Multiple Generations of Refractory Inclusions in the Metal‐Rich Carbonaceous Chondrites Acfer 182/214 and Isheyevo

Krot, Alexander N.; Nagashima, K.; Bizzarro, Martin; Huss, G. R.; Davis, A. M.; Meyer, B. S.; Ulyanov, A. A.

doi:10.1086/521973

Cited by 83 publications

(119 citation statements)

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“…Several observations suggest that these meteorites may have accreted a significant fraction of pristine material of possible outer Solar System origin. CH chondrites contain the highest abundance of early-formed 26 Al-poor CAIs (17); these solids are inferred to have been transported to larger radial distances from the Sun compared with the later-formed 26 Al-rich CAIs. In addition, CH chondrites have incorporated chondrite lithic clasts characterized by 15 N-rich (up to 1,000‰) compositions (18,19), which are thought to be indicative of lowtemperature environments that existed in the outer Solar System (20)(21)(22).…”

Section: Isotopic Evidence For Primordial Molecular Cloud Mattermentioning

confidence: 98%

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

Kooten

Wielandt

Schiller

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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170

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The short-lived 26 Al radionuclide is thought to have been admixed into the initially 26 Al-poor protosolar molecular cloud before or contemporaneously with its collapse. Bulk inner Solar System reservoirs record positively correlated variability in mass-independent 54 Cr and 26 Mg*, the decay product of 26 Al. This correlation is interpreted as reflecting progressive thermal processing of infalling 26 Al-rich molecular cloud material in the inner Solar System. The thermally unprocessed molecular cloud matter reflecting the nucleosynthetic makeup of the molecular cloud before the last addition of stellar-derived 26 Al has not been identified yet but may be preserved in planetesimals that accreted in the outer Solar System. We show that metal-rich carbonaceous chondrites and their components have a unique isotopic signature extending from an inner Solar System composition toward a 26 Mg*-depleted and 54 Cr-enriched component. This composition is consistent with that expected for thermally unprocessed primordial molecular cloud material before its pollution by stellar-derived 26 Al. The 26 Mg* and 54 Cr compositions of bulk metal-rich chondrites require significant amounts (25-50%) of primordial molecular cloud matter in their precursor material. Given that such high fractions of primordial molecular cloud material are expected to survive only in the outer Solar System, we infer that, similarly to cometary bodies, metal-rich carbonaceous chondrites are samples of planetesimals that accreted beyond the orbits of the gas giants. The lack of evidence for this material in other chondrite groups requires isolation from the outer Solar System, possibly by the opening of disk gaps from the early formation of gas giants. molecular cloud | outer Solar System | metal-rich chondrites | isotopes | chondrite accretion regions L ow-mass stars like our Sun form by the gravitational collapse of the densest parts of molecular clouds comprising stellarderived dust and gas. Collapsing clouds swiftly evolve into deeply embedded protostars that rapidly accrete material from their surrounding envelopes via a protoplanetary disk (1), in which planetesimals and planetary embryos form over timescales of several million years (2). Chondritic meteorites (chondrites) are fragments of early-formed planetesimals that avoided melting and differentiation and, therefore, provide a record of the earliest evolutionary stages of the Sun and its protoplanetary disk. Most chondrites contain calcium−aluminum-rich inclusions (CAIs) and chondrules, which formed by high-temperature processes that included evaporation, condensation, and melting during short-lived heating events (3). CAIs represent the oldest dated solids and, thus, define the age of the Solar System at 4,567.3 ± 0.16 Ma (4). It is inferred that CAIs formed near the proto-Sun during a brief time interval (<0

