摘要:

Abstract—A “carbonaceous chondrite” clast from Bencubbin was studied by analytical transmission electron microscopy and other electron beam techniques. In section, the clast consists of oval augen, with a preferred orientation, set in fine‐grained matrix. The augen comprise olivine microphenocrysts in fine‐grained to glassy mesostases. The olivines are heavily deformed, giving rise to mosaicism. Many sub‐grains have high densities of dislocations with [001] Burgers vectors. In some regions the dislocation configurations are recovered, causing a reduced dislocation density; recrystallization is rarer. Severe cataclasis is absent; there are few open fractures and little intergranular porosity except where a second phase occurs. Such porosity in olivine mainly occurs as finescale negative crystals, which appear to be healed cracks. Some mesostases consist of small amounts of microporous oxides and feldspathic glass but glassy veins are absent. Pyrhottite and Fe/Ni sulfides are major mesostasis constituents that rarely form tongues between the olivines, which commonly include smaller sulfide blebs.The matrix of the clast has abundant sulfides and fine‐grained, poorly crystalline Fe‐ and Fe/Ni‐oxides, with more sparse ferrihydrite. The oxides mostly occur in contact with, or within, aluminous and siliceous glassy material in which crystals of melilite, spinel, Ca‐pyroxene, feldspar, and other minor silicate phases have grown. Small patches of fibrous and/or sheet‐like Mg/Fe silicates with layer morphologies also occur. They are microporous, poorly crystalline and lack the layer spacings of phyllosilicates, of which they may be relics. Grains of anhydrite and calcite are interlaced with fibrous silicates.There is evidence that one, or more, intense shock‐heating event(s) produced local melting. A later shock event(s) involved less severe shock‐heating, to about 900 °C. The mineralogy indicates that the clast may have originated from a CM2 precursor or from material like Allan Hills 85085, although it is just possible that terrestrial weathering produced some

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01023.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—The purpose of this study is to examine, using light optical and electron optical techniques, the microstructure and composition of metal particles in ordinary chondritic meteorites. This examination will lead to the understanding of the low temperature thermal history of metal particles in their host chondrites. Two type 6 falls were chosen for study: Kernouvé (H6) and Saint Severin (LL6).In both meteorites, the taenite particles consisted of a narrow rim of high Ni taenite and a central region of cloudy zone similar to the phases observed in iron meteorites. The cloudy zone microstructure was coarser in Saint Severin than in Kernouvé due to the higher bulk Ni content of the taenite and the slower cooling rate, 3 K Ma−1vs.17 K Ma−1. Three microstructural zones were observed within the high Ni taenite region in both meteorites. The origin of the multiple zones is unknown but is most likely due to the high Ni taenite cooling into the two phase γ″ (FeNi) + γ′ (FeNi3) region of the low temperature Fe‐Ni phase diagram. Another explanation may be the presence of uniform size antiphase boundaries within the high Ni taenite. Finally, abnormally wide high Ni taenite regions are observed bordering troilite. The wide zones are probably caused by the diffusion of Ni from troilite into the high Ni taenite borders at low cooli

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01024.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—A crystal of clinopyroxene from the coarse‐grained refractory inclusion Egg 6 of the Allende meteorite has been studied in detail by transmission electron microscopy.The pyroxene crystal contains euhedral, dislocation‐free inclusions of pure spinel MgAl2O4, without any topotactic relation to the host. Extensive dislocation walls at equilibrium, characteristic of high‐temperature anneal, are present in the crystal. Alteration products are occasionally observed at the spinel‐pyroxene interface close to regions where dislocation walls decorated with bubbles (or voids) are present. The bubbles, often in the shape of tubes along the dislocation lines, are thought to be due to the precipitation of a fluid migrating along the dislocations.The observations are compatible with crystallization of the refractory inclusions from the melt and with the existence of a later stage of met

