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1.
A two-layered model is considered in which the upper layer is continuously stratified and the lower layer is homogeneous. The system is driven by atmospheric forces. Bottom stress and topography are included in the model. The linear three-dimensional hydrodynamical equations are used to describe the system. Taking the eddy viscosity in the upper layer as inversely proportional to the static stability, the dependent variables are expanded in terms of continuous functions in the vertical (eigenfunctions). Using this method it is possible to compute currents and internal displacements at any depth in the upper layer. The three-dimensional structure of the lower layer is not considered in this model. The equations describing the lower layer are integrated over depth to give depth mean currents. Using a staggered finite-difference grid in the horizontal and a forward time-stepping procedure, numerical test experiments are carried out for a cross section and for a closed rectangular basin. 相似文献
2.
The results of electron-microscopy investigations of calcite precipitated in a water-conducting fracture in a ca. 1800 Ma granitic rock from 207 m below sea level at the island of Aspo on the southeastern (Baltic) coast of Sweden are compared with measurements of carbon, oxygen, and sulfur isotope composition of the calcite and embedded pyrite. Parts of the calcite had extremely low delta 13C values, indicative of biological activity, and contained bacteria-like microfossils occurring in colonies and as typical biofllms. X-ray microanalysis demonstrated these fossils to be enriched in carbon. Our results provide evidence for ancient life in deep granitic rock aquifers and suggest that the modern microbial life found there is intrinsic. Modeling historical and present geochemical processes in deep granitic aquifers should, therefore, preferably include biologically catalyzed reactions. The results also suggest that the search for life on other planets, e.g., Mars, should include subsurface material. 相似文献
3.
Hege Fonneland-Jorgensen Harald Furnes Karlis Muehlenbachs Yildirim Dilek 《Island Arc》2005,14(4):517-541
Abstract The Solund‐Stavfjord ophiolite complex (SSOC) in western Norway represents a remnant of the Late Ordovician oceanic lithosphere, which developed in an intermediate‐ to fast‐spreading Caledonian back‐arc basin. The internal architecture and magmatic features of its crustal component suggest that the SSOC has a complex, multistage sea floor spreading history in a supra‐subduction zone environment. The youngest crustal section associated with the propagating rift tectonics consists of a relatively complete ophiolite pseudostratigraphy, including basaltic volcanic rocks, a transition zone between the sheeted dyke complex and the extrusive sequence, sheeted dykes, and high‐level isotropic gabbros. Large‐scale variations in major and trace element distributions indicate significant remobilization far beyond that which would result from magmatic processes, as a result of the hydrothermal alteration of crustal rocks. Whereas K2O is strongly enriched in volcanic rocks of the extrusive sequence, Cu and Zn show the largest enrichment in the dyke complex near the dyke–volcanic transition zone or within this transition zone. The δ18O values of the whole‐rock samples show a general depletion structurally downwards in the ophiolite, with the largest and smallest variations observed in volcanic rocks and the transition zone, respectively. δ18O values of epidote–quartz mineral pairs indicate 260–290°C for volcanic rocks, 420°C for the transition zone, 280–345°C for the sheeted dyke complex and 290–475°C for the gabbros. The 87Sr/86Sr isotope ratios show the widest range and highest values in the extrusive rocks (0.70316–0.70495), and generally the lowest values and the narrowest range in the sheeted dyke complex (0.70338–0.70377). The minimum water/rock ratios calculated show the largest variations in volcanic rocks and gabbros (approximately 0–14), and generally the lowest values and range in the sheeted dyke complex (approximately 1–3). The δD values of epidote (?1 to ?12‰), together with the δ18O calculated for Ordovician seawater, are similar to those of present‐day seawater. Volcanic rocks experienced both cold and warm water circulation, resulting in the observed K2O‐enrichment and the largest scatter in the δ18O values. As a result of metal leaching in the hot reaction zone above a magma chamber, Zn is strongly depleted in the gabbros but enriched in the sheeted dyke complex because of precipitation from upwelling of discharged hydrothermal fluids. The present study demonstrates that the near intact effect of ocean floor hydrothermal activity is preserved in the upper part of the SSOC crust, despite the influence of regional lower greenschist facies metamorphism. 相似文献
4.
