Measurement and Interpretation of Low Levels of Dissolved Oxygen in Ground Water   总被引：1，自引：0，他引：1  

Art F. White  Maria L. Peterson  Ray D. Solbau 《Ground water》1990,28(4):584-590

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Epithermal environments and styles of mineralization: Variations and their causes, and guidelines for exploration   总被引：3，自引：0，他引：3  

Noel C. White  Jeffrey W. Hedenquist 《Journal of Geochemical Exploration》1990,36(1-3)

Epithermal precious- and base-metal deposits are diverse, reflecting the different tectonic, igneous and structural settings in which they occur, the complexities of their local setting, and the many processes involved in their formation. Most epithermal deposits form at shallow crustal levels where abrupt changes in physical and chemical conditions result in metal deposition and attendant hydrothermal alteration. The principal factors that influence the conditions prevailing in the epithermal environment, and which ultimately determine the sites and character of mineralization, include: geology (structure, stratigraphy, intrusions and rock type, which affect the style and degree of permeability and the reactivity of the host); pressure and temperature (which in the epithermal environment are related on the boiling point with depth curve); hydrology (the relationship between permeability and topography which governs fluid flow, and discharge/recharge characteristics, as well as access of steam-heated waters); chemistry of the mineralizing fluid (which determines the metal-carrying capacity, as well as the associated vein and alteration assemblage); and syn-hydrothermal development of permeability and/or changes in hydraulic gradients.Many attempts have been made to classify epithermal deposits based on mineralogy and alteration, the host rocks, deposit form, genetic models, and standard deposits. All have their strengths and weaknesses. We prefer a simple approach using the fundamental fluid chemistry (high or low sulfidation, reflecting relatively oxidized or reduced conditions, respectively) as readily inferred from vein and alteration mineralogy and zoning, together with the form of the deposit, and using comparative examples to clarify the character of the deposit.Guidelines for exploration vary according to the scale at which work is conducted, and are commonly constrained by a variety of local conditions. On a regional scale the tectonic, igneous and structural settings can be used, together with assessment of the depth of erosion, to select areas for project area scale exploration. At project area scale, direct (i.e. geochemical) or indirect guidelines may be used. Indirect methods involve locating and interpreting hydrothermal alteration as a guide to ore, with the topographic and hydrologic reconstruction of the system being of high priority. These pursuits may involve mineralogic, structural, geophysical or remote sensing methods. On a prospect scale, both direct and indirect methods may be used; however, they can only be effective in the framework of a sound conceptual understanding of the processes that occur in the epithermal environment, and the signatures they leave.  相似文献   

The Continental Slope Experiment along the Tasman Project profile, southeast Australia     

Antony White 《Physics of the Earth and Planetary Interiors》1990,60(1-4):147-154

Three seafloor magnetometers made recordings for up to 95 days between July and October 1986 at site across the continental slope of southeast Australia along the profile of the 1984 Tasman Project of Seafloor Magnetotelluric Exploration (TPSME). Land magnetometers at TPSME sites at the coast and Canberra were reactivated to give simultaneous recordings. The seafloor magnetometers were at depths of 140 m, 2240 m and 3380 m and spanned the continental shelf between the coastline and the closest inshore TPSME ocean floor site (depth 4850 m). This experiment complements the TPSME by giving a much sharper definition of the geomagnetic coast effect in this critical region.

Data are presented in the form of Parkinson arrows for comparison with previously derived TPSME results along the profile. They show a strong coast effect with the maximum mid-way down the continental slope. The situation is closely two dimensional, and using this approximation some simple models have been computed. One which gives a relatively good fit at a period of 1 h comprises simply a conductive ocean overlying a uniform conductor at depth. Further work will be needed to determine whether lateral conductivity structure at depth is required to fit the data more closely.  相似文献   

Chemical equilibrium and mass balance relationships associated with the Long Valley hydrothermal system, California, U.S.A.     

