記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：European Association of Geochemistry  

DOI： 10.7185/geochemlet.2341

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

The nucleosynthetic isotope composition of planetary materials provides a record of the heterogeneous distribution of stardust within the early solar system. In 2020 December, the Japan Aerospace Exploration Agency Hayabusa2 spacecraft returned to Earth the first samples of a primitive asteroid, namely, the Cb-type asteroid Ryugu. This provides a unique opportunity to explore the kinship between primitive asteroids and carbonaceous chondrites. We report high-precision μ²⁶Mg* and μ²⁵Mg values of Ryugu samples together with those of CI, CM, CV, and ungrouped carbonaceous chondrites. The stable Mg isotope composition of Ryugu aliquots defines μ²⁵Mg values ranging from –160 ± 20 ppm to –272 ± 30 ppm, which extends to lighter compositions relative to Ivuna-type (CI) and other carbonaceous chondrite groups. We interpret the μ²⁵Mg variability as reflecting heterogeneous sampling of a carbonate phase hosting isotopically light Mg (μ²⁵Mg ∼ –1400 ppm) formed by low temperature equilibrium processes. After correcting for this effect, Ryugu samples return homogeneous μ²⁶Mg* values corresponding to a weighted mean of 7.1 ± 0.8 ppm. Thus, Ryugu defines a μ²⁶Mg* excess relative to the CI and CR chondrite reservoirs corresponding to 3.8 ± 1.1 and 11.9 ± 0.8 ppm, respectively. These variations cannot be accounted for by in situ decay of ²⁶Al given their respective ²⁷Al/²⁴Mg ratios. Instead, it requires that Ryugu and the CI and CR parent bodies formed from material with a different initial ²⁶Al/²⁷Al ratio or that they are sourced from material with distinct Mg isotope compositions. Thus, our new Mg isotope data challenge the notion that Ryugu and CI chondrites share a common nucleosynthetic heritage.

DOI： 10.3847/2041-8213/ad09d9

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その他リンク： https://iopscience.iop.org/article/10.3847/2041-8213/ad09d9/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Association for the Advancement of Science (AAAS)  

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 ⁵⁴ Cr-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 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.

DOI： 10.1126/sciadv.adi7048

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

We present oxygen isotopic analyses of fragments of the near-Earth C_b-type asteroid Ryugu returned by the Hayabusa2 spacecraft that reinforce the close correspondence between Ryugu and CI chondrites. Small differences between Ryugu samples and CI chondrites in ${ { \rm{\Delta } } }^{ { \prime} 17}{\rm{O } }$ can be explained at least in part by contamination of the latter by terrestrial water. The discovery that a randomly sampled C-complex asteroid is composed of CI-chondrite-like rock, combined with thermal models for formation prior to significant decay of the short-lived radioisotope ²⁶Al, suggests that if lithified at the time of alteration, the parent body was small (≪50 km radius). If the parent planetesimal was large (>50 km in radius), it was likely composed of high-permeability, poorly lithified sediment rather than consolidated rock.

DOI： 10.3847/psj/acea62

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その他リンク： https://iopscience.iop.org/article/10.3847/PSJ/acea62/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Association for the Advancement of Science (AAAS)  

Preliminary analyses of asteroid Ryugu samples show kinship to aqueously altered CI (Ivuna-type) chondrites, suggesting similar origins. We report identification of C-rich, particularly primitive clasts in Ryugu samples that contain preserved presolar silicate grains and exceptional abundances of presolar SiC and isotopically anomalous organic matter. The high presolar silicate abundance (104 ppm) indicates that the clast escaped extensive alteration. The 5 to 10 times higher abundances of presolar SiC (~235 ppm), N-rich organic matter, organics with N isotopic anomalies (1.2%), and organics with C isotopic anomalies (0.2%) in the primitive clasts compared to bulk Ryugu suggest that the clasts formed in a unique part of the protoplanetary disk enriched in presolar materials. These clasts likely represent previously unsampled outer solar system material that accreted onto Ryugu after aqueous alteration ceased, consistent with Ryugu’s rubble pile origin.

DOI： 10.1126/sciadv.adh1003

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

DOI： 10.1038/s41561-023-01226-y

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その他リンク： https://www.nature.com/articles/s41561-023-01226-y

掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

Rock fragments of the Cb-type asteroid Ryugu returned to Earth by the JAXA Hayabusa2 mission share mineralogical, chemical, and isotopic properties with the Ivuna-type (CI) carbonaceous chondrites. Similar to CI chondrites, these fragments underwent extensive aqueous alteration and consist predominantly of hydrous minerals likely formed in the presence of liquid water on the Ryugu parent asteroid. Here we present an in situ analytical survey performed by secondary ion mass spectrometry from which we have estimated the D/H ratio of Ryugu’s hydrous minerals, D/H_Ryugu, to be [165 ± 19] × 10⁻⁶, which corresponds to δD_Ryugu = +59 ± 121‰ (2σ). The hydrous mineral D/H_Ryugu’s values for the two sampling sites on Ryugu are similar; they are also similar to the estimated D/H ratio of hydrous minerals in the CI chondrites Orgueil and Alais. This result reinforces a link between Ryugu and CI chondrites and an inference that Ryugu’s samples, which avoided terrestrial contamination, are our best proxy to estimate the composition of water at the origin of hydrous minerals in CI-like material. Based on this data and recent literature studies, the contribution of CI chondrites to the hydrogen of Earth’s surficial reservoirs is evaluated to be ∼3%. We conclude that the water responsible for the alteration of Ryugu’s rocks was derived from water ice precursors inherited from the interstellar medium; the ice partially re-equilibrated its hydrogen with the nebular H₂ before being accreted on the Ryugu’s parent asteroid.

