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1.
Abstract— Some fraction of Zn, Cu, Se, Ga and Ge in chondritic interplanetary dust particles (IDPs) collected in the lower stratosphere between 1981 May and 1984 June has a volcanic origin. I present a method to evaluate the extent of this unavoidable type of stratospheric contamination for individual particles. The mass-normalised abundances for Cu and Ge as a function of mass-normalised stratospheric residence time show their time-integrated stratospheric aerosol abundances. The Zn, Se and Ga abundances show a subdivision into two groups that span approximately two-year periods following the eruptions of the Mount St. Helens (1980 May) and El Chichón (1982 April) volcanos. Elemental abundances in particles collected at the end of each two-year period indicate low, but not necessarily ambient, volcanic stratospheric abundances. Using this time-integrated baseline, I calculate the stratospheric contaminant fractions in nine IDPs and show that Zn, Se and Ga abundances in chondritic IDPs derive in part from stratospheric aerosol contaminants. Post-entry elemental abundances (i.e., the amount that survived atmospheric entry heating of the IDP) show enrichments relative to the CI abundances but in a smaller number of particles than previously suggested.  相似文献   

2.
Comet 81P/Wild 2 dust, the first comet sample of known provenance, was widely expected to resemble anhydrous chondritic porous (CP) interplanetary dust particles (IDPs). GEMS, distinctly characteristic of CP IDPs, have yet to be unambiguously identified in the Stardust mission samples despite claims of likely candidates. One such candidate is Stardust impact track 57 “Febo” in aerogel, which contains fine‐grained objects texturally and compositionally similar to GEMS. Their position adjacent the terminal particle suggests that they may be indigenous, fine‐grained, cometary material, like that in CP IDPs, shielded by the terminal particle from damage during deceleration from hypervelocity. Dark‐field imaging and multidetector energy‐dispersive X‐ray mapping were used to compare GEMS‐like‐objects in the Febo terminal particle with GEMS in an anhydrous, chondritic IDP. GEMS in the IDP are within 3× CI (solar) abundances for major and minor elements. In the Febo GEMS‐like objects, Mg and Ca are systematically and strongly depleted relative to CI; S and Fe are somewhat enriched; and Au, a known aerogel contaminant, is present, consistent with ablation, melting, abrasion, and mixing of the SiOx aerogel with crystalline Fe‐sulfide and minor enstatite, high‐Ni sulfide, and augite identified by elemental mapping in the terminal particle. Thus, GEMS‐like objects in “caches” of fine‐grained debris abutting terminal particles are most likely deceleration debris packed in place during particle transit through the aerogel.  相似文献   

3.
Abstract— The trace element compositions and noble gas contents of 32 individual interplanetary dust particles (IDPs) collected in the Earth's stratosphere were measured. Trace element compositions are generally similar to CI meteorites, with occasional depletions in Zn/Fe with respect to CI. Noble gases were detected in all but one of the IDPs. Noble gas elemental compositions are consistent with the presence of fractionated solar wind. A rough correlation between surface‐normalized He abundances and Zn/Fe ratios is observed; Zn‐poor particles generally have lower He contents than the other IDPs. This suggests that both elements were lost by frictional heating during atmospheric entry and confirms the view that Zn can serve as an entry‐heating indicator in IDPs.  相似文献   

4.
Abstract– Tucson is an enigmatic ataxitic iron meteorite, an assemblage of reduced silicates embedded in Fe‐Ni metal with dissolved Si and Cr. Both, silicates and metal, contain a record of formation at high temperature (～1800 K) and fast cooling. The latter resulted in the preservation of abundant glasses, Al‐rich pyroxenes, brezinaite, and fine‐grained metal. Our chemical and petrographic studies of all phases (minerals and glasses) indicate that they have a nebular rather than an igneous origin and give support to a chondritic connection as suggested by Prinz et al. (1987) . All silicate phases in Tucson apparently grew from a liquid that had refractory trace elements at approximately 6–20 × CI abundances with nonfractionated (solar) pattern, except for Sc, which was depleted (～1 × CI). Metal seems to have precipitated before and throughout silicate aggregate formation, allowing preservation of all evolutionary steps of the silicates by separating them from the environment. In contrast to most chondrites, Tucson documents coprecipitation of metal and silicates from the solar nebula gas and precipitation of metal before silicates—in accordance with theoretical condensation calculations for high‐pressure solar nebula gas. We suggest that Tucson is the most metal‐rich and volatile‐element‐poor member of the CR chondrite clan.  相似文献   

