Circumstellar Hibonite and Corundum and Nucleosynthesis in Asymptotic Giant Branch Stars

We report the discovery of two hibonite grains (CaAl_(12)O_(19)) whose isotopic compositions show that they formed in the winds of red giant and asymptotic giant branch (AGB) stars. While hibonite is the second major phase (after corundum, Al_2O_3) expected to condense from stellar ejecta with C/O < 1, it has not previously been found. One circumstellar hibonite grain is highly enriched in ^(17)O and slightly depleted in ^(18)O relative to the solar composition and has large excesses in ^(26)Mg and ^(41)K, decay products of ^(26)Al and ^(41)Ca. The inferred initial values (^(26)Al/^(27)Al)0 ≈ 5 × 10^(-3) and (^(41)Ca/^(40)Ca)0 ≈ 1.5 × 10^(-4) of this grain are consistent with models of nucleosynthesis in an AGB star. The other hibonite is enriched in ^(17)O, strongly depleted in ^(18)O, shows no evidence of ^(41)Ca and formed with (^(26)Al/^(27)Al)0 ≈ 2 × 10^(-2). The low ^(18)O/^(16)O and very high (^(26)Al/^(27)Al)_0 may indicate substantial proton exposure during cool bottom processing in a low-mass parent star. The low upper limit on ^(41)Ca/^(40)Ca (≤ 3.2 × 10^(-5)) implies that little or no He-shell material had been dredged into the envelope when this grain formed. We also report isotopic compositions for 12 new circumstellar corundum grains. The compositions of 11 of these grains are consistent with current models for red giant and AGB stars. One corundum grain has extremely high ^(17)O/^(16)O and near-solar ^(18)O/^(16)O and may have formed in a star that was initially enriched in ^(17)O and ^(18)O

Clastic matrix in EH3 chondrites

Patches of clastic matrix (15 to 730 μm in size) constitute 4.9 vol% of EH3 Yamato (Y-) 691 and 11.7 vol% of EH3 Allan Hills (ALH) 81189. Individual patches in Y-691 consist of 1) ~25 vol% relatively coarse opaque grain fragments and polycrystalline assemblages of kamacite, schreibersite, perryite, troilite (some grains with daubréelite exsolution lamellae), niningerite, oldhamite, and caswellsilverite; 2) ~30 vol% relatively coarse silicate grains including enstatite, albitic plagioclase, silica and diopside; and 3) an inferred fine nebular component (~45 vol%) comprised of submicrometer-size grains. Clastic matrix patches in ALH 81189 contain relatively coarse grains of opaques (~20 vol%; kamacite, schreibersite, perryite and troilite) and silicates (~30 vol%; enstatite, silica and forsterite) as well as an inferred fine nebular component (~50 vol%). The O-isotopic composition of clastic matrix in Y-691 is indistinguishable from that of olivine and pyroxene grains in adjacent chondrules; both sets of objects lie on the terrestrial mass-fractionation line on the standard three-isotope graph. Some patches of fine-grained matrix in Y-691 have distinguishable bulk concentrations of Na and K, inferred to be inherited from the solar nebula. Some patches in ALH 81189 differ in their bulk concentrations of Ca, Cr, Mn, and Ni. The average compositions of matrix material in Y-691 and ALH 81189 are similar but not identical—matrix in ALH 81189 is much richer in Mn (0.23 ± 0.05 versus 0.07 ± 0.02 wt%) and appreciably richer in Ni (0.36 ± 0.10 versus 0.18 ± 0.05 wt%) than matrix in Y-691. Each of the two whole-rocks exhibits a petrofabric, probably produced by shock processes on their parent asteroid

Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System

Little is known about the origin of the spectral diversity of asteroids and what it says about conditions in the protoplanetary disk. Here, we show that samples returned from Cb-type asteroid Ryugu have Fe isotopic anomalies indistinguishable from Ivuna-type (CI) chondrites, which are distinct from all other carbonaceous chondrites. Iron isotopes, therefore, demonstrate that Ryugu and CI chondrites formed in a reservoir that was different from the source regions of other carbonaceous asteroids. Growth and migration of the giant planets destabilized nearby planetesimals and ejected some inward to be implanted into the Main Belt. In this framework, most carbonaceous chondrites may have originated from regions around the birthplaces of Jupiter and Saturn, while the distinct isotopic composition of CI chondrites and Ryugu may reflect their formation further away in the disk, owing their presence in the inner Solar System to excitation by Uranus and Neptune

Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample

Studies of material returned from Cb asteroid Ryugu have revealed considerable mineralogical and chemical heterogeneity, stemming primarily from brecciation and aqueous alteration. Isotopic anomalies could have also been affected by delivery of exogenous clasts and aqueous mobilization of soluble elements. Here, we show that isotopic anomalies for mildly soluble Cr are highly variable in Ryugu and CI chondrites, whereas those of Ti are relatively uniform. This variation in Cr isotope ratios is most likely due to physicochemical fractionation between 54Cr-rich presolar nanoparticles and Cr-bearing secondary minerals at the millimeter-scale in the bulk samples, likely due to extensive aqueous alteration in their parent bodies that occurred 5:2þ11::84 Ma after Solar System birth. In contrast, Ti isotopes were marginally affected by this process. Our results show that isotopic heterogeneities in asteroids are not all nebular or accretionary in nature but can also reflect element redistribution by water

Clinical Practice Guidelines for the Endoscopic Management of Peripancreatic Fluid Collections

Endoscopic ultrasonography-guided intervention has gradually become a standard treatment for peripancreatic fluid collections (PFCs). However, it is difficult to popularize the procedure in Korea because of restrictions on insurance claims regarding the use of endoscopic accessories, as well as the lack of standardized Korean clinical practice guidelines. The Korean Society of Gastrointestinal Endoscopy (KSGE) appointed a Task Force to develope medical guidelines by referring to the manual for clinical practice guidelines development prepared by the National Evidence-Based Healthcare Collaborating Agency. Previous studies on PFCs were searched, and certain studies were selected with the help of experts. Then, a set of key questions was selected, and treatment guidelines were systematically reviewed. Answers to these questions and recommendations were selected via peer review. This guideline discusses endoscopic management of PFCs and makes recommendations on Indications for the procedure, pre-procedural preparations, optimal approach for drainage, procedural considerations (e.g., types of stent, advantages and disadvantages of plastic and metal stents, and accessories), adverse events of endoscopic intervention, and procedural quality issues. This guideline was reviewed by external experts and suggests best practices recommended based on the evidence available at the time of preparation. This will be revised as necessary to address advances and changes in technology and evidence obtained in clinical practice and future studies

Origin of Halogens and Nitrogen in Enstatite Chondrites

The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may have had high C/O and/or pH2/pH2O ratios. Enstatite chondrites (particularly EH) have higher CI- and Mg-normalized abundances of halogens (especially F and Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials. It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of the halogens remained in the gas. Nitrogen may have been incorporated into the enstatite chondrites as simple nitrides that did not condense under the more-oxidizing conditions in the regions where other chondrite groups formed. Literature data show that unequilibrated enstatite chondrites have light bulk N (δ 15N ≈ −20‰) compared to most ordinary (−5 to +20‰) and carbonaceous (+20 to +190‰) chondrites; this may reflect the contribution in enstatite chondrites of nitride condensates with δ15 N values close to the proposed nebular mean (~−400‰). In contrast, N in carbonaceous chondrites is mainly contained within 15N-rich organic matter. The major carrier of N in ordinary chondrites is unknown

Origin of Halogens and Nitrogen in Enstatite Chondrites

The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may have had high C/O and/or pH2/pH2O ratios. Enstatite chondrites (particularly EH) have higher CI- and Mg-normalized abundances of halogens (especially F and Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials. It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of the halogens remained in the gas. Nitrogen may have been incorporated into the enstatite chondrites as simple nitrides that did not condense under the more-oxidizing conditions in the regions where other chondrite groups formed. Literature data show that unequilibrated enstatite chondrites have light bulk N (δ 15N ≈ −20‰) compared to most ordinary (−5 to +20‰) and carbonaceous (+20 to +190‰) chondrites; this may reflect the contribution in enstatite chondrites of nitride condensates with δ15 N values close to the proposed nebular mean (~−400‰). In contrast, N in carbonaceous chondrites is mainly contained within 15N-rich organic matter. The major carrier of N in ordinary chondrites is unknown

Presolar Corundum and Spinel in Ordinary Chondrites: Origins from AGB Stars and a Supernova

On the basis of anomalous isotopic compositions of oxygen and magnesium, 14 oxide grains from two primitive meteorites (Bishunpur and Semarkona) have been identified as circumstellar condensates. One corundum grain has a high ^(18)O/^(16)O ratio and isotopic compositions of magnesium, calcium, and titanium that are compatible with a formation in ejecta of a type II supernova that was about 15 times the mass of the sun. The other grains have oxygen, magnesium, and titanium compositions that are consistent with a formation around asymptotic giant branch (AGB) stars with a range of mass and initial composition. The large range of aluminum/magnesium in circumstellar corundum and spinel is considered to reflect various stages of back-reaction between condensed corundum and gaseous magnesium in cooling stellar ejecta