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Philippe Machetel 《Comptes Rendus Geoscience》2003,335(1):91-97
The numerical models of mantle convection agree to depict avalanches behaviour according to the level of endothermicity of the spinel → perovskite phase change. Their potential effects on the global thermal and dynamical states of the mantle have been computed thanks to a numerical code, which takes into account both the 400-km exothermic and the 660-km endothermic phase changes. The cycle followed by the avalanches is: local layering, destabilization of the 660-km thermal layer, travelling and spreading on the core, and reappearing of the local layering. Therefore, mantle convection is characterized by quiet periods of partial layering embedded in catastrophic events. During the avalanche, the amplitude of the surface velocity is multiplied by two, which would imply an enhanced plate tectonic and ridge activities. The global thermal effects of the avalanche are compatible with a high mantle temperature and an acceleration of Earth's rotation during the Cretaceous. They also offer a coherent explanation to locate the origin of mantle plumes both within the CMB and just below the transition zone. 相似文献
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David J. Stevenson 《Comptes Rendus Geoscience》2003,335(1):99-111
Mantle convection is the method of heat elimination for silicate mantles in terrestrial bodies, provided they are not too small or too hot. Bodies that are small (～Moon or smaller, possibly even Mercury) may rely largely on conduction or melt migration, and bodies that are very hot (Io, very early Earth) may use massive melt migration (magma oceans) and heat pipes. In the standard, simple picture, we can use scaling laws to determine the secular cooling of a planet, likelihood and form of volcanism, and the possibility of a core dynamo. Contrary to popular belief, small planets do not cool faster than larger planets (provided they convect) but they do tend to have a slightly lower internal temperature at all times and thus may cease to be volcanically active at an earlier epoch. On the other hand, a larger volume fraction of a small planet may be involved in melt generation. However, our understanding of heat transfer by mantle convection is limited by three very important, largely unsolved problems: The complexities of rheology, the effects of compositional gradients, and the effects of phase transitions, especially melting. The most striking manifestation of the role of rheology lies in the difference between a mobile lid mode (plate tectonics for Earth) and a stagnant lid mode (other large terrestrial bodies). This difference may arise because of the role of water, but perhaps also because of melting, or size (gravity), or the vagaries of history. It has profound effects for the differences in history of Earth, Venus and Mars, including their surface geology, volatile reservoirs and magnetic fields. Since thermal convection is driven by small density differences, it can also be greatly altered or limited by compositional or phase effects. Melt migration introduces additional complications to the heat transport as well as being a source for the irreversible differentiation that might promote layering. Our limited understanding and ability to model these processes continues to limit the development of a predictive framework for the differences among the terrestrial planets. 相似文献
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RésuméDe nombreuses analogies entre les évolutions pétro-logiques et métamorphiques des ultrabasites de Lanzo et des ophiohtes liguro-piémontaises ont été soulignées récemment. Ces analogies concernent: la nature des ultrabasites (Iherzolites à plagioclase peu appauvries), la présence de gabbros et de filons basaltiques superficiels intrusifs dans les péridotites, l’existence de reliques de paragenèses métamorphiques de haute température attribuables à un épisode océanique, l’empreinte du métamorphisme alpin de haute pression. Ces faits ont conduit certains auteurs à proposer que les péridotites de Lanzo représentent un fragment du plancher océanique téthysien.Nous avons tenté de vérifier cette hypothèse en recherchant la couverture océanique (volcanique et/ou sédimentaire) des péridotites de Lanzo, dans la partie occidentale du massif où des pincées de métabasites et de schistes lustrés s.l. sont visibles associées aux serpentinites.Aux environs du village de Richiaglio, à l’Est de la cicatrice de Viù, les successions lithostratigraphiques reconstituées comprennent schémati-quement : les ultrabasites serpentinisées du corps de Lanzo, des métabasites, des quartzites manganésifères (comparables à celles du Mont Viso par exemple), et des calcschistes. Ces successions s’apparentent à celles décrites dans les couvertures d’autres ophiolites piémontaises, ce qui conduit à proposer que ces ensembles métavolcaniques et Dimetaires dimentaires représentent des résidus de la couverture océanique originelle des péridotites. L’analyse des paragenèses éclogitiques des métasédiments et des métabasites permet de montrer que l’évolution P,T alpine de ces lambeaux est compatible avec celle du corps de Lanzo. Cette interprétation et ses implications sont discutées. 相似文献
