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51.
Derek W. Sears Gregory W. Kallemeyn John T. Wasson 《Earth and Planetary Science Letters》1983,62(1):180-192
The concentrations of 25 major, minor and trace elements have been determined in four clasts, a metal-rich inclusion and two dark metal-poor inclusions from the Abee enstatite chondrite. The clasts are heterogeneous, displaying 2-fold enrichments or depletions in some elements. The data suggest that there are two generations of metal, one with low, the other with high concentrations of refractory siderophiles. The other elemental patterns can be understood in terms of variations in the abundance of major minerals. We infer that Sc and Mn are located largely in the niningerite ((Fe,Mg)S), V in the troilite (FeS) and rare earth elements in the oldhamite (CaS).Heterogeneities among the clasts are probably primary, resulting from the accretion-agglomeration process, although shock processes in a regolithic setting remain a possibility provided that they were followed by a period of metamorphism sufficient to erase petrologic evidence.In the dark inclusions the concentrations of the rare earths, Eu excepted, are 4 × higher than mean EH levels; this infers enhanced amounts of CaS. The dark inclusions are low in siderophiles, Sc, Mn, K, Na and Al, implying low amounts of metal, niningerite and feldspar. The origin of the dark inclusions is unclear; they do not appear to be the result of a simple, single-stage process. 相似文献
52.
Uranium-series ages have been obtained for 87 speleothems collected from nine major cave systems in the Craven district of northern England. Large systems such as Ease Gill Caverns, the West Kingsdale caves, and Gaping Gill-Ingleborough Cave, which contain relict high-level tunnels, are found to be older than 350,000 years (the limit of the 230Th/234U dating method). There is little evidence to indicate a significant enlargement of these passages since this time. Estimates of the age of Victoria Cave from 234U/238U isotopic ratios suggest that the cave has been fully relict for more than 500,000 years. Ages of in situ speleothems immediately adjacent to local base level cave streams show that mean maximum downcutting rates in limestone channels are about 2 to 5 cm 1000 yr. These rates are significantly lower than those obtained from direct measurements on limestone bedrock in stream channels but are comparable to areal denudation rates based on solute budgets. Using the present elevation of caves with respect to adjacent valley floor levels, mean maximum valley entrenchment rates are found to range between 5 cm/ky and < 20cm/ky, which corresponds to 6 m to < 24 m of lowering per glacial/interglacial cycle. These rates suggest that upper beds of the limestone were incised to form the Yorkshire Dales between 1 and 2 million years ago. The results indicate that the erosional effects of individual glaciations are not as severe as previously proposed in the literature. 相似文献
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56.
Derek. L. Smith 《The Australian geographer》1992,23(2):149-163
The cost of overcoming distance has played and continues to play an important role in the evolution of Australia's space‐economy. This paper examines the impact of the ‘friction of distance’, as well as the quality of the resource base, on the settlement and development of Eyre Peninsula in South Australia. A series of maps portraying the spread of pastoralism and then agriculture on Eyre Peninsula reveals clearly the importance of ‘accessibility’ in influencing the pattern and timing of settlement. It was not until well into this century that all the potentially agricultural land in the interior was occupied, for this required the construction of a railway and water pipeline up the centre of the peninsula. 相似文献
57.
John A. E. Gibson Warwick F. Vincent Patrick Van Hove Claude Belzile Xioawa Wang Derek Muir 《Aquatic Geochemistry》2002,8(2):97-119
The geochemical processes occurring within meromictic Lake A (maxdepth 120 m) on northern Ellesmere Island, Canada, were investigated to determinethe history of the lake and to provide a baseline for future studies. The lake contained seawaterdiluted by freshwater input that had been mixed prior to the lake's isolation from tidalaction. Input of freshwater after isolation of the lake created vertical stratification resultingin the creation of distinct oxic, suboxic and anoxic zones. Dissolved oxygen was present to 13 m,and sulphide beneath 32 m. Manganese and iron cycling dominated the redox chemistrybetween these depths. Total manganese concentrations reached 176 M, higher thanin most other natural stratified lake or marine environments. 相似文献
58.
