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1.
The presence of primary iron sulfides that appear to be aqueously altered in CM and CR carbonaceous chondrites provides the potential to study the effects and, by extension, the conditions of aqueous alteration. In this work, we have used SEM, TEM, and EPMA techniques to characterize primary sulfides that show evidence of secondary alteration. The alteration styles consist of primary pyrrhotite altering to secondary pentlandite (CMs only), magnetite (CMs and CRs), and phyllosilicates (CMs only) in grains that initially formed by crystallization from immiscible sulfide melts in chondrules (pyrrhotite‐pentlandite intergrowth [PPI] grains). Textural, microstructural, and compositional data from altered sulfides in a suite of CM and CR chondrites have been used to constrain the conditions of alteration of these grains and determine their alteration mechanisms. This work shows that the PPI grains exhibit two styles of alteration—one to form porous pyrrhotite‐pentlandite (3P) grains by dissolution of precursor PPI grain pyrrhotite and subsequent secondary pentlandite precipitation (CMs only), and the other to form the altered PPI grains by pseudomorphic replacement of primary pyrrhotite by magnetite (CMs and CRs) or phyllosilicates (CMs only). The range of alteration textures and products is the result of differences in conditions of alteration due to the role of microchemical environments and/or brecciation. Our observations show that primary sulfides are sensitive indicators of aqueous alteration processes in CM and CR chondrites.  相似文献   

2.
Abstract– CM chondrites were subjected to aqueous alteration and, in some cases, to secondary metamorphic heating. The effects of these processes vary widely, and have mainly been documented in silicate phases. Herein, we report the characteristic features of Fe‐Ni metal and sulfide phases in 13 CM and 2 CM‐related chondrites to explore the thermal history of these chondrites. The texture and compositional distribution of the metal in CM are different from those in unequilibrated ordinary and CO chondrites, but most have similarities to those in highly primitive chondrites, such as CH, CR, and Acfer 094. We classified the CM samples into three categories based on metal composition and sulfide texture. Fe‐Ni metal in category A is kamacite to martensite. Category B is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite. Opaque mineral assemblages of category C are typically kamacite, Ni‐Co‐rich metal, and pyrrhotite. These categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration. The characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration. Category A CM chondrites are unheated, whereas those in category B experienced small degrees of secondary heating. CMs in category C were subjected to the most severe secondary heating process. Thus, opaque minerals can provide constraints on the thermal history for CM chondrites.  相似文献   

3.
The Mighei-like carbonaceous (CM) chondrites, the most abundant carbonaceous chondrite group by number, further our understanding of processes that occurred in their formation region in the protoplanetary disk and in their parent body/bodies and provide analogs for understanding samples returned from carbonaceous asteroids. Chondrules in the CMs are commonly encircled by fine-grained rims (FGRs) whose origins are debated. We present the abundances, sizes, and petrographic observations of FGRs in six CMs that experienced varying intensities of parent body processing, including aqueous and thermal alteration. The samples studied here, in approximate order of increasing thermal alteration experienced, are Allan Hills 83100, Murchison, Meteorite Hills 01072, Elephant Moraine 96029, Yamato-793321, and Pecora Escarpment 91008. Based on observations of these CM chondrites, we recommend a new average apparent (2-D) chondrule diameter of 170 μm, which is smaller than previous estimates and overlaps with that of the Ornans-like carbonaceous (CO) chondrites. Thus, we suggest that chondrule diameters are not diagnostic for distinguishing between CM and CO chondrites. We also argue that chondrule foliation noted in ALH 83100, MET 01072, and Murchison resulted from multiple low-intensity impacts; that FGRs in CMs formed in the protoplanetary disk and were subsequently altered by both aqueous and thermal secondary alteration processes in their parent asteroid; and that the heat experienced by some CM chondrites may have originated from solar radiation of their source body/bodies during close solar passage as evidenced by the presence of evolved desiccation cracks in FGRs that formed by recurrent wetting and desiccation cycles.  相似文献   