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Section: Isotopic Evidence For Primordial Molecular Cloud Mattermentioning

confidence: 98%

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

Kooten

Wielandt

Schiller

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

170

140

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“…Injection during the solar nebula stage (i.e., protostar Class II, equivalently, a classical T Tauri star) may not be optimal because the small radius of the disk (∼30-45 AU) requires a nearby (∼0.3 pc) SNII to provide sufficient 26 Al, that potentially would disintegrate the solar nebula (Gounelle & Meibom 2008). Furthermore, the CAI-forming epoch could have been as short as 2 × 10 4 -10 5 yr according to radiometric dating (Bizzarro et al 2004;Thrane et al 2006;Krot et al 2008), presumably because sufficiently high temperatures for CAI formation only were met during the Class 0 and early Class I stages. These stages only last for ∼10 4 yr and ∼10 5 yr (André & Montmerle 1994).…”

Section: Radioactive Aluminum-26 In the Outer Solar Systemmentioning

confidence: 99%

The primordial nucleus of comet 67P/Churyumov-Gerasimenko

Davidsson

Sierks

Güttler

et al. 2016

A&A

187

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Context. We investigate the formation and evolution of comet nuclei and other trans-Neptunian objects (TNOs) in the solar nebula and primordial disk prior to the giant planet orbit instability foreseen by the Nice model. Aims. Our goal is to determine whether most observed comet nuclei are primordial rubble-pile survivors that formed in the solar nebula and young primordial disk or collisional rubble piles formed later in the aftermath of catastrophic disruptions of larger parent bodies. We also propose a concurrent comet and TNO formation scenario that is consistent with observations. Methods. We used observations of comet 67P/Churyumov-Gerasimenko by the ESA Rosetta spacecraft, particularly by the OSIRIS camera system, combined with data from the NASA Stardust sample-return mission to comet 81P/Wild 2 and from meteoritics; we also used existing observations from ground or from spacecraft of irregular satellites of the giant planets, Centaurs, and TNOs. We performed modeling of thermophysics, hydrostatics, orbit evolution, and collision physics. Results. We find that thermal processing due to short-lived radionuclides, combined with collisional processing during accretion in the primordial disk, creates a population of medium-sized bodies that are comparably dense, compacted, strong, heavily depleted in supervolatiles like CO and CO 2 ; they contain little to no amorphous water ice, and have experienced extensive metasomatism and aqueous alteration due to liquid water. Irregular satellites Phoebe and Himalia are potential representatives of this population. Collisional rubble piles inherit these properties from their parents. Contrarily, comet nuclei have low density, high porosity, weak strength, are rich in supervolatiles, may contain amorphous water ice, and do not display convincing evidence of in situ metasomatism or aqueous alteration. We outline a comet formation scenario that starts in the solar nebula and ends in the primordial disk, that reproduces these observed properties, and additionally explains the presence of extensive layering on 67P/Churyumov-Gerasimenko (and on 9P/Tempel 1 observed by Deep Impact), its bi-lobed shape, the extremely slow growth of comet nuclei as evidenced by recent radiometric dating, and the low collision probability that allows primordial nuclei to survive the age of the solar system. Conclusions. We conclude that observed comet nuclei are primordial rubble piles, and not collisional rubble piles. We argue that TNOs formed as a result of streaming instabilities at sizes below ∼400 km and that ∼350 of these grew slowly in a low-mass primordial disk to the size of Triton, Pluto, and Eris, causing little viscous stirring during growth. We thus propose a dynamically cold primordial disk, which prevented mediumsized TNOs from breaking into collisional rubble piles and allowed the survival of primordial rubble-pile comets. We argue that comets formed by hierarchical agglomeration out of material that remained after TNO formation, and that this slow growth was a necessity...

