Microstructures of metal grains in ordinary chondrites: Implications for their thermal histories     

Hugues LEROUX  Jean‐Claude DOUKHAN  Claude PERRON 《Meteoritics & planetary science》2000,35(3):569-580

Abstract— This paper reports one of the first attempts to investigate by analytical transmission electron microscopy (ATEM) the microstructures and compositions of Fe‐Ni metal grains in ordinary chondrites. Three ordinary chondrites, Saint Séverin (LL6), Agen (H5), and Tsarev (L6) were selected because they display contrasting microstructures, which reflects different thermal histories. In Saint Séverin, the microstructure of the Ni‐rich metal grains is due to slow cooling. It consists of a two‐phase assemblage with a honeycomb structure resulting from spinodal decomposition similar to the cloudy zone of iron meteorites. Microanalyses show that the Ni‐rich phase is tetrataenite (Ni = 47 wt%) and the Ni‐poor phase, with a composition of ～25% Ni, is either martensite or taenite, these two occurring adjacent to each other. The observation that the Ni‐poor phase is partly fcc resolves the disagreement between previous transmission electron microscopy (TEM) and Mössbauer studies on iron meteorites and ordinary chondrite metal. The Ni content of the honeycomb phase is much higher than in mesosiderites, confirming that mesosiderites cooled much more slowly. The high‐Ni tetrataenite rim in contact with the cloudy zone displays high‐Ni compositional variability on a very fine scale, which suggests that the corresponding area was destabilized and partially decomposed at low temperature. Both Agen and Tsarev display evidence of reheating and subsequent fast cooling obviously related to shock events. Their metallic particles mostly consist of martensite, the microstructure of which depends on local Ni content. Microstructures are controlled by both the temperature at which martensite forms and that at which it possibly decomposes. In high‐Ni zones (>15 wt%), martensitic transformation started at low temperature (<300 °C). Because no further recovery occurred, these zones contain a high density of lattice defects. In low‐Ni zones (<15 wt%), martensite grains formed at higher temperature and their lattice defects recovered. These martensite grains present a lath texture with numerous tiny precipitates of Ni‐rich taenite (Ni = 50 wt%) at lath boundaries. Nickel composition profiles across precipitate‐matrix interfaces show that the growth of these precipitates was controlled by preferential diffusion of Ni along lattice defects. The cooling rates deduced from Ni concentration profiles and precipitate sizes are within the range 1–10 °C/year for Tsarev and 10–100 °C/year for Agen.  相似文献   

Intertidal clay-drape couplets (Gironde estuary, France)     

Hugues Fenies  Amaury De Resseguier  & Jean-Pierre Tastet 《Sedimentology》1999,46(1):1-15

Clay-drape couplets on subaqueous dunes have been regarded as a diagnostic feature of the subtidal environment since Visser's seminal paper (1980). The new observation of clay-drape couplets in the intertidal zone on a present day tidal bar of the Gironde estuary shows that they are not restricted to the subtidal zone.
In the intertidal zone, low-tide slack-water clay drapes are deposited in the bottomsets of the dominant current dunes when the muddy water retained in the troughs is absorbed into the sand during the emergence of the intertidal bar. They drape emergence run-off ripples generated by the drainage currents in the bottomsets. High-tide slack-water clay drapes are deposited over the entire dune surface and are preserved on the lee side of the dunes and in the bottomsets. They drape the subordinate current ripples. Low-tide and high-tide slack-water clay drapes enclose one thin rippled sand layer (the subordinate current bundle) and are isolated from other adjacent clay-drape couplets by the dominant current bundle.
The clay-drape couplets deposited in the intertidal zone can be distinguished from their subtidal counterparts on the basis of two morphological differences:
1. In the intertidal zone, the low-tide clay drape is only present in the bottomsets of the dunes, whereas in the subtidal zone equivalent clay drapes are also present on the lower part of the lee side of the dunes.
2. In the intertidal zone, low-tide clay drapes are deposited in the bottomsets of the dunes over emergence run-off ripples oriented in the direction of the drainage currents (i.e. in a direction normal to the tidal currents). Conversely, in the subtidal zone, the equivalent clay drapes are typically deposited over ripples oriented in the tidal-current direction (ebb or flood). There is a difference of polarity of 90° between the intertidal and subtidal small-scale bedforms draped by the low-tide slack-water drapes.  相似文献   

Facies and geometry of tidal channel-fill deposits (Arcachon Lagoon, SW France)     

