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11.
Secular cooling and crystallization of partially molten Archaean continental crust over 1 Ga 总被引：1，自引：1，他引：0
Olivier Vanderhaeghe Célia Guergouz Cécile Fabre Stéphanie Duchêne David Baratoux 《Comptes Rendus Geoscience》2019,351(8):562-573
The protracted tectonic and magmatic record of cratons over the Archaean Eon has been classically interpreted in terms of long-lived shallow-dipping subduction or repeated mantle plumes. In this paper, we use the 1D conductive heat equation to model the evolution of the geotherm of a generic felsic-dominated Archaean cratonic nuclei solely considering the secular decay of radioactive isotopes (238U, 235U, 232Th, and 40K), responsible for heat production in the crust. Using a range of plausible parameters for crustal thickness, lithospheric thickness, and surface heat flux, this modelling shows that Archaean crust was characterized by an initially high geothermal gradient at 3.5 Ga, with a Moho temperature close to 900 °C, and that it might have remained partially molten for about one billion years. The existence of a partially molten crust for an extended period of time offers an alternative option to shallow-dipping subduction or repeated mantle plumes for the understanding of the peculiar tectonic evolution of Archaean cratons marked by (i) protracted high-temperature metamorphism and magmatism associated with crustal differentiation, and (ii) widespread deformation characterized by structural domes attributed to the development of crustal-scale gravitational instabilities. 相似文献
12.
The Fe-Mg thermometer widely used to infer the “equilibration” temperature of garnet-clinopyroxene assemblages of eclogites
records the progressive blocking of diffusion-limited exchanges between coexisting mineral phases. It is argued that equilibrium
is achieved through the fast grain boundary model in which Fe and Mg circulate in an interstitial medium fast enough for the
rate-limiting step to be the volume diffusion in each mineral phase. A semi-analytic solution is found and the influence of
the cooling history, grain size distribution, and rock composition on the temperature at which the Fe-Mg exchange between
garnet and clinopyroxene is frozen in are quantitatively evaluated. In particular, the model simulates the temperatures that
would be obtained from the concentration of Fe and Mg in the rim of adjacent garnet and clinopyroxene crystals such as those
commonly obtained by electron probe. For eclogites, the simulations show that correct peak temperatures are retrieved as long
as the temperature of the metamorphic climax does not exceed 650 °C. At higher climax temperatures, rim-rim temperatures underestimate
the peak temperature and cluster around 650 °C. Fast cooling, however, strongly limits diffusive equilibration. In crustal-type
eclogites, cooling rates of a few degrees per million years at 700 °C and of a few tens of degrees at 800 °C preserve the
record of the peak temperature. It is shown that these results are largely independent of the chemical composition of the
garnet and clinopyroxene. On the contrary, the mineral grain size and the respective proportions of garnet and clinopyroxene
have a major effect on retrogressive diffusion. For eclogites formed at a temperature of 800 °C and under conditions of slow
cooling, the record of the climax temperature is preserved by crystal rims as long as the volume of clinopyroxene is smaller
than that of garnet and crystal size is at least millimetric. The choice of a particular set of experimental data for the
diffusion coefficients is found to be immaterial. We suggest that the temperature of the metamorphic climax of eclogites is
best estimated from the composition of clinopyroxene rims combined with that of the inner edge of the diffusion boundary layer
fringing the coexisting garnet grains.
Received: 15 January 1998 / Accepted: 6 November 1998 相似文献
13.
S. Wolf F. Malbet R. Alexander J.-P. Berger M. Creech-Eakman G. Duchêne A. Dutrey C. Mordasini E. Pantin F. Pont J.-U. Pott E. Tatulli L. Testi 《Astronomy and Astrophysics Review》2012,20(1):1-83
We present a review of the interplay between the evolution of circumstellar disks and the formation of planets, both from the perspective of theoretical models and dedicated observations. Based on this, we identify and discuss fundamental questions concerning the formation and evolution of circumstellar disks and planets which can be addressed in the near future with optical and infrared long-baseline interferometers. Furthermore, the importance of complementary observations with long-baseline (sub)millimeter interferometers and high-sensitivity infrared observations is outlined. 相似文献
14.
Patrick Taillandier Cécile Duchêne Alexis Drogoul 《International journal of geographical information science》2013,27(12):1971-1999
Automating the generalisation process, a major issue for national mapping agencies, is extremely complex. Several works have proposed to deal with this complexity using a trial and error strategy. The performance of systems based on such a strategy is directly dependent on the quality of the control knowledge (i.e. heuristics) used to guide the trials. Unfortunately, most of the time, the definition and updation of knowledge is a fastidious task. In this context, automatic knowledge revision can not only improve the performance of the generalisation, but also allow it to automatically adapt to various usages and evolve when new elements are introduced. In this article, an offline knowledge revision approach is proposed, based on a logging of the system and on the analysis of outcoming logs. This approach is dedicated to the revision of control knowledge expressed by production rules. We have implemented and tested this approach for the automated generalisation of groups of buildings within a generalisation model called AGENT, from initial data that reference a scale of approximately 1:15,000 compared with the target map's scale of 1:50,000. The results show that our approach improves the quality of the control knowledge and thus the performance of the system. Moreover, the approach proposed is generic and can be applied to other systems based on a trial and error strategy, dedicated to generalisation or not. 相似文献
15.
