Multiple Tracers of Shallow Ground-Water Flow and Recharge in Hilly Loess     

R. O'Brien  C. K. Keller  J. L. Smith 《Ground water》1996,34(4):675-682

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Features of the distribution of azooxanthellata scleractinia (Anthozoa) on mid-pacific guyots     

N. B. Keller  I. G. Shcherba 《Oceanology》2006,46(2):238-241

Three species of azooxanthellate scleractinia were studied on two guyots of the East Mariana Basin. Two of them were encountered at depths 1000 m below their usual location. For one of the species, P. australiensis, the lower boundary of its existence is known to be confined to the Eocene. This allows us to indirectly confirm the post-Eocene time of the subsidence of the guyots studied below the zone of dwelling of the reefforming corals.  相似文献   

Diffusion along interphase boundaries and its effect on retrograde zoning patterns of metamorphic minerals     

L. M. Keller  C. A. Hauzenberger  R. Abart 《Contributions to Mineralogy and Petrology》2007,154(2):205-216

In this study we use two dimensional chemical patterns and numerical modeling to estimate the relative rates of chemical transport along interphase boundaries (ib) and through grain (s) interiors during retrograde Fe–Mg exchange between garnet and biotite at a garnet–biotite–quartz triple junction. We demonstrate that systematic variations in garnet–rim compositions, which are frequently observed along garnet–quartz interfaces, and deviations from concentric retrograde zoning patterns start to develop when chemical transport along the interphase boundaries becomes slow during cooling. The capacities for chemical transport along an interphase boundary depend on the product D ^ib K ^ib/s a, where D ^ib is the diffusion coefficient of the exchangeable components within the interphase boundary medium, K ^ib/s is the equilibrium partitioning coefficient between the cation exchange partners and the interphase boundary medium and a is the interphase boundary width. The model is applied to analyze the retrograde zoning patterns in garnets from the Mozambique belt (SE-Kenya), which cooled from 820°C at a rate of ca. 2°C/my. It is found that non-equilibrated compositions in garnet along garnet/quartz interphase boundaries started to develop below 700°C due to insufficient rates of chemical transport along these boundaries. The transport capacities of garnet/quartz interphase boundaries was estimated to have been between about 1E-23 cm³/s (575°C) and 1E-20 cm³/s (700°C) from modeling the observed X _Fe pattern in garnet close to a garnet–quartz–biotite triple junction and relying on published data on the diffusivity of Fe²⁺ in garnet. Similar transport capacities are obtained; when the interphase boundary is assumed to be filled with a material that has the transport properties and chemical composition of a free melt in equilibrium with garnet, biotite and quartz at the respective conditions. In contrast, if the transport properties of the interphase boundary medium are related to the diffusivities and solubility of Fe²⁺ and FeOH⁺ within a free aqueous solution, chemical transport along the interphase boundaries would be much more efficient, and exchange equilibrium would have been maintained during the entire cooling history of the rocks. The observation of systematic deviations from local equilibrium along the garnet–quartz interphase boundaries leads us to exclude the presence of an aqueous fluid along the interphase boundary at any time during cooling.  相似文献   

Deformation, mass transfer and mineral reactions in an eclogite facies shear zone in a polymetamorphic metapelite (Monte Rosa nappe, western Alps)   总被引：7，自引：0，他引：7  

L. M. Keller  R. Abart  H. Stünitz  C. De Capitani 《Journal of Metamorphic Geology》2004,22(2):97-118

This study analyses the mineralogical and chemical transformations associated with an Alpine shear zone in polymetamorphic metapelites from the Monte Rosa nappe in the upper Val Loranco (N‐Italy). In the shear zone, the pre‐Alpine assemblage plagioclase + biotite + kyanite is replaced by the assemblage garnet + phengite + paragonite at eclogite facies conditions of about 650 °C at 12.5 kbar. Outside the shear zone, only minute progress of the same metamorphic reaction was attained during the Alpine metamorphic overprint and the pre‐Alpine mineral assemblage is largely preserved. Textures of incomplete reaction, such as garnet rims at former grain contacts between pre‐existing plagioclase and biotite, are preserved in the country rocks of the shear zone. Reaction textures and phase relations indicate that the Alpine metamorphic overprint occurred under largely anhydrous conditions in low strain domains. In contrast, the mineralogical changes and phase equilibrium diagrams indicate water saturation within the Alpine shear zones. Shear zone formation occurred at approximately constant volume but was associated with substantial gains in silica and losses in aluminium and potassium. Changes in mineral modes associated with chemical alteration and progressive deformation indicate that plagioclase, biotite and kyanite were not only consumed in the course of the garnet‐and phengite‐producing reactions, but were also dissolved ‘congruently’ during shear zone formation. A large fraction of the silica liberated by plagioclase, biotite and kyanite dissolution was immediately re‐precipitated to form quartz, but the dissolved aluminium‐ and potassium‐bearing species appear to have been stable in solution and were removed via the pore fluid. The reaction causes the localization of deformation by producing fine‐grained white mica, which forms a mechanically weak aggregate.  相似文献   

Phase‐shifts in shear stress as an explanation for the maintenance of pool–riffle sequences     

Scott N. Wilkinson  Robert J. Keller  Ian D. Rutherfurd 《地球表面变化过程与地形》2004,29(6):737-753

