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1.
The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with 607 Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by phlogopite metasomatism. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial 293 Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic crustal residence ages of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.  相似文献   

2.
Studies of numerous shear zones hosting auriferous quartz veins suggest a three-stage model of formation for such veins. These stages are indicative of the progressive increase of the gold concentration with the evolution of the shear zone and comprise an early stage with invisible gold, an intermediate stage with finegrained gold, and a late stage with gold nuggets. The early stage, consisting of two consecutive episodes, comprises the development of the shear zone sensu stricto which results in the formation of a structure characterized by mylonite. This structure subsequently acts as a drain for hydrothermal fluids. Under the combined action of deformation and hydrothermal circulation, the rocks in this structure undergo mineralogical transformations that depend on the initial composition of the rock. These phenomena occur with increasing intensity towards the cores of the structures, the sites of substantial silicification and sulfidation. Gold is first fixed in the crystal lattice of pyrrhotite (up to 30 ppm of gold), disseminated throughout the structure as a whole. In the core, it is destabilized to pyrite-marcasite during the second episode of this early stage and the gold thus freed is trapped in the lattice of locally abundant ferriferous sulfides: pyrite, arsenopyrite (up to 1.6 wt% gold), and berthierite. At the intermediate stage, again consisting of two consecutive episodes, the shear zone can generate openings, allowing the emplacement of lenses and veins of milky white quartz. When these various veins are in turn crushed by continuing tectonic activity, microsaccharoidal quartz forms by cataclasis, and acts as a receptacle for the gold mineralization. Therefore textbook auriferous quartz veins are only one particular aspect of this stage, characterized by the appearance of visible native gold caused by destabilization of the auriferous sulfides of the early stage. This gold is generally very fine-grained (several micrometres) and relatively silverfree. The late stage, i.e. the fifth episode, in a brittle domain, can occur within a structure of the early or intermediate stage. It corresponds to a stage of opening, during which stockworks of quartz or of quartz and carbonate form, causing in situ remobilization of the auriferous mineralization from the earlier stages. This phenomenon is accompanied by the appearance of gold nuggets several millimetres in size, very often rich in silver (electrum).This work was supported by BRGM's scientific program Metallogeny of auriferous shear zones (Rm 13)  相似文献   

3.
We have experimentally determined the partitioning of REE (rare earth elements) between zoisite and hydrous silicate melt at 1,100 °C and 3 GPa. All REE behave moderately compatible in zoisite with respect to the melt and all show a smooth parabolic dependence on ionic radius. The partitioning parabola peaks at Nd , and the compatibility slightly decreases towards La and decreases by half an order of magnitude towards Yb . Application of the elastic strain model of Blundy and Wood (1994) to the available zoisite and allanite REE mineral/melt partitioning data and comparison with partitioning pattern calculated from a combination of structural and physical data (taken from the literature) with the elastic strain model suggest that in zoisite REE prefer the A1-site and that only La and Ce are incorporated into the A2-site in significant amounts. In contrast, in allanite, all REE are preferentially incorporated into the large and highly co-ordinated A2 site. As a result, zoisite fractionates the MREE effectively from the HREE and moderately from the LREE, while allanite fractionates the LREE very effectively from the MREE and HREE. Consequently, the presence of either zoisite or allanite during slab melting will lead to quite different REE pattern in the produced melt.Editorial responsibility: J. Hoefs  相似文献   

4.
This paper examines some aspects of the power and robustness of the test for complete subcompositional independence proposed by Aitchison (1982). Although the computed test statistics commonly do not approach being 2 distributed throughout their range, the upper tail of their distribution does mimic the 2 distribution sufficiently to yield a quite robust test when variates are drawn from identical distributions with different distribution parameters or even when variates are drawn from different distributions. But the magnitude of correlations among the variables and the proportion of correlated to independent variables that compose the closed data vectors affect the power of the test.  相似文献   

