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《Geochimica et cosmochimica acta》1987,51(4):869-878
K-Ar ages of illite alteration associated with Middle Proterozoic Athabasca unconformity-type U deposits in Saskatchewan range from 414 to 1493 Ma. The K-Ar ages correlate with water contents and δD values such that illites with young K-Ar ages have δD values as low as −169 and water contents as high as 7.7 wt.% whereas illites with older ages have δD values near −70 and water contents near 4 wt.%. Water extracted at 400°C from illites with low δD values and high water contents has low δD and δ18O values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration. The alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the K-Ar ages and introduction of excess structural water in the form of interlamellar water. The interlamellar water is enriched in 18O by about 7 per mil relative to the water that physically surrounded the clay particle. Further alteration decreases the δD value and increases the δ18O value of the illite by isotopic exchange between the mineral and the interlamellar water. Although the chemical compositions and XRD patterns of the altered illites indicate that no detectable smectite component is present in the samples, the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed-layer illite/smectite by retrograde alteration of pure illite. The wide variation of δD values of chlorite and kaolinite from these U deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial. The general association of altered clays with areas containing the highest concentrations of U is probably related to localized permeability within the ore zone. 相似文献
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The KAr isochron method is an attempt to obtain the initial age or reset age and the extraneous argon isotopic ratio of a suite of cogenetic samples of different K-contents. Some samples contain excess argon-40 and others lose argon-40. The resultant ages on single samples are not significant geologically. We have discussed the principles of three types of isochrons commonly used, the 40Ar vs 40K isochron, the 40Ar/36Ar vs 40K/30Ar isochron, and the 40Ar/36Ar vs 39Ar/36Ar isochron, and evaluated the first two types for ten different hypothetical cases. If a straight line is obtained in the 40Ar vs 40K isochron, a positive intercept indicates an approximately constant amount of excess argon, whereas a negative intercept indicates argon loss. A curved line or scattering of points indicates that the basic assumptions are not valid for the set of samples under consideration. The 40Ar/36Ar vs 40K/36Ar isochron method is valid, rigorously, only when all samples of the system under consideration have the same non-radiogenic argon isotope composition. This requires that either no excess argon is present in the system, or else each analysis contains the same proportion of excess and atmospheric argon. If these conditions do not hold, approximately, invalid ages and invalid intercepts are obtained. Any KAr isochron needs to be used with caution. 相似文献
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David Alan Spears 《Geochimica et cosmochimica acta》1974,38(2):235-244
The RbSr age dating method has been applied to a sequence of Carboniferous shales for which some geochemical information was already available. In the marine shales, the Rb/Sr ratio is higher than in the non-marine and brackish water shales. Although the range of values can be attributed to depositional processes, the range of Rb/Sr values is too small to give a depositional isochron, and thus the age of deposition can not be determined. If samples with a variable mineralogy had been selected for the isotope study, the range of Rb/Sr could have been extended. Only by chance, however, would the depositional age have been obtained.A comparison of the RbSr isotope values in the Carboniferous shales with those in probable source rocks shows that a reduction in the Sr87/Sr86 ratio relative to the Rb87/Sr86 ratio may have taken place. Such a reduction could have occurred on the Carboniferous land-mass during the formation of the clay minerals. 相似文献
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《Lithos》1987,20(1):59-76
