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1.
Abstract— Densities and porosities for 285 ordinary chondrites have been assembled and analyzed. Measured chondrite porosities are bimodal; finds have an average porosity of <3%, whereas fall porosities average 7% but range from zero to >30%. We conclude that mild degrees of weathering fill pore spaces, lowering grain densities and porosities without significantly changing the bulk size or mass of the sample. By assuming an original pristine grain density (as a function of the meteorite's mineralogy—determined by its class), we can derive model pristine porosities. These model porosities cluster around an average value of 10% for all classes of ordinary chondrites. Ordinary chondrites do not show any correlation of porosity (model or measured) with petrographic grade or sample size (over a range from 0.2 g to 2 kg). However, we do see a correlation between shock state and porosity. Shock-blackened meteorites are less porous than other meteorites. Furthermore, less severely shocked meteorites show a much broader range of porosities, with the maximum porosity seen among meteorites of a given shock class falling linearly as a function of that shock class. This is consistent with the idea that shock compresses and closes pore space. Analysis of meteorite porosity provides a lower bound to the fine-scale porosity of asteroids. Our densities, even with 10% primordial porosity, are significantly higher than inferred densities of possible asteroid parent bodies. These asteroids are probably loose piles of rubble.  相似文献   

2.
Abstract— In this review, we summarize the data published up to December 2001 on the porosity and density of stony meteorites. These data were taken from 925 samples of 454 different meteorites by a variety of techniques. Most meteorites have densities on the order of 3 to 4 g/cm3, with lower densities only for some volatile‐rich carbonaceous meteorites and higher densities for stony irons. For the vast majority of stones, porosity data alone cannot distinguish between different meteorite compositions. Average porosities for most meteorite classes are around 10%, though individual samples can range as high as 30% porosity. Unbrecciated basaltic achondrites appear to be systematically less porous unless vesicles are present. The measured density of ordinary chondrites is strongly controlled by the amount of terrestrial weathering the sample has undergone with porosities steadily dropping with exposure to the terrestrial environment. A theoretical grain density based on composition can model “pre‐weathered” porosities. The average model porosity for H and LL chondrites is 10%, while L chondrite model porosities average only 6%, a statistically significant difference.  相似文献   

3.
Abstract— We have developed a nondestructive technique to measure the densities and porosities of large (50 g to several kilogram) meteorite samples and have begun a systematic measurement program starting with meteorites from the Vatican Observatory collection. Our technique utilizes a modified Archimedean method (40 μm glass spheres substituting for the fluid) to determine bulk density, and an unusually large He pycnometer to determine grain density. From these, the porosity is calculated. We report here our first results for 89 samples (nearly doubling the published database on porosities of meteorite hand samples), including porosities of 35 different meteorites. These include several types not previously reported: eight iron meteorites, six stony-iron meteorites, an enstatite chondrite, the CI carbonaceous chondrite Orgueil, and the shergottite-nakhlatite-chassignite (SNC) meteorite Nakhla.  相似文献   

4.
Meteorites, generally 1 cm or larger in size that are believed to sample asteroids, and interplanetary dust particles (IDPs), generally 5–50 μm in size that are believed to sample both asteroids and comets, span the size range of the meteors. Thus, the physical properties of the meteorites and the IDPs are likely to constrain the properties of the meteors and their parent bodies. Measurements of the density, porosity, longitudinal and transverse speeds of sound, elastic modulus, and bulk modulus, as well as imaging of the internal structure by Computed Microtomography indicate that unweathered samples of chondritic meteorites are more porous and have lower sound velocities than compact terrestrial rocks. In general, the IDPs are even more porous than the chondritic meteorites. The impact energy per unit target mass required to produce a barely catastrophic disruption (Q * D) for anhydrous ordinary chondrite meteorites is twice that for terrestrial basalt or glass, indicating that collisional disruption of anhydrous meteorites requires more energy than for a compact basalt. These results indicate that most stone meteors are likely to be weak, porous objects, and that the parent bodies of the anhydrous stone meteorites are likely to be more difficult to disrupt than compact terrestrial basalt.  相似文献   

5.
A technique of impregnation by fluorescent resins followed by observations of thin sections in U V light has been applied to the study of porosity, brecciation and shock effects in chondritic meteorites. The following chondrites have been studied: Sena, Ikhraréne, Kernouve, Koutiaran, L'Aigle, Tiberrhamine. Tentative chronological orders of tectonic events undergone by those meteorites are proposed.  相似文献   

