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131.
95 analyses of ore lead isotope ratios from 23 Phanerozoic ore deposits from the Swedish segment of the Fennoscandian Shield form a marked linear trend on a 207Pb/204Pb versus 206Pb/204Pb diagram. The line may be interpreted in a two-stage model, the lead being derived from 1.8±0.15 Ga old Svecokarelian basement and mineralization occurring at 0.4±0.15 Ga. The initial composition of the Svecokarelian rock lead was similar to the lead in early Proterozoic volcanogenic sulfide ores in Sweden. — The large spread in the isotope ratios was caused by a combination of selective leaching of different minerals in the source rocks, mixing with less radiogenic Caledonian lead, and local or regional variations in the U, Th and Pb contents of the basement. As a consequence, conventional methods of identifying source rocks from lead isotopic data (e.g. mu-values, Th/U ratios) may not be directly applicable. Phanerozoic ore lead development in the Swedish section of the Fennoscandian Shield was ensialic. That is, the ore lead was almost entirely derived from the Precambrian basement, although this basement does not appear to be anomalously enriched in Pb. No juvenile or mantle lead was apparently contributed to this section of the crust after the Precambrian, except for that mechanically transported onto the western edge of the Shield by the Caledonian nappes. However, some of Europe's largest lead deposits are included in these Swedish Phanerozoic mineralizations, suggesting that it was the nature of the processes involved rather than the richness of the source, that determined their formation. 相似文献
132.
The geochemistry and petrogenesis of the lavas of the Vulsinian District,Roman province,Central Italy 总被引:1,自引:0,他引:1
The Vulsinian lavas are dominated by a suite of undersaturated leucite-bearing basic to intermediate compositions. The remaining lavas are mainly oversaturated and have shoshonitic affinities. One hundred and thirty-five samples have been analysed for major elements and most for 20 trace elements. Twenty-seven lavas have been analysed for REE. They are all perpotassic (for the undersaturated lavas: K2O/Na2O=2–8) and have very high LIL element concentrations, (e.g. Rb=400–800 ppm, Th=25–150 ppm, REE/REEcho=c.200, (LREE/HREE)cho=c.20) even in the most basic rocks.The undersaturated lavas appear to be interrelated by fractional crystallization of cpx±olivine (from 14 to 11 wt.% CaO), cpx+leu±plg±mica (from 11 to 8 wt.% CaO), cpx+leu+plg+apa+magnetite±mica (from 8 to 5 wt.% CaO), and additional sanidine (or hyalophane)±haüyne (from 5 to 3 wt.% CaO). The saturated lavas and the few slightly undersaturated shoshonite basalts are thought to be evolved from the undersaturated magma(s) by crustal contamination or mixing with silica-rich magmas. The parental Vulsinian magma having: Mg-value=c.73, Cr=300–700 ppm, Ni=100–125 ppm, Sc= 40–50 ppm, Fo89–92, Di77–97 approximates a primary, mantle-derived liquid. Enrichment in LIL elements (incl. REE) and LREE/HREE suggest a small degree of partial melting from fertile mantle; whereas the low concentrations of Na, Ti and P suggest larger degrees of partial melting. This indicates that either the primary magma or the parental mantle was metasomatized by a fluid, which previously equilibrated with subducted continental material. This model agrees with published high 18O, high 87Sr/86Sr and low 143Nd/144Nd. 相似文献
133.
Michiel Rutgers van der Loeff Manmohan M. Sarin Mark Baskaran Claudia Benitez-Nelson Ken O. Buesseler Matt Charette Minhan Dai
rjan Gustafsson Pere Masque Paul J. Morris Kent Orlandini Alessia Rodriguez y Baena Nicolas Savoye Sabine Schmidt Robert Turnewitsch Ingrid Vge James T. Waples 《Marine Chemistry》2006,100(3-4):190
The short-lived thorium isotope 234Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of 234Th in both marine and freshwater systems. The original 234Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of 234Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate 234Th analytical technique for a specific aquatic regime and known sampling constraints. 相似文献
134.
