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121.
Differentiation and crystallization conditions of basalts from the Kerguelen large igneous province: an experimental study 总被引:2,自引:2,他引:0
Marcus Freise Francois Holtz Marcus Nowak James S. Scoates Holger Strauss 《Contributions to Mineralogy and Petrology》2009,158(4):505-527
Phase relations of basalts from the Kerguelen large igneous province have been investigated experimentally to understand the
effect of temperature, fO2, and fugacity of volatiles (e.g., H2O and CO2) on the differentiation path of LIP basalts. The starting rock samples were a tholeiitic basalt from the Northern Kerguelen
Plateau (ODP Leg 183 Site 1140) and mildly alkalic basalt evolved from the Kerguelen Archipelago (Mt. Crozier on the Courbet
Peninsula), representing different differentiation stages of basalts related to the Kerguelen mantle plume. The influence
of temperature, water and oxygen fugacity on phase stability and composition was investigated at 500 MPa and all experiments
were fluid-saturated. Crystallization experiments were performed at temperatures between 900 and 1,160°C under oxidizing (log
fO2 ~ ΔQFM + 4) and reducing conditions (log fO2 ~ QFM) in an internally heated gas-pressure vessel equipped with a rapid quench device and a Pt-Membrane for monitoring the
fH2. In all experiments, a significant influence of the fO2 on the composition and stability of the Mg/Fe-bearing mineral phases could be observed. Under reducing conditions, the residual
melts follow a tholeiitic differentiation trend. In contrast, melts have high Mg# [Mg2+/(Mg2+ + Fe2+)] and follow a calk-alkalic differentiation trend at oxidizing conditions. The comparison of the natural phenocryst assemblages
with the experimental products allows us to constrain the differentiation and pre-eruptive conditions of these magmas. The
pre-eruptive temperature of the alkalic basalt was about 950–1,050°C. The water content of the melt was below 2.5 wt% H2O and strongly oxidizing conditions (log fO2 ~ ΔQFM + 2) were prevailing in the magma chamber prior to eruption. The temperature of the tholeiitic melt was above 1,060°C,
with a water content below 2 wt% H2O and a log fO2 ~ ΔQFM + 1. Early fractionation of clinopyroxene is a crucial step resulting in the generation of silica-poor and alkali-rich
residual melts (e.g., alkali basalt). The enrichment of alkalis in residual melts is enhanced at high fO2 and low aH2O. 相似文献
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Holger Penning 《Geochimica et cosmochimica acta》2006,70(9):2283-2297
In anoxic environments, microbial fermentation is the first metabolic process in the path of organic matter degradation. Since little is known about carbon isotope fractionation during microbial fermentation, we studied mixed-acid fermentation of different saccharides (glucose, cellobiose, and cellulose) in Clostridium papyrosolvens. The bacterium was grown anaerobically in batch under different growth conditions, both in pure culture and in co-culture with Methanobacterium bryantii utilizing H2/CO2 or Methanospirillum hungatei utilizing both H2/CO2 and formate. Fermentation products were acetate, lactate, ethanol, formate, H2, and CO2 (and CH4 in methanogenic co-culture), with acetate becoming dominant at low H2 partial pressures. After complete conversion of the saccharides, acetate was 13C-enriched (αsacc/ac = 0.991-0.997), whereas lactate (αsacc/lac = 1.001-1.006), ethanol (αsacc/etoh = 1.007-1.013), and formate (αsacc/form = 1.007-1.011) were 13C-depleted. The total inorganic carbon produced was only slightly enriched in 13C, but was more enriched, when formate was produced in large amounts, as 12CO2 was preferentially converted with H2 to formate. During biomass formation, 12C was slightly preferred (αsacc/biom ≈ 1.002). The observations in batch culture were confirmed in glucose-limited chemostat culture at growth rates of 0.02-0.15 h−1 at both low and high hydrogen partial pressures. Our experiments showed that the carbon flow at metabolic branch points in the fermentation path governed carbon isotope fractionation to the accumulated products. During production of pyruvate, C isotopes were not fractionated when using cellulose, but were fractionated to different extents depending on growth conditions when using cellobiose or glucose. At the first catabolic branch point (pyruvate), the produced lactate was depleted in 13C, whereas the alternative product acetyl-CoA was 13C enriched. At the second branch point (acetyl-CoA), the ethanol formed was 15.6-18.6‰ depleted in 13C compared to the alternative product acetate. At low hydrogen partial pressures, as normally observed under environmental conditions, fermentation of saccharides should mainly result in the production of acetate that is only slightly enriched in 13C (<3‰). 相似文献
124.
