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911.
Pierre Rochette Grard Fillion Jean-Luc Matti Marinus J. Dekkers 《Earth and Planetary Science Letters》1990,98(3-4)
A characteristic magnetic transition at 30–34 K is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low-temperature techniques as applied to magnetite and hematite, extended down to liquid helium temperature. A review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals. Unblocking temperature up to 350°C and extreme resistance against AF may be encountered in fine grained pyrrhotite. 相似文献
912.
Michel Cuney Pierre Sabat Philippe Vidal Moacyr M. Marinho Herbet Conceiao 《Journal of Volcanology and Geothermal Research》1990,44(1-2)
The Jacobina — Contendas Mirante belt represents a Transamazonian (2 Ga), N-S, 500-km long, elongated orogenic domain in the central part of the São Francisco craton, Bahia state. Numerous syntectonic to post-tectonic peraluminous leucogranites were emplaced along the major structures of the belt. Their mineralogical and geochemical and some of their metallogenetic characteristics are very similar to their Hercynian and Himalayan equivalents. However, their average peraluminous index varies from one granitic pluton to another and biotite is, on average, slightly more magnesian in the Transamazonian leucogranites. Higher oxygen fugacity is indicated by the general occurrence of magnetite, the stability of allanite and sometimes epidote in most of the plutons and by biotite chemistry. The peraluminous magmatism of the Jacobina-Contendas Mirante belt results from crustal partial melting during a continental collision event at 2 Ga.Trace-element geochemistry implies variable source composition and/or melting conditions for the different granitic plutons and some different facies within the same plutonic unit. The scarcity of ilmenite, the general occurrence of magnetite, and the relatively low peraluminous index of some of these granites suggest that graphite-beating sediments are not a significantly source material. From their mineralogical and geochemical characteristics, acid meta-igneous rocks such as the Sete Voltas TTG suite of presumed Archaean age, seem to represent a suitable source for these granites.Sn, W, Li, F and Be enrichment of most Transamazonian leucogranites is much weaker than in the mineralized Variscan equivalents. The Caetano-Aliança and Riacho das Pedras granites represent the most specialized granitic bodies. Beryl (emerald), molybdenite and scheelite mineralizations are related to some of these granites which intrude ultrabasic formations: the Campo Formoso and the Carnaiba granites. In many of these granites, uranium content is comparable to values measured in mineralized Hercynian leucogranites. The occurrence of hexavalent uranium minerals, mineralization and episyenitic alteration are favourable criteria for finding Variscan-type uranium ore deposits. 相似文献
913.
The radial-growth patterns of white spruce were studied on a number of trees growing in subarctic dunes along the eastern coast of Hudson Bay to calculate the rates of accumulation, erosion, and migration of cold-climate sand dunes. The average rate of sand accumulation in sheltered dunes (forest sites) was 2.5 to 3.3 cm/yr, which is two to three times lower than in highly exposed dunes with a rate of sedimentation of 7.65 cm/yr. The average erosion rate was 1.4–1.7 cm/yr, about two times lower than the accumulation rate. The migration rate of sheltered dunes was 18 to 30 cm/yr, three to five times lower than for an exposed dune which advanced at a speed of 74 cm/yr. This migration rate established for highly exposed dunes in the Subarctic with tree-ring methods is about 10 times lower than that established for a barchan in the Sahara with other methods. 相似文献
914.
Pierre Routhier 《Mineralium Deposita》1969,4(2):213-218
Des publications récentes conduisent à insister sur trois démarches, ou trois principes fondamentaux sans lesquels la Métallogénie (comme la Pétrologie) et la recherche minérale ne peuvent progresser. 1. Sortons des «types» connus et posons en principe que tout élément chimique peut se concentrer à diverses étapes du cycle géologique et géochimique; recherchons donc ces concentrations et traquons les par tous les moyens minéralogiques et géochimiques à notre disposition. 2. Récusons la confusion des concepts chronologiques et des concepts spatiaux, et en particulier la superposition totalement erronée d'«épigénétique» et d'«étranger». 3. Tout est hérité d'un passé plus ou moins lointain; principe qui rejoint le premier et nous conduira, dans certains cas favorables, vers des concentrations antérieures.
Recent publications lead to three steps without which metallogenic and thus petrologic investigations are impossible: 1. We should depart from known types in ore genesis and assume that any element can be enriched during various stages of the geological and geochemical cicle. These enrichments should thus be investigated with all possible methods of mineralogy and geochemistry. 2. We should reject the confusion of terms referring to time and space of ore genesis, especially the completely confused superposition of the terms epigenetic and introduced from the outside. 3. All deposits show signs of inheritance; this leads us back to point one which involves economic accumulations under certain favorable conditions.
