Hydrogeologic modeling of the genesis of carbonate-hosted lead-zinc ores

 Carbonate-hosted lead–zinc ore deposits in the Mississippi Valley region of North America and in the central midlands region of Ireland provide good examples where ancient groundwater migration controlled ore formation deep within sedimentary basins. Hydrogeologic and geochemical theories for ore genesis are explored in this paper with mathematical models that allow for complex permeability fields in two or three dimensions, hydrothermal flows in fault systems, and coupled effects of geochemical reactions. The hydrogeologic framework of carbonate-hosted ores is analyzed with the aim of developing a quantitative understanding of the necessary and sufficient processes required to form large ore deposits. Numerical simulations of basin-scale hydrodynamics and of deposit-scale reactive flow are presented to demonstrate the processes controlling low-temperature Pb–Zn ore genesis in two world-class ore districts, in southeast Missouri, USA, and central Ireland. The numerical models presented here provide a theoretical basis for the following observations: (1) topography-driven brine migration was the most effective mechanism for forming the large ore districts of the Mississippi Valley, such as the Viburnum Trend of southeast Missouri, during the uplift of the Appalachian–Ouachita mountain belt in late Paleozoic time; (2) three-dimensional flow fields were created by a dolomite facies of the Viburnum Trend, which acted as a giant lens for focusing metal and heat in southeast Missouri to produce the largest known concentration of lead in the Earth's crust; (3) ore-mineralization patterns were controlled locally by basement relief, permeability structure, and sandstone pinchouts, because of their effects on cooling and fluid-flow rates along the Viburnum Trend; (4) both density-driven and topography-driven fluid flow were important for ore genesis in the Irish midlands, where brines moved northward away from the Variscan orogen, leaked into the Hercynian basement, and discharged along normal faults up into the sedimentary cover; and (5) mixed convection within northeast–southwest fault planes elevated heat flow and flow rates that fed ore deposition by fluid mixing, in some cases near the Carboniferous seafloor in Ireland. Received, February 1998 · Revised, July 1998 · Accepted, September 1998     

Modeled Ground Water Age Distributions

The age of ground water in any given sample is a distributed quantity representing distributed provenance (in space and time) of the water. Conventional analysis of tracers such as unstable isotopes or anthropogenic chemical species gives discrete or binary measures of the presence of water of a given age. Modeled ground water age distributions provide a continuous measure of contributions from different recharge sources to aquifers. A numerical solution of the ground water age equation of Ginn (1999) was tested both on a hypothetical simplified one‐dimensional flow system and under real world conditions. Results from these simulations yield the first continuous distributions of ground water age using this model. Complete age distributions as a function of one and two space dimensions were obtained from both numerical experiments. Simulations in the test problem produced mean ages that were consistent with the expected value at the end of the model domain for all dispersivity values tested, although the mean ages for the two highest dispersivity values deviated slightly from the expected value. Mean ages in the dispersionless case also were consistent with the expected mean ages throughout the physical model domain. Simulations under real world conditions for three dispersivity values resulted in decreasing mean age with increasing dispersivity. This likely is a consequence of an edge effect. However, simulations for all three dispersivity values tested were mass balanced and stable demonstrating that the solution of the ground water age equation can provide estimates of water mass density distributions over age under real world conditions.     

Discriminating sources of PCB contamination in fish on the coastal shelf off San Diego, California (USA)

Management of coastal ecosystems necessitates the evaluation of pollutant loading based on adequate source discrimination. Monitoring of sediments and fish on the shelf off San Diego has shown that some areas on the shelf are contaminated with polychlorinated biphenyls (PCBs). Here, we present an analysis of PCB contamination in fish on the shelf off San Diego designed to discriminate possible sources. The analysis was complicated by the variability of species available for analysis across the shelf, variable affinities of PCBs among species, and non-detects in the data. We utilized survival regression analysis to account for these complications. We also examined spatial patterns of PCBs in bay and offshore sediments and reviewed more than 20 years of influent and effluent data for local wastewater treatment facilities. We conclude that most PCB contamination in shelf sediments and fish is due to the ongoing practice of dumping contaminated sediments dredged from San Diego Bay.     

