The primitive matrix components of the unique carbonaceous chondrite Acfer 094: a TEM study

The mineralogical and chemical characteristics of the fine-grained matrix (< or = 3 micrometers) of the unique primitive carbonaceous chondrite Acfer 094 have been investigated in detail by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). Generally, the fine-grained matrix represents a highly unequilibrated assemblage of an amorphous material, small forsteritic olivines (200-300 nm), low Ca-pyroxenes (300-400 nm), and Fe,Ni-sulfides (100-300 nm). The matrix is basically unaffected by secondary processes. Only minor amounts of serpentine and ferrihydrite, as products of hydrous alteration, are present. Texturally, the amorphous material acts as a groundmass to olivines, pyroxenes, and sulfides, mostly exhibiting rounded or elongated morphologies. Only very few clastic mineral grains have been found. The texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment. As such, the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites. The non-clastic matrix olivines (Fo98-99) and pyroxenes (En97-100) are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material. The formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons. Rapid cooling caused the observed intergrowths of clino/orthoenstatite in the Mg-rich matrix pyroxenes. Although some similarities exist comparing the fine-grained matrix of Acfer 094 with the matrices of the unequilibrated CO3 chondrite ALHA77307 and the unique type 3 chondrite Kakangari, Acfer 094 remains unique. Since it contains the highest measured concentrations of circumstellar SiC and the second highest of diamond (highest is Orgueil), it seems reasonable to suggested that at least parts of the amorphous material in the fine-grained matrix may be of circumstellar origin.     

NanoSIMS analysis and Auger electron spectroscopy of silicate and oxide stardust from the carbonaceous chondrite Acfer 094

We have detected 138 presolar silicate, 20 presolar oxide and three presolar complex grains within the carbonaceous chondrite Acfer 094 by NanoSIMS oxygen isotope mapping. These grains were further investigated by scanning electron microscopy (SEM) and Auger electron spectroscopy for morphological and chemical details and their distribution within the meteorite matrix. The three complex grains consist of Al-rich oxides (grossite and hibonite) attached to non-stoichiometric Si-rich silicates. Refractory Al-rich oxides therefore serve as seed nuclei for silicates to condense onto, which is proposed by condensation theory and astronomical observations. However, in the majority of presolar silicates we did not find any indications for large subgrains. Most of the grains (80%) belong to O isotope Group I (¹⁷O-enriched) and come from 1 to 2.5 M asymptotic giant branch (AGB) stars of close-to-solar or slightly lower-than-solar metallicity. About 60% of these grains are irregular in shape; 40% display elliptical morphologies together with smooth, platy surfaces. Three grains with large ¹⁷O enrichments (¹⁷O/¹⁶O > 3 × 10⁻³) have highly irregular shapes and are very small (<250 nm); these grains may have formed in binary star systems or around higher mass () AGB stars. About 10% of the presolar silicates in this study can be assigned to the O isotope Group IV, which most likely originate from type II supernovae (SNeII). These grains are also generally smaller than 300 nm and are often irregular in shape (88%), consistent with the SNII origin scenario. The presolar grains are generally evenly distributed within the matrix on an mm scale, although in one case a statistically significant clustering of five grains in one 10 × 10 μm² sized field is observed. This could be an important hint that the distribution of presolar material in the parental molecular cloud was heterogeneous on a very fine scale. The matrix-normalized abundance of silicate stardust in Acfer 094 is 163 ± 14 ppm, which is among the highest abundance of O-rich stardust in primitive meteorites. Oxide stardust comprises 26 ± 6 ppm of the matrix. Auger Nanoprobe measurements of 69 presolar silicates and oxides (30 on a quantitative, 39 on a qualitative basis) indicate that most of the grains are Fe-rich (Mg/(Mg + Fe) of 0.82 and lower), which is either due to non-equilibrium condensation, secondary alteration, or both. (Mg + Fe)/Si ratios of the silicates are mostly non-stoichiometric and scatter around pyroxene-like rather than olivine-like compositions, which is consistent with recent Auger and transmission electron microscopy observations and astrophysical predictions. Mg-rich grains (Mg/(Mg + Fe) > 0.5) more likely exhibit elliptical, smooth surfaces (14 out of 18 grains), which is an indication that these grains have not been strongly altered since their circumstellar condensation. We identified only one grain similar to the “glass with embedded metal and sulfides” (GEMS) with a statistically significant sulfur content (>2–3 at.%). It remains unclear why the typical high-sulfur GEMS grains are only found in interplanetary dust particles, but have not yet been unequivocally identified in primitive meteorites.     