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01025.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Infrared diffuse reflectance spectra were measured for several thermally metamorphosed carbonaceous chondrites with CI‐CM affinities which were recently found from Antarctica. Compared with other CI or CM carbonaceous chondrites, these Antarctic carbonaceous chondrites show weaker absorption bands near 3 μm due to hydrous minerals, and weaker absorption bands near 6.9 μm due to carbonates, interpreted as thermal metamorphic features. These absorption bands also disappear in the spectra of samples of the Murchison (CM) carbonaceous chondrite heated above 500 °C, implying that the metamorphic temperatures of the Antarctic carbonaceous chondrites considered here were higher than about 500 °C. Model calculations were performed to study thermal metamorphism of carbonaceous chondrites in a parent body internally heated by the decay of the extinct nuclide26Al. The maximum temperature of the interior of a body more than 20 km in radius is 500–700 °C for the bulk Al contents of CI and CM carbonaceous chondrites, assuming a ratio of26Al/27Al = 5 × 10−6which has been previously proposed for an ordinary‐chondrite parent body. The metamorphic temperatures experienced by the Antarctic carbonaceous chondrites considered here may be attainable by an internally heated body with an26Al/27Al ratio similar to that inferred for an ordinary‐chon

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01026.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Organic compounds in the Murchison (C2M) and Allende (CV3) carbonaceous chondrites were analyzed by photoionization time‐of‐flight mass spectrometry; thermal (25–850 °C) and stimulated (7 keV Ar+) desorption were combined with either nonresonant single‐photon ionization using 118 nm light or resonantly enhanced multiphoton ionization (selective for aromatic compounds) using 266 nm light. Samples weighing only 1–10 mg were sufficient for sensitive quantitative analysis of aromatic compounds using thermal desorption. The detection limits for phenanthrene and pyrene using 118 nm light were determined to be 0.8 and 1.4 picomoles, respectively, and the concentrations of these compounds (including their isomers anthracene and fluoranthene) in the Murchison meteorite were determined to be 9 and 12 μg/g, respectively, in good agreement with previously published values. Thermal‐desorption (–75–500 °C) field‐ionization mass spectra (activated foil‐type ionizing source and magnetic sector mass analyzer) of 20–40 mg of the same meteorite material were obtained to verify that the 118 nm photoionization mass spectra were not affected by photofragmentation or photodecomposition and were representative of the organic material extracted by thermal desorption. Photoionization mass spectrometry is a useful technique for studying small quantities (<1 nanomole) of organic matter in terrestrial and extraterrestrial samples. The present study aims to provide the background and analytical methods necessary for application to new and unsolved cosmochemical problems. Some potential

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01027.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—The pyroxenes in two new monomict eucrites from Antarctica, Yamato 791186 and Yamato 792510, have been studied and compared with those of other Antarctic and non‐Antarctic eucrites. The purpose of this study is to identify compositional and textural relationships shown by these pyroxenes which may be used as indicators of the thermal history of the meteorite. An attempt is made, using petrographic and compositional criteria, to distinguish between the initial cooling history and subsequent thermal events. We suggest that it is possible to identify stages of thermal “metamorphism” which may be used to indicate the conditions on the surface and crust of the parent body. A picture of the geological setting of the HED (Howardites, Eucrites, Diogenites) parent body is proposed, for which thermal metamorphism by impact heating is an important