Ophiolites within the Norwegian Caledonides were generated during at least two distinct periods, i.e. Tremadocian-Arenigian and Ashgillian. The older generation show a long-lived magmatic development (ca 500-470 Ma) with rocks that range from MORB, IAT, boninites, calc-alkaline to alkaline basalt in geochemical affinity. This development is compatible with generation in a subduction-influenced environment, and a plausible modern equivalent may be the ensimatic arc-basin evolution in the western Pacific.The metabasalts of the younger (Ashgill, 443±3 Ma) ophiolite occurrence are characterized by a dominance of N- to E-MORB compositions with subduction zone influence. The volvanics of the complex are intercalated with, and overlain by, continentally-derived sediments. Calc-alkaline as well as alkaline lavas and volcanic-lastics apparantly define late magmatic products. Associated with the youngest ophiolite is a sedimentary melange with olistoliths of mature island arc-derived volcanics, as well as clasts with MORB-IAT affinity, chert and sandstone, the latter probably derived from an accretionary prism. An Andaman Sea model appears most applicable to the youngest ophiolite/associated sediments assemblage.Contemporaneously with the renewed, Ashgillian spreading episode, widespread deposition of clastic sequences of Ashgillian age took place unconformably on the Lower Ordovician ophiolite complexes in fault controlled basins. 相似文献
5.
Harald Furnes 《Contributions to Mineralogy and Petrology》1975,50(2):105-113
The rate of palagonitization of three chemically different types of basaltic glasses has been determined experimentally as a function of temperature (20–90 ° C) and time (3.5–14 months) in both fresh and saline water. Between ca. 40 ° C and 70 ° C there is a marked increase in the rate of transformation of the glasses, especially those of alkali basalt composition. The alteration process also accelerates after ca. 10 months at temperature higher than 70 ° C. These phenomena are possibly related to stepwise losses of the major elements, and minimum activation temperatures for the oxide/ion—water metasomatism. 相似文献
6.
H Furnes 《Bulletin of Volcanology》1974,38(1):173-186
Hyaloclastites are particularly suitable material for palagonitization. This process of alteration leads to the formation in broadly equal proportions of authigenic minerals and palagonite. When the palagonitization process reachs the stage where voids between fragments, and vesicles within fragments are completely filled with authigenic minerals so as to make the rock more or less impermeable to water, the rate of the palagonitization process is thought to slow down considerably. Depending upon the original texture and porosity of the hyaloclastite, sideromelane may or may not remain at this stage. This may explain why sideromelane, which otherwise would have been completely palagonitized, can be found in relatively old hyaloclastites. Further, study of palagonitization taking place around glassy vesicle walls strongly suggests that the process is post-cruptional, being initiated at the time the glass encounters an equeous environment, and proceeds at low temperatures. 相似文献
7.
Four Quaternary volcanic ash zones in the southern Norwegian Sea have been investigated in core P57-7 from the Iceland Plateau. Both the geochemical composition and morphological variation of each ash layer have been studied. The four volcanic ash zones appear in the light oxygen isotope stages 1, 5, 7 and 11. The ash zones are composed of transparent platy grains, light brown transparent grains, brown blocky and black blocky grains and white/transparent pumice, and each zone shows a distinct stratigraphic evolution. The geochemical results show a mixture of basaltic and silicic grains in each ash zone, and that each zone contains grains from more than one eruption. The geochemical investigations strongly suggest that all the ashes are derived from Iceland. The youngest ash zone includes two layers, which based on their geochemical composition and stratigraphic position are correlated with the 14C-dated Vedde Ash (10 600 yr BP) and Saksunarvatn Ash (9 100 yr BP). Possible sources on Iceland for these layers are discussed. 相似文献
8.
Nils Rune Sandstå Brian Robins Harald Furnes Maarten de Wit 《Contributions to Mineralogy and Petrology》2011,162(2):365-377
Exceptionally well-preserved pillowed and massive phenocryst-free metabasaltic lava flows in the uppermost part of the Palaeoarchaean
Hooggenoeg Complex of the Barberton Greenstone Belt exhibit both flow banding and large leucocratic varioles. The flow banding
is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt (Robins
et al. in Bull Volcanol 72:579–592, 2010a). Varioles occur exclusively in the dark chlorite-, MgO- and FeO-rich metabasalt. Varioles are absent in the outermost rinds
of pillows and increase in both abundance and size towards the centres of pillows. In the central parts of some pillows, they
impinge to form homogeneous pale patches, bands or almost homogenous cores. Individual varioles consist essentially of radially
orientated or outwardly branching dendritic crystals of albite. Many varioles exhibit concentric zones and finer-grained rims.
