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The metastable, high-temperature portion of the microcline-lowalbite solvus has been experimentally determined at atmosphericpressure by the fused-salt alkali ion exchange technique (Bachinski),and by the homogenization—unmixing technique (Müller).Analytical smoothing of each set of pairs of coexisting compositionsyields critical temperatures and compositions of 885 °Cand 27 mol per cent KAISi3O8, and 887 °C and 24 mol percent KAISi3O8, respectively. The differences between the solviare real and the lack of coincidence of the two is attributable,to varying extents, to differences in the structural state,composition, twinning, and heat treatment of the starting materials.Previous workers' estimates of the position of the one-feldspar-two-feldsparfield boundary are in general accord with the experimental solvi. 相似文献
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A strong correlation in the geometry and mineralogy of two cement generations of a Quaternary with a Precambrian calcarenite enables us to reconstruct the diagenetic history of the Precambrian limestone. Both calcarenites contain two cement generations (A and B) of which A consists of dolomite, B of calcite. The following diagenetic stages can be recognized: after deposition of the allochems in a shallow marine environment, cementation in the intertidal zone with magnesian calcite (cement A) led to the formation of beachrock (Stage 1). By lowering of the sea level, the beach rock was shifted into the supratidal zone, but still remained under the predominant influence of the sea water (breakers, spray). During longer periods of aridity, the magnesian calcite of cement A and of the allochems was transformed into dolomite by brines derived from sea water with very high Mg/Ca ratio (> 15), whereas aragonite and calcite remained unaffected. After further lowering of the sea, an increasing influence of meteoric water caused the wet transformation of aragonitic allochems to sparry calcite and to the precipitation of sparry calcitic cement B. 相似文献
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CHRISTINE FLOSS AHMED EL GORESY ERNST ZINNER HERBERT PALME GERD WECKWERTH WERNER RAMMENSEE 《Meteoritics & planetary science》1998,33(2):191-206
Abstract— We have produced corundum-bearing residues through the evaporation of natural and synthetic hibonite samples. The sequence of major element losses as well as volatility related trace element fractionations in these residues are similar to those previously observed in residues from the evaporation of chondritic starting material, which suggests that the processes by which these fractionations occur may be largely independent of the starting material used. However, the mineralogy of the residues does depend on the composition of the starting material and, to some extent, on the conditions under which evaporation took place. Similarly, the degree of isotopic mass fractionation observed in the residues is composition-dependent. This observation means that it may be possible to use isotopic data for several elements to constrain the compositions of precursor materials of Ca-Al-rich inclusions, which have an evaporation origin. Although corundum-bearing inclusions are known, their origins are complex and variable, and the scarcity of such inclusions indicates that melting of hibonite, with or without concomitant evaporation, must have been a rare process in the solar nebula. By evaporating mixtures of synthetic oxides of the rare earth elements, we have reproduced the patterns of Group III inclusions and some of the characteristics of ultrarefractory patterns. However, the extreme conditions required to do so indicate that refractory inclusions with these patterns probably have a condensation rather than evaporation origin. 相似文献
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Evidence from a Late Pleistocene eolianite in Fuerteventura, Canary Islands, demonstrates that red algal clasts replaced by low-magnesian calcite in the subaerial environment show a partial re-constitution of their original high-magnesian calcite mineralogy when they are exposed subsequently to sea water. This process only affects the cell walls of the red algae, which, owing to their specific microarchitecture (extremely small crystal size, very large specific surface area), offer the most favourable conditions for dissolution-reprecipitation or diffusion processes. High-magnesian caicite precipitated as a second generation cement on fresh water calcitic cement is an equivalent of the high-magnesian calcite cement which at many locations on the island leads to the formation of beachrock in Holocene intertidal sediments. 相似文献