Plastically deformed ultramafic rocks in the Table Mountain and Blow-Me-Down Mountain ophiolites comprise a basal unit of slightly depleted Iherzolites, an intermediate sequence of strongly depleted harzburgites and an upper zone of dunites, also referred to as a transition zone intensively percolated by basaltic melts or magmatic fluids. Thirty-five samples including all of the above rock types have been investigated on 100 polished thin sections for CuFeNiS mineral assemblages. Most of them contain traces of CuFeNi sulfides, native metals and locally Ni arsenide. Compositional features of opaque assemblages as well as their textural sites in the rocks indicate that the present CuFeNiS minerals derive from an upper-mantle sulfide component through extensive subsolidus re-equilibration down to 100°C. The primitive component (predominant pentlandite, minor pyrrhotite and chalcopyrite) is preserved as sulfide inclusions in chromites of the transition zone, due to a subsolidus re-equilibration in a closed system. On the contrary, sulfide assemblages interstitial to silicates and spinel have extensively reacted with reducing serpentinizing fluids to produce sulfur-deficient sulfides such as heazlewoodite and mackinawite and native metals (native copper and awaruite). Microscale variations of redox conditions and the removal of Fe from the silicate during serpentinization may account for the peculiar “grain-by-grain” equilibrium state of intergranular assemblages. In spite of low-temperature alteration, a gradual depletion in sulfide component has been recognized from the basal lherzolites to the intermediate harzburgites while the sulfide content gradually increases in the transition zone (up to 0.2% by volume). The first pattern is consistent with the low-melting nature of the sulfide component in mantle melting processes. Microstructural criteria such as the absence of sulfide inclusions in olivine neoblasts demonstrate that the sulfide component postdates plastic deformation of the transition zone. The sulfide-enrichment pattern is thus ascribed to the percolation of a sulfur-saturated basaltic magma into residual dunites. 相似文献
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《Geochimica et cosmochimica acta》1987,51(6):1665-1673
An extraction-quench apparatus was used to measure calcite solubilities in supercritical CO2H2O mixtures. Experiments were conducted at 1 kbar and 2 kbar, between 240°C and 620°C and from XCO2 = .02 toXCO2 = .15 in order to determine the solubility behavior as a function of pressure, temperature and CO2 content. The results indicate that calcite solubilities under these conditions behave similarly to previously investigated calcite solubilities at lower pressures and temperatures (SHARP and Kennedy, 1965). At constant XCO2, the solubility increases with increasing pressure, but it decreases with increasing temperature. When the temperature and pressure are constant, the calcite solubility rises with increasing XCO2 to a maximum value at XCO2 between 0.02 and 0.05. For higher CO2 contents, up to XCO2 = .15, the calcite solubility decreases, probably due to the decrease of H2O activities to values significantly below unity.The solubility behavior can be successfully modeled by making the assumption that Ca++ is the dominant calcium species and that the carbon-bearing species are CO2(aq) and HCO−3. Since for these dilute H2OCO2 fluids, all activity coefficients can be assumed to not differ significantly from unity, ionization constants for the reaction H2O + CO2(aq) H+ + HCO−3 can be calculated at 1 and 2 kbar between 250°C and 550°C. These calculated values are in good agreement with the low temperature determinations of the ionization constants for this reaction determined by Read (1975). Values of the molal Gibbs free energy of CO2(aq) obtained in our study exhibit a much greater positive departure from ideality than those calculated with the modified Redlich-Kwong equations of either Flowers (1979) or Kerrick and Jacobs (1981) for dilute CO2 aqueous solutions. 相似文献
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《Geochimica et cosmochimica acta》1987,51(6):1651-1664
Microthermometry (MT) and laser Raman microprobe (LRM) spectroscopy (at room temperature and at about 0°C) were done on 33 synthetically produced CO2CH4H2O fluid inclusions in quartz (from R. Bodnar and M. Sterner). At room temperature, the inclusions consist of an aqueous liquid, a CO2CH4 supercritical carbonic fluid and (in most cases) graphite. In all these inclusions, the melting temperature for solid CO2 is less than that for the homogenization of the vapor bubble in the carbonic fluid.A method is described whereby MT data for CO2CH4H2O inclusions can be projected within the CO2CH4 binary phase diagram to infer CO2:CH4 ratios in the carbonic fluid (<2 to > 35 mole% CH4 in the inclusions under study). This method takes into account the formation of CO2CH4 clathrate hydrate during