5.
Abstract— The elemental compositions of 200 interplanetary dust particles (IDPs) collected in the stratosphere have been determined by energy dispersive X-ray (EDX) analysis. The results reasonably define the normal compositional range of chondritic interplanetary dust particles averaging 10 micrometers in size, and constitute a database for comparison with individual IDPs, meteorites, and spacecraft data from comets and asteroids. The average elemental composition of all IDPs analyzed is most similar to that of CI chondrites, but the data show that there are small yet discernable differences between mean IDP composition and the CI norm. Individual particles were classified into broad morphological groups, and the two major groups show unambiguous compositional differences. The “porous” group is a close match to bulk CI abundances, but the “smooth” group has systematic Ca and Mg depletions, and contains stoichiometric “excess” oxygen consistent with the presence of hydrous phases. Similar depletions of Ca and Mg in CI and CM matrix have been attributed to leaching, and by analogy we suggest that particles in the smooth group have also been processed by aqueous alteration. The occurrence of carbonates, magnetite framboids, and layer silicates provides additional evidence that at least a significant number of the smooth-class IDPs have been substantially processed by aqueous activity. The presence or absence of aqueous modification in members of a particle sub-class is an important clue to the origin. Although it cannot be proven, we hypothesize that extensive aqueous activity only occurs in asteroids and that, accordingly, the smooth class of IDPs has an asteroidal origin. If both comets and asteroids are major sources of interplanetary dust, then by default the porous particles are inferred to be dominated by cometary material.  相似文献   

6.
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   

7.
NASA’s Stardust spacecraft collected dust from the coma of Comet 81P/Wild 2 by impact into aerogel capture cells or into Al-foils. The first direct, laboratory measurement of the physical, chemical, and mineralogical properties of cometary dust grains ranging from <10−15 to ∼10−4 g were made on this dust. Deposition of material along the entry tracks in aerogel and the presence of compound craters in the Al-foils both indicate that many of the Wild 2 particles in the size range sampled by Stardust are weakly bound aggregates of a diverse range of minerals. Mineralogical characterization of fragments extracted from tracks indicates that most tracks were dominated by olivine, low-Ca pyroxene, or Fe-sulfides, although one track was dominated by refractory minerals similar to Ca–Al inclusions in primitive meteorites. Minor mineral phases, including Cu–Fe-sulfide, Fe–Zn-sulfide, carbonate and metal oxides, were found along some tracks. The high degree of variability of the element/Fe ratios for S, Ca, Ti, Cr, Mn, Ni, Cu, Zn, and Ga among the 23 tracks from aerogel capture cells analyzed during Stardust Preliminary Examination is consistent with the mineralogical variability. This indicates Wild 2 particles have widely varying compositions at the largest size analyzed (>10 μm). Because Stardust collected particles from several jets, sampling material from different regions of the interior of Wild 2, these particles are expected to be representative of the non-volatile component of the comet over the size range sampled. Thus, the stream of particles associated with Comet Wild 2 contains individual grains of diverse elemental and mineralogical compositions, some rich in Fe and S, some in Mg, and others in Ca and Al. The mean refractory element abundance pattern in the Wild 2 particles that were examined is consistent with the CI meteorite pattern for Mg, Si, Cr, Fe, and Ni to 35%, and for Ca, Ti and Mn to 60%, but S/Si and Fe/Si both show a statistically significant depletion from the CI values and the moderately volatile elements Cu, Zn, Ga are enriched relative to CI. This elemental abundance pattern is similar to that in anhydrous, porous interplanetary dust particles (IDPs), suggesting that, if Wild 2 dust preserves the original composition of the Solar Nebula, the anhydrous, porous IDPs, not the CI meteorites, may best reflect the Solar Nebula abundances. This might be tested by elemental composition measurements on cometary meteors.  相似文献   

8.
Abstract— We report the concentration of 50 elements, including rare earth elements (REEs) and platinum group elements (PGEs) in bulk samples of the Grove Mountains (GRV) 99027 lherzolitic shergottite. The abundances of REEs are distinctly lower than those of Allan Hills (ALH) A77005 and other lherzolitic shergottites, indicating that GRV 99027 is not paired with them. It may, nevertheless, sample the same igneous unit as the others (Lin et al. 2005b; Wang and Chen 2006). The CI‐normalized elemental pattern of GRV 99027 reveals low (0.004–0.008 × CI) and unfractionated PGEs (except for Pd of 0.018 × CI) without depletion of W. or Ga relative to lithophile element trends. Fractionation between siderophile and lithophile elements become less pronounced with increase of volatility, except for high abundances of Ni and Co. These characteristics are probably representative of the mantle of Mars, which is consistent with previous work that the Martian mantle formed in a deep magma ocean followed by a later accretion of chondritic materials.  相似文献   

