Paleomagnetism, geochemistry and Ar/Ar dating of the North-eastern Paraná Magmatic Province: tectonic implications     

M. Ernesto  M. I. B. Raposo  L. S. Marques  P. R. Renne  L. A. Diogo  A. de Min   《Journal of Geodynamics》1999,28(4-5)

Seventy sites of sills, flows and dikes from Northeastern Paraná Magmatic Province (PMP), were submitted to paleomagnetic, chemical and radiometric analyses. The rocks are high in TiO₂ content, and similar in composition to the rocks from the northern region of PMP. The sills intrude mainly Paleozoic sediments, and can be subdivided into two domains; the northern being characterized by sills showing reversed polarities, and the southern essentially by sills of normal polarities. ⁴⁰Ar/³⁹Ar dating of three distinct sills gave plateau ages (129.9 ± 0.1, 130.3 ± 0.1 and 131.9 ± 0.4 Ma) that are similar to surface-outcropping flows of the Northern Paraná Basin, and the Ponta Grossa dikes. The new paleomagnetic data combined with existing data from the northern PMP allowed the calculation of a paleomagnetic pole at 71.4° E and 83.0° S (N = 92; α₉₅=2.4°; k = 39). This pole is in good agreement with poles for central and southern PMP, which are slightly older than the northern PMP, as well as for the contemporaneous Central Alkaline Province (Paraguay) on the western side of PMP. In contrast, the coeval pole for the Ponta Grossa dikes (eastern border of PMP), however, is slightly displaced from that group of poles, suggesting that dikes in that area may have undergone some tectonic tilting.  相似文献   

The evolution of continental roots in numerical thermo-chemical mantle convection models including differentiation by partial melting     

J. H. de Smet  A. P. van den Berg  N. J. Vlaar 《Lithos》1999,48(1-4):153-170

Incorporating upper mantle differentiation through decompression melting in a numerical mantle convection model, we demonstrate that a compositionally distinct root consisting of depleted peridotite can grow and remain stable during a long period of secular cooling. Our modeling results show that in a hot convecting mantle partial melting will produce a compositional layering in a relatively short time of about 50 Ma. Due to secular cooling mantle differentiation finally stops before 1 Ga. The resulting continental root remains stable on a billion year time scale due to the combined effects of its intrinsically lower density and temperature-dependent rheology. Two different parameterizations of the melting phase-diagram are used in the models. The results indicate that during the Archaean melting occurred on a significant scale in the deep regions of the upper mantle, at pressures in excess of 15 GPa. The compositional depths of continental roots extend to 400 km depending on the potential temperature and the type of phase-diagram parameterization used in the model. The results reveal a strong correlation between lateral variations of temperature and the thickness of the continental root. This shows that cold regions in cratons are stabilized by a thick depleted root.  相似文献   

Remobilization of arsenic from buried wastes at an industrial site: mineralogical and geochemical control     

F. Juillot  Ph. Ildefonse  G. Morin  G. Calas  A. M. de Kersabiec  M. Benedetti 《Applied Geochemistry》1999,14(8):55

An industrial area contaminated by As was studied to determine the source of this element and its speciation in As-bearing solids and in run-off waters. Mineral precipitates and water samples were collected and analyzed to assess processes controlling As mobility at this site. The integrated study of a contaminated industrial area allowed identification of the source of the As and of the nature of secondary As-bearing phases. The results obtained both on solid and water samples were used to model As behavior during waste leaching on carbonate rocks. At the upper end of a topographic transect across the site, run-off waters (pH=7.9) interact with surficial waste piles (containing arsenolite, arsenopyrite and pyrite), becoming acidic (pH=2.2) and concentrated in dissolved arsenate species (As⁵⁺) (ΣAs ranging from 0.961 to 3.149·10⁻³ mol/l). Those acidic waters interact with the limestone substratum, providing dissolved Ca which reacts with As to precipitate 1:1 Ca arsenates (weilite CaHAsO₄, haidingerite CaHAsO₄.H₂O and pharmacolite CaHAsO₄.2H₂O) and, in minor amounts, Ca–Mg arsenates (picropharmacolite (Ca,Mg)₃(AsO₄)₂ 6H₂O). The 1:1 Ca arsenates identified are known to precipitate at low pH (3–6) and seem to be stable in media with high dissolved CO₂, in comparison with other types of Ca arsenates. However, due to their high solubilities, they are not strictly relevant candidates to immobilize As in contaminated surficial environments. Although reported solubilities decrease to values close to the French and US drinking standards in Ca-rich solutions, a thorough examination of the precipitation/dissolution kinetics of Ca arsenates should be undertaken to assess their long-term stability and their efficiency in rapidly immobilizing As in contaminated surficial environments.  相似文献   

