A new solid state diffusion model applied to inverse zoning and diffusion rims in minerals     

Friedemann Freund  Bruce V. King 《Physics and Chemistry of Minerals》1984,11(3):113-124

Any oxide and silicate mineral which is nominally anhydrous but crystallized in the presence of H₂O incorporates traces of H₂O in solid solution. In the case of MgO it can be shown that OH^? pairs convert into H₂+O ₂ ^2? . If the H₂ molecules are lost, the O ₂ ^2? remain in the lattice as excess oxygen stabilized by excess cation vacancies. When the O ₂ ^2? anions decay either thermally or by decompression unbound O^? states (positive holes) are generated which lead to surface charges and subsurface space charge layers. Calculated space charge profiles are presented. O^? concentrations as small as 10–20 ppm suffice to create electric surface fields of the order of 4·10⁷ V·m^?1. The diffusion mechanism which derives from these premises incorporates novel features: the cation diffusion is coupled to the counterdiffusion of unbound and vacancy-bound O^? states. The cation diffusion is predicted to be very fast because first, it is field-enhanced (electrochemically driven) and second, it is not rate-limited by the intrinsic cation vacancy concentration nor by the counter-diffusion of other cations. The model may apply to cases of inverse zoning and diffusion rim formation in minerals under certain P-T conditions.  相似文献   

Pyroclastic rocks: another manifestation of ultramafic volcanism on Gorgona Island,Colombia   总被引：5，自引：0，他引：5  

Lina M. Echeverría  Bruce G. Aitken 《Contributions to Mineralogy and Petrology》1986,92(4):428-436

Tertiary ultramafic volcanism on Gorgona Island, Colombia, is manifested not only by komatiite flows, but also by a more voluminous sequence of tuff breccias, which is cut by comagmatic picrite dikes. The ultramafic pyroclastic rocks are chaotic to stratified mixtures of angular to subrounded glassy picritic blocks and a fine grained volcaniclastic matrix that consists primarily of plastically-deformed, glassy globules. The entire deposit is interpreted to have formed by an explosive submarine eruption of phenocryst-laden picritic magma. MgO contents of tuff breccias and picrite dikes range from 21 to 27 wt%. Relative to nearby komatiite flows, these rocks are MgO-rich, and FeO-, TiO₂- and Ni-poor. HREE concentrations are very low (N<0.2), indicating derivation from a more highly depleted mantle source region. Nd isotopic data suggest a cogenetic relationship between the picritic magma that formed the tuff breccia and associated dikes and that which produced the komatiite flows. Nevertheless Pb isotopic data as well as whole rock geochemistry preclude such a connection, either due to olivine fractionation/accumulation or to different degrees of partial melting. These ultramafic rock types crystallized from magmas which most likely were extracted from distinct mantle source regions.  相似文献   

Water chemistry and sedimentological observations in littlefield lake,michigan: Implications for lacustrine marl deposition     

Nina M. Duston  Robert M. Owen  Bruce H. Wilkinson 《Environmental Geology》1986,8(4):229-236

A combination of both water chemistry and sedimentological information was used to investigate the carbonate-producing mechanism in Littlefield Lake, a small lake located in Isabella County, central Michigan. Data on temperature, dissolved oxygen, pH, calcium carbonate (CaCO₃) saturation, alkalinity, calcium, and magnesium were obtained on a monthly basis over a 13-month period, with each parameter determined at 1m intervals over a depth range of 20m. The loss of dissolved carbon dioxide (CO₂) from warm surface waters during direct degassing, and to a lesser extent during photosynthetic uptake by lacustrine macrophytes and phytoplankton during the summer, results in massive precipitation of the low-magnesium calcite which predominates in all Littlefield Lake sedimentary facies However, despite the fact that carbonate precipitation in this rather typical temperate-region marl lake is directly related to, and may be driven by, seasonal variation in these physiochemical parameters, most calcite forms as encrustations around cyanophytic and chlorophytic macrophytes. Such relationships demonstrate that carbonate precipitation in marl lakes may result from complex interactions between both biochemical and physiochemical processes. As such, marl formation in this, and probably many other calcareous lake systems, can not be simply ascribed to one or the other of these two general mechanisms.  相似文献   

