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21.
We have determined K, Rb and Sr concentrations and87Sr/86Sr ratios in fresh surface waters, a rain water sample and five geothermal waters from the Cantal volcanic area in the Massif Central, France. A comparison with appropriate rock types of the region showed no apparent chemical and isotopic fractionation occurring in the fresh water-surface rock system. The thermo-mineral water results suggest that all springs discharge dissolved Sr from the following contributors: Hercynian granito-metamorphic basement, lacustrian sediments underlying the volcano, Miocene-Pliocene volcanic rocks of basaltic to rhyolitic composition. 相似文献
22.
Annie Vitrac Michard Claude J. Allègre 《Contributions to Mineralogy and Petrology》1975,50(4):257-285
Rb-Sr whole rock data for gneisses and granites from the oriental Pyrénées yield cadomian ages for the basement gneisses and hercynian ages for the granites. Granites occur in two generations. An alkaline granite dated at 335±15 m.y. is syn.—or slightly post-tectonic, and a later calcalkaline granite dated at 275±12 m.y, is clearly intrusive and marks the end of hercynian tectono-metamorphism. Initial 87Sr/86Sr ratios of the granites put limits on the proportion of juvenile contribution from the mantle to their formation. Petrogenetic models are tested using Rb and Sr as tracers. 相似文献
23.
Bruno Hamelin Gérard Manhes Francis Albarede Claude J. Allègre 《Geochimica et cosmochimica acta》1985,49(1):173-182
A reappraisal of the “Double Spike Method” for correction of the mass discrimination in lead isotope analysis is presented in view of the possible efficiency of this method, especially as related to the improved performance of the new multi-collector mass spectrometers.First, the effects of mass discrimination during lead isotope analysis, using the silica-gel phosphoric acid method, are discussed in order to verify the validity of the linear discrimination law. In this discussion, we give evidence of a slight bias of the isotopic composition of the SRM 981 NBS lead standard, commonly used for mass discrimination calibrations.A rigorous way to deal with the propagation of the within-run statistical errors on the precision of the double spike correction is then presented. This allows the optimization of the parameters of the method (spike isotopic composition and precision, spiking proportion) and the calculation of the respective error associated with each corrected isotopic ratio. The perturbation of the double spike correction due to analytical contamination, which has caused previous failure in the application of the method, is avoided by preparation of the mixture between the spike and the sample at the very end of the chemical separation procedure.Using a single collector mass spectrometer, examples of application on lead standards and on a set of samples suggest a possible improvement in the precision by a factor of 3 compared to the usual method of fractionation normalization and confirm the usefulness of the method with the new generation of mass spectrometers. 相似文献
24.
Volcanic islands, being characterized by highly porous basaltic/andesitic lava flows and pyroclastic deposits, are subject
to important chemical weathering by subsurface waters. Moreover, such subsurface weathering is impacted by hydrothermal springs
in both active and non-active volcanic areas, thus increasing dissolved load concentrations. Here, we focus on the subsurface
water chemistry in the volcanic islands of the Lesser Antilles and Réunion and on the origin of these subsurface flows. We
are able, through the use of various isotopic tools (C, Sr, U–Th), to identify hydrothermal influences in river water. For
example, Li concentrations show a positive correlation with temperature of hot and cold springs and also a relationship with
δ13C; from this, we can show that several sources of hydrothermal activity influence the rivers of the Lesser Antilles and that
some rivers also reveal an important organic influence. As much as 20% of the subsurface hydrothermal springs go to feed the
rivers. The increasing temperatures result in more dissolved elements being mobilized and an increase in chemical weathering
rates. In addition, using the (230Th/238U) isochron for the well and river dissolved loads in Martinique, Guadeloupe and Réunion, we can evaluate residence times
in the river water, i.e. the average residence time in the water along the circulation path to the sampling point. Alteration
takes longer when the water circulates through thick soil, for example, 400–5,000 years when circulating under an ash profile
and 1,200–15,000 years when circulating through a collapse zone. It would appear that waters circulation is globally three
times longer for subsurface water than for surficial water. The weathering regime in tropical volcanic environments seems
to be controlled mainly by such subsurface circulation with high chemical concentration from hydrothermal inputs. The origin
of these compositions is varied and not controlled by a single hydrothermal spring. Consequently, it is subsurface circulation
that determines the weathering regime in tropical volcanic islands with the main controlling parameters being temperature
and residence time. 相似文献
25.