Sensitivity of model parameterizations for simulated latent heat flux at the snow surface for complex mountain sites

The snowcover energy balance is typically dominated by net radiation and sensible and latent heat fluxes. Validation of the two latter components is rare and often difficult to undertake at complex mountain sites. Latent heat flux, the focus of this paper, is the primary coupling mechanism between the snow surface and the atmosphere. It accounts for the critical exchange of mass (sublimation or condensation), along with the associated snowcover energy loss or gain. Measured and modelled latent heat fluxes at a wind‐exposed and wind‐sheltered site were compared to evaluate variability in model parameters. A well‐tested and well‐validated snowcover energy balance model, Snobal, was selected for this comparison because of previously successful applications of the model at these sites and because of the adjustability of the parameters specific to latent heat transfer within the model. Simulated latent heat flux and snow water equivalent (SWE) were not sensitive to different formulations of the stability profile functions associated with heat transfer calculations. The model parameters of snow surface roughness length and active snow layer thickness were used to improve latent heat flux simulations while retaining accuracy in the simulation of the SWE at an exposed and sheltered study site. Optimal parameters for simulated latent heat flux and SWE were found at the exposed site with a shorter roughness length and thicker active layer, and at the sheltered site with a longer roughness length and thinner active layer. These findings were linked to physical characteristics of the study sites and will allow for adoption into other snow models that use similar parameters. Physical characteristics of wind exposure and cover could also be used to distribute critical parameters in a spatially distributed modelling domain and aid in parameter selection for application to other watersheds where detailed information is not available. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance-based seismic design of nonstructural building components:The next frontier of earthquake engineering

With the development and implementation of performance-based earthquake engineering,harmonization of performance levels between structural and nonstructural components becomes vital. Even if the structural components of a building achieve a continuous or immediate occupancy performance level after a seismic event,failure of architectural,mechanical or electrical components can lower the performance level of the entire building system. This reduction in performance caused by the vulnerability of nonstructural components has been observed during recent earthquakes worldwide. Moreover,nonstructural damage has limited the functionality of critical facilities,such as hospitals,following major seismic events. The investment in nonstructural components and building contents is far greater than that of structural components and framing. Therefore,it is not surprising that in many past earthquakes,losses from damage to nonstructural components have exceeded losses from structural damage. Furthermore,the failure of nonstructural components can become a safety hazard or can hamper the safe movement of occupants evacuating buildings,or of rescue workers entering buildings. In comparison to structural components and systems,there is relatively limited information on the seismic design of nonstructural components. Basic research work in this area has been sparse,and the available codes and guidelines are usually,for the most part,based on past experiences,engineering judgment and intuition,rather than on objective experimental and analytical results. Often,design engineers are forced to start almost from square one after each earthquake event: to observe what went wrong and to try to prevent repetitions. This is a consequence of the empirical nature of current seismic regulations and guidelines for nonstructural components. This review paper summarizes current knowledge on the seismic design and analysis of nonstructural building components,identifying major knowledge gaps that will need to be filled by future research. Furthermore,considering recent trends in earthquake engineering,the paper explores how performance-based seismic design might be conceived for nonstructural components,drawing on recent developments made in the field of seismic design and hinting at the specific considerations required for nonstructural components.     

地层因素对陆相沉积储层特征的控制(英文)

在相同沉积相和沉积环境中 ,沉积记录中作为地层位置函数的不同尺度的地层学与沉积学性质的变化是有序的并可以预测。诸多沉积学和地层学性质 ,从孔隙度、渗透率到地质体的规模和非均质性对于储集性能和产能都是重要的。地层导致的上述性质的变化与由于原始沉积环境的差异产生的变化同等重要。运用地层观点对沉积物某种或多种性质变化进行分析 ,表明这些变化具有规律性并有向一些界限呈单向变化的趋势。与此对照 ,运用对时间和地层位置并不敏感的“相”对这些性质进行分析会发现数值具明显的非相关离散特征。因此 ,仅根据沉积环境或相组成对储集性能进行评价会造成预测的准确性和精确性变差。由此 ,文中提出了能够提高储层表征准确性和精确性的一种新的分析思路 ,即运用地层观点评价相的诸多性质。上述原理可用陆相地层中的一些实例加以说明。以河道砂岩储层为例 ,在地层剖面上 ,随着可容纳空间与沉积物补给的比值 (A/S)的增大 ,槽状交错层组的厚度和层的厚度向上增大 ,孔隙度、渗透率、保存下来的河道的宽度、连续性和连通程度降低 ,而底形多样性、相的分异性、相的保存程度和非均质性增加。这些性质所表现出来的单向变化趋势多数是由于作为A/S值函数的原始地貌单元的保存程度的变化而产生的。同时 ,地     

