Role of granite intrusions for the formation of ring structures in granulite complexes: Examples from the Limpopo belt,South Africa     

L. L. Perchuk  D. D. van Reenen  C. A. Smit  R. Boshoff  G. A. Belyanin  V. O. Yapaskurt 《Petrology》2008,16(7):652-678

Ring structures are cylindrical or sheath folds with concentrically distributed beds, often with granites in the cores. This paper reports structural, petrographic, and petrological evidence for four such structures from the Central Zone of the Limpopo complex, which were formed during granulite exhumation in Neoarchean time (event D2/M2). It was demonstrated that the orientation of linear and planar elements in the rocks are practically identical and independent of the position within the Central Zone. All existing measurements (hundreds for the four structures) project within the same fields in stereograms. The fold axes plunge SW at an angle of ∼40° within the whole area of the Central Zone of the Limpopo complex. This implies that the rocks were metamorphosed and deformed during event D2/M2, which is typical of the Neoarchean stage of the development of the Limpopo granulite complex. Local mineral equilibria and fluid inclusions were studied in a series of key rocks, and P-T paths were derived for them. A gravitation mechanism was substantiated for the ascent of granulites and accompanying granite bodies. The structure of ring complexes was evaluated on the basis of various erosion sections. It was shown that stocklike granite bodies occur at the base of each ring structure. Petrochemical and structural data were used to demonstrate that the granites (2627 Ma) had been derived by the complete or partial melting of the lower parts of the section of Neoarchean (2651 Ma) country rocks. The upwelling of a less dense granite magma synchronously with the exhumation resulted in the helical squeezing of the overlying gneisses. This led to the concentric arrangement of beds and development of a ring structure, a sheath fold containing a granite core in some sections. A preliminary numeric 2D model is considered for the ascent of a granite diapir accompanied by the downwelling of colder and denser country rocks. A better understanding of this process can be gained by 3D numerical simulation.  相似文献   

Towards a Validation of Scintillometer Measurements: The LITFASS-2009 Experiment   总被引：3，自引：2，他引：1  

Frank Beyrich  Jens Bange  Oscar K. Hartogensis  Siegfried Raasch  Miranda Braam  Dani?lle van Dinther  Doreen Gr?f  Bram van Kesteren  Aline C. van den Kroonenberg  Bj?rn Maronga  Sabrina Martin  Arnold F. Moene 《Boundary-Layer Meteorology》2012,144(1):83-112

Scintillometry has been increasingly used over the last decade for the experimental determination of area-averaged turbulent fluxes at a horizontal scale of a few kilometres. Nevertheless, a number of assumptions in the scintillometer data processing and interpretation still call for a thorough evaluation, in particular over heterogeneous terrain. Moreover, a validation of the path-averaged structure parameters derived from scintillometer data (and forming the basis for the flux calculations) by independent measurements is still missing. To achieve this, the LITFASS-2009 field campaign has been performed around the Meteorological Observatory Lindenberg ?C Richard-A?mann-Observatory of the German Meteorological Service (DWD) in July 2009. The experiment combined tower-based in-situ turbulence measurements, field-scale laser scintillometers, long-range optical (large-aperture) and microwave scintillometers, and airborne turbulence measurements using an automatically operating unmanned aircraft. The paper describes the project design and strategy, and discusses first results. Daytime near-surface values of the temperature structure parameter, ${C_{T}^{2}}$ , over different types of farmland differ by more than one order of magnitude in their dependence on the type and status of the vegetation. Considerable spatial variability in ${C_{T}^{2}}$ was also found along the flight legs at heights between 50 and 100?m. However, it appeared difficult to separate the effects of heterogeneity from the temporal variability of the turbulence fields. Aircraft measurements and scintillometer data agreed in magnitude with respect to the temporal variation of the path-averaged ${C_{T}^{2}}$ values during the diurnal cycle. The decrease of ${C_{T}^{2}}$ with height found from the scintillometer measurements close to the surface and at 43?m under daytime convective conditions corresponds to free-convection scaling, whereas the aircraft measurements at 54 and 83?m suggest a different behaviour.  相似文献   

Seasonality of tropospheric ozone and water vapor over Delhi,India: a study based on MOZAIC measurement data     

Lokesh K. Sahu  Shyam Lal  Valérie Thouret  Herman G. Smit 《Journal of Atmospheric Chemistry》2009,62(2):151-174

