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1.
2.
When viewed from the air, Scottish ‘hummocky moraine’ can be resolved into a series of linear ridges that resemble those found at the margins of actively retreating glaciers today. Recent work has supported the interpretation of these linear ridges as ice-marginal landforms and the authors believe that the majority of ‘hummocky moraine’ deposits can be interpreted in this way. Consequently the pattern of deglaciation can be established fairly precisely from the pattern of linear ridges. This approach is applied to the landforms of the northern part of the Loch Lomond Stadial ice-field in order to reconstruct the regional pattern of deglaciation. This leads to important inferences about the significance of topographic control during deglaciation and more importantly it provides fresh insight into the environment of the British Isles during the Loch Lomond Stadial. 相似文献
3.
C. Moreno R. Sáez F. González G. Almodóvar M. Toscano G. Playford A. Alansari S. Rziki A. Bajddi 《Mineralium Deposita》2008,43(8):891-911
The Draa Sfar mineralization consists of two main stratabound orebodies, Sidi M’Barek and Tazacourt, located north and south
of the Tensift River (“Oued Tessift”), respectively. Each orebody is comprised by at least two massive sulfide lenses. The
hosting rocks are predominantly black shales, although minor rhyolitic rocks are also present in the footwall to the southern
orebody. Shales, rhyolitic volcanic rocks, and massive sulfides are all included into the Sarhlef Series, which is recognized
as one of the main stratigraphic units of the Moroccan Variscan Meseta. Hydrothermal activity related with an anomalous thermal
gradient, together with a high sedimentation rate in a tectonically driven pull-apart marine basin, favored the accumulation
of organic-rich mud in the deepest parts of the basin and the sedimentary environment suitable for massive sulfide deposition
and preservation. This took place by replacement of the hosting unlithified wet mud below the sediment–water interface. Geochemical
data suggest a sedimentary environment characterized by oxic water column and anoxic sediment pile with the redox boundary
below the sediment–water interface. The low oxygen availability within the sediment pile inhibited oxidation and pyritization
of pyrrhotite. Biostratigraphic analysis, based on the palynological content of the hosting black shales, restricts the age
of the sulfides to the Asbian substage (mid-Mississippian). This age is consistent with earlier geochronological constraints. 相似文献
4.
Kilian Pollok Geoffrey E. Lloyd Hkon Austrheim Andrew Putnis 《Chemie der Erde / Geochemistry》2008,68(2):177-191
Eclogite-facies rocks within the Bergen Arcs, western Norway, have formed from granulites along shear zones and fluid pathways. Garnets that were inherited from granulite facies protoliths show different types of replacement patterns due to an incomplete eclogitisation process including concentric rim zoning, zoning along vein fillings and inclusion trails, and zoning bands without inclusions. The interfacial part between the granulitic core and the eclogitic rim of garnet as well as the microstructure of other relevant minerals (omphacite, plagioclase) has been analysed using analytical transmission electron microscopy (ATEM). In garnet, the interface is characterised by gradual changes in composition from Xalm=0.31, Xpyr=0.50 to Xalm=0.54, and Xpyr=0.25 within ≈20 μm and exhibits no distinct change in microstructure. Granulitic plagioclase shows exsolution lamellae of the Bøggild intergrowth. In omphacite, anti-phase domains (APDs) which potentially record the temperature of cation ordering after mineral growth have been observed and their size suggest eclogitisation at 600–700 °C. The electron backscatter diffraction (EBSD) analysis revealed that the lattice orientation of the granulitic feldspar is basically unrelated to tectonic axes whereas newly formed eclogitic minerals omphacite and kyanite show a crystallographic relation to the foliation. In garnet, no change in the basic crystallographic orientation between the eclogitic and granulitic garnet composition was confirmed. However, misorientation analysis suggests a cellular microstructure not more than 1° misorientation in the core of the garnets, which is missing in the eclogitic rim indicating textural equilibration of the latter. The heterogeneous replacement patterns are characteristic for dissolution and re-precipitation reactions in an open system limited to fluid availability. The appearance of the compositional profile in garnet is interpreted as a diffusional re-equilibration step after the time-limited, fluid-mediated eclogitisation event that apparently obscured the initially sharp interface within the further retrograde metamorphic history. 相似文献
5.