9.
Abstract— Submicron platey Sn-rich grains are present in chondritic porous interplanetary dust particle (IDP) W7029*A and it is the second occurrence of a tin mineral in a stratospheric micrometeorite. Selected Area Electron Diffraction data for the Snrich grains match with Sn2O3 and Sn3O4. The oxide(s) may have formed in the solar nebula when tin metal catalytically supported reduction of CO or during flash heating on atmospheric entry of the IDP. The presence of tin is consistent with enrichments for other volatile trace elements in chondritic IDPs and may signal an emerging trend towards non-chondritic volatile element abundances in chondritic IDPs. The observation confirms small-scale mineralogical heterogeneity in fine-grained chondritic porous interplanetary dust.  相似文献   

10.
Abstract— We report detailed chemical, petrological, and mineralogical studies on the Ningqiang carbonaceous chondrite. Ningqiang is a unique ungrouped type 3 carbonaceous chondrite. Its bulk composition is similar to that of CV and CK chondrites, but refractory lithophile elements (1.01 × CI) are distinctly depleted relative to CV (1.29 × CI) and CK (1.20 × CI) chondrites. Ningqiang consists of 47.5 vol% chondrules, 2.0 vol% Ca,Al‐rich inclusions (CAIs), 4.5 vol% amoeboid olivine aggregates (AOAs), and 46.0 vol% matrix. Most chondrules (95%) in Ningqiang are Mg‐rich. The abundances of Fe‐rich and Al‐rich chondrules are very low. Al‐rich chondrules (ARCs) in Ningqiang are composed mainly of olivine, plagioclase, spinel, and pyroxenes. In ARCs, spinel and plagioclase are enriched in moderately volatile elements (Cr, Mn, and Na), and low‐Ca pyroxenes are enriched in refractory elements (Al and Ti). The petrology and mineralogy of ARCs in Ningqiang indicate that they were formed from hybrid precursors of ferromagnesian chondrules mixed with refractory materials during chondrule formation processes. We found 294 CAIs (55.0% type A, 39.5% spinel‐pyroxene‐rich, 4.4% hibonite‐rich, and several type C and anorthite‐spinel‐rich inclusions) and 73 AOAs in 15 Ningqiang sections (equivalent to 20 cm2surface area). This is the first report of hibonite‐rich inclusions in Ningqiang. They are texturally similar to those in CM, CH, and CB chondrites, and exhibit three textural forms: aggregates of euhedral hibonite single crystals, fine‐grained aggregates of subhedral hibonite with minor spinel, and hibonite ± Al,Ti‐diopside ± spinel spherules. Evidence of secondary alteration is ubiquitous in Ningqiang. Opaque assemblages, formed by secondary alteration of pre‐existing alloys on the parent body, are widespread in chondrules and matrix. On the other hand, nepheline and sodalite, existing in all chondritic components, formed by alkali‐halogen metasomatism in the solar nebula.  相似文献   

11.
Abstract— The CBb chondrites are rare, primitive, metal‐rich meteorites that contain several features, including zoned metal, that have previously been interpreted as evidence for origins in the solar nebula. We have measured concentrations of Ni, Cu, Ga, Ru, Pd, Ir, and Au within both zoned and unzoned metal grains in the CBb chondrites Hammadah al Hamra (HaH) 237 and Queen Alexandra Range (QUE) 94627 using laser ablation inductively coupled plasma mass spectrometry. The refractory elements Ni, Ru, and Ir are enriched in the grain cores, relative to the rims, in the zoned metal. All refractory elements are uniform across the unzoned metal grains, at concentrations that are highly variable between grains. The volatile elements Cu, Ga, and Au are usually depleted relative to chondritic abundances and are most often uniform within the grains but are sometimes slightly elevated at the outermost rim. The Pd abundances are nearly uniform, at close to chondritic abundances, in all of the metal grains. A condensation origin is inferred for both types of metal. The data support a model in which the zoned metal formed at high temperatures, in a relatively rapidly cooling nebular gas, and the unzoned metal formed at lower temperatures and at a lower cooling rate. The CBb metal appears to have formed by a process very similar to that of the CH chondrites, but the CBb meteorite components experienced even less thermal alteration following their formation and are among the most primitive materials known to have formed in the solar nebula.  相似文献   

12.
Abstract— All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well‐defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare‐earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca‐Al‐Ti‐rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca‐Al‐Ti‐rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2‐0.4 wt% imposes an upper limit of ～1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O‐isotopic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.  相似文献   

13.
Abstract— Relative chemical abundances of 13 meteoroids were determined by averaging the composition of the radiating gas along the fireball path that originated during their penetration into the Earth's atmosphere. Mg, Fe, Ni, Cr, Mn, and Co abundances, relative to Si, are similar to those reported for CI and CM carbonaceous chondrites and interplanetary dust particles. In contrast, relative abundances of Ca and Ti in meteor spectra indicate that these elements suffer incomplete evaporation processes. The chemical composition of all meteoroids studied in this work differs from that of 1P/Halley dust.  相似文献   

14.
High dispersion photographic spectra of three Leonid and five Perseid meteors are used to derive relative abundances of nine chemical elements in the radiating meteoric vapors and in the meteoroids. Al and Ca were found to be incompletely evaporated in the main spectral component at 5000 K but completely evaporated in the second component at 10,000 K. Si lines are present in both components which enhances the reliability of determination of the Si abundance. The composition of the meteoroids was found to be more similar to comet Halley than to chondritic meteoroids. Fe, Cr, and Mn are depleted and Si, Na, and H are enhanced relative to Mg in comparison with CI chondrites.  相似文献   

15.
Abstract— The trace element distributions in the matrix of primitive chondrites were examined using four least‐contaminated matrix specimens from the polished sections of the Allende (CV) meteorite. Analysis of rare earth element (REE), Ba, Sr, Rb, and K abundances by isotope dilution mass spectrometry revealed that the elemental abundances of lithophile elements except for alkali metals (K, Rb) in the specimens of the Allende matrix studied here are nearly CI (carbonaceous Orgueil) chondritic (～1 × CI). Compared to refractory elements, all the matrix samples exhibited systematic depletion of the moderately volatile elements K and Rb (0.1–0.5 × CI). We suggest that the matrix precursor material did not carry significant amounts of alkali metals or that the alkalis were removed from the matrix precursor material during the parent body process and/or before matrix formation and accretion. The matrix specimens displayed slightly fractionated REE abundance patterns with positive Ce anomalies (CI‐normalized La/Yb ratio = 1.32–1.65; Ce/Ce* = 1.16–1.28; Eu/Eu* = 0.98–1.10). The REE features of the Allende matrix do not indicate a direct relationship with chondrules or calcium‐aluminum‐rich inclusions (CAIs), which in turn suggests that the matrix was not formed from materials produced by the breakage and disaggregation of the chondrules or CAIs. Therefore, we infer that the Allende matrix retains the REE features acquired during the condensation process in the nebula gas.  相似文献   

16.
Abstract— A controversially discussed and yet central question in interplanetary dust particle (IDP) research is the degree of alteration of these particles during their residence in the stratosphere. Especially, the typical enrichment of Br in chondritic IDPs (on the average ～21 × CI) has been inferred to be a result of contamination processes, probably invoking aerosol droplets. With time-of-flight secondary ion mass spectrometry (TOF-SIMS), we examined the surfaces of 13 stratospheric particles from the dust collector U2071. Six particles had severe, surface-bound, silicone oil residues preventing a proper analysis of their surfaces. Six other particles—-according to our scanning electron microscopy, energy dispersive x-ray spectrometer (SEM-EDS) studies preclassified as one (Fe,Ni)S-rich IDP, one Ca-rich particle, and four aluminum-oxide spheres—-carry the halogens F, Cl, and Br on the surface. At least for the aluminum-oxide spheres, we provide unequivocal evidence for a surface correlation of halogens. This evidence, taken together with that from previous studies, proves a general stratospheric contamination process which has to be considered in IDP research.  相似文献   

17.
Abstract— Siderophile elements have been used to constrain projectile compositions in terrestrial and lunar impact melt rocks. To obtain a better knowledge of compositional differences between potential chondritic projectile types, meteorite analyses of the elements Ru, Rh, Pd, Os, Ir, Pt, Cr, Co, Ni, and Au were gathered into a database. The presented compilation comprises 806 analyses of 278 chondrites including new ICP‐MS analyses of Allende and two ordinary chondrites. Each data set was evaluated by comparing element ratios of meteorites from the same chondrite group. Characteristic element abundances and ratios were determined for each group. Features observed in the element abundance patterns can be linked directly to the presence of certain components, such as the abundance of refractory elements Os, Ir, and Ru correlating with the occurrence of refractory inclusions in CV, CO, CK, and CM chondrites. The refined characteristic element ratios appear to be representative not only for meteorites, but also for related asteroidal bodies. Chondrite element ratios were compared to previously published values from impact melt rocks of the Popigai and Morokweng impact structures confirming that an identification of the specific type of projectile (L and LL chondrite, respectively) is possible. The assessment for Morokweng is supported by the recent discovery of an LL chondrite fragment in the impact melt rocks. Ultimately, the database provides valuable information for understanding processes in the solar nebula as they are recorded in chondrites. A new type of complementarity between element patterns of CK and EH chondrites is suggested to be the result of condensation, redox, and transportation processes in the solar nebula.  相似文献   

