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961.
Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution 40 Ar/39 Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2 , S3 , and S4 ) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40 Ar/39 Ar geochronology. Mean 40 Ar/39 Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4 . Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4 , these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region. 相似文献
962.
Arsenic concentrations exceeding the U.S. EPA's 10 μg/L standard are common in glacial aquifers in the midwestern United States. Previous studies have indicated that arsenic occurs naturally in these aquifers in association with metal-(hydr)oxides and is released to groundwater under reducing conditions generated by microbial oxidation of organic matter. Despite this delineation of the arsenic source and mechanism of arsenic mobilization, identification of arsenic-impacted aquifers is hindered by the heterogeneous and discontinuous nature of glacial sediments. In much of the Midwest, the hydrostratigraphy of glacial deposits is not sufficiently characterized to predict where elevated arsenic concentrations are likely to occur. This case study from southeast Wisconsin presents a detailed characterization of local stratigraphy, hydrostratigraphy, and geochemistry of the Pleistocene glacial deposits and underlying Silurian dolomite. Analyses of a single core, water chemistry data, and well construction reports enabled identification of two aquifers separated by an organic-rich aquitard. The upper, unconfined aquifer provides potable water, whereas arsenic generally exceeds 10 μg/L in the deeper aquifer. Although coring and detailed hydrostratigraphic characterization are often considered impractical, our results demonstrate that a single core improved interpretation of the complex lithology and hydrostratigraphy. This detailed characterization of hydrostratigraphy facilitated development of well construction guidelines and lays the ground work for further studies of the complex interactions among aquifer sediments, hydrogeology, water chemistry, and microbiology that lead to elevated arsenic in groundwater. 相似文献
963.
Ghylaine Quitt Eric Robin Sylvain Levasseur Franoise Capmas Robert Rocchia Jean‐Louis Birck Claude Jean AllGre 《Meteoritics & planetary science》2007,42(9):1567-1580
Abstract— It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re‐Os, Hf‐W, and Mn‐Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous‐Tertiary (K‐T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni‐rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite. 相似文献
964.
Paolo Saracco Sandro D'Odorico Alan Moorwood Jean G. Cuby 《Astrophysics and Space Science》2001,276(2-4):991-998
Deep J- and K
s-band images covering a 5 × 5 arcmin area centred on the NTT Deep Field have been obtained during the Science Verification
of SOFI at the NTT. These images were made available via the Web in early June, 1998. The preliminary results we have obtained
by the analysis of these data are the following: (i) the counts continue to rise with no evidence of a turnover or of a flattening
down to the limits of the survey (K
s = 22.5 and J = 24 mag); (ii) we find a slope d log(N)/dm≈ 0.37, in agreement with most of the faintest surveys but much steeper than the Hawaii survey; (iii) fainter than K
s ≈ 19and J ≈ 20 mag, the median J-K colour of galaxies shows a break in its reddening trend turning toward bluer colours; (iv) faint bluer galaxies also display
a larger compactness index, and a smaller apparent size.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
965.
966.
Jean‐Alix BARRAT 《Meteoritics & planetary science》2004,39(11):1767-1779
Abstract— An evaluation of trapped melts effects during crystallization and subsolidus equilibration of cumulates is necessary to constrain the composition of their parental magmas. In this paper, a simple mass balance approach is described. It allows estimation of trace element abundances in these parental melts from phase compositions. It is used to discuss the genesis of cumulate eucrites and diogenites. The REE behavior is in full agreement with the view that cumulate eucrites formed from melts similar to noncumulate eucrites. Trapped melt fractions ranging from <10 wt% for Moama to ?30 wt% for Moore County were involved. The origin of diogenites is more complex. The assumption that eucrites and diogenites shared the same parental melts cannot satisfactorily explain the diversity of incompatible trace element ratios (e.g., Dy/Yb) observed in diogenitic orthopyroxenes, even if interstitial melt effects are taken into account. Moreover, some diogenites unambiguously crystallized from magmas displaying significant HREE (heavy rare earth elements) enrichments. More likely, diogenites formed from distinct batches of parental magmas, as previously proposed by Mittlefehldt (1994), Fowler et al. (1995), and Shearer (1997). 相似文献
967.
Dr. Jean Renner 《Pure and Applied Geophysics》1948,13(1-2):6-10
Sans résumé 相似文献
968.
Jean H. Prevost 《国际地质力学数值与分析法杂志》1982,6(3):323-338
It is the objective of this note critically to assess the merits and shortcomings of two-surface and multi-surface plasticity theories. 相似文献
969.
970.
Peter von Ballmoos Hubert Halloin Jean Evrard Gerry Skinner Nikolai Abrosimov Jose Alvarez Pierre Bastie Bernard Hamelin Margarida Hernanz Pierre Jean Jürgen Knödlseder Bob Smither 《Experimental Astronomy》2005,20(1-3):253-267
The objective of the R&D project CLAIRE was to prove the principle of a gamma-ray lens for nuclear astrophysics. CLAIRE's Laue diffraction lens has a diameter of 45 cm and a focal length of 277 cm; 556 germanium-silicon crystals are tuned to focus 170 keV photons onto a 1.5 cm diameter focal spot. Laboratory measurements of the individual crystals and the entire lens have been used to validate a numerical model that we use to estimate the lens performance for a source at infinity. During a stratospheric balloon flight on 2001 June 14, CLAIRE was directed at the Crab nebula by a pointing system able to stabilize the lens to within a few arcseconds of the target. In 72 min of valid pointing time, 33 photons from the Crab were detected in the 3 keV bandpass of the lens: CLAIRE's first light! The performance of CLAIRE's gamma-ray lens, namely the peak reflectivity for a polychromatic source (9±1%), has been confirmed by ground data obtained on a 205 meter long test range. CLAIRE's measured performance validates the principle of a Laue lens for nuclear astrophysics, opening the way for a space-borne gamma-ray lens telescope that will achieve one to two orders of magnitude improvement in sensitivity over present technologies. 相似文献