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01028.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Interior samples of three different Nakhla specimens contain an iron‐rich silicate “rust” (which includes a tentatively identified smectite), Ca‐carbonate (probably calcite), Ca‐sulfate (possibly gypsum or bassanite), Mg‐sulfate (possibly epsomite or kieserite), and NaCl (halite); the total abundance of these phases is estimated as<0.01 weight percent of the bulk meteorite. Rust veins are truncated and decrepitated by fusion crust and are preserved as faulted segments in partially healed olivine crystals, indicating that the rust is pre‐terrestrial in origin. Because Ca‐carbonate and Ca‐sulfate are intergrown with the rust, they are also indicated to be of pre‐terrestrial origin. Similar textural evidence regarding origins of the NaCl and Mg‐sulfate is lacking. Impure and poorly crystallized sulfates and halides on the fusion crust of the meteorite suggest leaching of interior (pre‐terrestrial) salts from the interior after Nakhla arrived on Earth but coincidental addition of these same salts by terrestrial contamination cannot be excluded. At least the clay‐like silicate “rust,” Ca‐carbonate, and Ca‐sulfate were formed by precipitation from water‐based solutions on the Nakhla parent planet although temperature and pressure conditions of aqueous precipitation are unconstrained by currently available data. It is possible that aqueous alteration on the parent body was responsible for the previously observed disturbance of the Rb‐Sr geo

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01029.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—We report Rb‐Sr analyses of phosphates from nine ordinary chondrites, more than doubling the number of meteorites for which such data are available. Ordinary chondrite phosphates characteristically have Rb/Sr ratios sufficiently low to permit accurate identification of initial87Sr/86Sr, which is generally (but not in all cases) found to be significantly higher than the more primitive initial87Sr/86Sr ratios inferred for carbonaceous chondrite refractory inclusions (ALL), basaltic achondrites (BABI), or bulk ordinary chondrites (in the ALL‐BABI range). Such elevation of initial87Sr/86Sr is generally considered to reflect isotopic redistribution during metamorphism, and with a model for Rb/Sr in this environment can lead to an inferred metamorphic timescale. For whole rock Rb/Sr the inferred formation intervals are typically tens of Ma (range nil to>100 Ma).There is no evident relation between initial Sr elevation and metamorphic grade. There is not a clear difference in initial Sr effects between H and L chondrites; LL chondrites show much less (if any) elevation of initial Sr, but data are available for only two meteorites. For the first time it is possible to make a detailed comparison of initial Sr and I‐Xe chronologies for several meteorites: these two potential metamorphic chronometers conspicuously fail to agree, in terms of both age and sequence of ages. A comparably definitive assessment of the comparison between initial Sr and Pb‐Pb chronologies is not yet possible, but presently available data suggest that these two approaches to chondrite chronology also fail to agree. Without a correlation with metamorphic grade, or detailed agreement with an independent chronometer, it remains unclear whether initial87Sr/86Sr in phosphates can be translated into a reliable chronometer for ordinary chondrite metamorphism, at least within simple interpretational f

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01030.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Abundant tektites have been found in buried alluvial gravels at the south end of Lake Argyle in far northern Australia. The recovered tektites are unusual in that an astounding number are very large and 75% of those analyzed to date are HMg tektites, heretofore a very rare type in Australia. It is now clear that the paucity of tektites in northern Australia is due to the former high energy erosional environment rather than to crossing the strewnfield boundary, as has been previously suggested. The N‐S extent of the australite strewnfield encompasses the entire contin

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01031.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Natural and induced thermoluminescence (TL) data are reported for 12 meteorites recovered from the Allan Hills region of Antarctica by the European field party during the 1988/89 field season. The samples include one with extremely high natural TL, ALH88035, suggestive of exposure to unusually high radiation doses (i.e., low degrees of shielding), and one, ALH88034, whose low natural TL suggests reheating within the last 105years. The remainder have natural TL values suggestive of terrestrial ages similar to those of other meteorites from Allan Hills. ALH88015 (L6) has induced TL data suggestive of intense shock. TL sensitivities of these meteorites are generally lower than observed falls of their petrologic types, as is also observed for Antarctic meteorites in general. Acid‐washing experiments indicate that this is solely the result of terrestrial weathering rather than a nonterrestrial Antarctic—non‐Antarctic difference. However, other TL parameters, such as natural TL and induced peak temperature‐width, are unchanged by acid washing and are sensitive indicators of a meteorite's metamorphic and recent radiatio

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01032.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY