Tracing the exhumation of the Eclogite Zone (Tauern Window,Eastern Alps) by <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of white mica in eclogites     

Walter Kurz  Robert Handler  Christian Bertoldi 《Swiss Journal of Geoscience》2008,101(1):191-206

New radiometric ages from the Subpenninic nappes (Eclogite Zone and Rote Wand – Modereck Nappe, Tauern Window) show that phengites formed under eclogite-facies metamorphic conditions retain their initial isotopic signature, even when associated lithologies were overprinted by greenschist- to amphibolite-facies metamorphism. Different stages of the eclogite-facies evolution can be dated provided ⁴⁰Ar/³⁹Ar dating is combined with micro-structural analyses. An age of 39 Ma from the Rote Wand – Modereck Nappe is interpreted to be close to the burial age of this unit. Eclogite deformation within the Eclogite Zone started at the pressure peak along distinct shear zones, and prevailed along the exhumation path. An age of ca. 38 Ma is only observed for eclogites not affected by subsequent deformation and is interpreted as maximum age due to the possible influence of homogenously distributed excess argon. During exhumation deformation was localised along distinct mylonitic shear zones. This stage is mainly characterised by the formation of dynamically recrystallized omphacite2 and phengite. Deformation resulted in the resetting of the Ar isotopic system within the recrystallized white mica. Flat argon release spectra showing ages of 32 Ma within mylonites record the timing of cooling along the exhumation path, and the emplacement onto the Venediger Nappe. Ar-release patterns and ³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar isotope correlation analyses indicate no significant ⁴⁰Ar-loss after initial closure, and only a negligible incorporation of excess argon. From the pressure peak onwards, eclogitic conditions prevailed for almost 8–10 Ma.  相似文献   

Octahedral-site Fe2+ quadrupole splitting distributions from Mössbauer spectroscopy along the (OH, F)-annite join     

D. G. Rancourt  J. Y. Ping  B. Boukili  J. L. Robert 《Physics and Chemistry of Minerals》1996,23(1):63-71

We have performed a detailed Mössbauer study of synthetic annites on the (OH, F)-join. Recently developed data treatment and spectral analysis methods were used to extract true intrinsic Fe²⁺ quadrupole splitting distributions (QSDs) that represent the most information that can be resolved from the spectra. The overall room temperature (RT) QSDs can be consistently interpreted in terms of four QSD contributions (or populations) centered at: QS_HH2.55 mm/s for Fe²⁺O₄(OH)₂ octahedra (cis and trans not resolved), QS_HF 2.35 mm/s for Fe²⁺O₄(OH)F octahedra (cis and trans not resolved), QS_cFF2.15 mm/s for cis-Fe²⁺O₄F₂ octahedra, and QS_tFF 1.5 mm/s for trans-Fe²⁺O₄F₂ octahedra. Each such contribution has a width ( 0.2 mm/s) caused by distortions of the octahedra. Minor contributions due to Fe²⁺O₅(OH) and Fe²⁺O₅F octahedra probably also contribute to the overall Fe²⁺ QSDs. The ferric iron spectral components were also characterized. Here, two distinct types of octahedral Fe³⁺ contributions are seen and interpreted as being due mainly to Fe³⁺O₅OH and Fe³⁺O₅F octahedra, respectively. Tetrahedral Fe³⁺ is seen only in the OH-annite end-member and the total Fe³⁺ content drops significantly on addition of F. On leave from: Department of Materials Physics, University of Science and Technology Beijing, 100083 Beijing, China  相似文献   

Calorimetric study of the coesite-stishovite transformation and calculation of the phase boundary     

Jun Liu  Letitia Topor  Jianzhong Zhang  Alexandra Navrotsky  Robert C. Liebermann 《Physics and Chemistry of Minerals》1996,23(1):11-16