Hugues Fenies  Jean-Claude Faugres 《Marine Geology》1998,150(1-4):131-148

The Arcachon Lagoon has an important network of tidal channels and well developed tidal flats covered by the marine grass Zostera marina. Based on 66 piston cores taken from the Graveyron tidal channel, and observations on the neighbouring channels, this paper documents the facies and geometry of the channel-fill deposits. In the inner lagoon (studied area) the tidal channels are 80 to 150 m wide and have a meandering morphology with sandy point bars 2 to 5 m thick. The channel-fill does not consist of the classic inclined heterolithic bedding typical of many channel-fills (Reineck, 1958), but of cross-stratified sandy deposits characterized by the absence of slack-water clay-drapes. These unusual facies characteristics are due to the low turbidity of the lagoonal waters which is caused by the lack of significant river inflow and the dense coverage of Zostera marina on the tidal flats. The overall geometry of the channel-fill deposits is characterized by a narrow sand-ribbon shape, a few kilometres long, 80 to 150 m wide and 1 to 5 m thick. This sand ribbon is made of elliptical sand bodies, deposited as point bars, that coalesce longitudinally along the channel axis. This narrow shape is due to the fact that the lateral migration of the channel is virtually nil (reduced to a few metres). In spite of their characteristic meandering morphology, these channels do not deposit extensive tabular sand sheets of amalgamated point bars like the tidal creeks on the North Sea tidal flats. Two factors are thought to control this lack of channel migration. (1) The tidal flats adjacent to the tidal channels are made of 3- to 5-m-thick cohesive muddy sediments covered by Zostera marina that prevents the erosion of the channel banks. This first mechanism is supported by the observation that the tidal creeks that drain the muddy tidal flats covered by Zostera marina do not migrate laterally, whereas those that drain the sandy tidal flats devoid of a dense coverage of marine grass do have active lateral migration. (2) The tidal channels are not fed by any river and therefore do not receive any fluvial sand influx during the winter floods. Their morphology is in equilibrium with the tidal discharge and represents a stable stage in the development of the channel. This second mechanism is supported by the fact that the only tidal channels that actively migrate laterally in the lagoon receive sandy fluvial influx from the River Leyre located in the southeastern corner of the lagoon.  相似文献   

Facies and architecture of an estuarine tidal bar (the Trompeloup bar, Gironde Estuary, SW France)   总被引：1，自引：0，他引：1  

Hugues Fenies  Jean-Pierre Tastet 《Marine Geology》1998,150(1-4):149-169

This study describes the sedimentary facies and internal architecture of a modern estuarine tidal bar present in the bay-head delta of a wave- and tide-dominated estuary, the Gironde Estuary, in southwest France. Based on 51 cores (2 to 5 m long), this work demonstrates that such bars are composite sedimentary bodies made up of individual sigmoidal sand units isolated from one another by thick muddy layers. The bar appeared in the study area around 1874. Since then, its evolution has been characterized by phases of rapid downstream bar migration alternating with periods of bar abandonment. During the periods of active bar progradation, which commonly last between 10 and 20 years, the bar progrades up to 7 km seaward and forms a narrow (200–800 m wide) sand ridge up to 6 m thick. During abandonment phases which also last around 10–20 years, the bar is starved of sand and a mud layer (1 to 3 m thick) is deposited. The vertical succession formed by the seaward progradation of the tidal bar records these alternating phases of growth and abandonment. A typical vertical succession in the bar system is 8 m thick and comprises several superimposed, upward-coarsening units of cross-stratified sand, deposited during phases of seaward progradation, separated by a thick layer of thinner-bedded mud and sand laminae deposited during phases of bar abandonment. When a tidal bar builds up to the supratidal zone, marshes accumulate and the bar becomes a permanent island in the estuary. The large-scale architecture of the bar is thus made of an alternation of potential sandy reservoir units and muddy internal seals. The reservoir units are characterized by a sigmoidal shape. The muddy layer, which is deposited between the two sigmoidal units, is a thick, continuous potential internal seal. This complex internal architecture is due to the fact that there is a considerable amount of mud present in the bay-head delta of the Gironde Estuary. It is thought to be representative of the architecture of tidal bars in the inner part of wave- and tide-dominated estuaries.  相似文献   

On the limits of linear moment-tensor inversion of teleseismic body wave spectra     

H. Dufumier 《Pure and Applied Geophysics》1996,147(3):467-482

The limits of linear moment-tensor inversions from long-period teleseismic body waves are analysed in detail, using inverse methods. We focus our attention on single-station and few-stations methods. Information on the feasibility of full or deviatoric moment-tensor determinations prior to inversion are deduced from the system conditioning. The resolution and correlation of the momenttensor components are analysed using the resolution matrix. Conclusions on the importance and independence of the data are drawn from the information matrix. The single--station case and its implications are discussed in greater detail, as well as possible constraints on the inversion.  相似文献   

A coastal reservoir of terrestrial resources for neanderthal populations in north‐eastern Iberia: palaeoenvironmental data inferred from the small‐vertebrate assemblage of Cova del Gegant,Sitges, Barcelona     

Juan Manuel López‐García  Hugues‐Alexandre Blain  Montserrat Sanz  Joan Daura 《第四纪科学杂志》2012,27(1):105-113

Coastal areas as reservoirs of resources for hominid groups have been widely studied in recent years. These areas combine marine with terrestrial and wetland resources and would have been optimum sites for hominids, including Neanderthals. This is the case with the Cova del Gegant, a cave that today opens directly onto the Mediterranean Sea and is located in the north‐eastern Iberian Peninsula. The geomorphological evolution of the Massis del Garraf has provided evidence that during the late Pleistocene there was a littoral platform between 8 and 13 km wide in front of the Cova del Gegant. Within this framework, the data derived from analysis of the small vertebrates and large mammals recovered from Cova del Gegant, including taxa currently absent from the Massis del Garraf, suggest that the landscape surrounding the cave provided a richer terrestrial ecosystem for Neanderthals than is available in this zone today. Analysis of the small‐vertebrate association from the cave reveals that the landscape surrounding the cave was dominated by woodland‐edge and open environments and that the climate was Mediterranean. The results have been compared with the only Iberian site with similar characteristics to the Cova del Gegant, Gorham's cave (southern Iberia, Gibraltar), revealing differences and similarities in the landscape and climate on the basis of the small‐mammal assemblages as well as the differences in the accessibility to terrestrial mammalian resources for the Neanderthal groups. The landscape and the climate were reasonably similar at the two sites, but the differences in the accessibility of resources for the Neanderthals are directly related to the location of the sites and the coastal position. Whereas the Cova del Gegant was on a route of mammal migration (between the Ebro Valley and France) suited for securing terrestrial resources, Gorham's cave is located on a small peninsula with a lower abundance of terrestrial mammal resources. This is probably why the Neanderthal groups at Gorham's cave exploited marine resources, whereas there is no evidence of marine resources having been exploited at Cova del Gegant, even though the seashore was nearby. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Unconformity between a Late Miocene–Pliocene accretionary prism (Nishizaki Formation) and Pliocene trench‐slope sediments (Kagamigaura Formation), central Japan     

Yuzuru Yamamoto  Shun Chiyonobu  Toshiyuki Kurihara  Asuka Yamaguchi  Shoko Hina  Mari Hamahashi  Hugues Raimbourg  Romain Augier  Leslie Gadenne 《Island Arc》2012,21(4):231-234

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Limited Consequences of Seagrass Decline on Benthic Macrofauna and Associated Biotic Indicators     

V. Tu Do  Hugues Blanchet  Xavier de Montaudouin  Nicolas Lavesque 《Estuaries and Coasts》2013,36(4):795-807

Marine phanerogams are ecosystem engineers, as their presence induces major environmental changes that impact on the benthic fauna. Consequently, modifications to the structure of benthic communities would be expected to be associated with seagrass decline. Since 2005, Zostera noltii seagrass beds in Arcachon Bay (France), the largest in Europe, have undergone a severe decline. Twelve stations distributed throughout the lagoon were sampled in 2002, and all were found to be densely planted at that time. Subsequently, the same stations were revisited in 2010 and seagrass cover had drastically decreased by that time. Based on benthic macrofauna, multidimensional scaling (MDS) analysis identified four groups. Years were separated. In 2002, two groups were distinct in relation to the water body, since in 2010 separation between the two other groups was related to seagrass occurrence. When looking at community structure and dominant species there were moderate differences within and between years, independent of seagrass decline. Seagrass loss did not drastically modify the species composition as they were preserved in the remaining seagrass patches. However, there was a drop in macrofauna abundance in unvegetated muddy compared with abundance in the remaining seagrass areas. Epifauna was particularly affected by seagrass decline. Among biotic indicators based on macrofauna, multivariate indicator MISS (Macrobenthic Index in Sheltered Systems) was in agreement with the similarity of macrofauna structure among groups, while other tested indicators performed badly in relation to seagrass occurrence. However, no index detected seagrass loss, highlighting the necessity of maintaining a separate survey on seagrass cover.  相似文献   

Preliminary study on the preservation of giant clam (Tridacnidae) shells from the Balobok Rockshelter archaeological site,south Philippines     

Marie Grace Pamela G. Faylona  Claire E. Lazareth  Anne‐Marie Smah  Sandrine Caquineau  Hugues Boucher  Wilfredo P. Ronquillo 《Geoarchaeology》2011,26(6):888-901

Tridacnidae shells, a valuable archive of past environments, are common in the Balobok Rockshelter archaeological site on Sanga‐Sanga Island in the south Philippines. This site was occupied during the mid‐Holocene (ca. 5000–8800 ¹⁴C yr B.P.), a period of Neolithic cultural expansion in the Philippines. This paper focuses on the preservation of two shell specimens, Hippopus hippopus and Tridacna maxima, unearthed from two mid‐Holocene layers within the rockshelter. The shells' mineralogy and microstructure (prismatic and crossed‐lamellar) were studied using micro‐Fourier transform infrared spectroscopy and scanning electron microscopy to determine if the samples were suitable as paleoenvironmental records. Both shells are still aragonitic but aragonite crystals of both microstructure types are partly dissolved. This dissolution, characteristic of meteoric water alteration, precludes their utility in paleoenvironmental geochemical studies. Nevertheless, these shells are abundant in archaeological sites in the region and may be better preserved in other depositional contexts; more studies on Philippine Tridacnidae shell diagenesis are needed. © 2011 Wiley Periodicals, Inc.  相似文献