Some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and
a finer-grained rim. The matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain-like,
skeletal Ca-rich pyroxenes that are partly overgrown by the margins of varioles. Varioles are enriched in the chemical constituents
of feldspar but contain concentrations of immobile TiO2, Cr, Zr and REE that are similar to the host metabasalts. The shape, distribution, texture and composition of the varioles
exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled
basalt melt and glass. Nucleation of plagioclase was strongly inhibited and took place on vesicles, on the bases of drainage
cavities and along early fractures. Eruption in deep water and retention of relatively high concentrations of volatiles in
the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins
as in younger submarine flows. 相似文献
9.
The Paleoarchean (ca. 3.5–3.3 Ga) Onverwacht Suite (OS) of the Barberton Greenstone Belt consists of a 15‐km thick imbricate tectonic stack of seven complexes consisting predominantly of volcanic rocks and intrusions. Tectonostratigraphically from base to top they are the Sandspruit, Theespruit, Komati, Hooggenoeg, Noisy, Kromberg and Mendon Complexes. The Hooggenoeg and Noisy Complexes in the middle of the OS are separated by a significant unconformity resulting from the uplift of the submarine lavas and deep erosion, demonstrating the onset of tectonic accretion prior to 3455 Ma. The basic lavas of the tectonostratigraphic lower (Theespruit, Sandspruit and Komati) and upper (Mendon) complexes are composed of komatiite, komatiitic basalt and high-MgO basalt, whereas those in the middle part (Hooggenoeg and Kromberg) are predominantly high- to low-MgO tholeiitic basalts. Felsic volcanic rocks and intrusions are important in two of the complexes (Theespruit and Noisy). The ultramafic to basaltic lavas show REE patterns that are almost flat and resemble those of modern MORB, whereas those of the felsic rocks are flat from Lu to Gd and moderately to strongly enriched in LREE, similar to modern arcs. Average εNd (T) values are close to depleted mantle growth curves. In MORB-normalised multi-element diagrams, the komatiitic to basaltic rocks exhibit flat patterns from Lu through La and consistent relative enrichment in the elements Pb, U, Th, Ba and Cs. Apart from the Komati Complex, the majority of the lavas show significant negative Nb and Ta anomalies. Enrichment in non-conservative incompatible elements (Cs, Ba, Th, LREE) relative to conservative elements (Ta, Nb, Zr, Hf, Ti, Y, HREE) shows that the komatiitic to basaltic magmas were generated from metasomatised mantle above subducting altered oceanic crust. The geochemistry of the felsic rocks indicates an origin by melting of subducted amphibolite and eclogite. The tectonostratigraphy and the geochemical characteristics of the lavas and intrusions are consistent with successive obduction and accretion of segments of oceanic crust formed in back-arc basins and volcanic arcs. 相似文献
10.
Earth's oldest preserved conglomerates and basaltic pillow lavas at Isua, Greenland, provide robust field evidence for deep- and shallow-water environments on our planet within the first billion years of its formation. The conglomerates represent the first Paleoarchaean archive of sub-aerial erosion and shallow water sedimentation. These ca. 3700 million year old sedimentary rocks, now metamorphosed at amphibolite grade, comprise units of rounded quartz pebbles set in a sandy to muddy matrix that unconformably overlie an ophiolite sequence. The pillow lavas of the ophiolite are variolitic and essentially vesicle-free, indicative of formation in a deep water environment. Locally, an unconformity separates the conglomerates from the deformed ophiolite-related rocks; elsewhere the contact between these units is tectonic. Such field relationship between deep and shallow water environments resemble those preserved in younger orogenic belts, where obducted oceanic crust has been tectonically emplaced across terrestrial platforms or subaerial parts of forearc and backarc basins. 相似文献