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

2.
The size frequency distributions of fluid drop chondrules in 11 ordinary chondrites (five H3, one H4, four L3, one LL3) have been determined by optical measurements in petrographic thin sections. The extreme range of median size of the fluid drop chondrules in individual meteorites is only slightly greater than lφ unit, and the grain size frequency distributions are approximately log normal. Chondrule size frequency distributions generally are fine skewed, platykurtic, and indicate moderate sorting. The size frequency distributions of fluid drop chondrules in ordinary chondrites are distinctly coarser than similar chondrules measured previously in CM2 and C03 carbonaceous chondrites.  相似文献   

3.
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   

4.
Scott A. Sandford 《Icarus》1984,60(1):115-126
Infrared transmission spectra from 53 meteorites in the spectral range from 2.5 to 25 μm were measured to permit comparisons with data of astronomical objects that are potential meteorite sources. Data were taken for 14 carbonaceous chondrites, 5 LL ordinary chondrites, 6 L ordinary chondrites, 10 H ordinary chondrites, 1 enstatite chondrite, 4 aubrites, 3 eucrites, 4 howardites, 1 diogenite, 1 mesosiderite, 2 nakhlites, 1 shergottite, and the anomalous achondrite Angra dos Reis. The CO and CV carbonaceous chondrites have spectra similar to each other, with 10-μm features characteristic of olivine. The CM carbonaceous chondrites have distinctive 10-μm features that are attributed to layer lattice silicates. Members of both the CI and CR classes have spectra distinct from those of other carbonaceous chondrites. The LL, L, and H ordinary chondrites have spectra that match those of olivine and pyroxene mixtures. The enstatite chondrites and enstatite achondrites (aubrites) all exhibit spectra diagnostic of the pyroxene enstatite. The angrite, howardites, aucrites, nakhlites, shergottite, and diogenite all have similar spectra also dominated by pyroxene. The single mesosiderite examined had a spectrum distinct from all the other meteorites.  相似文献   

5.
Abstract— Meteoritical Bulletin No. 83 lists information for 898 newly described meteorites. These include 473 from Antarctica, 341 from the Sahara, and 22 from dry lakes in the southwestern United States. Seven of the meteorites are falls: Kunya-Urgench (H5), Lohawat (howardite), Ourique (H4), Portales Valley (H6), San Pedro de Quiles (L6), Talampaya (eucrite), and Zag (H3-6). Also included are a dozen new iron meteorites; several mesosiderites; a pallasite; several HED meteorites; several ureilites; a variety of CM, CO, CV, CR, and R chondrites; and numerous unequilibrated ordinary chondrites. All shock classifications are after Stöffler et al. (1991) and weathering grades are after Wlotzka (1993), except as noted. All italicized abbreviations refer to addresses tabulated at the end of this document.  相似文献   

6.
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   

7.
Abstract— Quantifying the peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent bodies. Various geothermometers have been used to estimate equilibration temperatures for chondrites of the highest metamorphic grade (type 6), but results are inconsistent and span hundreds of degrees. Because different geothermometers and calibration models were used with different meteorites, it is unclear whether variations in peak temperatures represent actual ranges of metamorphic conditions within type 6 chondrites or differences in model calibrations. We addressed this problem by performing twopyroxene geothermometry, using QUILF95, on the same type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer (Nakamuta and Motomura 1999). We also calculated temperatures for published pyroxene analyses from other type 6 H, L, and LL chondrites to determine the most representative peak metamorphic temperatures for ordinary chondrites. Pyroxenes record a narrow, overlapping range of temperatures in H6 (865–926 °C), L6 (812–934 °C), and LL6 (874–945 °C) chondrites. Plagioclase temperature estimates are 96–179 °C lower than pyroxenes in the same type 6 meteorites. Plagioclase estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak. The average temperature for H, L, and LL chondrites (～900 °C), which agrees with previously published oxygen isotope geothermometry, is at least 50 °C lower than the peak temperatures used in current asteroid thermal evolution models. This difference may require minor adjustments to thermal model calculations.  相似文献   