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André Giret Sylvie Tourpin Sabine Marc Olivier Verdier Jean-Yves Cottin 《Comptes Rendus Geoscience》2002,334(7):481-488
Despite its small area (5.6 km2), the Penguins Island brings magmatic information concerning mantle geochemical heterogeneities in southwestern Indian Ocean. The volcanism that built the island was firstly associated with marine deposits, and secondly with aerial, giving then abundant volcanic breccias and lava flows. The rocks are weakly differentiated and result of magmatic fractionation from picritic to tephritic types. KAr ages are near 1.1 Ma. The magmatic source may be related to a HIMU reservoir or to EMI ± EMII ones, depending on the geochemical evidences that are taken into account. However, comparisons with the Marion Island, on the same oceanic plateau and far to the west, as well as with Kerguelen Islands, far to the east, suggest a very heterogeneous mantle source. To cite this article: A. Giret et al., C. R. Geoscience 334 (2002) 481–488. 相似文献
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Carlos Villaseca David Orejana Christian Pin José-Angel López Garcı́a Pilar Andonaegui 《Comptes Rendus Geoscience》2004,336(10):877-888
Basic magmatism in central Spain occurred sporadically during a long period of ca. 140 Ma. The geochemical data on representative samples from the five igneous episodes show that different mantle reservoirs were tapped. Crustal involvement (via recycling in mantle sources or by hybridization at emplacement levels) was important in the basic magmas related to the Hercynian orogeny (Gb1 to Gb3 suites). Samples occurring as later dykes have signatures ranging from OIB-like values (Gb4 alkaline lamprophyres) to those of a formerly enriched subcontinental lithospheric mantle (tholeiites Gb5). To cite this article: C. Villaseca et al., C. R. Geoscience 336 (2004). 相似文献
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High‐Precision Mass‐Dependent Molybdenum Isotope Variations in Magmatic Rocks Determined by Double‐Spike MC‐ICP‐MS 
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下载免费PDF全文 Matthias Willbold Kate Hibbert Yi‐Jen Lai Heye Freymuth Remco C. Hin Christopher Coath Flurin Vils Tim Elliott 《Geostandards and Geoanalytical Research》2016,40(3):389-403
Small mass‐dependent variations of molybdenum isotope ratios in oceanic and island arc rocks are expected as a result of recycling altered oceanic crust and sediments into the mantle at convergent plate margins over geological timescales. However, the determination of molybdenum isotope data precise and accurate enough to identify these subtle isotopic differences remains challenging. Large sample sizes – in excess of 200 mg – need to be chemically processed to isolate enough molybdenum in order to allow sufficiently high‐precision isotope analyses using double‐spike MC‐ICP‐MS techniques. Established methods are either unable to process such large amounts of silicate material or require several distinct chemical processing steps, making the analyses very time‐consuming. Here, we present a new and efficient single‐pass chromatographic exchange technique for the chemical isolation of molybdenum from silicate and metal matrices. To test our new method, we analysed USGS reference materials BHVO‐2 and BIR‐1. Our new data are consistent with those derived from more involved and time‐consuming methods for these two reference materials previously published. We also provide the first molybdenum isotope data for USGS reference materials AGV‐2, the GSJ reference material JB‐2 as well as metal NIST SRM 361. 相似文献
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Mohamed Zerka Jean-Yves Cottin Michel Grégoire Jean-Pierre Lorand M&#x;Hamed Megartsi Mohamed Midoun 《Comptes Rendus Geoscience》2002,334(6):387-394
Numerous ultramafic xenoliths occur within the A??n–Temouchent volcanic complex (Northwestern Oranie, Algeria). Most of them are type I mantle tectonites (lherzolites and harzburgites) and composite xenoliths (harzburgite/clinopyroxenite) are rare. Only a few samples of spinel lherzolites display relatively fertile compositions when the major part of type I xenoliths have refractory major element compositions but enriched LREE contents showing that they have been affected by mantle metasomatism. The composite xenoliths are witnesses of reactions of alkaline magmas with the upper mantle. An asthenospheric rising, in relation with the large strike slip fault affecting the North African plate margin at Trias time is proposed as a possible geodynamical setting. To cite this article: M. Zerka et al., C. R. Geoscience 334 (2002) 387–394. 相似文献
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