DOI： 10.3847/2041-8213/acc393

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その他リンク： https://iopscience.iop.org/article/10.3847/2041-8213/acc393/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Japan Association of Mineralogical Sciences  

DOI： 10.2465/jmps.221201

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掲載種別：研究論文（学術雑誌）   出版者・発行元：American Association for the Advancement of Science (AAAS)  

The extraterrestrial materials returned from asteroid (162173) Ryugu consist predominantly of low-temperature aqueously formed secondary minerals and are chemically and mineralogically similar to CI (Ivuna-type) carbonaceous chondrites. Here, we show that high-temperature anhydrous primary minerals in Ryugu and CI chondrites exhibit a bimodal distribution of oxygen isotopic compositions: ¹⁶ O-rich (associated with refractory inclusions) and ¹⁶ O-poor (associated with chondrules). Both the ¹⁶ O-rich and ¹⁶ O-poor minerals probably formed in the inner solar protoplanetary disk and were subsequently transported outward. The abundance ratios of the ¹⁶ O-rich to ¹⁶ O-poor minerals in Ryugu and CI chondrites are higher than in other carbonaceous chondrite groups but are similar to that of comet 81P/Wild2, suggesting that Ryugu and CI chondrites accreted in the outer Solar System closer to the accretion region of comets.

DOI： 10.1126/sciadv.ade2067

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

Initial analyses showed that asteroid Ryugu’s composition is close to CI (Ivuna-like) carbonaceous chondrites (CCs) – the chemically most primitive meteorites, characterized by near-solar abundances for most elements. However, some isotopic signatures (for example, Ti, Cr) overlap with other CC groups, so the details of the link between Ryugu and the CI chondrites are not yet fully clear. Here we show that Ryugu and CI chondrites have the same zinc and copper isotopic composition. As the various chondrite groups have very distinct Zn and Cu isotopic signatures, our results point at a common genetic heritage between Ryugu and CI chondrites, ruling out any affinity with other CC groups. Since Ryugu’s pristine samples match the solar elemental composition for many elements, their Zn and Cu isotopic compositions likely represent the best estimates of the solar composition. Earth’s mass-independent Zn isotopic composition is intermediate between Ryugu/CC and non-carbonaceous chondrites (NCs), suggesting a contribution of Ryugu-like material to Earth’s budgets of Zn and other moderately volatile elements.

DOI： 10.1038/s41550-022-01846-1

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その他リンク： https://www.nature.com/articles/s41550-022-01846-1

掲載種別：研究論文（学術雑誌）   出版者・発行元：American Association for the Advancement of Science (AAAS)  

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.

DOI： 10.1126/sciadv.add8141

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掲載種別：研究論文（学術雑誌）   出版者・発行元：European Association of Geochemistry  

DOI： 10.7185/geochemlet.2238

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Astronomical Society  

Abstract

We have conducted a NanoSIMS-based search for presolar material in samples recently returned from C-type asteroid Ryugu as part of JAXA's Hayabusa2 mission. We report the detection of all major presolar grain types with O- and C-anomalous isotopic compositions typically identified in carbonaceous chondrite meteorites: 1 silicate, 1 oxide, 1 O-anomalous supernova grain of ambiguous phase, 38 SiC, and 16 carbonaceous grains. At least two of the carbonaceous grains are presolar graphites, whereas several grains with moderate C isotopic anomalies are probably organics. The presolar silicate was located in a clast with a less altered lithology than the typical extensively aqueously altered Ryugu matrix. The matrix-normalized presolar grain abundances in Ryugu are ${4.8}_{-2.6}^{+4.7}$ ppm for O-anomalous grains, ${25}_{-5}^{+6}$ ppm for SiC grains, and ${11}_{-3}^{+5}$ ppm for carbonaceous grains. Ryugu is isotopically and petrologically similar to carbonaceous Ivuna-type (CI) chondrites. To compare the in situ presolar grain abundances of Ryugu with CI chondrites, we also mapped Ivuna and Orgueil samples and found a total of 15 SiC grains and 6 carbonaceous grains. No O-anomalous grains were detected. The matrix-normalized presolar grain abundances in the CI chondrites are similar to those in Ryugu: ${23}_{-6}^{+7}$ ppm SiC and ${9.0}_{-3.6}^{+5.4}$ ppm carbonaceous grains. Thus, our results provide further evidence in support of the Ryugu–CI connection. They also reveal intriguing hints of small-scale heterogeneities in the Ryugu samples, such as locally distinct degrees of alteration that allowed the preservation of delicate presolar material.

