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111.
Andreas Tennie Radegund Hoffbauer Stephan Hoernes 《Contributions to Mineralogy and Petrology》1998,133(4):346-355
The oxygen isotope fractionation between kyanite and calcium carbonate has been investigated experimentally at four temperatures
in the range between 625 and 775 °C at 13 kbar. Because of low exchange rates, the isotopic reaction was enhanced by polymorphic
transformation of andalusite to kyanite. With this experimental modification a close approach to equilibrium was reached in
all runs. The temperature dependence of the equilibrium fractionation is described by the equation 1000 ln ky-cc=−2.62×106/T
2. Application of the experimental results to natural quartz-kyanite-garnet assemblages indicates the preservation of the oxygen
isotope composition of kyanite acquired during its formation, reflecting its extremely low oxygen diffusivity. This refractory
behaviour restricts the use of kyanite for thermometry but opens the possibility to use its O-isotope composition as an indicator
for recognition of polymetamorphic rock histories and reconstruction of the prograde evolution of a metamorphic sequence.
Received: 8 June 1998 / Accepted: 24 August 1998 相似文献
112.
113.
Pipe G3b is part of the Upper Cretaceous carbonatitic Gross Brukkaros Volcanic Field in southern Namibia. The pipe represents
the root zone of a diatreme and is located 2800 m west of the rim of Gross Brukkaros, a downsag caldera. The pipe is exposed
approximately 550 m below the original Upper Cretaceous land surface. It cuts down into its own feeder dyke, 0.3 m thick.
The pipe coalesced from two small pipes and in plan view is 19 m long and 12 m wide. It consists of fragmented Cambrian Nama
quartzites and shales of the Fish River subgroup. Despite intensive brecciation, the stratigraphic sequence of the country
rocks is almost preserved in the pipe. In addition, the feeder dyke became fragmented too and can be traced in a 2- to 3-m-wide
zone full of carbonatite blocks along the southern margin of the pipe. The void space of the breccia is 30–50% in volume.
Finally, after the disruption of country rocks and feeder dyke, a little carbonatite magma intruded some of the void space.
The breccia of pipe G3b is considered to represent a root zone at the transition from the feeder dyke into a diatreme above.
Formation of the breccia required a shock wave thought to have been associated with a last explosion of the diatreme immediately
above the present level of exposure. The explosion can be shown to have been phreatomagmatic in origin.
Received: 11 October 1996 / Accepted: 6 March 1997 相似文献
114.
In the Upper Cretaceous Gross Brukkaros Volcanic Field, southern Namibia, a radial dyke system surrounds a dome structure
and its 74 closely related carbonatite diatremes. This paper focuses on volcanological features which seem to be typical for
a low-viscosity melt in various settings such as dykes, sills and diatremes. The total or near absence of vesicles in carbonatite
ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a
phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes.
Received: 15 August 1996 / Accepted: 13 January 1997 相似文献
115.
116.
Stephan R. McCandliss Bruce Bohannan Carmelle Robert Anthony F. J. Moffat 《Astrophysics and Space Science》1994,221(1-2):155-167
The original temporal analysis of a 12 night spectral timeseries of WR-134 has been found to be flawed and a re-analysis shows that the line profile variations are indeed periodic. When combined with a 4 night timeseries taken 45 days earlier, a period near 2.27 d is found in periodograms of the Heii 5412 line centroid,rms line width, and line skew variations. When the emission line residuals are ordered as a function of phase, a sinuous feature appears to snake about the line center with an amplitude of ± 500 km s–1. This is 20 larger than the line centroid amplitude; the calculation of which is heavily weighted by static portions of the line profile. In addition to the snake, emission residuals appear that move away from line center on unbound trajectories and are thought to result from the interaction of a periodic driver with the unstable flow of the radiation driven wind. The nature of the periodic driver is a topic for discussion. 相似文献
117.
Martin Bizzarro Antonio Simonetti Stephan Kurszlaukis 《Geochimica et cosmochimica acta》2003,67(2):289-302
In situ Sr isotopic compositions of coexisting apatite and carbonate for carbonatites from the Sarfartoq alkaline complex, Greenland, have been determined by laser-ablation multicollector inductively coupled plasma mass spectrometry. This study is the first to examine the extent of Sr isotopic homogeneity among coexisting igneous minerals containing high Sr (>3000 ppm) and low Rb (?1 ppm) contents within a single ∼50-μm-thick thin-section mount. This technique is capable of producing measured 87Sr/86Sr values with analytical precision (∼0.005%, 2σ) approaching those obtained by thermal ionization mass spectrometry but in a much shorter interval of time (100 s vs. >1 h, respectively). The combined total analyses (n = 107) of apatite and carbonate yield 87Sr/86Sr compositions ranging from ∼0.7025 to ∼0.7031. This relatively large variation in Sr isotopic compositions (∼0.0006) is ∼1 order of magnitude larger than the estimated external reproducibility (∼0.00005, 2σ) of the method. The large range in 87Sr/86Sr values suggests that apatite and carbonate precipitated predominantly under nonequilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger (regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources or from a common isotopically heterogeneous mantle. 相似文献
118.