High temperature drop-solution calorimetry in molten 2 PbO · B₂O₃ at 1044 K for coesite and stishovite polymorphs of silica was carried out to determine the enthalpy of the coesite-stishovite transition. These experiments were performed on high-purity, single-phase samples of coesite and stishovite. Our new value for the enthalpy of the coesitestishovite transition (ΔH ₂₉₈ ⁰ ) is 29.85 ± 0.78 kJ/mol, which is about 35% lower than previously reported by Akaogi and Navrotsky (1984) and Holm et al. (1967), but which compares well with new measurements by Akaogi et al. (1994b). Using these new data, we have calculated the equilibrium phase boundary between coesite and stishovite and obtained a slope, dP/dT=0.0031 (2) GPa/K. This calculated slope is in good agreement with that determined [0.0026 (2) GPa/K] from the in-situ X-ray diffraction study of Zhang et al. (1996).  相似文献   

The ‘ultimate objective’ of the framework convention on climate change requires a new approach in climate change research     

Robert J. Swart  Pier Vellinga 《Climatic change》1994,26(4):343-349

In the Framework Convention on Climate Change an ultimate objective is formulated that calls for stabilization of the concentrations of greenhouse gases in the atmosphere at a level that would allow ecosystems to adapt naturally, safeguard food supply and enable sustainable development to proceed in a sustainable manner. This paper addresses the possible contribution of science to translate this rather vague and ambiguous objective into more practicable terms. We propose a regionalized, risk-based six-step approach that couples an analysis of ecosystem vulnerability to the results of simulations of climate change. An ultimate objective level could be determined in terms of stabilized concentrations of greenhouse gases in the atmosphere. The level and timing of this stabilization would be determined by a political appreciation of associated risks for managed and unmanaged ecosystems. These risks would be assessed by region in an internationally coordinated scientific effort, followed by a global synthesis.  相似文献   

Solar and stellar chromospheric contrast     

Robert A. Donahue  Sergio R. Restaino  Stephen L. Keil 《Solar physics》1994,149(2):257-265

Positions of active regions estimated from observations of the whole solar disk in Caii K _iv during the period 1977–1989 at the Coimbra Astronomical Observatory are compared with the time-dependent latitudinal distribution of background solar magnetic fields and with the latitudinal shifts of boundaries of their polarities. We confirm that the sunspot groups are located near the zonal boundaries between the opposite polarities of the solar background magnetic field during different phases of the two recent consecutive cycles of activity. We demonstrate a probable connection between the increased number of groups and the commencement of poleward migration of zonal boundaries in both hemispheres. But the influence of the dominant convective rolls seems to he still unclear. A new problem of interrelation between the zonal and sector boundaries has also appeared.  相似文献   

Electron microscopy study of domain structure due to phase transitions in natural perovskite     

Yanbin Wang  Robert C. Liebermann 《Physics and Chemistry of Minerals》1993,20(3):147-158

Transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90° rotation twins about the [001] axis (referred to as [001]_90° twin). Single crystals that were heattreated and quenched from above 1585 K exhibit a dramatic change in domain structure compared with the starting material and specimens quenched from T < 1470=" k.=" mutually=" perpendicular=" {110}=" and=">_90° twins are observed throughout the crystal, forming a cross-hatched domain texture. 1/2[001] antiphase domains, which are very rarely observed in the starting material, also become dominant in the crystal. This change in domain structure is interpreted as due to a structural phase transition in perovskite at a temperature below 1585 K. From the point symmetry elements that describe the twin laws and the translational elements that relate the antiphase domains, the most likely phase near 1585 K is tetragonal with space group P4/mbm. These results are consistent with high-temperature powder X-ray diffraction study. On the other hand, density of the {112} twins is increased significantly in the crystal quenched from 1673 K. Twin domains are either bound by mutually perpendicular {110} and (001) walls, or by {112} walls with {110} twin domains within the polygonal {112} domains. Both twin density variation and domain morphology suggest that the crystal may be cubic at this temperature. Microstructure of a single crystal deformed at 1273 K and 3.5 GPa (within the orthorhombic stability field) is morphologically quite distinct from that of the heat-treated specimens. Dislocations dominate the microstructure and often interact with twin domain boundaries.A National Science Foundation Science and Technology Center  相似文献   