135.
Energy spectra and angular distributions of auroral electrons in the energy range 0.2–16 keV measured by the low-altitude polar orbiting satellite ESRO 4 are presented. The observations were made in the altitude range 800–1000 km near magnetic midnight. Energy-time spectrograms show inverted-V structures with peaked energy spectra. The inverted-V events are associated with anisotropic electron pitch angle distributions peaked at 0 deg. Frequently these distributions have a maximum also at 90 deg. Measurements of >43 keV electrons indicate that the acceleration probably occurs on closed field lines. It is found that many properties of the observed particle distributions can be explained by acceleration in an electric field parallel to the magnetic field lines, if trapping of particles under an increasing potential drop is included in the model. 相似文献
136.
137.
直流电阻率测深中二维反演程序对三维数据的近似解释 总被引:5,自引:0,他引:5
直流电阻率测深剖面数据解释最好的方法是二维反演,这是由于它反演速度快,而三维反演则需要很多的实测数据、很长的计算时间。而且通过对几种三维模型测深数据的二维反演结果的研究,发现尽管对无走向的良导矿体,二维反演方法所圈定的异常中心与真实矿体中心有一定的误差,但基本上还是能反映矿体的真实形态和中心位置。当矿体具有一定的走向长度时,则无论对高阻矿体还是良导矿体二维反演的效果都很好。 相似文献
138.
The geochemical and mineralogical characteristics of bulk samples and various size fractions of sulfide-bearing fine-grained
sediments in the Petalax area, western Finland, were studied with ICP-AES (aqua regia digestion and "total" digestion) and
X-ray diffraction. The sediments, which are dominated by particles <0.06 mm (clay and silt size), are composed of phyllosilicates
(micas, kaolinite, chlorite, vermiculite), quartz, feldspars, amphiboles, organic matter, and secondary S minerals. The chemical
analyses show that Na, Ca, and Sr are enriched in the silt and sand fractions, that Zr is enriched in the silt fraction, and
that the concentrations of all other studied metals (Al, Ba, Co, Cr, Cu, Fe, K, La, Mg, Mn, Nb, Ni, Sc, Th, Ti, V, Y, and
Zn) increase from the sand to the clay fraction. Sodium, Ca, and Sr occur largely in poorly dissolved feldspars, which explains
the distribution of these metals in the sediments. The increase in the concentrations of most metals in the clay fraction
is interpreted to be the result mainly of an increase in phyllosilicates in this size fraction. A geochemical comparison between
the sulfide-bearing fine-grained sediments and glacial till shows that the former are enriched in several potentially toxic
metals. It is therefore argued that the hydrological and ecological problems associated with the sulfide-bearing sediments
are related not only to the production of acidity in oxidized layers, but also to mobilization and dispersion of toxic metals.
Strategies to minimize damage of freshwater systems in areas covered with sulfide-bearing fine-grained sediments are suggested.
Received: 20 February 1997 · Accepted: 25 August 1997 相似文献
139.
Anne-Marie Lézine Jean-François Saliège Christian Robert Frédéric Wertz Marie-Louise Inizan 《Quaternary Research》1998,50(3):290-299
Paleoecology and paleohydrology of the Ramlat as-Sab'atayn (Southern Arabia) are reconstructed from a comparative study including sedimentology, mineralogy, stable isotope ratios of carbonates, and palynology of lacustrine sediments recovered from the al-Hawa depression. The section dates from 8700 to 7200 yr B.P. and records an early phase of flooding followed by distinct lacustrine development from 7800 to 7200 yr B.P., coeval with maximum activity of the Indian monsoon. Comparison of the pollen record with modern pollen deposition suggests that regional vegetation was then already of desert type and was related to strong seasonal trade winds. 相似文献
140.