6.
Raman spectroscopy was used to investigate insoluble organic matter (IOM) from a range of chondritic meteorites, and a suite of interplanetary dust particles (IDPs). Three monochromatic excitation wavelengths (473 nm, 514 nm, 632 nm) were applied sequentially to assess variations in meteorite and IDP Raman peak parameters (carbon D and G bands) as a function of excitation wavelength (i.e., dispersion). Greatest dispersion occurs in CVs > OCs > CMs > CRs with type 3 chondrites compared at different excitation wavelengths displaying conformable relationships, in contrast to type 2 chondrites. These findings indicate homogeneity in the structural nature of type 3 chondrite IOM, while organic matter (OM) in type 2 chondrites appears to be inherently more heterogeneous. If type 2 and type 3 chondrite IOM shares a common source, then thermal metamorphism may have a homogenizing effect on the originally more heterogeneous OM. IDP Raman G bands fall on an extension of the trend displayed by chondrite IOM, with all IDPs having Raman parameters indicative of very disordered carbon, with almost no overlap with IOM. The dispersion effect displayed by IDPs is most similar to CMs for the G band, but intermediate between CMs and CRs for the D band. The existence of some overlapping Raman features in the IDPs and IOM indicates that their OM may share a common origin, but the IDPs preserve more pristine OM that may have been further disordered by ion irradiation. H, C, and N isotopic data for the IDPs reveal that the disordered carbon in IDPs corresponds with higher δ15N and lower δ13C.  相似文献   

7.
The size distribution, abundance, and physical and chemical characteristics of chondritic inclusions are key features that define the chondrite groups. We present statistics on the size and abundance of the macroscopic components (inclusions) in the Murchison (CM2) and Allende (CV3) chondrites and measure their general chemical trends using established X‐ray mapping techniques. This study provides a fine‐scale assessment of the two meteorites and a semiquantitative evaluation of the relative abundances of elements and their distribution among meteorite components. Murchison contains 72% matrix and 28% inclusions; Allende contains 57% and 43%, respectively. A broad range of inclusion sizes and relative abundances has been reported for these meteorites, which demonstrates the necessity for a more standardized approach to measuring these characteristics. Nonetheless, the characteristic mean sizes of inclusions in Allende are consistently larger than those in Murchison. We draw two significant conclusions (1) these two meteorites sampled distinct populations of chondrules and refractory inclusions, and (2) complementary Mg/Si ratios between chondrules and matrix are observed in both Murchison and Allende. Both support the idea that chondrules and matrix within each chondrite group originated in single reservoirs of precursors with approximately solar Mg/Si ratios, providing a constraint on astrophysical models of the origin of chondrite parent bodies.  相似文献   

8.
Abstract— The Rumuruti meteorite shower fell in Rumuruti, Kenya, on 1934 January 28 at 10:43 p.m. Rumuruti is an olivine-rich chondritic breccia with light-dark structure. Based on the coexistence of highly recrystallized fragments and unequilibrated components, Rumuruti is classified as a type 3–6 chondrite breccia. The most abundant phase of Rumuruti is olivine (mostly Fa～39) with about 70 vol%. Feldspar (～14 vol%; mainly plagioclase), Ca-pyroxene (5 vol%), pyrrhotite (4.4 vol%), and pentlandite (3.6 vol%) are major constituents. All other phases have abundances below 1 vol%, including low-Ca pyroxene, chrome spinels, phosphates (chlorapatite and whitlockite), chalcopyrite, ilmenite, tridymite, Ni-rich and Ge-containing metals, kamacite, and various particles enriched in noble metals like Pt, Ir, arid Au. The chemical composition of Rumuruti is chondritic. The depletion in refractory elements (Sc, REE, etc.) and the comparatively high Mn, Na, and K contents are characteristic of ordinary chondrites and distinguish Rumuruti from carbonaceous chondrites. However, S, Se, and Zn contents in Rumuruti are significantly above the level expected for ordinary chondrites. The oxygen isotope composition of Rumuruti is high in δ17O (5.52 ‰) and δ18O (5.07 ‰). Previously, a small number of chondritic meteorites with strong similarities to Rumuruti were described. They were called Carlisle Lakes-type chondrites and they comprise: Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, PCA91002, and PCA91241, as well as clasts in the Weatherford chondrite. All these meteorites are finds from hot and cold deserts having experienced various degrees of weathering. With Rumuruti, the first meteorite fall has been recognized that preserves the primary mineralogical and chemical characteristics of a new group of meteorites. Comparing all chondrites, the characteristic features can be summarized as follows: (a) basically chondritic chemistry with ordinary chondrite element patterns of refractory and moderately volatile lithophiles but higher abundances of S, Se, and Zn; (b) high degree of oxidation (37–41 mol% Fa in olivine, only traces of Fe, Ni-metals, occurrence of chalcopyrite); (c) exceptionally high Δ17O values of about 2.7 for bulk samples; (d) high modal abundance of olivine (～70 vol%); (e) Ti-Fe3+?rich chromite (～5.5 wt% TiO2); (f) occurrence of various noble metal-rich particles; (g) abundant chondritic breccias consisting of equilibrated clasts and unequilibrated lithologies. With Rumuruti, nine meteorite samples exist that are chemically and mineralogically very similar. These meteorites are attributed to at least eight different fall events. It is proposed in this paper to call this group R chondrites (rumurutiites) after the first and only fall among these meteorites. These meteorites have a close relationship to ordinary chondrites. However, they are more oxidized than any of the existing groups of ordinary chondrites. Small, but significant differences in chemical composition and in oxygen isotopes between R chondrites and ordinary chondrites exclude formation of R chondrites from ordinary chondrites by oxidation. This implies a separate, independent R chondrite parent body.  相似文献   