4.
Here, we present the results of a multitechnique study of the bulk properties of insoluble organic material (IOM) from the Tagish Lake meteorite, including four lithologies that have undergone different degrees of aqueous alteration. The IOM C contents of all four lithologies are very uniform and comprise about half the bulk C and N contents of the lithologies. However, the bulk IOM elemental and isotopic compositions vary significantly. In particular, there is a correlated decrease in bulk IOM H/C ratios and δD values with increasing degree of alteration—the IOM in the least altered lithology is intermediate between CM and CR IOM, while that in the more altered lithologies resembles the very aromatic IOM in mildly metamorphosed CV and CO chondrites, and heated CMs. Nuclear magnetic resonance (NMR) spectroscopy, C X‐ray absorption near‐edge (XANES), and Fourier transform infrared (FTIR) spectroscopy confirm and quantitate this transformation from CR‐like, relatively aliphatic IOM functional group chemistry to a highly aromatic one. The transformation is almost certainly thermally driven, and probably occurred under hydrothermal conditions. The lack of a paramagnetic shift in 13C NMR spectra and 1s‐σ* exciton in the C‐XANES spectra, both typically seen in metamorphosed chondrites, shows that the temperatures were lower and/or the timescales were shorter than experienced by even the least metamorphosed type 3 chondrites. Two endmember models were considered to quantitatively account for the changes in IOM functional group chemistry, but the one in which the transformations involved quantitative conversion of aliphatic material to aromatic material was the more successful. It seems likely that similar processes were involved in producing the diversity of IOM compositions and functional group chemistries among CR, CM, and CI chondrites. If correct, CRs experienced the lowest temperatures, while CM and CI chondrites experienced similar more elevated temperatures. This ordering is inconsistent with alteration temperatures based on mineralogy and O isotopes.  相似文献   

5.
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   

6.
The highly hydrated, petrologic type 1 CM and CI carbonaceous chondrites likely derived from primitive, water‐rich asteroids, two of which are the targets for JAXA's Hayabusa2 and NASA's OSIRIS‐REx missions. We have collected visible and near‐infrared (VNIR) and mid infrared (MIR) reflectance spectra from well‐characterized CM1/2, CM1, and CI1 chondrites and identified trends related to their mineralogy and degree of secondary processing. The spectral slope between 0.65 and 1.05 μm decreases with increasing total phyllosilicate abundance and increasing magnetite abundance, both of which are associated with more extensive aqueous alteration. Furthermore, features at ~3 μm shift from centers near 2.80 μm in the intermediately altered CM1/2 chondrites to near 2.73 μm in the highly altered CM1 chondrites. The Christiansen features (CF) and the transparency features shift to shorter wavelengths as the phyllosilicate composition of the meteorites becomes more Mg‐rich, which occurs as aqueous alteration proceeds. Spectra also show a feature near 6 μm, which is related to the presence of phyllosilicates, but is not a reliable parameter for estimating the degree of aqueous alteration. The observed trends can be used to estimate the surface mineralogy and the degree of aqueous alteration in remote observations of asteroids. For example, (1) Ceres has a sharp feature near 2.72 μm, which is similar in both position and shape to the same feature in the spectra of the highly altered CM1 MIL 05137, suggesting abundant Mg‐rich phyllosilicates on the surface. Notably, both OSIRIS‐REx and Hayabusa2 have onboard instruments which cover the VNIR and MIR wavelength ranges, so the results presented here will help in corroborating initial results from Bennu and Ryugu.  相似文献   

7.
Abstract— Most studies of Tagish Lake have considered features that were either strongly affected by or formed during the extensive hydrous alteration experienced by this meteorite. This has led to some ambiguity as to whether Tagish Lake should be classified a CI, a CM, or something else. Unlike previous workers, we have focused upon the primary, anhydrous component of Tagish Lake, recovered through freeze‐thaw disaggregation and density separation and located by thin section mapping. We found many features in common with CMs that are not observed in CIs. In addition to the presence of chondrules and refractory forsterite (which distinguish Tagish Lake from the CIs), we found hibonite‐bearing refractory inclusions, spinel‐rich inclusions, forsterite aggregates, Cr‐, Al‐rich spinel, and accretionary mantles on many clasts, which clearly establishes a strong link between Tagish Lake and the CM chondrites. The compositions of isolated olivine crystals in Tagish Lake are also like those found in CMs. We conclude that the anhydrous inclusion population of Tagish Lake was, originally, very much like that of the known CM chondrites and that the inclusions in Tagish Lake are heavily altered, more so than even those in Mighei, which are more heavily altered than those in Murchison.  相似文献   