Oxygen-isotopic compositions of low-FeO relicts in high-FeO host chondrules in Acfer 094, a type 3.0 carbonaceous chondrite closely related to CM

With one exception, the low-FeO relict olivine grains within high-FeO porphyritic chondrules in the type 3.0 Acfer 094 carbonaceous chondrite have Δ¹⁷O (= δ¹⁷O − 0.52 × δ¹⁸O) values that are substantially more negative than those of the high-FeO olivine host materials. These results are similar to observations made earlier on chondrules in CO3.0 chondrites and are consistent with two independent models: (1) Nebular solids evolved from low-FeO, low-Δ¹⁷O compositions towards high-FeO, more positive Δ¹⁷O compositions; and (2) the range of compositions resulted from the mixing of two independently formed components. The two models predict different trajectories on a Δ¹⁷O vs. log Fe/Mg (olivine) diagram, but our sample set has too few values at intermediate Fe/Mg ratios to yield a definitive answer.Published data showing that Acfer 094 has higher volatile contents than CO chondrites suggest a closer link to CM chondrites. This is consistent with the high modal matrix abundance in Acfer 094 (49 vol.%). Acfer 094 may be an unaltered CM chondrite or an exceptionally matrix-rich CO chondrite. Chondrules in Acfer 094 and in CO and CM carbonaceous chondrites appear to sample the same population. Textural differences between Acfer 094 and CM chondrites are largely attributable to the high degree of hydrothermal alteration that the CM chondrites experienced in an asteroidal setting.     

Ca,Al-rich inclusions, amoeboid olivine aggregates, and Al-rich chondrules from the unique carbonaceous chondrite Acfer 094: I. mineralogy and petrology

Based on their mineralogy and petrography, ∼200 refractory inclusions studied in the unique carbonaceous chondrite, Acfer 094, can be divided into corundum-rich (0.5%), hibonite-rich (1.1%), grossite-rich (8.5%), compact and fluffy Type A (spinel-melilite-rich, 50.3%), pyroxene-anorthite-rich (7.4%), and Type C (pyroxene-anorthite-rich with igneous textures, 1.6%) Ca,Al-rich inclusions (CAIs), pyroxene-hibonite spherules (0.5%), and amoeboid olivine aggregates (AOAs, 30.2%). Melilite in some CAIs is replaced by spinel and Al-diopside and/or by anorthite, whereas spinel-pyroxene assemblages in CAIs and AOAs appear to be replaced by anorthite. Forsterite grains in several AOAs are replaced by low-Ca pyroxene. None of the CAIs or AOAs show evidence for Fe-alkali metasomatic or aqueous alteration. The mineralogy, textures, and bulk chemistry of most Acfer 094 refractory inclusions are consistent with their origin by gas-solid condensation and may reflect continuous interaction with SiO and Mg of the cooling nebula gas. It appears that only a few CAIs experienced subsequent melting. The Al-rich chondrules (ARCs; >10 wt% bulk Al₂O₃) consist of forsteritic olivine and low-Ca pyroxene phenocrysts, pigeonite, augite, anorthitic plagioclase, ± spinel, FeNi-metal, and crystalline mesostasis composed of plagioclase, augite and a silica phase. Most ARCs are spherical and mineralogically uniform, but some are irregular in shape and heterogeneous in mineralogy, with distinct ferromagnesian and aluminous domains. The ferromagnesian domains tend to form chondrule mantles, and are dominated by low-Ca pyroxene and forsteritic olivine, anorthitic mesostasis, and Fe,Ni-metal nodules. The aluminous domains are dominated by anorthite, high-Ca pyroxene and spinel, occasionally with inclusions of perovskite; have no or little FeNi-metal; and tend to form cores of the heterogeneous chondrules. The cores are enriched in bulk Ca and Al, and apparently formed from melting of CAI-like precursor material that did not mix completely with adjacent ferromagnesian melt. The inferred presence of CAI-like material among precursors for Al-rich chondrules is in apparent conflict with lack of evidence for melting of CAIs that occur outside chondrules, suggesting that these CAIs were largely absent from chondrule-forming region(s) at the time of chondrule formation. This may imply that there are several populations of CAIs in Acfer 094 and that mixing of “normal” CAIs that occur outside chondrules and chondrules that accreted into the Acfer 094 parent asteroid took place after chondrule formation. Alternatively, there may have been an overlap in the CAI- and chondrule-forming regions, where the least refractory CAIs were mixed with Fe-Mg chondrule precursors. This hypothesis is difficult to reconcile with the lack of evidence of melting of AOAs which represent aggregates of the least refractory CAIs and forsterite grains.     