DOI： 10.3847/2041-8213/ac83bd

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その他リンク： https://iopscience.iop.org/article/10.3847/2041-8213/ac83bd/pdf

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Association for the Advancement of Science (AAAS)  

Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measure the mineralogy, bulk chemical and isotopic compositions of Ryugu samples. They are mainly composed of materials similar to carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37 ± 10°C, (Stat.) (Syst.) million years after formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles the Sun’s photosphere than other natural samples do.

DOI： 10.1126/science.abn7850

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier BV  

The end-Triassic mass extinction (ETE) is thought to have been triggered by widespread eruption and emissions of volcanic and contact metamorphic carbon and sulfur gases from the Central Atlantic Magmatic Province (CAMP) flood basalts. Although palynological studies of shallow marine deposits in the basins of Denmark and Germany suggest that deforestation and catastrophic soil loss occurred during the CAMP volcanism and ETE interval, the intensity and timing of continental weathering in the region of northwestern Tethys Ocean remain unclear. Here we present Sr, C, and O isotope data, as well as multivariate statistical analyses of major element contents, for carbonate-clastic deposits in the Kardolina section, Slovakia, to develop a continental weathering record in the NW Tethys during the ETE event. This section consists of a shallow marine carbonate sequence in the Rhaetian Fatra Formation and the overlying Hettangian Kopieniec Formation. The Fatra Formation represents ramp facies deposits that formed in a restricted pull-apart basin of the Fatric Zone along the southern margin of the Bohemian Massif in the region of northwestern Tethys. Carbon isotope analysis of the limestones revealed two negative carbon isotope excursions (NCIEs) in the uppermost Fatra Formation. These two NCIEs occurred in the latest Rhaetian and can be compared to the "precursor" and "initial" NCIEs reported for the northwestern Tethys. Strontium isotope analysis of the limestones revealed an abrupt increase in Sr isotope ratios between the precursor and initial NCIEs, which indicates that continental weathering of the Bohemian Massif increased rapidly in the latest Rhaetian. Multivariate analysis of major element contents in the carbonate rocks also shows that intense chemical weathering of the hinterland was accelerated after the precursor NCIE, supporting the inferences drawn from Sr isotopes. Furthermore, our study reveals that the carbonate depositional environments of the Fatra Formation changed to the formation of specific Fe-rich oolites with increased continental weathering after the precursor NCIE. The concentrations of redox-sensitive major elements (e.g., Mn and Fe) and multivariate analysis of the major element data suggest that the Fe-rich oolites were formed by an influx of O-depleted water into the shallow water depositional environment of the Fatra Formation. A possible origin for this O-depleted water is the oxygen minimum zone (OMZ) that formed at intermediate water depths in European basins (e.g., the German and Eiberg basins) during the latest Rhaetian. Our results suggest that the marine environment in the European basins may have developed an OMZ due to the increase in continental weathering during the latest Rhaetian, and these environmental changes may have had an important role in the marine ETE event.

DOI： 10.1016/j.palaeo.2022.110934

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Wiley  

DOI： 10.1002/ecs2.3921

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/ecs2.3921

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Wiley  

DOI： 10.1111/iar.12389

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of the National Academy of Sciences  

Dynamic models of the protoplanetary disk indicate there should be large-scale material transport in and out of the inner Solar System, but direct evidence for such transport is scarce. Here we show that the ε⁵⁰Ti-ε⁵⁴Cr-Δ¹⁷O systematics of large individual chondrules, which typically formed 2 to 3 My after the formation of the first solids in the Solar System, indicate certain meteorites (CV and CK chondrites) that formed in the outer Solar System accreted an assortment of both inner and outer Solar System materials, as well as material previously unidentified through the analysis of bulk meteorites. Mixing with primordial refractory components reveals a “missing reservoir” that bridges the gap between inner and outer Solar System materials. We also observe chondrules with positive ε⁵⁰Ti and ε⁵⁴Cr plot with a constant offset below the primitive chondrule mineral line (PCM), indicating that they are on the slope ∼1.0 in the oxygen three-isotope diagram. In contrast, chondrules with negative ε⁵⁰Ti and ε⁵⁴Cr increasingly deviate above from PCM line with increasing δ¹⁸O, suggesting that they are on a mixing trend with an ordinary chondrite-like isotope reservoir. Furthermore, the Δ¹⁷O-Mg# systematics of these chondrules indicate they formed in environments characterized by distinct abundances of dust and H₂O ice. We posit that large-scale outward transport of nominally inner Solar System materials most likely occurred along the midplane associated with a viscously evolving disk and that CV and CK chondrules formed in local regions of enhanced gas pressure and dust density created by the formation of Jupiter.

DOI： 10.1073/pnas.2005235117

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その他リンク： https://syndication.highwire.org/content/doi/10.1073/pnas.2005235117

記述言語：日本語  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.scitotenv.2019.135449

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Elsevier B.V.  