Nahon S Pruski AM Duchêne JC Méjanelle L Vétion G Desmalades M Charles F 《Marine environmental research》2011,71(4):266-274
The growth, tentacle development and feeding activity of the benthic polychaete Eupolymnia nebulosa were examined to determine whether UV might affect marine deposit-feeders indirectly through the modification of the nutritional quality of their resources. Since marine invertebrates have higher nutritional requirements during the period following settlement, we tested the effect of UV-altered phytodetritus on freshly settled juveniles of E. nebulosa. Phytodetritus was prepared from cultures of the diatom Skeletonema costatum either grown under or sheltered from UVB radiation. Sterol content of phytodetritus was unmodified by UV radiation. Conversely, phytodetritus was noticeably depleted in polyunsaturated fatty acids. Growth and tentacle development of juveniles fed on altered phytodetritus were reduced by 35% and 15% respectively, suggesting potential deficiencies in essential nutrients. In response to the lower quality of the phytodetritus, juveniles explored a wider area as they search for food, a strategy that could compensate for low food quality. 相似文献
16.
MUGABOWINDEKWE Maurice MUYIZERE Aline LI Fadong QIAO Yunfeng RWANYIZIRI Gaspard 《资源与生态学报(英文版)》2018,9(3):273-280
Crop calendar is an important tool providing relevant information on crops cycles in a specific area for effective agricultural management. Crop calendars vary in different areas given dissimilarities in agro-ecosystems’ characteristics. This research used multi-temporal MODIS NDVI stratification to assess differences in practiced maize crop calendars in various areas of Rwanda. Four (4) sample NDVI strata dominated by agriculture were purposively chosen, and 433 local farmers were randomly selected from the strata for interviews. The collected information helped to know about their maize planting as well as harvesting dates in order to generate maize calendars per NDVI strata. The generated crop calendars were later classified using k-means unsupervised classification, and produced 4 groupings of practiced maize calendars irrespective of NDVI strata. ANOVA results revealed significant differences between both the generated maize crop calendars by NDVI strata and the practiced crop calendars irrespective of NDVI strata, at p = 0.05. Moreover, chi-square tests and t-tests revealed not only a significant relationship between maize calendars and number of crop growing seasons, but also a significant relationship between maize calendars and NDVI strata, at p = 0.05. Finally, findings of this research contrasted the present conviction that there exist a single general maize calendar all over the country. Instead, the results were in accordance with the fact that Rwanda agro-ecosystems differ from East to West in terms of, mainly, altitude and rainfall patterns variations. 相似文献
17.
Cécile Duchêne Anne Ruas Christophe Cambier 《International journal of geographical information science》2013,27(9):1533-1562
Our research is concerned with automated generalisation of topographic vector databases in order to produce maps. This article presents a new, agent-based generalisation model called CartACom (Cartographic generalisation with Communicating Agents), dedicated to the treatment of areas of low density but where rubber sheeting techniques are not sufficient because some eliminations or aggregations are needed. In CartACom, the objects of the initial database are modelled as agents, that is, autonomous entities, that choose and apply generalisation algorithms to themselves in order to increase the satisfaction of their constraints as much as possible. The CartACom model focuses on modelling and treating the relational constraints, defined as constraints that concern a relation between two objects. In order to detect and assess their relational constraints, the CartACom agents are able to perceive their spatial surroundings. Moreover, to make the good generalisation decisions to satisfy their relational constraints, they are able to communicate with their neighbours using predefined dialogue protocols. Finally, a hook to another agent-based generalisation model – AGENT – is provided, so that the CartACom agents can handle not only their relational constraints but also their internal constraints. The CartACom model has been applied to the generalisation of low-density, heterogeneous areas like rural areas, where the space is not hierarchically organised. Examples of results obtained on real data show that it is well adapted for this application. 相似文献
18.
Magnetic field is believed to play an important role in the collapse of a molecular cloud. In particular, due to the properties of magnetic forces, collapse should be easier along magnetic field lines. This is supported by the large-scale sheet-like structures observed in the Taurus giant molecular cloud for instance. Here we investigate whether such a preferred orientation for collapse is present at a much smaller scale, that of individual objects, i.e., about 100AU. We use recent high-angular resolution images of T Tauri stars located in the Taurus star-forming region to find the orientation of the symmetry axis of each star+jet+disk system and compare it with that of the local magnetic field. We find that (i) T Tauri stars that are associated to a jet or an outflow are generally oriented parallel to the magnetic field, as previously demonstrated. More surprising, given our current knowledge of these objects, we also find that (ii) T Tauri stars that are not at present believed to be associated to a jet or an outflow are oriented very differently, i.e., mostly perpendicular to the magnetic field. We present some implications of this puzzling new result. 相似文献
19.
Pierre Bastien François Ménard Patrice Corporon Nadine Manset Frédérick Poidevin Gaspard Duchêne Jean-Louis Monin 《Astrophysics and Space Science》2004,292(1-4):427-433
We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south–east to the north–west. The cloud is threaded by a very uniform magnetic field. 相似文献