Petrogenesis of lavas from the AMAR Valley and Narrowgate region of the FAMOUS Valley, 36°–37°N on the Mid-Atlantic Ridge     

A. P. le Roex  Frederick A. Frey  Stephen H. Richardson 《Contributions to Mineralogy and Petrology》1996,124(2):167-184

Basaltic lavas from the AMAR Valley and the Narrowgate region of the FAMOUS Valley on the Mid-Atlantic Ridge (36° to 37°?N) range in texture from aphyric to highly plagioclase phyric (>25% large plagioclase phenocrysts). Based on ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, most of these lavas can be subdivided into two distinct, isotopically homogeneous, groups: Group I has lower ⁸⁷Sr/⁸⁶Sr (0.70288±1) and higher ¹⁴³Nd/¹⁴⁴Nd (0.51312±1) ratios; Group II has higher ⁸⁷Sr/⁸⁶Sr (0.70296±1) and lower ¹⁴³Nd/¹⁴⁴Nd (0.51309±2) ratios. Most Group II lavas are aphyric, whereas Group I lavas are primarily plagioclase phyric. Lavas from both groups show a wide range in incompatible element abundance ratios (e.g., Zr/Nb =6–29; (La/Sm)_n=0.6–1.7). Aphyric lavas have relatively constant Sc (40±1.5?ppm) abundances and CaO/Al₂O₃ ratios (0.80±0.02). Group I lavas are confined primarily to the AMAR rift valley floor whereas Group II lavas are found along the east and west marginal highs. We interpret the isotopic differences between the two groups as reflecting a temporal change in the upwelling mantle beneath this region of the Mid-Atlantic Ridge which is south of the Azore Islands. For each group, a petrogenetic model consistent with the geochemical data is multi-stage decompression melting of an initially enriched, homogeneous, mantle source region. If the early derived, incompatible-element enriched, melt increments are not always pooled with subsequent increments, the erupted magma batches may have the major element characteristics of melts derived by 10 to 20% melting, but with incompatible element abundance ratios reflecting the change from an enriched to depleted source during the incremental melting process. In this process an initially homogeneous source can generate primary magmas with the required range in incompatible element abundance ratios shown by each group. The nearly constant CaO/Al₂O₃ ratios and Sc contents of the aphyric lavas with decreasing Mg?? reflects subsequent polybaric fractionation of clinopyroxene, plagioclase and olivine over the pressure interval 8–6?kbar (24–18?km), followed by rapid transport to the surface and eruption. There is no geochemical evidence for a crustal magma chamber beneath this section of the Mid-Atlantic Ridge.  相似文献   

Estimation of in situ viscoelastic parameters of weak floor strata by plate-loading tests     

Gang Chen  Yoginder P. Chugh 《Geotechnical and Geological Engineering》1996,14(2):151-167

Summary This paper explores an approach to estimation of the viscoelastic parameters of weak floor strata using plate-loading tests. Mathematical equations are derived to describe deformation-time history for the standard Burgers model under three-dimensional stress-strain conditions, which are verified with a finite element model. A number of time-dependent in-mine plate-loading tests were conducted and the values of the viscoelastic parameters were estimated based on the derived equation. It is shown that large discrepancies between the parameter values estimated from the plate-loading tests and those acquired from the in-mine convergence data are the results of neglecting adjacent pillar interaction. Finite element modelling was conducted to investigate the effects of adjacent pillar interactions upon the pillar settlement. Based on the finite element analyses, a correction curve was developed to adjust the viscoelastic parameter values to take into account the interaction between adjacent pillars. With the correction, the parameter values estimated from the plate-loading tests compared favourably with those acquired from the in-mine convergence results.  相似文献   

Composition of hydrous melts in equilibrium with quartz eclogites   总被引：1，自引：0，他引：1  

Prof. Dr. I. D. Ryabchikov  Dr. Ch. Miller  Prof. Dr. P. W. Mirwald 《Mineralogy and Petrology》1996,58(1-2):101-110

Summary Compositions of the hydrous melts in equilibrium with garnet, omphacitic clinopyroxene and quartz have been investigated experimentally at 28.5 and 35 kbar. They are represented by silica-rich liquids (> 70% SiO₂) with low MgO, FeO and CaO contents. The removal of ca 10–15% of the magma of this composition may be sufficient to convert quartz eclogite formed after subduction of altered MORB into a quartz-free bimineralic eclogite assemblage, which is a common type of xenoliths in kimberlites.At 28.5 kbar the solidus temperature is between 700 and 750° C in the system quartz eclogite—water, and the high pressure amphibole-out boundary lies at ca 25 kbar in accord with the previous studies.