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The identification of the remains of organisms contributing to carbonate sediments by means of scanning electron-microscopy is limited to particles of the 2–20 μm size class. Mineralogy and the content of Mg, Sr and trace elements alone are usually insufficient to solve the problem of identification, especially in the differentiation between algal and coral aragonite. The organic matrix of calcareous organisms consists of stable biopolymers such as polysaccharides and glycoproteins which are intimately associated with the carbonate skeleton. Analysis of these hydrolysed compounds gives rise to characteristic arrays of monosaccharides which provide independent criteria for producer identification. The calcareous green algae Halimeda, Penicillus and Udotea show high xylose and low fucose levels. Xylose and fucose levels are elevated in the red algae Amphiroa but only fucose is prominent in the brown algae Padina. The corals Oculina, Porites, Millipora and Montastrea are relatively rich in fucose and show little or no xylose. In the bivalves Arca, Codakia and in Argopecten mannose may be characteristic. Analysis of artificial and natural sediments demonstrates that coral and algal aragonite can be distinguished on the basis of the total sugar concentration and respective xylose and fucose levels. The applicability of the technique in comparison to geochemical and mineralogical methods has been demonstrated for surface sediments from varying water depths of Harrington Sound, Bermuda. 相似文献
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Major eustatic fall has been invoked to explain Lower–Middle Cambrian boundary sandstones and faunal replacements on a number of Cambrian palaeocontinents. This proposal has been tested on the Moroccan and Spanish margins of West Gondwana and found to be inadequate to explain stratigraphical developments. In these regions, sandstone intervals long presumed to be regressive and late Early Cambrian in age are now shown to be early Middle Cambrian, and composed of a lower regressive and an overlying transgressive sandstone separated by a regional unconformity. Only the lower tidalites (i.e. Tazlaft Formation in Morocco and lower Daroca sandstones in Spain) record the Hawke Bay eustatic regression in West Gondwana. The Tazlaft is overlain by a newly recognized, unconformably overlying sandstone (Talelt Formation) that onlapped southern Morocco with reactivation of a pull‐apart or transcurrent regime. Up to 150 m of erosion on uplifted blocks in the High Atlas range and foundering of the Souss Basin to the south preceded onlap and deposition of the volcanic‐rich Tatelt, the correlative and depositional analogue of the upper Daroca and lower Valdemides Formations in northern Spain. With folding and erosion, a type 1 depositional sequence boundary also caps the Tatelt at its contact with an overlying, lower Middle Cambrian mudstone‐dominated succession. This unconformity probably occurs in Spain within the Valdemiedes Formation and corresponds to a faunal discontinuity called the ‘Valdemiedes geoevent’. The Iberian ‘Daroca regression’ and Moroccan ‘Asrir regression’ are misnomers, as the sandstones on which they are based are composite units with a lower regressive interval that records eustatic fall and an upper transgressive unit that records epeirogenically driven onlap. 相似文献
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Abstract. Biofacies development, sedimentology, and geochemical properties of intertidal Mytilus beds in the German Wadden Sea have been studied in vertical profiles in situ and in box core samples. Characteristic features of both the live mussel beds and the underlying sediment sequences were detected. The deposition of faeces, pseudofaeces, and trapped fine-grained material results in the formation of well-defined biosedimentary mud layers of several centimetres thickness. The initial stages of this deposition process were observed in a young, growing Myrilus colony over a 17-month period. In an old mussel bed, being in state of decline, both reworking of dead shells and erosion of the biosedimentary mud layers were recognisable. However, deeper parts of the sediment column reveal shell horizons rich in "pairs" of dead Mytilus shells, i.e ., corresponding right and left valves found close together. These represent subrecent records of former Mytilus colonisation. Historically, they point to long-term occurrence of mussel beds at typical localities, in the framework of fluctuation and patchiness patterns, in the more recent past. On the other hand, mud banks at tidal flat margins adjacent to channels suggest a high preservation potential of biosedimentary mud layers in the low-water level environment. Geochemical properties of these muddy sediments corroborate their biodepositional origin. 相似文献
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DAVID W. KICKLIGHTER MICHELE BRUNO SILKE DÖNGES GERD ESSER MARTIN HEIMANN JOHN HELFRICH FRANK IFT FORTUNAT JOOS JÖRG KADUK GUNDOLF H. KOHLMAIER A. DAVID McGUIRE JERRY M. MELILLO ROBERT MEYER BERRIEN MOORE III REAS NADLER I. COLIN PRENTICE WALTER SAUF ANNETTE L. SCHLOSS STEPHEN SITCH UWE WITTENBERG GUDRUN WÜRTH 《Tellus. Series B, Chemical and physical meteorology》1999,51(2):343-366
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