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121.
This paper reviews spectra obtained with the SWS on board of ISO of dust shells around O-rich objects. These spectra reveal the presence of many new emission features between 10 and 45 μm. These bands are generally much narrower than the well-known 10 and 20 μm silicates features. The strength of these features relative to the underlying broad continuum varies from source to source (≅ 5-50%). The 10 μm region shows evidence for the presence of Al2O3 grains. At longer wavelength, the spectra are dominated by features due to crystalline olivine and pyroxene. The exact peak position of these features shows that the emitting grains consist of the Mg-rich end-members of these minerals with an Fe-content of < 10%. The underlying continuum is attributed to amorphous silicate grains. These observations of aluminum-rich and magnesium-rich compounds compare well with the thermodynamic condensation sequence of minerals expected for O-rich outflows. The observations also imply that freeze out (ie., kinetics) of this condensation sequence at different temperatures is an important characteristic of dust formation in these objects. It is suggested that the absence of Fe-rich silicates is a natural consequence of the low temperature at which gaseous Fe reacts with Mg-rich silicates in these outflows, resulting in amorphous grains with little characterizing spectral detail. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
122.
Abstract— The Crow Creek Member is one of several marl units recognized within the Upper Cretaceous Pierre Shale Formation of eastern South Dakota and northeastern Nebraska, but it is the only unit that contains shock‐metamorphosed minerals. The shocked minerals represent impact ejecta from the 74‐Ma Manson impact structure (MIS). This study was aimed at determining the bulk chemical compositions and analysis of planar deformation features (PDFs) of shocked quartz; for the basal and marly units of the Crow Creek Member. We studied samples from the Gregory 84‐21 core, Iroquois core and Wakonda lime quarry. Contents of siderophile elements are generally high, but due to uncertainties in the determination of Ir and uncertainties in compositional sources for Cr, Co, and Ni, we could not confirm an extraterrestrial component in the Crow Creek Member. We recovered several shocked quartz grains from basal‐unit samples, mainly from the Gregory 84‐21 core, and results of PDF measurements indicate shock pressures of at least 15 GPa. All the samples are composed chiefly of SiO2 (29–58 wt%), Al2O3 (6–14 wt%), and CaO (7–30 wt%). When compared to the composition of North American Shale Composite, the samples are significantly enriched in CaO, P2O5, Mn, Sr, Y, U, Cr, and Ni. The contents of rare earth elements (REE), high field strength elements (HFSE), Cr, Co, Sc, and their ratios and chemical weathering trends, reflect both felsic and basic sources for the Crow Creek Member, an inference, which is consistent with the lithological compositions in the environs of the MIS. The high chemical indices of alteration and weathering (CIA' and CIW': 75–99), coupled with the Al2O3‐(CaO*+Na2O)‐K2O (A‐CN'‐K) ratios, indicate that the Crow Creek Member and source rocks had undergone high degrees of chemical weathering. The expected ejecta thicknesses at the sampled locations (409 to 219 km from Manson) were calculated to range from about 1.9 to 12.2 cm (for the present‐day crater radius of Manson), or 0.4 to 2.4 cm (for the estimated transient cavity radius). The trend agrees with the observed thicknesses of the basal unit of the Crow Creek Member, but the actually observed thicknesses are larger than the calculated ones, indicating that not all of the basal unit comprises impact ejecta.  相似文献   
123.
Abstract— Using an H‐plot analysis, we identify 234 currently known near‐Earth objects that are accessible for rendezvous with a “best case” delta‐V of less than 7 km/s. We provide a preliminary compositional interpretation and assessment of these potential targets by summarizing the taxonomic properties for 44 objects. Results for one‐half (22) of this sample are based on new spectroscopic measurements presented here. Our approach provides an easy‐to‐update method for giving guidelines to both observers and mission analysts for focusing on objects for which actual mission opportunities are most likely to be found. Observing prospects are presented for categorizing the taxonomic properties of the most accessible targets that are not yet measured.  相似文献   
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The Strand Fiord Formation is a volcanic unit of early Late Cretaceous age which outcrops on west-central and northwestern Axel Heiberg Island in the Canadian Arctic Archipelago. The formation is part of the thick Sverdrup Basin succession and immediately precedes the final basin foundering event. The Strand Fiord volcanics are encased in marine strata and thin southward from a maximum thickness of 789+ m on northwestern Axel Heiberg to a zero edge near the southern shore of the island. Tholeiitic icelandite flows are the main constituent of the formation with volcaniclastic conglomerates, sandstones, mudrocks and rare coal seams also being present. The lava flows range in thickness from 6 to 60 m and subaerial flows predominate. Both pahoehoe and aa lava types are common and the volcanic pile accumulated mostly by the quiet effusion of lavas. The volcaniclastic lithologies become more common near the southern and eastern edges of the formation and represent lahars and beach to shallow marine reworked deposits. The Strand Fiord volcanics are interpreted to represent the cratonward extension of the Alpha Ridge, a volcanic ridge that was active during the formation of the Amerasian Basin.  相似文献   
126.
The accuracy of the AE index as a measure of the joule heat production rate is examined for a typical substorm event on 18 March 1978 by estimating the global joule heat production rate, as a function of time, using data obtained from the IMS six meridian chains. In spite of the fact that the AE index had had an initial slow growth which was followed by a rapid growth, the joule heat production rate attained a high level during the slow growth and thus their variations were considerably different from those of the AE index. Therefore, although the AE index is statistically linearly related to the global joule heat production rate, one should be cautious in assuming that details of time variations of the AE index during individual events represent those of the joule heat production rate.  相似文献   
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Various tectonic features are present in the Meso-Cenozoic basin units of the Sabina region (Central Apennines, Italy): Mio-Pliocene northeasterly verging thrusts are followed by Plio-Pleistocene, N-S oriented right-lateral strike-slip faults. Stable isotope geochemistry and examination of meso- and microstructures show that strain conditions differed through the course of tectonic history. Carbon and oxygen isotope analyses of the calate-filled extensional fractures, the sigmoidal veins present between stylolitic cleavage surfaces, and fault plane surfaces with differing motion, demonstrate those different geneses.
The "C/'" C of the older calcite-filling fractures (present both in the thrust and the strike-slip systems) suggests a deposition from shallow, fresh water circulation. Furthermore, the calcite fill of en echelon systems, that occur in the southernmost Plio-Pleistocene units, is clearly the result of a more recent, right-lateral strike-slip movement, connected with shallow water circulation within Mesozoic limestones.
The sigmoidal vein fills are derived from solid-state pressure solution processes which were the result of strike-slip movement. The deformation pattern related to the older thrust system is similar, but less intense; this also demonstrates general recrystallization processes in a closed system.
This suggests that the total shortening of the deformed sections is lower than that obtained on the basis of solution on stylolitic planes, because a sigruficant volume of dissolved carbonates remained in the system.
Stable isotope analysis also confirms that the deformational history of strongly cleaved rocks in the Sabina region took place in two phases and that extensional fractures formed before stylolithic planes, as suggested by structural and field observations.  相似文献   
130.
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