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The palynomorph assemblage of lake sediments younger than 51.9 kaBP from Wulagai Gobi in Inner Mongolia was analyzed to reconstruct the vegetation and climate.From 51.9 to 30.6kaBP,the vegetation was arid to semi-arid grassland with only slight changes.According to the palynomorphs,trees and shrubs were very rare.The large number and diversity of algae indicate the presence of a lake.Quantitative climatic conditions were reconstructed using the Best Analogues Method.The results indicate that the annual mean temperature was higher than that at present.The combination of temperature and annual precipitation suggests a change in the climate from cool dry to warm dry and then cool humid.Our results show that the annual precipitation values were mostly higher than that at present but were lower than 400 mm.It infers that the study area was already within the arid to semi-arid regions but with a stronger influence of the summer monsoon during 51.9to 30.6 kaBP than at present.With slight differences mainly in time scale,the changing trend of the annual temperature curve is consistent with the other climatic records from Antarctica,Greenland,Hulu Cave(East China),and the Tibetan Plateau during the last glacial period.From 30.6 kaBP to present,very few palynomorphs were detected in the samples.Hence,no information about the vegetation and climate could be extracted.Combined with other studies during Late Pleistocene,we presume that the reason for the lack of pollen during this period was caused by an abrupt temperature fall after 30.6 kaBP or that the lacustrine conditions were unsuitable for pollen deposition.It was probably incurred by the oxidation on land prior to deposition.But for those samples only with algae,it might be caused by the fact that algae could finish their life history in a very short time in a seasonal lake. 相似文献
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Evidence of Liquid Immiscibility in Alkaline Ultrabasic Dikes at Callander Bay, Ontario 总被引：1，自引：1，他引：1
Monchiquitic and camptonitic dikes radiating from an alkalinecarbonatite intrusion display sharply bounded, ellipsoidal,coarse-grained segregations of predominantly leucocratic minerals,identical in both composition and zoning to minerals in thematrix. These oeelli are not found in chilled margins, but elsewhereoccur in layers or segregations within the intrusives. Similarphenomena are seen in rocks of similar composition, both plutonicand volcantic, the world over. Field relations, detailed mineralogy,and macro- and micro-chemical studies strongly suggest thatthey result from liquid immiscibility in ultra basic alkalinemagma. Hydrothermal melting experiments show that olivine-bearing dikeswith carbonate-rich ocelli melt to a homogeneous silicate liquid,and an immiscible CO2-rich fluid phase. Kaersutite-bearing lamprophyrewith felsic ocelli yields a homogeneous silicate liquid on firstmelting, but on further heating this liquid dissociates intotwo immiscsible portions. The data strongly suggest that aninitial carbonated nepheliniti magma could gives rise by successiveimmiscibility to carbonatite, melteigite, and nepheline syeniteliquids. 相似文献
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Yuki KIMURA Joseph A. NUTH III Frank T. FERGUSON 《Meteoritics & planetary science》2006,41(5):673-680
Abstract— We demonstrate a new formation route for TiC core‐graphitic mantle spherules that does not require carbon‐atom addition and the very long time scales associated with such growth (Bernatowicz et al. 1996). Carbonaceous materials can be formed from C2H2 and its derivatives, as well as from CO gas. In this paper, we will demonstrate that large‐cage‐structure carbon particles can be produced from CO gas by the Boudouard reaction. Since the sublimation temperature for such fullerenes is low, the large cages can be deposited onto previously nucleated TiC and produce TiC core‐graphitic mantle spherules. New constraints for the formation conditions and the time scale for the formation of TiC core‐graphitic mantle spherules are suggested by the results of this study. In particular, TiC core‐graphitic mantle grains that are found in primitive meteorites that have never experienced hydration could be mantled by fullerenes or carbon nanotubes rather than by graphite. In situ observations of these grains in primitive anhydrous meteoritic matrix could confirm or refute this prediction and would demonstrate that the graphitic mantle on such grains is a metamorphic feature due to interaction of the presolar fullerenes with water within the meteorite matrix. 相似文献
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The formation of ooids 总被引：5，自引：0，他引：5
Field and laboratory studies suggest that different types of ooids form during quiet and agitated water conditions. Both types have been synthesized in the laboratory. Quiet water types exhibit a radial orientation of carbonate crystals, whereas in those formed in agitated conditions, a tangential orientation is prevalent. Successful laboratory formation of quiet water ooids was accomplished in supersaturated seawater solutions containing humic acids. Negative results were obtained from strictly inorganic solutions, and from those containing simple amino acids, single proteins, mixtures of proteins or mucopolysaccharides, soil and sediment extracts. Partly successful results were obtained using an organic extract from Bahamian ooids. The organic parameters most important in quiet water ooid formation are molecular weight, the presence of carboxyl groups and an ability to participate in hydrophobic/hydrophilic interactions, all of which are critical to membrane formation. Membranes form concentric shells which act as growth surfaces for carbonate and also induce the periodicity in carbonate precipitation. Ooids exhibiting a tangential orientation of batten-like crystals have been synthesized under conditions of agitation, supersaturation and without the intervention of organic processes during the precipitation. Complete growth may be divided into agitation, resting and sleeping stages In the agitation stage, quartz nuclei induce an inorganic, heterogeneous nucleation from a supersaturated solution, which finally ceases as a result of Mg2+ and possibly H+ poisoning of the carbonate surfaces. No further precipitation occurs until the crystal surfaces are reactivated by removal of Mg2+ and H+ during the resting stage. Following a series of agitation and resting stages, precipitation is inhibited by a degree of poisoning which is not totally removed during the resting stage. For further growth, a new substrate is required and is provided by the development of organic membranes around the grains. This occurs when the grains are buried in the subsurface, the period of organic growth constituting the sleeping stage. Only 2% of an ooid's life is spent growing in the agitated environment, while 95% of its life is spent accreting organic membranes in the subsurface. Our experimental work indicates that ooids of Bahamian type are inorganic precipitates. The tangential arrangement of battens is the result of suspension in an environment where the degree of turbulence is sufficient to induce grain to grain contact of sufficient strength and frequency to inhibit any crystal growth other than tangential. The role of organics is to provide a substrate for further growth after precipitation has slowed to a point when no further accretion is occurring. 相似文献
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The Tatara shield volcano and subsequent San Pedro cone arethe youngest edifices of the San Pedro-Pellado volcanic complexat 36S in the Chilean Andes. There are multiple basaltic andesitecompositional types present in the Tatara volcano, which couldresult from either contrasting source regions or interactionof primitive liquids with heterogeneous crust. The eruptivestratigraphy of the magma types implies concurrent, isolatedmagma chambers beneath Tatara-San Pedro. Open-system processesand multiple crustal endmembers were involved in calcalkalinedifferentiation series, whereas a tholeitiic series evolvedmainly by fractional crystallization. The glaciated Tatara shield comprises two cycles of compositionallydiverse basaltic andesite lavas, each of which is capped byvolumetrically minor andesite to dacite lavas. Four types (I-IV)of basaltic andesite are defined on the basis of chemical criteria,two in each cycle. The early cycle consists of calcalkalinetype I basaltic andesites, and tholeiitic type II basaltic andesitesand andesites; it culminated in the eruption of a dacite dome.The later cycle comprises intercalated calcalkaline type IIIand IV basaltic andesites, and they are overlain by San Pedroandesites and dacites which appear to be the differentiationproducts of type IV magmas. Tatara lavas were erupted from acommon vent situated beneath the modern San Pedro cone. Althoughthey overlap temporally and spatially, there is little evidenceof chemical interaction among the different lava types, indicatingthat there were two or more magma reservoirs beneath Tatara-SanPedro. Chemical differences among the basaltic andesite types precludederivation of any one from any of the others by fractional crystallization,assimilation-fractional crystallization (AFC), or magma mixing.The differences seem to reflect chemically different parentmagmas. The type I and IV parent liquids were relatively highin MgO, low in CaO and AI2O3, and had high incompatible andcompatible element abundances. The type II and III parents werelower in MgO, higher in A12O3 and CaO, and had lower compatibleand incompatible element abundances. Tholeiitic type II lavasappear to have evolved mainly by fractional crystallization,whereas there is evidence of open-system processes such as AFCand magma mixing in the evolution of the calcalkaline I, III,and IV suites. The chemical evolution of the type III and type IV-San Pedromagma suites has been simulated by assimilation and mixing modelsusing local granites and xenoliths as assimilants. The xenolithsprobably represent portions of a sub-caldera pluton associatedwith the Quebrada Turbia Tuff, which erupted from the Rio Coloradocaldera within the San Pedro-Pellado complex at 0–487Ma. Chemical and textural variations in type III lavas correlatewith stratigraphic position and appear to represent mixing betweena parental type III magma and remnant, evolved type I magmathat was progressively flushed from its chamber concurrent withmixing. The youngest San Pedro flow is chemically zoned fromdacite to basaltic andesite and may have formed by mixing withina conduit during eruption. 相似文献
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