Ordered mixed-layer structures in the Mighei carbonaceous chondrite matrix

High resolution transmission electron microscopy of the Mighei carbonaceous chondrite matrix has revealed the presence of a new mixed layer structure material. This mixed-layer material consists of an ordered arrangement of serpentine-type (S) and brucite-type (B) layers in the sequence … SBBSBB. … Electron diffraction and imaging techniques show that the basal periodicity is ~ 17 Å. Discrete crystals of SBB-type material are typically curved, of small size (<1 μm) and show structural variations similar to the serpentine group minerals. Mixed-layer material also occurs in association with planar serpentine. Characteristics of SBB-type material are not consistent with known terrestrial mixed-layer clay minerals. Evidence for formation by a condensation event or by subsequent alteration of preexisting material is not yet apparent.     

Chemistry, petrology and bulk oxygen isotope compositions of chondrules from the Mokoia CV3 carbonaceous chondrite

We report bulk chemical compositions and physical properties for a suite of 94 objects, mostly chondrules, separated from the Mokoia CV3_ox carbonaceous chondrite. We also describe mineralogical and petrologic information for a selected subset of the same suite of chondrules. The data are used to examine the range of chondrule bulk compositions, and to investigate the relationships between chondrule mineralogy, texture and bulk compositions, as well as oxygen isotopic properties that we reported previously. Most of the chondrules show minimal metamorphism, corresponding to petrologic subtype <3.2. In general, elemental fractionations observed in chondrule bulk compositions are reflected in the compositions of constituent minerals. For chondrules, mean bulk compositions and compositional ranges are very similar for large (>2 mg) and small (<2 mg) size fractions. Two of the objects studied are described as matrix-rich clasts. These have similar bulk compositions to the chondrule mean, and are potential chondrule precursors. One of these clasts has a similar bulk oxygen isotopic composition to Mokoia chondrules, but the other has an anomalously high value of Δ¹⁷O (+3.60‰).Chondrules are diverse in bulk chemical composition, with factor of 10 variations in most major element abundances that cannot be attributed to secondary processes. The chondrules examined show evidence for extensive secondary oxidation, and possible sulfidization, as expected for an oxidized CV chondrite, but minimal aqueous alteration. Some of the bulk chondrule compositional variation might be the result of chemical (e.g. volatilization or condensation) or physical (e.g. metal loss) processes during chondrule formation. However, we suggest that it is mainly the result of significant variations in the assembly of particles that constituted chondrule precursors. Precursor material likely included a refractory component, possibly inherited from disaggregated CAIs, an FeO-poor ferromagnesian component such as olivine or pyroxene, an oxidized ferromagnesian component, and a metal component. Bulk oxygen isotope ratios of chondrules can be explained if refractory and ferromagnesian precursor materials initially shared similar oxygen isotopic compositions of δ¹⁷O, δ¹⁸O around −50‰, and then significant exchange occurred between the chondrule and surrounding ¹⁶O-poor gas during melting.     

Mineral associations and character of isotopically anomalous organic material in the Tagish Lake carbonaceous chondrite

We report a coordinated analytical study of matrix material in the Tagish Lake carbonaceous chondrite in which the same small (?20 μm) fragments were measured by secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS), and X-ray absorption near-edge spectroscopy (XANES). SIMS analysis reveals H and N isotopic anomalies (hotspots), ranging from hundreds to thousands of nanometers in size, which are present throughout the fragments. Although the differences in spatial resolution of the SIMS techniques we have used introduce some uncertainty into the exact location of the hotspots, in general, the H and N isotopic anomalies are spatially correlated with C enrichments, suggesting an organic carrier. TEM analysis, enabled by site-specific extraction using a focused-ion-beam scanning-electron microscope, shows that the hotspots contain an amorphous component, Fe-Ni sulfides, serpentine, and mixed-cation carbonates. TEM imaging reveals that the amorphous component occurs in solid and porous forms, EDS indicates that it contains abundant C, and EELS and XANES at the C K edge reveal that it is largely aromatic. This amorphous component is probably macromolecular C, likely the carrier of the isotopic anomalies, and similar to the material extracted from bulk samples as insoluble organic matter. However, given the large sizes of some of the hotspots, the disparity in spatial resolution among the various techniques employed in our study, and the phases with which they are associated, we cannot entirely rule out that some of the isotopic anomalies are carried by inorganic material, e.g., sheet silicates. The isotopic composition of the organic matter points to an initially primitive origin, quite possibly within cold interstellar clouds or the outer reaches of the solar protoplanetary disk. The association of organic material with secondary phases, e.g., serpentine and carbonates, suggests that the organic matter was susceptible to parent-body processing, and thus, isotopic dilution.     