---

Die Zusammensetzung wasserhältiger Schmelzen im Gleichgewicht mit Quarz-Eklogiten

Zusammenfassung Um Prozesse zu simulieren, die bei der Subduktion von Ozeanbodenbasalten durch partielle Anatexis im Stabilitätsfeld von Eklogiten ablaufen, wurde die Zusammensetzung wasserhältiger Schmelzen in Gleichgewicht mit Granat, Omphacit und Quarz bei 28.5 und 35 Kbar experimentell untersucht. Diese Schmelzen sind reich an SiO₂ (> 70 Gew%) und arm an Mg0, Fe0 and CaO. Die Extraktion von ca. 10–15% derartiger Schmelzen würde genügen, um quarzführende Eklogite, die durch die Subduktion von alteriertem MORB Material entstanden sind, in quarzfreie bimineralische Eklogite umzuwandeln wie sie häufig als Xenolithe in Kimberliten beobachtet werden.Im System Quarz-Eklogit-Wasser liegt die Solidustemperatur bei 28.5 Kbar zwischen 700 und 750°C. Die obere Stabilitätsgrenze von Amphibol liegt in diesem Temperaturbereich bei ca. 25 Kbar.

---

---

With 1 Figures  相似文献   

Comparison of the pseudo-static and dynamic behaviour of gravity retaining walls     

P. K. Woodward  D. V. Griffiths 《Geotechnical and Geological Engineering》1996,14(4):269-290

Summary Pseudo-static and dynamic non-linear finite element analyses have been performed to assess the dynamic behaviour of gravity retaining walls subjected to horizontal earthquake loading. In the pseudo-static analysis, the peak ground acceleration is converted into a pseudo-static inertia force and applied as a horizontal incremental gravity load. In the dynamic analysis, an actual measured earthquake acceleration time history has been scaled to provide peak ground acceleration values of 0.1 g and 0.3 g. Good agreement is obtained between the pseudo-static analysis and analytical methods for the calculation of the active coefficient of earth pressure. However, the results from the dynamic analysis require careful interpretation. In the pseudo-static analysis, the increase in the point of application of the resultant active force with the horizontal earthquake coefficient k _h from the one-third point to the mid-height of the wall is clearly observed. In the dynamic analysis, the variation in the point of application is shown to be a function of the type of wall deformation. Both finite element analyses indicate the importance of determining the magnitude of the predicted displacements when assessing the behaviour of the wall to seismic loading.  相似文献   

Characterisation of brown coal humic acids and modified humic acids using pyrolysis gcms and other techniques     

W. R. Jackson  G. D. Bongers  P. J. Redlich  G. Favas  Y. Fei  A. F. Patti  R. B. Johns 《International Journal of Coal Geology》1996,32(1-4)

It has been proposed that Victorian brown coal can be considered as a two-component structure — a lignocellulosic “host”, containing various amounts of weakly bound or entrapped “guest” material together with very small amounts of inorganic and/or mineral matter. The latter predominantly consists of wax esters and/or terpenoid material. In this paper we describe attempts to gain structural information regarding the more complex, “host” component of the coal. Our initial model compound has been humic acid that can be readily obtained from the coal by alkaline extraction. It has been found that “pure” humic acid, free from material associated with the “guest” components of the coal, can be obtained by a highly selective, low-yielding alkaline extraction. This humic acid has been studied by nmr spectroscopy and pyrolysis gas chromatography-mass spectroscopy (py-gc/ms). The products arising from py-gc/ms have been compared with those obtained from similar pyrolysis of whole coals. Alkylation of humic acids using alkyl halides in the presence of base has been successfully carried out and reactivity of the resulting materials compared with those of the parent coal and humic acid.  相似文献   

Paleogeography and structure of the central Mediterranean: Sicily and its offshore area   总被引：1，自引：0，他引：1  