8.
Abstract— Six ordinary chondrite breccias from the Museo Nacional de Ciencias Naturales, Madrid (Spain), are described and classified as follows: the solar gas-rich regolith breccia Oviedo (H5); the pre-metamorphic fragmental breccias Cabezo de Mayo (type 6, L-LL), and Sevilla (LL4); the fragmental breccias Cañellas (H4) and Gerona (H5); and the impact melt breccia, Madrid (L6). We confirm that chondrites with typical light-dark structures and petrographic properties typical of regolith breccias may (Oviedo) or may not (Cañellas) be solar gas-rich. Cabezo de Mayo and Sevilla show convincing evidence that they were assembled prior to peak metamorphism and were equilibrated during subsequent reheating. These meteorites contain small melt rock clasts that were incorporated into the host chondrite while still molten and/or plastic and cooled rapidly and, yet, are totally equilibrated with their hosts. Compositions of olivine and low-Ca pyroxene in host chondrite and breccia clasts in Cabezo de Mayo are transitional between groups L and LL. It is suggested, based on mineralogic and oxygen isotopic compositions of host and clasts, that the rock formed on the L parent body by mixing, prior to peak metamorphism. This was followed by partial equilibration of two different materials: the indigenous L chondrite host and exotic LL melt rock clasts.  相似文献   

9.
Abstract— This Meteoritical Bulletin is again dominated by meteorite finds from hot and cold deserts: 99 from the Nullarbor, 12 from the Sahara, and 35 from Antarctica. Besides 161 ordinary chondrites, it lists 5 irons (Cotton, Hidden Valley, Miles, Tagounite, Tres Castillos), 2 ureilites (FRO90168, Hughes 009), 1 howardite (ALH 88135), 1 CV3 (Axtell), 1 CK4 (Sleeper Camp 006), and 2 enstatite chondrites (ALH 88070, Forrest 033). Three of the meteorites are falls.  相似文献   

10.
Abstract— The mineralogy and composition of six Mongolian meteorites were studied in some detail. Previously, only limited information existed about these rocks, and some were still unclassified. The six meteorites include three ordinary chondrites and three irons. The ordinary chondrite Adzhi-Bogdo (stone) is a regolith breccia (LL3–6) containing various types of clasts (some of foreign origin) embedded within a fine-grained clastic matrix. Tugalin Bulen (H6) and Noyan Bogdo (L6) meteorites are typical, well-metamorphosed ordinary chondrites. Adzhi-Bogdo (iron) has to be regarded as an IA iron meteorite like Campo del Cielo or Canyon Diablo; although the sample studied had been heated to about 900 °C–950 °C some time in the past, thus eradicating all original structural elements. Manlai is structurally closely related to the IIC iron meteorites; but based on its chemistry, which does not fit into this group, it is suggested that Manlai is an anomalous iron meteorite. The third iron, Sargiin Gobi, is certainly a normal member of the IA iron meteorites. The concentrations and isotopic compositions of He, Ne, and Ar were measured for all meteorites and their gas retention ages and exposure ages are discussed.  相似文献   

11.
Abstract— Thirteen new meteorites and three meteorite inclusions have been analyzed. Their results have been incorporated into earlier published data for a comprehensive reference to all analyzed meteorites at the Smithsonian Institution. The six tables facilitate a convenient overlook of meteorite data. Table 1 presents an alphabetical list of analyzed meteorites, Table 2 chemical analyses of stony meteorites, Table 3 chemical analyses of iron meteorites, Table 4 elemental composition of stony meteorites, Table 5 average composition of carbonaceous chondrites and achondrites (falls and finds), and Table 6 presents average composition of H, L, LL, and Antarctic chondrites (falls and finds). The tables are available online at the journal's Web site http:meteoritics.org .  相似文献   

12.
Mineral compositions and abundances derived from visible/near-infrared (VIS/NIR or VNIR) spectra are used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid belt. Using a suite of 48 equilibrated (types 4-6) ordinary (H, L, and LL) chondrites containing orthopyroxene, clinopyroxene, and olivine, new relationships between spectra and mineralogy have been established. Contrary to previous suggestions, no meaningful correlation is observed between band parameters and cpx/(opx + cpx) ratios. We derive new calibrations for determining mineral abundances (ol/(ol + px)) and mafic silicate compositions (Fa in olivine, Fs in pyroxene) from VIS/NIR spectra. These calibrations confirm that band area ratio (BAR) is controlled by mineral abundances, while Band I center is controlled by mafic silicate compositions. Spectrally-derived mineralogical parameters correctly classify H, L and LL chondrites in ∼80% of cases, suggesting that these are robust relationships that can be applied to S(IV) asteroids with ordinary chondrites mineralogies. Comparison of asteroids and meteorites using these new mineralogical parameters has the advantage that H, L and LL chemical groups were originally defined on the basis of mafic silicate compositions.  相似文献   

13.
Nine twentieth-century ordinary chondrite falls from China are described and classified. They include: Nantong (H6), Zaoyang (H5), Zhaodong (L4), Qidong (L-LL5), Raoyang (L6), Sheyang (L6), Guangnan (L6), Suizhou (L6) and Nan Yang Pao (L6). Kamacite in Qidong is rare and contains much more Co (15 mg/g) than is characteristic of L-group chondrites; Qidong may be a member of a chondrite group intermediate in its properties between L and LL. Zhaodong, Qidong, Raoyang, Sheyang and Suizhou have several olivine and/or low-Ca pyroxene grains with aberrant Fel(Fe + Mg) ratios; it is probable that these five chondrites are fragmental breccias. The lack of correlation between shock facies and occurrence of aberrant silicate grains suggests that breccia lithification caused only minimal shock effects in many meteorites. Alternatively, postshock annealing may have resulted in the recrystallization of shock-indicating phases, leading to assignment of a shock facies that is lower than that present immediately after the shock event.  相似文献   

14.
Abstract— We report here new analyses of S and Se in carbonaceous chondrites (2 CIs, 11 CMs, 6 CO3s, 7 CV3s, 2 C4s, 4 CRs, and 1 CH), 2 rumurutiites, ordinary chondrites (2 Hs, 2 Ls, and 1 LL), 3 anomalous chondrites, 3 acapulcoites, 3 lodranites, and in silicate inclusions of the Landes IAB iron meteorite. To avoid problems from inhomogeneous distribution of sulfides, the same samples that had been analysed for Se by INAA were analysed for S using a Leybold Heraeus Carbon and Sulfur Analyser (CSA 2002). With the measured CI contents of 5.41% S and 21.4 ppm Se a CI S/Se ratio of 2540 is obtained. A nearly identical S/Se ratio of 2560 ± 150 is found for carbonaceous chondrites (average of falls). The average ratio of all meteorite falls analysed in this study was 2500 ± 270. These data suggest that the new S content of Orgueil with 5.41% provides a reliable estimate for the average Solar System. The new solar system abundance of S of 4.62 × 105 (atoms/106 Si) is in good agreement with the solar photospheric abundance of 7.21 (log (a(H)) = E12) (Anders and Grevesse, 1989). Among the 50 analysed meteorites, 24 were finds from hot (Australia, Africa) and cold (Antarctica) deserts. Weathering effects in the carbonaceous chondrites and in one lodranite from the hot deserts resulted in losses of S, Se, Na and occasionally Ni. Sulfur is apparently more affected by weathering than Se. No losses were observed in ordinary chondrite finds and in meteorites collected in the Antarctica, except for the obvious loss of Na in the CM-chondrite Y 74662. The low S-content of 0.096% in Gibson, a lodranite, is probably not representative of this group of meteorites. Gibson is a find from the Australian desert and has lost S and also Se by weathering. Two other lodranites, finds from Antarctica, have about 2% S.  相似文献   

15.
Abstract— The nature and isotopic composition of carbonaceous components in a variety of ordinary chondrites have been studied using stepped combustion. The samples were chosen to include falls, finds and Antarctic meteorites; specimens from all three chemical groups (H, L and LL) have been analysed. Effort was concentrated mostly on the low petrologic type meteorites (i.e., type 3); however, types 4–6 were also included in the study. Apart from terrestrial contaminants and weathering products, some of the unequilibrated ordinary chondrites appear to contain an indigenous organic component. In addition, most of the samples studied show evidence for an amorphous/graphitic component. This exists as C-rich aggregates or as carbon associated with “Huss” matrix. There does not appear to be any difference in δ13C for this carbon between Antarctic and non-Antarctic meteorites. In contrast, low temperature carbon in Antarctic samples is characterized by a 13C-enrichment. This is thought to be due to the influence of terrestrial weathering products introduced in the Antarctic. Curiously, the low temperature carbon in non-Antarctic finds appears to be intermediate in δ13C between Antarctic finds and non-Antarctic falls. This suggests that the weathering processes which are so obviously apparent from Antarctic samples may also extend, albeit in a more limited way, to non-Antarctic meteorites.  相似文献   