Al-Mg systematics of chondrules in a primitive CO chondrite

We have conducted systematic investigations of formation age, chemical compositions, and mineralogical characteristics of ferromagnesian chondrules in Yamato-81020 (CO3.05), one of the most primitive carbonaceous chondrites, to get better understanding of the origin of chemical groups of chondrites. The ²⁶Al-²⁶Mg isotopic system were measured in fourteen FeO-poor (Type I), six FeO-rich (Type II) and two aluminum-rich (Al-rich) chondrules using a secondary ion mass spectrometer. Excesses of ²⁶Mg in plagioclase (1.0-13.5‰) are resolved with sufficient precision (mostly 0.4-6.6‰ at 2σ level) in all the chondrules studied except one. Chemical zoning of Mg and Na in plagioclase were investigated in detail in order to evaluate the applicability of ²⁶Al-²⁶Mg chronometer. We conclude that the Al-Mg isotope system of the chondrules in Y-81020 have not been disturbed by parent-body metamorphism and can be used as chronometer assuming homogeneous distribution of ²⁶Al. Assuming an initial ²⁶Al/²⁷Al ratio of 5 × 10⁻⁵ in the early solar system, ²⁶Al-²⁶Mg ages were found to be 1.7-2.5 Ma after CAI formation for Type I, 2.0-3.0 Ma for Type II and 1.9 and 2.6 Ma for Al-rich chondrules.The formation ages of ferromagnesian chondrules in Y-81020 are in good agreement with those of L and LL (type 3.0-3.1) chondrites in the literature, which indicates that common chondrules in the CO chondrite were formed contemporaneously with those in L and LL chondrites. The concurrent formation of chondrules of CO and L/LL chondrites suggests that the chemical differences between CO and L/LL chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk.     

Chemical and crystallographic study of cronstedtite in the matrix of the Cochabamba (CM2) carbonaceous chondrite

Summary The principal mineral component in the matrix of the Cochabamba carbonaceous chondrite is a phyllosilicate, which is identified as cronstedtite mainly on the basis of its chemical composition. Its approximate idealized formula is given by M₆ Fe _0.7 ³⁺ Al_0.5Si_2.7O₁₀ (OH)₈ with M=Fe²⁺, Fe³⁺, and Mg in somewhat variable amounts. TEM studies reveal the presence of three polytypes, and show a high degree of stacking disorder parallel to (001) with the displacement vector ±b/3 or ±2b . Crumpled amorphous masses in the matrix may contain structural building blocks of phyllosilicates. They, rather than the anhydrous minerals, seem to be the most likely progenitors of cronstedtite. Some constraints on its origin are reviewed. In addition to cronstedtite, observations on some other matrix phases are also reported.

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Chemische und kristallographische Untersuchung von Cronstedtit in der Matrix des kohligen Chondrits (CM2) Cochabamba

Zusammenfassung Der Hauptbestandteil der Matrix im kohligen Chondrit Cochabamba ist ein Schichtsilikat, das hauptsächlich aufgrund seiner chemischen Zusammensetzung als Cronstedtit identifiziert wurde. Die idealisierte Formel entspricht ungefähr M₆Fe _0.7 ⁺³ Al_0.5Si_2.7O₁₀(OH)₈ mit M=Fe²⁺, Fe³⁺ und Mg in wechselnden Mengen. TEM-Untersuchungen zeigen das Vorkommen von drei Modifikationen, sowie einen hohen Grad von Versetzungsfehlern parallel zu (001), mit dem Versetzungsfaktor ±b/3 oder ±2b/3. Deformierte amorphe Aggregate in der Matrix scheinen primitive Bausteine der Schichtsilikate zu sein. Sie (und nicht die wasserfreien Mineralien) dürften das Material darstellen, aus dem Cronstedtit gebildet wurde. Die Bildungsbedingungen von Cronstedtit werden diskutiert. Außerdem wird über Beobachtungen an anderen Matrixmineralien berichtet.