18.
Abstract— Inductively coupled plasma mass spectrometry (ICP-MS) was successfully applied to bulk samples of Allende, Jilin, Modoc, Saint-Séverin and Atlanta for the determination of rare earth elements (REE) (Y and 14 lanthanoids), Th and U. The results of ICP-MS showed good agreement with recommended values, and their reproducibilities were high enough to discuss the detailed abundances of lanthanoids and actinoids in chondritic meteorites. For the Allende reference sample issued by the Smithsonian Institution, a positive anomaly of Tm, a fractionation between light REE and heavy REE and a high Th/U ratio were observed in the CI-normalized abundances of REE, Th and U. These features are common for group II inclusions in Allende, suggesting that the abundances of refractory lithophiles in Allende are somewhat influenced by those in a specific constituent. For the other chondritic meteorites, a zigzag alteration was commonly observed in the heavy-REE region of their CI-normalized abundance patterns. It is suggested that such a zigzag pattern is attributable to erratically high abundances of monoisotopic REE (Tb, Ho and Tm) in the CI values. Abundances of REE, Th and U in the bulk samples are also discussed separately in detail.  相似文献   

19.
The Colony meteorite is an accretionary breccia containing several millimeter-to centimeter-size chondritic clasts embedded in a chondritic host. Colony is one of the least equilibrated CO3 chondrites; it has an unrecrystallized texture and contains compositionally heterogeneous olivine and low-Ca pyroxene, kamacite with low Ni and Co and high Cr, amoeboid inclusions with low FeO and MnO, a fine-grained silicate matrix with very high FeO, and numerous small chondrules with clear pink glass. However, Colony differs from normal CO chondrites in several respects: Although Al, Sc, V, Cr, Ir, Fe, Au and Ga abundances are consistent with a CO chondrite classification, certain lithophiles (Mg and Mn), siderophiles (Ni and Co) and chalcophiles (Se and Zn) are depleted by factors of 10–40%. The shape of Colony's thermoluminescence (TL) glow curve is similar to that of Allan Hills A77307 (another unequilibrated chondrite with CO3 petrological characteristics) and different from those of normal CO chondrites. [ALHA77307 also resembles Colony in having low Mg, Mn, Ni and Co, compared to normal CO chondrites, but it possesses CO-CV levels of Se and Zn and nearly CV levels of Cd.] Colony is badly weathered; it contains 22.7 wt.% Fe2O3 and 5.7 wt.% H2O. Recalculating the analysis on an H2O-free basis with all Fe2O3, NiO and CoO converted to metal, yields an inferred original metallic Fe, Ni abundance of ～ 19 wt.%. This is similar to that of Kainsaz (an unweathered CO3 fall), but much higher than that of all other CO3 chondrites (< 6.3 wt.%). Although it is possible that Colony and either ALHA77307 or Kainsaz constitute distinct CO3 chemical subgroups, the weathered nature of Colony and ALHA77307 preclude the drawing of firm conclusions. Nevertheless, it is clear that CO3 chondrites vary more in compositional and petrological properties than was previously recognized.  相似文献   

20.
Abstract– Nineteen nonporphyritic pyroxene and pyroxene/olivine chondrules, chondrule fragments, and irregular objects were studied from two equilibrated chondrites, the ordinary (L/LL5) Knyahinya chondrite and the Rumuruti type (R4) Ouzina chondrite. Major element contents for almost all objects in the chondrites are disturbed from their chondritic ratios, most probably during metamorphic re‐equilibration. However, the volatile elements (Na2O + K2O) in Ouzina scatter around the CI line, probably the result of being generated and/or processed in different environments as compared with those for Knyahinya. All studied objects from Knyahinya and Ouzina possess systematically fractionated trace element abundances. Depletion of LREE with respect to HREE and ultra‐refractory HFSE documents variable degrees of LREE transport into an external mineral sink and restricted mobility of most of the HREE and HFSE. Moderately volatile elements preserve volatility‐controlled abundances. Strongly fractionated Rb/Cs ratios (up to 10× CI) in all studied objects suggest restricted mobility of the large Cs ion. All studied objects sampled and preserved Y and Ho in solar proportions, a feature that they share with the nonporphyritic chondrules of unequilibrated ordinary chondrites.  相似文献   

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