8.
Abstract— We determined the mineralogical and chemical characteristics and the He, Ne, and Ar isotopic abundances of 2 meteorites that fell in China and of 2 meteorites that were recovered by the 15th Chinese Antarctic Research Expedition. Guangmingshan (H5), Zhuanghe (H5), and Grove Mountain (GRV) 98002 (L5) yield cosmic ray exposure (CRE) ages of 68.7 ± 10.0 Ma, 3.8 ± 0.6 Ma, and 17.0 ± 2.5 Ma, respectively. These ages are within the range typically observed for the respective meteorite types. GRV 98004 (H5) had an extremely short parent body‐Earth transfer time of 0.052 ± 0.008 Ma. Its petrography and mineral chemistry are indistinguishable from other typical H5 chondrites. Only 3 other meteorites exist with similarly low CRE ages: Farmington (L5), Galim (LL6), and ALH 82100 (CM2). We show that several asteroids in Earth‐crossing orbits, or in the main asteroid belt with orbits close to an ejection resonance, are spectrally matching candidates and may represent immediate precursor bodies of meteorites with CRE ages ≤0.1 Ma.  相似文献   

9.
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   

10.
Abstract— Enstatite is one of the major constituent minerals in carbonaceous chondrites. Hydrothermal alteration experiments (26 in total) of enstatite were carried out at pH 0, 6, 7, 12, 13, and 14, at temperatures of 100, 200, and 300 °C, and for run durations of 24, 72, 168, and 336 h in order to provide constraints on the aqueous‐alteration conditions of the meteorites. The recovered samples were studied in detail by using powder X‐ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Under acidic and mildly acidic conditions (pH 0, 6), no significant alteration occurred, whereas under neutral to alkaline conditions (pH 7–14), serpentine and saponite formed in various proportions by replacing enstatite. At 300 °C for 168 h, serpentine formed under neutral to moderately alkaline conditions (pH 7, 12), and serpentine and saponite formed as unit cell‐scale coherent intergrowths under highly alkaline conditions (pH 13, 14). The amounts of phyllosilicates have a tendency to increase with increasing pH, temperature, and run duration. There is also a tendency for saponite to form at higher pH and temperature and under longer run‐durations than serpentine. The results indicate that alteration of enstatite is strongly dependent on the experimental conditions, especially pH. They suggest that CM chondrites experienced aqueous alteration under neutral to alkaline conditions, whereas CV and CI chondrites experienced aqueous alteration under more alkaline conditions. The results also suggest that aqueous alteration in CI chondrites occurred at higher temperatures than in CM chondrites, and aqueous alteration in CV chondrites occurred at even higher temperatures than in CI chondrites.  相似文献   

11.
We report a petrographic and mineralogical survey of Paris, a new CM chondrite considered to be the least‐altered CM identified so far (Hewins et al. 2014 ). Compared to other CMs, Paris exhibits (1) a higher concentration of Fe‐Ni metal beads, with nickel contents in the range 4.1–8.1 wt%; (2) the systematic presence of thin lamellae and tiny blebs of pentlandite in pyrrhotite grains; and (3) ubiquitous tochilinite/cronstedtite associations with higher FeO/SiO2 and S/SiO2 ratios. In addition, Paris shows the highest concentration of trapped 36Ar reported so far for a CM chondrite (Hewins et al. 2014 ). In combination with the findings of previous studies, our data confirm the reliability of (1) the alteration sequence based on the chemical composition of tochilinite/cronstedtite associations to quantify the fluid alteration processes and (2) the use of Cr content variability in type II ferroan chondrule olivine as a proxy of thermal metamorphism. In contrast, the scales based on (1) the Fe3+ content of serpentine in the matrix to estimate the degree of aqueous alteration and (2) the chemical composition of Fe‐Ni metal beads for quantifying the intensity of the thermal metamorphism are not supported by the characteristics of Paris. It also appears that the amount of trapped 36Ar is a sensitive indicator of the secondary alteration modifications experienced by chondrites, for both aqueous alteration and thermal metamorphism. Considering Paris, our data suggest that this chondrite should be classified as type 2.7 as it suffered limited but significant fluid alteration and only mild thermal metamorphism. These results point out that two separated scales should be used to quantify the degree of the respective role of aqueous alteration and thermal metamorphism in establishing the characteristics of CM chondrites.  相似文献   