Calcium‐aluminum‐rich inclusions in enstatite chondrites (I): Mineralogy and textures

Abstract— Like calcium‐aluminum‐rich inclusions (CAIs) from carbonaceous and ordinary chondrites, enstatite chondrite CAIs are composed of refractory minerals such as spinel, perovskite, Al, Ti‐diopside, melilite, hibonite, and anorthitic plagioclase, which may be partially to completely surrounded by halos of Na‐(±Cl)‐rich minerals. Porous, aggregate, and compact textures of the refractory cores in enstatite chondrite CAIs and rare Wark—Lovering rims are also similar to CAIs from other chondrite groups. However, the small size (<100μm), low abundance (<1% by mode in thin section), occurrence of only spinel or hibonite‐rich types, and presence of primary Ti‐(±V)‐oxides, and secondary geikelite and Ti, Fe‐sulfides distinguish the assemblage of enstatite chondrite CAIs from other groups. The primary mineral assemblage in enstatite chondrite CAIs is devoid of indicators (e.g., oldhamite, osbornite) of low O fugacities. Thus, high‐temperature processing of the CAIs did not occur under the reducing conditions characteristic of enstatite chondrites, implying that either (1) the CAIs are foreign to enstatite‐chondrite‐forming regions or (2) O fugacities fluctuated within the enstatite‐chondrite‐forming region. In contrast, secondary geikelite and Ti‐Fe‐sulfide, which replace perovskite, indicate that alteration of perovskite occurred under reducing conditions distinct from CAIs in the other chondrite groups. We have not ascertained whether the reduced alteration of enstatite chondrite CAIs occurred in a nebular or parent‐body setting. We conclude that each chondrite group is correlated with a unique assemblage of CAIs, indicating spatial or temporal variations in physical conditions during production or dispersal of CAIs.     

Origin of lunar high‐titanium ultramafic glasses: Constraints from phase relations and dissolution kinetics of clinopyroxene‐ilmenite cumulates

Abstract— Phase equilibrium and dissolution kinetics experiments on synthetic late‐stage magma ocean cumulates are used to place constraints on hypotheses for the origin of lunar high‐Ti ultramafic glasses. Models for the production of high‐Ti lunar magmas have called for either (1) assimilation of late‐stage clinopyroxene‐ilmenite cumulates at shallow levels or (2) sinking of clinopyroxene‐ilmenite cumulates to form a hybrid mantle source. To satisfy the constraints of our experiments, we propose an alternative model that involves shallow‐level reaction and mixing of cumulates, followed by sinking of hybrid high‐Ti materials. This model can fulfill compositional requirements imposed by the pristine lunar glass suite that are difficult to satisfy in assimilation models. It also avoids difficulties that arise in overturn models from the low solidus temperatures of clinopyroxene‐ilmenite cumulates. Partially molten clinopyroxene‐ilmenite cumulates become gravitationally unstable with respect to underlying mafic cumulates only when they have cooled to within ～30°C of their solidus (～1125°C at 100 km depth). At these temperatures, the viscosity of mafic cumulates is too high to allow for growth and descent of clinopyroxene‐ilmenite diapirs on the appropriate time scale. Reaction and mixing between late‐stage liquids and mafic cumulates at shallow levels would produce a refractory hybrid material that is negatively buoyant at higher temperatures and could sink more efficiently to the depths inferred for production of high‐Ti ultramafic glasses.