Art F. White  Maria L. Peterson 《Journal of Volcanology and Geothermal Research》1991,48(3-4)

Recent drilling and sampling of hydrothermal fluids from Long Valley permit an accurate characterization of chemical concentrations and equilibrium conditions in the hydrothermal reservoir. Hydrothermal fluids are thermodynamically saturated with secondary quartz, calcite, and pyrite but are in disequilibrium with respect to aqueous sulfide-sulfate speciation. Hydrothermal fluids are enriched in ¹⁸O by approximately 1‰ relative to recharge waters. ¹⁸O and Cl concentrations in well cuttings and core from high-temperature zones of the reservoir are extensively depleted relative to fresh rhyolitic tuff compositions. Approximately 80% of the Li and 50% of the B are retained in the altered reservoir rock. Cl mass balance and open-system ¹⁸O fractionation models produce similar water-rock ratios of between 1.0 and 2.5 kg kg⁻¹. These water-rock ratios coupled with estimates of reservoir porosity and density produce a minimum fluid residence time of 1.3 ka. The low fluid Cl concentrations in Long Valley correlate with corresponding low rock concentrations. Mass balance calculations indicate that leaching of these reservoir rocks accounts for Cl losses during hydrothermal activity over the last 40 ka.  相似文献   

Boundary-layer depth and entrainment zone characterization with a boundary-layer profiler   总被引：13，自引：0，他引：13  

Wayne M. Angevine  Allen B. White  S. K. Avery 《Boundary-Layer Meteorology》1994,68(4):375-385

A technique for determining the height of the convective atmospheric boundary layer (CBL) with a 915 MHz boundary-layer profiler is discussed. The results are compared with CBL heights determined from radiosonde measurements. The profiler provides continuous CBL height measurements with very good time resolution (30 minutes or less), allowing for detailed understanding of the growth and fluctuations of the CBL. In addition, the profiler provides information about the degree of definition of the CBL top and the thickness of the entrainment zone. The measurements discussed were taken during the Rural Oxidants in the Southern Environment II (ROSE II) experiment.  相似文献   

Monitoring and reducing greenhouse gas emissions from agricultural,forestry and other human activities     

S. Boag  D. H. White  S. M. Howden 《Climatic change》1994,27(1):5-11

Agriculture and forestry are significant sources and sinks of greenhouse gases. A holistic systems approach to estimating and reducing greenhouse gas emissions from agricultural, forestry and other systems requires that the major inputs, components and outputs of the production system are defined. Fluxes of greenhouse gases in natural systems may be estimated by mathematical modelling of the major biological processes and activities. Field and laboratory experiments and information from satellites provide the raw data on which such models are based. Such an approach can have a significant role in guiding key decision makers and policy analysts. We conclude that management strategies that reduce greenhouse gas emissions from agriculture and forestry are likely to be strategies that will also contribute to ecologically sustainable development.  相似文献   

Direct measurements of latent heat during crystallization and melting of a ugandite and an olivine basalt     

Rebecca A. Lange  Katharine V. Cashman  Alexandra Navrotsky 《Contributions to Mineralogy and Petrology》1994,118(2):169-181

Step-scanning calorimetric measurements using a Setaram HT1500 calorimeter were performed between 800 and 1400°C on two natural samples: a ugandite from the East African rift and an olivine basalt from the western Mexican arc. Our measurements provide the first in-situ quantitative assessment of enthalpy during melting of initially crystalline natural samples. The distribution of latent heat across the liquidus-solidus intervals of the two samples is distinctly different, reflecting significant variation in the sequence and abundance of mineral phases during melting (clinopyroxene and leucite in the ugandite; olivine, clinopyroxene, and plagioclase in the basalt). Our data further indicate that the common assumption of a uniform distribution of latent heat across the liquidus-solidus interval of a magma is a reasonable approximation for the olivine basalt, but is grossly in error for the ugandite. This is due to cotectic precipitation of leucite and clinopyroxene, leading to a large, disproportionate release of latent heat early in the crystallization sequence. The implication for the thermal history of a crystallizing ugandite magma is that therate of heat loss during conductive cooling will unitially be more rapid than the average rate. The net result will be to produce lower magmatic temperatures after a given cooling interval relative to models assuming a uniform release of latent heat. An additional series of scanning calorimetric experiments were performed at variable rates (1,2 and 3°/min) to evaluate the role of kinetics on the distribution of enthalpy during both melting and crystallization of the ugandite and olivine basalt. The results indicate that clinopyroxene is the most important mineral phase in controlling the shapes of the enthalpy profiles during cooling; this is due to its large enthalpy of fusion and its tendency for sluggish nucleation, followed by rapid crystallization at temperatures that vary with cooling rate. The resolution of the calorimeter (in terms of heat detected per unit time) is also important in determining the shapes of theobserved enthalpy profiles during these rapid scans. Estimates based on the observed calorimetric signal associated with melting of olivine, and the lack of a calorimetric signal during melting of leucite, combined with known enthalpies of fusion for the two phases, indicate detection limits of approximately 0.6–1.2 kJ per 5 min increments.  相似文献   

Speleogenesis in aeolian calcarenite: A case study in western Victoria     

S. White 《Environmental Geology》1994,23(4):248-255

Most studies of karst landscapes and their processes have been concerned with consolidated, often well-jointed limestones. There are particular problems involved in the study of karst procesess in softer, less-compact limestones such as chalk, coral reefs, and aeolian calcarenite. Previous studies in aeolian calcarenite indicated these problems and a scheme was developed of speleogenesis in aeolian calcarenite. A study of karst processes in aeolian calcarenite at Bats Ridge in western Victoria has developed this scheme further. The karst features and processes at Bats Ridge are an integral part of the landscape of a mid-Pleistocene calcarenite dune system. The resolution of problems of the rapid subaerial speleogenesis in the area is achieved by the synthesis of the known karst features of the ridge and the geology and geomorphology of the area. Karst development on this aeolianite ridge depends on lithological conditions as well as the availability of aggressive water capable of solution. The diagenesis of the calcarenite is occurring now and must have been occurring by the mid-Pleistocene. This simultaneous lithification of the carbonate dunes into aeolian calcarenite rock and the development of solutional karst features in the dunes is the characteristic feature of the speleogenesis in this area. It is the formation of a hardened kankar layer (cap rock) in the dunes of sufficient compressive and tensile strength to support cavities, which is the result of these interrelated factors, that has strongly determined the formation of the karst features.  相似文献   

Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region,Atlantic Ocean     

William M. White  Bernard Dupré  Philippe Vidal 《Geochimica et cosmochimica acta》1985,49(9):1875-1886

Twenty-four piston core sediment samples and 13 sediments and 3 basalts from DSDP Leg 78 Site 543 were analyzed for Sr, Nd and Pb isotopic compositions. The results show sediment with highly radiogenic Pb${}^{206}\text{Pb}{}^{204}\text{Pb}$ up to 19.8) and rather radiogenic Sr and unradiogenic Nd has been deposited in the region since the Cretaceous. The source of this sediment is probably the Archean Guiana Highland, which is drained by the Orinoco River. Pb and Sr isotopic compositions and sediment thickness decrease and${}^{143}\text{Nd}{}^{144}\text{Nd}$ increases northward due to a decrease in turbiditic component. This decrease is partly due to the damming action of basement ridges. Rare earth concentrations in the sediments are somewhat low, due to the abundance of detrital and biogenic components in the sediment and rapid sedimentation rates. Both positive and negative Ce anomalies occur in the surface sediments, but only positive Ce anomalies occur in the Site 543 sediments. It is unlikely that sediment subducted to the source region of Lesser Antilles arc magmas could be the cause of negative Ce anomalies in those magmas.Isotopic compositions of Site 543 basalts show some effect of contamination by seawater-basalt reaction products and sediments. Beyond this, however, they are typical of “normal” depleted MORB.  相似文献   

Hafnium/rare earth element fractionation in the sedimentary system and crustal recycling into the Earth's mantle     

P.J. Patchett  W.M. White  H. Feldmann  S. Kielinczuk  A.W. Hofmann 《Earth and Planetary Science Letters》1984,69(2):365-378

Among long-lived radioactive parent-daughter element pairs, the ratio Lu/Hf is strongly fractionated relative to constant Sm/Nd in the Earth's sedimentary system. This is caused by high resistance to chemical weathering of the mineral zircon (Zr,Hf)SiO₄. Zircon-bearing sandy sediments on and near continents have very low Lu/Hf, while deep-sea clays have up to three times the chondritic Lu/Hf ratio. Turbidity currents mechanically carry the low-Lu/Hf sandy material onto the ocean floor. The results are important for the crust-to-mantle recycling discussion, where most recycled materials would be subducted oceanic sediments. Such sediment should be capable of explaining the HfNd mantle isotopic variation by mixing with peridotite, but in fact any average pelagic sediment has Nd/Hf and Lu/Hf too high to allow mixing curves to pass through the Hf/Nd isotopic array. The array could only be reproduced by subduction of turbidite sandstone with pelagic sediment in the approximate ratio 1.2 to 1, and by maintaining a good mixture between the two components. At least today, turbidites are available for subduction only at locations quite different and distant from those where pelagic sediments may be recycled; furthermore, mantle isotopic variation shows that the mantle often cannot mix itself well enough to homogenize these widely-separated sedimentary components to the degree required. The Lu/Hf fractionations place a severe restriction on the ability of recycled sediments to explain mantle isotopic patterns.  相似文献