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291.
Metamictisation of natural zircon: accumulation versus thermal annealing of radioactivity-induced damage 总被引:12,自引:0,他引:12
Lutz Nasdala Marita Wenzel Gerhard Vavra Gert Irmer Thomas Wenzel Bernd Kober 《Contributions to Mineralogy and Petrology》2001,141(2):125-144
Annealing of the radiation damage generated in !-decay events is a common phenomenon in natural zircons. We have studied relationships between radiation damage, age and radionuclide content of natural zircons on a micro-scale. The degree of metamictisation was estimated by means of confocal laser-Raman spectroscopic measurements. Raman band broadening in well-ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth, shows a nearly linear dependence on the !-fluence. This dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage, which may contribute to the understanding of the low-T thermal history of their host rocks. Seven examples for the application are presented. Radiation damage may anneal heterogeneously, with preferred re-ordering in more damaged micro-areas. Results suggest that single point defects do not anneal continuously over geologic periods of time but are, in contrast, a comparably stable type of radiation damage. In contrast to recrystallisation processes, structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic Pb. The most prominent example for this are concordant zircons from Sri Lanka. Although they are excellent standards for U-Pb dating, Sri Lankan zircons should not be used as standards for structural radiation damage. The experimental work is completed by Monte Carlo simulations of !-decay events in zircon. 相似文献
292.
293.
Taking varves to bits: Scanning electron microscopy in the study of laminated sediments and varves 总被引:2,自引:0,他引:2
Jean M. Dean Alan E.S. Kemp David Bull Jennifer Pike Gunilla Patterson Bernd Zolitschka 《Journal of Paleolimnology》1999,22(2):121-136
Conventional high resolution studies of varved sediments are able to identify clastic and biogenic laminae, but are often unable to resolve the nature of fine-scale lamination contained therein. This intra-annual signal provides us with the highest potential resolution from the sedimentary record and can be resolved using scanning electron microscopy (SEM). Six case studies from lacustrine and marine settings are presented to illustrate the combination of clastic and biogenic fabric types typically found in laminated sediments. Clastic laminae fabrics include those which originate through grain settling and those which are event deposits. The correct identification of event deposits is essential if varves are to be used chronologically. SEM-based biogenic laminae fabric studies have identified seasonal faunal successions where individual laminae may be less than 100 thick and most recently, deep chlorophyll maxima (DCM) summer diatom floras, providing an insight into seasonal scale processes. High resolution lamina fabric studies can provide a basis for generating records of seasonal and inter-annual variability, thus contributing to our understanding of lacustrine and marine processes and palaeoenvironmental interpretation. 相似文献
294.
Prof. Dr. Bernd Andreae 《GeoJournal》1978,2(4):331-342
The agricultural and pastoral development of the huge dry areas of the world are subject of this paper. The author describes the possible stages of development of the pastoral industry in accordance with the general economic development of a country or region.The development of the pastoral industry is much related to the availability of fodder during the year and over the years. Typical of the pastoral industry in the dry areas are the different ways of fodder compensation. The author points out that each development stage of a ranch has its typical way of fodder compensation in accordance with the general economic development stage of the region. 相似文献
295.
Jaroslava Plomerová Libuše Ruprechtová Bernd Tittel Joachim Weyrauch Kurt Wylegalla Reviewer A. Zátopek 《Studia Geophysica et Geodaetica》1979,23(2):189-196
n¶rt; auau uu ¶rt; mnu¶rt; n, n, g, Sg, m anmam anauu ¶rt; numa amu 2.6° ¶rt; 4.7° m aa amu uu (mau). aa, m m auum Sg(D) mam mua ¶rt; m n¶rt;u aum¶rt; uma a uu mu. 相似文献
296.
Synthesis, structure, thermodynamic properties, and stability relations of K-cymrite, K[AlSi3O8]·H2O
Detlef W. Fasshauer Niranjan D. Chatterjee Bernd Marler 《Physics and Chemistry of Minerals》1997,24(6):455-462
Single-phase K-cymrite, K[AlSi3O8]·H2O, has been synthesized in the P-T range 3≤P(GPa)≤4 and 350≤T(°C)≤650, and characterized by a variety of techniques like SEM, FTIR, and 29Si MAS-NMR. Its thermal expansivity and compressibility have been measured up to 375?°C and 6.0?GPa, respectively. Within the uncertainty of the microchemical determination of H2O by Karl-Fischer titration, it invariably contains 1?mol of H2O per mol of KAlSi3O8. Under the SEM, it appears a small idiomorphic prisms. It is optically negative, with n o=1.553(1) and n e=1.521(1). FTIR spectrum identifies the water in its structure as molecular H2O. Its lattice constants are a=5.3348(1)?Å, c=7.7057(1) Å, V= 189.924 Å3, the space group being P6/mmm. The 29Si MAS-NMR suggests a weak short-range order of Al and Si in the symmetrically equivalent tetrahedral sites. A Rietveld structure refinement demonstrates that it is isostructural with cymrite (BaAl2Si2O8·H2O), the structure comprising double tetrahedral sheets with H2O molecules residing in their cavities, K serving as an interlayer cation. Whereas cymrite, with its ordered tetrahedral Al/Si distribution, shows a Pm symmetry, the weak short-range Al/Si order in K-cymrite (abbreviated below as KCym) makes it crystallize in the space group P6/mmm. Three reversal experiments on the reaction K[AlSi3O8]·H2O (KCym)=K[AlSi3O8] (Kfs)+H2O, executed in this study, confirm the earlier results of Thompson (1994) and supplement her data. A simultaneous treatment of those reversals, together with the thermodynamic data for Kfs and H2O available in the literature, helps derive the standard enthalpy of formation (?4233±9.4?kJ/mol) and standard entropy (276.3±10.2 J/K·mol) for K-cymrite. The computed phase relations of KCym in the KAlSi3O8-H2O binary are shown in Figure 4 for three different values of aH 2O. Given a 5?°C/km isotherm in a subducting slab of metasediments in a ultra-high-pressure metamorphic environment, KCym will be expected to grow by hydration of Kfs, unless the aH 2O had been substantially less than 0.5. Nevertheless, how far it can survive exhumation of the subducted terrain will depend critically on the rate of uplifting and on the aH 2O prevailing during that process. 相似文献
297.