The area from the Greater Caucasus to the southeast Turkey is characteri：;.ed and shaped by several major continental blocks. These are Scythian Platform, Pontian-Transcaucasu.,; Continent-Arc System （PTCAS）, the Anatolian-lranian and the Arabian Platforms. The aim of this paper is to define these continental blocks and describe and also compare their boundary relationships along the suture zones. The Scythian Platform displays the evidence of the Hercynian and Alpine orogens. This platform is separated from the PTCAS by the Greater Caucasus Suture Zone. The incipient collision began along this suture zone before middle-late Carboniferous whereas the final collision occurred before Oligocene. The PTCAS can be divided into four structural units： （1） the Georgian Block - northern part of the Pontian-Transcaucasian island-arc, （2） the southern and eastern Black Sea Coast-Adjara-Trialeti Unit, （3） the Artvin-Bolnisi Unit, comprising the northern part of the southern Transcaucasus, and （4） the Imbricated Bayburt-Garabagh Unit. The PTCAS could be separated from the Anatolian Iranian Platform by the North Anatolian-Lesser Caucasus Suture （NALCS） zone. The initial collision was developed in this suture zone during Senonian-early Eocene and final collision before middle Eocene or Oligocene-Miocene. The Anatolian-lranian Platform （AIP） is made up of the Tauride Platform and its metamorphic equivalents together with Iranian Platform. It could be separated from the Arabian Platform by the Southeastern Anatolian Suture （SEAS） zone. The collision ended before late Miocene along this suture zone. The southernmost continental block of the geotraverse is the Arabian Platform, which constitutes the northern part of the Arabian-African Plate. This platform includes a sequence from the Precambrian felsic volcanic and clastic rocks to the Campanian-early Maastrichtian fiyschoidal clastics. All the suture zones include MORB and SSZ-types ophiolites in different ages. However, the ages of the suture 相似文献
26.
Dr. L. G. Gwalani Prof. N. M. S. Rock Dr. W. -J. Chang Dr. S. Fernandez Prof. C. J. Allégre Prof. A. Prinzhofer 《Mineralogy and Petrology》1993,47(2-4):219-253
Summary Rift-related, late Eocene ( 60 Ma) alkaline-carbonatitec intrusions cover 1200 km2 south of the town of Chhota Udaipur, and form a subprovince within the alkaline magmatism that accompanies the tholeiitic Deccan Traps. They were emplaced temporally between late Deccan Trap flows and late dykes of basalt and picritic basalt. The subprovince comprises five main geographic occurrences (sectors): (1)Amba Dongar: a ring-complex of Ca-Mg-Fe-carbonatites, nephelinites to tephriphonolites, and fluorite deposits; (2)Siriwasan-Dugdha: an intrusive complex of Ca—carbonatites, nephelinites and trachytic rocks; (3)Phenai Mata: a nepheline syenite plug and dykes, plus dykes of tephrites to phonolites and lamprophyres (intimately associated with a layered tholeiitic gabbro—granophyre intrusion); (4)Panwad-Kawant: dykes and plugs of lamprophyres and tephrites to phonolites; (5)Bakhatgarh-Phulmahal: late basic-ultrabasic dykes only. The alkaline rocks range from ultrasodic to ultrapotassic, but are mostly nonperalkaline. Silica-undersaturated examples show higher incompatible and LIL element contents (Rb, K, Nb, Zr, Sr, Ba, LREE, etc.) than the associated tholeiites. However, the late basic-ultrabasic dykes display an continuum of alkaline-tholeiitic compositions. Possible parent magmas are represented among the primitive undersaturated basic dykes (including lamprophyres). The trachytic rocks are subalkaline, and may be genetically related to a tholeiitic rather than alkaline parent magma.