The Late Triassic Cimmerian Orogeny was a result of the final closure of the Palaeotethys Ocean and the accretion of Gondwana-derived (Cimmerian) continents to southern Eurasia. Determining the timing of the Cimmerian Orogeny is crucial to our understanding of the large-scale climate changes driven by the uplift of the Cimmerian Mountains. Here we present stratigraphic variations in 87Sr/86Sr values of Upper Triassic pelagic limestone from the Pizzo Mondello section, Sicily, Italy, that constrain the timing of uplift of the Cimmerian Mountains. The 87Sr/86Sr values remain relatively constant in the lower part of the section, decreasing slightly in the Tuvalian (upper Carnian) and Lacian (lower Norian). However, 87Sr/86Sr ratios rise sharply at the Lacian–Alaunian (lower–middle Norian) boundary and continue to rise through to the Sevatian (upper Norian). This observation indicates an increased input of radiogenic strontium derived from continental weathering, which resulted from the rapid uplift and erosion of the Cimmerian Mountains at this time. The climatic and environmental changes following the uplift of the Cimmerian Mountains provide an explanation for (1) an intense sea-surface-temperature warming event in the western Tethys Ocean, and (2) a rapid increase in precipitation on the northern coast of the Tethys during the Alaunian–Sevatian.

DOI： 10.1016/j.palaeo.2018.03.025

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER WIEN  

To clarify the processes that occur in hydrous basaltic magma chambers, we have undertaken detailed petrological and geochemical analyses of mafic and intermediate rocks from the Ikoma gabbroic complex, southwest Japan. The complex consists mainly of hornblende gabbros, hornblende gabbronorites, and hornblende leucogabbros. The hornblende leucogabbros are characterized by low TiO2 and high CaO contents, whereas the hornblende gabbronorites have high TiO2 and low CaO contents. The initial Sr-87/Sr-86 ratios (Sr-I) of the hornblende gabbronorites and hornblende gabbros are higher than those of the hornblende leucogabbros and plagioclase, and they may have resulted from a higher degree of assimilation of metasediments. The geochemical features of the hornblende leucogabbros and hornblende gabbronorites can be explained by accumulation of plagioclase and ilmenite, respectively, in a hybrid magma that formed by chemical interaction between mafic magma and metasediment, whereas the hornblende gabbros were produced by a high degree of crustal assimilation and fractional crystallization of this hybrid magma. As a result of the density differences between crystals and melt, the Ikoma gabbroic rocks formed by the accumulation of plagioclase in the middle of the magma chamber and by the accumulation of ilmenite in the bottom of the chamber. Taking into account the subsequent assimilation and fractional crystallization, our observations suggest an enriched mantle (Sr-I = similar to 0.7071) as the source material for the Ikoma gabbros.

DOI： 10.1007/s00710-016-0423-9

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

The Black Bear Ridge section in northeastern British Columbia, Canada, consists of a continuously exposed sequence of Upper Carnian through Lower Norian (Upper Triassic) continental margin strata. The section has been proposed as a candidate Global Stratotype Section and Point (GSSP) for the Carnian/Norian boundary (CNB). In order to assess Late Camian to Early Norian environmental changes recorded in the section, we examined stratigraphic variations in Sr-87/Sr-86, delta C-13, and delta O-18 values, and also values of redox sensitive elements (V. Ni and Cr), in the CNB interval. The study section is located along the north shore of Williston Lake, northeastern British Columbia. The Black Bear Ridge section was deposited in a distal ramp environment on the passive western margin of the North American craton.
The strata across the CNB display a positive shift in delta C-13 values and a corresponding increase in the redox indices V/(V + Ni) and V/Cr. The synchronous increase in delta C-13 values and redox indices suggests that burial rates of marine organic carbon increased in response to the development of anoxic conditions in the water column. An increase in delta C-13 values in carbonate rocks across the CNB has also been reported from Upper Triassic sections in the western Tethys (e.g., in the Pizzo Mondello section, Sicily), which suggests that the development of anoxic conditions within the CNB interval was widespread, affecting both the Panthalassan Ocean and Tethyan Sea. The geochemical data from this study, as well as from research into conodont biostratigraphy in the Black Bear Ridge section, show that the onset of oceanic anoxic conditions may have been responsible for the faunal turnover event at the CNB. The cause of this anoxic event is unknown, but the Sr-87/Sr-85 and delta C-13 isotope data largely exclude the possibility that the event was triggered by dissociation of methane hydrates and degassing related to large-scale volcanic activity. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.palaeo.2015.10.008

