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1.
Abstract– Dhofar (Dho) 225 and Dho 735 are carbonaceous chondrites found in a hot desert and having affinities to Belgica‐like Antarctic chondrites (Belgica [B‐] 7904 and Yamato [Y‐] 86720). Texturally they resemble CM2 chondrites, but differ in mineralogy, bulk chemistry and oxygen isotopic compositions. The texture and main mineralogy of Dho 225 and Dho 735 are similar to the CM2 chondrites, but unlike CM2 chondrites they do not contain any (P, Cr)‐sulfides, nor tochilinite 6Fe0.9S*5(Fe,Mg)(OH)2. H2O‐contents of Dho 225 and Dho 735 (1.76 and 1.06 wt%) are lower than those of CM2 chondrites (2–18 wt%), but similar to those in the metamorphosed carbonaceous chondrites of the Belgica‐like group. Bulk compositions of Dho 225 and Dho 735, as well as their matrices, have low Fe and S and low Fe/Si ratios relative to CM2 chondrites. X‐ray powder diffraction patterns of the Dho 225 and Dho 735 matrices showed similarities to laboratory‐heated Murchison CM2 chondrite and the transformation of serpentine to olivine. Dho 225 and 735’s oxygen isotopic compositions are in the high 18O range on the oxygen diagram, close to the Belgica‐like meteorites. This differs from the oxygen isotopic compositions of typical CM2 chondrites. Experimental results showed that the oxygen isotopic compositions of Dho 225 and Dhofar 725, could not be derived from those of typical CM2 chondrites via dehydration caused by thermal metamorphism. Dho 225 and Dho 735 may represent a group of chondrites whose primary material was different from typical CM2 chondrites and the Belgica‐like meteorites, but they formed in an oxygen reservoir similar to that of the Belgica‐like meteorites.  相似文献   

2.
The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?17O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?17O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.  相似文献   

3.
Abstract— Two pallasites, Vermillion and Yamato (Y)‐8451, have been studied to obtain petrologic, trace element, and O‐isotopic data. Both meteorites contain low‐Ca and high‐Ca pyroxenes (<2% by volume) and have been dubbed “pyroxene pallasites.” Pyroxene occurs as large individual grains, as inclusions in olivine and in other pyroxene, and as grains along the edges of olivine. Symplectic overgrowths, sometimes found in Main Group and Eagle Station pallasites, are not seen in the pyroxene pallasites. Olivine compositions are Fa10–12, similar to those of Main Group pallasites. Siderophile trace element data show that metal in the two meteorites have significantly differing compositions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites. These compositions also differ from those of IAB and IIIAB iron meteorites. Rare earth element (REE) patterns in merrillite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inheriting its pattern from the surrounding silicates. The O‐isotopic compositions of Vermillion and Y‐8451 are similar but differ from Main Group or Eagle Station pallasites, as well as other achondrite and primitive achondrite groups. Although Vermillion and Y‐8451 have similar mineralogy, pyroxene compositions, REE patterns, and O‐isotopic compositions, there is sufficient evidence to resist formally grouping these two meteorites. This evidence includes the texture of Vermillion, siderophile trace element data, and the presence of cohenite in Vermillion.  相似文献   

4.
Abstract— We report the mineralogy and oxygen isotopic compositions of FeO‐rich silicates in the Sahara 97159 EH3 chondrite. This component is referred to as FeO‐rich because it contains substantially more FeO than the characteristic FeO‐poor silicates in the highly reduced enstatite meteorites. These FeO‐rich silicates are mostly low‐Ca pyroxene (Fs5–35) and their compositions suggest an origin under more oxidizing conditions, like those for the ordinary chondrites. However, the mafic silicates in ordinary and carbonaceous chondrites are dominantly olivine, and the FeO‐rich silicates in the E chondrites are less commonly olivine. The oxygen isotopic compositions of the FeO‐rich silicates are indistinguishable from those of FeO‐poor silicates in Sahara 97159. These observations suggest that both the FeO‐rich silicates and the FeO‐poor silicates in EH chondrites formed from the same oxygen reservoir where redox conditions varied widely.  相似文献   

