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891.
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Timothy J. FAGAN Kevin D. McKEEGAN Alexander N. KROT Klaus KEIl 《Meteoritics & planetary science》2001,36(2):223-230
Abstract— In situ io n microprobe analyses of spinel in refractory calcium‐aluminium‐rich inclusions (CAIs) from type 3 EH chondrites yield 16O‐rich compositions (δ 18O and δ 17O about‐40‰). Spinel and feldspar in a CAI from an EL3 chondrite have significantly heavier isotopic compositions (δ 18O and δ 17O about ?5‰). A regression through the data results in a line with slope 1.0 on a three‐isotope plot, similar to isotopic results from unaltered minerals in CAIs from carbonaceous chondrites. The existence of CAIs with 16O‐rich and 16O‐poor compositions in carbonaceous as well as enstatite chondrites indicates that CAIs formed in at least two temporally or spatially distinct oxygen reservoirs. General similarities in oxygen isotopic compositions of CAIs from enstatite, carbonaceous, and ordinary chondrites indicate a common nebular mechanism or locale for the production of most CAIs. 相似文献
894.
Periodic Doppler width fluctuations have been observed in Fe xiv spectra above an active region. The oscillations have a period of 6.1 ± 0.6 min and a peak-to-peak amplitude of 0.07 ± 0.006 Å. The amplitude of the oscillation increases with height above the limb, and is enhanced at specific heights where we marginally detect line center intensity oscillations. The intensity fluctuations have a period of 6.1 min, an amplitude of 2.0 ± 1.4%, and are 180 ° out of phase with the width oscillations. A comparison region in the quiet corona showed no evidence of oscillatory phenomena.Visiting Student, Sacramento Peak Observatory.NAS/NRC Resident Research Associate.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation. 相似文献
895.
Barbara Anne COHEN Timothy D. SWINDLE David A. KRING 《Meteoritics & planetary science》2005,40(5):755-777
Abstract— We studied 42 impact‐melt clasts from lunar feldspathic regolith breccias MacAlpine Hills (MAC) 88105, Queen Alexandra Range (QUE) 93069, Dar al Gani (DaG) 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to the impactmelt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from them, having lower Fe, Ti, K, and P, thus representing previously unsampled impacts. The 40Ar‐39Ar ages on 31 of the impact melts, the first ages on impact‐melt samples from outside the region of the Apollo and Luna sampling sites, range from ~4 to ~2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact‐melt rock age cluster at 3.9 Ga, but the meteorite impact‐melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact‐melt clast ages is a lunar‐wide phenomenon. No meteorite impact melts have ages more than 1s? older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm. 相似文献
896.
897.
E. H. Timothy Whitten 《Mathematical Geology》1970,2(2):141-152
The advantages of using orthogonal rather than nonorthogonal polynomials for trend-surface analysis are discussed briefly.
A method for calculating orthogonal polynomial trend surfaces of any degree on the basis of irregularly spaced data is described.
The method is illustrated with subsurface data for the elevation on top of the Devonian Dundee Limestone, Michigan. 相似文献
898.
899.
Timothy W. Collins Sara E. Grineski María de Lourdes Romo Aguilar 《Applied geography (Sevenoaks, England)》2009,29(3):448-461
The purpose of this paper is to present a geographic information system (GIS)-based method for mapping risk to environmental hazards. Framed by the hazards literature, the method has been developed to specifically overcome issues of data compatibility associated with transnational contexts. The approach is elaborated in reference to a project in which risk was spatially characterized, using a suite of biophysical and social indicators, for the Ciudad Juárez (Mexico)–El Paso (USA) metropolis. Results reveal clear spatial disparities in hazard vulnerability, both within and between the two cities, based on the differential allocation of selected risk factors. The case indicates that future international analyses will be advanced by the clear definition of concepts, the systematic mining of compatible variables, and the selection of valid risk indicators based on criteria that balance the need to incorporate contextual specificity with general comparability. 相似文献
900.
The reliability of single-grain optically stimulated luminescence (OSL) dating of young (<500 years old) samples is assessed by examining the results of 166 single-grain OSL analyses of Australian alluvial, lacustrine, estuarine, coastal and marine sediment samples. All samples have been analysed using the modified SAR protocol of Olley et al. [2004b. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology 60, 337–358], with burial doses (Db) calculated by fitting a single Gaussian curve to the peak of a multi-Gaussian summed probability distribution (PDFGaussian). The near-equivalence of this approach to currently accepted methods of palaeodose estimation – namely the minimum age and central age models of Galbraith et al. [1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: part I, experimental design and statistical models. Archaeometry 41, 339–364] – is demonstrated. Considered as a whole, OSL ages calculated using the PDFGaussian Db determination method are consistent with other chronometric, geomorphological, botanical and historical indicators of depositional age, while four alternative (previously published) methods are generally less reliable when applied to the samples analysed here. The single-grain data from 12 samples of known age (reported by Olley et al. [2004b. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology 60, 337–358]) are re-analysed using the PDFGaussian approach, with all but one yielding a Db consistent with the independently known age. Examples of luminescence data (dose recovery tests, preheat plateaux, continuous-wave and linear-modulated OSL, growth curves, single-grain dose distributions) are provided that together indicate the suitability of Australian quartz for dating young sediments. 相似文献