The current study provides an investigation of abiotic reduction of an oversaturated uranyl solution driven by iron nanoparticle oxidation. The reactivity of nano-scale zero-valent iron (ZVI) under mildly oxic conditions (1.2% O2 and 0.0017% CO2) was studied in 1000 ppm uranyl solution in the pH range 3-7, at reaction times from 10 min to 4 h. Reductive precipitation of UO2 was observed as the main process responsible for the removal of uranium from solution with the kinetics of reaction becoming increasingly favourable at higher pH. Despite working with an oversaturated uranium solution, the precipitation of UO2 occurred in preference to precipitation of UO3·2H2O (metaschoepite) at reaction times between 1 and 4 h and for uranyl solutions initially set up at pH ?5. Characterisation of both solid and solution phases was performed using X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) imaging, X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). 相似文献
6.
Summary A geometrically simple volcano is considered, havig a spherical magma chamber of 2.5 km radius centred at 10 km depth. The Curie point isotherm is assumed to be a plane at 20 km depth, except for the spherical volume which is also non-magnetic. The stress pattern in the vicinity of the spherical chamber, due to regional stress of sufficient intensity to cause an eruptions, is used to calculate the change in magnetization which results from the piezomagnetic effect through the volume of solid rock. The consequent magnetic field anomaly at the surface is then obtaied by numerical integration of the dipole law of force over the stressed volume. For rocks of the type found on the volcanic island of St. Vincent (West Indies), this model gives a maximum local volcano-magnet c effect of about 7 gammas. 相似文献
7.
The dissolution of opaline silica of diatom tests in sea water 总被引:3,自引:0,他引:3
Two different series of measurements have been made of the solubilities and rates of solution in sea water of acid-washed siliceous tests of the diatomsThalassiosira decipiens andRhizosolenia hebetata. The kinetic approach was carried out on the assumption that the tests behave as identical solid spheres. If the tests were present in considerable excess over that required for saturation of the water with respect to silica, the dissolution obeyed first order reaction kinetics and its rate was proportional to the surface area of the exposed tests. When the tests were not in excess the kinetics of dissolution appeared to be more complex. This was considered to be due to a decrease in specific surface area as dissolution proceeded. An expression developed to allow for the change of surface area as a sphere dissolves offers a partial explanation of the kinetics when the tests are not present in excess. 相似文献
8.
9.
Elizabeth B. Kujawinski Rossana Del Vecchio Neil V. Blough Geoffrey C. Klein Alan G. Marshall 《Marine Chemistry》2004,92(1-4):23
Molecular-level characterization of natural organic matter (NOM) has been elusive due to the inherent complexity of natural organic mixtures and to the fact that individual components are often polar and macromolecular. Electrospray ionization (ESI) is a “soft” ionization technique that ionizes polar compounds from aqueous solution prior to injection into a mass spectrometer. The highest resolution and mass accuracy of compounds within NOM have been achieved when ESI is combined with an ultrahigh-resolution mass spectrometer such as the Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). With this technique, individual molecules within a variety of natural organic mixtures can be detected and their elemental composition can be determined. At low mass-to-charge (m/z) ratio, the resolution is high enough to assign exact molecular formulas allowing specific components of these mixtures to be identified. In addition to molecular identification, we can now use ESI FT-ICR MS to examine molecular-level changes in different organic mixtures as a function of relevant geo-processes, such as microbial alterations and photochemistry. Here we present the results from the application of ESI FT-ICR MS to two geochemical questions: (1) the effect of photoirradiation on the molecular composition of fulvic acids and (2) the role of protozoan grazers in the modification of DOM in aquatic systems. 相似文献
10.
The Magellan seamounts began forming as large submarine shield volcanoes south of the equator during the Cretaceous. These
volcanoes formed as a cluster on the small Pacific plate in a period when tectonic stress was absent. Thermal subsidence of
the seafloor led to sinking of these volcanoes and the formation of guyots as the seamounts crossed the equatorial South Pacific
(10–0°S) sequentially and ocean surface temperatures became too high for calcareous organisms to survive. Guyot formation
was completed between about 59 and 45 Ma and the guyots became phosphatized at about 39–34 and 27–21 Ma. Ferromanganese crusts
began formation as proto-crusts on the seamounts and guyots of the Magellan Seamount cluster towards the end of the Cretaceous
up to 55 Ma after the formation of the seamounts themselves. The chemical composition of these crusts evolved over time in
a series of steps in response to changes in global climate and ocean circulation. The great thickness of these crusts (up
to 15–20 cm) reflects their very long period of growth. The high Co contents of the outer parts of the crusts are a consequence
of the increasing deep circulation of the ocean and the resulting deepening of the oxygen minimum zone with time. Growth of
the Co-rich Mn crusts in the Magellan Seamount cluster can be considered to be the culmination of a long journey through time. 相似文献