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排序方式: 共有320条查询结果,搜索用时 15 毫秒
31.
Francesco Di Benedetto Francesco D’Acapito Gabriele Fornaciai Massimo Innocenti Giordano Montegrossi Luca A. Pardi Silvia Tesi Maurizio Romanelli 《Physics and Chemistry of Minerals》2010,37(5):283-289
An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of α-quartz, SiO2) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe2+ is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell: = 1.78(2) Å, the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe3+ variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe4+ is stabilised by the favourable local structural arrangement. 相似文献
32.
Guido Schreurs Jörg Giese Alfons Berger Edwin Gnos 《International Journal of Earth Sciences》2010,99(8):1827-1847
The Ranotsara shear zone in Madagascar has been considered in previous studies to be a >350-km-long, intracrustal strike-slip
shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara
shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment
using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the
Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent
NW–SE trending brittle faulting along most of its length. We therefore prefer the more neutral term “Ranotsara Zone”. Lithologies,
tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara
Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of
E–W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar
Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550–520 Ma). The Ranotsara Zone shows significant
NW–SE striking brittle faulting that reactivates part of the NW–SE striking ductile structures in the flexure zone, but also
extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment
of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin
underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central
segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern
Madagascar. It can therefore not be used as a piercing point in Gondwana reconstructions. 相似文献
33.
Gianluigi Di Paola Pietro Patrizio Ciro Aucelli Guido Benassai Germán Rodríguez 《Natural Hazards》2014,71(3):1795-1819
This paper presents a new method for coastal vulnerability assessment (CVA), which relies upon three indicators: run-up distance (as a measurement of coastal inundation), beach retreat (as a measurement of potential erosion), and beach erosion rate (obtained through the shoreline positions in different periods). The coastal vulnerability analysis of Sele Coastal Plain to storm impacts is examined along a number of beach profiles realized between 2008 and 2009. This particular study area has been selected due to its low-lying topography and high erosion propensity. Results are given in terms of an impact index, performed by combining the response due to coastal inundation, storm erosion, and beach erosion rate. This analysis is implemented on the basis of morphosedimentary characteristics of the beach, wave climate evaluation, and examination of multitemporal aerial photographs and topographic maps. The analysis of the final results evidences different coastal responses as a function of the beach width and slope, which in turn depend on the local anthropization level. The comparison of this method with a Coastal Vulnerability Index method evidences the better attitude of CVA index to take into account the different beach features to explain the experienced damages in specific stretches of the coastline considered. 相似文献
34.
C. M. Giordano A. R. Plastino A. Plastino 《Celestial Mechanics and Dynamical Astronomy》1996,66(2):229-242
Robe's restricted three-body problem is investigated with regards to the effects of a linear drag force. In particular. the stability of the model's equilibrium points is studied in this respect. Two scenarios are envisaged: the one originally discussed by Robe himself and the one suggested by him and recently analyzed by the present authors, that assumes for the fluid body the structure of a Roche's ellipsoid. 相似文献
35.
36.
Probabilistic estimates of future changes in California temperature and precipitation using statistical and dynamical downscaling 总被引:1,自引:0,他引:1
David W. Pierce Tapash Das Daniel R. Cayan Edwin P. Maurer Norman L. Miller Yan Bao M. Kanamitsu Kei Yoshimura Mark A. Snyder Lisa C. Sloan Guido Franco Mary Tyree 《Climate Dynamics》2013,40(3-4):839-856
Sixteen global general circulation models were used to develop probabilistic projections of temperature (T) and precipitation (P) changes over California by the 2060s. The global models were downscaled with two statistical techniques and three nested dynamical regional climate models, although not all global models were downscaled with all techniques. Both monthly and daily timescale changes in T and P are addressed, the latter being important for a range of applications in energy use, water management, and agriculture. The T changes tend to agree more across downscaling techniques than the P changes. Year-to-year natural internal climate variability is roughly of similar magnitude to the projected T changes. In the monthly average, July temperatures shift enough that that the hottest July found in any simulation over the historical period becomes a modestly cool July in the future period. Januarys as cold as any found in the historical period are still found in the 2060s, but the median and maximum monthly average temperatures increase notably. Annual and seasonal P changes are small compared to interannual or intermodel variability. However, the annual change is composed of seasonally varying changes that are themselves much larger, but tend to cancel in the annual mean. Winters show modestly wetter conditions in the North of the state, while spring and autumn show less precipitation. The dynamical downscaling techniques project increasing precipitation in the Southeastern part of the state, which is influenced by the North American monsoon, a feature that is not captured by the statistical downscaling. 相似文献
37.
Diffuse CO<Subscript>2</Subscript> degassing at Vesuvio,Italy 总被引:1,自引:0,他引:1
Francesco?FrondiniEmail author Giovanni?Chiodini Stefano?Caliro Carlo?Cardellini Domenico?Granieri Guido?Ventura 《Bulletin of Volcanology》2004,66(7):642-651
At Vesuvio, a significant fraction of the rising hydrothermal–volcanic fluids is subjected to a condensation and separation process producing a CO2–rich gas phase, mainly expulsed through soil diffuse degassing from well defined areas called diffuse degassing structures (DDS), and a liquid phase that flows towards the outer part of the volcanic cone. A large amount of thermal energy is associated with the steam condensation process and subsequent cooling of the liquid phase. The total amount of volcanic–hydrothermal CO2 discharged through diffuse degassing has been computed through a sequential Gaussian simulation (sGs) approach based on several hundred accumulation chamber measurements and, at the time of the survey, amounted to 151 t d–1. The steam associated with the CO2 output, computed assuming that the original H2O/CO2 ratio of hydrothermal fluids is preserved in fumarolic effluents, is 553 t d–1, and the energy produced by the steam condensation and cooling of the liquid phase is 1.47×1012 J d–1 (17 MW). The location of the CO2 and temperature anomalies show that most of the gas is discharged from the inner part of the crater and suggests that crater morphology and local stratigraphy exert strong control on CO2 degassing and subsurface steam condensation. The amounts of gas and energy released by Vesuvio are comparable to those released by other volcanic degassing areas of the world and their estimates, through periodic surveys of soil CO2 flux, can constitute a useful tool to monitor volcanic activity.Editorial responsibility: H. Shinohara 相似文献
38.
39.
Sandro Conticelli Sara Marchionni Davide Rosa Guido Giordano Elena Boari Riccardo Avanzinelli 《Contributions to Mineralogy and Petrology》2009,157(1):41-63
The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera
collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics
similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca.
400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera
Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic
rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes.
The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte
Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised
by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at
Cs, K, Th, U, and Pb. Initial values of 87Sr/86Sr range from 0.70926 to 0.70999, 143Nd/144Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for 206Pb/204Pb, 15.685–15.701 for 207Pb/204Pb, and 39.048–39.076 for 208Pb/204Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing
rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand,
shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks.
87Sr/86Sr ranges from 0.70665 to 0.70745, 143Nd/144Nd ranges from 0.51234 to 0.51238, 206Pb/204Pb ranges from 18.924 to 19.153, 207Pb/204Pb ranges from 15.661 to 15.694, and 208Pb/204Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase
leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic
magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic
agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the
veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional
isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature,
of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition
of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
40.
Matteo Correnti Francesco Paresce Rossella Aversa Giacomo Beccari Guido De Marchi Marcella Di Criscienzo Xiaoying Pang Loredana Spezzi Elena Valenti Paolo Ventura 《Astrophysics and Space Science》2012,340(2):263-279
We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (∼1–3 M⊙) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ∼2 to ∼30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20–30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East–North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction. 相似文献