alloys are identified, which correspond to the early-stage condensation products and the secondary alteration products of primordial metals, respectively.

지구의 나이: 재평가

이 논문은 지구의 나이를 알기 위한 초기의 다양한 시도를 간략히 소개하고, 운석의 납동위원소 연대측정으로 처음으로 지구의 나이가 <TEX>$4,550{\pm}70Ma$</TEX>임을 밝힌 Patterson (1956)의 연구를 재조명한다. 태양계 초기의 진화과정은 성운가스가 식으면서 응축되어 만들어지는 고체입자 -> 이들 입자들이 서로 들러붙어 커지는 첨합과정을 통하여 행성 크기의 물체가 생성되는 것으로 생각되고 있다. 또한 이때 생긴 원시지구가 화성 크기의 물체와 충돌하면서 그 잔해가 달을 만들었다고 생각되고 있다. 이 일련의 과정에서 지구가 생성된 시기를 꼭 집어 말하기 힘들긴 하나, 현재 지구는 이 충돌의 직접적인 결과로 생각할 수 있기 때문에 이 충돌시기를 지구의 나이로 정할 것을 제안한다. 기존 연구를 고려하면 충돌시기는 태양계에서 가장 먼저 만들어진 운석물질의 나이(즉, 태양계의 나이) <TEX>$4567.30{\pm}0.16Ma$</TEX>와 지구와 달 암석에서 가장 오래된 나이 <TEX>$4,456{\pm}40Ma$</TEX> 사이로 개략적으로 제한될 수 있다. 이 충돌시기는 태양계 초기 행성 크기의 물체가 만들어지는 시간간격을 밝히고, 충돌 이후 생성된 지구와 달의 마그마 바다의 열 역사를 규명하는데 매우 중요하기 때문에, 앞으로 보다 정확한 충돌시기를 측정하려는 노력이 요구된다. This paper presents a brief historical review of various attempts to estimate the age of the Earth, and reappraises the study of Patterson (1956) which revealed for the first time that the age of the Earth is <TEX>$4550{\pm}70Ma$</TEX> by measuring Pb isotope ratios of several meteorites and a marine sediment. The standard model for the planetary formation of early solar system is: formation of solid particles condensed from the cooling of hot nebular gas -> formation of planet-sized bodies by accretion of those solid particles. The Moon is supposed to have formed from the accretion of the relicts produced by the collision of proto-Earth with Mars-sized body. It is not easy to pinpoint the age of the Earth, considering the series of events related to the formation of the Earth. So, I propose that the collision age as that of the Earth, since the present status of the Earth is thought to be the direct product of the collision. According to the previous studies, the collision age can be broadly constrained between the age (<TEX>$4567.30{\pm}0.16Ma$</TEX>) of the earliest condensates (CAI, calcium-aluminum rich inclusion) of the nebula gas, i.e., the age of the solar system, and the oldest age (<TEX>$4,456{\pm}40Ma$</TEX>) among rocks and minerals of the Earth and the Moon. We need more precise estimation of the collision age, since it is important in estimating time scale for the formation of planet-size body and in revealing thermal evolution of magma oceans of the Earth and the Moon presumably developed right after the collision.

Samarium–neodymium chronology and rubidium–strontium systematics of an Allende calcium–aluminum-rich inclusion with implications for 146Sm half-life

Calcium–aluminum-rich inclusions (CAIs) are primitive objects that formed within the protoplanetary disk surrounding the young Sun. Recent Pb–Pb chronologic studies have demonstrated that CAIs are the oldest solar system solids, crystallizing 4567 Ma ago (Amelin et al., 2002; Connelly et al., 2012). The isotope systematics of CAIs therefore provide critical insight into the earliest history of the Solar System. Although Sm–Nd and Rb–Sr geochronometers are highly effective tools for investigating cosmochemical evolution in the early Solar System, previous studies of CAIs have revealed evidence for isotopically disturbed systems. Here we report new age data for Allende CAI Al3S4 derived from both the long-lived (147Sm–143Nd) and short-lived (146Sm–142Nd) isotopic systems. The 147Sm–143Nd chronometer yields an age of 4560±34 Ma that is concordant with 207Pb–206Pb ages for CAIs and indicates that the Sm–Nd system was not significantly disturbed by secondary alteration or nucleosynthetic processes. The slope of the 146Sm–142Nd isochron defines the Solar System initial 146Sm/144Sm of 0.00828±0.00044. This value is significantly different from the value of 0.0094 determined by Kinoshita et al. (2012). Ages recalculated from all published 146Sm–142Nd isochron data using the traditional 103 Ma half-life and the initial 146Sm/144Sm value determined here closely match Pb–Pb and 147Sm–143Nd ages determined on the same samples. In contrast, ages recalculated using the 68 Ma half-life determined by Kinoshita et al. (2012) and either of the initial 146Sm/144Sm values are often anomalously old. This is particularly true for the youngest samples with 146Sm–142Nd isochron ages that are most sensitive to the choice of 146Sm half-life used in the age calculation. In contrast to the Sm–Nd isotope system, the Rb–Sr system is affected by alteration but yields an apparent isochron with a slope corresponding to a much younger age of 4247±110 Ma. Although the Rb–Sr system in CAIs appears to be disturbed, the initial 87Sr/86Sr value determined from the isochron is 0.698942±0.000008, and closely approximates estimates of the initial Solar System value. Although this isochron may be a mixing line, it might also record alteration on the Allende parent body in which Rb was added to the Al3S4 CAI that was initially largely devoid of Rb.