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1.
Mireille Christophe Michel-Levy Michle Bourot-Denise 《Meteoritics & planetary science》1992,27(1):73-80
Abstract— The mineral compositions of 250 micrometeorites have been studied and olivines and low-calcium pyroxenes with crystals larger than 5 μm have been analyzed. While magnesium-rich grains dominate, the Fa content of olivine may reach 50% and the Fs content of pyroxene may reach 26%. The Ca and Mn of the olivine show no consistent trends with increasing Fe, but Cr shows a negative correlation. For low-Ca pyroxene, Al and Cr contents are generally higher than in pyroxenes of equilibrated chondrites but similar to those of highly unequilibrated chondrites. Calcium-bearing pyroxene, feldspar and chromite are rare in the micrometeorites which were selected because of their high Mg, Si, Fe and their low Ca and Al content. All these minerals are found as coarse-grained particles often with adhering iron-rich scoria or as clasts in fine-grained or scoriaceous micrometeorites. Apart from a few particles which could be the debris of ordinary chondrites, most micrometeorites probably come from a common source similar, but not identical to carbonaceous chondrites, as shown by their lower Ni and S content and their different oxygen isotopic composition assuming two measurements performed on olivine grains prove to be typical. 相似文献
2.
Abstract— Platinum group elements (PGE) enrichments are found in Fe-Ni blebs (<1 μm) in Al- and Cr-rich objects in the ordinary chondrites Ste. Marguerite, Forest Vale, Montferré. In Ste. Marguerite, high ZrO2 concentrations (baddeleyite) are also present in a chromite inclusion. Iridium enrichment in this material compared to its metal content is confirmed by INAA. The widespread occurrence of PGE in Al- and Cr-rich objects, mostly present in H-group chondrites of nearly equilibrated types, must be taken into account to understand their conditions of formation. 相似文献
3.
Abstract Dahmani is a shocked LL6 fragmental breccia. According to the composition of the silicates (olivine Fa30,32.6, orthopyroxene Fs24.5–26.3) and of the metal (a 60% Ni taenite) it is one of the most oxidised known. 相似文献
4.
Eloïse Gaillou Aurlien Delaunay Benjamin Rondeau Martine Bouhnik-le-Coz Emmanuel Fritsch Guy Cornen Christophe Monnier 《Ore Geology Reviews》2008,34(1-2):113
Seventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondrite-normalized REE pattern, are the keys to separating sedimentary opals (Ba > 110 ppm, Eu and Ce anomalies) from volcanic opals (Ba < 110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si4+ by Al3+ and Fe3+. The induced charge imbalance is compensated chiefly by Ca2+, Mg2+, Mn2+, Ba2+, K+, and Na+. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to “chocolate brown”). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (≤ 1 ppm) induces a green luminescence. 相似文献
5.