Formation of the Trimouns talc-chlorite deposit (Pyrenees) from persistent hydrothermal activity between 112 and 97 Ma   总被引：1，自引：0，他引：1  

Schärer  de Parseval  Polvé  & de Saint Blanquat 《地学学报》1999,11(1):30-37

For many economically important deposits, precise duration and age of hydrothermal ore-formation are poorly known. We tackle these questions for the large Trimouns talc-chlorite deposit by U–Pb analyses of xenotime and monazite, which crystallized within cm-size geodes in the hanging wall dolostones. Formation of geodes and REE-minerals occurred in relation to 300–350°C/0.2–0.3 GPa metasomatism along a host shear zone. Sixteen xenotime fragments, broken off from four single euhedral grains, define an age-range from 112 to 97 Myr, and a monazite age of 99 Myr. Some ages of individual fragments are grain-specific, whereas others show the same range of dates. Given the analytical uncertainties on individual fragments (about ± 1 Myr), the absence of inherited components, and the fact that no thermal event has affected these crystals after their formation, these ages are interpreted to record continuous crystallization between 112 and 97 Ma. This means that the Trimouns deposit was produced by hydrothermal activity lasting for at least 16 million years, occurring along a shear zone through which fluids continuously (or episodically) circulated to allow ion transport from the country gneisses to the deposit locality. Persistent deformation and high heat flow, related to the transtensional regime that opened the Bay of Biscay, seems to be the most likely tectonic scenario to explain this activity.  相似文献   

Assessment of mercury contamination and human exposure associated with coastal disposal of waste from a cinnabar mining operation, Palawan, Philippines   总被引：2，自引：0，他引：2  

T. M. Williams  J. M. Weeks  A.N. Apostol Jr.  C. R. Miranda 《Environmental Geology》1999,39(1):51-60

 An integrated geochemical and toxicological assessment of environmental mercury contamination and attendant human exposure in Honda Bay, Palawan was undertaken in 1995 following a nationally reported pollution scare centered on a coastal jetty, Sitio Honda Bay, constructed using approximately 1 million tons of tailings and beneficiation waste from a cinnabar mine. Mercury (Hg) data for marine and fluvial sediments, fish tissues and human hair indicate that the toxicological hazard is considerably lower than initially reported by state environment and health officials. Typical Hg concentrations in surficial Honda Bay sediments were found to lie within the global background range (<60 μg/kg). Downcore profiles provide no evidence of enhanced Hg fluxes coincident with the onset of mining and/or coastal tailings disposal. The mean and median Hg concentrations recorded in tissues of six species of Honda Bay fish are compliant with thresholds established by the US Environmental Protection Agency (US-EPA) for marketable stocks. Earlier reports of 'Minamata range' Hg concentrations in fish and shellfish from Honda Bay remain unsubstantiated. Geochemical analyses of samples of the Sitio Honda Bay substrate have confirmed the prevalence of solid-phase Hg concentrations to ca. 340 mg/kg. The speciation of Hg is, however, dominated by secondary oxides of low bioavailability. The mean Hg concentration in hair from Sitio Honda Bay residents (4.41 mg/kg) was found to be statistically analogous to that for a neighbouring coastal community unimpacted by the coastal disposal of mine waste. A negligible residential exposure factor is thus inferred for the former. Relatively high hair Hg burdens prevail throughout the coastal Honda Bay population, consistent with significant methyl Hg ingestion through daily fish consumption. The data presented provide no environmental or toxicological justification for immediate remedial action. Received: 14 May 1998/Accepted: 1 September 1998  相似文献   

