Mg-metasomatism and formation conditions of Mg-chlorite-muscovite-quartzphyllites (leucophyllites) of the Eastern Alps (W. Hungary) and their relations to Alpine whiteschists   总被引：1，自引：0，他引：1  

Attila Demény  Zachary D. Sharp  Hans-Rudolf Pfeifer 《Contributions to Mineralogy and Petrology》1997,128(2-3):247-260

The lower Austroalpine orthogneiss-micachist complex of the Sopron-Fertörákos area of W. Hungary contains Mg-chlorite-muscovite-quartzphyllites (leuco- phyllite) and Mg-chlorite-bearing kyanite quartzites whose chemical compositions differ greatly from their surrounding rocks. Formation of leucophyllites took place in shear zones and was associated with depletion in alkalies and iron and enrichment of magnesium and H₂O. Mg-zonation of relict igneous muscovites of leucophyllites and changes in the whole rock chemical compositions suggest Mg-metasomatism. Material gains and losses have been assessed using the composition-volume relationship approach. Proceeding from metagranite through transition rocks to leucophyllites, MgO, H₂O, FeO, and alkalies show continuously increasing dispersion in isocon plots with Mg-enrichment even in sheared gneiss not in contact with leucophyllite. The metasomatic processes that formed the Mg-rich rocks may be similar to those responsible for the formation of high pressure whiteschists in the Central and Western Alps. The geochemical characteristics of the Dora Maira whiteschists (Italy) and their country gneisses are very similar to those of the Sopron leucophyllites, supporting the theory that Mg-metasomatism produced the whiteschist chemistry. On the basis of oxygen isotope compositions of relict igneous muscovites, the precursor granitic rock had a δ¹⁸O value around 13‰ proving its crustal anatectic origin. The leucophyllites have whole rock oxygen isotope compositions around 8.5‰ which is in conflict with the theory of an Mg-rich sedimentary protolith. Rather, the low δ¹⁸O values reflect fluid/rock interaction with a low δ¹⁸O fluid. Quartz-mineral oxygen isotope fractionations yield a metamorphic temperature of 560 ± 30 °C which agrees with earlier estimates from mineral stabilities. Silicon contents of phengites correspond to a metamorphic pressure of ～13 GPa at this temperature indicating eclogite facies metamorphism. The fluids in equilibrium with leucophyllites had oxygen isotope compositions around 7.9‰, similar to those calculated for the ultrahigh pressure Dora Maira whiteschists (7.6‰), further supporting the genetic link between the leucophyllites and whiteschists. Hydrogen isotope compositions of mixed white mica + chlorite samples from leucophyllites range from ?40 to ?35‰, correlating with chlorite contents. The calculated endmember chlorite and white mica have δD values of ?30 and ?40‰, respectively. The similar δD values of the white micas in leucophyllites, gneisses and metagranites suggest an overall equilibration with respect to H isotopes. The calculated δD value of the fluid is approximately 0‰, suggesting a seawater origin. This conclusion was also reached for the Dora Maira whiteschists. A possible fluid source that satisfies both metasomatic and isotopic data is dehydration of hydrothermally altered oceanic crust. The mafic–ultramafic complex of the Alpine Penninic unit underlying the Austroalpine nappes is a likely candidate. The subduction and subsequent dehydration of the ophiolite series would supply the Mg-rich fluids whose migration brought about the metasomatic alteration of the overlying gneiss-micaschist complexes.  相似文献   

A unified hydrogeological conceptual model of the Milk River transboundary aquifer,traversing Alberta (Canada) and Montana (USA)     

Marie-Amélie Pétré  Alfonso Rivera  René Lefebvre  M. Jim Hendry  Attila J. B. Folnagy 《Hydrogeology Journal》2016,24(7):1847-1871

A conceptual model of the transboundary Milk River Aquifer (MRA), extending across the Canada–USA border, was developed based on literature, focused fieldwork and a three-dimensional geological model. The MRA corresponds to the Virgelle Member of the Milk River Formation (Eagle Formation in Montana, USA) and it is an important groundwater resource over a large area (25,000 km²). The Virgelle outcrops near the international border and along the Sweet Grass Arch in Montana. The down-gradient limit of the MRA is the unconformity separating the Virgelle from the gas-bearing sandy shale of the Alderson Member. The MRA is confined above by the Pakowki/Claggett Formations aquitards and below by the Colorado Group aquitard. The MRA contains higher transmissivity areas resulting in preferential flowpaths, confirmed by natural geochemical tracers. Tritium and ¹⁴C delineate restricted recharge areas along the outcrops on both sides of the international border. Drastic decreases in horizontal hydraulic gradients indicate that the Milk River intercepts a large proportion of groundwater flowing to the north from the recharge area. Downgradient of the Milk River, groundwater movement is slow, as shown by ³⁶Cl residence times exceeding 1 Ma. These slow velocities imply that groundwater discharge downgradient of the Milk River is via vertical leakage through the Colorado Group and upward along buried valleys, which act as drains and correspond to artesian areas. When confined, the MRA contains a fossil groundwater resource, not significantly renewed by modern recharge. Groundwater exploitation thus far exceeds recharge, a situation requiring properly managed MRA groundwater depletion.  相似文献   

