The isotopic composition and concentration of Ag in iron meteorites and the origin of exotic silver     

T Kaiser  G.J Wasserburg 《Geochimica et cosmochimica acta》1983,47(1):43-58

The isotopic composition of Ag and the concentration of Ag and Pd have been determined in Canyon Diablo (IA), Grant (IIIB), Hoba, Santa Clara, Tlacotepec and Warburton Range (IVB), Piñon and Deep Springs (anom.). Troilite from Grant and Santa Clara have also been analyzed. All of these meteorites, with the exception of Canyon Diablo, give ${}^{107}\text{Ag}{}^{109}\text{Ag}$ in the metal phase that is greater than the terrestrial value with the enrichments of ¹⁰⁷Ag ranging from ~2% to 212%. These data show that Ag of anomalous isotopic composition is common to all IVB and anomalous meteorites. The results on Grant suggest that the anomalies may be widespread including more common meteorite groups. There is a general correlation of ${}^{107}\text{Ag}{}^{109}\text{Ag}$ with $\text{Pd}\text{Ag}$ except for the data from FeS of Santa Clara. It is concluded that the excess ¹⁰⁷Ag is the result of decay of ¹⁰⁷Pd, a nuclide that is extinct at present with an abundance of ${}^{107}\text{Pd}{}^{108}\text{Pd}$ of about 3 × 10^?5. The troilite in Grant exhibits normal ${}^{107}\text{Ag}{}^{109}\text{Ag}$ to within errors, a high Ag concentration and a low ratio of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~0.17. Grant metal has ${}^{107}\text{Ag}{}^{109}\text{Ag}$ that is ~2% greater than normal and a high ratio of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~ 10³. The data from Grant appear to represent a ¹⁰⁷Pd-¹⁰⁷Ag isochron and indicate that the cooling rate at elevated temperatures was sufficiently rapid to preserve substantial isotopic differences between metal and troilite. Troilite in Santa Clara was found to contain Ag with a very high ${}^{107}\text{Ag}{}^{109}\text{Ag}$ ratio (108% above normal), an Ag concentration only a factor of three above the metal and a high value of ${}^{108}\text{Pd}{}^{109}\text{Ag}$ ~1.3 × 10⁴. The troilite has a higher ${}^{107}\text{Ag}{}^{109}\text{Ag}$ than the metal. These data are not compatible with a simple model of in situ decay and subsequent local Ag redistribution between metal and troilite during cooling. These data suggest that Ag in Santa Clara and possibly other IVB meteorites is made up of almost pure ¹⁰⁷Ag produced from ¹⁰⁷Pd decay and ¹⁰⁹Ag produced by nuclear reactions with only a small amount of “normal” Ag. This indicates an intense energetic particle bombardment history in the early solar system (~10²⁰ p/m²) which occurred after the formation of small planetary bodies. We infer that a T-Tauri activity by the early sun contributed to some late stage “nucleosynthesis” and the heating of a dust cloud. In addition, implications on the early thermal evolution of iron meteorites are presented based on ¹⁰⁷Pd decay and models of the cooling history.  相似文献   

Effects of rock mass anisotropy and non-linearity on the near face stresses in deep tunnels     

F. Pelli  P. K. Kaiser  N. R. Morgenstern 《Rock Mechanics and Rock Engineering》1995,28(2):125-132

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Book reviews     

L. Müller  P. Egger  G. E. A. H.  P. Duffaut  W. Kaiser  A. E. Scheidegger 《Rock Mechanics and Rock Engineering》1972,4(2):130-134

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Estimation of Block Sizes for Rock Masses with Non-persistent Joints   总被引：2，自引：3，他引：2  