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：AMER GEOPHYSICAL UNION  

Pb, Nd, and Sr isotopic compositions and the (40)Ar-(39)Ar ages of lavas from seamounts in the Joban seamount chain, which is the northernmost Cretaceous seamount chain in the northwest Pacific, suggest that these seamounts were produced by HIMU-type (high-mu or high(-238U)/(204P)b) magmatism around 120 Ma. In addition, seamount chains in the western Pacific, which are coeval with the Joban seamount chain, also exhibit a HIMU isotopic signature. Since the seamount chains in the western Pacific are considered to have formed in the present-day South Pacific region, these isotopic features suggest that HIMU-type magmatism must have been active in the South Pacific since the Cretaceous. A notable geochemical characteristic of this magmatism is a correlation between the elemental ratios (La/Yb, Sm/Yb, Sr/Y, Nb/Zr, Ta/Zr, and Th/Ta) of the seamounts/islands and a "relative age" that is an index of the thickness of the lithosphere beneath hot spots. These correlations suggest the importance of the tectonic environment in determining the chemical composition of the magmas. They also suggest a genetic relationship between oceanic plateaus and island/seamount chains because the elemental ratios of oceanic plateaus follow the same trends. Another important geochemical feature is that the elemental ratios of mid-ocean ridge basalts (MORBs) from the East Pacific Rise (EPR) do not follow these trends. Since the source material for EPR MORBs is probably representative of the composition of the upper mantle beneath the South Pacific, this difference could imply an upward flow of material from the deep mantle.

DOI： 10.1029/2011GC003531

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP PUBLISHING LTD  

High-precision Cr isotope ratios for chondrules and metal grain separated from CB chondrite Gujba were determined. The epsilon Cr-54 values (epsilon Cr-i = [(Cr-i/Cr-52)(sample)/(Cr-i/Cr-52)(standard) - 1] x 104) for all samples were identical within the analytical uncertainty, with a mean value of +1.29 +/- 0.02. Uniform epsilon Cr-54 signatures of both chondrules and metal grains imply that the Cr isotope systematics of the meteorite was once completely equilibrated. The epsilon Cr-53 values of the chondrules and metal grain, on the other hand, display a strong correlation with the Mn-55/Cr-52 ratio. The Mn-53/Mn-55 calculated from the slope of the isochron is (3.18 +/- 0.52) x 10(-6). This corresponds to absolute ages of 4563.7 +/- 1.2 Ma and 4563.5 +/- 1.1 Ma using angrites D'Orbigny and LEW 86010, respectively, as time anchors. These ages are consistent with the ages obtained using other short-and long-lived radio nuclides, supporting the uniform distribution of Mn-53 in the early solar nebula.

DOI： 10.1088/0004-637X/723/1/20

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担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP PUBLISHING LTD  

The standard planetary formation models assume that primitive materials, such as carbonaceous chondrites, are the precursor materials of evolved planetesimals. Past chronological studies have revealed that planetesimals of several hundred kilometers in size, such as the Howardite-Eucrite-Diogenite (HED) parent body (Vesta) and angrite parent body, began their differentiation as early as similar to 3 million years of the solar system formation, and continued for at least several million years. However, the timescale of planetesimal formation in distinct regions of the inner solar system, as well as the isotopic characteristics of the reservoirs from which they evolved, remains unclear. Here we present the first report for the precise (53)Mn-(53)Cr ages of monomict ureilites. Chemically separated phases from one monomict ureilite (NWA 766) yielded the Mn-Cr age of 4564.60 +/- 0.67 Ma, identical within error to the oldest age preserved in other achondrites, such as angrites and eucrites. The (54)Cr isotopic data for this and seven additional bulk ureilites show homogeneous epsilon(54)Cr of similar to-0.9, a value distinct from other achondrites and chondrites. Using the epsilon(54)Cr signatures of Earth, Mars, and Vesta (HED), we noticed a linear decrease in the epsilon(54)Cr value with the heliocentric distance in the inner region of the solar system. If this trend can be extrapolated into the outer asteroid belt, the epsilon(54)Cr signatures of monomict ureilites will place the position of the ureilite parent body at similar to 2.8 AU. These observations imply that the differentiation of achondrite parent bodies began nearly simultaneously at similar to 4565 Ma in different regions of the inner solar system. The distinct epsilon(54)Cr value between ureilite and carbonaceous chondrite also implies that a genetic link commonly proposed between the two is unlikely.

DOI： 10.1088/0004-637X/720/1/150

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

A new determination method for Cr, Mn, Fe, Co and Ni in silicate samples by an isotope dilution-internal standardization method (ID-IS)-high resolution (HR)-MC-ICP-MS has been developed. A silicate sample is dissolved into solution, and a Cr-spike solution is added into an aliquot of the sample solution. Then the solution is introduced into HR-MC-ICP-MS without chemical separation. Concentration of Cr is determined by ID, and those of Mn, Fe, Co and Ni are by ID-IS. Matrix effects were examined in detail, and negligible down to a dilution factor (DF; the weight of the sample solution over that of the sample in the solution) of similar to 3000 for basalt, and similar to 10(4) for peridotite and chondrite within an error range of +/-1%. ID-IS-FIR-MC-ICP-MS showed intermediate precision of similar to 1% for basalt and chondrite, which is comparable to ID-TIMS for Cr, Fe and Ni, and >10 times better than HR-ICP-MS, suggesting that this method is suitable for precise Mn-Cr and Fe-Ni chronometers. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.chemgeo.2010.03.018

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担当区分：筆頭著者   記述言語：日本語  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：GEOCHEMICAL SOC JAPAN  

A total evaporation negative thermal ionization mass spectrometry (TE-N-TIMS) technique for the isotopic analysis of chlorine was developed. This technique provides fast and reliable way to determine the isotopic signature of chlorine samples as small as 100 ng. Compared to the conventional N-TIMS method, the precision of the Cl isotopic analysis is improved by a factor of up to 3 by minimizing the effect of mass fractionation. Using this method, reproducibility of 0.9 parts per thousand (R.S.D.: n = 25) can be achieved for Cl-37/Cl-35 ratio of 200 ng Cl. The analyzed results of the AgCl reagent expressed as a per-mil deviation (delta Cl-37) relative to the Standard Mean Ocean Chloride showed good concordance with the value obtained by conventional positive thermal ionization mass spectrometry (P-TIMS).

