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21.
Gavin L. England Birger Rasmussen Neal J. McNaughton Ian R. Fletcher David I. Groves & Bryan Krapez 《地学学报》2001,13(5):360-367
SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime 207 Pb/206 Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event. 相似文献
22.
We report intermediate resolution H spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85±6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106±13 kms−1 with respect to template field stars, leading to a system mass ratio of q =M1/M2 = 7.8−2.0+3.4. Doppler imaging of the H line shows strong emission coming from the secondary star (EW=3.1±0.6Å) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods. The surface brightness distribution of the accretion disk in H follows a relation I∝R−1.1, less steep than typically observed in cataclysmic variables. Updated system parameters are also presented. 相似文献
23.
R. Sarmiento M. Gallardo A. Laquidara J. Reyna Almandos 《Astrophysics and Space Science》1997,256(1-2):359-363
This work presents preliminaries spectroscopic results about ionized xenon and xenon-helium mixture using a capillary pulsed discharge under several experimental results. 相似文献
24.
25.
T. H. BELL S. E. JOHNSON B. DAVIS A. FORDE N. HAYWARD C. WILKINS 《Journal of Metamorphic Geology》1992,10(3):295-307
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens. 相似文献
26.
27.
The Siberian Solar Radio Telescope: the current state of the instrument,observations, and data 总被引:2,自引:0,他引:2
Grechnev V.V. Lesovoi S.V. Smolkov G. Ya. Krissinel B.B. Zandanov V.G. Altyntsev A.T. Kardapolova N.N. Sergeev R.Y. Uralov A.M. Maksimov V.P. Lubyshev B.I. 《Solar physics》2003,216(1-2):239-272
The Siberian Solar Radio Telescope (SSRT) is one of the world's largest solar radio heliographs. It commenced operation in
1983, and since then has undergone several upgrades. The operating frequency of the SSRT is 5.7 GHz. Since 1992 the instrument
has had the capability to make one-dimensional scans with a high time resolution of 56 ms and an angular resolution of 15 arc sec.
Making one of these scans now takes 14 ms. In 1996 the capability was added to make full, two-dimensional images of the solar
disk. The SSRT is now capable of obtaining images with an angular resolution of 21 arc sec every 2 min. In this paper we describe
the main features and operation of the instrument, particularly emphasizing issues pertaining to the imaging process and factors
limiting data quality. Some of the data processing and analysis techniques are discussed. We present examples of full-disk
solar images of the quiet Sun, recorded near solar activity minimum, and images of specific structures: plages, coronal bright
points, filaments and prominences, and coronal holes. We also present some observations of dynamic phenomena, such as eruptive
prominences and solar flares, which illustrate the high-time-resolution observations that can be done with this instrument.
We compare SSRT observations at 5.7 GHz, including computed `light curves', both morphologically and quantatively, with observations
made in other spectral domains, such as 17 GHz radio images, Hα filtergrams and magnetograms, extreme-ultraviolet and X-ray
observations, and dynamic radio spectra. 相似文献
28.
Summary ?In the south-eastern Altenbergkar–Silbereck area in the eastern Tauern window (Lungau, Salzburg) structurally controlled
precious-metal (Au–Ag) mineralization is hosted in marbles of the Permo(?)-Mesozoic Silbereck Formation and in the underlying
Variscan Central gneiss. During the Alpine otogeny both lithologies were affected by ductile deformation (shearing, D1; folding,
D2/D3) and subsequent brittle deformation (tension gashes, D4; normal faulting, D5) related to the uplift and exhumation of
the Tauern window. Mineralization is controlled by brittle D4 structures. NE–SW trending steeply dipping tension gashes of
the “Tauerngoldgang” type occur within the Central gneiss. Three different marble-hosted ore types following fracture systems
as well as foliation and bedding planes can be distinguished: 1) metasomatic replacement ores, 2) ores in tension gashes and
3) ores in talc-bearing structures, often containing high-grade gold and silver mineralization (native gold in association
with Ag–Pb–Bi–sulfosalts). Four stages of mineralization can be distinguished which occur in all ore types: arsenopyrite–pyrite–pyrrhotite
(first stage), Au–(Ag–Pb–Bi–sulfosalts) (second stage), base-metal sulfides and tetrahedrite–tennantite (third stage) and
Ag-rich galena (fourth stage). Preliminary fluid inclusion data indicate temperatures of ore formation well above 300 °C (346 °C
mean) for the second stage within the Central gneiss and temperatures between 310 and 230 °C for the second and third stages
in the marble.
Received October 12, 2001; revised version accepted September 5, 2002
Published online March 10, 2003 相似文献
29.
J. Dommanget 《Journal of Astrophysics and Astronomy》2003,24(3-4):99-109
A research that we conducted in 1963 on the evolution of the binaries based on the available orbital data to obtain a philosophical
degree, led to the establishment of an interesting and new diagram between the logarithm of the total mass and a particular
parameterX, bound to the areal constant. This appeared to have a real physical significance but the basic observational material was
insufficiently extended to assure its undeniable existence. In 1981, a new research based on a more extended orbital material,
has confirmed this diagram. Presently, another important increase in the orbital material and the availability of highly accurate
trigonometric parallaxes produced by the Hipparcos satellite, gave us the opportunity to confirm once more the stability of
this diagram. This last research is here described. 相似文献
30.
R. J. DELMAS P. WAGNON K. GOTO-AZUMA K. KAMIYAMA O. WATANABE 《Tellus. Series B, Chemical and physical meteorology》2003,55(1):71-79
We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarctic ice sheet and in particular the new Dome F profile. At all the four sites studied, concentration profiles exhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phase migration are responsible for a marked release of gaseous MSA to interstitial firn air as well as probably to the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modify MSA concentration. It is proposed that, below the upper few metres where the communication with the free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later in air bubbles at pore close-off, i.e. when firn is transformed into ice. Chemical measurements on the firn core do not take into account the MSA released to the gaseous phase, but this fraction is measurable in ice samples. In spite of these alterations occurring in the firn layers, relative changes of the atmospheric MSA concentration in the past are probably still there deep within the Antarctic ice sheet. However, for glacial periods, different processes have to be considered in relation to modified aerosol properties. 相似文献