5.
Knowledge of Martian igneous basaltic compositions is crucial for constraining mantle evolution, including early differentiation and mantle convection. Primitive magmas provide direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The recently discovered Martian meteorite Northwest Africa (NWA) 5789 is an olivine‐phyric shergottite. NWA 5789 has special significance among the Martian meteorites because it appears to represent one of the most magnesian Martian magmas known, other than Yamato (Y) 980459. Its most magnesian olivine cores (Fo85) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that the bulk represents a magma composition. Based on the Al/Ti ratio of its pyroxenes, we infer that the rock began to crystallize at a high pressure consistent with conditions in Mars’ lower crust/upper mantle. It continued and completed its crystallization closer to the surface, where cooling was rapid and produced a mesostasis of radiating sprays of plagioclase and pyroxene. The mineralogy, petrology, mineral chemistry, and bulk rock composition of NWA 5789 are very similar to those of Y‐980459. The similarities between the two meteorites suggest that NWA 5789 (like Y‐980459) represents a primitive, mantle‐derived magma composition. They also suggest the possibility that NWA 5789 and Y‐980459 formed in the same lava flow. However, based on the mineralogy and texture of its mesostasis, NWA 5789 must have cooled more slowly than Y‐980459. NWA 5789 will help elucidate the igneous geology and geochemistry of Mars.  相似文献   

6.
Except for asteroid sample return missions, measurements of the spectral properties of both meteorites and asteroids offer the best possibility of linking meteorite groups with their parent asteroid(s). Visible plus near‐infrared spectra reveal distinguishing absorption features controlled mainly by the Fe2+ contents and modal abundances of olivine and pyroxene. Meteorite samples provide relationships between spectra and mineralogy. These relationships are useful for estimating the olivine and pyroxene mineralogy of stony (S‐type) asteroid surfaces. Using a suite of 10 samples of the acapulcoite–lodranite clan (ALC), we have developed new correlations between spectral parameters and mafic mineral compositions for partially melted asteroids. A well‐defined relationship exists between Band II center and ferrosilite (Fs) content of orthopyroxene. Furthermore, because Fs in orthopyroxene and fayalite (Fa) content in olivine are well correlated in these meteorites, the derived Fs content can be used to estimate Fa of the coexisting olivine. We derive new equations for determining the mafic silicate compositions of partially melted S‐type asteroid parent bodies. Stony meteorite spectra have previously been used to delineate meteorite analog spectral zones in Band I versus band area ratio (BAR) parameter space for the establishment of asteroid–meteorite connections with S‐type asteroids. However, the spectral parameters of the partially melted ALC overlap with those of ordinary (H) chondrites in this parameter space. We find that Band I versus Band II center parameter space reveals a clear distinction between the ALC and the H chondrites. This work allows the distinction of S‐type asteroids as nebular (ordinary chondrites) or geologically processed (primitive achondrites).  相似文献   

7.
Abstract— The Burnwell, Kentucky, meteorite fell as a single stone on 1990 September 4. The Burnwell meteorite has lower Fa in olivine (15.8 mol%), Fs in orthopyroxene (13.4 mol%), Co in kamacite (0.36 wt%), FeO from bulk chemical analysis (9.43 wt%), and Δ17O (0.51 ± 0.02%), and higher Fe, Ni, Co metal (19.75 wt% from bulk wet chemical analysis) than observed in H chondrites. The Burnwell meteorite plots on extensions of H-L-LL chondrite trends for each of these properties towards more reducing compositions than in H chondrites. Extensions of this trend have been previously suggested in the case of other low-FeO chondrites or silicate inclusions in the HE iron Netschaëvo, but interpretation of the evidence in these meteorites is complicated by terrestrial weathering, chemical disequilibrium or reduction. In contrast, the Burn-well meteorite is an equilibrated fall that exhibits no evidence for reduction. As such, it provides the first definitive evidence for extension of the H-L-LL ordinary chondrite trend beyond typical H values towards more reducing compositions.  相似文献   