Magnesium isotope systematics of the lithologically varied Moselle river basin, France 总被引:2,自引:0,他引:2
Agnès Brenot Christophe Cloquet Nathalie Vigier Jean Carignan Christian France-Lanord 《Geochimica et cosmochimica acta》2008,72(20):5070-5089
Magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the Moselle basin, northeastern France. This small basin is remarkable for its well-constrained and varied lithology on a small distance scale, and this is reflected in river water Sr isotope compositions. Upstream, where the Moselle River drains silicate rocks of the Vosges mountains, waters are characterized by relatively high 87Sr/86Sr ratios (0.7128-0.7174). In contrast, downstream of the city of Epinal where the Moselle River flows through carbonates and evaporites of the Lorraine plateau, 87Sr/86Sr ratios are lower, down to 0.70824.Magnesium in river waters draining silicates is systematically depleted in heavy isotopes (δ26Mg values range from −1.2 to −0.7‰) relative to the value presently estimated for the continental crust and a local diorite (−0.5‰). In comparison, δ26Mg values measured in soil samples are higher (∼0.0‰). This suggests that Mg isotope fractionation occurs during mineral leaching and/or formation of secondary clay minerals. On the Lorraine plateau, tributaries draining marls, carbonates and evaporites are characterized by low Ca/Mg (1.5-3.2) and low Ca/Sr (80-400) when compared to local carbonate rocks (Ca/Mg = 29-59; Ca/Sr = 370-2200), similar to other rivers draining carbonates. The most likely cause of the Mg and Sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer. δ26Mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year (from −1.4‰ to −1.6‰ and from −1.2‰ to −1.4‰, respectively), within the range defined for the underlying rocks. Downstream of Epinal, the compositions of the Moselle River samples in a δ26Mg vs. 87Sr/86Sr diagram can be explained by mixing curves between silicate, carbonate and evaporite waters, with a significant contribution from the Vosgian silicate lithologies (>70%). Temporal co-variation between δ26Mg and 87Sr/86Sr for the Moselle River throughout year is also observed, and is consistent with a higher contribution from the Vosges mountains in winter, in terms of runoff and dissolved element flux. Overall, this study shows that Mg isotopes measured in waters, rocks and soils, coupled with other tracers such as Sr isotopes, could be used to better constrain riverine Mg sources, particularly if analytical uncertainties in Mg isotope measurements can be improved in order to perform more precise quantifications. 相似文献
6.
Atmospherically-promoted photosynthetic activity in a well-mixed ecosystem: Significance of wet deposition events of nitrogen compounds 总被引:1,自引:0,他引:1
C. Boulart P. Flament V. Gentilhomme K. Deboudt C. Migon F. Lizon M. Schapira A. Lefebvre 《Estuarine, Coastal and Shelf Science》2006,69(3-4):449
Wet atmospheric deposition of dissolved N, P and Si species is studied in well-mixed coastal ecosystem to evaluate its potential to stimulate photosynthetic activities in nutrient-depleted conditions. Our results show that, during spring, seawater is greatly depleted in major nutrients: Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Phosphorus (DIP) and Silicic acid (Si), in parallel with an increase of phytoplanktonic biomass. In spring (March–May) and summer (June–September), wet atmospheric deposition is the predominant source (>60%, relative to riverine contribution) for nitrates and ammonium inputs to this N-limited coastal ecosystem. During winter (October–February), riverine inputs of DIN predominate (>80%) and are annually the most important source of DIP (>90%). This situation allows us to calculate the possibility for a significant contribution to primary production in May 2003, from atmospheric deposition (total input for DIN ≈300 kg km−2 month−1). Based on usual Redfield ratios and assuming that all of the atmospheric-derived N (AD-N) in rainwater is bioavailable for phytoplankton growth, we can estimate new production due to AD-N of 950 mg C m−2 month−1, during this period of depletion in the water column. During the same episode (May 2003), photosynthetic activity rate, considered as gross primary production, was estimated to approximately 30 300 mg C m−2 month−1. Calculation indicates that new photosynthetic activity due to wet atmospheric inputs of nitrogen could be up to 3%. 相似文献
7.
Geochemical and hydrodynamic constraints on the distribution of trace metal concentrations in the lagoon of Nouméa, New Caledonia 总被引:1,自引:0,他引:1
Christophe Migon Sylvain Ouillon Xavier Mari Emmanuel Nicolas 《Estuarine, Coastal and Shelf Science》2007,74(4):756
Seawater samples were collected in the lagoon of Nouméa (southwest New Caledonia) along two transects from eutrophic coastal bays to the oligotrophic barrier reef. Land-based emissions to the lagoon were measured with dissolved and particulate concentrations of chromium (Cr) and nickel (Ni), used as tracers of both terrigenous and industrial (Ni ore treatment) activities, as well as dissolved and particulate concentrations of zinc (Zn), used as a tracer of urban effluents. The spatial variability of metal concentrations was related to geochemical and hydrodynamic conditions, i.e., respectively: (1) natural and anthropogenic emission sources, and chemical processes occurring in the water column; and (2) water residence times. The parameter used to describe the residence time of water masses was the local e-flushing time, i.e. the time required for a tracer mass contained within a given station to be reduced by a factor 1/e. High metal concentrations were found in coastal areas (up to 9000 ng dissolved Ni L−1), and steeply decreased with distance from the coast (down to 101 ng dissolved Ni L−1 near the barrier reef) to reach levels similar to those found in remote Pacific waters, suggesting a rapid renewal of waters close to the barrier. Distributions of metals in the lagoon are controlled upstream by land-based emission sources and later chemical processes. Then hydrodynamics constrain metal distributions, as shown by the observed relationship between local e-flushing times and the spatial variability of metal concentrations. In addition, a change in the direction of prevailing winds yielded a decrease of dissolved metal concentrations at the same site by a factor of 2.5 (Cr and Ni) and 2.9 (Zn). It is suggested that the residence time is a key parameter in the control of elemental concentrations in the lagoon waters, as much as land-based emission sources. 相似文献
8.
We present new sea-level data from the coasts of southern Tunisia, between the Gulf of Gabès and the Libyan border. The work tests, previously, published evidence on Holocene shorelines, and confirmed that a distinct emergence has occurred in this area during this time. The emergence peak lies at least 186 ± 11 cm above present and is inferred from: (1) AMS radiocarbon dates of subtidal vermetids and boring shells collected in growth position, and (2) careful assessment of tidal heights. Maximum emergence took place between about 6000 and 5000 14C years BP; it cannot be ascribed to tectonics and is probably related to post-glacial hydro-isostatic effects. It challenges the inference of a 3-m global sea-level rise since 6000 years BP due to residual Antarctic melting. 相似文献
9.
Since a state law issued in 1989, every big town in Colombia has to promote its own preparedness programme for disaster relief in the framework of the Oficina Nacional para la Atencion de Desastres (1987). Such a programme includes a very detailed hazard-zone map for flooding and debris flows in the densely inhabited suburbs of Ibagué city (capital of Tolima Department), as well as enquiries about risk knowledge and consciousness among people who live at high risk. We stress the fact that both detailed hazard-zone maps and enquiries should be closely undertaken at the same time, and that distinguishing previous knowledge, existing consciousness and risk perception to forecast the behaviour of people and of the community in case of disaster, are criteria for a better risk assessment. 相似文献
10.
On the role of heat flow, lithosphere thickness and lithosphere density on gravitational potential stresses 总被引:1,自引:0,他引:1
Gravitational potential stresses (GPSt) are known to play a first-order role in the state of stress of the Earth's lithosphere. Previous studies focussed mainly on crust elevation and structure and little attention has been paid to modelling GPSt using realistic lithospheric structures. The aim of the present contribution is to quantify gravitational potential energies and stresses associated with stable lithospheric domains. In order to model realistic lithosphere structures, a wide variety of data are considered: surface heat flow, chemical depletion of mantle lithosphere, crustal thickness and elevation. A numerical method is presented which involves classical steady-state heat equations to derive lithosphere thickness, geotherm and density distribution, but additionally requires the studied lithosphere to be isostatically compensated at its base. The impact of varying surface and crustal heat flow, topography, Moho depth and crust density on the signs and magnitudes of predicted GPSt is systematically explored. In clear contrast with what is assumed in most previous studies, modelling results show that the density structure of the mantle lithosphere has a significant impact on the value of the predicted GPSt, in particular in the case of thick lithospheres. Using independent information from the literature, the method was applied to get insights in the state of stress of continental domains with contrasting tectono-thermal ages. The modelling results suggest that in the absence of tectonic stresses Phanerozoic and Proterozoic lithospheres are spontaneously submitted to compression whereas Archean lithospheres are in a neutral to slightly tensile stress state. These findings are in general in good agreement with global stress measurements and observed geoid undulations. 相似文献