排序方式： 共有114条查询结果，搜索用时 19 毫秒
71.
Fred Jourdan Sebastien Nomade Michael T. D. Wingate Ela Eroglu Al Deino 《Meteoritics & planetary science》2019,54(10):2573-2591
The Australasian tektites are quench melt glass ejecta particles distributed over the Asian, Australian, and Antarctic regions, the source crater of which is currently elusive. New 40Ar/39Ar age data from four tektites: one each from Thailand, China, Vietnam, and Australia measured using three different instruments from two different laboratories and combined with published 40Ar/39Ar data yield a weighted mean age of 788.1 ± 2.8 ka (±3.0 ka, including all sources of uncertainties) (P = 0.54). This age is five times more precise compared to previous results thanks, in part, to the multicollection capabilities of the ARGUS VI noble gas mass spectrometer, which allows an improvement of almost fourfold on a single plateau age measurement. Diffusion experiments on tektites combined with synthetic age spectra and Monte Carlo diffusion models suggest that the minimum temperature of formation of the Thai tektite is between 2350 °C and 3950 °C, hence a strict minimum value of 2350 °C. 相似文献
72.
Potential-based molecular dynamics simulations of aqueous uranyl carbonate species (MxUO2(CO3)y2+2x−2y with M = Mg, Ca, or Sr) were carried out to gain molecular-level insight into the hydration properties of these species. The simulation results were used to estimate the self-diffusion coefficients of these uranyl carbonate species, which often dominate uranyl speciation in groundwater systems. The diffusion coefficients obtained for the monoatomic alkaline-earth cations and polyatomic ions (uranyl, carbonate, and uranyl tri-carbonate) were compared with those calculated from the Stokes-Einstein (SE) equation and its variant formulation by Impey et al. (1983). Our results show that the equation of Impey et al. (1983), originally formulated for monovalent monoatomic ions, can be extended to divalent monoatomic ions, with some success in reproducing the absolute values and the overall trend determined from the molecular dynamics simulations, but not to polyatomic ions, for which the hydration shell is not spherically symmetrical. Despite the quantitative failure of both SE formulations, a plot of the diffusion coefficients of the uranyl carbonate complexes as a function of the inverse of the equivalent spherical radius showed that a general linear dependence is observed for these complexes as expected from the SE equation. The nature of the alkaline-earth cation in the uranyl carbonate complexes was not found to have a significant effect on the ion’s diffusion coefficient, which suggests that the use of a single diffusion coefficient for different alkaline-earth uranyl carbonate complexes in microscopic diffusion models is appropriate.The potential model reproduced well published quantum mechanical and experimental data of and of the individual constituent ions, and therefore is expected to offer reliable predictions of the structure of magnesium and strontium uranyl carbonate aqueous species, for which there is no structural data available to date. In addition, the interatomic distances reported for could help with the refinement of the interpretation of EXAFS data of these species, which is made difficult by the similar uranium-distant carbonate oxygen and uranium-calcium distances.An analysis of the dynamics of water exchange around the alkaline-earth cations revealed that the presence of the uranyl tri-carbonate molecule has a strong influence on the geometry of the cation’s first hydration shell, which, in turn, can considerably affect the water exchange kinetics depending on whether the imposed geometry matches that around the isolated alkaline-earth cation. This result shows that the alkaline-earth uranyl carbonate complexes have distinct water exchange dynamics, which may lead to different reactivities. Finally, significant changes in water residence time were also predicted when replacing carbonate for water ligands in the uranyl coordination shell. 相似文献
73.
