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91.
《Applied Geochemistry》2006,21(3):419-436
Geochemical maps expressing areal distributions of chemical elements in the earth’s land surface have been published in several countries in relation to various global environment issues. The authors have applied a radiogenic isotope ratio, 87Sr/86Sr, to geochemical mapping in order to understand the geological origin, transportation and dispersion system of chemical elements in the earth’s land surface. The Sr isotope ratio is a useful tracer for distinguishing the geological origin of surficial deposits, especially in areas where surface exposure of bedrocks is low, because it is not significantly altered by the processes of weathering and transportation. Most bedrocks in the Japanese islands are covered by plants, soils and urban areas. In this study, 142 of 1219 stream sediments (<180 μm) collected from the northeastern part of Aichi Prefecture, in the central part of Japan (75 km × 30 km), were analyzed. Their Sr isotope ratios range from 0.7086 to 0.7315 with an average of 0.7129, except for one sample. This average is higher than the mean of the upper crust of the Japan Arc (the Japanese Island Crustal Composite, JICC), 0.7077. This difference can be attributed to the below-average presence of young volcanic rocks, generally having lower 87Sr/86Sr values, and the above-average presence of granitic rocks, in the study area compared with the surface exposure of the Japan Arc. The first factor controlling the distribution of Sr isotope ratios is the bedrock distributed around the sampling points. Regional variation in the 87Sr/86Sr value shows that it is higher in the western and southeastern parts, where sedimentary rocks and metamorphic rocks are distributed, and that it is lower mainly in the central part, where granitic rocks are distributed. The 87Rb/86Sr–87Sr/86Sr plot for stream sediments more clearly reveals the differences and similarities of bedrocks. In some locations, the distribution of Sr isotope ratios does not correspond to that of bedrocks on the geological map. One reason is the existence of unmapped bedrock, for example, small intrusive masses of granite. The other is fluvial transportation and dispersion. The distribution of the isotope ratios suggests that some stream sediments include surficial deposits from a few km upstream. Application of the Sr isotope ratio to geochemical mapping is useful for revealing both the distribution of unexposed bedrocks and the transportation of surficial deposits. Information on unexposed bedrocks will be expected to contribute to the improvement of geological mapping. 相似文献
92.
Four southeastern Wisconsin mammoth localities located within a glacial landscape had well-preserved remains found in inter-morainal depressions filled with lacustrine clays covered with peat. Numerous radiocarbon ages dated the mammoths to the late Pleistocene. The taphonomic analysis focused on determining the agency or agencies involved in site formation and the agency or agencies involved in modification to the bones. Statistical approaches to bone orientation data underscored that water transport was not a factor in the formation of the bone beds nor was water movement a disturbance factor. The bone modification profile of the overall assemblage was dominated by chemical weathering, followed by other chemical and microbiological processes. Root etching was the most common biological modification, and the most frequent modifications are involved with the microenvironments of the surface to burial substrate. Potentially cultural modification accounted for only a small portion of the cortical damage to the bones. This general profile indicated an assemblage far more influenced by the immediate environment than by passing animals or people. 相似文献
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北京市深基坑工程中土钉墙支护技术应用比较广泛,对于边坡变形要求严格的基坑,采用土钉墙与预应力锚杆联合支护技术可有效控制变形,结合工程实例,介绍了该技术的设计思路及应用过程,说明了该技术的适用性,并就施工中遇到的问题进行分析,给出相应的处理措施。 相似文献
96.
Integrated, in situ textural, chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western Musgrave Block, central Australia has identified evidence for multiple events of growth and recrystallisation during poly-metamorphism in the Mesoproterozoic. Garnet + sillimanite-bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 Ma, with monazite grains in leucosome recording crystallisation at 1300 Ma. Monazite breakdown during melting is inferred to have occurred in the palaeosome. During a subsequent granulite facies event at 1200 Ma, deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 Ma monazite in leucosome. Growth of new, high-Y (+HREE) monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel, as a result of post-peak isothermal decompression. Diffusive enrichment of resorbed garnet rims in Y + HREE suggests garnet breakdown occurred slower than volume diffusion of REE. Monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150–1130 Ma. The intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or ‘fertile’ local environments provided by the palaeosome/mesosome volumes, which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite. Like reaction textures, it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite REE and U–Th–Pb chemistry and hence ages. 相似文献
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Two Early Cenozoic rifts in Southeast Asia (beneath the Pattani and Malay basins) experienced only limited upper-crustal extension (β≤1.5); yet very thick post-rift sequences are present, with 6–12 km of Late Cenozoic terrestrial and shallow-marine sediment derived from adjacent sources. Conventional post-rift backstripping requires depth-dependent lithospheric thinning by β=2–4 to explain these tremendous thicknesses. We assess an alternative explanation for this post-rift subsidence, involving lower-crustal flow from beneath these basins in response to lateral pressure-gradients induced by the sediment loads and the negative loads arising from the erosion of their sediment sources. We calculate that increased rates of erosion in western Thailand in the Early Miocene placed the crust in a non-steady thermal state, such that the depth (and thus, the pressure) at the base of the brittle upper crust subsequently varied over time. Following such a perturbation, thermal and mass-flux steady-state conditions took millions of years to re-establish. In the meantime, the lateral pressure-gradient caused net outflow of lower crust, thinning the crust beneath the depocentre by several kilometres (mimicking the isostatic effect of greater crustal extension having occurred beforehand) and thickening it beneath the sediment source region. The local combination of hot crust and high rates of surface processes, causing lower-crustal flow to be particularly vigorous and thus making its effects more readily identifiable, means that the Pattani and Malay basins represent a set of conditions different from basins in many other regions. However, lower-crustal flow induced by surface processes will also occur to some extent, but less recognisably, in many other continental crustal provinces, but its effects may be mistaken for those of other processes, such as larger-magnitude stretching and/or depth-dependent stretching. 相似文献
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