环北极地区冰川（盖）物质平衡研究进展   总被引：6，自引：4，他引：2  

效存德  秦大河 《冰川冻土》1999,21(3):200-206

回顾了环北极各分区冰川物质平衡观测,研究的历史与现状,过去３０ａ间,北极冰川（帽）波动与全球平均变化保持同步。相对于中低纬地区冰川,北极地区的小型冰川（帽）对全球气候变化反应更灵敏,北极冰川物质平衡的变化对气候变化具有更好的指示意义。１９世纪末以来的百年时间里,除本世纪６０年代出现短暂的相对冰时外,环北极冰川表现出普遍退缩趋势。  相似文献   

Asymmetries in the jets of weak radio galaxies     

R. A. Laing  P. Parma  H. R. de Ruiter  R. Fanti 《Monthly notices of the Royal Astronomical Society》1999,306(3):513-530

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Gravitational waves in a string-like fluid cosmology     

Júlio César Fabris  Sérgio Vitorino de Borba Gonçalves 《Monthly notices of the Royal Astronomical Society》1999,306(3):679-683

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Global velocity field and bubbles in the blue compact dwarf galaxy Mrk 86     

A. Gil de Paz  J. Zamorano  J. Gallego 《Monthly notices of the Royal Astronomical Society》1999,306(4):975-987

We have studied the velocity field of the blue compact dwarf galaxy Mrk 86 (NGC 2537) using data provided by 14 long-slit optical spectra obtained in 10 different orientations and positions. This kinematical information is complemented with narrow-band ([O  iii ]5007 Å and H α ) and broad-band ( B , V , Gunn r and K ) imaging. The analysis of the galaxy global velocity field suggests that the ionized gas could be distributed in a rotating inclined disc, with projected central angular velocity of Ω=34 km s⁻¹ kpc⁻¹. The comparison between the stellar, H  i and modelled dark matter density profile indicates that the total mass within its optical radius is dominated by the stellar component. Peculiarities observed in its velocity field can be explained by irregularities in the ionized gas distribution or local motions induced by star formation.
Kinematical evidences for two expanding bubbles, Mrk 86–B and Mrk 86–C, are given. They show expanding velocities of 34 and 17 km s⁻¹, H α luminosities of 3×10³⁸ and 1.7×10³⁹ erg s⁻¹, and physical radii of 374 and 120 pc, respectively. The change in the [S  ii ]/H α , [N  ii ]/H α , [O  ii ]/[O  iii ] and [O  iii ]/H β line ratios with the distance to the bubble precursor suggests a diminution in the ionization parameter and, in the case of Mrk 86–B, an enhancement of the shock-excited gas emission. The optical–near-infrared colours of the bubble precursors are characteristic of low‐metallicity star‐forming regions (∼0.2 Z_⊙) with burst strengths of about 1 per cent in mass.  相似文献   

Soft X-ray emission lines of Ni XVIII in the solar spectrum     

Keenan  F.P.  Mathioudakis  M.  Pinfield  D.J.  Brown  W.A.  Bruner  M.E. 《Solar physics》1999,185(2):289-296

R-matrix calculations of electron impact excitation rates in Nixviii are used to derive theoretical electron-temperature-sensitive emission line ratios involving 3s–4p,3p–4d,3p –4s, and 3d–4f transitions in the 41–53 Å wavelength range. A comparison of these with solar flare observations from a rocket-borne X-ray spectrograph (XSST) reveals generally excellent agreement between theory and experiment (within the experimental and theoretical uncertainties), which provides support for the atomic data adopted in the analysis. However the 3s 2S–4p 2P1/2 line of Nixviii at 41.22 Å appears to be blended with the Fexix 13.74 Å feature observed by XSST in third order. In addition, the measured Nixviii intensity ratio I(3p 2P3/2– 4s 2S)/I(3p 2P1/2–4s 2S)=I(51.02 Å)/I(50.26 Å)=0.56, a factor of 3.8 smaller than the theoretical (temperature and density-insensitive) value of 2.1. The reason for this discrepancy is currently unexplained, but is unlikely to be due to blending of the 50.26 Å line, as the intensity of this feature is consistent with that expected from the other Nixviii lines in the XSST spectrum. Future observations of the Nixviii lines by the Advanced X-ray Astrophysics Facility (AXAF) should allow this problem to be resolved, and may also permit the use of the lines as electron-temperature diagnostics.  相似文献