Shock-induced amorphous textures in plagioclase,Manicouagan, Quebec,Canada     

Joseph Clancy White 《Contributions to Mineralogy and Petrology》1993,113(4):524-532

The textural relationships and structural states of optically isotropic labradorite from the Manicouagan, Quebec, impact structure have been examined by light (optical) and transmission electron (TEM) microscopy. Two distinct diaplectic glasses have been recognized based on their contrasting morphology, timing and the inferred modes of formation. The earliest isotropic bands and grain-scale isotropism (maskelynite) optically exhibit a gradational,in situ transformation from crystalline plagioclase with preservation of relict textures (twins, grain boundaries). The same transformation from crystalline to amorphous structure is observed in TEM to occur heterogeneously at scales on the order of the unit cell. The progressive transformation of optical properties reflects an increase in the volume fraction and eventual coalescence of these amorphous units. This maskelynite-type diaplectic glass is interpreted to form in the solid-state directly from crystalline material during the compressional phase of the shock wave. The other isotropic material occurs in spatially discrete tensiongashes and planar deformation features (PDFs) that overprint the maskelynite-type glass. This second type of diaplectic glass (PDF-type) is developed homogeneously within a given glass band and exhibits sharp crystal-glass boundaries, in contrast to the gradational boundaries of the maskelynite-type glass. PDF-type glass is interpreted to form by melting in tensional release zones during passage of the rarefaction wave. These observations emphasize the ability of naturally shocked rocks to preserve subtle evidence of variations in the shock process from highly transient events.  相似文献   

SH propagation in rocks with planar fractures–I. Excess slowness     

L. Neil Frazer 《Geophysical Journal International》1995,122(1):33-62

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Sub-Milankovitch palaeoclimatic events: Their recognition and correlation     

Neil Roberts 《Climatic change》1993,24(3):175-178

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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   

New evidence on the hydrothermal system in Long Valley caldera, California, from wells, fluid sampling, electrical geophysics, and age determinations of hot-spring deposits     

Michael L. Sorey  Gene A. Suemnicht  Neil C. Sturchio  Gregg A. Nordquist 《Journal of Volcanology and Geothermal Research》1991,48(3-4)

Data collected since 1985 from test drilling, fluid sampling, and geologic and geophysical investigations provide a clearer definition of the hydrothermal system in Long Valley caldera than was previously available. This information confirms the existence of high-temperature (> 200°C) reservoirs within the volcanic fill in parts of the west moat. These reservoirs contain fluids which are chemically similar to thermal fluids encountered in the central and eastern parts of the caldera. The roots of the present-day hydrothermal system (the source reservoir, principal zones of upflow, and the magmatic heat source) most likely occur within metamorphic basement rocks beneath the western part of the caldera. Geothermometer-temperature estimates for the source reservoir range from 214 to 248°C. Zones of upflow of hot water could exist beneath the plateau of moat rhyolite located west of the resurgent dome or beneath Mammoth Mountain. Lateral flow of thermal water away from such upflow zones through reservoirs in the Bishop Tuff and early rhyolite accounts for temperature reversals encountered in most existing wells. Dating of hot-spring deposits from active and inactive thermal areas confirms previous interpretations of the evolution of hydrothermal activity that suggest two periods of extensive hot-spring discharge, one peaking about 300 ka and another extending from about 40 ka to the present. The onset of hydrothermal activity around 40 ka coincides with the initiation of rhyolitic volcanism along the Mono-Inyo Craters volcanic chain that extends beneath the caldera's west moat.  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