12.
Abstract— The fall and recovery of the Tagish Lake meteorite in British Columbia in January 2000 provided a unique opportunity to study relatively pristine samples of carbonaceous chondrite material. Measurements of the oxygen isotopic composition of water extracted under stepped pyrolysis from a bulk sample of this meteorite have allowed us to make comparisons with similar data obtained from CI and CM chondrites and so further investigate any relationships that may exist between these meteorites. The much lower yield of water bearing a terrestrial signature in Tagish Lake is indicative of the pristine nature of the meteorite. The relationship between the isotopic composition of this water and reported isotopic values for carbonates, bulk matrix and whole rock have been used to infer the extent and conditions under which parent‐body aqueous alteration occurred. In Tagish Lake the difference in Δ17O isotopic composition between the water and other phases is greater than that found in either CM or CI chondrites suggesting that reaction and isotopic exchange between components was more limited. This in turn suggests that in the case of Tagish Lake conditions during the processes of aqueous alteration on the parent body, which ultimately controlled the formation of new minerals, were distinct from those on both CI and CM parent bodies.  相似文献   

13.
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   

14.
We report light noble gas (He, Ne, and Ar) concentrations and isotopic ratios in 11 achondrites, Tafassasset (unclassified primitive achondrite), Northwest Africa (NWA) 12934 (angrite), NWA 12573 (brachinite), Jiddat al Harasis (JaH) 809 (ureilite), NWA 11562 (ungrouped achondrite), four lodranites (NWA 11901, NWA 7474, NWA 6685, and NWA 6484), NWA 2871 (acapulcoite), and Sahara 02029 (winonaite), most of which have not been previously studied for noble gases. We discuss their noble gas isotopic composition, determine their cosmogenic nuclide content, and systematically calculate their cosmic ray exposure (CRE) and gas retention ages. In addition, we estimate their preatmospheric radii and preatmospheric masses based on the shielding parameter (22Ne/21Ne)cos. None of the studied meteorites shows evidence of contribution from solar cosmic rays (SCRs). JaH 809 and NWA 12934 show evidence of 3He diffusive losses of >90% and 40%, respectively. The winonaite Sahara 02029 has lost most of its noble gases, either during or before analysis. The average CRE age of Tafassasset of ~49 Ma is lower than that reported by Patzer et al. (2003), but is consistent with it within the uncertainties; this confirms that Tafassasset and CR chondrites are not source paired, CR chondrites having CRE ages from 1 to 25 Ma (Herzog & Caffee, 2014). The ureilite JaH 809 has a CRE age of ~5.4 Ma, which falls into the typical range of exposure ages for ureilites; the angrite NWA 12934 has a CRE age of ~49 Ma, which is within the main range of exposure ages reported for angrites (0.2–56 Ma). We calculate a CRE age of ~2.4 Ma for the brachinite NWA 12573, which falls into a possible “cluster” in the brachinite CRE age histogram around ~3 Ma. Three lodranites (NWA 11901, NWA 7474, and NWA 6685) have CRE ages higher than the average CRE ages of lodranites measured so far, NWA 11901 and NWA 6685 having CRE ages far higher than the CRE age already reported by Li et al. (2019) on NWA 8118. The measured 40K-40Ar gas retention ages fit well into established systematics. The gas retention age of Tafassasset is consistent, within respective uncertainties, with that previously calculated by Patzer et al. (2003). Our study indicates that Tafassasset originates from a meteoroid with a preatmospheric radius of ~20 cm, however discordant with the radius of ~85 cm inferred in a previous study (Patzer et al., 2003).  相似文献   

15.
CM chondrites are complex impact (mostly regolith) breccias, in which lithic clasts show various degrees of aqueous alteration. Here, we investigated the degree of alteration of individual clasts within 19 different CM chondrites and CM‐like clasts in three achondrites by chemical analysis of the tochilinite‐cronstedtite‐intergrowths (TCIs; formerly named “poorly characterized phases”). To identify TCIs in various chondritic lithologies, we used backscattered electron (BSE) overview images of polished thin sections, after which appropriate samples underwent electron microprobe measurements. Thus, 75 lithic clasts were classified. In general, the excellent work and specific criteria of Rubin et al. (2007) were used and considered to classify CM breccias in a similar way as ordinary chondrite breccias (e.g., CM2.2‐2.7). In BSE images, TCIs in strongly altered fragments in CM chondrites (CM2.0‐CM2.2) appear dark grayish and show a low contrast to the surrounding material (typically clastic matrix), and can be distinguished from TCIs in moderately (CM2.4‐CM2.6) or less altered fragments (CM2.7‐CM2.9); the latter are bright and have high contrast to the surroundings. We found that an accurate subclassification can be obtained by considering only the “FeO”/SiO2 ratio of the TCI chemistry. One could also consider the TCIs’ S/SiO2 ratio and the metal abundance, but these were not used for classification due to several disadvantages. Most of the CM chondrites are finds that have suffered terrestrial weathering in hot and cold deserts. Thus, the observed abundance of metal is susceptible to weathering and may not be a reliable indicator of subtype classification. This study proposes an extended classification scheme based on Rubin’s scale from subtypes CM2.0‐CM2.9 that takes the brecciation into account and includes the minimum to maximum degree of alteration of individual clasts. The range of aqueous alteration in CM chondrites and small spatial scale of mixing of clasts with different alteration histories will be important for interpreting returned samples from the OSIRIS‐REx and Hayabusa 2 missions in the future.  相似文献   

16.
Abstract– CM chondrites are primitive solar‐system materials that have undergone high degrees of aqueous alteration, resulting in the formation of secondary minerals including carbonates. Two different carbonate minerals (calcite/aragonite and dolomite) together constitute 1.4–2.8 vol% of CM chondrites. In contrast, CI chondrites contain four different carbonate minerals: calcite/aragonite, dolomite, breunnerite, and siderite. CI chondrites have abundant dolomite, a mineral that seems to be absent in the most aqueously altered CM chondrites. In this study, carbonates in seven CM chondrites (Y‐791198, LaPaz Icefield 04796, Cold Bokkeveld, Nogoya, Queen Alexandra Range 93005, Allan Hills 83100, and Meteorite Hills 01070) were studied petrographically and by electron microprobe. The results indicate that carbonate formation in CM chondrites differs from that in CI chondrites and is more complex than previously recognized. Our studies of CM chondrites indicate that (1) carbonates formed on the parent asteroid in an aqueous environment that gradually changed in composition, (2) at some stage, Ca and Mg activities in the environment were high enough to form metastable dolomite, and (3) dolomites disappeared in the most aqueously altered CM chondrites.  相似文献   

17.
Abstract— In most groups of carbonaceous chondrites, minerals occur that are formed due to aqueous alteration in the nebula and/or within meteorite parent bodies. For determining the evolution of materials in the early solar system, it is of significant importance to clearly identify evidence for either nebular or planetary aqueous alteration. Therefore, results from the study of chondrites have fundamental implications for ideas concerning nebular dynamics, gas-solid interactions in the nebula, and accretionary processes. Considering the topic of this review, it is important to define nebular and parent body processes. The solar nebula activity should include condensation processes of high- and low-temperature components as well as processes of chemical fractionation and grain-size sorting, mixing of solids and gas, and interactions between early formed solids with the remaining gas; but it should exclude processes that occurred in small uncompacted protoplanetary objects that may have been totally destroyed again before accretion of the final meteorite parent bodies. Therefore, the term “preaccretionary” instead of “nebula” is used in this paper to include all these processes that may have occurred in small precursor planetesimals. Currently, there is no doubt that parent body aqueous alteration is a fundamental process in the evolution of several groups of carbonaceous chondrites. However, due to textural and mineralogical observations and chemical analyses, strong arguments have been found also indicating preaccretionary aqueous alteration of distinct components in carbonaceous chondrites. In this paper, evidence for preaccretionary aqueous alteration in carbonaceous chondrites and their components taken from previous studies is discussed in detail. The strongest evidence for preaccretionary alteration comes from studies of CM chondrites. The survival of highly unequilibrated mineral assemblages in accretionary rims, and sharp contacts between chondrule glass and surrounding phyllosilicates, are only two important arguments for preaccretionary alteration features among many others discussed in the paper. Similar observations were also made in CR, CH and related chondrites. Due to the small abundance of water-bearing phases in CO and CV chondrites, the origin of phyllosilicates in these groups is less clear. A preaccretionary origin of hydrous phases in Ca-Al-rich inclusions (CAIs) has also been suggested by several scientists. In CI chondrites, no strong indications for such processes have been found, mainly due to heavy brecciation and severe parent body alteration.  相似文献   

18.
Abstract— We measured the concentrations of noble gases in 32 ordinary chondrites from the Dar al Gani (DaG) region, Libya, as well as concentrations of the cosmogenic radionuclides 14C, 10Be, 26Al, 36Cl, and 41Ca in 18 of these samples. Although the trapped noble gases in five DaG samples show ratios typical of solar or planetary gases, in all other DaG samples, they are dominated by atmospheric contamination, which increases with the degree of weathering. Cosmic ray exposure (CRE) ages of DaG chondrites range from ?1 Myr to 53 Myr. The CRE age distribution of 10 DaG L chondrites shows a cluster around 40 Myr due to four members of a large L6 chondrite shower. The CRE age distribution of 19 DaG H chondrites shows only three ages coinciding with the main H chondrite peak at ?7 Myr, while seven ages are <5 Myr. Two of these H chondrites with short CRE ages (DaG 904 and 908) show evidence of a complex exposure history. Five of the H chondrites show evidence of high shielding conditions, including low 22Ne/21Ne ratios and large contributions of neutron‐capture 36Cl and 41Ca. These samples represent fragments of two or more large pre‐atmospheric objects, which supports the hypothesis that the high H/L chondrite ratio at DaG is due to one or more large unrecognized showers. The 14C concentrations correspond to terrestrial ages <35 kyr, similar to terrestrial ages of chondrites from other regions in the Sahara but younger than two DaG achondrites. Despite the loss of cosmogenic 36Cl and 41Ca during oxidation of metal and troilite, concentrations of 36Cl and 41Ca in the silicates are also consistent with 14C ages <35 kyr. The only exception is DaG 343 (H4), which has a 41Ca terrestrial age of 150 ± 40 kyr. This old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr. A possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods, such as the current one, which started ?3000 years ago. Finally, based on the 26Al/21Ne and 10Be/21Ne systematics in 16 DaG meteorites, we derived more reliable estimates of the 10Be/21Ne production rate ratio, which seems more sensitive to shielding than was predicted by the semi‐empirical model of Graf et al. (1990) but less sensitive than was predicted by the purely physical model of Leya et al. (2000).  相似文献   

19.
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   

20.
As part of an integrated consortium study, we have undertaken O, Cd, Cr, Si, Te, Ti, and Zn whole rock isotopic measurements of the Winchcombe CM2 meteorite. δ66Zn values determined for two Winchcombe aliquots are +0.29 ± 0.05‰ (2SD) and +0.45 ± 0.05‰ (2SD). The difference between these analyses likely reflects sample heterogeneity. Zn isotope compositions for Winchcombe show excellent agreement with published CM2 data. δ114Cd for a single Winchcombe aliquot is +0.29 ± 0.04‰ (2SD), which is close to a previous result for Murchison. δ130Te values for three aliquots gave indistinguishable results, with a mean value of +0.62 ± 0.01‰ (2SD) and are essentially identical to published values for CM2s. ε53Cr and ε54Cr for Winchcombe are 0.319 ± 0.029 (2SE) and 0.775 ± 0.067 (2SE), respectively. Based on its Cr isotopic composition, Winchcombe plots close to other CM2 chondrites. ε50Ti and ε46Ti values for Winchcombe are 3.21 ± 0.09 (2SE) and 0.46 ± 0.08 (2SE), respectively, and are in line with recently published data for CM2s. The δ30Si composition of Winchcombe is −0.50 ± 0.06‰ (2SD, n = 11) and is essentially indistinguishable from measurements obtained on other CM2 chondrites. In conformity with petrographic observations, oxygen isotope analyses of both bulk and micromilled fractions from Winchcombe clearly demonstrate that its parent body experienced extensive aqueous alteration. The style of alteration exhibited by Winchcombe is consistent with relatively closed system processes. Analysis of different fractions within Winchcombe broadly support the view that, while different lithologies within an individual CM2 meteorite can be highly variable, each meteorite is characterized by a predominant alteration type. Mixing of different lithologies within a regolith environment to form cataclastic matrix is supported by oxygen isotope analysis of micromilled fractions from Winchcombe. Previously unpublished bulk oxygen isotope data for 12 CM2 chondrites, when combined with published data, define a well-constrained regression line with a slope of 0.77. Winchcombe analyses define a more limited linear trend at the isotopically heavy, more aqueously altered, end of the slope 0.77 CM2 array. The CM2 slope 0.77 array intersects the oxygen isotope field of CO3 falls, indicating that the unaltered precursor material to the CMs was essentially identical in oxygen isotope composition to the CO3 falls. Our data are consistent with earlier suggestions that the main differences between the CO3s and CM2s reflect differing amounts of water ice that co-accreted into their respective parent bodies, being high in the case of CM2s and low in the case of CO3s. The small difference in Si isotope compositions between the CM and CO meteorites can be explained by different proportions of matrix versus refractory silicates. CMs and COs may also be indistinguishable with respect to Ti and Cr isotopes; however, further analysis is required to test this possibility. The close relationship between CO3 and CM2 chondrites revealed by our data supports the emerging view that the snow line within protoplanetary disks marks an important zone of planetesimal accretion.  相似文献   

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