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1.
浙东南中生代晚期火山岩Nd同位素组成及其他地层学意义   总被引：21，自引：3，他引：21  
浙江东南部中生代晚期火山地层可划分为三个火山旋回，通过对不同层位火岩岩Nd同位素组成的研究，得出结果：第一至第三旋回火山岩（自下而上）εNd(t)值依次为－14．1－6．2，平均－9．9；－7．7－－6．4，平均－7．0及－7．5－＋0．1，平均－4．6。t2DM值依次为1．6－1．8Ga，平均1．7Ga；1．4－1．6Ga，平均1．5Ga及0．9－1．5Ga。由此可见，在不同的火山旋回之间具有明显的Nd同位素组成特征差异，它反映了该区中生代晚期火山活动随时间的推移，在构造环境和岩浆来源上所发生的有序变化，从而，Nd同位素示踪信息也就有可能作为火山地层划分和对比的重要依据之一。  相似文献   

2.
The efficiency of neutron activation as an analytical technique has been evaluated for exploration of lateritic and karstic bauxites in Sierra Leone, Guinea, Australia (Gove), Philippines and Jamaica. The 10-g bauxite samples are prepared in the field and sent to a central laboratory. With the “Automatic Bauxite Analyzer” — equipped with an isotopic neutron source — a single Al and Si determination can be made in 5 minutes. For calibration, standards are required which are similar in composition to the samples to be analyzed. Using sample pairs and averaging the results of three subsequent determinations an accuracy of ±2% Al2 O3 and about ±0.6% SiO2 can be obtained. Time requirements, accuracy and cost of this technique are compared with wet chemistry and X-ray fluorescence. For an exploration programme incorporating for example 15,000 drill core samples, a 50% reduction in expenditure for analytic determinations can be achieved by applying neutron activation instead of X-ray fluorescence alone.  相似文献   

3.
Siliceous nodules in the upper Eocene to middle Miocene Cypress Hills Formation in southern Saskatchewan formed on the distal portions of an extensive braidplain. The nodules are similar to silcretes described elsewhere in the literature but their morphology and occurrence suggests that they are of a rare variety. The silcretes are discrete equant to disc-shaped nodules, 1–3 cm thick, and up to 15 cm long. The silicretes form horizontal, discontinous layers parallel to bedding within an unweathered profile. Void spaces and fractures within the nodules are lined or filled with drusy quartz. The silcretes contain less than 0.07% TiO2. The TiO2/SiO2/Al2O3 and TiO2/SiO2/Fe2O3 ratios are similar to values obtained from nodules formed in an arid to semi-arid environment based on comparison with modern silcretes. This interpretation is confirmed by independent sedimentological and palaeontological climatic evidence for an arid to semi-arid climate throughout Oligocene time in the western Canadian plains.  相似文献   

4.
This paper presents an elastoplastic model for saturated expansive and non-expansive clays. The original feature of this model is that a plastic mechanism is introduced during unloading to take into account the irreversible swelling of the macroporosities. These strains are induced by the repulsive stresses which are unbalanced at the scale of the microporosities. Thus two yield surfaces are activated: a classical contact yield surface (FC) similar to an associated modified Cam-clay approach and a swelling yield surface (FR−A) based on the non-associated plasticity. The formulation considers that for the normally consolidated stress states, the strains are mainly produced by an increase of the contact stresses. For the overconsolidated stress states, the repulsive stresses balance the external stresses. The rheological parameters are easily determined from the results of either triaxial or oedometer tests. The model is then used in a finite element program, using the classical concepts of plasticity, especially for the loading–unloading criterion based on the sign of the plasticity multiplier. Simulations of the convergence of a gallery (under an earth retaining structure) sunk at great depth in Boom clay are presented. The results are compared with those obtained with the Cam-clay model. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

