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Fawn M. Last William M. Last Mostafa Fayek Norman M. Halden 《Journal of Paleolimnology》2013,50(4):505-517
Micron-scale pseudomorphs of calcite after ikaite were discovered in microbialites from Manito Lake, a large hypersaline lake in the Great Plains of western Canada. Although environmental conditions in the lakes of this region (seasonally cold temperatures, high salinities and elevated productivity) suggest that ikaite should be common, this is the first documentation of lacustrine ikaite in Canada and the Great Plains region of North America. The calcite pseudomorphs form a porous dendritic fabric, comprise the interiors of massive shoreline microbialite mounds and pinnacles, and are encased in centimeter-scale, laminated dolomite-aragonite rinds. Because of the limited thermodynamic stability of ikaite at or near freezing temperatures, the psuedomorphs of this mineral provide evidence of an extended cold episode during the first millennium AD. The Manito deposits also show high δ13C values, indicating elevated productivity and δ18O signatures are consistent with precipitation in a cold, somewhat fresher lake than present. 相似文献
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Photogrammetry-based method is gaining popularity in many fields. One of the main tasks of photogrammetry is to identify the homologous points in multiple images, which is commonly referred to as the corresponding problem. Coded targets are often placed on the surfaces of the targeted objects and have been widely used as a reliable method for solving the corresponding problem in photogrammetry for high-accuracy three-dimensional measurements. Automated recognition and identification of coded targets are of great importance in the coded target-based photogrammetry. However, false coded target identifications are inevitable due to large perspective distortion, unfavorable lighting conditions, and low-resolution, low-quality images, etc. As a result, manual corrections are often required, which are tedious, prone to error, and inefficient. In this paper, a faster R-CNN-based method has been proposed to recognize coded targets. Then, a table method has been developed to automatically identify and reject the falsely identified coded targets by taking advantages of the prior knowledge of the geometric arrangement of the coded targets. Based on that, missing coded targets can be recovered using either interpolation or extrapolation method. The effectiveness and accuracy of the proposed method are validated by implementing it into three-dimensional reconstruction of soil specimens during triaxial testing in geotechnical engineering. Experimental validation results indicate that the proposed method can achieve accurate and efficient coded target recognition and identification.
相似文献4.
西准白杨河铍矿床萤石及流体包裹体特征 总被引：3，自引：0，他引：3
利用激光测温学和激光拉曼光谱方法,通过镜下观察,研究了白杨河铍矿床与铍成矿作用有关的脉石矿物(萤石),将萤石分为4个期次,其中第三期萤石与铍成矿关系最密切。萤石流体包裹体具低盐(0.7%~7.0%)、低温(89.7℃~188.9℃)的特征,第三期萤石脉流体包裹体成矿温度99.4℃~120.5℃,成矿盐度3.5%~5.4%,与铍矿物属同一期,揭示出该矿床成矿流体为低温、低盐度热液。 相似文献
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Grant M. Cox Christopher J. Lewis Alan S. Collins Galen P. Halverson Fred Jourdan John Foden David Nettle Fayek Kattan 《Gondwana Research》2012,21(2-3):341-352
The Ad Dawadimi Terrane is an Ediacaran basin of the Arabian Nubian Shield (ANS), Saudi Arabia. This basin terrane is situated in the far eastern part of the ANS and represents the youngest accretion event of the exposed ANS. Therefore, the timing of events within the basin is key to understanding both the closure of the Mozambique Ocean and the amalgamation of Gondwana along the northern East African Orogen. Here we present U/Pb detrital zircon data for the Abt Formation, the principle basin sediments of the Ad Dawadimi Terrane, along with 40Ar/39Ar ages on muscovite and whole rock Sm/Nd data. These data indicate that deep-water deposition in the Abt Basin did not end until after ca. 620 Ma and that deformation and greenschist-facies metamorphism of the Abt Formation occurred at 620 ± 3 (2σ) Ma along an active margin. This is the youngest terrane amalgamation event reported so far in the Arabian–Nubian Shield, but we suggest even younger sutures lie further east beneath the Phanerozoic cover of eastern Saudi Arabia. Our results suggest that the Ediacaran basins of the eastern ANS were not part of the Huqf basin in Oman, which was instead part of a passive margin of Neoproterozoic India, separated from the active margin of Africa by the Mozambique Ocean that probably did not close until the late Ediacaran or early Cambrian. 相似文献
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Samuel Angiboust Mostafa Fayek Ian M. Power Alfredo Camacho Georges Calas Gordon Southam 《Mineralium Deposita》2012,47(8):859-874
Epithermal uranium deposits of the Sierra Pe?a Blanca are classic examples of volcanic-hosted deposits and have been used as natural analogs for radionuclide migration in volcanic settings. We present a new genetic model that incorporates both geochemical and tectonic features of these deposits, including one of the few documented cases of a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit. Four tectono-magmatic faulting events affected the volcanic pile. Uranium occurrences are associated with breccia zones at the intersection of fault systems. Periodic reactivation of these structures associated with Basin and Range and Rio Grande tectonic events resulted in the mobilization of U and other elements by meteoric fluids heated by geothermal activity. Focused along breccia zones, these fluids precipitated under reducing conditions several generations of pyrite and uraninite together with kaolinite. Oxygen isotopic data indicate a low formation temperature of uraninite, 45–55°C for the uraninite from the ore body and ～20°C for late uraninite hosted by the underlying conglomerate. There is geochemical evidence for biological activity being at the origin of these reducing conditions, as shown by low δ34S values (～?24.5‰) in pyrites and the presence of low δ13C (～?24‰) values in microbial patches intimately associated with uraninite. These data show that tectonic activity coupled with microbial activity can play a major role in the formation of epithermal uranium deposits in unusual near-surface environments. 相似文献