5.
Basalt recovered beneath Jurassic sediments in the western Atlantic at Deep Sea Drilling Project sites 100 and 105 of leg 11 has petrographic features characteristic of water-quenched basalt extruded along modern ocean ridges. Site 100 basalt appears to represent two or three massive cooling units, and an extrusive emplacement is probable. Site 105 basalt is less altered and appears to be a compositionally homogeneous pillow lava sequence related to a single eruptive episode.Although the leg 11 basalts are much more closely related in time to the Triassic lavas and intrusives of eastern continental North America, their geochemical features are closely comparable to those of modern Mid-Atlantic Ridge basalts unrelated to postulated mantle plume activity. Projection of leg 11 sites back along accepted spreading flow lines to their presumed points of origin shows that these origins are also outside the influence of modern plume activity. Thus, these oldest Atlantic seafloor basalts provide no information on the time of initiation of these plumes. The Triassic continental diabases show north to south compositional variations in Rb, Ba, La, and Sr which lie within the range of plume-related basalt on the Mid-Atlantic Ridge (20 °–40 °N). This suggests that these diabases had mantle sources similar in composition to those beneath the present Mid-Atlantic Ridge. Plumes related to deep mantle sources may have contributed to the LIL-element enrichment in the Triassic diabase and may alos have been instrumental in initiating the rifting of the North Atlantic. Systematically high values for K and Sr87/Sr86 in the Triassic diabases may reflect superimposed effects of crustal contamination in the Triassic magmas.Contribution Number 3953 from the Woods Hole Oceanographic Institution  相似文献   

6.
In closed magma systems SiO2 approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO2. The main role of 18O is as a sensitive indicator of contamination, either at the start of differentiation ( 18Oinit) or as a proportion of fractionation in AFC. Plots of 18O vs SiO2-allow to determine initial 18O values for different sequences for source comparison. For NBS-28=9.60, the 18O at 48% SiO2-varies between a high 6.4 for Kiglapait (Kalamarides 1984), 5.9 for Transhimalaya, 5.8 for Hachijo-Jima (Matsuhisa 1979), 5.6 for Koloula (Chivas et al. 1982) and a low 5.3 for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the Ladakh-Lhasa terrane, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous ( 18O vs SiO2-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in 18O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low- 18O (5.0–0) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya 18O trend line, but the distribution of low total rock or feldspar 18O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any 18O-contamination by crustal material.  相似文献   

7.
In the context of robust statistics, the breakdown point of an estimator is an important feature of reliability. It measures the highest fraction of contamination in the data that an estimator can support before being destroyed. In geostatistics, variogram estimators are based on measurements taken at various spatial locations. The classical notion of breakdown point needs to be extended to a spatial one, depending on the construction of most unfavorable configurations of perturbation. Explicit upper and lower bounds are available for the spatial breakdown point in the regular unidimensional case. The difficulties arising in the multidimensional case are presented on an easy example in IR2 , as well as some simulations on irregular grids. In order to study the global effects of perturbations on variogram estimators, further simulations are carried out on data located on a regular or irregular bidimensional grid. Results show that if variogram estimation is performed with a 50% classical breakdown point scale estimator, the number of initial data likely to be contaminated before destruction of the estimator is roughly 30% on average. Theoretical results confirm the previous statement on data in IRd , d 1.  相似文献   

8.
    
For any distribution of grades, a particular cutoff grade is shown here to exist at which the indicator covariance is proportional to the grade covariance to a very high degree of accuracy. The name mononodal cutoff is chosen to denote this grade. Its importance for robust grade variography in the presence of a large coefficient of variation—typical of precious metals—derives from the fact that the mononodal indicator variogram is then linearly related to the grade variogram yet is immune to outlier data and is found to be particularly robust under data information reduction. Thus, it is an excellent substitute to model in lieu of a difficult grade variogram. A theoretical expression for the indicator covariance is given as a double series of orthogonal polynomials that have the grade density function as weight function. Leading terms of this series suggest that indicator and grade covariances are first-order proportional, with cutoff grade dependence being carried by the proportionality factor. Kriging equations associated with this indicator covariance lead to cutoff-free kriging weights that are identical to grade kriging weights. This circumstance simplifies indicator kriging used to estimate local point-grade histograms, while at the same time obviating order relations problems.This paper is based in part on a PhD thesis submitted to the Department of Applied Earth Sciences, Stanford University, Stanford, California 94305, in 1984 (unpublished).  相似文献   

9.
Summary Metamorphosed carbonatites and related skarn deposits, located in Fuerteventura Basal Complex, contain unusual Sr-rich minerals. Maximum SrO concentration in the following minerals are: calcite, 7.23wt%; apatite, 5.22wt%; epidote, 11.64wt%; clinozoisite, 1.25wt%; allanite, 5.63wt%; britholite, 4.11wt% and a Sr–Na aluminosilicate (probably stronalsite), 16.44wt% SrO. Calcite and apatite are chemically similar to those found in carbonatites and are therefore considered to be of igneous origin. Textural evidence indicates that the first skarn stage garnet+diopside+Sr–Na aluminosilicate formed as the result of chemical interaction between carbonatites and adjacent silicate rocks. The formation of Sr-bearing epidote/clinozoisite, allanite and britholite appears to be related to the release of Sr into the fluid phase from the breakdown of high temperature assemblages during the retrograde skarn stage. During the final evolution stages, further alteration of britholite by bastnäsite and törnebohmite took place. The occurrence of REE minerals shows that the fluids responsible for this metasomatism must also have transported significant quantities of REE.  相似文献   

10.
Major-element and REE compositions of 14 diamondiferous eclogites from the Udachnaya kimberlite in Yakutia, Siberia have been determined by electron microprobe and secondary ion mass spectrometer (SIMS). Based on previous clinopyroxene classification schemes (e.g., Taylor and Neal 1989), all of these eclogite xenoliths belong to Group B/C, although some of the garnet compositions and mineral REE abundances are inconsistent with the indicated groups. This demonstrates the inadequacy of the classification scheme based on African eclogites for application to Siberian samples. Because of the coarse grain size of the Udachnaya nodules, meaningful modal abundances could not be obtained. However, reconstructed REE compositions using various garnet: clinopyroxene ratios demonstrate relative insensitivity to changes in mode for common eclogitic assemblages. Many of these reconstructed REE compositions show LREE depletions. Some depletions are consistent with an origin (either directly or through partial melting) as normal or Type-I ocean floor basalt. Others, however, require material of eclogitic or pyroxenitic affinities to undergo partial melting; this facilitates the depletion of LREE while leaving the HREE at nearly original levels. Many of the eclogites of South Africa are consistent with a protolith of anomalous or Type II ocean floor basalt. This fundamental difference between the two regions is the likely cause of the inconsistencies with the chemicallybased classification.  相似文献   

11.
Conclusions Central place preference by social geographic groups is in the Tokyo area less determined by social status than by age and life cycles differentiated by sex.In case of the tributary area of Shibuya a simple classification into young people (male and female bachelors) and married people (husbands and housewives) indicates essential differences in central place preferences between these groups.The bachelors, particularly the young men, are most important for central place orientations in that they show distinctive spatial behavioural patterns in terms of complexity, mobility and intensity of distance-movements. The young people being employed — today joculously called dokushin kizoku (bachelors' aristocracy) because of their salary raised remarkably as well as of their extensive concumer demands and high buying power — are representative for the present-day economically independent young generation. The bachelors judge urban center attractiveness on the basis of shopping facilities and also in terms of atmosphere — meeting friends, strolling through town and amusing themselves. The young people are very much courted by commercial enterprises and are regarded as special trend setters having great influence on the image and vitality of Tokyo's inner urban centers.Most attractive for the young people are the ring centers. In the southwestern suburban areas Shibuya predominates; however generally speaking it is Shinjuku, the western and leading (ring)-center, which draws the young people most strongly. Special centers such as Harajuku, Aoyama and Roppongi are also preferred by this group, often in combination with a ring-center visit. Towards the inner city core (Ginza-Nihonbashi) central place preferences chiefly of the young male bachelors are declining gradually.Compared with young people the married people prefer the ring centers only slightly before the Ginza area, which for shopping is primarily visited by housewives, for going out mainly by husbands. Particularly in the inner city core near the main office districts a typical feature for Tokyo of central place preference is evident. For the salaryman — the most representative business type in the Tokyo area — going out is identical with not going home. This phenomenon depends on the place of work in two aspects. First, spatially in terms of a close connection between office and inner city amusement/shopping centers and additionally distance-movements towards a ring center nearest of the place of work or residence. Second, socially — social activities after working hours mainly carried out with colleagues result in a behavioural pattern, in which a clear differentiation between company and private life appears to be difficult. For not going home there are many reasons — among others the constricted living conditions of Japanese homes as Westerners believe — however, other factors play a much more important role: (i) Mainly the extremely long commuting distances, which allow the salaryman a real leaving home to go out only on Sundays or holidays. Other reasons for this typically Japanese feature closely related to inner city tavern and bar visits, may be seen in (ii) the traditionally very limited participation of Japanese housewives to their husbands' out-door social activities; (iii) a system of charging and crediting expenses to one's personal bank account or the company account; (iv) the personal atmosphere of taverns and bars providing the best chances for communication/relaxation and for overcoming stress and frustrations experienced in the office.Shopping is particularly for housewives the most important motivation for visiting urban centers, in this respect two Japanese characteristics should be mentioned: First, fashion and being well-groomed are, as far as the mass of people particularly the young women is concerned, more highly esteemed than in other urban societies. As a consequence good clothes are much in demand. Purchases of these kinds of articles are most often made in department stores and cause the longest distance-movements. Second, in rank of central goods next to fashion and quality clothes follow standard gifts such as seibo and chugen. The purchase of these articles is apart from the unique custom of giving gifts twice a year typically Japanese also in so far as it contradicts the least-effort criterion in central place theory. Though these patterns mostly bought by housewives or middle-aged and older people are substantially more of daily or periodical than episodical demand, they are related to surprisingly long distance-movements because of usually being bought for image reasons in leading department stores located in higher-ranking more distant urban centers.Typical behaviour for all groups visiting higher-order urban centers is an often used combination of shopping and taking part in out-door activities such as meeting friends/acquaintances, strolling around, going to the cinema or other show performances, eating, drinking and chatting together. This is due to the outstanding facilities available not only in retail-trade but also in the gastronomic, entertainment and amusement sectors.Taken all together, complexity and intensity of central place preferences in Tokyo reflect the higher and different attractiveness of Japanese urban centers compared with that of other urban societies. The urban centers are highly esteemed because of their excellent facilities as well as of their vitality, crowds and activity. These latter characteristics are summarized by the word nigiyaka meaning vivacity, which is most typical for Japanese urban centers. The inner city centers of Tokyo can be regarded in the true sense of this term as urban foci compensating for diverse problems such as environmental pollution or lack of inner urban green areas, making life in the most densely populated area of the world worthwhile.  相似文献   

12.
New single crystal diffraction data for natural and heat-treated anorthite crystals (Angel et al. 1990) allow the determination of their states of Al/Si order in terms of a macroscopic order parameter,Q OD , for the transition. Numerical values ofQ OD obtained from estimates of site occupancies are shown to vary with the scalar spontaneous strain, s , as s Q OD 2 , and with the ratio of the sums of typeb (superlattice) reflections and typea (sublattice) reflections asI b/I a Q OD 2 . An empirical calibration for pure anorthite is obtained giving varies between 0.92 and 0.87 in samples equilibrated at T1300° C, but then falls off relatively rapidly with increasing temperature, reaching 0.7 near the melting point ( 1557° C). The observed temperature dependence does not conform to the predictions of the simplest single order parameter models; coupling ofQ OD withQ of the transition is suspeeted.  相似文献   