Stephan Königer Volker Lorenz Harald Stollhofen Richard A. Armstrong 《International Journal of Earth Sciences》2002,91(2):341-356
Thin, widespread, fallout tuff layers interbedded within fluvio-lacustrine successions of the Carboniferous-Permian Saar-Nahe Basin provide important tephrostratigraphic markers. In addition, radiogeochronometric data derived from the tuffs serve as calibration points for the adjustment to regional chronostratigraphy and to numerical time scales. The Pappelberg-Tuff in the Meisenheim Formation (Glan Group) has been dated by U/Pb zircon SHRIMP technique at 297.0Dž.2 Ma. Taking the Carboniferous/Permian boundary at 296 Ma, the Meisenheim Formation coincides approximately with this boundary. Consequently, underlying strata, lithostratigraphically regarded as the basal part of the 'Rotliegend', chronostratigraphically belong to the Upper Carboniferous. Bed thicknesses, grain size and sorting characteristics of the tuffs and the absence of contemporaneously emplaced volcanics within the Saar-Nahe Basin point to an extrabasinal derivation of the wind-drifted volcanic ash. Decreasing grain sizes of juvenile pyroclastic particles towards the north suggest source areas south of the basin within 300 km distance. The majority of the tuffs are rhyolitic to rhyodacitic and indicate petrographic and geochemical affinities to Moldanubian S-type granitoids, in particular to highly differentiated two-mica granites, and related volcanic effusives. Within the time frame considered here, such potential source rocks were emplaced in the northern and central Black Forest (SW Germany) and the northern Vosges (E France) at 100-150 km distance south of the Saar-Nahe Basin. 相似文献
119.
In this article, we present a straightforward random walk model for fast evaluation of push‐pull tracer tests. By developing an adaptive algorithm, we overcome the problem of manually defining how many particles have to be used to simulate the transport problem. Beside this, we validate the random walk model by evaluating a push‐pull tracer test with drift phase and confirm the results with MT3DMS. The random walk model took less than 1% of computational time of MT3DMS, thus allowing a remarkable faster evaluation of push‐pull tracer tests. 相似文献
120.
Michael Schäfer Tanja Schäfer Matthew R. M. Izawa Edward A. Cloutis Stefan E. Schröder Thomas Roatsch Frank Preusker Katrin Stephan Klaus‐Dieter Matz Carol A. Raymond Christopher T. Russell 《Meteoritics & planetary science》2018,53(9):1925-1945
Ceres’ surface has commonly been linked with carbonaceous chondrites (CCs) by ground‐based telescopic observations, because of its low albedo, flat to red‐sloped spectra in the visible and near‐infrared (VIS/NIR) wavelength region, and the absence of distinct absorption bands, though no currently known meteorites provide complete spectral matches to Ceres. Spatially resolved data of the Dawn Framing Camera (FC) reveal a generally dark surface covered with bright spots exhibiting reflectance values several times higher than Ceres’ background. In this work, we investigated FC data from High Altitude Mapping Orbit (HAMO) and Ceres eXtended Juling (CXJ) orbit (~140 m/pixel) for global spectral variations. We found that the cerean surface mainly differs by spectral slope over the whole FC wavelength region (0.4–1.0 μm). Areas exhibiting slopes 10% μm?1 constitute only ~3% of the cerean surface and mainly occur in the bright material in and around young craters, whereas slopes ≥?10% μm?1 occur on more than 90% of the cerean surface; the latter being denoted as Ceres’ background material in this work. FC and Visible and Infrared Spectrometer (VIR) spectra of this background material were compared to the suite of CCs spectrally investigated so far regarding their VIS/NIR region and 2.7 μm absorption, as well as their reflectance at 0.653 μm. This resulted in a good match to heated CI Ivuna (heated to 200–300 °C) and a better match for CM1 meteorites, especially Moapa Valley. This possibly indicates that the alteration of CM2 to CM1 took place on Ceres. 相似文献