An Experimental Study of Nepheline-Kalsilite Exsolution     

YUND  RICHARD A.; MCCALLISTER  ROBERT H.; SAVIN  SAMUEL M. 《Journal of Petrology》1972,13(2):255-272

The nepheline-kalsilite exsolution reaction was studied isothermallybetween 400 and 700°C. Under nonaqueous conditions the mechanisminvolves nucleation of kalsilite and growth by diffusion ofthe alkalis. As predicted by simple nucleation theory, the nucleationrate and hence the over-all exsolution rate are strongly dependenton the supersaturation of the nepheline. A decrease in temperatureat constant composition increases the supersaturation and therebythe nucleation rate. This increased nucleation rate is opposedby the decrease in the growth rate due to slower volume diffusion.At a supersaturation of more than 8–10 mole per cent thenumber of nuclei is large and the over-all exsolution rate isdetermined primarily by the growth rate. The activation energyfor growth is 28 kcal/mole. An increase of two kilobars in thehydrostatic pressure has little effect on the kinetics of thereaction. Under nonhydrostatic conditions the exsolution rateincreases significantly because the nucleation rate is faster. Under hydrothermal conditions the ‘exsolution’ rateis approximately two orders of magnitude faster due to a modificationin the mechanism. Partial dissolution of the original solidsolution in distilled water creates a condition of nonequilibriumin which the fluid is sodium-rich. Rapid alkali exchange eliminatesthis condition but produces the equilibrium compositions ofthe solids because kalsilite nucleates and grows in contactwith the fluid. The experimental evidence for this mechanismincludes X-ray diffraction data showing a gradual change inthe composition of the initial supersaturated solid, essentiallyidentical activation energies for growth under aqueous and nonaqueousconditions, and a lower percentage of oxygen isotope exchangethan ‘exsolution’ in the same experiment.  相似文献   

TEXTURAL DIFFERENTIATION ON THE SHORE FACE DURING EROSIONAL RETREAT OF AN UNCONSOLIDATED COAST, CAPE HENRY TO CAPE HATTERAS, WESTERN NORTH ATLANTIC SHELF     

DONALD J. P. SWIFT  ROBERT B. SANFORD  CHARLES E. DILL JR  NICHOLAS F. AVIGNONE 《Sedimentology》1971,16(3-4):221-250

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Lithified carbonate sediment and zeolitic tuff in basalts, Mid-Atlantic Ridge   总被引：1，自引：0，他引：1  

ROBERT E. GARRISON  JAMES R. HEIN  THOMAS F. ANDERSON 《Sedimentology》1973,20(3):399-410

Basaltic boulders dredged from the Mid-Atlantic Ridge contain lithified coccolith-foraminiferal ooze in fractures and small pockets. Textural and isotopic studies of this sediment provide no evidence for high temperature metamorphism. The lithified carbonate sediment occurs together with palagonitized basaltic glass and zeolitic tuff, and appears to have been squeezed into cracks and other voids within the congealed margins of partly-cooled basalt pillows which intruded soft carbonate ooze. Thermal metamorphism probably was precluded by rapid heat dissipation in convecting pore waters and by the thermal stability of calcareous microplankton. Low temperature alteration of basaltic glass appears to have provided the chemical milieu for precipitation of calcite cement as well as zeolites in the sediment.  相似文献   

Stability Relations of the Pyroxenes and Olivine in Certain High Grade Metamorphic Rocks     

MUELLER  ROBERT F. 《Journal of Petrology》1966,7(3):363-374

The stability relations of mineral assemblages consisting ofvarious combinations of Capyroxene, orthopyroxene, olivine,quartz, dolomite, calcite, and graphite are treated by analyticalmeans with emphasis on reactions of the following type: MgSiO₃CaCO₃SiO₂CaMgSi₂O₆CO₂ (1) 3MgSiO₃CaCO₃ Mg₂SiO₄CaMgSi₂O₆CO₂ (2) and the associated reactions involving ferrous iron. In particular,reaction (1) is discussed in detail in terms of uncertaintiesinherent in the thermochemical data and the limits placed onits equilibrium constant by the geological conditions of theoccurrence of the observed assemblages. It is shown from theequation of equilibrium of (1) that the composition of orthopyroxeneis a linear function of the carbon dioxide fugacity and thatthe latter is lowered by the presence of the FeSiO₃ component.The latter conclusion is compatible with the rarity of the associationcalcite-orthopyroxcne and its apparent confinement to iron-richrocks. An analysis of the same assemblage involved in reaction (1)indicates that when graphite coexists with the iron-rich pyroxene,the fugacity of oxygen falls near the center of the magnetitefield for a wide range of CO₂ fugacities and temperatures. It is shown that reaction (2) is also displaced strongly tothe right under most metamorphic conditions, which is compatiblewith fairly common occurrence of the association of Capyroxeneand olivine. Finally, a critique is presented on the possibility of the useof the pyroxene-bearing assemblages as indicators of metamorphicgrade and it is concluded that this is not rigorously possiblesince the systems almost invariably possess too many degreesof freedom.  相似文献   

A REVIEW OF GRAIN-SIZE PARAMETERS   总被引：28，自引：0，他引：28  

ROBERT L. FOLK 《Sedimentology》1966,6(2):73-93

This paper comprises a review of the many graphical and mathematical techniques that have been proposed for the statistical summary of grain-size data. Satellitic problems, such as laboratory techniques, choice of size scales, and interpretation, are only considered briefly.  相似文献   

Grain scale simulations of loose sedimentary beds: the example of grain-bed impacts in aeolian saltation   总被引：4，自引：0，他引：4  

PETER K. HAFF  ROBERT S. ANDERSON† 《Sedimentology》1993,40(2):175-198

Sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role. In order to illustrate the modelling of collections of frictional, inelastic sedimentary grains with the particle dynamics method (PDM), we use the grain impact process in aeolian saltation as a specific example. In PDM, all the forces on each particle are evaluated at a sequence of small time-steps, and the Newtonian equations of motion are integrated forward in time. Interparticle forces at grain contacts are treated as springs with prescribed stiffness (normal force) and by a Coulomb friction law (tangential force); particle inelasticity is represented by spring damping. The granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties, and the integration time-step. We find that for the range of impact speeds and impactor masses relevant to aeolian settings, grain splashes are relatively insensitive to grain stiffness, grain inelasticity and grain friction, and that the pattern of ejection from the bed is largely controlled by bed microtopography. A large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process. The splash function representing these statistics is then available for use in calculations over longer time-scales, such as the evolution of the saltation curtain. The details given here will enable the interested reader to adapt PDM modelling to other types of clastic sedimentary systems.  相似文献   

Porosity evolution of the Cambrian Bonneterre Dolomite, south-eastern Missouri, USA     

JAY M. GREGG  PETER R. LAUDON†  ROBERT E. WOODY  KEVIN L. SHELTON‡ 《Sedimentology》1993,40(6):1153-1169

The Cambrian Bonneterre Dolomite of south-eastern Missouri, USA, hosts the large Mississippi Valley type (MVT) lead-zinc-copper ore deposits of the region. The Bonneterre Dolomite consists of dolomitized algal bioherms, oolitic grainstone and associated lithologies that were deposited on a carbonate platform surrounding the Precambrian age St Francois Mountains. Porosity was determined by point counting thin sections from cores and mines in the Bonneterre Dolomite and by gas porosimetry. Volumes of epigenetic cements were estimated by point counting cement filling micro- and mesoporosity using cathodoluminescence. Cement volumes were added to present porosity to estimate porosities during various stages of mineralization. Prior to the onset of mineralization, micro- and mesoporosity in the Bonneterre Dolomite averaged approximately 19%. Precipitation of early dolomite cement (roughly concurrent with the main period of sulphide deposition) reduced average porosity to approximately 7% and closed off much of the intercrystalline pore space. Later cementation by dolomite (prior to late stage sulphides) reduced porosity to approximately 5%, and late cementation by quartz further reduced porosity to the present average value of <4%. Periods of carbonate dissolution during MVT mineralization enhanced large scale megaporosity associated with fractures and breccias but did not significantly increase smaller scale porosity. Dolomite cementation associated with MVT mineralization, porosity and permeability were facies controlled. Today, as a result of mineralization, large scale fractures and breccias control porosity and permeability. This study indicates that dolomite porosity may undergo significant change during basinal brine migrations associated with MVT mineralization.  相似文献   

DIFFERENTIAL ROCK WEATHERING IN THE 'VALLEY OF THE BOULDERS', KÄRKEVAGGE, SWEDISH LAPLAND   总被引：1，自引：1，他引：0  

ROBERT G. DARMODY  COLIN E. THORN  JOHN C. DIXON 《Geografiska Annaler: Series A, Physical Geography》2008,90(3):201-209

Kärkevagge is an alpine valley in the low arctic of Swedish Lapland. It is named after, and famous for, its large deposit of immense (c. 10–15 m) boulders that almost fill the lower valley. Above the boulder deposit, on the flanks of the valley, are more recent and generally much smaller (c. 1–3 m) individual boulders that have fallen from the valley-wall cliff face, presumably from post-glacial valley-side unloading. Some of these smaller boulders are seemingly fresh and unweathered while others have been reduced to no more than mounds in the tundra. These boulders must be younger than the larger, lower giant boulder deposit, but are not particularly recent rockfalls as they are partially buried in colluvium. Comparisons of mineralogy and chemistry indicate that the possibility exists that the incompetent, 'rotten' rocks, if not considerably older than their competent neighbors, are inherently self-destructive. They have evidence of increased sulfur content, which is a proxy for pyrite, a known weathering accelerant in Kärkevagge.  相似文献   

A Response to Bradwell's Commentary on Recent Statistical Studies in Lichenometry     

VINCENT JOMELLI  PHILIPPE NAVEAU  DANIEL COOLEY  DELPHINE GRANCHER  DANIEL BRUNSTEIN  ANTOINE RABATEL 《Geografiska Annaler: Series A, Physical Geography》2010,92(4):485-487

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EFFECTS OF CLIMATE CHANGE ON INLAND WATERS OF THE PACIFIC COASTAL MOUNTAINS AND WESTERN GREAT BASIN OF NORTH AMERICA     

JOHN M. MELACK  JEFF DOZIER  CHARLES R. GOLDMAN  DAVID GREENLAND  ALEXANDER M. MILNER  ROBERT J. NAIMAN 《水文研究》1997,11(8):971-992

The region designated as the Pacific Coastal Mountains and Western Great Basin extends from southern Alaska (64°N) to southern California (34°N) and ranges in altitude from sea level to 6200 m. Orographic effects combine with moisture-laden frontal systems originating in the Pacific Ocean to produce areas of very high precipitation on western slopes and dry basins of internal drainage on eastern flanks of the mountains. In the southern half of the region most of the runoff occurs during winter or spring, while in the northern part most occurs in summer, especially in glaciated basins. Analyses of long-term climatic and hydrological records, combined with palaeoclimatic reconstructions and simulations of future climates, are used as the basis for likely scenarios of climatic variations. The predicted hydrological response in northern California to a climate with doubled CO₂ and higher temperatures is a decrease in the amount of precipitation falling as snow, and substantially increased runoff during winter and less in late spring and summer. One consequence of the predicted earlier runoff is higher salinity in summer and autumn in San Francisco Bay. In saline lakes, the incidence of meromixis and the associated reduction in nutrient supply and algal abundance is expected to vary significantly as runoff fluctuates. In subalpine lakes, global warming will probably will lead to increased productivity. Lacustrine productivity can also be altered by changes in wind regimes, drought-enhanced forest fires and maximal or minimal snowpacks associated with atmospheric anomalies such as El Niño–Southern Oscillation (ENSO) events. Reduced stream temperature from increased contributions of glacial meltwater and decreased channel stability from changed runoff patterns and altered sediment loads has the potential to reduce the diversity of zoobenthic communities in predominately glacier-fed rivers. Climatic warming is likely to result in reduced growth and survival of sockeye salmon in freshwater, which would, in turn, increase marine mortality. Further research activities should include expanded studies at high elevations and of glacier mass balances and glacial runoff, applications of remote sensing to monitor changes, further refinement of regional climatic models to improve forecasts of future conditions and continued analyses of long-term physical, chemical and biological data to help understand responses to future climates. © 1997 John Wiley & Sons, Ltd.  相似文献