Oxygen isotope fractionations between zoisite and water have been studied at 400–700°C, PH2O = 13.4 kbar, using the three-isotope method described by Matsuhisaet al. (1978) and Matthewset al. (1983a). The zoisite-waier exchange reaction takes place extremely slowly and consequently direct-exchange calibration of equilibrium fractionation factors was possible only at 600 and 700°C. Fractionation factors at 400–600°C were determined from samples hydrothermally crystallized from a glass of the anhydrous zoisite composition. At 600°C, both exchange procedures gave identical fractionations within experimental error. Scanning electron microscope studies showed that the zoisite-water exchange reaction occurs largely by solution-precipitation mass-transfer mechanisms. The slow kinetics of zoisite-water exchange may be typical of hydrous silicates, since additional experiments on tremolite-water and chlorite-water exchange also showed very low rates. When the zoisite-water fractionation factors determined in this study are combined with the quartz and albite-water data of Matsuhisaet al. (1979) and the calcite-water data of O'Nellet al. (1969), mineral-pair fractionations are obtained for which the coefficients “A” in the equation 1000 In α = A × 106T?2 are:
Many geophysical problems involve solving for the time derivative of the Laplacian of a streamfunction, particularly three-dimensional ocean circulation studies. This paper demonstrates the existence of two instabilities when relaxation methods are used to solve the resulting Poisson equation. One occurs as a result of over-relaxation. The other occurs even if under-relaxation is used, provided that there are variations in Coriolis parameter and that extrapolation provides an initial guess for the relaxation process. Both instabilities equilibrate with (erroneous) amplitudes determined by the degree of dissipation present. In the case of the latter instability, the amplitude can be six orders of magnitude larger than the error criterion for the relaxation; its structure resembles basic modes, which persist indefinitely. Some ways of circumventing the problem are discussed. 相似文献
Sphalerite is a common sulphide and is the dominant ore mineral in Zn-Pb sulphide deposits. Precise determination of minor and trace element concentrations in sulphides, including sphalerite, by Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS) is a potentially valuable petrogenetic tool. In this study, LA-ICP-MS is used to analyse 19 sphalerite samples from metamorphosed, sphalerite-bearing volcanic-associated and sedimentary exhalative massive sulphide deposits in Norway and Australia. The distributions of Mn, Fe, Co, Cu, Ga, Se, Ag, Cd, In, Sn, Sb, Hg, Tl, Pb and Bi are addressed with emphasis on how concentrations of these elements vary with metamorphic grade of the deposit and the extent of sulphide recrystallization. Results show that the concentrations of a group of trace elements which are believed to be present in sphalerite as micro- to nano-scale inclusions (Pb, Bi, and to some degree Cu and Ag) diminish with increasing metamorphic grade. This is interpreted as due to release of these elements during sphalerite recrystallization and subsequent remobilization to form discrete minerals elsewhere. The concentrations of lattice-bound elements (Mn, Fe, Cd, In and Hg) show no correlation with metamorphic grade. Primary metal sources, physico-chemical conditions during initial deposition, and element partitioning between sphalerite and co-existing sulphides are dominant in defining the concentrations of these elements and they appear to be readily re-incorporated into recrystallized sphalerite, offering potential insights into ore genesis. Given that sphalerite accommodates a variety of trace elements that can be precisely determined by contemporary microanalytical techniques, the mineral has considerable potential as a geothermometer, providing that element partitioning between sphalerite and coexisting minerals (galena, chalcopyrite etc.) can be quantified in samples for which the crystallization temperature can be independently constrained. 相似文献
From 2009 to 2011, the Canadian Prairies were subjected to exceptionally variable precipitation regimes, ranging between record drought and unprecedented flooding. Adjacent regions concurrently experienced droughts and floods, and individual areas transitioned rapidly from pluvial to drought conditions and vice versa. Such events had major impacts; for example, damages from floods in the Assiniboine River Basin (ARB) have exceeded $1 billion, and forest fires ravaged the town of Slave Lake, Alberta. This study first characterizes, and then assesses, these devastating natural hazards in terms of their physical processes (across multiple spatial and temporal scales) related to both the spatially contrasting precipitation states and rapid temporal transitions between these states. Subtle differences in large-scale atmospheric flow had marked impacts on precipitation. Primary factors controlling the distribution and amount of precipitation included the location and persistence of key surface and upper-air features, as well as their interaction. Additionally, multiple events—rather than individual extremes—were responsible for the flooding over the Saskatchewan River Basin and the ARB. Very heavy rainfall events (≥25 mm d?1) accounted for up to 55 % of warm season rain at some locations, and the frequency of heavy rainfall events was critical for determining whether a region experienced drought or pluvial conditions. This study has increased our knowledge of the characteristics, impacts and mechanisms of rapidly transitioning disparate precipitation states on the Canadian Prairies and will aid in better understanding both past and projected future hydro-climatic extremes in the region. 相似文献
Flexible steel barriers are commonly constructed on steep hillsides to mitigate rockfall. The evaluation of the dynamic response of proprietary flexible barriers is conventionally performed using full-scale field tests by dropping a weight onto the barriers in accordance with the European test standard ETAG 27. The weight typically has a spherical or polyhedral shape and cannot reproduce more complex rockfall scenarios encountered in the field. A rigid slab may load a barrier over a larger area and its effect has not been investigated. In this study, a calibrated three-dimensional finite-element model was developed to study the performance of vertically and horizontally orientated rockfall barriers under concentrated areal impact loads. A new bilinear force-displacement model was incorporated into the model to simulate the behavior of the energy-dissipating devices on the barriers. The effect of different weight geometries was studied by considering impacts by a rigid single spherical boulder and a rigid slab. Results reveal that areal loading induced by a rigid slab increases the loading on the barrier foundation by up to 40 % in both horizontally and vertically positioned barriers when compared to a concentrated load scenario with a single boulder. This indicates that barriers tested under the current test standard does not give the worst-case scenario in terms of foundation loads, and barrier designers should take into account the possible effect of increased foundation loads by reinforcing the barrier posts and/or increasing their spacing. 相似文献