B. H. Kim  M. Cai  P. K. Kaiser  H. S. Yang 《Rock Mechanics and Rock Engineering》2007,40(2):169-192

Summary  Discontinuities or joints in the rock mass have various shapes and sizes. Along with the joint orientation and spacing, the joint persistence, or the relative size of the joint, is one of the most important factors in determining the block sizes of jointed rock masses. Although the importance of joint persistence on the overall rock mass strength has long been identified, the impact of persistence on rock strength is in most current rock mass classification systems underrepresented. If joints are assumed to be persistent, as is the case in most designs, the sizes of the rock blocks tend to be underestimated. This can lead to more removable blocks than actually exist in-situ. In addition, a poor understanding of the rock bridge strength may lead to lower rock mass strengths, and consequently, to excessive expenditure on rock support. In this study, we suggest and verify a method for the determination of the block sizes considering joint persistence. The idea emerges from a quantitative approach to apply the GSI system for rock mass classification, in which the accurate block size is required. There is a need to statistically analyze how the distribution of rock bridges according to the combination of joint orientation, spacing, and persistence will affect the actual size of each individual block. For this purpose, we generate various combinations of joints with different geometric conditions by the orthogonal arrays using the distinct element analysis tools of UDEC and 3DEC. Equivalent block sizes (areas in 2D and volumes in 3D) and their distributions are obtained from the numerical simulation. Correlation analysis is then performed to relate the block sizes predicted by the empirical equation to those obtained from the numerical model simulation. The results support the concept of equivalent block size proposed by Cai et al. (2004, Int. J. Rock Mech. Min. Sci., 41(1), 3–19).  相似文献   

The lower symmetric regime and its transition to waves in rotating annulus convection     

Jack A.C. Kaiser 《Dynamics of Atmospheres and Oceans》1977,1(4):323-343

Measurements of the temperature and zonal velocity fields which develop in a rotating annulus of fluid with an upper surface, differentially heated from the inner to outer cylinder, are described for the lower symmetric regime (small radial temperature differences). The temperature field is essentially conductive for moderate to large rotation rates, Ω (>1.0 sec⁻¹). The zonal velocity field is poorly approximated by the thermal wind equation.Measurements of the transition to waves from the lower symmetric regime at very large rotation rates are presented for positive and negative radial temperature differences. They suggest that the centrifugal buoyancy force and the free surface curvature may be important factors for the lower symmetric-wave transition at large Ω. By varying the stratification of the fluid over a range of 10³ independently of the radial temperature difference, Δr_wT, it is conclusively shown that several theories are correct in predicting that the lower symmetric transition is independent of the stratification at small Δr_wT > 0 for large enough Ω.  相似文献   

Charcoal and fossil wood from palaeosols,sediments and artificial structures indicating Late Holocene woodland decline in southern Tibet (China)     

Knut Kaiser  Lars Opgenoorth  Werner H. Schoch  Georg Miehe 《Quaternary Science Reviews》2009,28(15-16):1539-1554

Charcoal and fossil wood taken from palaeosols, sediments and artificial structures were analysed in order to evaluate the regional pedoanthracological potential and to obtain information on Holocene environmental changes, particularly on possible past tree occurrences in southern Tibet. This research was initiated by the question to what extent this area is influenced by past human impact. Even recent evaluations have perceived the present treeless desertic environment of southern Tibet as natural, and the previous Holocene palaeoenvironmental changes detected were predominantly interpreted to be climate-determined. The material analysed – comprising a total of 53 botanical spectra and 55 radiocarbon datings from 46 sampling sites (c. 3500–4700 m a.s.l.) – represents the largest systematically obtained data set of charcoal available from Tibet so far. 27 taxa were determined comprising trees, (dwarf-) shrubs and herbs as well as grasses. The predominant tree taxa were Juniperus, Hippophae, Salix and Betula. According to their present-day occurrence in the region, the genera Juniperus and Hippophae can be explicitly attributed to tree species. Further, less frequently detected tree taxa were Populus, Pinus, Quercus, Taxus and Pseudotsuga. Charcoal of Juniperus mainly occurred on southern exposures, whereas Betula was associated with northern exposures. In contrast, the (partly) phreatophytic taxa Hippophae and Salix showed no prevalent orientation. The distribution of radiocarbon ages on charcoal revealed a discontinuous record of burning events cumulating in the Late Holocene (c. 5700–0 cal BP). For southern Tibet, these results indicated a Late Holocene vegetation change from woodlands to the present desertic pastures. As agrarian economies in southern and south-eastern Tibet date back to c. 3700 and 5700 cal BP, respectively, and the present-day climate is suitable for tree growth up to c. 4600 m a.s.l., we concluded that the Late Holocene loss or thinning out of woodlands had been primarily caused by humans.  相似文献   