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With 4 Figures     

Microscopic oxygen isotopic homogeneity/heterogeneity in the matrix of the Vigarano CV3 chondrite

Two-dimensional ¹⁸O/¹⁶O isotopic analysis of the Vigarano matrix was conducted by secondary ion-imaging using a novel two-dimensional ion-imager. Quantitative oxygen-isotope images (isotopographs) of the Vigarano matrix show that ¹⁶O-rich micrograins are scattered within ¹⁶O-poor matrix. This heterogeneous O-isotopic distribution indicates that matrix is composed of different O-isotopic components that formed in different locations and/or at different times. However, the O-isotopic composition of groundmass in the matrix is the same as the bulk isotopic composition of the matrix within ±5‰ uncertainty. The spatial resolution and isotopic precision of our technique should allow submicron-size objects (>0.2 μm) with extreme O-isotopic anomalous characteristics (δ¹⁸O_SMOW ∼250‰) to be detectable in isotopographs. Because the mean grain size of the matrix is ∼0.2 μm, the inability to detect such O-isotopic anomalous objects indicates that isotopically anomalous micrograins (e.g., presolar grains) are extremely rare in the Vigarano matrix and that most objects in the matrix were formed in the solar nebula or in the parent body.     

Composition of matrix in the CR chondrite LAP 02342

We report evidence of interchondrule matrix heterogeneity on a scale of ∼50 μm in the well-preserved CR2 chondrite LAP 02342. Despite minor effects resulting from asteroidal aqueous alteration, the matrix in this CR chondrite seems to preserve much of the compositional record of nebular fines. We carried out electron-microprobe studies using a 3-μm-diameter beam; we analyzed 10 elements in 36- or 49-point grids on 11 ca. 50 × 50-μm rectangular areas of matrix. Each grid area has a distinct composition, inconsistent with a simple model of matrix material having a uniform composition throughout the nebular formation region of the CR chondrites. On S-Fe, Mg-Si, K-Na and K-Al scatter diagrams, the grid areas (i.e., different matrix patches) are largely separated from each other; plots of means with 95% confidence limits demonstrate that the compositions are resolvable. Five matrix areas were analyzed again in duplicate runs; excellent agreement was observed between duplicate studies. LAP 02342 experienced two forms of mild aqueous alteration - as patchy enrichments in Ca (inferred to reflect CaCO₃) and as regions in which sulfide laths are embedded within phyllosilicates. Despite this evidence of aqueous transport, the effect on the composition of matrix is not resolvable. For example, matrix points that were adjacent to points with high CaCO₃ contents show elemental concentrations similar to those in regions having only one or two points with a Ca enrichment. It appears that secondary minerals are found in areas where there are suitable precursor phases and voids into which new phases could grow unimpeded. Calcium appears to be unique in forming a phase that greatly lowers the Ca⁺⁺ content of the aqueous medium, thus enhancing the rate of diffusion. Because chondrules vary widely in bulk composition, the formation of chondrules in small sets (100 or less) could generate “smoke” and mesostasis spray with compositions unique to each set. However, if these nanoparticles were moving independently in the nebula, it would not have been possible to have preserved compositional variations. We therefore infer that the anomalous materials were preserved in small nebular structures, probably as porous chondrules formed by low degrees of melting.     

Mineralogy, aqueous alteration, and primitive textural characteristics of fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002

Transmission electron microscopy (TEM) was used to study the microtextural and mineralogical characteristics of fine-grained rims in the unbrecciated CM2 chondrites, Y-791198 and ALHA81002, in an effort to provide constraints on the origins of the rims themselves. Our TEM observations show that the rims in Y-791198 are composed of two distinct types of region, sulfide-poor and sulfide-rich, that are intermixed in a complex manner at the micron to submicron level. The sulfide-poor regions are largely composed of amorphous silicate material or nanocrystalline serpentine, but rare fibrous and coarse-grained serpentine grains have also been identified. No fine-grained cronstedtite or tochilinite were observed, although coarse-grained lumps of tochilinite are present in the rims. In contrast, the sulfide-rich regions are characterized by the presence of a myriad, nanometer-sized Fe, Ni sulfide grains (pentlandite with some Ni-rich pyrrhotite) embedded within an amorphous silicate similar in composition to that of the sulfide-poor regions. The sulfide-rich regions also contain rare phases such as olivine, and Fe, Ni metal grains with grain sizes that are always >100 nm in size. Z-contrast scanning transmission electron microscopy (STEM) reveals that the fine-grained rims consist of a mosaic of irregularly-shaped sulfide-poor and sulfide-rich-regions with sizes of about 0.2-0.5 μm, that have been compacted together during parent body lithification. Despite aqueous alteration, the distinct mineralogical characteristics of these different regions are preserved on a fine-scale and probably represent primitive heterogeneity in the dust from which these rims formed.Serpentine is much better developed and more widespread in the fine-grained rims of ALHA81002 than Y-791198. Complex mats of serpentine fibers are commonly found and cronstedtite and tochilinite are plentiful. Anhydrous minerals such as olivine are rare and have usually been replaced by serpentine. Like Y-791198, all the fine-grained rims studied in ALHA81002 show the same mineral assemblages and textural characteristics throughout and between rims. The homogeneity of the mineralogy, textural relationships and degree of hydration in the rims of these two chondrites is more consistent with parent-body alteration than with pre-accretionary alteration.     