DOI： 10.2343/geochemj.1.0053

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担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：AMER CHEMICAL SOC  

A sequential chemical separation technique for Cr, Fe, Ni, Zn, and Cu in terrestrial and extraterrestrial silicate rocks was developed for precise and accurate determination of elemental concentration by the isotope dilution method (ID). The technique uses a combination of cation-anion exchange chromatography and Eichrom nickel specific resin. The method was tested using a variety of matrixes including bulk meteorite (Allende), terrestrial peridotite (JP-1), and basalt (JB-1b). Concentrations of each element was determined by thermal ionization mass spectrometry (TIMS) using W filaments and a Si-B-Al type activator for Cr, Fe, Ni, and Zn and a Re filament and silicic acid-H(3)PO(4) activator for Cu. The method can be used to precisely determine the concentrations of these elements in very small silicate samples, including meteorites, geochemical reference samples, and mineral standards for microprobe analysis. Furthermore, the Cr mass spectrometry procedure developed in this study can be extended to determine the isotopic ratios of (53)Cr/(52)Cr and (54)Cr/(52)Cr with precision of similar to 0.05 epsilon and similar to 0.10 epsilon (1 epsilon = 0.01%), respectively, enabling cosmochemical applications such as high precision Mn-Cr chronology and investigation of nucleosynthetic isotopic anomalies in meteorites.

DOI： 10.1021/ac901762a

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記述言語：英語   出版者・発行元：国立極地研究所  

We have undertaken Sm-Nd isotopic studies on Yamato-793605 lherzolitic shergottite. The Sm-Nd internal isochron obtained for acid leachates and residues of whole-rock and separated mineral fractions yields an age of 185±16Ma with an initial εNd value of +9.7±0.2. The obtained Sm-Nd age is, within analytical errors, identical to the Rb-Sr age of this meteorite as well as to the previous Rb-Sr and Sm-Nd ages of Allan Hills-77005 and Lewis Cliff 88516, although the ε_(Nd) values are not identical to each other. Elemental abundances of lithophile trace elements remain nearly unaffected by aqueous alteration on the Martian surface. The isotopic systems of lherzolitic shergottites, thus, are considered to be indigenous, although disturbances by shock metamorphism are clearly observed. "Young ages of ～180Ma" have been consistently obtained from this and previous Rb-Sr, Sm-Nd and U-Pb isotopic studies and appear to represent crystallization events.

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：NATURE PUBLISHING GROUP  

Determining the chronology for the assembly of planetary bodies in the early Solar System is essential for a complete understanding of star- and planet-formation processes. Various radionuclide chronometers (applied to meteorites) have been used to determine that basaltic lava flows on the surface of the asteroid Vesta formed within 3 million years (3 Myr) of the origin of the Solar System(1-3). Such rapid formation is broadly consistent with astronomical observations of young stellar objects, which suggest that formation of planetary systems occurs within a few million years after star formation(4,5). Some hafnium-tungsten isotope data, however, require that Vesta formed later 6 (similar to16 Myr after the formation of the Solar System) and that the formation of the terrestrial planets took a much longer time(7-10) (62(-14)(+4504) Myr). Here we report measurements of tungsten isotope compositions and hafnium-tungsten ratios of several meteorites. Our measurements indicate that, contrary to previous results(7-10), the bulk of metal-silicate separation in the Solar System was completed within <30 Myr. These results are completely consistent with other evidence for rapid planetary formation(1-5), and are also in agreement with dynamic accretion models(11-13) that predict a relatively short time (∼10 Myr) for the main growth stage of terrestrial planet formation.

DOI： 10.1038/nature00995

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：NATURE PUBLISHING GROUP  

Variations in the isotopic composition of some components in primitive meteorites demonstrate that the pre-solar material was not completely homogenized, nor was it processed at sufficiently high temperatures to erase the signatures of the diverse stellar sources(1). This is in accord with the observation that accretion disks of young stellar objects are at relatively low temperatures(2). Carbonaceous chondrites are considered to represent the 'average' Solar System composition; the rare pre-solar grains in the matrixes of carbonaceous chondrites(3) have been used to identify some sources of the pre-solar material. Here we report that the molybdenum isotopic composition of bulk carbonaceous chondrites is distinctly different from the accepted average solar value. We show that the Mo data require the presence of material produced in at least two different r-processes, and that the contribution from the p-process material is decoupled from the r-process, all occurring in supernova explosions. This is consistent with the emerging picture of diverse sources inferred from short-lived isotopes in the early Solar System(4) and elemental analyses of metal-poor stars(5,6).