9.
Abstract— Densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors. We report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study. Based on our measurements, we find that no significant correlation exists between porosity and petrologic grade, chemical group, sample mass, bulk and grain density, or shock level. To investigate variations in porosity and density between pieces of a meteorite, we examined stones from two showers, Holbrook and Pultusk. Examination of nine samples of Holbrook suggests relative homogeneity in porosity and density between pieces of this shower. Measurements of three samples of Pultusk show homogeneity in bulk density, in contrast to Wilkison and Robinson (2000), a study that reported significant variations in bulk density between 11 samples of Pultusk. Finally, examination of two friable ordinary chondrites, Bjurböle and Allegan, reveal variability in friability and porosity among pieces of the same fall. We suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid.  相似文献   

10.
Density,porosity, and magnetic susceptibility of carbonaceous chondrites   总被引：1，自引：0，他引：1  
Abstract– We report physical properties (bulk and grain density, magnetic susceptibility, and porosity) measured using nondestructive and noncontaminating methods for 195 stones from 63 carbonaceous chondrites. Grain densities over the whole population average 3.44 g cm?3, ranging from 2.42 g cm?3 (CI1 Orgueil) to 5.66 g cm?3 (CB Bencubbin). Magnetic susceptibilities (in log units of 10?9 m3 kg?1) averaged log χ = 4.22, ranging from 3.23 (CV3 Axtell) to 5.79 (CB Bencubbin). Porosities averaged 17%, ranging from 0 (for a number of meteorites) to 41% (for one stone of the CO Ornans). Notably, we found significant differences in porosity between the oxidized and reduced CV subgroups, with the porosities of CVo averaging approximately 20% and CVr porosities approximately 4%. Overall, porosities of carbonaceous chondrite falls trend with petrographic type, from type 1 (CI) near 35%, type 2 (CM, CR) averaging 23%, type 3 (CV, CO) 21%, to type 4 (CK and some CO) averaging 15%. There is also a significant decrease in porosity between meteorites of shock stage S1 and those of S2, indicative of shock compression.  相似文献   

11.
The Kumtag 016 strewn field was found in the eastern part of the Kumtag desert, Xinjiang Province, China. In this study, 24 recovered meteorites have been characterized by a suite of different analytical techniques to investigate their petrography, mineralogy, bulk trace elements, noble gas isotopic composition, density, and porosity. We attribute to the strewn field 22 L5 chondrites with shock stage S4 and weathering grade W2–W3. Two different meteorites, Kumtag 021, an L4 chondrite and Kumtag 032, an L6 chondrite, were recognized within the strewn field area. Moreover, Kumtag 003, an H5 chondrite, was previously found in the same area. We infer that the Kumtag 016 strewn field most likely consists of at least four distinct meteorite falls. The effects of terrestrial weathering on the studied meteorites involve sulfide/metal alteration, chemical changes (Sr, Ba, Pb, and U enrichments and depletion in Cr, Co, Ni, and Cs abundances), and physical modifications (decrease of grain density and porosity). Measurements of the light noble gases indicate that the analyzed Kumtag L5 samples contain solar wind-implanted noble gases with a 20Ne/22Ne ratio of ~12.345. The cosmic-ray exposure (CRE) ages of the L5 chondrites are in a narrow range (3.6 ± 1.4 Ma to 5.2 ± 0.4 Ma). For L4 chondrite Kumtag 021 and L6 chondrite Kumtag 032, the CRE ages are 5.9 ± 0.4 Ma and 4.7 ± 0.8 Ma, respectively.  相似文献   

