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排序方式: 共有178条查询结果,搜索用时 31 毫秒
81.
Etienne Balan Simon Delattre Damien Roche Loïc Segalen Guillaume Morin Maxime Guillaumet Marc Blanchard Michele Lazzeri Christian Brouder Ekhard K. H. Salje 《Physics and Chemistry of Minerals》2011,38(2):111-122
The crystallinity of natural and synthetic apatite samples is often determined from the broadening of ν
4 PO4 infrared absorption bands. However, various physical mechanisms contribute to the observed linewidth. In the present study,
the factors determining the linewidth in the powder spectrum of synthetic fluorapatite and hydroxyapatite samples are investigated.
The temperature dependence of the infrared spectrum (10–270 K) is used to assess the respective contributions of homogeneous
broadening, related to the decay of phonons through anharmonic coupling, and heterogeneous broadening related to elastic strain
and macroscopic electrostatic effects. This latter contribution is dominant in the investigated samples and depends on the
shape of powder particles. It is discussed under the light of the theoretical modeling of the low-frequency dielectric properties
of apatite based on first-principles density functional theory calculations. The linewidth of the weak ν
1 PO4 absorption band provides a reliable information on microscopic sources of broadening, i.e., apatite crystallinity. In comparison,
the other more intense PO4 bands are more sensitive to long-range electrostatic effects. 相似文献
82.
The structural and sedimentological study of fault-propagation folds in Southern Tunisia highlights a special geometry of the growth strata (strata deposited simultaneously with the formation or growth of a fold). This distinct geometry is visible in the uppermost growth-strata beds and consists of one flank with unconformity as opposed to the other flank with perfect conformity. This geometry can be explained by the mechanism of fault-propagation folding, with asymmetrical flank dips and hinge migration kinematics. This kinematics was originally predicted by the fault-propagation fold model, which facilitates the study of this special geometry in a narrow domain of sedimentation-to-shortening ratios. A plot projection provides a generalisation of the results of all types of fault-propagation folds by revealing the expected geometry of the growth strata. This study constitutes one of the most complete examples of kinematic model validation on a field scale. 相似文献
83.
The purpose of the study was to provide further chronological data for the microlithic assemblages in the region of the Dogon Country (Mali). In two sites, Kobo 1 and L'Abri aux Vaches, the absence of any characteristic material could not help in concluding whether the microliths were of Pleistocene or Holocene age. Radiocarbon dating failed because the analysed charcoals appeared to have been introduced following recent bioturbation. These were visible in the archaeological sections and therefore, an attempt at dating with single-grain optically stimulated luminescence was made. After discussing the appropriate way of applying the Finite Mixture Model, we are able to conclude that the sediments containing the microliths from L'Abri aux Vaches were deposited during the beginning and the middle of the Holocene period (13.5 ± 1.0 and 5.9 ± 0.4 ka). For Kobo 1, the analysis reveals extensive bioturbation, precluding the obtaining of any reliable result. 相似文献
84.
Xuefeng Sun Norbert Mercier Christophe Falgueres Jean-Jacques Bahain Jackie Despriee Grégory Bayle Huayu Lu 《Quaternary Geochronology》2010,5(2-3):342-347
The site of Bonneval (France) exhibits a 6 m thick stratigraphical sequence in which Middle and Lower Palaeolithic artefacts were discovered. The stratigraphy of the site consists of an alternation of loess-like silt layers deposited along glacial stages, and paleosoil horizons corresponding to the vegetal stabilization of the area during interglacials. These layers hence provide past environmental informations and can be used as stratigraphical markers and compared with well dated sequences of Northern France. Eight sediment samples from this Middle and Upper Pleistocene sequence were dated using the recuperated optically stimulated luminescence (Re-OSL) signal of quartz grains. The middle-size fraction (40–63 μm) shows a good reproducibility of the OSL signals which allows the application of a multiple-aliquot additive-dose approach for De determination. For all samples, the growth curves are linear up to an accumulated dose of 3000 Gy. The Re-OSL ages range from ~100 to ~450 ka and are in good agreement with the independent chronological information; they establish when handaxe industries have been made during the Middle and Upper Pleistocene in the Paris Basin. 相似文献
85.
Geotectonics - Through several stations on the forelimb of the Es Satah anticline belonging to Gafsa basin part of the southern Tunisian Atlas, an analysis of the striations encountered on the... 相似文献
86.
It has been shown earlier that energy balance processes play a very important role in the determination of the reconnection regime in the central diffusive region of a steady Petschek flow (usually considered elsewhere as isothermal and incompressible): as a consequence of the plasma thermal properties, abrupt transitions in the reconnection regime may occur for special external conditions. The regime becomes then a dynamical one, and it was suggested that onset of plasma microturbulence may result and act as a primary triggering mechanism in solar flares.In this paper we will reexamine the problem of onset of such dynamical transition and conclude that plasma microturbulence does not appear in a straightforward way. However it is possible that the canonical Petschek regime may evolute into a new one in which the dissipative sheet is no longer infinitesimal with respect to the dimensions of the structure, and in which gravity plays an important role. Flare triggering, if related to the reconnection regime, must then proceed by more complex processes, possibly related to tearing mode dynamics, or to more global properties of the magnetic structure of the active region. 相似文献
87.
