Metasomatism and melting history of a Variscan lithospheric mantle domain: evidence from the Puy Beaunit xenoliths (French Massif Central)     

Olivier?FéméniasEmail author  Nicolas?Coussaert  Julien?Berger  Jean-Claude?C.?Mercier  Daniel?Demaiffe 《Contributions to Mineralogy and Petrology》2004,148(1):13-28

Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe.  相似文献   

Atomic oxygen distribution in the Venus mesosphere from observations of O₂ infrared airglow by VIRTIS-Venus Express     

Jean-Claude Gérard  Adem Saglam  Pierre Drossart  Jean-Loup Bertaux 《Icarus》2009,199(2):264-272

This VIRTIS instrument on board Venus Express has collected spectrally resolved images of the Venus nightside limb that show the presence of the (0,0) band of the infrared atmospheric system of O₂ at 1.27 μm. The emission is produced by three-body recombination of oxygen atoms created by photodissociation of CO₂ on the dayside. It is consistently bright so that emission limb profiles can be extracted from the images. The vertical distribution of O₂() may be derived following Abel inversion of the radiance limb profiles. Assuming photochemical equilibrium, it is combined with the CO₂ vertical distribution to determine the atomic oxygen density. The uncertainties on the O density caused by the Abel inversion reach a few percent at the peak, increasing to about 50% near 120 km. We first analyze a case when the CO₂ density was derived from a stellar occultation observed with the SPICAV spectrometer simultaneously with an image of the O₂ limb airglow. In other cases, an average CO₂ profile deduced from a series of ultraviolet stellar occultations is used to derive the O profile, leading to uncertainties on the O density less than 30%. It is found that the maximum O density is generally located between 94 and 115 km with a mean value of 104 km. It ranges from less than 1×¹⁰¹¹ to about 5×¹⁰¹¹ cm⁻³ with a global mean of 2.2×¹⁰¹¹ cm⁻³. These values are in reasonable agreement with the VIRA midnight oxygen profile. The vertical O distribution is generally in good agreement with the oxygen profile calculated with a one-dimensional chemical-diffusive model. No statistical latitudinal dependence of the altitude of the oxygen peak is observed, but the maximum O density tends to decrease with increasing northern latitudes. The latitudinal distribution at a given time exhibits large variations in the O density profile and its vertical structure. The vertical oxygen distribution frequently shows multiple peaks possibly caused by waves or variations in the structure of turbulent transport. It is concluded that the O₂ infrared night airglow is a powerful tool to map the distribution of atomic oxygen in the mesosphere between 90 and 115 km and improve future Venus reference atmosphere models.  相似文献   

On the swelling potential of compacted high plasticity clays     

Valry Ferber  Jean-Claude Auriol  Yu-Jun Cui  Jean-Pierre Magnan 《Engineering Geology》2009,104(3-4):200-210

We conducted a study of the spatial distributions of seismicity and earthquake hazard parameters for Turkey and the adjacent areas, applying the maximum likelihood method. The procedure allows for the use of either historical or instrumental data, or even a combination of the two. By using this method, we can estimate the earthquake hazard parameters, which include the maximum regional magnitude Mˆ_max, the activity rate of seismic events and the well-known bˆ value, which is the slope of the frequency-magnitude Gutenberg-Richter relationship. These three parameters are determined simultaneously using an iterative scheme. The uncertainty in the determination of the magnitudes was also taken into consideration. The return periods (RP) of earthquakes with a magnitude M ≥ m are also evaluated. The whole examined area is divided into 24 seismic regions based on their seismotectonic regime. The homogeneity of the magnitudes is an essential factor in such studies. In order to achieve homogeneity of the magnitudes, formulas that convert any magnitude to an M_S-surface scale are developed. New completeness cutoffs and their corresponding time intervals are also assessed for each of the 24 seismic regions. Each of the obtained parameters is distributed into its respective seismic region, allowing for an analysis of the localized seismicity parameters and a representation of their regional variation on a map. The earthquake hazard level is also calculated as a function of the form Θ = (Mˆ_max,RP_6.0), and a relative hazard scale (defined as the index K) is defined for each seismic region. The investigated regions are then classified into five groups using these parameters. This classification is useful for theoretical and practical reasons and provides a picture of quantitative seismicity. An attempt is then made to relate these values to the local tectonics.  相似文献   

Introduction     

Erzinger  Jörg  Hinz  Karl  Sibuet  Jean-Claude  Talwani  Manik 《Marine Geophysical Researches》1998,20(4):251-252

Marine Geophysical Research -  相似文献   

Himalayan inverted metamorphism constrained by oxygen isotope thermometry   总被引：3，自引：0，他引：3  

Jean-Claude Vannay  Zachary D. Sharp  Bernhard Grasemann 《Contributions to Mineralogy and Petrology》1999,137(1-2):90-101