Tropospheric distributions of ozone (O₃) and water vapor (H₂O) have been presented based on the Measurements of OZone and water vapor by Airbus In-Service AirCraft (MOZAIC) data over the metro and capital city of Delhi, India during 1996–2001. The vertical mixing ratios of both O₃ and H₂O show strong seasonal variations. The mixing ratios of O₃ were often below 40 ppbv near the surface and higher values were observed in the free troposphere during the seasons of winter and spring. In the free troposphere, the high mixing ratio of O₃ during the seasons of winter and spring are mainly due to the long-range transport of O₃ and its precursors associated with the westerly-northwesterly circulation. In the lower and middle troposphere, the low mixing ratios of ∼20–30 ppbv observed during the months of July–September are mainly due to prevailing summer monsoon circulation over Indian subcontinent. The summer monsoon circulation, southwest (SW) wind flow, transports the O₃-poor marine air from the Arabian Sea and Indian Ocean. The monthly averages of rainfall and mixing ratio of H₂O show opposite seasonal cycles to that of O₃ mixing ratio in the lower and middle troposphere. The change in the transport pattern also causes substantial seasonal variation in the mixing ratio of H₂O of 3–27 g/kg in the lower troposphere over Delhi. Except for some small-scale anomalies, the similar annual patterns in the mixing ratios of O₃ and H₂O are repeated during the different years of 1996–2001. The case studies based on the profiles of O₃, relative humidity (RH) and temperature show distinct features of vertical distribution over Delhi. The impacts of long range transport of air mass from Africa, the Middle East, Indian Ocean and intrusions of stratospheric O₃ have also been demonstrated using the back trajectory model and remote sensing data for biomass burning and forest fire activities.  相似文献   

Determination of humidity and temperature fluctuations based on MOZAIC data and parametrisation of persistent contrail coverage for general circulation models     

K. M. Gierens  U. Schumann  H. G. J. Smit  M. Helten  G. Zangl 《Annales Geophysicae》1997,15(8):1057-1066

Humidity and temperature fluctuations at pressure levels between 166 and 290 hPa on the grid scale of general circulation models for a region coveredn by the routes of airliners, mainly over the Atlantic, have been determined by evaluation of the data obtained with almost 2000 flights within the MOZAIC programme. It is found that the distributions of the fluctuations cannot be modelled by Gaussian distributions, because large fluctuations appear with a relatively high frequency. Lorentz distributions were used for the analytical representation of the fluctuation distributions. From these a joint probability distribution has been derived for simultaneous temperature and humidity fluctuations. This function together with the criteria for the formation and persistence of contrails are used to derive the maximum possible fractional coverage of persistent contrails in a grid cell of a GCM. This can be employed in a statistical formulation of contrail appearance in a climate model.  相似文献   

Proximal impact deposits at the Cretaceous-Tertiary boundary in the Gulf of Mexico: a restudy of DSDP Leg 77 Sites 536 and 540     

Alvarez W  Smit J  Lowrie W  Asaro F  Margolis SV  Claeys P  Kastner M  Hildebrand AR 《Geology》1992,20(8):697-700

Restudy of Deep Sea Drilling Project Sites 536 and 540 in the southeast Gulf of Mexico gives evidence for a giant wave at Cretaceous-Tertiary boundary time. Five units are recognized: (1) Cenomanian limestone underlies a hiatus in which the five highest Cretaceous stages are missing, possibly because of catastrophic K-T erosion. (2) Pebbly mudstone, 45 m thick, represents a submarine landslide possibly of K-T age. (3) Current-bedded sandstone, more than 2.5 m thick, contains anomalous iridium, tektite glass, and shocked quartz; it is interpreted as ejecta from a nearby impact crater, reworked on the deep-sea floor by the resulting tsunami. (4) A 50-cm interval of calcareous mudstone containing small Cretaceous planktic foraminifera and the Ir peak is interpreted as the silt-size fraction of the Cretaceous material suspended by the impact-generated wave. (5) Calcareous mudstone with basal Tertiary forams and the uppermost tail of the Ir anomaly overlies the disturbed interval, dating the impact and wave event as K-T boundary age. Like Beloc in Haiti and Mimbral in Mexico, Sites 536 and 540 are consistent with a large K-T age impact at the nearby Chicxulub crater.  相似文献   

On the Exchange of NO3 Radicals with Aqueous Solutions: Solubility and Sticking Coefficient     

Karin Thomas  Andreas Volz-Thomas  Djuro Mihelcic  Herman G. J. Smit  Dieter Kley 《Journal of Atmospheric Chemistry》1998,29(1):17-43

The exchange of NO₃ radicals with the aqueous-phase was investigated at room temperature (293 K) in a series of wetted denuders. From these experiments, the uptake coefficient of NO₃ was determined on 0.1 M NaCl solutions and was found to be (NO₃) 2 × 10^-3 in good agreement with recent studies. The Henry coefficient of NO₃ was estimated to be K_H(NO₃) = 1.8 M · atm^-1, with a (2) uncertainty of ±3 M · atm^-1. From the upper limit for the Henry coefficient (K_H = 5 M · atm^-1) and available thermodynamic data, the redox potential of dissolved NO₃/NO ₃ ^– is estimated to be in the range of 2.3 to 2.5 V. This range is at the lower boundary of earlier estimates. The results are discussed in the light of a recent publication. Based on our data and a model of the transport and chemistry in the liquid film, an upper limit is derived for the product of the Henry coefficient K_H and the rate coefficient k ₁₀ of the potential reaction NO₃ + H₂O HNO₃ + OH. For K_H = 0.6 M · atm^-1, we find k ₁₀ < 0.05 s^-1 · atm^-1, i.e., about 100 times smaller than what was suggested by Rudich and co-workers. Because of its small solubility, heterogeneous removal of NO₃ is only important under conditions where the dissolved NO₃ is removed quickly from equilibrium, for example by reactions with Cl^– or HSO ₃ ^– ions in the liquid-phase. Otherwise, heterogenous removal should mainly proceed via N₂O₅.  相似文献   