A water movement study in Lianzhou Bay,Guangxi Province,China     

SUN Ting  Andreas Macrander  and David Kaiser 《中国海洋大学学报(英文版)》2014,13(1):13-22

This study investigates the physical conditions (water depth, current speed, salinity, temperature) in Lianzhou Bay, a shallow coastal bay in southern China, during two expeditions in the dry and wet seasons of 2011. Based on these expedition data, basic hydrodynamic parameters like Brunt-Väisälä Frequency, Richardson Number, Rossby radius, and Resonance Period are calculated. The results show that Lianzhou Bay is characterized by comparatively small quantity of freshwater input and weak stratification. Strong tides, which are spatially uniform within the bay, cause turbulent mixing. Residence time of the water is shorter in winter due to a stronger coastal current in that season. Consideration of the water movement may help to reduce the harmful ecological impact of aquaculture waste water discharge.  相似文献   

DEM Simulation of Direct Shear: 1. Rupture Under Constant Normal Stress Boundary Conditions     

R. P. Bewick  P. K. Kaiser  W. F. Bawden  N. Bahrani 《Rock Mechanics and Rock Engineering》2014,47(5):1647-1671

A particle-based distinct element method and its grain-based method are used to generate and simulate a synthetic specimen calibrated to the rupture characteristics of an intact (non-jointed) low-porosity brittle rock deformed in direct shear. The simulations are compared to the laboratory-generated ruptures and used to investigate rupture at various normal stress magnitudes. The fracturing processes leading to shear rupture zone creation and the rupture mechanism are found to be normal stress dependent (progressing from tensile splitting to shear rupture) and show partial confirmation of rupture zone creation in nature and in experiments from other materials. The normal stress dependent change is found to be due to the orientation of the major principal stress and local stress concentrations internal to the synthetic specimens being deformed. The normal stress dependent rupture creation process results in a change to the rupture zone’s geometry, shear stress versus horizontal displacement response, and thus ultimate strength.  相似文献   

Crack Initiation and Crack Propagation in Heterogeneous Sulfate-Rich Clay Rocks   总被引：2，自引：1，他引：1  

Florian Amann  Ömer Ündül  Peter K. Kaiser 《Rock Mechanics and Rock Engineering》2014,47(5):1849-1865

Brittle fracture processes were hypothesized by several researches to cause a damage zone around an underground excavation in sulfate-rich clay rock when the stress exceeds the crack initiation threshold, and may promote swelling by crystal growth in newly formed fractures. In this study, laboratory experiments such as unconfined and confined compression tests with acoustic emission monitoring, and microstructural and mineralogical analyses are used to explain brittle fracture processes in sulfate-rich clay rock from the Gipskeuper formation in Switzerland. This rock type typically shows a heterogeneous rock fabric consisting of distinct clayey layers and stiff heterogeneities such as anhydrite layers, veins or nodules. The study showed that at low deviatoric stress, the failure behavior is dominated by the strength of the clayey matrix where microcracks are initiated. With increasing deviatoric stress or strain, growing microcracks eventually are arrested at anhydrite veins, and cracks develop either aligned with the interface between clayey layers and anhydrite veins, or penetrate anhydrite veins. These cracks often link micro-fractured regions in the specimen. This study also suggest that fracture localization in sulfate-rich clay rocks, which typically show a heterogeneous rock fabric, does not take place in the pre-peak range and renders unstable crack propagation less likely. Sulfate-rich clay rocks typically contain anhydrite veins at various scales. At the scale of a tunnel, anhydrite layers or veins may arrest growing fractures and prevent the disintegration of the rock mass. The rock mass may be damaged when the threshold stress for microcrack initiation is exceeded, thus promoting swelling by crystal growth in extension fractures, but the self-supporting capacity of the rock mass may be maintained rendering the possibility for rapidly propagating instability less likely.  相似文献   

The death of FR II radio sources and their connection with radio relics     

Christian R. Kaiser  Garret Cotter 《Monthly notices of the Royal Astronomical Society》2002,336(2):649-658

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