MT analysis. Unless clathrate formation is properly considered, errors arise in the determination of the bulk CO2CH4 ratio. For the inclusions in our study, these errors are on the order of 5 to 8 mole% CH4. Our interpretation of the MT data indicates that CH4 is preferentially partitioned into the clathrate over the coexisting carbonic fluid, in contradiction to the prediction from Parrish and Prausnitz's (1972) model for clathrate equilibria. Comparison of LRM analyses on the bulk carbonic fluid (in the absence of clathrate) and the residual carbonic fluid (in the presence of clathrate) confirm the preferential partitioning of CH4 into the clathrate. LRM analyses of the clathrate itself indicate that CH4 occupies both types of cage sites in the clathrate structure, whereas CO2 may only occupy one site. Two by-products of the combined LRM and MT analyses of the same inclusions are derivation of empirical ratios of Raman quantification factors for high-density CO2CH4 fluids and the ability to determine CO2:CH4 ratios of inclusions whose MT data lie near the critical region for CO2CH4. Thus, the joint use of LRM and MT techniques provides information that could not be obtained by either technique alone. 相似文献
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《Precambrian Research》1987,37(4):343-355
Major and trace elements including REE as well as SmNd isotope analyses of eight granulite samples from the Madras area are presented. The data provide evidence for the origin of some of the granulites, but in other cases demonstrate metasomatic changes in major and trace element contents, including LREE. The data furthermore demonstrate that the analysed granulites are not cogenetic. However, when plotted in a SmNd isochron diagram, the samples yield a linear array which corresponds to an age T = 2555 ± 140 Ma and ϵNd(T) = +1 ± 1.4. This age of 2555 ± 140 Ma could represent the time of granulite facies metamorphism immediately preceded by the formation of the protolith. The ϵNd(T) = 1 value may be indicative of a near chondritic mantle source, or alternatively of mixing between a depleted mantle and a crustal component. 相似文献
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《Organic Geochemistry》1987,11(2):91-101
Nuclear magnetic resonance (NMR) techniques have been applied to the structural characterization of humic substances isolated from an organic-rich sediment in Loch Thurnaig, northwest Scotland. Both the sedimentary humic acid (SHA) and sedimentary fulvic acid (SFA) fractions from Loch Thurnaig contained substantial quantities of carboxylic and alkyl carbon with a small contribution (26 and 22% respectively) of aromatic/olefinic carbon atoms. The latter structures were shown by dipolar dephasing 13C NMR to be largely non-protonated. Differences in the alkyl structures of the two fractions were noted, with the SHA containing a larger amount of branched chain aliphatic material. As branched chain compounds are more resistant to microbial degradation than straight chain compounds, this may indicate that SHA is the older, more refractory fraction of the sedimentary organic matter, although it is possible that the results reflect different algal precursor material. The fraction of aromatic carbon was higher in the SHA than in the SFA as is generally found for humic and fulvic acids isolated from the same source. The low aromaticities and highly branched aliphatic structures show that the sedimentary humic substances from the loch resemble dissolved marine humic substances rather than their terrestrial counterparts. 相似文献
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《Geochimica et cosmochimica acta》1987,51(6):1379-1384
The occurrence of C12C22n-alkanes with a strong even carbon-number preference is reported in recent and Miocene sediments, encompassing marine and freshwater systems, deposited under oxic and anoxic conditions. It is therefore shown that these n-alkanes can be found in a wider type of sedimentary environment than those previously described. The occurrence in some cases of a parallel distribution of C14C20n-alk-1-enes is also illustrated. A variety of biological sources, including both marine and freshwater bacteria as well as fungi and yeast species is suggested to account for such distributions. 相似文献
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《Journal of African Earth Sciences》2000,30(3):641-650
Biotite separates from Archaean granitoid lithologies on the Kaapvaal Craton north of the roterozoic Namaqua-Natal Belt in south eastern South Africa exhibit RbSr model dates of 967 ± 24 Ma for samples from within 25 km of the present northern limit of the Proterozoic thrust front. Samples from further north (>50 km to 170 km) have model RbSr dates of 2614 ± 74 Ma. The younger dates are interpreted as dating cooling after northwards emplacement of Proterozoic crust onto the Kaapvaal Craton, whereas the older dates are presumed to relate to an Archaæan metamorphic episode, possibly associated with intrusion of the post-Pongola granites. 相似文献
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《Geochimica et cosmochimica acta》1987,51(4):1011-1018
The extent of fractionation of Rb and Sr is routinely used in petrogenetic modelling of igneous processes, including internal fractionation of individual pegmatites as well as large-scale evolution of pegmatite groups and fields. However, highly evolved granitic pegmatites may contain as much as 14000 ppm Rb and less than 150 ppm Sr. The total Sr in K-feldspar and micas from geologically old and Rb-rich pegmatites may consist predominantly of radiogenic 87Sr, which obscures the original relationship of Rb to common Sr at the time of crystallization. A subtraction of radiogenic 87Sr calculated from the Rb content and age of emplacement is possible, but it commonly results in negative concentrations of Sr. The relative immobility of Rb, analytically determined isotopic composition of Sr, apparent ages of the Rb, Sr-bearing minerals, high concentration of 87Sr in coexisting Rb-poor phases, and experimental evidence indicate that post-crystallization migration of radiogenic 87Sr is significant. Where isotopic data are not available, RbSr trends in geologically old and highly fractionated pegmatites are misleading and cannot be used for geochemical interpretation of pegmatite derivation or evolution. 相似文献
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《Organic Geochemistry》1987,11(2):103-113
Four novel hexacyclic alkanes, fairly common in crude oils and rock extracts from evaporitic series, have been tentatively identified on the basis of GC/MS data as C32, C33, C34 and C35 hexahydrobenzohopanes. These structures, only recorded in carbonate-anhydrite sequences, i.e. very anoxic paleoenvironments, tend to concentrate when pristane to phytane ratios increase. Changes in their relative concentration to αβ hopanes appear to be more related to variations in source and/or environmental conditions than to maturity. The ratio of the novel hexacylic C35 hopane to C35 αβ hopane, compared with other biomarker ratios, suggests that hexacyclic alkanes and hopanes are byproducts of the same hopanoid precursors via different chemical reactions. In addition the novel hexacyclic alkanes are bacterially resistant and may serve as a useful family to define the paleoenvironment (viz. very anoxic) of the parent source rock of a drastically biodegraded oil. 相似文献
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《Precambrian Research》1987,36(1):1-20
UPb zircon systematics for granitic and gneissic clasts from the Moodies conglomerate of the Barberton Greenstone Belt, South Africa, exhibit discordancy patterns that suggest variable and multi-episodic lead loss and indicate crystallisation ages for the parent granitoids between 3.3 and 3.47 Ga. Metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2.8–2.9 Ga ago that is ascribed to a regional thermal event as also shown by published RbSr data for a nearby granitoid pluton. Since the Moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments, a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early Archaean granite-greenstone terrains. The maximum age of 3.3 Ga for the Moodies Group also shows that greenstone belt evolution in the Barberton region took place over a period of at least 150 Ma from about 3450 to 3300 Ma ago.The Kaap Valley and Stentor plutons intrude the lower part of the greenstone succession, and their diapiric emplacement caused large-scale deformation of earlier structures in the entire greenstone belt sequence at about 3.23–3.25 Ga ago.Detrital zircons in the sediments of the Fig Tree Group date the mean age of their source terrain at c. 3520 Ma which is also the maximum age of deposition for these strata. This age is indistinguishable from the oldest age reported from the Barberton greenstones and suggests the possibility that pre-greenstone granitoid crust may exist in the eastern Kaapvaal craton. 相似文献
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《Precambrian Research》2001,105(2-4):289-314
The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative εNd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture. 相似文献