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“…Isotopic analyses are much needed to gain additional constrains on the formation of CAIs in SaU 290. Krot et al (2008) inferred that some more refractory inclusions (composed mainly of hibonite, grossite, Al-rich pyroxene, perovskite, and gehlenitic melilite) in one CH chondrite and a CH/CB-like chondrite formed before less refractory inclusions on the basis of their Al-Mg isotopic system and oxygen isotopic compositions. Recently, Krot and Nagashima (2009) (Krot et al 2006a).…”

Section: Pristine Nature Of Refractory Inclusions In Sau 290 and Compmentioning

confidence: 99%

“…The latter two characteristics suggest that CAIs in CH chondrites are more refractory and primitive than those in other chondrites (e.g., CV3 and CO3). However, some investigators (MacPherson et al 1989;Kimura et al 1993;Weber et al 1995;Krot et al 2008) found that most grossiteand hibonite-bearing CAIs lack excess of 26 Mg, the decay product of short-lived radionuclide 26 Al. Furthermore, CH CAIs have a bimodal distribution of oxygen isotopic compositions (Sahijpal et al 1999;Krot et al 2008) whereas oxygen isotopic compositions are homogeneous within individual CAI (Kimura et al 1993;Sahijpal et al 1999).…”

Section: Introductionmentioning

confidence: 99%

“…These characteristics are different from those of CAIs found in other primitive carbonaceous chondrites (e.g., CV3). Recently, Krot et al (2008) suggested that 16 O-rich CAIs without excess of 26 Mg in CH chondrites probably formed prior to the injection of 26 Al into the solar system. To better understand these unusual features (e.g., petrographic, mineralogical, and isotopic) of CAIs in CH chondrites, more detailed and systematic investigations are needed for CH chondrites.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Refractory inclusions and aluminum‐rich chondrules in Sayh al Uhaymir 290 CH chondrite: Petrography and mineralogy

Zhang¹,

Hsu²

2009

Meteorit & Planetary Scien

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Abstract-Here we report the petrography, mineralogy, and bulk compositions of Ca,Al-rich inclusions (CAIs), amoeboid olivine aggregate (AOA), and Al-rich chondrules (ARCs) in Sayh al Uhaymir (SaU) 290 CH chondrite. Eighty-two CAIs (0.1% of the section surface area) were found. They are hiboniterich (9%), grossite-rich (18%), melilite ± spinel-rich (48%), fassaite ± spinel-rich (15%), and fassaiteanorthite-rich (10%) refractory inclusions. Most CAIs are rounded in shape and small in size (average = 40 µm). They are more refractory than those of other groups of chondrites. CAIs in SaU 290 might have experienced higher peak heating temperatures, which could be due to the formation region closer to the center of protoplanetary disk or have formed earlier than those of other groups of chondrites. In SaU 290, refractory inclusions with a layered texture could have formed by gas-solid condensation from the solar nebula and those with an igneous texture could have crystallized from melt droplets or experienced subsequent melting of pre-existing condensates from the solar nebula. One refractory inclusion represents an evaporation product of pre-existing refractory solid on the basis of its layered texture and melting temperature of constituting minerals. Only one AOA is observed (75 µm across). It consists of olivine, Al-diopside, anorthite, and minor spinel with a layered texture. CAIs and AOA show no significant low-temperature aqueous alteration. ARCs in SaU 290 consist of diopside, forsterite, anorthite, Al-enstatite, spinel, and mesostasis or glass. They can be divided into diopsiderich, Al-enstatite-rich, glass-rich, and anorthite-rich chondrules. Bulk compositions of most ARCs are consistent with a mixture origin of CAIs and ferromagnesian chondrules. Anorthite and Al-enstatite do not coexist in a given ARC, implying a kinetic effect on their formation.

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Multiple Generations of Refractory Inclusions in the Metal‐Rich Carbonaceous Chondrites Acfer 182/214 and Isheyevo

Abstract: Ca,Al-rich inclusions (CAIs) are believed to have formed by evaporation, condensation, and melting of the pre-existing solids during the earliest stages of the solar system evolution.

Cited by 83 publications

References 41 publications

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

The primordial nucleus of comet 67P/Churyumov-Gerasimenko

Refractory inclusions and aluminum‐rich chondrules in Sayh al Uhaymir 290 CH chondrite: Petrography and mineralogy

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