Late Holocene high precipitation events recorded in lake sediments and catchment geomorphology,Lake Vuoksjávrátje,NW Sweden 
下载免费PDF全文
下载免费PDF全文 Annika Berntsson Krister N. Jansson Malin E. Kylander Francois De Vleeschouwer Sebastien Bertrand 《Boreas: An International Journal of Quaternary Research》2015,44(4):676-692
In this paper, we show the potential of combining multi‐proxy analysis of lake sediments with catchment geomorphology to better understand palaeoenvironmental changes. Previous studies have suggested that alpine lakes in N Sweden may be highly sensitive to variations in catchment erosion and precipitation, and that this sensitivity may influence ecologically based reconstructions of past temperature changes. We analysed lake sediments covering the last 5100 years from the alpine Lake Vuoksjávrátje in NW Sweden in order to identify different erosional regimes in the lake catchment, sediment sources and lake sedimentary processes, which ultimately affect the palaeoecological record. The measured proxies include elemental geochemistry from XRF core scanning, grain size, sediment accumulation rates, fraction of terrestrial organic carbon and geomorphological mapping, supported by previously published chironomid and total organic carbon data. From the integrated results we identified time intervals when increased intensity of precipitation altered sedimentation and lake catchment erosional processes. The most prominent event occurred c. 2900 cal. a BP and is interpreted to be the result of excessive precipitation, possibly related to the climatic shift towards cooler and wetter conditions referred to as the 2.8 ka event. 相似文献
74.
Sebastien Owona Bernhard Schulz Lothar Ratschbacher Joseph Mvondo Ondoa Georges E. Ekodeck Félix M. Tchoua Pascal Affaton 《Journal of African Earth Sciences》2011,59(1):125-139
Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D1–D2, D3 and D4. The garnet-bearing assemblages crystallized in course of the deformation stage D1–D2 which led to the formation of the regional main foliation S2. In XCa–XMg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing XMg and variably decreasing XCa signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P–T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P–T trends. The P–T evolution started at ～450 °C/7 kbar, passed high-pressure conditions at 11–12 kbar at variable temperatures (600–700 °C) and involved a marked decompression toward 6–7 kbar at high temperatures (700–750 °C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613 ± 33 Ma and 586 ± 15 Ma. The EMP-monazite age populations between 613 ± 33 Ma enclosed in garnet and 605 ± 12 Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P–T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P–T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes. 相似文献
75.
Kaori Nagashima Björn Löptien Laurent Gizon Aaron C. Birch Robert Cameron Sebastien Couvidat Sanja Danilovic Bernhard Fleck Robert Stein 《Solar physics》2014,289(9):3457-3481
The Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) filtergrams, taken at six wavelengths around the Fe i 6173.3 Å line, contain information about the line-of-sight velocity over a range of heights in the solar atmosphere. Multi-height velocity inferences from these observations can be exploited to study wave motions and energy transport in the atmosphere. Using realistic convection-simulation datasets provided by the STAGGER and MURaM codes, we generate synthetic filtergrams and explore several methods for estimating Dopplergrams. We investigate at which height each synthetic Dopplergram correlates most strongly with the vertical velocity in the model atmospheres. On the basis of the investigation, we propose two Dopplergrams other than the standard HMI-algorithm Dopplergram produced from HMI filtergrams: a line-center Dopplergram and an average-wing Dopplergram. These two Dopplergrams correlate most strongly with vertical velocities at the heights of 30?–?40 km above (line center) and 30?–?40 km below (average wing) the effective height of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity information from two layers separated by about a half of a scale height in the atmosphere, at best. The phase shifts between these multi-height Dopplergrams from observational data as well as those from the simulated data are also consistent with the height-difference estimates in the frequency range above the photospheric acoustic-cutoff frequency. 相似文献
76.
Ching‐Hua LO Kieren T. HOWARD Sun‐Lin CHUNG Sebastien Meffre 《Meteoritics & planetary science》2002,37(11):1555-1562
Abstract— Three samples of Darwin Glass, an impact glass found in Tasmania, Australia at the edge of the Australasian tektite strewn field were dated using the 40Ar/39Ar single‐grain laser fusion technique, yielding isochron ages of 796–815 ka with an overall weighted mean of 816 ± 7 ka. These data are statistically indistinguishable from those recently reported for the Australasian tektites from Southeast Asia and Australia (761–816 ka; with a mean weighted age of 803 ± 3 ka). However, considering the compositional and textural differences and the disparity from the presumed impact crater area for Australasian tektites, Darwin Glass is more likely to have resulted from a distinct impact during the same period of time. 相似文献
77.