Towards Earth AntineutRino TomograpHy (EARTH)     

R. J. de Meijer  F. D. Smit  F. D. Brooks  R. W. Fearick  H. J. Wörtche  F. Mantovani 《Earth, Moon, and Planets》2006,99(1-4):193-206

The programme Earth AntineutRino TomograpHy (EARTH) proposes to build ten underground facilities each hosting a telescope. Each telescope consists of many detector modules, to map the radiogenic heat sources deep in the interior of the Earth by utilising direction sensitive geoneutrino detection. Recent hypotheses target the core-mantle boundary (CMB) as a major source of natural radionuclides and therefore of radiogenic heat. A typical scale of the processes that take place at the CMB is about 200 km. To observe these processes from the surface requires an angular resolution of about 3°. EARTH aims at creating a high-resolution 3D-map of the radiogenic heat sources in the Earth’s interior. It will thereby contribute to a better understanding of a number of geophysical phenomena observed at the Earth’s surface. This condition requires a completely different approach from the monolithic detector systems as e.g. KamLAND. This paper presents, for such telescopes, the boundary conditions set by physics, the estimated count rates, and the first initial results from Monte-Carlo simulations and laboratory experiments. The Monte-Carlo simulations indicate that the large volume telescope should consist of detector modules each comprising a very large number of detector units, with a cross section of roughly a few square centimetres. The signature of an antineutrino event will be a double pulse event. One pulse arises from the slowing down of the emitted positron, the other from the neutron capture. In laboratory experiments small sized, ¹⁰B-loaded liquid scintillation detectors were investigated as candidates for direction sensitive, low-energy antineutrino detection.  相似文献   

Trace-element patterns at the Cretaceous-Tertiary boundary—Consequences of a large impact     

J. Smit  W.G.H.Z. ten Kate 《Cretaceous Research》1982,3(3):307-332

The earlier discovery of high concentrations of iridium and osmium at the very base of the Tertiary in marine pelagic sequences, has led to the hypothesis that the impact of a large projectile on Earth terminated the Mesozoic era.This paper describes the distribution of other trace elements in samples closely spaced around the Cretaceous-Tertiary boundary. We found an enrichment of Co, Cr, Ni, As, Sb and Se, and a depletion of the rare earth elements (REE), coupled with the anomalously high concentration of Ir and Os. Also, in the enrichment layer, problematic sanidine spherules occur which may be spheres, ablated from the impacting projectile or microtektites.The REE depletion could not only be demonstrated in the basal millimetres of the relatively thick (～ 10 cm) boundary marl bed of the Barranco del Gredero section, Caravaca, Spain, but also in the boundary clay of the Stevns Klint section—the “Fiskeler”—Denmark. In these boundary clay beds are also the highest concentrations of Ir recorded [44 and 86.7 ppb (1 ppb = 10^?9 g/g), respectively].This REE depletion is tentatively connected with the low REE concentration in common meteorites. In that case the basal millimetres are dominated by a meteoritic component and represent a “fall-out” layer of the impact event. This in turn would imply low terrestrial dilution at impact on Earth, suggesting that the projectile might have fallen into the ocean.Identification of the projectile remains speculative; especially the high As, Sb and Se anomalies are problematic. These latter may point to a projectile of uncommon composition, possibly a comet.An integrated scenario of a possible sequence of events is presented, chiefly based on the interpretation of micropaleontological, lithological, geochemical and stable isotope data of the studied sections.The direct and indirect effects of an impact leading to mass extinction constitute a complicated problem and may comprise a sudden temperature rise, a poisoning by cyanide or arsenic, and a few years suppression of sunlight, or preferably, a combination of these effects.  相似文献   

Variability of the Structure Parameters of Temperature and Humidity Observed in the Atmospheric Surface Layer Under Unstable Conditions     

Miranda Braam  Arnold F. Moene  Frank Beyrich 《Boundary-Layer Meteorology》2014,150(3):399-422

The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as $\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$ ) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of $\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$ depends on instability and on the size of the averaging window over which $\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$ is calculated. If instability increases, differences in $\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$ between upward motions (large $C_{s}^2$ ) and downward motions (small $C_{s}^2$ ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of $\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$ decrease. A linear regression of the variances of $\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$ versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in $C_{s}^2$ along a scintillometer path or aircraft flight leg are just within the range of local variability in $C_{s}^2$ or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.  相似文献