5.
The Peuyuk kimberlite is divisable into three petrographically distinct phases (A, B, and C) on the basis of oxide mineralogy and the presence or absence of an immiscible carbonate liquid.Phase A contains spinels (titan-magnesian-aluminous chromite) and perovskite with no reaction rims or complex mantles, and no evidence of an immiscible carbonate liquid.Phase B contains complexly zoned and mantled oxides with no evidence of an immiscible carbonate liquid. The spinels are zoned from titan-magnesian-aluminous chromite to members of the magnesian ulvospinel-ulvospinel-magnetite series. The zoned spinels are mantled by a serpentine-carbonate mixture followed by a mixture of Ti free magnetite and minor rutile. Perovskites are mantled by nickeliferous pyrite (0–11 % Ni) and rutile.Phase C is characterized by the presence of an immiscible carbonate liquid. Spinels are either discrete crystals of magnesian ulvospinel-ulvospinel-magnetite or as discrete rims of this composition upon cores of titan-magnesian-aluminous chromite. Rutile rims are poorly developed upon perovskites. Sulphides associated with the silicate groundmass are Cu-pentlandite, heazle-woodite and chalcopyrite. Nickeliferous pyrite is associated with the carbonate fraction.Common to all phases of the intrusion is a red titanium poor aluminous-magnesian chromite which is interpreted to have formed prior to the fluidized intrusion of the kimberlite. All other spinels being considered to have crystallized after fluidization.The spinel composition trends may reflect decreasing oxygen fugacities (10–19 to 10–22 bars) in response to falling temperatures (800–600° C) along the quartz-fayalite-magnetite buffer.In the immediate post-fluidization history of the diatreme all phases crystallized titan-magnesian-aluminous chromite but the trends of Ti and Fe enrichment observed in phases B and C was prevented in phase A possibly by rapid cooling. Slower cooling coupled with reaction of earlier formed phases with residual H2O and CO2 rich fluids resulted in the phase B assemblage. Extensive reaction of this type was prevented in phase C by the separation of an immiscible carbonate liquid.  相似文献   

6.
Redox processes are ubiquitous in Earth science and are often associated with large isotope fractionations. In a previous study, voltage-dependent amplification of stable isotope fractionation was observed for an Fe reduction process. Here, we describe experiments showing a similar effect for a second transition metal, zinc. After electrochemical reduction, the composition of plated Zn metal is enriched in the light isotope (64Zn) with respect to the Zn2+ leftover in solution, with a voltage-dependent fractionation factor. Results from voltage-dependent electroplating experiments are in good agreement with a second data set following equilibrium fractional isotope evolution of Zn isotopes during an electroplating process which stepwise removes most of the Zn from the aqueous reservoir. Taken together, the results indicate a voltage-dependent isotope fractionation (in permil) of 66Zn with respect to 64Zn to be equal to −3.45 to 1.71 V. The negative slope trend is in contrast with previously published results on iron isotope fractionation during electroplating which shows a positive slope. These results are interpreted using an extension of Marcus theory, which predicts isotope fractionations as a function of driving force in an electrochemical system. Taken together with observations of natural fractionation of redox-sensitive and non redox-active elements, our modified Marcus theory provides a framework for quantitatively predicting transition metal isotope geochemical signatures during environmentally relevant redox processes in terms of simple energetic parameters.  相似文献   

7.
This study presents the geochemical characteristics of granitic rocks located on the northern margin of Chotanagpur Gneissic Complex (CGC), exposed in parts of Gaya district, Bihar and discusses the possible petrogenetic process and source characteristics. These granites are associated with Barabar Anorthosite Complex and Neo-proterozoic Munger–Rajgir group of rocks. The granitic litho-units identified in the field are grey, pink and porphyritic granites. On the basis of geochemical and petrographic characteristics, the grey and pink granites were grouped together as GPG while the porphyritic granites were named as PG. Both GPG and PG are enriched in SiO2, K2O, Na2O, REE (except Eu), Rb, Ba, HFSE (Nb, Y, Zr), depleted in MgO, CaO, Sr and are characterised by high Fe* values, Ga/Al ratios and high Zr saturation temperatures (GPGavg～ 861 °C and PGavg～ 835 °C). The REE patterns for GPG are moderately fractionated with an average (La/Yb)N～ 4.55 and Eu/Eu* ～ 0.58, than PG which are strongly fractionated with an average (La/Yb)N～ 31.86 and Eu/Eu* ～ 0.75. These features indicate that the granites have an A-type character. On the basis of geochemical data, we conclude that the granites are probably derived from a predominant crustal source with variable mantle involvement in a post-collisional setting.  相似文献   