Corona textures in Proterozoic olivine melanorites of the Equeefa Suite,Natal Metamorphic Province,South Africa     

Dr. G. H. Grantham  Dr. R. J. Thomas  Dr. B. M. Eglington  Dr. D. de Bruin  Dr. A. Atanasov  Dr. M. J. Evans 《Mineralogy and Petrology》1993,49(1-2):91-102

Summary Olivine-plagioclase and phlogopite-plagioclase coronas have been identified from olivine melanorites of the Mid- to Late Proterozoic Equeefa Suite in southern Natal, South Africa. Olivine, in contact with plagioclase, is mantled by a shell of clear orthopyroxene, in turn rimmed by pale green (pargasitic) clinoamphibole. Locally a third rim, composed of a fine pargasite-spinel symplectite is developed adjacent to the plagioclase. The second corona reaction has produced greenish-brown pargasite at phlogopite-plagioclase interfaces. A third, less obvious reaction, between olivine and phlogopite is also noted. Analytical data of all the mineral phases present, along with the coronas, are given. Two-pyroxene thermometry yields magmatic core temperatures ( 1120°C), with rim compositions indicating equilibration at 850°C. Consistent with this, the modelled olivine-plagioclase reaction occurs between 830–1050°C with a_waterbetween 0.1 and 1.0 at 7 kbar. The three reactions took place during a prolonged history of cooling and partial hydration of the magmatic olivine melanorites from over 1000°C down to 600°C. The P-T conditions indicated by the reactions suggest this cooling process was essentially isobaric, indicating that the area was not subjected to rapid uplift or burial throughout this entire period.

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Korona-Texturen in proterozoischen Olivin-Melanoriten der Equeefa Suite, Natal Metamorphic Province, Südafrika

Zusammenfassung Aus Olivin-Melanoriten der mittel- bis spätproterozoischen Equeefa Suite im südlichen Natal, Südafrika, wurden Olivin-Plagioklas und Phlogopit-Plagioklas-Koronartexturen beobachtet. Olivin, der mit Plagioklas im Kontakt steht, wird von einem klaren Saum vom Orthopyroxen ummantelt, der seinerseits von blaßgrünem (pargasitischem) Klinoamphibol umsäumt wird. Stellenweise ist ein dritter Saum, bestehend aus feinkörnigem symplektitischem Pargasit-Spinell im Kontakt mit Plagioklas ausgebildet. Die zweite koronabildende Reaktion resultiert in Bildung eines grünbraunen Pargasites an Phlogopit-Plagioklas Kornkontakten. Eine dritte, weniger auffällige Reaktion zwischen Olivin und Phlogopit wurde ebenfalls beobachtet. Zwei-Pyroxen-Thermometrie ergab magmatische Temperaturen der Kernbereiche ( 1120°C) und belegt eine Gleichgewichtseinstellung in den Randzonen bei ca. 850°C. Olivin-Plagioklas-Modellreaktionen liegen ebenfalls in einem Temperaturbereich von 830–1050°C bei Wasseraktivitäten von 0.1 bis 1.0 und einem Druck von 7 kbar. Die drei Reaktionen liefen im Zuge einer länger andauernden Abkühlung unter teilweiser Hydratisierung der magmatischen Melanorite in einem Temperaturbereich von 1000°C bis ca. 600°C ab. Die aus den Reaktionen ableitbaren P-T-Bedingungen sprechen für eine im wesentlichen isobare Abkhlungsgeschichte und zeigen, daß dieses Gebiet wáhrend dieser gesamten Periode keiner raschen Hebung bzw. keiner Versenkung unterworfen worden ist.