Martin Bak Hansen Magdalena Scheck-Wenderoth Christian Hübscher Holger Lykke-Andersen Ali Dehghani Benjamin Hell Dirk Gajewski 《Tectonophysics》2007,437(1-4):1-16
A 3D structural model for the entire southwestern Baltic Sea and the adjacent onshore areas was created with the purpose to analyse the structural framework and the sediment distribution in the area. The model was compiled with information from several geological time-isochore maps and digital depth maps from the area and consists of six post-Rotliegend successions: The Upper Permian Zechstein; Lower Triassic; Middle Triassic; Upper Triassic–Jurassic; Cretaceous and Cenozoic. This structural model was the basis for a 3D backstripping approach, considering salt flow as a consequence of spatially changing overburden load distribution, isostatic rebound and sedimentary compaction for each backstripping step in order to reconstruct the subsidence history in the region. This method allows determination of the amount of tectonic subsidence or uplifting as a consequence of the regional stress field acting on the basin and was followed by a correlation with periods of active salt movement. In general, the successions above the highly deformed Zechstein evaporites reveal a thickening trend towards the Glückstadt Graben, which also experienced the highest amount of tectonic subsidence during the Mesozoic and Cenozoic. Two periods of accelerating salt movement in the area has been correlated with the E–W directed extension during the Late Triassic–Early Jurassic and later by the Late Cretaceous–Early Cenozoic inversion, suggesting that the regional stress field plays a key role in halokinesis. The final part of this work dealt with a neotectonic forward modelling in an attempt to predict the future topography when the system is in a tectonic equilibrium. The result reveals that many of the salt structures in the region are still active and that future coastline will run with a WNW–ESE trend, arguing that the compressional stresses related to the Alpine collision are the prime factor for the present-day landscape evolution. 相似文献
125.
The efficiency and sustainability of carbon dioxide (CO2) storage in deep geological formations crucially depends on the integrity of the overlying cap-rocks. Existing oil and gas
wells, which penetrate the formations, are potential leakage pathways. This problem has been discussed in the literature,
and a number of investigations using semi-analytical mathematical approaches have been carried out by other authors to quantify
leakage rates. The semi-analytical results are based on a number of simplifying assumptions. Thus, it is of great interest
to assess the influence of these assumptions. We use a numerical model to compare the results with those of the semi-analytical
model. Then we ease the simplifying restrictions and include more complex thermodynamic processes including sub- and supercritical
fluid properties of CO2 and non-isothermal as well as compositional effects. The aim is to set up problem-oriented benchmark examples that allow
a comparison of different modeling approaches to the problem of CO2 leakage. 相似文献
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Exsolution systems in synthetic pyroxenes were studied by transmission electron microscopy. An iron free sample En80Wo20 was prepared by devitrifying glass at 1300°C. Samples with bulk composition En50Fs30Wo20 and En35Fs38Wo27 were given various but well-defined heat treatments. The exsolution systems observed cannot unambiguously be related to the heat treatment. Periodic lamellar exsolution was observed parallel to (001) and (100) with sharp satellite reflections in the diffraction diagram. In more complex exsolution systems coarse (100) lamellae were found together with fine lamellae parallel to (001) and (100). An unusual phenomenon occurs at a (100) twin boundary where both individuals display exsolution lamellae parallel to (001). Pigeonite lamellae in one twin meet augite lamellae of the other individual at the twin boundary and vice-versa. The precise matching is achieved by a change in width near the boundary. Smoothly curved phase boundaries are developed in the obtuse angle of crosshatched (100) and (001) pigeonite lamellae in augite, whereas the boundaries in the acute angle are straight with sharp edges. This is consistent with elastic energy constraints. 相似文献