8.
Statistics of ultimate strain of the earth's crust are obtained on the basis of levelling and triangulation data over earthquake areas. The mean value of ultimate strain e0 is obtained as 5.3 · 10?5 with a standard deviation σ amounting to 3.3 · 10?5 on the assumption that the deviation from the mean value is described by a Gaussian distribution.Assuming that crustal strain increases linearly with time t from an approximately zero value immediately after a large earthquake, which occurred at t = 0, the probability of having a crustal rupture or an earthquake occurrence during a time-interval from 0 to t can be calculated from e0 and a along with the data for strain accumulation over the area concerned as brought out by repetitions of geodetic survey.Applying the above theory to an area southwest of Tokyo, where an earthquake of magnitude 7.9 took place in 1923, the probabilities for repetition of an earthquake there are estimated as 0.2, 0.5 and 0.8 respectively for periods 1925–1980, 1925–2030, and 1925–2080.Similar studies are made for the areas off eastern Hokkaido and the Tokai district in Central Japan. No geodetic data over focal regions are available in these cases because observations are made only on land more than 100 km distant from epicentral area off the coast. In the circumstances theoretical land deformations caused by a plate subduction, which is believed to be taking place at the trench axis, are compared to the deforma tions actually detected by repeated surveys. Although the reliability of probability calculated on the basis of such processes may be substantially lower than that based on data taken in an area immediately covering a focal region, it is striking that the probabilities of reoccurrence of a large earthquake for a time-interval from the last shock to the present are so high that they exceed 0.8 ~ 0.9 for reasonable values of parameters involved.  相似文献   

9.
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10.
Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi3O8 in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi3O8 in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica.  相似文献   

11.
Calibrations are presented for an independent set of four equilibria between end-members of garnet, hornblende, plagioclase and quartz. Thermodynamic data from a large internally-consistent thermodynamic dataset are used to determine the ΔG° of the equilibria. Then, with the known mixing properties of garnet and plagioclase, the non-ideal mixing in amphibole is derived from a set of 74 natural garnet–amphibole–plagioclase–quartz assemblages crystallised in the range 4–13 kbar and 500–800 °C. The advantage of using known thermodynamic data to calculate ΔG° is that correlated variations of composition with temperature and pressure are not manifested in fictive derived entropies and volumes, but are accounted for with non-ideal mixing terms. The amphibole is modelled using a set of ten independent end-members whose mixing parameters are in good agreement with the small amount of data available in the literature. The equilibria used to calibrate the amphibole non-ideal mixing reproduce pressures and temperatures with average absolute deviations of 1.1 kbar and 35 °C using an average pressure–temperature approach, and 0.8 kbar with an average pressure approach. The mixing data provide not only a basis for thermobarometry involving additional phases, but also for calculation of phase diagrams in complex amphibole-bearing systems. Received: 8 November 1999 / Accepted: 7 July 2000  相似文献   

12.
Weights of evidence modeling for combining indicator patterns in mineral resource evaluation is based on an application of Bayes' rule. Two weights are defined for each indicator pattern and Bayes' rule is applied repeatedly to combine indicator patterns. If all patterns are conditionally independent with respect to deposits, the logit of the posterior probability can be calculated as the sum of the logit of the prior probability plus the weights of the overlay patterns. The information to be integrated for gold exploration in Xiong-er Mountain Region comes from a geological map, an interpreted map of a Thematic Mapper (TM) image, and the locations of known gold deposits. Favorable stratigraphic units, structural control factors, and alteration factors are considered. The work was conducted on an S600 I2S image-processing system. FORTRAN programs were developed for creating indicator patterns, statistical calculations, and pattern integration. Six indicator patterns were selected to predict mineral potential. They are conditionally independent according to pairwiseG 2 tests, and an overall chi-square test. The potential area predicted using the 32 known deposits generally coincides with the prospect areas determined by geological fieldwork.  相似文献   