Indicators of parent-body processes: Hydrated chondrules and fine-grained rims in the Mokoia CV3 carbonaceous chondrite

A petrographic and electron microscopic study of the Mokoia CV3 carbonaceous chondrite shows that all of the chondrules and inclusions (>400 μm in diameter) and most of their fine-grained rims studied (referred to as chondrules/rims) contain various amounts of hydrous phyllosilicates (mostly saponite) formed by aqueous alteration of anhydrous silicates. The rims mainly consist of fine-grained olivine and saponite in varying proportions and contain crosscutting veins of Fe-rich olivine. The boundaries between the chondrules and their rims are irregular and show abundant evidence of aqueous alteration interactions between them. In contrast, the host matrix contains very minor amounts of saponite and shows no evidence of such extensive aqueous alteration. The boundaries between the chondrules/rims and the matrix are sharp and show no traces of the matrix having been involved in the alteration of the chondrules/rims. These observations indicate that the aqueous alteration in the chondrules/rims did not occur in the present setting.We suggest that the chondrules/rims are actually clasts transported from a location on the meteorite parent body different from where the Mokoia meteorite was from. The aqueous alteration of the chondrules/rims probably occurred there. The veins in the rims were originally fractures produced in an interchondrule matrix by impacts; these were later filled by Fe-rich olivine during aqueous activity. This location was then involved in impact brecciation, and individual chondrules were ejected as clasts with remnants of the matrix surrounding them. During the continuing brecciation, those chondrule/rim clasts were transported, mixed with anhydrous matrix grains, and finally lithified to the present meteorite. Therefore, the rims are fragmented remnants of a former matrix.Textures characterized by fine-grained rims surrounding chondrules in chondrites have been widely thought to have formed in the solar nebula before they accreted into their parent bodies. However, our results suggest that some textures may not be explained by such an accretionary model; instead, the multi-stage parent-body process modeled for the Mokoia rim formation may be a more plausible explanation.     

The mineral chemistry and origin of inclusion matrix and meteorite matrix in the Allende CV3 chondrite

The two textural varieties of olivine-rich Allende inclusions (rimmed and unrimmed olivine aggregates) consist primarily of a porous, fine-grained mafic constituent (inclusion matrix) that differs from the opaque meteorite matrix of CV3 chondrites by being relatively depleted in sulfides, metal grains, and (perhaps) carbonaceous material. Olivine is the most abundant mineral in Allende inclusion matrix; clinopyroxene, nepheline, sodalite, and Ti-Al-pyroxene occur in lesser amounts. Olivine in unrimmed olivine aggregates (Type 1A inclusions) is ferrous and has a narrow compositional range (Fo_50–65). Olivine in rimmed olivine aggregates (Type 1B inclusions) is, on average, more magnesian, with a wider compositional range (Fo_53–96). Olivine grains in the granular rims of Type 1B inclusions are zoned, with magnesian cores (Fo_>80) and ferrous rinds (Fo_<70). Ferrous olivines (Fo_<65) in both varieties of inclusions commonly contain significant amounts of Al₂O₃ (as much as ~0.7 wt%), CaO (as much as ~0.4 wt%), and TiO₂ (as much as ~0.2 wt%), refractory elements that probably occur in submicroscopic inclusions of Ca,Al,Ti-rich glass (rather than in the olivine crystal structure). Defocussed beam analyses of Allende matrix materials demonstrate that: (1) inclusion matrix in Type 1A inclusions is more enriched in olivine and FeO than inclusion matrix in the cores of Type 1B inclusions; (2) opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix; (3) the bulk compositions of Type 1A and Type 1B inclusions overlap; and (4) excluding sulfides and metal, the bulk compositions of Allende matrix materials cluster in a complementary pattern around the bulk composition of C1 chondrites.Inclusion matrix and meteorite matrix in Allende and other CV3 chondrites are probably relatively primitive nebular material, but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition. Allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing, non-equilibrium conditions from supercooled, supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula; CV3 meteorite matrix contains, in addition, a proportion of interstellar material that was heated (but not vaporized) in the nebula. Granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula. The mineral chemistry of matrix olivine in Allende seems to have been established by three different processes: non-equilibrium vapor → solid condensation; recrystallization and partial melting in the nebula; and $\text{Fe}\text{Mg}$ equilibration (without textural homogenization) in the meteorite parent body.     