78.
安徽西冲钼矿床细粒花岗岩的岩石定年、岩石化学及与成矿的关系研究 总被引：1，自引：0，他引：1
西冲钼矿是大别成矿带东段近年新发现的斑岩型钼-多金属矿床,其矿床学和矿床地球化学研究少见报导。该研究成果丰富了我国斑岩-矽卡岩型钼钨-多金属矿床研究成果,同时对指导区域找矿具有一定意义。本文在详细的野外地质调研基础上,对大别成矿带西冲钼矿区出露的主要岩石单元、矿化和蚀变特征、细粒花岗岩的岩石学、岩石化学和成岩年代学进行了系统的研究。结果表明,矿区出露的主要侵入岩单元由老至新依次为:石英二长岩、细粒花岗岩(130.8±1.1Ma)及双峰式脉岩组合(131.9±1.4Ma和131.3±1.6Ma),其均为燕山期岩浆活动产物,产于华北与扬子陆块拼合后的陆内环境。矿化类型以脉状、网脉状和浸染状为主,为一斑岩型钼-多金属矿床。矿区蚀变发育,包括黑云母-磁铁矿化、钾长石化、硅化、石英-绢云母化、石英-绿帘石化、绿泥石-碳酸盐化和泥化,且空间上具有一定的分带规律。矿区已知矿化、蚀变及Cu-Mo化探异常的空间展布与细粒花岗岩关系密切,且细粒花岗岩中具有高的Mo含量,表明该矿床的形成主要与细粒花岗岩有关。岩石化学结果表明,细粒花岗岩属过铝质-准铝质、高钾钙碱性-钾玄质过渡岩石,锆石U-Pb年龄与双峰式脉岩的在误差范围内一致,表明其均形成于大别造山带大规模伸展背景。 相似文献
79.
Jason W. Barnes Jason M. Soderblom Laurence A. Soderblom Ralf Jaumann Sebastien Rodriguez Bonnie J. Buratti Roger N. Clark 《Icarus》2011,211(1):722-731
Stephan et al. (Stephan, K. et al. [2010]. Geophys. Res. Lett. 37, 7104-+.) first saw the glint of sunlight specularly reflected off of Titan’s lakes. We develop a quantitative model for analyzing the photometric lightcurve generated during a flyby in which the specularly reflected light flux depends on the fraction of the solar specular footprint that is covered by liquid. We allow for surface waves that spread out the geographic specular intensity distribution. Applying the model to the VIMS T58 observations shows that the waves on Jingpo Lacus must have slopes of no greater than 0.15°, two orders of magnitude flatter than waves on Earth’s oceans. Combining the model with theoretical estimates of the intensity of the specular reflection allows a tighter constraint on the waves: ?0.05°. Residual specular signal while the specular point lies on land implies that either the land is wetted, the wave slope distribution is non-Gaussian, or that 5% of the land off the southwest edge of Jingpo Lacus is covered in puddles. Another specular sequence off of Kraken Mare acquired during Cassini’s T59 flyby shows rapid flux changes that the static model cannot reproduce. Points just 1 min apart vary in flux by more than a factor of two. The present dataset does not uniquely determine the mechanism causing these rapid changes. We suggest that changing wind conditions, kilometer-wavelength waves, or moving clouds could account for the variability. Future specular observations should be designed with a fast cadence, at least 6 points per minute, in order to differentiate between these hypotheses. Such new data will further constrain the nature of Titan’s lakes and their interactions with Titan’s atmosphere. 相似文献
80.
A cluster analysis methodology is developed to recover facies realizations from observed reservoir attributes. A maximum likelihood
estimator allows us for identifying the most probable underlying facies using a spatial clustering algorithm. In seismic characterization,
this algorithm can yield relevant geological models for subsequent history-matching studies. In history-matching procedures,
it provides informative facies maps as well as starting points for further studies. 相似文献