摘要:

Abstract—Evolutionary processes in meteorites and magnetic fields in the early solar system, both spatial and localised in planetary bodies, can leave their imprint in meteorites through the natural remanent magnetization (NRM) and other magnetic properties they impart to them. In the present investigation the Estherville mesosiderite has been studied to enquire whether its magnetic properties can help to resolve any of the uncertainties associated with mesosiderite history and evolution, and to examine evidence for any magnetic fields to which it or its constituent fragments have been subjected.The Estherville sample as received is strongly magnetized, with an initial NRM intensity of 1.4 × 10−3Am2kg−1. The NRM of individual fragments broken from the main mass, when referred to common reference axes, is scattered in direction on a scale which ranges from ∼ 1 cm down to ∼ 1 mm. Alternating field and thermal demagnetization show a range of magnetic stability among the samples and also some secondary NRM, indicating a variety of magnetic histories. Thermomagnetic analyses of matrix and iron‐nickel separates show that the dominant magnetic carriers are kamacite and tetrataenite.The non‐coherent directions of NRM within the matrix imply the acquisition of an initial NRM by kamacite in the fragments prior to their final accumulation into the mesosiderite material, and the presence of an ambient magnetic field when the fragment material cooled after its formation. If the tetrataenite carrying the primary NRM was formed from the previously magnetized kamacite/taenite during slow cooling after later metamorphic heating, the maximum temperature during the latter event could not have been higher than ∼700 °C or the kamacite would have been remagnetized uniformly or demagnetized, according to whether or nor there was an ambient magnetic field present.Susceptibility anisotropy observations indicate the acquisition of anisotropic properties occurred before final accumulation of the meteorite. Shock and flow processes were probably important in producing foliation and lineation respectively in the fragments resulting from brecciation, and there could also be a contribution from larger metal fragments and/or veins. The scattered NRM of the iron‐nickel fragments also indicates magnetization prior to emplacement, therefore favouring introduction in the solid rather t

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01008.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—The Sfax meteorite fell on 16 October 1989. Four pieces totaling less than 10 kg were recovered from a much larger meteoroid, according to the cosmogenic gas measurements. It is an L6 chondrite, strongly degassed and shocked, with olivine of composition Fa2

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01009.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Twenty ∼100‐mg random fragments of the Bruderheim chondrite were analyzed by INAA to determine extents of heterogeneity and to check earlier results that gave anomalous LREE concentrations. Considerable compositional heterogeneity was observed in both concentrations (maximum/minimum of 1.7 for Ir, 1.8 for Na, 6.8 for La, 7.4 for Co) and concentration ratios (>3 × for Ni/Co and La/Eu). Compositions could be modelled quantitatively in terms of end‐member components of constant composition representing the minerals olivine + orthopyroxene, clinopyroxene, plagioclase, kamacite, taenite, troilite, chromite, and phosphate. The model yields the proportions of minerals in each sample as well as the meteorite as a whole; accurate determination of the end members depends on high intrasample precision for many elements. No evidence of compositional heterogeneity of end members for trace elements was found. It is shown that these analyses, which used only ∼2 grams of the meteorite, are equivalent in accuracy to a single sample of several tens of grams, and that knowledge of the extent of heterogeneity is essential to knowing the quality of sampling as well as for comparing results from one analysis with another and of one meteorite wi

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01010.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Accompanied by loud thunder at about 1.30 pm (local time) on 3 February 1974, a single stone weighing approx. 480 g was seen to fall and was recovered near the Jolomba village, Huambo District, Angola. The stone is strongly brecciated, has dark‐greyish angular fragments within a lighter matrix and does not exhibit any chondritic textures. Jolomba consists of olivine (Fa 31), orthopyroxene (Fs 25), sodic plagioclase (An 11), sulphides (troilite), very scarce nickel‐iron (Ni up to 56%), oxides (chromite and ilmenite) and apatite. Pervasive fracturing of silicates suggests that Jolomba suffered strong brecciation and partial recrystallization. Mineralogy and bulk chemical analysis indicate that Jolomba belongs to the LL group of chondrites (amphoterites). Uniform olivine and pyroxene composition, well‐crystallized plagioclase and textural features in general indicate that Jolomba belongs to the petrologic