12.
Abstract— Twenty-two carbonaceous chondrite clasts from the two howardites Bholghati and EET87513 were analyzed. Clast N from EET87513 is a fragment classified as CM2 material on the basis of texture, bulk composition, mineralogy, and bulk O isotopic composition. Carbonaceous chondrite clasts from Bholghati, for which less data are available because of their small size, can be divided into two petrologic types: C1 and C2. C1 clasts are composed of opaque matrix with rare coarse-grained silicates as individual mineral fragments; textures resemble CI meteorites and some dark inclusions from CR meteorites. Opaque matrix is predominantly composed of flaky saponite; unlike typical CI and CR meteorites, serpentine is absent in the samples we analyzed. C2 clasts contain chondrules, aggregates, and individual fragments of coarse-grained silicates in an opaque matrix principally composed of saponite and anhydrous ferromagnesian silicates with flaky textures similar to phyllosilicates. These anhydrous ferromagnesian silicates are interpreted as the product of heating of pre-existing serpentine. The carbonaceous chondrite clasts we have studied from these two howardites are, with one notable exception (clast N from EET87513), mineralogically distinct from typical carbonaceous chondrites. However, these clasts have very close affinities to carbonaceous chondrites and have also experienced thermal metamorphism and aqueous alteration, but to different degrees.  相似文献   

13.
Acfer 217-A new member of the Rumuruti chondrite group (R)   总被引：1，自引：0，他引：1  
Abstract— Previously, three meteorites from Australia and Antarctica were described as a new chondritic “grouplet” (Carlisle Lakes, Allan Hills (ALH) 85151, Yamato (Y) ?75302; Rubin and Kallemeyn, 1989). This grouplet was classified as the “Carlisle Lakes-type” chondrites (Weisberg et al., 1991). Recently, one Saharan sample and four more Antarctic meteorites were identified to belong to this group (Acfer 217, Y-793575, Y-82002, PCA91002, PCA91241). The latter two are probably paired. With the meteorite Rumuruti, the first fall of this type of chondrite is known (Schulze et al., 1994). We report here on the Saharan meteorite Acfer 217 which has chemical and mineralogical properties very similar to Rumuruti and Carlisle Lakes. All eight members of this group, Rumuruti, Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, and the paired samples PCA91002 and PCA91241 justify the introduction of a new group of chondritic meteorites, the Rumuruti meteorites (R). Acfer 217 is a regolith breccia consisting of up to cm-sized clasts (～33 vol%) embedded in a fine-grained, well-lithified clastic matrix. The most abundant mineral is olivine (～72 vol%), which has a high Fa-content of 37–39 mol%. The major minerals (olivine, low-Ca pyroxene, Ca-pyroxene, and plagioclase) show some compositional variability indicating a slightly unequilibrated nature of the meteorite. Considering the mean olivine composition of Fa37.8 ± 5.7, a classification of Acfer 217 as a R3.8 chondrite would result; however, Acfer 217 is a regolith breccia consisting of clasts of various petrologic types. Therefore, we suggest to classify Acfer 217 as a R3–5 chondrite regolith breccia. The bulk meteorite is very weakly shocked (S2). The bulk composition of Acfer 217 and other R-meteorites show that the R-meteorites are basically chondritic in composition. The pattern of moderately volatile elements is unique in R chondrites; Na and Mn are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The oxygen isotopic composition in Acfer 217 is similar to that of Rumuruti, Carlisle Lakes, ALH 85151, and Y-75302. In a δ17O vs. δ18O-diagram, the R-meteorites form a group well resolved from other chondrite groups. Acfer 217 was a meteoroid of common size with a radius between 15–65 cm and with a single stage exposure history. Based on 21Ne, an exposure age of about 35 Ma was calculated.  相似文献   

14.
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   

15.
The existence of mass‐independent chromium isotope variability of nucleosynthetic origin in meteorites and their components provides a means to investigate potential genetic relationship between meteorites and planetary bodies. Moreover, chromium abundances are depleted in most surficial terrestrial rocks relative to chondrites such that Cr isotopes are a powerful tool to detect the contribution of various types of extra‐terrestrial material in terrestrial impactites. This approach can thus be used to constrain the nature of the bolide resulting in breccia and melt rocks in terrestrial impact structures. Here, we report the Cr isotope composition of impact rocks from the ~0.57 Ma Lonar crater (India), which is the best‐preserved impact structure excavated in basaltic target rocks. Results confirm the presence of a chondritic component in several bulk rock samples of up to 3%. The impactor that created the Lonar crater had a composition that was most likely similar to that of carbonaceous chondrites, possibly a CM‐type chondrite.  相似文献   

16.
Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio 20Ne/22Ne of 0.80 ± 0.03 and a trapped solar 4He/3He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped 20Ne and 36Ar concentrations in the range of 10?6–10?9 cm3 STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced 3He and of radiogenic 4He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a 20Ne/22Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.  相似文献   

17.
We report all available measurements on strength of meteorites, primarily focusing on compressive and tensile strengths and supplementary data such as Young's modulus, Poisson's ratio, elastic sound wave velocities, density, porosity, and sample sizes. These data are solely taken from the original papers to avoid misprints and other issues. The data are provided as originally presented by the authors with the exception of standardization of units to the SI system. A brief overview of methods for each original work is also provided as a guide to “data quality” since individual papers go to varying levels of detail on their experimental setup and procedures. From this data set, we confirm that the compressive strength of ordinary chondrites (varying in the range of 10s to 100s of MPa) is about an order of magnitude larger compared to their tensile strength and the difference increases with iron content. For carbonaceous chondrites, the tensile strength seems to be about an order of magnitude below the tensile strength of ordinary chondrites and at least an order of magnitude below their compressive strength. We also provide a statistical relation between the strength of meteorites and their densities and porosities and discuss the role of strain rate and sample size on the resultant measured strength. Finally, the data do not provide sufficient statistics to support a size scale effect of strength of meteorites.  相似文献   

18.
Abstract— Concentration and isotopic composition of the light noble gases as well as of 84Kr, 129Xe, and 132Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary‐type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high 129Xe/132Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects. Cosmic‐ray exposure ages are calculated from cosmogenic 21Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic‐ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ～9 and ～29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls. In general, we confirm the results of Mazor et al. (1970) regarding cosmic‐ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic‐ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar‐gas‐bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.  相似文献   

19.
Abstract– Transmission electron microscopy examination of 87 large fragments from 16 carrot‐shaped and bulbous Stardust (SD) tracks was performed to study the range and diversity of materials present in comet Wild 2. Olivines and low‐Ca pyroxenes represent the largest proportions of fragments observed; however, a wide range of minerals and rocks were found including probable ferromagnesian, Al‐rich and Si‐rich chondrule fragments, a refractory inclusion, possible matrix mineral/lithic clasts, and probable condensate minerals. These materials, combined with fine‐grained components in the tracks, are analogous to components in unequilibrated chondrite meteorites and cluster interplanetary dust particles (IDPs). Two unusual lithologies in the bulbous tracks are only observed in chondritic porous IDPs and may have direct links to IDPs. The absence of phyllosilicates indicates that comet Wild 2 may be a “dry” comet that did not accrete or form significant amounts of hydrated phases. Some large mineral fragments in the SD tracks are analogous to large mineral IDPs. The large variations of the coarse‐grained components within and between all 16 tracks show that comet Wild 2 is mineralogically diverse and unequilibrated on nearly all scales and must have accreted materials from diverse source regions that were widely dispersed throughout the solar nebula.  相似文献   

20.
Abstract— Modal abundances of Ca,Al‐rich inclusions (CAIs) are poorly known and reported data scatter across large ranges. CAIs are Poisson distributed, and if only small areas (<1000 mm2) are studied, the data are probably not representative of the true CAI modal abundances, explaining their reported large scatter in a single chondrite group. We combine reported CAI modal abundances and our own set, and present a complete list of CAI modal abundances in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer 094: 1.12, CO: 0.99, CK/CV (Ningqiang and Dar al Gani [DaG] 055): 0.77, CK: 0.2, CR: 0.12 and CB: 0.1. CAIs are Poisson distributed and if only small areas are studied, the data are probably not representative of the true CAI modal abundances, Carbonaceous chondrites have excess bulk Al concentrations when compared to the CI‐chondritic value. We find a correlation between this excess and CAI modal abundances and conclude that the excess Al was delivered by CAIs. The excess Al is only a minor fraction (usually ?10 rel%, but 25 rel% in case of CVs) of the bulk chondrite Al and cannot have contributed much 26Al to heat the chondrite parent body. Ordinary, enstatite, R and K chondrites have an Al deficit relative to CI chondrites and only very low CAI modal abundances, if any are present at all. Carbonaceous chondrites also had an initial Al deficit if the contribution of Al delivered by CAIs is subtracted. Therefore all chondrites probably lost a refractory rich high‐T component. Only minor amounts of CAIs are present in the matrix or have been present in the chondrule precursor aggregates. Most CAI size distributions contain more than one size population, indicating that CAIs from within a single meteorite group had different origins.  相似文献   

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