Water concentrations and hydrogen isotope compositions of alkaline basalt-hosted clinopyroxene megacrysts and amphibole clinopyroxenites: the role of structural hydroxyl groups and molecular water     

István Kovács  Attila Demény  György Czuppon  Christophe Lécuyer  Francois Fourel  Qun-Ke Xia  Jia Liu  Zsanett Pintér  Edit Király  Kálmán Török  Ábel Szabó  Etienne Deloule  György Falus  Tamás Fancsik  Zoltán Zajacz  Judit Sándorné Kovács  Beatrix Udvardi 《Contributions to Mineralogy and Petrology》2016,171(5):38

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Volcanic ash and tsunami record of the Minoan Late Bronze Age Eruption (Santorini) in a distal setting,southwestern Turkey     

Erkan Aydar  Attila ÇİNer  Orkun Ersoy  Emilie ÉCochard  Eric G. Fouache 《第四纪科学杂志》2021,36(4):586-597

We present the volcanic ash and tsunami record of the Minoan Late Bronze Age Eruption of Santorini (LBAES) in a distal setting in southwestern Turkey. In one of the drilled cores at the Letoon Hellenic antique site on Eşençay Delta, we encountered a 4 cm thick tephra deposit underlain by 46 cm thick tsunami-deposited sand (tsunamite), and an organic-rich layer that we ¹⁴C dated to 3295 ± 30 bp or 1633 bc. The relationship between Santorini distal volcanic ash and underlying tsunamite is described and interpreted. LBAES occurred in four main phases: (1) plinian; (2) phreatomagmatic; (3) phreatomagmatic with mudflows; and (4) ignimbritic flows and co-ignimbrite tephra falls. In this study, we aim to understand which eruptive phases generate distal ash during the Minoan eruptive sequence by examining the 3D surface morphology of ash formed by different fragmentation processes. To that end, we used numerous statistical multivariates, 3D fractal dimension of roughness, and a new textural parameter of surface area-3D/plotted area-2D to characterise the eruption dynamics. Based on ash surface morphologies and the calculated statistical parameters, we propose that that distal ash is represented by a single layer composed of well-mixed (coarse to fine) magmatic and phreatomagmatic ash.  相似文献   

Numerical sensitivity test of three-electrode laterolog borehole tool     

Márk Szijártó  László Balázs  Dezső Drahos  Attila Galsa 《Acta Geophysica》2017,65(4):701-712

Finite element numerical simulation has been carried out to investigate quantitatively the response of the three-electrode laterolog borehole tool (LL3) on radial and vertical heterogeneity of the rock. In order to calculate the apparent resistivity from the electric potential and the current discharge of the measurement electrode the probe coefficient of the LL3 tool with finite electrode extent was determined. Two independent methods, a finite element modeling and a semi-analytical solution, resulted in the probe coefficient of approx. 0.15 m with a relative deviation of 2.4% due to the different geometry, resolution and electronics of the models. It was established that LL3 is only slightly sensitive to the presence of mud when the borehole diameter is d ≤ 30 cm and the ratio of the resistivity of rock and the borehole mud is 1 ≤ R _t/R _m ≤ 1000. Vertical heterogeneity test pointed out that the layer boundaries can be localized exactly even for thin bedded layer (with a thickness of 1 m) and the presence of low-resistive borehole mud. Correction factors were suggested to decrease the biasing effect of the low-resistive borehole mud and the shoulder beds on the apparent resistivity observed by LL3. Finally, it was verified that the probe has large penetration depth with excellent vertical resolution, what explains the enduring popularity of the LL3 tool in well logging.  相似文献   

Modelling Last Glacial Maximum ice cap with the Parallel Ice Sheet Model to infer palaeoclimate in south-west Turkey     

Adem Candaş  M. Akif Sarikaya  Oğuzhan KÖSE  Ömer L. Şen  Attila Çiner 《第四纪科学杂志》2020,35(7):935-950