Mass-independent fractionation of oxygen isotopes in the mesostasis of a chondrule from the Semarkona LL3.0 ordinary chondrite

A large chondrule from Semarkona, the most primitive ordinary chondrite known, has been discovered to contain a record of mass transport during its formation. In most respects, it is a normal Type I, group A1, low-FeO chondrule that was produced by reduction and mass-loss during the unidentified flash-heating event that produced the chondrules, the most abundant structural component in primitive meteorites. We have previously measured elemental abundances and abundance profiles in this chondrule. We here report oxygen isotope ratio abundances and ratio abundance profiles. We have found that the mesostasis is zoned in oxygen isotope ratio, with the center of the chondrule containing isotopically heavier oxygen than the outer regions, the outer regions being volatile rich from the diffusion of volatiles into the chondrule during cooling. The δ¹⁷O values range from −2.0‰ to 9.9‰, while δ¹⁸O range from −1.9‰ to 9.6‰. More importantly, a plot of δ¹⁷O against δ¹⁸O has a slope of 1.1 ± 0.2 (1σ) and 0.88 ± 0.10 (1σ) when measured by two independent methods. Co-variation of δ¹⁷O with δ¹⁸O that does not follow mass-dependent fractionation has often been seen in primitive solar system materials and is usually ascribed to the mixing of different oxygen reservoirs. We argue that petrographic and compositional data indicate that this chondrule was completely melted at the time of its formation so that relic grains could not have survived. Furthermore, there is petrographic and compositional evidence that there was no aqueous alteration of this chondrule subsequent to its formation. Although it is possible to formulate a series of exchanges between the chondrule and external ¹⁶O-rich and ¹⁶O-poor reservoirs that may explain the detailed oxygen isotope systematics of this chondrule, such a sequence of events looks very contrived. We therefore hypothesize that reduction, devolatilization, and crystallization of the chondrule melt may have produced ¹⁶O-rich olivines and ¹⁶O-poor mesostasis plotting on a slope-one line as part of the chondrule-forming process in an analogous fashion to known chemical mass-independent isotopic fractionation mechanisms. During cooling, volatiles and oxygen near the terrestrial line in oxygen isotope composition produced the outer zone of volatile rich and ¹⁶O-rich mesostasis. The chondrule therefore not only retains a record of considerable mass transport accompanying formation, but also may indicate that the isotopes of oxygen underwent mass-independent fractionation during the process.     

Carbonaceous chondrites—I. Characterization and significance of carbonaceous chondrite (CM) xenoliths in the Jodzie howardite

Mineralogical, chemical, textural, and isotopic studies of the abundant carbonaceous inclusions in the Jodzie howardite are consistent with CM characteristics. These CM xenoliths show regolith alteration on a level comparable to the Murray and Murchison meteorites but less than Nogoya, flow-oriented development of phyllosilicates and ‘poorly characterized phases’, and partial oxidation of sulfides. Temperature-programmed pyrolysis mass spectrometry (25°–1400°C) indicates that gas release patterns of volatiles and hydrocarbon components and percent contents of N(0.15), C(2.3) and S(2.4) are typical of CM meteorites. Release of significant amounts of SO₂ is attributed to the thermal breakdown of ‘poorly characterized phases’ (Fe-Ni-C-S-O) that formed during low temperature aqueous alteration in the CM parent body.Noble gas abundances are well within the reported range of CM meteorites. The fact that the Ne composition is typical for ‘solar’ values and the isotopic structure of Xe is ‘planetary’ argues that these gases were entrapped by different mechanisms. Cosmic ray exposure ages for the xenoliths (³He, 5 × 10⁶; ²¹Ne, 6.7 × 10⁶; ³⁸Ar, 6.9 × 10⁶ yr) agree with the reported exposure age for the eucritic host. Volatile abundances, presence of intact organic molecules, and phyllosilicates in the CM xenoliths preclude regolith temperatures in excess of 200°C after CM incorporation. Mixing of the host and xenoliths probably occurred during a low-velocity collision of main belt asteroids.     