8.
Abstract– Xenoliths are inclusions of a given meteorite group embedded in host meteorites of a different group. Xenoliths with dimensions between a few μm and about 1 mm (microxenoliths) are “meteorite‐trapped” analogues of micrometeorites collected on the Earth. However, they have the unique features of sampling the zodiacal cloud (1) at more ancient times than those sampled by micrometeorites and (2) at larger distances from the Sun (corresponding to the asteroid Main Belt) than that sampled by micrometeorites (1 AU). Herein we describe a systematic search for new xenoliths and microxenoliths in H chondrites, aimed at determining their abundance in these ordinary chondrites, analyzing their mineralogy, and searching for possible correlations with host meteorite properties. Sixty‐six sections from 40 meteorites have been analyzed. Twenty‐four new xenoliths have been discovered. About 87% of them are microxenoliths (i.e., <1 mm), only three are >1 mm in their largest dimension. All the newly discovered xenoliths and microxenoliths are composed of carbonaceous chondritic material. Hence, the zodiacal cloud was dominated by carbonaceous material even in past epochs. All the new xenoliths and microxenoliths have been found in regolith breccias. Hydrous‐phase‐rich xenoliths and microxenoliths in H4 and H5 chondrites attest that their embedding happened after the end of the thermal metamorphism. All these data suggest that xenoliths and microxenoliths were embedded when their host meteorites were part of the parent body regolith. This, combined with the H chondrite impact age distribution, attests that the embedding may have happened as early as 3.5 Gyr ago.  相似文献   

9.
We report the mineralogy and texture of magnetite grains, a magnetite‐dolomite assemblage, and the adjacent mineral phases in five hydrated fine‐grained Antarctic micrometeorites (H‐FgMMs). Additionally, we measured the oxygen isotopic composition of magnetite grains and a magnetite‐dolomite assemblage in these samples. Our mineralogical study shows that the secondary phases identified in H‐FgMMs have similar textures and chemical compositions to those described previously in other primitive solar system materials, such as carbonaceous chondrites. However, the oxygen isotopic compositions of magnetite in H‐FgMMs span a range of ?17O values from +1.3‰ to +4.2‰, which is intermediate between magnetites measured in carbonaceous and ordinary chondrites (CCs and OCs). The δ18O values of magnetites in one H‐FgMM have a ~27‰ mass‐dependent spread in a single 100 × 200 μm particle, indicating that there was a localized control of the fluid composition, probably due to a low water‐to‐rock mass ratio. The ?17O values of magnetite indicate that H‐FgMMs sampled a different aqueous fluid than ordinary and carbonaceous chondrites, implying that the source of H‐FgMMs is probably distinct from the asteroidal source of CCs and OCs. Additionally, we analyzed the oxygen isotopic composition of a magnetite‐dolomite assemblage in one of the H‐FgMMs (sample 03‐36‐46) to investigate the temperature at which these minerals coprecipitated. We have used the oxygen isotope fractionation between the coexisting magnetite and dolomite to infer a precipitation temperature between 160 and 280 °C for this sample. This alteration temperature is ~100–200 °C warmer than that determined from a calcite‐magnetite assemblage from the CR2 chondrite Al Rais, but similar to the estimated temperature of aqueous alteration for unequilibrated OCs, CIs, and CMs. This suggests that the sample 03‐36‐46 could come from a parent body that was large enough to attain temperatures as high as the OCs, CIs, and CMs, which implies an asteroidal origin for this particular H‐FgMM.  相似文献   

10.
Mineral compositions and abundances derived from visible/near-infrared (VIS/NIR or VNIR) spectra are used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid belt. Using a suite of 48 equilibrated (types 4-6) ordinary (H, L, and LL) chondrites containing orthopyroxene, clinopyroxene, and olivine, new relationships between spectra and mineralogy have been established. Contrary to previous suggestions, no meaningful correlation is observed between band parameters and cpx/(opx + cpx) ratios. We derive new calibrations for determining mineral abundances (ol/(ol + px)) and mafic silicate compositions (Fa in olivine, Fs in pyroxene) from VIS/NIR spectra. These calibrations confirm that band area ratio (BAR) is controlled by mineral abundances, while Band I center is controlled by mafic silicate compositions. Spectrally-derived mineralogical parameters correctly classify H, L and LL chondrites in ∼80% of cases, suggesting that these are robust relationships that can be applied to S(IV) asteroids with ordinary chondrites mineralogies. Comparison of asteroids and meteorites using these new mineralogical parameters has the advantage that H, L and LL chemical groups were originally defined on the basis of mafic silicate compositions.  相似文献   

11.
Carbonaceous chondrites are classified into several groups. However, some are ungrouped. We studied one such ungrouped chondrite, Y‐82094, previously classified as a CO. In this chondrite, chondrules occupy 78 vol%, and the matrix is distinctly poor in abundance (11 vol%), compared with CO and other C chondrites. The average chondrule size is 0.33 mm, different from that in C chondrites. Although these features are similar to those in ordinary chondrites, Y‐82094 contains 3 vol% Ca‐Al‐rich inclusions and 5% amoeboid olivine aggregates (AOAs). Also, the bulk composition resembles that of CO chondrites, except for the volatile elements, which are highly depleted. The oxygen isotopic composition of Y‐82094 is within the range of CO and CV chondrites. Therefore, Y‐82094 is an ungrouped C chondrite, not similar to any other C chondrite previously reported. Thin FeO‐rich rims on AOA olivine and the mode of occurrence of Ni‐rich metal in the chondrules indicate that Y‐82094 is petrologic type 3.2. The extremely low abundance of type II chondrules and high abundance of Fe‐Ni metal in the chondrules suggest reducing condition during chondrule formation. The depletion of volatile elements indicates that the components formed under high‐temperature conditions, and accreted to the parent body of Y‐82094. Our study suggests a wider range of formation conditions than currently recorded by the major C chondrite groups. Additionally, Y‐82094 may represent a new, previously unsampled, asteroidal body.  相似文献   

12.
Abstract— Modal mineralogies of individual, equilibrated (petrologic type 4–6 L and LL chondrites have been measured using an electron microprobe mapping technique, and the chemical compositions of coexisting silicate minerals have been analyzed. Progressive changes in the relative abundances and in the molar Fe/Mn and Fe/Mg ratios of olivine, low‐Ca pyroxene, and diopside occur with increasing metamorphic grade. Variations in olivine/low‐Ca pyroxene ratios (Ol/Px) and in metal abundances and compositions with petrologic type support the hypothesis that oxidation of metallic iron accompanied thermal metamorphism in ordinary chondrites. Modal Ol/Px ratios are systematically lower than normative Ol/Px ratios for the same meteorites, suggesting that the commonly used C.I.P.W. norm calculation procedure may not adequately estimate silicate mineral abundances in reduced chondrites. Ol/Px ratios calculated from visible and near‐infrared (VISNIR) reflectance spectra of the same meteorites are not in agreement with other Ol/Px determinations, possibly because of spectral complexities arising from other minerals in chondrites. Characteristic features in VISNIR spectra are sensitive to the proportions and compositions of olivine and pyroxenes, the minerals most affected by oxidative metamorphism. This work may allow spectral calibration for the determination of mineralogy and petrologic type, and thus may be useful for spectroscopic studies of asteroids.  相似文献   

13.
Abstract— Isheyevo is a metal‐rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7–90 vol%). The metal‐rich lithologies with 50–60 vol% of Fe,Ni metal are dominant. The metal‐rich and metal‐poor lithologies are most similar to the CBb and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components—Fe,Ni metal, chondrules, refractory inclusions (Ca, Al‐rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts—but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non‐porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in 15N (δ15N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non‐magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CBb chondrites, ～20% of Fe,Ni‐metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni‐rich (>20 wt% Ni). In contrast to CBb and CH chondrites, most metal grains are thermally decomposed into Ni‐rich and Ni‐poor phases. Similar to CH chondrites, chondrules have porphyritic and non‐porphyritic textures and ferromagnesian (type I and II), silica‐rich, and aluminum‐rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite‐, grossite‐, and melilite‐rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico‐chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine‐grained dust. Such a scenario is consistent with the presence of solar wind—implanted noble gases in Isheyevo and with its comparatively old K‐Ar age. We cannot exclude that the K‐Ar system was affected by a later collisional event. The cosmic‐ray exposure (CRE) age of Isheyevo determined by cosmogenic 38Ar is ～34 Ma, similar to that of the Bencubbin (CBa) meteorite.  相似文献   

14.
Abstract— CM2 carbonaceous chondrites are the most primitive material present in the solar system, and some of their subtypes, the CM and CI chondrites, contain up to 2 wt% of organic carbon. The CM2 carbonaceous chondrites contain a wide variety of complex amino acids, while the CI1 meteorites Orgueil and Ivuna display a much simpler composition, with only glycine and β‐alanine present in significant abundances. CM1 carbonaceous chondrites show a higher degree of aqueous alteration than CM2 types and therefore provide an important link between the CM2 and CI1 carbonaceous chondrites. Relative amino acid concentrations have been shown to be indicative for parent body processes with respect to the formation of this class of compounds. In order to understand the relationship of the amino acid composition between these three types of meteorites, we have analyzed for the first time three Antarctic CM1 chondrites, Meteorite Hills (MET) 01070, Allan Hills (ALH) 88045, and LaPaz Icefield (LAP) 02277, using gas chromatography‐mass spectrometry (GC‐MS) and high performance liquid chromatography‐fluorescence detection (HPLC‐FD). The concentrations of the eight most abundant amino acids in these meteorites were compared to those of the CM2s Murchison, Murray, Mighei, Lewis Cliff (LEW) 90500, ALH 83100, as well as the CI1s Orgueil and Ivuna. The total amino acid concentration in CM1 carbonaceous chondrites was found to be much lower than the average of the CM2s. Relative amino acid abundances were compared in order to identify synthetic relationships between the amino acid compositions in these meteorite classes. Our data support the hypothesis that amino acids in CM‐ and CI‐type meteorites were synthesized under different physical and chemical conditions and may best be explained with differences in the abundances of precursor compounds in the source regions of their parent bodies in combination with the decomposition of amino acids during extended aqueous alteration.  相似文献   

15.
Northwest Africa (NWA) 6112, Miller Range (MIL) 090206 (plus its pairs: MIL 090340 and MIL 090405), and Divnoe are olivine‐rich ungrouped achondrites. We investigated and compared their petrography, mineralogy, and olivine fabrics. We additionally measured the oxygen isotopic compositions of NWA 6112. They show similar petrography, mineralogy, and oxygen isotopic compositions and we concluded that these five meteorites are brachinite clan meteorites. We found that NWA 6112 and Divnoe had a c axis concentration pattern of olivine fabrics using electron backscattered diffraction (EBSD). NWA 6112 and Divnoe are suggested to have been exposed to magmatic melt flows during their crystallization on their parent body. On the other hand, the three MIL meteorites have b axis concentration patterns of olivine fabrics. This indicates that the three MIL meteorites may be cumulates where compaction of olivine grains was dominant. Alternatively, they formed as residues and were exposed to olivine compaction. The presence of two different olivine fabric patterns implies that the parent body(s) of brachinite clan meteorites experienced diverse igneous processes.  相似文献   

16.
High‐precision isotope data of meteorites show that the long‐standing notion of a “chondritic uniform reservoir” is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this “isotopic crisis” and to better understand the genetic relations of meteorites and the Earth‐forming reservoir, we performed a comprehensive petrographic, elemental, and multi‐isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent‐body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium‐tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent‐body is not the “missing link” that could explain the composition of the Earth by the mixing of known meteorites. Until this “missing link” or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth.  相似文献   

17.
Tibooburra, a new meteorite find from western New South Wales, belongs to the Vigarano subgroup of the carbonaceous chondrites and, on the basis of its opaque mineralogy, appears to be oxidised. Petrological evidence suggests that, like the Allende meteorite, Tibooburra is a CV3 chondrite which has experienced greater metamorphic effects than other CV3 meteorites. Tibooburra has a bulk composition intermediate between the CO and less altered CV chondrites. This transitional nature is exhibited by several elements and is convincingly displayed by the multivariate techniques of cluster analysis and principal component analysis. Tibooburra thus resembles several other CV chondrites, such as Coolidge and Karoonda, which have been strongly metamorphosed. This group of meteorites is believed to have accreted early in the history of the Vigarano parent body and, as a result, contain greater quantities of high temperature Ca-Al-rich inclusions but less low temperature matrix and volatile phases than other CV chondrites. Furthermore, in these meteorites both the matrix and magnesium silicate phases appear to be more iron rich than those in later accreted meteorites. Subsequently, these deeper seated meteorites have undergone more pronounced thermal metamorphism than those located in shallower portions of the parent body.  相似文献   

18.
Abstract— Oxygen‐isotopic compositions were determined for a suite of enstatite chondrites and aubrites. In agreement with previous work (Clayton et al., 1984), most samples have O‐isotopic compositions close to the terrestrial fractionation line (TFL), and there appear to be no significant differences in O‐isotopic compositions between individual EH and EL chondrites and aubrites. Five enstatite meteorites have O‐isotopic compositions that are significantly different from the other samples and >0.2% away from the TFL. Two of these have petrographic evidence of brecciation and interaction between other meteorite types; for the other three, similar scenarios are suggested. There appears to be a systematic increase in δ18O from enstatite chondrites (both EH and EL) of petrologic type 3 to those of type 6. There is also good evidence that the EH meteorites do not fall along a mass fractionation line but along a line slope 0.66. At the present time, detailed understanding of the origin of these O‐isotopic systematics remain elusive but clearly point to a complex accretion history, parent‐body evolution, or both.  相似文献   

19.
Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass‐independent and mass‐dependent isotopic fractionations. Due to the analytical challenges to obtain high‐precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high‐precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from ?2.34‰ to ?0.18‰). However, K isotopic compositions of nearly all enstatite meteorites scatter around the bulk silicate earth (BSE) value. The average K isotopic composition of the eight enstatite chondrites (?0.47 ± 0.57‰) is indistinguishable from the BSE value (?0.48 ± 0.03‰), thus further corroborating the isotopic similarity between Earth's building blocks and enstatite meteorite precursors. We found no correlation of K isotopic compositions with the chemical groups, petrological types, shock degrees, and terrestrial weathering conditions; however, the variation of K isotopes among enstatite meteorite can be attributed to the parent‐body processing. Our sample of the main‐group aubrite MIL 13004 is exceptional and has an extremely light K isotopic composition (δ41K = ?2.34 ± 0.12‰). We attribute this unique K isotopic feature to the presence of abundant djerfisherite inclusions in our sample because this K‐bearing sulfide mineral is predicted to be enriched in 39K during equilibrium exchange with silicates.  相似文献   

20.
Abstract– A method is described for imaging in 3‐D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X‐ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long‐term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment‐dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non‐destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two‐thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth.  相似文献   

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