Philipp Steinmann Bernd Eilrich Markus Leuenberger 《Geochimica et cosmochimica acta》2008,72(24):6015-6026
Vertical profiles of concentration and C-isotopic composition of dissolved methane and carbon dioxide were observed over 26 months in the catotelm of a deep (6.5 m) peat bog in Switzerland. The dissolved concentrations of these gases increase with depth while CO2 predominates over CH4 (CO2 ca. 5 times CH4). This pattern can be reproduced by a reaction-advection-ebullition model, where CO2 and CH4 are formed in a ratio of 1:1. The less soluble methane is preferentially lost via outgassing (bubbles). The isotopic fractionation between CO2 and CH4 also increases with depth, with αC values ranging from 1.045 to 1.075. The isotopic composition of the gases traces the passage of respiration-derived CO2 (from the near surface) through a shallow zone with methanogenesis of low isotopic fractionation (splitting of fermentation-derived acetate). This solution then moves through the catotelm, where methanogenesis occurs by CO2 reduction (large isotopic fractionation). In the upper part of the catotelm the C-13-depleted respiration-derived CO2 pool buffers the isotopic composition of CO2; the δ13C of CO2 increases only slowly. At the same time strongly depleted CH4 is formed as CO2 reduction consumes the depleted CO2. In the lower part of the catotelm, the respiration-derived CO2 and shallow CH4 become less important and CO2 reduction is the dominant source of CO2 and CH4. Now, the δ13C values of both gases increase until equilibrium is reached with respect to the isotopic composition of the substrate. Thus, the δ13C values of methane reach a minimum at intermediate depth, and the deep methane has δ13C values comparable to shallow methane. A simple mixing model for the isotopic evolution is suggested. Only minor changes of the observed patterns of methanogenesis (in terms of concentration and isotopic composition) occur over the seasons. The most pronounced of these is a slightly higher rate of acetate splitting in spring. 相似文献
298.
Functionalised aromatic constituents of Chukurovo resinite are compared here with the chemical composition of a macrofossil assigned paleobotanically as Taxodium dubium (Sternb.) Heer. Terpenoids and lipids of fossil wood tissue embedded in a clay sediment were extracted and analyzed for comparison. The information was interpreted in relation to the biomarker compositions of different conifer families, as well as to the effects of contamination by inward migration of organic matter from the adjacent sediments.Many of identified geolipids, i.e., alkanes, alkanols, alkan-2-ones, and steranes/triterpanes have no chemosystematic value because they are ubiquitous in the plant kingdom. Ferruginol, sugiol, and products of their diagenetic transformations were identified as the dominant specific biomarkers in the resinite. Ferruginol and its analogues were the most abundant diterpenoids in Taxodium dubium sample. Thus, the biomarker composition of the extractable matter of the resinite strongly suggests that species of Cupressaceae contributed significantly to the Chukurovo paleoenvironment. 相似文献
299.
The Urals are characterized by a depression of the Moho to a depth of 57 km. This structure is interpreted as a relic orogenic root, which has been conserved because no significant post-collisional processes occurred. However, there is evidence that voluminous post-collisional magmatism affected the lower crust. In this paper, we use thermal finite element models to quantify the influence of the post-collisional magmatism on the stabilization of the root. We show that at least 70% of the heat producing elements migrated in granitic melts from the lower crust to the upper crust. As a result the crustal heat flow reduced and the lithosphere could stabilize at a thickness of 180 km. Furthermore, we propose that a granulite metamorphic event during the thermal relaxation of the collision zone prevented the 57 km thick crust from delamination. These results strongly indicate that post-collisional processes were necessary for the stabilization of the Uralian crust and lithosphere. 相似文献
300.
Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation 总被引:2,自引:1,他引:1
Christian Meyer Bernd Wunder Anette Meixner Rolf L. Romer Wilhelm Heinrich 《Contributions to Mineralogy and Petrology》2008,156(2):259-267
The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments
between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water
that contained boron in excess. In one series of experiments, [B]fluid/[B]tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along
with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated
boron in tourmaline. 11B always preferentially fractionated into the fluid. For experiments where [B]fluid/[B]tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ11B(tour–fluid) = −4.20 · [1,000/T (K)] + 3.52; R
2 = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰
at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where [B]fluid/[B]tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition
of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization
of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating
larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron
excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations. 相似文献