13.
The P/T gradient or baric type of low-grade metamorphic terranes can be characterized on the basis of cumulative frequency curves of the lattice parameter b 0 of dioctahedral micas (Sassi and Scolari 1974), which essentially reflects celadonite content and increases with pressure. The method is here applied to illite/muscovite of sub-greenschist (incipient metamorphism) and low-greenschist facies terranes from the marginal zones of the Swedish Caledonides, Swiss Alps and Venezuelan Andes, where the grade of metamorphism was determined on the basis of illite crystallinity.Complications in the determinations of b 0 arising from (a) shifts in the spacing of the apparent 060 diffraction peaks of the illites due to the presence of inter-stratified expandable layers, and (b) persistence of more crystalline clastic micas are pointed out.In contrast to the earlier-reported decrease of b 0 with progressive metamorphism at higher grades, b 0 tends to increase with grade during incipient metamorphism: it is markedly lower in the diagenetic and low-grade anchimetamorphic grades in the Cambro-Silurian of the Caledonides of Jämtland, western Sweden (zone A), and the Lower Tertiary of the Helvetic zone of the Swiss Alps, than in the higher grade anchizone and epizone of the former terrane.Mean b 0 values for the intermediate and highest (epizone) grades of the Swedish Caledonides are respectively 9.030 and 9.037 Å compared to the mean value of 9.035 Å for the medium-high pressure, Barrovian-type, terrane of Otago, N.Z. Mean b 0 of 9.005 Å for the mainly epizonal slates of the Venezuelan Andes lies between those for the low-pressure terranes of Bosost ( 8.992 Å), and the low-medium pressure terrane of northern New Hampshire (9.010 Å). These baric estimates agree well with those for the metamorphic facies series at higher grade produced by the same events.Cumulative b 0 curves are concluded to be useful for the characterization of P/T gradients of incipient metamorphism, particularly in the higher grade part of the anchizone and the epizone.  相似文献   

14.
Summary ¶Sm–Nd, Rb–Sr and Pb–Pb isotopic compositions of 34 intrusive AMCG (anorthosite–mangerite–charnockite–granite) suite rocks and spatially related ferrodiorites and gabbros from the Lofoten Islands, northern Norway, suggest that almost all Lofoten intrusive rocks can be modelled as a mixture of mainly two components: Archean lower crustal material and an about 1.8Ga mantle-derived component. Isotopically, the gabbros and anorthosites overlap the mangeritic rocks in all three isotope systems. Hence, the isotopic data are in agreement with a model that relates the mangeritic rocks to the fractionation of crustally contaminated mantle-derived basaltic melts. Overlap in all three isotopic systems indicates that at least part of the assimilation process predates or accompanies fractionation. Whole rock geochemistry supports this model: based on major, minor, and trace element data, primitive 1.79–1.86Ga mangerites could have formed from anorthosite residual liquids – the ferrodiorites – by fractionation processes involving Fe–Ti oxides, Fe–Mg silicates and apatite combined with contamination of the ferrodiorites with about 50wt.% crustal anatectic melt.Received May 7, 2002; revised version accepted November 26, 2002 Published online: April 14, 2003  相似文献   

15.
The effect of Alpine (40 my) low amphibolite facies metamorphism on strontium isotopes in the Variscan (250–300 my) granitic complex of the Zentralgneis was investigated by analysing samples of a leucogranite which forms small intrusions into a tonalite. The leucogranite has Rb/Sr ratios up to 30.Two muscovites and three biotites underwent complete reequilibration during the metamorphism. The ages of 12–20 my are consistent with published K-Ar ages on micas from the same area, and reflect post-metamorphic cooling.Six whole-rock samples define an isochron with an age of 244±7 my (=1.47×10–11yr–1). Three further samples, collected within 30 cm of contacts with the tonalite, plus 1 cm sawn slabs of leucogranite and tonalite from either side of a contact fit a line with 10% greater slope; this line is interpreted as a mixing line, resulting from contamination of the leucogranite, near its contacts, with strontium from the enclosing tonalite. The time of contamination is uncertain but petrological evidence suggests it is the result of the metamorphism.Neither leucogranite whole rocks nor minerals from the tonalite, at contacts and in a xenolith, show any evidence for preferential mobilisation of radiogenic strontium in the course of metamorphism.The work was carried out in Heidelberg during successive tenure of the Michael Foster Memorial Scholarship from the University of Oxford and a Wates Travelling Fellowship of the Royal Society of London. Field work and sample collection was supported in part by the Geologische Bundesanstalt, Wien. For support from each of these sources I am very grateful. It is also a pleasure to acknowledge with thanks the support and facilities afforded me at the Laboratorium für Geochronologie by Dr. Hans-J. Lippolt.  相似文献   

16.
Summary Microprobe analyses of olivines, orthopyroxenes and clinopyroxenes of the Ivrea-Verbano basic formation reveal that in the Balmuccia Periodotites (regarded as residual mantle) the phases are more magnesian than in the overlaying ultramafic-mafic series (interpreted as a cumulitic complex formed by fractionation of mantle-derived magma).Pyroxenes underwent sub-solidus re-equilibration. Whole phase (host + exsolved lamellae) compositions give a temperature in the range 946°C–1236°C, compatible with the igneous crystallisation. The compositions of the host phases give temperatures in the range 712°C to 919°C, indicating that the samples did not attain the same degree of equilibrium during the sub-solidus unmixing. The sub-solidus unmixing of the pyroxenes may have occurred either during the superimposed granulite-facies metamorphism or during the slow cooling of the complex intruded into deep crustal levels.
Pyroxene und Olivine als Indikatoren der petrologischen Entwicklung der basischen Formation von Ivrea-Verbano (Italienische Westalpen)
Zusammenfassung Mikrosondenanalysen von Olivinen, Orthopyroxenen und Klinopyroxenen der basischen Formation von Ivrea-Verbano zeigen, daß in den Balmuccia Peridotiten (die als residuales Mantelmaterial angesehen werden) die Phasen Mg-reicher sind als in den überlagernden ultramafischen bis mafischen Serien (die als Kumulationskomplex gedeutet werden, der durch Fraktionierung eines aus dem Mantel stammenden Magmas gebildet wurde).Die Pyroxene wurden unter Subsolidus-Bedingungen reequilibriert. Die Zusammensetzungen der Gesamtphasen (Wirt + Entmischungslamellen) geben Temperaturen im Bereich von 946–1236°C, was mit einer magmatischen Kristallisation verträglich ist. Die Zusammensetzungen der Wirt-Phasen geben Temperaturen im Bereich von 712–919°C an, was anzeigt, daß die Proben während der Subsolidus-Entmischung nicht denselben Grad der Equilibirierung erreichten. Die Subsolidus-Entmischung der Pyroxene kann entweder während der überlagerten Metamorphose in Granulitfazies oder während der langsamen Abkühlung des in tiefe Krustenschichten intrudierten Komplexes geschehen sein.

With 4 Figures  相似文献   

17.
New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite 34 S-values range from 17–21, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have 34 S-values ranging from –13 to –36; vein sulphide 34 S-values range from –8 to 4; sulphide 34 S-values increase upwards and outwards respectively in the related discordant and stratiform G orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite 18O-values range from –13 to –17, stratiform barites ranging from 13 to 14.5; sulphides separated from a single stratiform ore lens hand specimen usually have 34 Ssl > 34 Sga > 34 Spy; the outward decrease in 34 S-values in the stratiform G orebody is confined to the first few hundred feet only; pyrite 34 S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13 occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.  相似文献   

18.
An experimental study of the system Zr-Fe-Ti-O in equilibrium with metallic iron has been conducted at atmospheric pressure and in the temperature range 950–1300° C, with the purpose of modelling sub-solidus relations between Zr-bearing Fe-Ti oxide phases in lunar rocks. The phase relations are governed by the coexistence of the Fe-Ti oxides with ZrO2 at temperatures below 1247° C, but with the new cubic ZFT phase at higher temperatures. ZFT, approximately Zr Fe Ti O5, is probably related to the so-called stabilized cubic zirconia and could represent a synthetic equivalent of some lunar Zr-Fe-Ti rich oxide minerals.The zirconium contents in all Fe-, Ti and Fe-Ti-oxide phases increase with increasing temperature. At each temperature, Zr is incorporated preferentially in the Ti-oxide (up to 11 wt% ZrO2), followed by ferropseudobrookite (up to 6 wt% ZrO2), ilmenite (max. 4 wt%) and wüstite (max. 2.5 wt%). Ulvöspinel coexisting with ilmenite (+Fe0+ ZrO2 and/or ZFT) always contains less than 1.2 wt% ZrO2, whereas the same mineral in assemblage with wüstite-ZrO2-Fe0 displays even higher ZrO2 contents than ilmenite (above 4 wt%). Considering that the values determined here in the synthetic parageneses represent saturation concentrations at high temperatures in a simple model system, the ZrO2 concentrations in the corresponding lunar minerals should generally be significantly lower. This is realized, except for the so-called Cr-Zr-Ca armalcolites which display ZrO2 contents in the range 3.5–7 wt% and are thus probably related rather to Zr-rich oxide minerals than to armalcolite itself.  相似文献   

19.
It is valuable in earthquake prediction to determine the occurrence probability of major earthquakes by making use of data obtained from precursory phenomena up to the time of the evaluation. In this study, the time evolution of the state determined by earthquakes and precursory phenomena was modelled using Markov chains. Various probabilities suitable for earthquake prediction were derived from the transition probability of the Markov chain with a chosen length of memory time. As an example, earthquake sequence records for northern China, which covered a period of about two thousand years, were examined and the results were also obtained from modern scientific observations of the radon anomaly which covered a period of about 10 years.Assuming moderate-sized earthquakes of two magnitude ranges 4 3/4 M 5 3/4, 6 M 6 3/4 as precursors to large earthquakes of a magnitude range, 7 M 8 1/2, transition probabilities were calculated for a time interval from 2 to 40 yr. The results showed that the precursory time of moderate-sized earthquakes is mainly distributed in a time span of around several years and that the earthquake occurrence probability is considerably large when the precursory earthquakes occur successively. Furthermore, it was shown that a larger moderate-sized earthquake (4 3/4 M 5 3/4) is a more effective precursor than an entire moderate-sized earthquake (4 3/4 M 6 3/4).Second, a multiple precursor case was tried by means of simulation based on the radon anomaly data obtained during a limited observation period. Simultaneous occurrence of two precursors makes the earthquake occurrence probability increase by 1.5–2.0 for a reasonable choice of a mean recurrence interval of the radon anomaly compared with the case where only a moderate-sized earthquake was treated as the precursor. However, the probability is much the same if the average recurrence interval of the radon anomaly is the same through time, including the preparatory period before the earthquake.  相似文献   

20.
Cappadocia is essentially covered by nine rhyolitic ignimbrite units, being the products of a multi-phase volcanism of Upper Miocene-Pliocene age. Around Ürgüp and the Kizilirmak River, these ignimbrites constitute a volcanic-sedimentary succession together with lacustrine sediments, called the Ürgüp formation. In the zeolite occurrences of Cappadocia that were found in the lacustrine parts of pyroclastics outcropping around the Tuzköy, Sarihidir, Karain, Çökek, Ibrahim Pasa and Karadag areas, clinoptilolite is the most common mineral and is associated with chabazite, erionite and phillipsite in some areas. A gain of alkaline-earths from the lake water, which is compensated by a loss of alkalis from the glass, took place during the formation of the different zeolite assemblages, which were probably controlled by the composition of the parent glasses. The distribution of erionite confirms its relation with mesothelioma cases in Tuzköy, Sarihidir and Karain villages that are located outside tourist areas. The high incidence of malignant mesothelioma in Karain may be explained by uncontrolled occupational exposure to erionite. The fairy chimneys, canyons and underground cities, which are situated in the unaltered or slightly altered parts of the Kavak, Zelve, Cemilköy, Gördeles and Kizilkaya ignimbrites do not constitute any health risk for the inhabitants or for visitors.  相似文献   

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