Trace element distribution in the soils above deeply weathered pegmatites, Virginia, U.S.A.: implications for exploration     

Bruce T. Marshall  Janet S. Herman 《Applied Geochemistry》1986,1(6)

Chemical partitioning data are of fundamental interest to exploration geochemists. This paper is one of the few studies which has investigated the relative proportions of the rare elements in various soil extracts. The dispersion of trace elements from weathering pegmatites in Powhatan Country, Virginia, was found to be restricted to the immediate vicinity of the pegmatites. A sequential extraction procedure was used to measure the distribution of Be, La, Nb, Sn, U, Li, and Ni,among the following fractions of the B soil horizons: exchangeable, Fe and Mn oxyhydroxide, residual, and aqua regia digestion. The elements Sn, Be, Li, and U were found to be associated with soils over the complex Herbb No. 2 pegmatite, whereas La and Ni were generally associated with the background soils.A geochemical exploration model was developed using stepwise discriminant function analysis to determine the combination of elements and soil extracts that best differentiates between complex pegmatitic, simple pegmatitic, and background soils. Log-transformed aqua regia extract concentrations of Sn, La, U, and Li were the most effective variables when used to separate complex pegmatitic from simple pegmatitic soils.  相似文献   

A reassessment of the economic effects of global climate change on U.S. agriculture     

Richard M. Adams  Ronald A. Fleming  Ching-Chang Chang  Bruce A. McCarl  Cynthia Rosenzweig 《Climatic change》1995,30(2):147-167

This study uses recent GCM forecasts, improved plant science and water supply data and refined economic modeling capabilities to reassess the economic consequences of long-term climate change on U.S. agriculture. Changes in crop yields, crop water demand and irrigation water arising from climate change result in changes in economic welfare. Economic consequences of the three GCM scenarios are mixed; GISS and GFDL-QFlux result in aggregate economic gains, UKMO implies losses. As in previous studies, the yield enhancing effects of atmospheric CO₂ are an important determinant of potential economic consequences. Inclusion of changes in world food production and associated export changes generally have a positive affect on U.S. agriculture. As with previous studies, the magnitude of economic effects estimated here are a small percentage of U.S. agricultural value.  相似文献   

Use of Electric Logs to Estimate Water Quality of Pre-Tertiary Aquifers     

J. B. Lindner-Lunsford  Breton W. Bruce 《Ground water》1995,33(4):547-555

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Developing A National Drinking Water Regulation for Disinfection of Ground Water     

Bruce A. Macler 《Ground Water Monitoring & Remediation》1995,15(4):77-84

The Safe Drinking Water Act directs EPA to promulgate requirements for disinfection of ground water-based drinking water systems. The Ground- water Disinfection Rule regulatory workgroup, made up of representatives from EPA, the states, and other interested parties, is actively considering the issues for the wide range of elements necessary to ensure a regulation that will protect public health and can be feasibly implemented. This regulation is likely to require disinfection of ground water sources and systems found to be contaminated or vulnerable to contamination.  相似文献   

Book reviews     

Robert Maples  Bruce W. Nelson  Peter G. Verity 《Estuaries and Coasts》1987,10(2):177-179

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SPECTROPHOTOMETRIC DETERMINATION OF FERROUS IRON IN EIGHTEEN UNITED STATES GEOCHEMICAL REFERENCE STANDARDS     

Charles B. Bruce  Jr.  Nathan W. Bower 《Geostandards and Geoanalytical Research》1987,11(1):41-42

Iron (II) oxide in 50 mg samples of silicate rocks, deposits, and soils is measured after digestion with HF and HCl in the presence of ICl. The iodine produced is measured spectrophotometrically. Results are presented for the eighteen available USGS geochemical reference standards including the GXR series.  相似文献   

Diffusion of dissolved carbonate in magmas: Experimental results and applications     

E. Bruce Watson  Mark A. Sneeringer  Alan Ross 《Earth and Planetary Science Letters》1982,61(2):346-358

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