Modelling palaeoglaciers in mountainous terrain is challenging due to the need for detailed ice flow computations in relatively narrow and steep valleys, high-resolution climate estimations, knowledge of pre-ice topography, and proxy-based palaeoclimate forcing. The Parallel Ice Sheet Model (PISM), a numerical model that approximates glacier sliding and deformation to simulate large ice sheets such as Greenland and Antarctica, was recently adapted to alpine environments. In an attempt to reconstruct the climate conditions during the Last Glacial Maximum (LGM) on Mount Dedegöl in SW Turkey, we used PISM and explored palaeoglacier dynamics at high spatial resolution (100 m) in a relatively small domain (225 km²). Palaeoice-flow fields were modelled as a function of present temperature and precipitation. Nine different palaeoclimate simulations were run to reach the steady-state glacier extents and the modelled glacial areas were compared with the field-based and chronologically well-established ice extents. Although our results provide a non-unique solution, best-fit scenarios indicate that the LGM climate on Mount Dedegöl was between 9.2 and 10.6 °C colder than today, while precipitation levels were the same as today. More humid (20% wetter) or arid (20% drier) conditions than today bring the palaeotemperature estimates to 7.7–8.8 or 11.5–13.2 °C lower than present, respectively.  相似文献   

A framework for automatic modeling of underground excavations and optimizing three‐dimensional boundary and finite element meshes derived from them—framework     

Mohammad Hazegh  Attila M. Zsaki 《国际地质力学数值与分析法杂志》2013,37(6):641-660

Many problems in mining and civil engineering require using numerical stress analysis methods to repeatedly solve large models. Widespread acceptance of tunneling methods, such as New Austrian Tunneling Method, which depend heavily on numerical stress analysis tools and the fact that the effects of excavation at the face of a tunnel are distinctively three–dimensional (3D), necessitates the use of 3D numerical analysis for these problems. Stress analysis of a practical mining problem can be very lengthy, and the processing time can be measured in days or weeks at times. A framework is developed to facilitate efficient modeling of underground excavations and to create an optimal 3D mesh by reducing the number of surface and volume elements while keeping the result of stress analysis accurate enough at the region of interest, where a solution is sought. Fewer surface and volume elements mean fewer degrees of freedom in the numerical model, which directly translates into savings in computational time and resources. The mesh refinement algorithm is driven by a set of criteria that are functions of distance and visibility of points from the region of interest, and the framework can be easily extended by adding new types of criteria. This paper defines the framework, whereas a second companion paper will investigate its efficiency, accuracy and application to a number of practical mining problems. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Comparison of absorbing boundary conditions: A correlation of reflection coefficients and determination of the optimum grid interval for migration     

Attila Aydemir 《Journal of Applied Geophysics》2009,67(2):143-149

Modeling and migration couple is one of the most important steps in seismic data processing and interpretation. Absorbing boundary conditions used in the modeling were studied with the wave-equation by different authors. In this study, reflection coefficient analyses of recent solutions are compared to each other for the different incident angles of seismic waves to the modeling boundaries. According to the reflection coefficients correlation, the A3 condition is the most appropriate solution which greatly reduces artificial reflections from the boundaries. However, multi-transmitting Formula is better for relatively high angles between 32–58° with the usage of a special parameter. On the contrary, this formula is not an appropriate condition for angles lower than 32°, although it allows setting the boundary at any preferred angle. Considering that most of the boundaries are set in low angles, A3 solution is still most preferential condition. In this study, it is also aimed to find out the optimum grid intervals for minimizing the ill-posedness arose from the combination of the 45° finite difference migration equation and the B₂ absorbing boundary condition for migration. Appropriate values are determined as ωΔx = 0.2 and ωΔz = 0.4 or neighbouring coarser values. It is also concluded that finer mesh spacing can increase the ill-posedness, in contrast to existence of some fine grid size values providing well-posedness. In addition, ill-posedness is obviously standard after ωΔx = 0.6 for all values of ωΔz.  相似文献   

On the origin of solar cycle periodicity     

Attila Grandpierre 《Astrophysics and Space Science》1996,243(2):393-400

Recently, the origin of the solar cycle is considered to be rooted in the dynamics of the solar core (Grandpierre, 1996). The dynamic solar core model requires macroscopic flow and magnetic field as basic inputs. The macroscopic flow cannot be generated by the quasistatic solar structure and it has to reach a larger than critical size (Grandpierre, 1984) in order to survive dissipation. Therefore the flow must be generated by outer agents. The most significant outer agents to the Sun are the planets of the Solar System. These theoretical arguments are supported by observations showing that planetary tides follow a pattern correlating with the solar cycle in the last three and a half centuries (Wood, 1972; Desmoulins, 1995). Therefore the pulsating-ejecting solar core model gives a firm theoretical basis for the interpretation of these largely ignored observations. In this paper a new and simple calculation is presented which enlightens the planetary origin of the eleven-year period and gives a physical basis for a detailed modelling of the dynamo and the solar cycle.  相似文献   

Non-static fluid spheres in general relativity     

Attila Mészáros 《Astrophysics and Space Science》1985,116(2):415-417

We critically comment upon the paper: The properties of large particles in the zodiacal cloud and in the interstellar medium, and their relation to recent IRAS observations, by F. Hoyle and N. C. Wickramasinghe, which appeared inAstrophysics and Space Science 107 (1984) 224.  相似文献