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Paleohydrology and paleoclimate records in a saline prairie lake core: mineral,isotope and organic indicators     

D. R. Van Stempvoort  T. W. D. Edwards  M. S. Evans  W. M. Last 《Journal of Paleolimnology》1993,8(2):135-147

This pilot study examines the potential of obtaining a sedimentary record of paleoenvironmental/climatic/hydrologic conditions for saline Redberry Lake in southern Saskatchewan, Canada. The tools are mineralogy, stable isotopes and pigments. The upper meter of an offshore sediment core contains 10 to 20% by weight aragonite (CaCO₃), which apparently precipitated in the water column. The ¹⁸O and ¹³C of the bulk aragonite (corrected for content of detrital calcite) vary by 4 to 5. Enrichment in ¹⁸O in aragonite is significantly correlated with pigment concentrations (chlorophyll a, phaeophytin). The ¹⁸O and pigment data provide evidence for relatively dry and/or warm conditions and high limnetic productivity for the period 2500 to 1500 yrs B.P. After 1500 B.P., the climate was apparently similar to the present, with two episodes of relatively enhanced productivity, dryness and/or warmth, at around 1000 to 900 and 500 to 200 B.P. During the past century, Redberry Lake has decreased approximately 8 m in depth and its salinity has doubled. No clear sedimentary signal was observed in response to these recent hydrologic trends. These changes have not been associated with a significant climate trend in the region, but may have been induced by land use changes in the catchment.This publication is the third of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr. Evans is serving as Guest Editor for this series.  相似文献   

New paleomagnetic results from the Aegean extensional province     

Haubold  Herbert  Scholger  Robert  Kondopoulou  Despina  Mauritsch  Hermann J. 《Geologie en Mijnbouw》1997,76(1-2):45-55

Various Oligocene formations from NE Greece (ignimbrites from the Medousa area, rhyolites from Zagradenia, granodiorites from Elatia) show discordant paleomagnetic signatures, in each case indicating small cw (clockwise) rotation and also inclination flattening. Marls from Pithion were partly remagnetized in a present-day field. Samples that contain ancient magnetization components also indicate small cw rotation and inclination flattening. However, the magnetization of andesites from Peplos reflects a considerably larger rotation, likely owing to local tectonics. In the context of previous work in the area, these results are used to propose a subdivision of NE Greece into four structural zones of distinctive rotational behaviour (from east to west): sites in zone 1, east of the Kavala-Xanthi-Komotini fault (KXK), show various cw and ccw (counterclockwise) rotation angles owing to complex kinematics resulting from the interaction of the KXK and the north-Anatolian fault zone. However, zone 2, between the KXK and the Strymon valley, is structurally homogeneous ( 10° cw rotation). The paleomagnetic signature of the Vertiskos massif (zone 3) implies a larger (> 30°) cw rotation, whereas sites in the Vardar basin (zone 4) contain a paleomagnetic signature similar to that of zone 2. This suggests a motion of the Vertiscos massif, a meta-ophiolitic nappe, relative to underlying strata. Indeed, zones 2 and 4 may be parts of the same structural unit which underlies this nappe.  相似文献   

Ozone production due to emissions from vegetation burning     

Günter Helas  Jürgen Lobert  Dieter Scharffe  Luise Schäfer  Johann Goldammer  Jean Baudet  Brou Ahoua  Ayité-Lô Ajavon  Jean-Pierre Lacaux  Robert Delmas  Meinrat O. Andreae 《Journal of Atmospheric Chemistry》1995,22(1-2):163-174

Ozone has been observed in elevated concentrations by satellites over areas previously believed to be background. There is meteorological evidence, that these ozone plumes found over the Atlantic Ocean originate from vegetation fires on the African continent.In a previous study (DECAFE-88), we have investigated ozone and assumed precursor compounds over African tropical forest regions. Our measurements revealed large photosmog layers at altitudes from 1.5 to 4 km. Both chemical and meteorological evidence point to savanna fires up to several thousand km upwind as sources.Here we describe ozone mixing ratios observed over western Africa and compare ozone production ratios from different field measurement campaigns related to vegetation burning. We find that air masses containing photosmog ingredients require several days to develop their oxidation potential, similar to what is known from air polluted by emissions from fossil fuel burning. Finally, we estimate the global ozone production due to vegetation fires and conclude that this source is comparable in strength to the stratospheric input.  相似文献