The stability of the pool–rif?e sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of pool–rif?e sequence bars during high ?ows, offsetting gradual scour of rif?es and deposition in pools during low ?ows. Despite several attempts, reversal has rarely been recorded in ?eld measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase‐shifted with respect to the pool–rif?e sequence bedform pro?le, so that maximum shear stress occurs upstream of rif?e crests at high ?ow, and downstream at low ?ow. Such phase‐shifts produce gradients of shear stress that explain rif?e deposition, and pool scour, at high ?ow, in accord with sediment continuity. The proposal is supported by results of a one‐dimensional hydraulic model applied to the surveyed bathymetry of a pool–rif?e sequence in a straight reach of a gravel‐bed river. In the sequence studied, the upstream phase‐shift in shear stress at high ?ow was associated with variations in channel width, with width minima occurring upstream of rif?e crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of rif?e crests. Assuming that the width variation is itself the result of ?ow de?ection by rif?e crests at low ?ow, and associated bank‐toe scour downstream, low and high ?ow can be seen to have complementary roles in maintaining alluvial pool–rif?e sequences. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

Lower lithospheric structure beneath the Trans-European Suture Zone from POLONAISE''97 seismic profiles     

M. Grad  G. R. Keller  H. Thybo  A. Guterch  POLONAISE Working Group 《Tectonophysics》2002,360(1-4):153-168

The large-scale POLONAISE'97 seismic experiment investigated the velocity structure of the lithosphere in the Trans-European Suture Zone (TESZ) region between the Precambrian East European Craton (EEC) and Palaeozoic Platform (PP). In the area of the Polish Basin, the P-wave velocity is very low (Vp <6.1 km/s) down to depths of 15–20 km, and the consolidated basement (Vp5.7–5.8 km/s) is 5–12 km deep. The thickness of the crust is 30 km beneath the Palaeozoic Platform, 40–45 km beneath the TESZ, and 40–50 km beneath the EEC. The compressional wave velocity of the sub-Moho mantle is >8.25 km/s in the Palaeozoic Platform and 8.1 km/s in the Precambrian Platform. Good quality record sections were obtained to the longest offsets of about 600 km from the shot points, with clear first arrivals and later phases of waves reflected/refracted in the lower lithosphere. Two-dimensional interpretation of the reversed system of travel times constrains a series of reflectors in the depth range of 50–90 km. A seismic reflector appears as a general feature at around 10 km depth below Moho in the area, independent of the actual depth to the Moho and sub-Moho seismic velocity. “Ringing reflections” are explained by relatively small-scale heterogeneities beneath the depth interval from 90 to 110 km. Qualitative interpretation of the observed wave field shows a differentiation of the reflectivity in the lower lithosphere. The seismic reflectivity of the uppermost mantle is stronger beneath the Palaeozoic Platform and TESZ than the East European Platform. The deepest interpreted seismic reflector with zone of high reflectivity may mark a change in upper mantle structure from an upper zone characterised by seismic scatterers of small vertical dimension to a lower zone with vertically larger seismic scatterers, possible caused by inclusions of partial melt.  相似文献   

Buchbesprechungen     

Hans M. Keller  Peter Perret  H. R. Preisig  Peter Perret 《Aquatic Sciences - Research Across Boundaries》1979,41(1):161-163

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Xenoliths of lawsonite-ferroglaucophane rocks from a Quaternary volcano of Milos (Aegean Sea,Greece)     

Christoph Hoffman  Jörg Keller 《Lithos》1979,12(3):209-219

Xenoliths of metamorphic rocks with ferroglaucophane + albite + stilpnomelane + muscovite ± lawsonite ± pumpellyite are described fromn the rhylolitic tephra deposits of the Late Quaternary Phyriplaka Volcano, Milos, Aegean Island, Greece. The chemical bulk analyses reveal metabasaltic parent rocks with strongly modified chemical compositions. Unusual for common fresh basalt types is extreme Na₂O combined with low K₂O and rather low CaO. These are chemical characteristics of spilitic basalts. High iron in the bulk rock and very high Fe/Mg is probably the cause of extreme ferroglaucophane compositions. The metamorphic assemblages point to a high-p/low-T metamorphism in the pressure range 4–8 kb and with temperatures of about 350°C. Phyriplaka Volcano is likely to have only a shallow explosive center and ferroglaucophane rocks were ejected from the immediate basement of the island, which was uplifted after the metamorphic event.  相似文献   

Pools,riffles, and channelization   总被引：2，自引：0，他引：2  

Edward A. Keller 《Environmental Geology》1978,2(2):119-127

The addition of regularly spaced deeps (pools) and shallows (riffles) that provide a variety of flow conditions, areal sorting of stream-bed material, cover for wildlife, and a positive aesthetic experience, may be desirable in many channel projects. Such designs will reduce adverse environmental impacts of stream channel modifications. Analysis of variance for pool-to-pool spacing data suggests that there is no significant difference with respect to channel width between pools that form in natural streams and those in streams affected by a variety of human uses. Short of channelization, which changes the channel width, pools and riffles, within limits, are not particularly sensitive to environmental stress. Experiments in Gum Branch near Charlotte, North Carolina, support the hypothesis that channel form and process evolve in harmony and that manipulation of cross-channel morphology can influence the development of desired channel processes. Planned manipulation of its channel form induced Gum Branch to develop as desired. Morphologic stability consisting of incipient point bars, pools, and riffles was maintained over a period of high magnitude flood events, only to be degraded later by a wave of sediment derived from upstream construction and stream-bank failures. Thus, environmentally desirable channel morphology in urban streams cannot remain stable if changes in the sediment load or storm-water runoff exceed the limits of the stream's ability to make internal adjustments while maintaining morphologic stability.  相似文献   

Xenolithe in vulkanischen Gesteinen der Äolischen Inseln (Sizilien)     

Dr. José Honnorez  Dr. Jörg Keller 《International Journal of Earth Sciences》1968,57(3):719-736

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