R. Catalano  P. Di Stefano  A. Sulli  F.P. Vitale 《Tectonophysics》1996,260(4):291-323

The geology of the mainland and offshore of Sicily is illustrated by a few geologic sections and seismic profiles across the late Cenozoic orogenic belt of central and western Sicily and across the Sardinia Channel and Sicily Straits. This belt is the result of several tectonic events. Deformation involved mainly the sedimentary cover of the old African continental margin characterized by a broad basinal domain, flanked along its external (southern) margin by a shallow-water carbonate platform attached to Africa in the Triassic. Compressional deformation started in the more internal basinal rock assemblages overlying a thinned crust. The most important structural characteristic of the early phase of thrusting is the duplex pile forming the bulk of the chain in Central Western Sicily. The structure consists of a basal allochthon, made up of Permian to Middle Triassic layers, an intermediate duplex wedge, composed of competent Mesozoic carbonates, and a roof complex, including Upper Mesozoic-Lower Tertiary less competent rocks. Large-scale clockwise rotation of the thrusts accompanied transpressional movements in the hinterland during the Pliocene. Right oblique reverse faults modified the previous tectonic contacts between the allochthons in the hinterland zones. Contemporaneous south-directed imbrications affected the southern external areas, progressively incorporating foreland and piggyback basins. Development of the Gela Thrust System appears to be linked to the transpressional event; its accretion is also related to contemporaneous underthrusting at deeper levels of Mesozoic carbonate substratum. The older buried thrust sheets were pushed up to the surface breaching the deformed Tertiary cover of the Gela TS. Northwards in the belt post-Messinian normal growth faults opened half graben whose sedimentary fill underwent structural inversion. Alternation of extension and compression tectonics characterizes the Sicilian continental margin in the last million years.  相似文献   

Recognition, classification and mechanical description of debris flows   总被引：21，自引：0，他引：21  

P. Coussot  M. Meunier 《Earth》1996,40(3-4):209-227

Various types of flow or mass movement involving water and sediments occur on steep slopes in mountainous areas. Among them, debris flows are peculiar events during which a large volume of a highly concentrated viscous water-debris mixture flows through a stream channel. Throughout the world these phenomena cause considerable damage but remain poorly understood although a basic knowledge is already available concerning their recognition and propagation.

Firstly, a synthesis of the useful practical criteria of recognition is proposed. Debris flows must be seen as intermediate phenomena between hyperconcentrated flows (intense bed load transport) and landslides separated from them by sharp transitions of some characteristics (celerity, deposit nature and flow type). Two parameters, solid fraction and material type, thought to be appropriate for a sound and practical classification, are brought out, and the corresponding complete classification of flow and mass movements in mountain areas is presented. Two extreme debris flow types are thus distinguished: muddy debris flows and granular debris flows. A critical review of recent advances in debris flow dynamic is then proposed. It is pointed out that adequate work must be carried out in the field of non-Newtonian fluid mechanics. In particular, one fundamental rheological property of debris flow materials is the yield stress, which explains thick deposits on steep slopes and can be inferred from field measurements. Furthermore it can be used to estimate viscous dissipation within the bulk during flow. Relevant models predicting muddy debris flow dynamics are already available whereas further progress is needed concerning granular flows.  相似文献   

The dynamic earth: An introduction to physical geology (3d edition)     

P.A. Ziegler 《Earth》1996,40(3-4):307-308

  相似文献   

Heat-induced moisture movement in a clay barrier II. Computational modelling and comparison with experimental results     

A.P.S. Selvadurai 《Engineering Geology》1996,41(1-4):219-238

The paper presents the application of a Galerkin finite-element technique for the numerical solution of the differential equations governing coupled heat flow and moisture movement in a clay buffer. Attention is focussed on the axisymmetric modelling of the hygro-thermal processes encountered in the single borehole emplacement configuration developed for laboratory simulation. The numerical results derived for the time-dependent temperature distributions within the granite block and the residual moisture distribution within the buffer are compared with the respective experimental results.  相似文献   

Effects of heat/water flow interaction on compacted bentonite: Preliminary results     

M.V. Villar  J. Cuevas  P.L. Martín 《Engineering Geology》1996,41(1-4):257-267

Compacted bentonite blocks have been heated and hydrated in a stainless steel cell in order to simulate, in the laboratory, the conditions of the clay barrier in a high-level radioactive waste repository. Temperature distributions at different times, rate of hydration, final water content and dry density have been measured. Some chemical parameters, as electrical conductivity in an aqueous extractable amorphous silica, have also been obtained. For the periods of time considered (up to 2500 h), the hydration process is not affected by the thermal gradient, the high suction of the bentonite being the critical factor in the initial water uptake of the clay barrier. A remarkable saline environment has been detected near the heater, due to salt migration towards dried areas. This phenomenon should be taken into account in further investigations of the mechanical and geochemical behaviour of the clay barrier.  相似文献