Inverted metamorphic field gradients are preserved in two amphibolite facies metapelitic sequences forming the crystalline core zone of the Himalayan orogen in the Sutlej valley (NW India). In the High Himalayan Crystalline Sequence (HHCS), metamorphic conditions increase upwards from the staurolite zone at the base, through the kyanite-in and sillimanite-in isograds, finally to reach partial melting conditions at the top. The structurally lower Lesser Himalayan Crystalline Sequence (LHCS) shows a gradual superposition of garnet-in, staurolite-in and kyanite + sillimanite-in isograds. Although phase equilibria constraints imply inverted temperature field gradients in both units, garnet-biotite (GARB) rim thermometry indicates final equilibration at a nearly uniform temperature around T ≈ 600 °C across these sequences. The P-T path and garnet zoning data show that this apparent lack of thermal field gradient is mainly the consequence of a resetting of the GARB equilibria during cooling. In order to constrain peak temperature conditions, 20 samples along the studied section have been analysed for oxygen isotope thermometry. The isotopic fractionations recorded by quartz-garnet and quartz-aluminosilicate mineral pairs indicate temperatures consistent with phase equilibria and P-T path constraints for metamorphic peak conditions. Together with barometry results, based on net transfer continuous reactions, the oxygen isotope thermometry indicates peak conditions characterized by: (1) a temperature increase from T ≈ 570 to 750 °C at a nearly constant pressure around P ≈ 800 MPa, from the base to the top of the HHCS unit; (2) a temperature increase from T ≈ 610 to 700 °C and a pressure decrease from P ≈ 900 to 700 MPa, from the base to the top of the LHCS metapelites. Oxygen isotope thermometry thus provides the first quantitative data demonstrating that the Himalayan inverted metamorphism can be associated with a complete inversion of the thermal field gradient across the crystalline core zone of this orogen. Received: 1 April 1999 / Accepted: 12 July 1999  相似文献   

Late quaternary glacial stades in the Cordillera Central,Colombia, based on glacial geomorphology,tephra–soil stratigraphy,palynology, and radiocarbon dating     

Jean-Claude Thouret  Thomas Van Der Hammen  Barry Salomons  Etienne Juvign 《第四纪科学杂志》1997,12(5):347-369

Using data from glacial geomorphology, tephra–soil stratigraphy and mineralogy, palynology, and radiocarbon dating, a sequence of glacial and bioclimatic stades and interstades has been identified for the last ca. 50000 yr in the Ruiz-Tolima massif, Cordillera Central, Colombia. Six Pleistocene cold stades separated by warmer interstades occurred: before 48000, between 48000 and 33000, between 28000 and 21000, from ≥16000 to ca. 14000, ca. 13000–12400, and ca. 11000–10000 yr BP. Although these radiocarbon ages are minimum-limiting ages obtained from tephra layers on top of tills, the tills are not significantly older because most are bracketed by dated tephra sets in measured stratigraphic sections. Two minor moraine stages likely reflect glacier standstill during cold intervals ca. 7400 yr BP and slightly earlier. Finally, glaciers readvanced between the seventeenth and nineteenth centuries. In contrast to the ice-clad volcanoes of the massif, ca. 34 km² in area above an altitude of ca. 4800 m, the ice cover expanded to 1200 km² during the Last Glacial Maximum (LGM) and was still 800 km² during Late-glacial time (LGT). Glacier reconstructions based on the moraines suggest depression of the equilibrium line altitude (ELA) by ca. 1100 m during the LGM and 500–600 m during LGT relative to the modern ELA, which lies at ca. 5100 m in the Cordillera Central. Glaciers in this region apparently reached their greatest extent when the climate was cold and wet, e.g. during stades corresponding to Oxygen Isotope Stage 3; glaciers were still expanding during the LGM ca. 28000–21000 yr BP, but they shrank considerably after 21000 yr BP because of greatly reduced precipitation. © 1997 John Wiley & Sons, Ltd.  相似文献   

Erosion and aggradation on persistently active volcanoes—a case study from Semeru Volcano,Indonesia     

Jean-Claude Thouret  Jean-François Oehler  Avijit Gupta  Akhmad Solikhin  Jonathan N. Procter 《Bulletin of Volcanology》2014,76(10):1-26

Erosion processes on active volcanoes in humid climates result in some of the highest sediment yields on Earth. Episodic sediment yields after large eruptions have been evaluated, but not the long-term and continuous patterns on persistently active volcanoes. We have used high-spatial resolution satellite imagery and DEMs/DSMs along with field-based geologic mapping to assess accurately sediment budgets for the active Semeru Volcano in Java, Indonesia. Patterns of aggradation and degradation on Semeru differ from that of other active volcanoes because (1) both episodic pyroclastic density currents (PDC) and continuous supplies of tephra generate pulses of sediment, (2) sediment is transferred via cycles of aggradation and degradation that continue for >15 years in river channels after each PDC-producing eruption, and (3) rain-triggered lahars remove much greater material than fluvial transport during long, intense rainfall events. The geomorphic response of two of Semeru’s rivers to volcanic sediment migration indicates that (1) each river experiences alternating aggradation and degradation cycles following PDC-producing eruptions and (2) spatial patterns of sediment transfer are governed by geomorphic characteristics of the river reaches. Usually high degradation in the steep source reach is followed by a long bypassing middle reach. Aggradation predominates in the depositional reaches further down valley on the ring plain. Average sediment yields (10³–10⁵ t/km²/year) at persistently active volcanoes are two to three orders of magnitude lower than sediment yields after large and infrequent eruptions, but the continuous and steady sediment transfer in rivers removes more sediment on a mid-term (10 years) to long-term (30 years) basis. In contrast to the trend observed on composite cones after large and infrequent eruptions, decay of sediment yields is not exponential and river channels do not fully recover at steadily active volcanoes as episodic inputs from BAF eruptions, superimposed on the background remobilization of daily tephra, have a greater cumulative effect.  相似文献   

Are the eastern and western basins of the English Channel two separate ecosystems?     

Dauvin JC 《Marine pollution bulletin》2012,64(3):463-471

The English Channel is part of the "Greater North Sea" sub-region in the Marine Strategy Framework Directive. However, the Channel is characterized by hydrologic, oceanographic and biogeographic features that support its division into two main entities: the western basin and the eastern basin. This paper summarizes the Channel's main natural features and principal human activities to examine the similarities and differences between both basins. The differences between the basins support an ecosystem-based management approach at the combined basin scale, rather than the present approach of separating the France and UK sides.  相似文献   

Benthic indicators: From subjectivity to objectivity - Where is the line?   总被引：1，自引：0，他引：1  

Jean-Claude Dauvin  Gérard Bellan 《Marine pollution bulletin》2010,60(7):947-953

Over the last few years, the interest in using benthic indicators to assess marine environments has increased dramatically after a rather long period of relative stagnation, mostly due to the need to assess the status of coastal marine waters required by North American and European regulations. Numerous papers on this topic have been published in the domain of ecology, using a variety of different terms to refer to two categories of information: benthic species and the status of benthic communities. Nowadays, the abundant literature on these two categories makes it possible to comment on (1) the definition of the different terms used by benthic researchers, (2) the current increase of papers of rising complexity about benthic indicators, and (3) the subjectivity and objectivity involved in using benthic indicators. Faced with the increase in the number of methods, we recommend pragmatism and thus the transfer of simple methods to the research consultancies that are responsible for assessing benthic quality in numerous impact studies. Using certain procedures, such as the “sentinel species”, the best professional judgement (BPJ) and taxonomic sufficiency (TS), should clearly be encouraged.  相似文献   

Reconstruction of the recent history of a large deep prealpine lake (Lake Bourget,France) using subfossil chironomids,diatoms, and organic matter analysis: towards the definition of a lake-specific reference state     

Laurent Millet  Charline Giguet-Covex  Valérie Verneaux  Jean-Claude Druart  Thierry Adatte  Fabien Arnaud 《Journal of Paleolimnology》2010,44(4):963-978

This paper presents the recent history of a large prealpine lake (Lake Bourget) using chironomids, diatoms and organic matter analysis, and deals with the ability of paleolimnological approach to define an ecological reference state for the lake in the sense of the European Framework Directive. The study at low resolution of subfossil chironomids in a 4-m-long core shows the remarkable stability over the last 2.5 kyrs of the profundal community dominated by a Micropsectra-association until the beginning of the twentieth century, when oxyphilous taxa disappeared. Focusing on this key recent period, a high resolution and multiproxy study of two short cores reveals a progressive evolution of the lake’s ecological state. Until AD 1880, Lake Bourget showed low organic matter content in the deep sediments (TOC less than 1%) and a well-oxygenated hypolimnion that allowed the development of a profundal oxyphilous chironomid fauna (Micropsectra-association). Diatom communities were characteristic of oligotrophic conditions. Around AD 1880, a slight increase in the TOC was the first sign of changes in lake conditions. This was followed by a first limited decline in oligotrophic diatom taxa and the disappearance of two oxyphilous chironomid taxa at the beginning of the twentieth century. The 1940s were a major turning point in recent lake history. Diatom assemblages and accumulation of well preserved planktonic organic matter in the sediment provide evidence of strong eutrophication. The absence of profundal chironomid communities reveals permanent hypolimnetic anoxia. From AD 1995 to 2006, the diatom assemblages suggest a reduction in nutrients, and a return to mesotrophic conditions, a result of improved wastewater management. However, no change in hypolimnion benthic conditions has been shown by either the organic matter or the subfossil chironomid profundal community. Our results emphasize the relevance of the paleolimnological approach for the assessment of reference conditions for modern lakes. Before AD 1900, the profundal Micropsectra-association and the Cyclotella dominated diatom community can be considered as the Lake Bourget reference community, which reflects the reference ecological state of the lake.  相似文献