DOI： 10.1038/415881a

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

A geochronological, major/trace element geochemical and Nd-Pb isotopic study of early- to mid-Archean basement granitoids/gneisses, similar to 2.68 Ga greenstone belt and 2.58-2.62 Ga late-Archean granitoids from the Hanikahimajuk Lake area of the northern Slave province was undertaken to constrain the tectonomagmatic evolution of the northcentral area of Slave province, Northwest Territories, Canada. The basement tonalites are precisely dated at 3378 +/- 1 Ma by U-Pb zircon methods and are characterized by steeply fractionated REE patterns with no negative Eu anomalies. They were likely produced through partial melting of mafic crust at >0.8 GPa. The epsilon Nd(T) of these rocks range from -0.3 to +2.1, implying involvement of even older crustal material in their genesis. Two different types of similar to 2.68 Ga mafic to intermediate volcanic rock are recognized in the Hanikahimajuk Lake area: tholeiitic rocks with no Ta-Nb depletions and calc-alkaline rocks with slight negative Ta-Nb depletions. All of these rocks show clear evidence for contamination by the similar to 3.4 Ga basement tonalite, thus precluding the possibility that some of these tholeiitic mafic rocks were produced in a mid-oceanic ridge setting. From the data presented here, we conclude that the tholeiitic rocks were produced in a back-are setting, likely similar to that of the Japanese are. The calc-alkaline rocks may either be a remnant of the volcanic are or part of the same back-are system. However, the close proximity with the tholeiitic rocks in the field favours the latter possibility. Mafic to intermediate rocks with similar geochemical and isotopic characteristics are also found from the Yellowknife area of the southwestern Slave province, and from the Indin Lake area of the westernmost Slave province (south of Grenville Lake area). This observation may indicate that the mafic-dominated volcanic belts of the western Slave province may be a remnant of a single, now collapsed and shortened back-are basin. The existence of syn-volcanic calc-alkaline plutonic rocks with similar geochemical and isotopic features as the calc-alkaline volcanic rocks supports this possibility. The geochemistry of late-Archean granitoids suggests that they are produced mainly through partial melting of mafic igneous protoliths and elastic sedimentary rocks (and possibly tonalitic rocks). Similarity in the epsilon Nd(2.6) values between these granitoids and the supracrustal rocks in the area is in accord with this model. Such protoliths are also consistent with the tectonic setting at which the similar to 2.68 Ga volcanic rocks were generated, because mafic igneous rocks and elastic sedimentary rocks (with lesser felsic igneous rocks) are the major constituents of a back-are basin. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0301-9268(99)00058-3

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

A combined U-Pb geochronology and Nd isotopic analysis of elastic metasedimentary rocks from the Archean Slave Province, Northwest Territories, Canada is presented. A series of elastic sedimentary rocks with deposition age of similar to 3.13 Ga to similar to 2.58 Ga was collected from the west-central Slave Province. These samples include conglomerates as well as finer sedimentary rocks such as greywacke, arkose and quartzite, Although it is generally agreed that the Nd model age (T-DM) of elastic sedimentary rocks represents the average sedimentary provenance age, T-DM of samples studied here is generally older than the average U-Pb age of detrital zircons extracted from the same sample. Rather, there seems to be a better similar to 1 : 1 relationship between the T-DM and U-Pb age of the oldest zircon from each sample. A simple model relationship between U-Pb geochronology and T-DM suggests that such correlation is a result of Archean crustal evolution of the Slave Province bring dominated by crustal reworking (i.e, intracrustal processes) rather than addition of juvenile materials from the mantle. One exception is the period between similar to 3.1 and similar to 3.2 Ga, during which period a significant portion of detrital zircons have U-Pb ages exceeding the T-DM, indicating a large input of juvenile material and/or decreasing level of crustal reworking, An alternative explanation for the U-Pb vs. T-DM trend may be a mixing of older mafic juvenile crust (containing no zircon) and younger felsic juvenile crust. However, the general lack of extensive pre-2.8 Ga mafic crust in the Slave Province makes this scenario unlikely, The results of this study cannot constrain the change in the volume of continental crust at any given period of time, since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle. However, a dominance of crustal reworking during the early- to mid-Archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events. Since the majority of zircon may be lost during crustal reworking processes (unless they are preserved as an inherited core), absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early Earth, (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0012-821X(99)00265-4

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：PERGAMON-ELSEVIER SCIENCE LTD  

