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931.
Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bedforms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bedforms respond to rapid changes in flow rate,... 相似文献
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Jonathon Dale Tim Holland Roger Powell 《Contributions to Mineralogy and Petrology》2000,140(3):353-362
Calibrations are presented for an independent set of four equilibria between end-members of garnet, hornblende, plagioclase
and quartz. Thermodynamic data from a large internally-consistent thermodynamic dataset are used to determine the ΔG° of the equilibria. Then, with the known mixing properties of garnet and plagioclase, the non-ideal mixing in amphibole is
derived from a set of 74 natural garnet–amphibole–plagioclase–quartz assemblages crystallised in the range 4–13 kbar and 500–800 °C.
The advantage of using known thermodynamic data to calculate ΔG° is that correlated variations of composition with temperature and pressure are not manifested in fictive derived entropies
and volumes, but are accounted for with non-ideal mixing terms. The amphibole is modelled using a set of ten independent end-members
whose mixing parameters are in good agreement with the small amount of data available in the literature. The equilibria used
to calibrate the amphibole non-ideal mixing reproduce pressures and temperatures with average absolute deviations of 1.1 kbar
and 35 °C using an average pressure–temperature approach, and 0.8 kbar with an average pressure approach. The mixing data
provide not only a basis for thermobarometry involving additional phases, but also for calculation of phase diagrams in complex
amphibole-bearing systems.
Received: 8 November 1999 / Accepted: 7 July 2000 相似文献
936.
Abstract— The composition of surface deposits on vesicle walls in irghizites (i.e., impact glasses at site) from the Zhamanshin meteorite crater were studied using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS). The cavity walls are unique interfaces for condensation of gases from the superheated, high‐silica melt during the impact. Initially, signals from the cavity wall are dominated by hydrocarbon fragments whereas the glass fracture face surrounding the cavity gave only signals corresponding to glass components. After 12 h in ultra high vacuum (UHV), signals from the cavity wall are dominated by peaks corresponding to fragments normally measured from organosilanes and organosiloxanes with the majority of the hydrocarbon signals markedly reduced. Characteristic hydrocarbon fragments are now observed on the glass fracture surface next to the cavity in an annulus around the cavity perimeter. There are also minor signals in this region from organosilanes and organosiloxanes. In contrast, four tektites (Australites) (i.e., glassy distal ejecta) gave no organosilane or organosiloxane signals after the same preparation and vacuum evaporation procedure. These species appear to be formed only at the impact site where higher levels of organic material are likely to be present in soil and are trapped before evaporation. This appears to be the first report of naturally occurring silicon‐organic compounds. 相似文献
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Susumu Honjo Roger Francois Steven Manganini Jack Dymond Robert Collier 《Deep Sea Research Part II: Topical Studies in Oceanography》2000,47(15-16)
An array of five bottom-tethered moorings with 19 PARFLUX time-series sediment trap at three depths (1 and 2 km below the surface, and 0.7 km above the sea-floor) was deployed in the western Pacific sector of the Southern Ocean, along 170°W. The five stations were selected to sample settling particles in the main hydrological zones of the Southern Ocean. The sampling period spanned 425 days (November 28, 1996–January 23, 1998) and was divided into 13 or 21 synchronized time intervals. A total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass (TMF), organic carbon, carbonate, biogenic silica, and lithogenic particles. The fluxes of biogenic material were higher than anticipated, challenging the notion that the Southern Ocean is a low-productivity region. Organic carbon fluxes at 1 km depth within the Polar Frontal Zone and the Antarctic Zone were relatively uniform (1.7–2.3 g m−2 yr−1), and about twice the estimated ocean-wide average (ca. 1 g m−2 yr−1). Carbonate fluxes were also high and uniform between the Subantarctic Front and ca. 64°S (11–13 g m−2 yr−1). A large fraction of the carbonate flux in the Antarctic Zone was due to the presence of pteropod shells. Coccoliths were found only to the north of the Polar Front, and calcium carbonate became the dominant phase in the Subantarctic Zone. In contrast, carbonate particles were nearly absent near 64°S. Latitudinal variations in biogenic silica fluxes were substantial. The large opal flux (57 g m−2 yr−1) measured in the Antarctic Zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the Polar Front. Unlike biogenic material, fluxes of lithogenic particles were among the lowest measured in the open-ocean (0.12–0.05 g m−2 yr−1), reflecting a very low dust input. 相似文献
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1 Introduction A series of studies have indicated that there were two extensional phases in the North Sea (Fig. 1). An earlier period (Late Permian-Early Triassic) of rifting occurred widely in these areas, with predominant extension direction of W-E (F?rseth, 1996; F?rseth et al., 1997). In contrast to the widely distributed Permo-Triassic extension, Jurassic extension in the North Sea were generally much more localized into the three main rift arms (Fig. 1): the Viking Graben, Moray… 相似文献