Neuere Veröffentlichungen zeigen die Notwendigkeit, drei Umstände anzuerkennen, ohne die eine petrologische Forschung nicht mehr möglich ist: 1. Wir müssen davon Abstand nehmen, von den bekannten Lagerstätten-Typen zu reden. Richtig ist vielmehr, daß jedes chemische Element in den verschiedenen Etappen des geologischen und geochemischen Geschehens angereichert werden kann; forschen wir also nach diesen Anreicherungen und verfolgen wir sie mit allen mineralogischen und geochemischen Mitteln, die uns zur Verfügung stehen. 2. Wir müssen die Verwirrung der chronologischen und räumlichen Vorstellungen vermeiden, vor allem die vollkommen falsche Überlagerung von epigenetisch und fremd. 3. Alles was wir beobachten, ist das Erbe einer mehr oder weniger fernen Vergangenheit. Nur wenn wir uns ständig über dieses Faktum klar sind, dann können wir unter günstigen Umständen zu frühen Anreicherungen hingeführt werden.相似文献
915.
916.
Pierre Dansereau 《Geoforum》1978,9(3):161-210
This paper attempts to place within an ecological framework the way in which man has appropriated and changed landscape. The ecosystem concept is used in order to facilitate a realistic examination of the ever changing relationship of man and nature. The resultant ecological grading of human settlements covers the full range of environments from “wild” to “urban”. It is hoped that such an approach will help to alert, educate, and instruct planners and decision makers as to the ecological cost of a number of problems which are normally viewed in economic, political, or social terms. 相似文献
917.
918.
Sediment cores and sediment traps were collected twice a month in two 35 m deep stations of Lake Geneva (Switzerland). The
organic input sedimenting to the bottom is equal to 157 g C m−2y−1 in station 1, to 214 g C in station 2. In spite of this difference, the oxygen uptake by the sediment (OUS) is similar in
both locations (46–47 g C m−2y−1). The oxygen uptake by the matter sedimenting to the bottom (OUSM) is respectively 45 g C m−2y−1 and 41 g C in stations 1 and 2. The equivalence between OUS and OUSM implies that most of the sedimented matter arriving
to the bottom is directly oxidized at the sediment surface. In station 1, OUS is positively correlated to OUSM, and OUSM is
positively correlated to chlorophyll-a concentrations in the water column (0–20 m) one week before sediment sampling. In location
2, OUS is positively correlated to the percentage of organic carbon and nitrogen in the sedimented matter, negatively to its
C:N ratio. Increasing allochthonous inputs have a negative influence on benthic respiration. At both sites, OUS is not directly
related to macrobenthic biomass or to temperature of bottom water. 相似文献
919.
Mössbauer studies of micas on the polylithionite-side-rophyllite join show the existence of a relation between the quadrupole splitting (ΔE Q) values of Fe2+ high spin doublets and both cationic and anionic composition of micas. This linear relation is positive as Li2O content increases and negative as F content increases. In the lithium iron micas, the inner ferrous quadrupole doublet is assigned to the cis-site M(2), while the outer doublet is assigned to the trans-site M(1). A random distribution of Fe2+ is observed in fluorine-rich compositions, while slight enrichment of the M(1) site is noticed in hydroxyl compositions, perhaps due to a more sensitive oxidation in situ in M(2) than M(1) sites. The Mössbauer spectrum of siderophyllite K2(Fe 4 2+ Al2)(Si4Al4)O20(OH)4 shows the presence of only one ferrous doublet, which is assigned to M(2) sites. Hence from Mössbauer data we must consider a clintonite (“xanthophyllite”) structure for this mica. The ordered octahedral layer has two distorted ferrous cis-sites and one, more symmetrical, aluminum trans-site. 相似文献
920.
Pierre Gangloff 《Geoforum》1981,12(3):273-280
Evidence of villafranchian permafrost has been found at the southern end of the Rhine graben, 20km west of Basel. Two syngenetic ice-wedge casts were discovered in the “alluvions du Sundgau”. From a stratigraphical point of view, these coarse gravels were deposited after the Miocene and before the Günz glaciation. The occurrence of syngenetic periglacial structures shows that they must be of Pleistocene (Villafranchian) age. A SEM study of sand from the infilling of one of the cryogenic structures showed numerous marks of glacial origin on the quartz grains. They are indicative of a pre-Günz glaciation in the Alps. It is suggested that permafrost in villafranchian time developed only on the fringe of the alpine glaciation, but further studies will be needed for a better picture of villafranchian palaeogeography. 相似文献