A detailed geochemical and isotopic study of Late Archean turbidites and volcanic rocks from the Yellowknife area, Slave province, was undertaken to constrain the nature of exposed crust at the time of 2.6 to 2.7 Ga crustal consolidation. The epsilon Nd-T values of the volcanic rocks range from +1.7 to -4.4. This variation can be produced by assimilation of pre-2.8 Ga basement by a depleted mantle-derived magma, possibly followed by fractional crystallization. The turbidites are typically metamorphosed to greenschist to amphibolite facies, and where metamorphosed to greenschist facies, different units of Bouma sequence can be observed. The different units of Bouma sequence were sampled and analyzed separately to evaluate the possible differences in geochemical and isotopic signatures. The geochemical data presented here is in accord with the previously proposed model that argues for a mixture of 20% mafic-intermediate volcanic rocks, +55% felsic volcanic rocks, and +25% granitic rocks as a source of these turbidites. However, our revised calculation with the new data presented here argues for 1 to 2% input from an ultramafic source, as well as somewhat higher input from mafic-intermediate volcanic sources in the upper shale units compared to the lower sand units. The epsilon Nd-T values of the turbidites generally are lower in the upper shale units compared to the lower sand units. Detailed inspection of trace-element data suggest that this is not an artifact of ran earth element-rich heavy minerals concentrating in the lower sand units of the turbidites, but rather is a result of "unmixing" of detritus with different epsilon Nd-T values during sediment transportation and deposition. The upper shale units of the turbidites are isotopically compatible with a derivation mainly from crustally contaminated volcanic rocks, similar to those exposed in the Yellowknife area. The lower sand units contain a higher proportion of westerly derived plutonic rock detritus, characterized by higher epsilon Nd-T, suggesting that there are area(s) west of Yellowknife not underlain by older (2.8-4.0 Ga) basement. The trace-element characteristics of these turbidites (i.e., Cr, Ni, La, Th, Sc, Eu/Eu*, and Gd-N/Yb-N) are distinct from those of typical post-Archean turbidites. This observation is consistent with the models that predict that the chemical composition of the upper continental crust was slightly different in the Archean compared to post-Archean time. Copyright (C) 1999 Elsevier Science Ltd.

DOI： 10.1016/S0016-7037(99)00167-2

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：CANADIAN SCIENCE PUBLISHING, NRC RESEARCH PRESS  

New geochemical and Nd-Pb isotopic data for similar to 2.62-2.59 Ga granitoids from the southwest Slave Province are used to determine the source(s) of granitoid magmas, to evaluate the role of pre-2.8 Ga basement during this magmatism, and to refine the existing Nd-Pb isotopic structure of the western Slave Province. The Pb isotopic data require crust older than similar to 3.2 Ga as a granitoid protolith, whereas the Nd isotopic data require input from juvenile crustal material. This discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and similar to 2.7 Ga juvenile crust in varying proportions. Specifically, granitoids from the southwestern Slave Province require 10-30% basement, whereas granitoids from other parts of the western Slave Province require > 50%. Incorporation of basement as a protolith may be achieved indirectly, by assimilation of basement during juvenile similar to 2.7 Ga magmatism, or directly during similar to 2.62-2.59 Ga magmatism. The granitoid isotopic data suggest that indirect basement input was important on a regional scale, but direct input may have also taken place in some areas of the western Slave Province, particularly along the similar to 111W "isotopic boundary" zone previously recognized. The geochemical characteristics of these granitoids are compatible with an origin by partial melting of dominantly amphibolite and metasedimentary rocks to produce the similar to 2.61 Ga and similar to 2.59Ga magmatism, respectively; partial melting occurred in response to regional crustal thickening at this time.

DOI： 10.1139/e98-047

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

In the Japan context, the following characteristics of arc volcanic rocks are important for crustal growth: (1) intimate association of volcanic rocks of tholeiitic and calc-alkaline series; (2) repeated eruption of basaltic and dacitic lavas in a volcanic succession; (3) gradual change in chemistry from tholeiitic to calc-alkaline volcanic rocks; (4) crystallization differentiation which includes the crystallization of plagioclase along with other mafic minerals; (5) marked enrichment of K2O associated with slight depletion of compatible elements; (6) an evolution limit represented by, for instance, Rb/Sr ratios of 0.23-0.28 and SiO2 contents of 60-66 wt.% and (7) repetitive magma mixing.
Batch fractionation in refilled magma chambers best accounts for these characteristics, transforming mantle-derived magmas through calc-alkaline magmas to magmas of composition very close to the bulk composition of the upper continental crust. Systems of periodically refilled magma chambers are general in the Japanese arcs. Differentiates and accumulated crystals may form the compositional stratification of the continental crust.

DOI： 10.1016/0024-4937(94)90061-2

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：GEOCHEMICAL SOC JAPAN  

U-Pb, dating of zircons in a pelitic gneiss from the Oki metamorphic rocks gives an upper intercept age of 1.96 +/- 0.04 Ga on the concordia diagram and an average Pb-Pb age of 1.96 +/- 0.04 Ga. Rb-Sr biotite ages vary with crystal size and indicate that large biotite crystals are zoned in age. The relation between ages and crystal sizes reveals two different stages of crystallization. The age of the first stage was estimated to be about 180 Ma, which represents the time of the main phase of metamorphism. The second stage was estimated to have occurred at 161 +/- 2 Ma, which represents the final phase of metamorphism.
The U-Pb and Pb-Pb zircon ages represent the time of prominent plutonic activity in the provenance from which the elastic precursor of the pelitic gneiss was derived.

DOI： 10.2343/geochemj.28.333

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