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281.
282.
Volker Lüders Rolf L. Romer Alexandre R. Cabral Christian Schmidt David A. Banks Jens Schneider 《Mineralium Deposita》2005,40(3):289-306
Fluid inclusions hosted in quartz and specular hematite from auriferous (jacutinga) and barren veins in the Quadrilátero Ferrífero (QF) have been studied using conventional and near infrared microscopy, respectively.
The mineralization consists of veins that cross-cut metamorphosed iron formation (itabirite) of the Paleoproterozoic Itabira
Group. The sample suite comprises hematite from veins from the low-strain domain in the W and SW of the study area, as well
as hematite samples from the eastern high-strain domain in the central and NE parts of the QF. Halogen ratios of fluid inclusions
in quartz and hematite from all studied deposits are consistent with a fluid evolved from dissolving and reprecipitating halite
that was subsequently diluted. Fluid inclusions hosted in quartz and hematite are characterized by consistent Na/K ratios
and considerable SO4 contents, and suggest similar formation conditions and, perhaps, fluid origin from a common source. Na/K and Na/Li fluid
mineral geothermometers indicate water–rock interaction at approximately 340±40°C. Hematites from the high-strain domain contain
fluid inclusion assemblages of high-temperature aqueous-carbonic and multiphase high-salinity, high-temperature aqueous inclusions
probably due to fluid immiscibility in the system H2O–NaCl–CO2. Fluid inclusions hosted in hematite from barren veins in the low-strain domain, as well as in hematite from jacutinga-type mineralization from the central part of the QF, only host multiphase aqueous fluid inclusions all showing narrow ranges
of salinity (7.2–11.7 wt.% NaCl equiv.) and homogenization temperatures (148 to 229°C). Lower homogenization temperatures
and the absence of CO2-rich inclusions in specular hematite from these occurrences are attributed to carbonate precipitation and/or CO2 escape due to cooling during fluid migration from the high- to the low-strain domain. Pb–Pb and U–Pb systematics of gold,
hematite and hematite-hosted fluid inclusions in combination with geochemical evidence indicate distinct sources for Pd, Au,
and Pb. The formation of specular hematite veins may be related to retrograde metamorphic fluids being released during the
Brazilian orogenic cycle (600–700 Ma). The Pb isotopic characteristics of all samples are readily reconciled in a simple model
that involves two different Paleoproterozoic or Archean source lithologies for lead and reflects contrasting depths of fluid
percolation during the Brasiliano orogeny. 相似文献
283.
With the increasing use of permanently installed seismic installations, many of the issues in time‐lapse seismic caused by the lack of repeatability can be reduced. However, a number of parameters still influence the degree of reliability of 4D seismic data. In this paper, the specific impact of seawater velocity variations on time‐lapse repeatability is investigated in a synthetic study. A zero‐lag time‐lapse seabed experiment with no change in the subsurface but with velocity changes in the water column is simulated. The velocity model in the water column is constant for the baseline survey while the model for the repeat survey is heterogeneous, designed from sea salinity and temperature measurements in the West of Shetlands. The difference section shows up to 80% of residual amplitude, which highlights the poor repeatability. A new dynamic correction which removes the effect of seawater velocity variations specifically for permanent installations is developed. When applied to the synthetic data, it reduces the difference residual amplitude to about 3%. This technique shows substantial improvement in repeatability beyond conventional time‐lapse cross‐equalization. 相似文献
284.
A new natural,super-hard,transparent polymorph of carbon from the Popigai impact crater,Russia 总被引:1,自引:0,他引:1
Ahmed El Goresy Leonid S Dubrovinsky Philippe Gillet Smail Mostefaoui Günther Graup Michael Drakopoulos Alexandre S Simionovici Varghese Swamy Victor L Masaitis 《Comptes Rendus Geoscience》2003,335(12):889-898
A natural shockwave event led to the formation of a new crystalline polymorph of carbon in gneisses from the Popigai crater, Russia. The new species occupies the interior of a multiphase assemblage and is entirely enveloped by lonsdaleite and graphite. Polishing hardness of this new phase is greater than that of lonsdaleite. Micro-beam synchrotron X-ray diffraction, imaging and fluorescence studies revealed a pure transparent carbon phase. The diffraction pattern is indexed in terms of a cubic cell (a=14.697 Å, space group Pm3m.). This species was neither encountered in static or dynamic high-pressure experiment nor predicted by theoretical calculations. To cite this article: A. El Goresy et al., C. R. Geoscience 335 (2003). 相似文献
285.