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01011.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Approximately 1500 g of float samples of impact melt rocks have been recovered from gravel deposits ∼4 km north and northeast of the rim of the 3.4 km diameter New Quebec Crater (61°17′N; 73°40′W) in northern Quebec, Canada. Previously, only two small samples of impact melt rocks were known. The newly recovered samples have cryptocrystalline to microcrystalline matrices with microlites of andesine and pigeonite. Mineral clasts of quartz and feldspar occur and, in some cases, show shock metamorphic features. The melt rocks have a normative mineralogy corresponding to ∼70% quartz, orthoclase and albite and are compositionally similar. Their major element composition can be modeled as a mix of granitic gneisses that make up the target rocks. The melt rocks show enrichments, however, in Cr (21 ppm), Co (9 ppm), Ni (12 ppm) and Ir (1.5 ppb) over the target rocks. Interelement ratios suggest a chondritic impacting body, although they do not define a specific type. Assuming a C‐1 chondrite, the impact melt rocks average ∼2% meteoritic contamination. Stepwise40Ar‐39Ar dating using a laser on three chips from three samples give integrated ages of 0.6–2.5 Ma. From the best plateau ages, the age of the New Quebec impact is taken to be 1.4 ± 0.1 Ma, which places it before the first major northern hemisphere continental glaciation of the Pleistocene. A number of considerations suggest that the impact melt rocks were originally deposited in fractures in the crater wall and later transported to their discovery site by glacia

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01012.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—We have analyzed fluorine and boron in nine tektites from all four strewn fields, and in a suite of impact glasses and target rocks from the Zhamanshin and Darwin impact craters, as well as Libyan Desert Glass and Aouelloul impact glass samples. Fluorine and boron are useful indicators for the volatilization and temperature history of tektites and impact glasses. Tektites from different strewn fields show a limited range of F and B contents and have F/B ratios near unity. Most splash‐form tektites have lower average F and B contents than Muong Nong type tektites, which is similar to the relation between irghizites and zhamanshinites. The F and B contents in target rocks from the Zhamanshin and Darwin impact craters are similar to normal terrestrial sediments. Fluorine in impact glasses and tektites is more depleted compared to their (known or inferred) target rocks than is boron, which is caused by the higher volatility of F. The F/B ratios therefore decrease with increasing temperature of formation (suggesting that irghizites were formed at a higher temperature than zhamanshinites, and Muong Nong type tektites at a lower temperature than splash‐form tektites). Mixing of local country rocks together with partial loss of the volatiles F and B can reproduce the F and B contents of impact gl

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01013.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—The Versailles cryptoexplosion structure, located in central Kentucky, contains a partially brecciated central uplift and a boundary marked by arcuate faulting. Seismic refraction data were used to divide the structure into distinct units on the basis of velocity. A zone of brecciated material shows a decrease in velocity compared with surrounding rocks. Locally thickened strata are present on the flanks of the central uplift. An increase of the bedrock velocity in the area surrounding the structure suggests localized dolomization of fractured bedrock, and a plug of fractured strata near the base of the brecciation appears to be uplifted. Based on the geometry of deformation seen in known impact structures, and the integration of this study with previous geological and geophysical work in the area, it is suggested that our results support the interpretation of the Versailles structure as an eroded astrobleme (Black, 1964b; Seeger, 1972

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01014.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—Ten type 1 and 2 carbonaceous chondrites have been analysed by stepped combustion to determine the nature of their sulphur‐bearing components. All samples show complex release patterns which, when combined with published petrographic observations, can be interpreted in terms of sulphur derived from a mixture of at least seven components (elemental sulphur, organic sulphur, sulphide, FESON or tochilinite and sulphates: gypsum, bloedite and epsomite). Estimated concentrations of individual components show that most of the sulphur in the meteorites exists in oxidised form. The concentrations of reduced and oxidised components can be used to derive approximate oxidised/reduced sulphur ratios for the meteorites. These ratios are indicators of the extent of aqueous alteration and increase in the order CM

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01015.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Abstract—The Raghunathpura iron meteorite fell on 20 November 1986 around 8 p.m.; it weighed 10.2 kg. It is a group IIA hexahedrite based on metallographic, x‐ray diffraction and chemical studies. Eighteen major, minor and trace elements were determined using classical, atomic‐absorption spectrometry and semiquantitative spectrographic proce

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01016.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

ISSN:0026-1114    

DOI:10.1111/j.1945-5100.1991.tb01017.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY