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941.
Problems with biogenic silica measurement in marginal seas 总被引:5,自引:0,他引:5
Surface sediment samples from the Bohai, Yellow Sea, and the Pacific were used to assess biogenic silica (BSi) content and to study uncertainties in BSi measurements. The contents of BSi in the Bohai and Yellow Sea are all less than 1%. The dissolution of BSi in sediments from the Bohai and Yellow Sea is very important to maintain high levels of silicate in the water column. The non-biogenic silica from clay minerals has an obvious effect on BSi of sediment samples in the Bohai and Yellow Sea with low BSi and high clay minerals. The solid to solution ratio was found to have a great influence on BSi measurement, which can induce uncertainties up to 75%. The effect of loss by sorption and centrifugation is negligible. Interlaboratory comparison of techniques for BSi measurement by the wet alkaline extraction technique of Mortlock and Froelich [Deep-Sea Res. 36 (1989) 1415-1426] with clay correction was suggested to give no significant differences. However, differences in sediment compositions and reagent to sample ratio may limit the application of the wet alkaline extraction method. 相似文献
942.
Homogenization temperatures and salinity data are presented for fluid inclusions from hydrothermal gangue minerals (quartz and fluorite) associated with porphyry wolframite-molybdenite-arsenopyrite-sphaleritebismuth-chalcopyrite-cassiterite mineralization within the Fire Tower ore zone, Mt Pleasant, New Brunswick. The data indicate that ore mineral precipitation occurred within a temperature range of 260° to 490°C from moderate to high salinity (10–42 wt% NaCl equivalent) aqueous fluids. Two stages of hydrothermal activity characterized by high (>30 wt% NaCl equivalent) salinity fluids are recognized; one which occurred at relatively high temperature (350°–490°C); and one which took place at lower temperature (180°–250°C). The high salinity, high temperature stage is interpreted to be the result of resurgent boiling. Dilution of these early fluids by convecting meteoric water resulted in low to moderate salinity fluids, which dominate the inclusion population. The low temperature, high salinity fluid inclusions are interpreted to represent late residual fluids derived from boiling which occurred as a result of a change in the pressure regime from dominantly lithostatic to hydrostatic conditions. 相似文献
943.
944.
Refraction data taken from ocean bottom seismograph recordings in the western Arafura Sea indicate a continental‐type structure for the region. This structure is characterised by a thin column (2 km) of sediments, with velocities ranging from about to 2 to 4 km s‐1, overlying an essentially two layer crust. The compressional wave velocities in the upper and lower crust are 5.97 and 6.52 km s‐1, respectively, with the boundary between the layers at a depth of 11 km. Very weak mantle‐refracted arrivals with a velocity of about 8.0 km s‐1 were recorded. Large‐amplitude, later arrivals, beginning at distances near 100 and 150 km, have been interpreted to be part of the retrograde branches from the 8.0 and 7.33 km s‐1 layers, respectively. Model studies indicate that a small positive velocity gradient is required between 17 and 30 km, and that the Moho is at a depth of 34 km. A third set of large amplitude, later arrivals starting at a distance near 250 km has been interpreted as most probably multiple refraction‐reflection arrivals from the 5.97 and 6.52 km s‐1 layers. Correlation of this structure with the stratigraphic logs from exploratory oil wells in the Arafura Sea using layer velocities indicates that rocks younger than Jurassic appear to thin towards the east. 相似文献
945.
946.
Tectonometamorphic Evolution of the Eastern Tibet Plateau: Evidence from the Central Songpan-Garze Orogenic Belt, Western China 总被引:3,自引:0,他引:3
The SongpanGarzê Orogenic Belt (northeastern TibetPlateau) experienced polyphase deformation and metamorphismthat is best developed in the Danba Domal Metamorphic Terrane(DDMT), in which three tectonometamorphic events can be identified.The first event (D1M1) is characterized by prograde sericite-to kyanite-grade Barrovian metamorphism during late Indosinian( 相似文献
947.
R. Thomas J.D. Webster D. Rhede W. Seifert K. Rickers H.-J. Frster W. Heinrich P. Davidson 《Lithos》2006,91(1-4):137-149
Fractional crystallization of peraluminous F- and H2O-rich granite magmas progressively enriches the remaining melt with volatiles. We show that, at saturation, the melt may separate into two immiscible conjugate melt fractions, one of the fractions shows increasing peraluminosity and the other increasing peralkalinity. These melt fractions also fractionate the incompatible elements to significantly different degrees. Coexisting melt fractions have differing chemical and physical properties and, due to their high density and viscosity contrasts, they will tend to separate readily from each other. Once separated, each melt fraction evolves independently in response to changing T/P/X conditions and further immiscibility events may occur, each generating its own conjugate pair of melt fractions. The strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained. Consequently, the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions.
We demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore-forming and accessory minerals, and are visible on scales from a few micrometers to hundreds of meters. 相似文献
948.
Calcite dendrite crystals are important but poorly understood components of calcite travertine that forms around many hot springs. The Lýsuhóll hot-spring deposits, located in western Iceland, are formed primarily of siliceous sinters that were precipitated around numerous springs that are now inactive. Calcite travertine formed around the vent and on the discharge apron of one of the springs at the northern edge of the area. The travertine is formed largely of two types (I and II) of complex calcite dendrite crystals, up to 1 cm high, that grew through the gradual addition of trilete sub-crystals. The morphology of the dendrite crystals was controlled by flow direction and the competition for growth space with neighbouring crystals. Densely crowded dendrites with limited branching characterize the rimstone dams whereas widely spaced dendrites with open branching are found in the pools. Many dendrite bushes in the pools nucleated around plant stems. Growth of the dendrite crystals was seasonal and incremental. Calcite precipitation was driven by rapid CO2 degassing of CO2-rich spring waters during the spring and summer. During winter, when snow covered the ground and temperatures were low, opal-A precipitated on the exposed surfaces of the dendrites. Segmentation of dendrite branches by discontinuities coated with opal-A and overgrowth development around sub-crystals resulted from this seasonal growth cycle. The calcite dendrite crystals in the Lýsuhóll travertine differ in morphology from those at other hot springs, such as those at Lake Bogoria, Kenya, and Waikite in New Zealand. Comparison with the calcite dendrite crystals found at those sites shows that dendrite morphology is site-specific and probably controlled by carbonate saturation levels that, in turn, are controlled by the rate of CO2 degassing and location in the spring outflow system. 相似文献
949.
Jacek?MajorowiczEmail author Jan??afanda Rajmund?Przybylak Gabriel?Wójcik 《Pure and Applied Geophysics》2004,161(2):351-363
Ground Surface Temperature (GST) history in Poland was derived from the inversion of temperature-depth profiles in over 20 wells. Temperature histories for the period 1500 A.D. through 1977 A.D. agree well with the instrumental record of the surface-air temperature available for the last two centuries. A statistical correlation of the reconstructed histories (from the well temperature data) with the instrumental record (air temperature) from the homogeneous Warsaw series is high (>0.8). Functional space inversion (FSI) of the temperature data with depth shows that beginning in the early 19th century, temperatures warmed by 0.9 ± 0.1°C following a long period of colder climate before. The last number could be a minimal as higher warming was calculated using a simple model based on surface temperature for the observational period (homogenized Warsaw surface temperature series, Lorenc, 2000) and POM (pre-observational mean; Harris and Chapman, 1998) of –1.53oC below the 1951–1980 mean temperature level. 相似文献
950.