天山北部石炭纪埃达克岩-高镁安山岩-富Nb岛弧玄武质岩:对中亚造山带显生宙地壳增生与铜金成矿的意义 总被引:28,自引:30,他引:28
新疆天山北部地区存在有石炭纪的埃达克岩-高镁安山岩-富Nb玄武质岩组合,并且其中许多岩石与铜(金)矿床伴生(如达巴特、阿希、土屋-延东、赤湖,等等)。埃达克岩富钠、高Sr但亏损Y与Yb,无明显Eu-正Eu异常以及正Sr异常与Nb、Ti亏损。高镁安山(闪长)岩是本次研究首次报道的,这些岩石无明显Eu-正Eu异常以及Nb、Ti亏损,普遍具有高的MgO和Cr、Ni含量,其中阿希金矿区一些样品类似于日本西南新生代Setouchi弧火山岩带中的赞岐岩类。富Nb玄武质岩富钠贫钾,具有微弱负.正Ba、Nb和Ti异常以及高的Nb/La比值,不同于大多数正常岛弧玄武岩。天山北部地区石炭纪埃达克岩具有高的8Nd(t)(+3.4-+9.0)和低的(^87Sr/^86Sr)i(0.7032—0.7043)。富Nb玄武质岩具有变化的εNd(t)(+3.6-+11.6)和(^87Sr/^86Sr);(0.7007—0.7067)。我们的研究表明,天山北部地区石炭纪埃达克岩-高镁安山岩-富Nb玄武质岩组合可能是“埃达克岩交代的岛弧岩浆岩系列”。埃达克岩最有可能由石炭纪北天山洋的年轻洋壳在俯冲过程中熔融形成。另外,俯冲板片产生的熔体以及所释放的少量流体在上升过程中可能交代地幔楔橄榄岩或与其发生反应:一方面,触发地幔楔橄榄岩发生熔融形成富Nb岛弧玄武质岩;另一方面,地幔组分迅速进入到板片熔体中,导致其地幔组分增加,乃至形成高镁安山岩。因此,天山北部地区石炭纪埃达克岩-高镁安山岩-富Nb玄武质岩组合表明:(1)天山北部地区石炭纪可能为岛弧环境而非裂谷环境;(2)天山地区石炭纪的地壳生长可能以侧向增生为主;(3)除了亏损地幔之外,俯冲洋壳的熔融可能也在地壳的生长中发挥了重要的作用;(4)俯冲板片产生的埃达克质岩浆具有高的氧逸度,而其与地幔楔橄榄岩的强烈相互作用将导致地幔中的金属硫化物分解,成矿金属元素进入到岩浆中。这可能是新疆北部铜金矿化与一些埃达克岩、高镁安山(闪长)岩或富Nb岛弧玄武质岩密切共生的基本原因。 相似文献
59.
Angular resolution of terrestrial laser scanners 总被引:5,自引:0,他引:5
Knowledge of a laser scanner's spatial resolution is necessary in order to prevent aliasing and estimate the level of detail that can be resolved from a scanned point cloud. Spatial resolution can be decoupled into range and angular components. The latter is the focus of this paper and is governed primarily by sampling interval and laser beamwidth. However, emphasis is often placed on one of these—typically sampling interval—as an indicator of resolution. Since both affect the resolution of a scanned point cloud, consideration of only one factor independent of the other can lead to a misunderstanding of a system's capabilities. This will be demonstrated to be inappropriate except under very specific conditions. A new, more appropriate resolution measure for laser scanners, the effective instantaneous field of view (EIFOV), is proposed. It is derived by modelling the shift variance of the equal angular increment sampling process, laser beamwidth-induced positional uncertainty and observed angle quantisation with ensemble average modulation transfer functions (AMTFs). Several commercially available terrestrial laser scanner systems are modelled and analysed in terms of their angular resolution capabilities using the EIFOV. 相似文献
60.
Extensive computer simulations aimed at testing a hypothesis that impact craters may explain the scaling behaviour of surface
spectra are presented. The simulations show that indeed crater effects alone may explain the spectral scaling of Mars’ topography
revealed from MOLA data. The range of the scaling exponents obtained for a wide range of simulation parameters does not exceed
the limits observed for Mars. The simulations suggest that the shape of large craters is the key factor leading to the two
scaling ranges in the surface spectra. Particular values of the scaling exponents may additionally depend on the depth-diameter
relationship and the crater size distribution. 相似文献