C. Mercier 《Solar physics》1975,45(1):169-179
On July 1,1971, about ten groups of type III radiobursts were observed with the 169 MHz Nançay radioheliograph. Each group consisted of two or several bursts exhibiting short delays from east to west (~0.5 s) and separated by distance of ~ 1.5 × 105 km. Explanations by simultaneous emission at the fundamental and harmonic of the local plasma frequency, or by reflexion of electromagnetic radiations at dense regions are shown to be inconsistent with the observations. We suggest that distinct exciters are simultaneously accelerated at low levels in very close regions and propagate upward along very widely diverging magnetic field lines. 相似文献
88.
K-rich glass-bearing wehrlite xenoliths from Yitong,Northeastern China: petrological and chemical evidence for mantle metasomatism 总被引:9,自引:1,他引:9
Y. Xu J.-C. C. Mercier Chuanyong Lin Lanbin Shi M. A. Menzies J. V. Ross B. Harte 《Contributions to Mineralogy and Petrology》1996,125(4):406-420
Ultramafic xenoliths in Cenozoic alkali basalts from Yitong, northeast China comprise three types in terms of their modal
mineralogy: lherzolite, pyroxenite and wehrlite. The wehrlite suite always contains interstitial pale/brown glass which occupies
several per cent by volume of the whole rock. The texture of the wehrlites is porphyroclastic with some large strained grains
of olivine (0.5–1 mm) scattered in a very fine grained matrix (0.1 mm), implying a metamorphic origin for the protolith rather
than an igneous origin. The host minerals are compositionally zoned, showing evidence of reaction with a melt. Petrological
evidence for resorption of spinel (lherzolite) and orthopyroxene (wehrlite) by infiltrating melt further supports the hypothesis
that the wehrlites result from interaction between a partial melting residue and a melt, which preferentially replaced primary
spinel, Cr-diopside and enstatite to produce secondary clinopyroxene (cpx) + olivine (ol) ± chromite ± feldspar (fd). The
composition of the mineral phases supports this inference and, further indicates that, prior to melt impregnation, the protoliths
of these wehrlites must have been subjected to at least one earlier Fe-enrichment event. This explanation is consistent with
the restricted occurrence of glasses in the wehrlite suite. The glass is generally associated with fine-grained (0.1 mm) minerals
(cpx+ol+chromite ±fd). Electron microprobe analyses of these glasses show them to have high SiO2 content (54–60 wt%), a high content of alkalis (Na2O, 5.6–8.0%; K2O, 6.3–9.0%), high Al2O3 (20–24%), and a depletion in CaO (0.13–2.83%), FeO (0.89–4.42%) and MgO (0.29–1.18%). Ion probe analyses reveal a light rare
earth element-enrichment in these glasses with chondrite normalised (La)n = 268–480. The high K2O contents in these glasses and their mode of occurrence argue against an origin by in-situ melting of pre-existent phases. Petrographic characteristics and trace element data also exclude the possibility of percolation
of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of
mantle metasomatic melts which may have been produced by partial melting of peridotites with involvement of deep-seated fluids.
Such melts may have been significantly modified by subsequent fractional crystallization of ol, cpx and sp, extensive reaction
with the mantle conduit and the xenolith transport process.
Received: 1 August 1995 / Accepted: 19 June 1996 相似文献
89.
John L Berkley H.Gassaway Brown Klaus Keil N.L Carter J-C.C Mercier Glenn Huss 《Geochimica et cosmochimica acta》1976,40(12):1429-1437
The Kenna ureilite was found in February, 1972 near the town of Kenna, Roosevelt County, New Mexico U.S.A., weighed 10.9 kg, and measured 26.7 × 14.7 × 14.2 cm; it is the seventh known ureilite. The meteorite is composed of xenoblastic olivine (Fo79.2), commonly rimmed by forsterite (Fo99), and pigeonite (En73Wo9Fs18), in a volumetric ratio of 3:1, set in a matrix of three carbon polymorphs (graphite, lonsdaleite, and diamond) plus nickel-iron metal and troilite. Some thin metalliferous veins penetrating silicate grains contain secondary inclusions of melt with high-calcium clinopyroxene (high-Ca, Mg-rich augite to augite), andesine, K-feldspar, chromite, and siliceous CaO- and alkali-rich glasses of variable compositions.Textural, mineralogical and fabric information suggest a complex history for Kenna, involving igneous, metamorphic and shock processes. The rock appears to have originated as an ultramafic cumulate whose texture and structure was modified by adcumulus processes and by solution and redeposition in a weak deviatoric stress field. A strong mineral elongation lineation was produced during this high-temperature phase accompanied by mild plastic deformation of olivine on the system 0kl[100]. Superimposed on this original texture and fabric are processes resulting from light to moderate (50–250 kbar) shock deformation, as manifested by fracturing of the silicates, slip parallel to (001) in olivine, and twin and translation gliding parallel to (100) in the clinopyroxene. Lonsdaleite and diamond probably formed during this shock phase, which may be associated with the break-up of the parent body, but the relative time of introduction of the carbon-rich matrix is still unresolved. 相似文献
90.
Both elastic and anelastic properties of the earth simultaneously play an important role in the high-attenuation, low-velocity zone (H.A.L.V.Z.), in the upper mantle, where the drop of velocity appears to be correlated with high attenuation. Internal friction studies and experiments suggest the possibility of relaxation mechanisms occurring within the H.A.L.V.Z. and clearly it is important to compare assumptions of relaxation processes with seismic data and investigate the consequences of such a model. A variety of physical explanations of the H.A.L.V.Z. are discussed and it is concluded that three mechanisms could produce the H.A.L.V.Z., namely partial melting, viscous grain-boundary effects and dislocation-impurity interactions. Only more experiments and more data on the Q-structure of the H.A.L.V.Z. would allow us to decide which is the most probable of these mechanisms. 相似文献