16.
Electron microprobe analyses and petrographic observations demonstrate that Almelo Township (L6), Beeler (LL6), Kalvesta (H4), Phillips County (L6), and Yocemento (L5) are typical ordinary chondrites. Selden has abnormally iron-rich silicates and nickel-rich metal, and it may have been a lower petrologic type that has been shocked to an LL5. Shielding corrected exposure ages range from 0.80 m.y. (Kalvesta) to 47.0 m.y. (Yocemento).  相似文献   

17.
Abstract Experimentally produced analogues of porphyritic olivine (PO) chondrules in ordinary chondrites provide an important insight into chondrule formation processes. We have studied experimental samples with PO textures grown at three different cooling rates (2, 5 and 100 *C/h), and samples that have been annealed at high temperatures (1000–1200 °C) subsequent to cooling. These are compared with natural chondrules of similar composition and texture from the ordinary chondrites Semarkona (LL3.0) and ALH 81251 (LL3.3). Zoning properties of olivine grains indicate that the Semarkona chondrules cooled at comparable rates to the experiments. Zoning in olivine from chondrules in ALH 81251 is not consistent with cooling alone but indicates that the chondrules underwent an annealing process. Chromium loss from olivine is very rapid during annealing and calculated diffusion coefficients for Cr in olivine are very similar to those of Fe-Mg interdiffusion coefficients under the same conditions. Annealed experimental samples contain an aluminous, low-Ca pyroxene which forms by reaction of olivine and liquid. No similar reaction texture is observed in ALH 81251 chondrules, and this may be evidence that annealing of the natural samples took place at considerably lower temperatures than the experimental analogues. The study supports the model of chondrule formation in a cool nebula and metamorphism of partly equilibrated chondrites during reheating episodes on the chondrite parent bodies.  相似文献   

18.
Abstract— The cumulative mass distributions (mass range > 100 g) of each type of Japanese and U.S. Antarctic ordinary chondrites are compared with those of non-Antarctic falls and those obtained from the present-day flux of meteorites. The steeper slope of the mass distribution of Antarctic chondrites is indicative of the presence of several chondrite showers. The fall-to-specimen ratio of Antarctic ordinary chondrites larger than 100 g is about 1:2, indicating that half of them are shower components. The fall-to-specimen ratios of each group range from 1:1 to 1:6; those of the Japanese and U.S. Antarctic meteorite collections are 1:1 to 1:2 and 1:4 for H chondrites, 1:1 to 1:2 and 1:2 for L chondrites, and 1:2 and 1:6 for LL chondrites, indicating that the Japanese collection includes less abundant shower components than the U.S. collection. The fall-to-specimen ratios of each H4-6 and L4-6 type range from 1:1 to 1:4, and U.S. H6 and Japanese H4 have the low ratios of 1:4.  相似文献   

19.
We describe the geological, geomorphological, and paleoclimatic setting of the Sahara of North Africa in particular, focused on the main meteorite dense collection areas (DCA; Morocco, Algeria, Tunisia, and Libya). We report on the outcome of several meteorite recovery field expeditions in Morocco and Tunisia since 2008, by car and by foot, that applied systematic search methods. The number of meteorites collected is 41 ordinary chondrites and one brachinite. The statistics of unpaired ordinary chondrites indicates that H chondrites are more abundant (21) than L chondrites (12), while LL chondrites are rare (2). Our meteorite density estimates for Tunisia and Morocco are in the order of magnitude of 1 met km?2. An estimate of the total maximum number of meteorites that could be recovered from the Sahara is 780,000 meteorites. We selected 23 meteorites from Aridal, Bou Kra, Bir Zar, and Tieret DCAs for 14C dating. The results show a wide range of terrestrial ages from 0.4 to more than 40 kyr with a majority of meteorites showing ages between 0.4 and 20 kyr. The weathering degree of these meteorites is ranges from minor (W1) to strong (W4). The highest weathering grades result from repeated oscillations between high and low humidity in the Sahara. However, there appears to be no correlation between weathering grade and terrestrial age of meteorites.  相似文献   

20.
Abstract— Reconnaissance searches for meteorites were made in five selected areas of western Namibia in 1991. Three new ordinary chondrites (H5, L5, L4/5) were recovered from one of the areas searched.  相似文献   

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