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Evidence of uranium biomineralization in sandstone-hosted roll-front uranium deposits, northwestern China 总被引：13，自引：0，他引：13
We show evidence that the primary uranium minerals, uraninite and coffinite, from high-grade ore samples (U3O8>0.3%) in the Wuyiyi, Wuyier, and Wuyisan sandstone-hosted roll-front uranium deposits, Xinjiang, northwestern China were biogenically precipitated and psuedomorphically replace fungi and bacteria. Uranium (VI), which was the sole electron acceptor, was likely to have been enzymically reduced. Post-mortem accumulation of uranium may have also occurred through physio-chemical interaction between uranium and negatively-charged cellular sites, and inorganic adsorption or precipitation reactions. These results suggest that microorganisms may have played a key role in formation of the sandstone- or roll-type uranium deposits, which are among the most economically significant uranium deposits in the world. 相似文献
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The Central Asian Orogenic Belt,as one of world-class gold economic belts,preserves a number of giant,large black shale-hosted gold deposits,while it is still debated for origin of sulfides and gold mainly due to lack of identification for multiple stages of sulfides.The Haoyaoerhudong gold deposit is hosted in a sequence of Mesoproterozoic carbonaceous and pyritic slate,phyllite,and schist that form a tight syncline along the north margin of the North China Craton.Detailed petrography of the host rocks and mineralization have defined five stages of pyrites.The earliest form of pyrite(Py1)occurs as fine-grained dispersed pyrite in black carbonaceous slate and medium-to coarse-grained disseminated pyrite in pyrite-rich layers,contains relative low gold and high arsenic content,indicating a syn-sedimentary or diagenetic in origin.Stage Ⅱ pyrite(Py2)occurs with garnet and quartz inclusions and Py3 occurs as pyrite veins,contains higher gold and lower As content,and are interpreted to have formed from the dissolution-reprecipitation of Py1 during the peak metamorphism or post-peak metamorphism.Stage Ⅳ pyrite(Py4)from the pyrite-quartz veins crosscut the metamorphic garnet,contains the highest gold concentrations and other trace elements,and is considered to have formed post-peak metamorphism.Abundant native gold,electrum,and maldonite occur as inclusions within Py4 and monazite and in fractures that crosscut garnet.While,Py5 with typical remobilized feature is thought to be a product of melting of former pyrites(Py1 to Py4)triggered by the large-scale Hercynian magmatism.The sedimentary/diagenetic Py1 have δ34S values that range from +12.4‰to +16.2‰.Later generations of sulfides,including Py2 to Py5,and Ccp2 to Ccp3,have δ34S values from +9.5‰to +12.7‰.Monazite with maldonite inclusions from quartz-pyrite veins yielded an intercept age of 341.3±6.6 Ma,while coarse grained monazite associated biotite along fractures in the reefs yielded an intercept age of 254.6±8.2 Ma.The paragenetic,textural,chemical,and isotopic data suggest three distinct gold producing episodes at Haoyaoerhudong gold deposit.Gold and arsenic were clearly initially concentrated in organic muds,and enriched along the structures of diagenetic arsenic-rich pyrite.Subsequently,accompanying metamorphism and deformation,gold was liberated from the dissolution of diagenetic pyrites to form the pyrite veins.Finally,accompanying transformation of pyrite into pyrrhotite,gold was released into the metamorphic fluids to become concentrated as native gold,electrum,and maldonite in pyrite-quart veins.Monazite with age of 341 Ma from quartz-pyrite veins suggests that the third major gold mineralizing event in Haoyaoerhudong occurred before the Hercynian magmatism,suggesting that the Haoyaoerhudong deposit is a typical orogenic gold deposit rather than intrusion-related deposit. 相似文献
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Mostafa Fayek Lawrence M. Anovitz Debra A. Bostick 《Geochimica et cosmochimica acta》2011,75(13):3677-3686
Diffusion coefficients for oxygen and hydrogen were determined from a series of natural uraninite-H2O experiments between 50 and 700 °C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature range of 400-600 °C, diffusion coefficients for oxygen can be represented by D = 1.90e−5 exp (−123,382 J/RT) cm2/s and for temperatures between 100 and 300 °C the diffusion coefficients can be represented by D = 1.95e−10 exp (−62484 J/RT) cm2/s, where the activation energies for uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in uraninite appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between 50 and 700 °C, diffusion coefficients for hydrogen can be represented by D = 9.28e−6 exp (−156,528 J/RT) cm2/s for temperatures between 450 and 700 °C and D = 1.39e−14 exp (−34518 J/RT) cm2/s for temperatures between 50 and 400 °C, where the activation energies for uraninite are 156.5 and 34.5 kJ/mol, respectively.Results from these new experiments have implications for isotopic exchange during natural UO2-water interactions. The exceptionally low δ18O values of natural uraninites (i.e. −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental uraninite-water and UO3-water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ18O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ18O values of ca. −18‰, despite petrographic and U-Pb isotope data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the uraninite from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals that can be used to track the route of fissile material. 相似文献