With 8 Figures 相似文献
Alkaligesteine und Karbonatite von Amba Dongar und Umgebung, Dekkan-Provinz, Gujarat, Indien: 1. Geologie, Petrographie und Gesteinschemie
Zusammenfassung Alkalisch-karbonatitische Intrusionen aus dem Obereozän (60 Ma) erstrecken sich über 1200 km2 südlich der indischen Stadt Chhota Udaipur und bilden einen Teil des alkalischen Komplexes, der die tholeiitischen Deccan Traps begleitet. Die Gesteine wurden während eines Rifting Prozesses intrudiert und lagern stratigraphisch zwischen Deccan Trap Tholeiiten und basaltischen bzw. pikritischen Ganggesteinen. Das Gebiet läßt sich in fünf geographische Regionen unterteilen: (1) Amba Dongar: ein ringförmiger Komplex aus Ca-Mg-Fe-reichen Karbonatiten, Nepheliniten und Tephriphonoliten mit Fluorit-Lagerstätten; (2) Siriwasan-Dugdha: eine Ca-reiche Karbonatit-Intrusion mit Nepheliniten und Trachyten; (3) Phenai Mata: Eine Nephelin-syenitische Intrusion und begleitende Ganggesteine aus Tephriten, Phonoliten und Lamprophyren, die mit einer magmatisch geschichteten tholeiitischen Gabbro Intrusion vergesellschaftet sind; (4) Panwad-Kawant: Lamprophyrische Ganggesteine neben Tephriten und Phonoliten; (5) Bakhatgarh-Phulmahal: späte basisch bis ultrabasische Ganggesteine. Die alkalischen Gesteine besitzen eine ultrapotassische bis extrem Na-reiche Zusammensetzung, nur vereinzelte Proben weisen jedoch peralkalischen Charakter auf. Silizium-untersättigte (alkalische) Proben besitzen in der Regel höhere Konzentrationen an Mantel-inkompatiblen Elementen (z.B. LREE, Nb und Zr) und LILE (z.B. Rb, Sr und Ba) als die benachbarten Tholeiite. Die späten basisch bis ultrabasischen Ganggesteine stellen ein Kontinuum von alkalischer bis tholeiitischer Geochemie dar. Die primitiven Silizium-untersättigten Ganggesteine (und Lamprophyre) repräsentieren vermutlich die Ausgangssehmelzen; die subalkalischen Trachyte dürften in genetischem Zusammenhang mit den Tholeiiten stehen.
With 8 Figures 相似文献
27.
利用塔克拉玛干沙漠大气环境观测试验站(塔中站)实测的长波辐射资料,分析了流动沙漠区的长波辐射特征.结果表明:地表长波辐射能具体地反映地表失去热能的状况.在沙漠区,由于地表层的湍流运动较强以及下垫面温度与空气温度的差值较大,使得地表向大气传输的热通量较大,所以地面长波辐射值较大,有时接近于总辐射,最小值出现在日出前,最大值出现在15:00左右;由于大气中的水汽含量及云量较少,而沙尘气溶胶含量较高,所以大气长波辐射较大,沙漠区有效辐射也较大,为我国有效辐射年总量最高的地区之一.地表放射的净长波辐射在太阳总辐射中所占的比例小于地表反射辐射,只有在春、夏季两者较接近,且以反射辐射大些.区域气候变化过程中,沙漠加热场的强度是沙漠局地天气气候变化的主要影响因素之一. 相似文献
28.
29.
Claude J. Allègre Bernard Dupré Bernard Lambret Pierre Richard 《Earth and Planetary Science Letters》1981,52(1):85-92
Pb, Nd and Sr isotopic compositions have been determined in lherzolite-xenolith-bearing alkali-basalts from the center of the African shield. The present data are very similar to those reported for ocean-island basalts and do not support the hypothesis of different mantle sources for alkali-basalts from continental and oceanic areas. From these observations and on the basis of data obtained for xenolith in kimberlite and for tholeiitic continental basalts one may infer the following terrestrial mantle structure: whereas oceanic tholeiites would originate in upper oceanic mantle, oceanic and continental alkali basalts would come from the lower mantle and tholeiitic continental basalts from the continental lithosphere. 相似文献
30.
Using terrestrial osmium-enriched samples of known ages, we have shown that187Os/186Os varies with time in result of the187Re β? radioactivity. Such a variation in the earth's mantle can be fitted by a straight line corresponding to187Re/186Os = 3.15 for the mantle, comparable to C1 carbonaceous chondrites. Using this result and the Re and Os contents of various rocks, several theoretical considerations and predictions can be deduced for the chemical evolution of the earth, such as a method for distinguishing between different processes of development of the continental crust. The special result of187Os/186Os from Bushveld is discussed with respect to the possible existence of an “enriched” subcontinental mantle. 相似文献