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A Laplace-transform analytic element method (LT-AEM) is described for the solution of transient flow problems in porous media. Following Laplace transformation of the original flow problem, the analytic element method (AEM) is used to solve the resultant time-independent modified Helmholtz equation, and the solution is inverted numerically back into the time domain. The solution is entirely general, retaining the mathematical elegance and computational efficiency of the AEM while being amenable to parallel computation. It is especially well suited for problems in which a solution is required at a limited number of points in space–time, and for problems involving materials with sharply contrasting hydraulic properties. We illustrate the LT-AEM on transient flow through a uniform confined aquifer with a circular inclusion of contrasting hydraulic conductivity and specific storage. Our results compare well with published analytical solutions in the special case of radial flow. 相似文献
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Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe,Tanzania 总被引：1，自引：0，他引：1
Tanya Furman 《Contributions to Mineralogy and Petrology》1995,122(1-2):97-115
This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer
the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites
and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern
portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K2O/Na2O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An82–90), clinopyroxene (4.5–9.5 wt% Al2O3), and olivine (Fo79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances
of incompatible elements (e.g., 0.7–2.2 wt% P2O5) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout
the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global
Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent
correlation with P2O5/Al2O3, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element
abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite,
and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO2-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas
from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province
is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the
source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range
in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting
that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric
mantle.
Received: 30 May 1994 / Accepted: 5 April 1995 相似文献
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CL de Moor SJ Johnston A Brandão RA Rademeyer JP Glazer LB Furman 《African Journal of Marine Science》2015,37(3):285-311
The waters off South Africa's coastline boast a rich mix of commercially fished species. Quantitative assessments of these marine resources have developed from simple methods first applied in the 1970s, to models that encompass a wide range of methodologies. The more valuable resources have undergone regular assessments in recent decades, with frequencies closely related to the management approach employed for each fishery. Many of these assessments form the operating models used to simulation-test candidate management procedures. This paper provides a comprehensive review of the assessments of 11 of the most important fisheries resources in South Africa. Some assessments use simple biomass dynamics models, whereas others are a hybrid of age- and length-based models, each designed to model the specific characteristics of the resource and fishery concerned. Many of the assessments have been disaggregated by species/stock and/or area as related multispecies/stock/ distribution hypotheses have arisen. This paper explores the similarities and differences in the data available and the methods applied. The review indicates that, whereas the status of three of these resources cannot be estimated reliably at present, the status of six resources is considered to be reasonable to good, whereas that of abalone Haliotis midae and West Coast rock lobster Jasus lalandii remains poor. 相似文献
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This study examined the stream chemistry changes in Staunton River (a second‐order headwater stream with an average annual discharge 704 m3 ha?1 yr?1, Shenandoah National Park, Virginia) resulting from a catastrophic flood in June 1995. This flood, which followed after 800 mm of rain in a 4‐day period, caused large‐scale debris flows and complete scouring of riparian soils down to bedrock in the lower 2 km of the stream, and has been estimated to be a 1000‐year flood. The flood affected stream chemistry on both short‐ and long‐term time scales. The primary short‐term response was elevations in stream concentration of Ca2+, Mg2+, and K+ by 59%, 87%, and 49%, respectively, for 6 months immediately following the flood. The long‐term impact of decreased concentration of all base cations and SiO2 during summer months (8% average) lasted about 2 years. At the episodic time scale, Ca2+, Mg2+, and K+ flushed from soil sources during pre‐flood storms while Na+ and SiO2 diluted; these trends generally reversed during post‐flood storms for 2 years. Short‐term effects are attributed to the leaching of unconsolidated soil and upturned organic matter that clogged the streambed after the flood. The long‐term and superimposed episodic impacts may have resulted from the loss of riparian soils and vegetation in the flood. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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The problem of the theoretical computation of the emission intensities and ion composition in a weak aurora which has been preceded by a stronger event is examined. For this purpose a model auroral precipitation consisting of biexponential primaries is considered. The softer of the two components is brighter, and begins to decay after remaining steady for ten to fifteen minutes. The other, harder component starts to build up at that instant. Our results suggest that at least a part of the high ratios could be attributed to the retention, by the atmosphere, of the memory of previous auroral precipitations. Thus, the serious energy paradox in the context of 1·27 μ intensity need not arise, and, in the context of the large NO+ density, it may perhaps be unnecessary to invoke any major conversion of O2 to NO thus avoiding the associated energy problem. 相似文献
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The Kostomuksha greenstone belt is the main iron district of Karelia. Gold, diamonds, and molybdenum mineralization have been found in this belt over the last two decades. The gold occurrences and deposits pertaining to the gold-sulfide and gold-quartz types were formed at the orogenic stage. The Taloveis deposit and the Faktorny and Berendei ore occurrences are located in the South Kostomuksha area and related to a complex of porphyritic diorite and granite porphyry. The West Ruvinvaara, Niemijarvi, Eastern, and Kurgelampi gold-sulfide and gold-sulfide-quartz occurrences are controlled by metasomatized shear zones and recrystallized sulfide ore hosted in the Shurlovaara and Ruvinvaara formations in the contact aureole of granite of the framework of the greenstone belt near its margin. The gold-arsenopyrite mineralization of the South Kostomuksha open pit is related to shear and metasomatic zones at the contact between the Kostomuksha Formation and helleflinta. Significant concentrations of gold are related to the complex of diorite and granite porphyry and the shear and metasomatic zones formed at the collision stage after emplacement of potassium granite. The small Taloveis deposit is located in the southern portion of the Kostomuksha greenstone belt and related to an intrusion of diorite and granite porphyry dated at 2720 Ma. The gold-sulfide mineral type in beresitized granite is combined with the gold-quartz type in shear zones trending at an azimuth of 20 NNE and coinciding in orientation with shear zones in the Kostomuksha belt. The Au grade varies from 0.5 to 60 g/t. 相似文献
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The Austurhorn intrusive complex in southeastern Iceland represents an exhumed Tertiary central volcano. The geometry of the intrusion and geochemistry of the mafic and felsic rocks indicate Austurhorn was a volcanic center analogous to Eyjafjallajökull and Torfajökull in Iceland's eastern neovolcanic zone (EVZ). Early transitional tholeiitic basalt magmatism at Austurhorn formed a shallow crustal chamber 5 km in diameter. Apparent rhythmic modal layering of, and intrusive contacts within, the gabbro indicate the mafic chamber was replenished frequently as it cooled and crystallized. Felsic activity postdated near-solidification of the gabbro; numerous granitic magmas intruded along gabbro margins and within the adjacent crust. Field relations indicate that infrequent felsic replenishment prevented convective mixing of the Austurhorn chamber during this time, although commingled mafic and felsic magmas are observed in an extensive net veined complex. Late stage mafic dikes intrude the entire complex, suggesting that magmatic heat was abundantly available throughout the evolution of the Austurhorn system. Plagioclase and clinopyroxene compositions in mafic through felsic rocks, including gabbros, support a model of progressive differentiation. Field relations constrain the felsic magmas to originate at P1 kbar, presumably by fractional crystallization. The structure and geochemistry of the Austurhorn intrusive complex suggest formation in an immature rift environment similar to the modern EVZ. The proposed rift segment was parallel to the western and eastern neovolcanic zones, and probably resulted from a reorganization of plate boundaries 7 Ma (Saemundsson 1979; Helgason 1985; Jancin et al. 1985) triggered by activity of the Iceland mantle plume. 相似文献
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