13.
Tourmaline synthesised in an experiment with low boron excess was analysed in situ by secondary ion mass spectrometry. It revealed significant B isotope zonation with 11B/10B ratios increasing in the growth direction of the crystals. Trend, magnitude and absolute values strongly support results from high-B-excess isotope fractionation experiments. Furthermore, the closed system B-isotopic evolution of the experimental fluid was modelled by Rayleigh fractionation. The model results are in excellent agreement with the measured B-isotope composition of the run-product fluid. Consequently, low-element-excess experiments are proposed as an ideal approach to determine fluid-solid isotope fractionation factors for systems that are characterised by Rayleigh fractionation.  相似文献   

14.
Proxy climate data can be obtained from reconstructions of hydrological changes on ombrotrophic (rain‐fed) peatlands using biological indicators, such as testate amoebae. Reconstructions are based on transfer functions, relating modern assemblage composition to water table and moisture content, applied to fossil sequences. Existing transfer functions in Europe and elsewhere are limited geographically and there are often problems with missing or poor analogues. This paper presents a new palaeohydrological transfer function based on sampling raised mires from across Europe. Relationships between assemblages and hydrological variables are described using ordination analyses. Transfer functions are developed for depth to water table (n = 119) and moisture content (n = 132) with root mean squared errors (RMSEP) of 5.6 cm and 2.7% respectively. Both transfer functions have an r2 of 0.71, based on ‘leave one out’ cross‐validation. Comparisons with an existing transfer function for Britain show that the European transfer function performs well in inferring measured water tables in Britain but that the British data cannot be used to infer water tables for other European sites with confidence. Several of the key missing and poor analogue taxa problems encountered in previous transfer functions are solved. The new transfer function will be an important tool in developing peat‐based palaeoclimatic reconstructions for European sites. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

15.
Lattice defects along (100) planes in an orthopyroxene, En92.8-Fs7.0-Wo0.2, have been observed in the transmission electron microscope. From an analysis of the electron diffraction contrast at these defects it is proposed that the defects are regions of iron and possibly calcium enrichment. The defects are completely coherent with the lattice of the matrix and appear to be metastable or transition zones. They do not have any direct relationship with the spinodal decomposition proposed for the exsolution textures in some of the lunar pyroxenes of Apollo 11 and 12. Possible cationic ordering within these zones is discussed and the effects of such ordering on magnetic properties are considered.  相似文献   

16.
The Bogda Shan orogenic belt is interpreted to be an Upper Palaeozoic continental rift, which was closed towards the end of the Carboniferous period. Intrusive activities in that belt are represented mainly by a large number of diabasic sills, dykes and stocks with sparse dioritic and granitic bodies. Determinations on a group of samples from the Shangdaheyan intrusion yielded a mineral-rock Rb-Sr isochron with an age of 298.4±0.76 Ma and an initial87Sr/86Sr ratio of 0.7041. In combination with field evidence, these results indicate that most of the intusions of the Bogda Shan orogenic belt were emplaced during the Hercynian cycle as a result of initial extension following rift closure, and that post-Hercynian intrusive activities are not important in that belt.  相似文献   

17.
Evaluation of the hydraulic conductivity of aquifers with piles   总被引：2，自引：0，他引：2  
Distributed piles are often installed in the foundation pit in sandy soil before dewatering. Hydraulic conductivity should be estimated considering the block effect of piles on groundwater seepage. Research shows that the effective medium theory (EMT) could be used to calculate the equivalent hydraulic conductivity (k eq) of a heterogeneous medium with other material inclusions. In order to verify the applicability of EMT in an aquifer with piles, an experimental investigation is conducted. In this experiment, a sandy soil is considered as an aquifer and polyvinylchloride pipes are adopted as piles. Piles are distributed in rectangular and triangular layouts. The relationship between k eq of the aquifer and the volume replacement ratio is plotted for these two layouts. The results indicate that EMT could well predict k eq for the cases with the rectangular layout; however, for triangular layouts, the prediction becomes poor. To provide a better prediction of k eq of the aquifer with piles distributed in a triangular layout, a modified EMT (MEMT) is proposed, in which a correction factor is adopted considering different layouts of piles. The MEMT can provide better k eq predictions for an aquifer with piles in the triangular layout pattern than those obtained using the EMT.  相似文献   

18.
The study of mafic and aluminous granulites from the Monte CasteloGabbro (Órdenes Complex, NW Spain) reveals an anticlockwiseP–T path that we interpret as related to the tectonothermalactivity in a magmatic arc, probably an island arc. The P–Tpath was obtained after a detailed study of the textural relationshipsand mineral assemblage succession in the aluminous granulites,and comparing these with an appropriate petrogenetic grid. Additionalthermobarometry was also performed. The granulites are highlyheterogeneous, with distinct compositional domains that mayalternate even at thin-section scale. Garnets are generallyidiomorphic to subidiomorphic, and in certain domains of thealuminous granulites they show overgrowths forming xenomorphiccoronas around a more or less idiomorphic core. Both types ofgarnets show significant Ca enrichment at the crystal rims,which, together with the other mineralogical and textural characteristics,is compatible with a pressure increase with low T variation.P–T estimations indicate a peak of T > 800°C andP  相似文献   

19.
This contribution describes primary basalt mineral saturation in surface- and up to 90°C ground waters in a tholeiite flood basalt region in northern Iceland. It is based on data on 253 water samples and the mineralogical composition of the associated basalts. Surface waters are significantly under-saturated with plagioclase and olivine of the compositions occurring in the study area, saturation index (SI) values ranging from −1 to −10 and −5 to −20, respectively. With few exceptions these waters are also significantly under-saturated with pigeonite and augite of all compositions (SI = −1 to −7) and with ilmenite (SI = −0.5 to −6). The surface waters are generally over-saturated with respect to the titano-magnetite of the compositions occurring in the basalts of the study area, the range in SI being from −2 to +10. For crystalline OH-apatite, SI values in surface waters range from strong under-saturation (−10) to strong over-saturation (+5) but for crystalline F-apatite they lie in the range 0 to 15. Systematic under-saturation is, on the other hand, observed for “amorphous apatite,” i.e. an apatite of the kind Clark (1955) prepared by mixing Ca(OH)2 and H3PO4 solutions. Like surface waters, ground waters are under-saturated with plagioclase and olivine, its degree increasing with increasing Ca content of the plagioclase and increasing Fe content of the olivine, the SI values being −2 to −7 and 0 to −4 for the Ca-richest and Ca-poorest plagioclase, respectively, and about −3 to −18 and 0 to −15 for forsterite and fayalite, respectively. Ground waters are generally close to saturation with pigeonite and augite of all compositions. However, some non-thermal ground waters in highland areas are strongly under-saturated. Above 25°C the ground waters are ilmenite under-saturated but generally over-saturated at lower temperatures. These waters are titano-magnetite over-saturated at temperatures below 70°C, the SI values decreasing with increasing temperature from about 6 to 8 at 10°C to 0 at 70°C. The ground waters are highly over-saturated with both crystalline OH- and F-apatite, or by approximately 10 to 15 SI units but close to saturation with “amorphous apatite” containing about equal amounts of F and OH. The results presented here for the pyroxenes carry an unknown error because available thermodynamic data do not permit but a simple solid solution model for the calculation of their solubility. Published values on the dissociation constants for ferrous iron hydroxide complexes are very variable and those for ferric iron are limited. This casts an error of an unknown magnitude on the calculated SI values for all iron bearing minerals. This error may not be large for waters with a pH of less than 9 but it is apparently high for waters with a higher pH. Improved experimental data on the stability of ferrous and ferric hydrolysis constants are needed to improve the accuracy by which Fe-mineral saturation can be calculated in natural waters.  相似文献   

20.
The Dôme de l'Agout consists of an ellipsoidal mass of gneisses and migmatites emerging from a large area of chlorite-sericite schists. Fluid inclusions in syn-metamorphic quartz segregations are typically one-phase at room temperature. Analyses by gas chromatography indicate that their main constituents are N2, CH4 and CO2; such compositions are confirmed by freezing studies on individual inclusions. Nitrogen contents of the inclusions range from 2 to 72 mol% and tend to increase with increasing degree of metamorphism. CH4 and CO2 show an erratic distribution with the exception of the lowest grade samples, which invariably have very high CO2 contents. The origin of the nitrogen remains unsolved; however, it seems likely that the nitrogen was not produced by mineral reactions in the presently exposed rocks but came from an external source and moved from the centre of the dome outwards.  相似文献   

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