Mineralogical evidence for a local volcanic origin of the parent material of Bermuda Quaternary paleosols

The alternation of carbonate deposits and paleosols compose the emerged part of the Bermuda archipelago. The pedological units present a complex and diversified mineralogy. Former studies demonstrated that the paleosols are not primarily a product of the unique dissolution of the surrounding carbonates, but contain a massive input of allochthonous non-carbonate detrital material. Researchers during more than the past three decades have attributed this flux of insoluble residues (IR) to Saharan dusts. We carried out systematic field and mineralogical analyses on the Quaternary paleosols from the Bermuda archipelago. Their mineralogical assemblage predominantly includes carbonates, clay minerals (kaolinite, chlorite and chlorite/vermiculite), phosphates, and aluminium and iron oxides/hydroxides. This assemblage is strikingly close to the mineralogy of the weathered volcanic substrate of Bermuda, but noticeably different from the mineralogy of Saharan dust. Moreover, we found volcanic lithoclasts in numerous paleosol profiles all over the archipelago and in all the recorded time intervals. We thus consider the volcanic seamount underlying Bermuda as the main source of non-carbonate minerals detected in the paleosols. This hypothesis further resolves the anomalous maturity of Bermudan paleosols compared to their southern counterparts in the Bahamas and Barbados.     

Oxygen isotope systematics of chondrules in the Allende CV3 chondrite: High precision ion microprobe studies

The oxygen three-isotope systematics of 36 chondrules from the Allende CV3 chondrite are reported using high precision secondary ion mass spectrometer (CAMECA IMS-1280). Twenty-six chondrules have shown internally homogenous Δ¹⁷O values among olivine, pyroxene, and spinel within a single chondrule. The average Δ¹⁷O values of 19 FeO-poor chondrules (13 porphyritic chondrules, 2 barred olivine chondrules, and 4 chondrule fragments) show a peak at −5.3 ± 0.6‰ (2SD). Another 5 porphyritic chondrules including both FeO-poor and FeO-rich ones show average Δ¹⁷O values between −3‰ and −2‰, and 2 other FeO-poor barred olivine chondrules show average Δ¹⁷O values of −3.6‰ and 0‰. These results are similar to those for Acfer 094 chondrules, showing bimodal Δ¹⁷O values at −5‰ and −2‰. Nine porphyritic chondrules contain olivine grains with heterogeneous Δ¹⁷O values as low as −18‰, indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions. However, most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules. The Δ¹⁷O values of four barred olivine chondrules range from −5‰ to 0‰, indicating that not all BO chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses. Based on these data, we suggest the presence of multiple oxygen isotope reservoirs in local dust-rich protoplanetary disk, from which the CV3 parent asteroid formed.A compilation of 225 olivine and low-Ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between carbonaceous chondrite anhydrous mineral (CCAM) and Young and Russell lines. These data define a correlation line of δ¹⁷O = (0.982 ± 0.019) × δ¹⁸O − (2.91 ± 0.10), which is similar to those defined by chondrules in CV3 chondrites and Acfer 094 in previous studies. Plagioclase analyses in two chondrules plot slightly below the CCAM line with Δ¹⁷O values of −2.6‰, which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the CV parent body.     

Peculiarities of the oxygen isotope ratio in precious opals

This paper presents the results of the δ¹⁸O study of the precious opals from Primor’e (Raduzhnoe deposit), Australia, and Ethiopia and the modern opals from the hydrotherms of the Mendeleev Volcano (Kunashir Island, Kuril Islands). It is established that the oxygen isotope ratio in opals may serve as a criterion for the estimation of their formation temperature. The low-temperature sedimentary opals are relatively enriched in the heavy oxygen isotope independently of the sedimentary or volcanic host rocks. Examples are the Australian and Slovakian opals of the A-type. The hydrothermal opals are enriched in the light oxygen isotope, which depends on the precipitation temperature. The higher the temperature, the lighter the oxygen isotope ratio of the precipitating opal is and the closer it is to that of the hydrothermal fluid.     

Metamorphism of carbonaceous material in the Tono contact aureole, Kitakami Mountains, Japan

Abstract Optical and X-ray studies of carbonaceous material in the Tono contact metamorphic aureole, Kitakami Mountains, northeast Japan, have revealed that metamorphic graphitization proceeded through two discontinuous changes: first, optically anisotropic domains develop within the coaly phytoclast, forming transitional material, and then, ordered graphite crystallizes by the decomposition of pre-existing carbonaceous materials. Coaly material disappears in the uppermost chlorite zone. Transitional material appears in the middle of the lower chlorite zone. Graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso-grad to the cordierite isograd where all the carbonaceous materials have converted to graphite. The apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials. The temperature of graphitization in the present area is at least 100°C higher than the temperature in the Sanbagawa and New Caledonia high-pressure metamorphic terrains, probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism.