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1.
Abstract— We present the results of the infrared (IR) microscopic study of the anomalous carbonaceous chondrites Dhofar (Dho) 225 and Dhofar 735 in comparison to typical CM2 chondrites Cold Bokkeveld, Murray, and Mighei. The Fourier transform infrared (FTIR) 2.5–14 μm reflectance measurements were performed on conventional polished sections using an infrared microscope with a synchrotron radiation source. We demonstrate that the synchrotron‐based IR microspectroscopy is a useful, nondestructive tool for studying hydration states of meteorite constituents in situ. Our results show that the matrices of Dho 225 and Dho 735 are dehydrated compared to the matrices of typical CM2 chondrites. The spectra of the Dho 225 and Dho 735 matrices lack the 2.7–2.8 μm absorption feature present in the spectra of Cold Bokkeveld, Murray, and Mighei. Spectral signatures caused by Si‐O vibrations in fine‐grained, Fe‐rich olivines dominate the 10 μm spectral region in the spectra of Dho 225 and Dho 735 matrices, while the spectra of normal CM2 chondrites are dominated by spectral signatures due to Si‐O vibrations in phyllosilicates. We did not detect any hydrated phases in the spectra of Dho 225 and Dho 735 polished sections. In addition, the near‐infrared reflectance spectra of Dho 225 and Dho 735 bulk powders show spectral similarities to the Antarctic metamorphosed carbonaceous chondrites. We confirm the results of previous mineralogical, chemical, and isotopic studies indicating that the two meteorites from Oman are the first non‐Antarctic metamorphosed carbonaceous chondrites.  相似文献   

2.
Abstract– We used instrumental neutron activation analysis and petrography to determine bulk and phase compositions and textural characteristics of 15 carbonaceous chondrites of uncertain classification: Acfer 094 (type 3.0, ungrouped CM‐related); Belgica‐7904 (mildly metamorphosed, anomalous, CM‐like chondrite, possibly a member of a new grouplet that includes Wisconsin Range (WIS) 91600, Dhofar 225, and Yamato‐86720); Dar al Gani (DaG) 055 and its paired specimen DaG 056 (anomalous, reduced CV3‐like); DaG 978 (type 3 ungrouped); Dominion Range 03238 (anomalous, magnetite‐rich CO3.1); Elephant Moraine 90043 (anomalous, magnetite‐bearing CO3); Graves Nunataks 98025 (type 2 or type 3 ungrouped); Grosvenor Mountains (GRO) 95566 (anomalous CM2 with a low degree of aqueous alteration); Hammadah al Hamra (HaH) 073 (type 4 ungrouped, possibly related to the Coolidge‐Loongana [C‐L] 001 grouplet); Lewis Cliff (LEW) 85311 (anomalous CM2 with a low degree of aqueous alteration); Northwest Africa 1152 (anomalous CV3); Pecora Escarpment (PCA) 91008 (anomalous, metamorphosed CM); Queen Alexandra Range 99038 (type 2 ungrouped); Sahara 00182 (type 3 ungrouped, possibly related to HaH 073 and/or to C‐L 001); and WIS 91600 (mildly metamorphosed, anomalous, CM‐like chondrite, possibly a member of a new grouplet that includes Belgica‐7904, Dhofar 225, and Y‐86720). Many of these meteorites show fractionated abundance patterns, especially among the volatile elements. Impact volatilization and dehydration as well as elemental transport caused by terrestrial weathering are probably responsible for most of these compositional anomalies. The metamorphosed CM chondrites comprise two distinct clusters on the basis of their Δ17O values: approximately ?4‰ for PCA 91008, GRO 95566, DaG 978, and LEW 85311, and approximately 0‰ for Belgica‐7904 and WIS 91600. These six meteorites must have been derived from different asteroidal regions.  相似文献   

3.
We have conducted hydration–dehydration experiments on terrestrial olivine to investigate the behavior of oxygen isotopic fractionation to test the hypothesis that multiple cycles of aqueous and thermal processing on a parent asteroid comprise a genetic relationship between CM2s and metamorphosed carbonaceous chondrites (MCCs). Two experiments were undertaken. In the first experiment, serpentine was obtained by hydrating terrestrial olivine (Fo90.9) in the laboratory. During this experiment, olivine was reacted with isotopically heavy water (δ18O 21.5‰) at T = 300 °C,  = 300 bar, for 100 days. The oxygen isotopic composition of the experimental serpentine was enriched in 18O (by 10 ‰ in δ18O) due to exchange of oxygen isotopes between olivine and the 18O‐rich water. Dehydrated serpentine was then produced during laboratory heating experiment in vacuum, at T = 930 °C, for 1 h. The oxygen isotopic composition of the dehydrated serpentine was enriched in 18O by a further 7 ‰. The net result of the hydration–dehydration process was an enrichment of 18O in the final material by approximately 17‰. The new experimental results suggest that the oxygen isotopic compositions of MCCs of the Belgica‐like group, including Dhofar 225 and Dhofar 725, could be derived from those of typical CM2 chondrites via several cycles of hydration–dehydration caused by aqueous alteration and subsequent thermal metamorphism within their parent asteroids.  相似文献   

4.
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   

5.
Abstract— In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)‐792947, Y‐93408 and Y‐82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole‐rock oxygen isotopes and the chemical compositions are indicative of H chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2–3.4 (S1), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.  相似文献   

6.
We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine‐grained chondrule rims in six Mighei‐type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM‐related carbonaceous chondrite Sutter's Mill. Sixteen O‐anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix‐normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O‐rich grains are enriched in 17O, and could originate from low‐mass asymptotic giant branch stars. One grain is enriched in 17O and significantly depleted in 18O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core‐collapse supernova origin showing enhanced 18O/16O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O‐anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O‐rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe‐rich silicates, one forsterite‐like Mg‐rich silicate, two Al‐oxides with spinel‐like compositions, and one Fe‐(Mg‐)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm × 735 nm) revealed that it was crystalline åkermanite (Ca2Mg[Si2O7]) or a an åkermanite‐diopside (MgCaSi2O6) intergrowth.  相似文献   

7.
We have sampled sulfide grains from one pristine CM2 chondrite (Yamato [Y‐] 791198), one thermally metamorphosed CM2 chondrite (Y‐793321), and two anomalous, metamorphosed CM/CI‐like chondrites (Y‐86720 and Belgica [B‐] 7904) by the focused ion beam (FIB) technique and studied them by analytical transmission electron microscopy (TEM). Our study aims at exploring the potential of sulfide assemblages and microstructures to decipher processes and conditions of chondrite petrogenesis. Complex exsolution textures of pyrrhotite (crystallographic NC‐type with N ≈ 6), troilite, and pentlandite occur in grains of Y‐791198 and Y‐793321. Additionally, polycrystalline 4C‐pyrrhotite‐pentlandite‐magnetite aggregates occur in Y‐791198, pointing to diverse conditions of gas–solid interactions in the solar nebula. Coarser exsolution textures of Y‐793321 grains indicate higher long‐term average temperatures in the <100 °C range compared to Y‐791198 and other CM chondrites. Sulfide mineralogy of Y‐86720 and B‐7904 is dominated by aggregates of pure troilite and metal, indicating metamorphic equilibration at sulfur fugacities (fS2) of the iron‐troilite buffer. Absence of magnetite in equilibrium with sulfide and metal in Y‐86720 indicates higher peak temperatures compared with B‐7904, in which coexistence of troilite, metal, and magnetite constrains metamorphic temperature to less than 570 °C. NC‐pyrrhotite occurs in both meteorites as nm‐wide rims on troilite grains and, together with frequent anhydrite, indicates a retrograde metamorphic stage at higher fS2 slightly above the fayalite‐magnetite‐quartz‐pyrrhotite buffer. Fine‐grained troilite‐olivine intergrowths in both meteorites suggest the pre‐metamorphic presence of tochilinite‐serpentine interlayer phases, pointing to mineralogical CM affinity. Pseudomorphs after euhedral pyrrhotite crystals in Y‐86720 in turn suggest CI affinity as do previously published O isotopic data of both meteorites.  相似文献   

8.
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   

9.
Abstract— The petrogenesis of four lunar highlands meteorites, Dhofar 025 (Dho 025), Dhofar 081 (Dho 081), Dar al Gani 262 (DaG 262), and Dar al Gani 400 (DaG 400) were studied. For Dho 025, measured oxygen isotopic values and Fe‐Mn ratios for mafic minerals provide corroboratory evidence that it originated on the Moon. Similarly, Fe‐Mn ratios in the mafic minerals of Dho 081 indicate lunar origin. Lithologies in Dho 025 and Dho 081 include lithic clasts, granulites, and mineral fragments. A large number of lithic clasts have plagioclase AN# and coexisting mafic mineral Mg# that plot within the “gap” separating ferroan anorthosite suite (FAN) and high‐magnesium suite (HMS) rocks. This is consistent with whole rock Ti‐Sm ratios for Dho 025, Dho 081, and DaG 262, which are also intermediate compared to FAN and HMS lithologies. Although ion microprobe analyses performed on Dho 025, Dho 081, DaG 262, and DaG 400 clasts and minerals show far stronger FAN affinities than whole rock data suggest, most clasts indicate admixture of ≤12% HMS component based on geochemical modeling. In addition, coexisting plagioclase‐pyroxene REE concentration ratios in several clasts were compared to experimentally determined plagioclase‐pyroxene REE distribution coefficient ratios. Two Dho 025 clasts have concordant plagioclase‐pyroxene profiles, indicating that equilibrium between these minerals has been sustained despite shock metamorphism. One clast has an intermediate FAN‐HMS composition. These lunar meteorites appear to represent a type of highland terrain that differs substantially from the KREEP‐signatured impact breccias that dominate the lunar database. From remote sensing data, it is inferred that the lunar far side appears to have appropriate geochemical signatures and lithologies to be the source regions for these rocks; although, the near side cannot be completely excluded as a possibility. If these rocks are, indeed, from the far side, their geochemical characteristics may have far‐reaching implications for our current scientific understanding of the Moon.  相似文献   

10.
Abstract– Stable hydrogen, carbon, and nitrogen isotopic ratios (δD, δ13C, and δ15N) of organic compounds can reveal information about their origin and formation pathways. Several formation mechanisms and environments have been postulated for the amino acids detected in carbonaceous chondrites. As each proposed mechanism utilizes different precursor molecules, the isotopic signatures of the resulting amino acids may indicate the most likely of these pathways. We have applied gas chromatography with mass spectrometry and combustion isotope ratio mass spectrometry to measure the compound‐specific C, N, and H stable isotopic ratios of amino acids from seven CM and CR carbonaceous chondrites: CM1/2 Allan Hills (ALH) 83100, CM2 Murchison, CM2 Lewis Cliff (LEW) 90500, CM2 Lonewolf Nunataks (LON) 94101, CR2 Graves Nunataks (GRA) 95229, CR2 Elephant Moraine (EET) 92042, and CR3 Queen Alexandra Range (QUE) 99177. We compare the isotopic compositions of amino acids in these meteorites with predictions of expected isotopic enrichments from potential formation pathways. We observe trends of decreasing δ13C and increasing δD with increasing carbon number in the α‐H, α‐NH2 amino acids that correspond to predictions made for formation via Strecker‐cyanohydrin synthesis. We also observe light δ13C signatures for β‐alanine, which may indicate either formation via Michael addition or via a pathway that forms primarily small, straight‐chain, amine‐terminal amino acids (n‐ω‐amino acids). Higher deuterium enrichments are observed in α‐methyl amino acids, indicating formation of these amino acids or their precursors in cold interstellar or nebular environments. Finally, individual amino acids are more enriched in deuterium in CR chondrites than in CM chondrites, reflecting different parent‐body chemistry.  相似文献   

11.
The CM carbonaceous chondrite meteorites experienced aqueous alteration in the early solar system. They range from mildly altered type 2 to almost completely hydrated type 1 chondrites, and offer a record of geochemical conditions on water‐rich asteroids. We show that CM1 chondrites contain abundant (84–91 vol%) phyllosilicate, plus olivine (4–8 vol%), magnetite (2–3 vol%), Fe‐sulfide (<5 vol%), and calcite (<2 vol%). The CM1/2 chondrites contain phyllosilicate (71–88 vol%), olivine (4–20 vol%), enstatite (2–6 vol%), magnetite (2–3 vol%), Fe‐sulfides (1–2 vol%), and calcite (~1 vol%). As aqueous alteration progressed, the abundance of Mg‐serpentine and magnetite in the CM chondrites increased. In contrast, calcite abundances in the CM1/2 and CM1 chondrites are often depleted relative to the CM2s. The modal data support the model, whereby metal and Fe‐rich matrix were the first components to be altered on the CM parent body(ies), before further hydration attacked the coarser Mg‐rich silicates found in chondrules and fragments. Based on the absence of tochilinite, we suggest that CM1 chondrites experienced increased alteration due to elevated temperatures (>120 °C), although higher water/rock ratios may also have played a role. The modal data provide constraints for interpreting the composition of asteroids and the mineralogy of samples returned from these bodies. We predict that “CM1‐like” asteroids, as has been proposed for Bennu—target for the OSIRIS‐REx mission—will have a high abundance of Mg‐rich phyllosilicates and Fe‐oxides, but be depleted in calcite.  相似文献   

12.
Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for 26Al‐26Mg systematics, only one aluminum‐rich chondrule contains excess 26Mg from the in situ decay of 26Al; the inferred initial ratio (26Al/27Al)o = (6.8 ± 2.4) × 10?6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ?0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more 16O‐rich compositions with a range in δ18O of about ?12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess 26Mg from the in situ decay of 26Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of 26Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that 26Al was at least approximately homogeneously distributed across the chondrite‐forming region.  相似文献   

13.
Abstract— In situ io n microprobe analyses of spinel in refractory calcium‐aluminium‐rich inclusions (CAIs) from type 3 EH chondrites yield 16O‐rich compositions (δ 18O and δ 17O about‐40‰). Spinel and feldspar in a CAI from an EL3 chondrite have significantly heavier isotopic compositions (δ 18O and δ 17O about ?5‰). A regression through the data results in a line with slope 1.0 on a three‐isotope plot, similar to isotopic results from unaltered minerals in CAIs from carbonaceous chondrites. The existence of CAIs with 16O‐rich and 16O‐poor compositions in carbonaceous as well as enstatite chondrites indicates that CAIs formed in at least two temporally or spatially distinct oxygen reservoirs. General similarities in oxygen isotopic compositions of CAIs from enstatite, carbonaceous, and ordinary chondrites indicate a common nebular mechanism or locale for the production of most CAIs.  相似文献   

14.
Abstract— Dhofar 287 (Dho 287) is a new lunar meteorite, found in Oman on January 14, 2001. The main portion of this meteorite (Dho 287A) consists of a mare basalt, while a smaller portion of breccia (Dho 287B) is attached on the side. Dho 287A is only the fourth crystalline mare basalt meteorite found on Earth to date and is the subject of the present study. The basalt consists mainly of phenocrysts of olivine and pyroxene set in a finer‐grained matrix, which is composed of elongated pyroxene and plagioclase crystals radiating from a common nucleii. The majority of olivine and pyroxene grains are zoned, from core to rim, in terms of Fe and Mg. Accessory minerals include ilmenite, chromite, ulvöspinel, troilite, and FeNi metal. Chromite is invariably mantled by ulvöspinel. This rock is unusually rich in late‐stage mesostasis, composed largely of fayalite, Si‐K‐Ba‐rich glass, fluorapatite, and whitlockite. In texture and mineralogy, Dho 287A is a low‐Ti mare basalt, with similarities to Apollo 12 (A‐12) and Apollo 15 (A‐15) basalts. However, all plagioclase is now present as maskelynite, and its composition is atypical for known low‐Ti mare basalts. The Fe to Mn ratios of olivine and pyroxene, the presence of FeNi metal, and the bulk‐rock oxygen isotopic ratios, along with several other petrological features, are evidence for the lunar origin for this meteorite. Whole‐rock composition further confirms the similarity of Dho 287A with A‐12 and A‐15 samples but requires possible KREEP assimilation to account for its rare‐earth‐element (REE) contents. Cooling‐rate estimates, based on Fo zonation in olivine, yield values of 0.2–0.8°C/hr for the lava, typical for the center of a 10–20 m thick flow. The recalculated major‐element concentrations, after removing 10–15% modal olivine, are comparable to typical A‐15 mare basalts. Crystallization modeling of the recalculated Dho 287A bulk‐composition yields a reasonable fit between predicted and observed mineral abundances and compositions.  相似文献   

15.
CV (Vigarano type) carbonaceous chondrites, comprising Allende‐like (CVoxA) and Bali‐like (CVoxB) oxidized and reduced (CVred) subgroups, experienced differing degrees of fluid‐assisted thermal and shock metamorphism. The abundance and speciation of secondary minerals produced during asteroidal alteration differ among the subgroups: (1) ferroan olivine and diopside–hedenbergite solid solution pyroxenes are common in all CVs; (2) nepheline and sodalite are abundant in CVoxA, rare in CVred, and absent in CVoxB; (3) phyllosilicates and nearly pure fayalite are common in CVoxB, rare in CVred, and virtually absent in CVoxA; (4) andradite, magnetite, and Fe‐Ni‐sulfides are common in oxidized CVs, but rare in reduced CVs; the latter contain kirschsteinite instead. Thus, a previously unrecognized correlation exists between meteorite bulk permeabilities and porosities with the speciation of the Ca‐, Fe‐rich silicates (pyroxenes, andradite, kirschsteinite) among the CVox and CVred meteorites. The extent of secondary mineralization was controlled by the distribution of water ices, permeability, and porosity, which in turn were controlled by impacts on the asteroidal parent body. More intense shock metamorphism in the region where the reduced CVs originated decreased their porosity and permeability while simultaneously expelling intergranular ices and fluids. The mineralogy, petrography, and bulk chemical compositions of both the reduced and oxidized CV chondrites indicate that mobile elements were redistributed between Ca,Al‐rich inclusions, dark inclusions, chondrules, and matrices only locally; there is no evidence for large‐scale (>several cm) fluid transport. Published 53Mn‐53Cr ages of secondary fayalite in CV, CO, and unequilibrated ordinary chondrites, and carbonates in CI, CM, and CR chondrites are consistent with aqueous alteration initiated by heating of water ice‐bearing asteroids by decay of 26Al, not shock metamorphism.  相似文献   

16.
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   

17.
Abstract— Iron‐rich aureoles in CM carbonaceous chondrites are previously unidentified domains of aqueously altered matrix material, whose FeO content may exceed that of the surrounding matrix by up to more than 15 wt%. We describe the petrography and mineralogy of these objects in the CM chondrites Murray, Murchison, and Allan Hills (ALH) 81002. The size of Fe‐rich aureoles ranges from a few hundred microns to several millimeters in diameter and appears to be a function of the degree of alteration of the host chondrite. The origin of Fe‐rich aureoles is related to the alteration of large metal grains that has resulted in the formation of characteristic PCP‐rich reaction products that are frequently observed at the centers of the aureoles. This suggests that Fe‐rich aureoles in CM chondrites are the result of the mobilization of Fe from altering metal grains into the matrix. The fact that Fe‐rich aureoles enclose numerous chondritic components such as chondrules, calcium‐aluminum‐rich inclusions (CAIs), and mineral fragments, as well as their radial symmetric appearance, are strong evidence that they formed in situ and that significant directional fluid flow was not involved in the alteration process. This and additional constraints, such as the distribution of S and other elements, as well as the inferred alteration conditions, are consistent with in situ parent‐body alteration. The observations are, however, entirely incompatible with preaccretionary alteration models in which the individual CM chondrite components have experienced diverse alteration histories. The presence of numerous intact aureoles in the brecciated CM chondrites Murray and Murchison further suggests that the alteration occurred largely after brecciation affected these meteorites. Therefore, the progressive aqueous alteration of CM chondrites may not be necessarily coupled to brecciation as has been previously proposed.  相似文献   

18.
Abstract— I have determined the composition via instrumental neutron activation analysis of a bulk pristine sample of the Tagish Lake carbonaceous chondrite fall, along with bulk samples of the CI chondrite Orgueil and of several CM chondrites. Tagish Lake has a mean of refractory lithophile element/Cr ratios like those of CM chondrites, and distinctly higher than the CI chondrite mean. Tagish Lake exhibits abundances of the moderately volatile lithophile elements Na and K that are slightly higher than those of mean CM chondrites. Refractory through moderately volatile siderophile element abundances in Tagish Lake are like those of CM chondrites. Tagish Lake is distinct from CM chondrites in abundances of the most volatile elements. Mean CI‐normalized Se/Co, Zn/Co and Cs/Co for Tagish Lake are 0.68 ± 0.01, 0.71 ± 0.07 and 0.76 ± 0.02, while for all available CM chondrite determinations, these ratios lie between 0.31 and 0.61, between 0.32 and 0.58, and between 0.39 and 0.74, respectively. Considering petrography, and oxygen isotopic and elemental compositions, Tagish Lake is an ungrouped member of the carbonaceous chondrite clan. The overall abundance pattern is similar to those of CM chondrites, indicating that Tagish Lake and CMs experienced very similar nebular fractionations. Bells is a CM chondrite with unusual petrologic characteristics. Bells has a mean CI‐normalized refractory lithophile element/Cr ratio of 0.96, lower than for any other CM chondrite, but shows CI‐normalized moderately volatile lithophile element/Cr ratios within the ranges of other CM chondrites, except for Na which is low. Iridium, Co, Ni and Fe abundances are like those of CM chondrites, but the moderately volatile siderophile elements, Au, As and Sb, have abundances below the ranges for CM chondrites. Abundances of the moderately volatile elements Se and Zn of Bells are within the CM ranges. Bells is best classified as an anomalous CM chondrite.  相似文献   

19.
CM chondrites are a group of primitive meteorites that have recorded the alteration history of the early solar system. We report the occurrence, chemistry, and oxygen isotopic compositions of P‐O‐rich sulfide phase in two CM chondrites (Grove Mountains [GRV] 021536 and Murchison). This P‐O‐rich sulfide is a polycrystalline aggregate of nanometer‐size grains. It occurs as isolated particles or aggregates in both CM chondrites. These grains, in the matrix and in type‐I chondrules from Murchison, were partially altered into tochilinite; however, grains enclosed by Ca‐carbonate are much less altered. This P‐O‐rich sulfide in Murchison is closely associated with magnetite, FeNi phosphide, brezinaite (Cr3S4), and eskolaite (Cr2O3). In addition to sulfur as the major component, this sulfide contains ~6.3 wt% O, ~5.4 wt% P, and minor amounts of hydrogen. Analyses of oxygen isotopes by SIMS resulted in an average δ18O value of ?22.5 ‰ and an average Δ17O value of 0.2 ± 9.2 ‰ (2σ). Limited variations in both chemical compositions and electron‐diffraction patterns imply that the P‐O‐rich sulfide may be a single phase rather than a polyphase mixture. Several features indicate that this P‐O‐rich sulfide phase formed at low temperature on the parent body, most likely through the alteration of FeNi metal (a) close association with other low‐temperature alteration products, (b) the presence of hydrogen, (c) high Δ17O values and the presence in altered mesostasis of type‐I chondrules and absence in type‐II chondrules. The textural relations of the P‐O‐rich sulfide and other low‐temperature minerals reveal at least three episodic‐alteration events on the parent body of CM chondrites (1) formation of P‐O‐rich sulfide during sulfur‐rich aqueous alteration of P‐rich FeNi metal, (2) formation of Ca‐carbonate during local carbonation, and (3) alteration of P‐O‐rich sulfide and formation of tochilinite during a period of late‐stage intensive aqueous alteration.  相似文献   

20.
Abstract– Although iron isotopes are increasingly used for meteorites studies, no attempt has been made to evaluate the effect of terrestrial weathering on this isotopic tracer. We have thus conducted a petrographic, chemical, and iron isotopic study of equilibrated ordinary chondrites (OC) recovered from hot Moroccan and Algerian Saharan deserts environment. As previously noticed, we observe that terrestrial desertic weathering is characterized by the oxidation of Fe‐Ni metal (Fe0), sulfide and Fe2+ occurring in olivine and pyroxene. It produces Fe‐oxides and oxyhydroxides that partially replace metal, sulfide grains and also fill fractures. The bulk chemical compositions of the ordinary chondrites studied show a strong Sr and Ba enrichment and a S depletion during weathering. Bulk meteoritic iron isotope compositions are well correlated with the degree of weathering and S, Sr, and Ba contents. Most weathered chondrites display the heaviest isotopic composition, by up to 0.1‰, which is of similar magnitude to the isotopic variations resulting from meteorite parent bodies’ formation and evolution. This is probably due to the release of isotopically light Fe2+ to waters on the Earth’s surface. Hence, when subtle Fe isotopic effects have to be studied in chondrites, meteorites with weathering grade above W2 should be avoided.  相似文献   

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