Peter Shebalin Alexandre SolovievJean-Louis Le Mouël 《Physics of the Earth and Planetary Interiors》2002,131(2):141-153
Some elements of the model of scaling organization of fracture tectonics (SOFT) are incorporated into the model of block structure dynamics (BSD). The resulting new model permits to obtain a power law relationship between earthquake energy and fault area with an exponent 3/2, as for observed seismicity. This is an important advantage of the new model in comparison with the BSD model, in which this dependence is generally linear. 相似文献
286.
An extended narrative dataset of the Medieval time AD 708-1426 in continental western Europe was set up. Some 30-year-moving
mean seasonal temperature deviation series were reconstructed. A- warming trend occurred around AD 1200. During the warm stage,
seasonal cycle might be weak due to frequent cool summers. Significant warm summer conditions did not occur until the late
14th century, when the annual temperature level began decreasing. The mean warm season temperature level during the late 14th
to the early 15th century might be about 0.3°C higher than the present. It was suggested to cautiously comment on historical
climates for different seasons. 相似文献
287.
Baud Alexandre Jenny Jean-Philippe Francus Pierre Gregory-Eaves Irene 《Journal of Paleolimnology》2021,66(4):453-467
Journal of Paleolimnology - Sediment Accumulation Rate (SAR; measured as mm yr?1) and Mass Accumulation Rate (MAR; measured as g cm?2 yr?1) data were collected from... 相似文献
288.
Alexandre Raphael Cabral Orlando Garcia Rocha Filho Richard David Jones 《Journal of Geochemical Exploration》2008,96(1):69-76
Mercury contents in Precambrian banded iron formation-hosted hematite ores are virtually unknown. In an attempt to provide information on the abundance and distribution of Hg in Fe ore, we present analyses for Hg in samples of high-grade soft hematite ore from Gongo Soco, Minas Gerais, Brazil. Bulk samples contain from < 5 to 25 ppb Hg without obvious correlation with major elements. Granulometric fractions of follow-up samples have amounts of Hg from 6 to 48 ppb and display positive linear correlations with total Mn as MnO (r = 0.87), LOI (r = 0.87) and SiO2 (r = 0.76), as well as a negative linear correlation with total Fe as Fe2O3 (r = − 0.87). The correlations suggest that Hg is associated with a hydrated ferruginous groundmass bearing residual Mn, Al and Si, which replaced gangue minerals in itabirite in the process of formation of the Gongo Soco soft hematite ore. 相似文献
289.
Alexandre Corgne Shantanu Keshav Bernard J. Wood Yingwei Fei 《Geochimica et cosmochimica acta》2008,72(2):574-589
We present the results of new partitioning experiments between metal and silicate melts for a series of elements normally regarded as refractory lithophile and moderately siderophile and volatile. These include Si, Ti, Ni, Cr, Mn, Ga, Nb, Ta, Cu and Zn. Our new data obtained at 3.6 and 7.7 GPa and between 2123 and 2473 K are combined with literature data to parameterize the individual effects of oxygen fugacity, temperature, pressure and composition on partitioning. We find that Ni, Cu and Zn become less siderophile with increasing temperature. In contrast, Mn, Cr, Si, Ta, Nb, Ga and Ti become more siderophile with increasing temperature, with the highly charged cations (Nb, Ta, Si and Ti) being the most sensitive to variations of temperature. We also find that Ni, Cr, Nb, Ta and Ga become less siderophile with increasing pressure, while Mn becomes more siderophile with increasing pressure. Pressure effects on the partitioning of Si, Ti, Cu and Zn appear to be negligible, as are the effects of silicate melt composition on the partitioning of divalent cations. From the derived parameterization, we predict that the silicate Earth abundances of the elements mentioned above are best explained if core formation in a magma ocean took place under increasing conditions of oxygen fugacity, starting from moderately reduced conditions and finishing at the current mantle-core equilibrium value. 相似文献
290.
Studies on Brazilian archaeofauna from prehistoric and historic sites have attempted to explain human settlement and cultural trajectories in different geographical regions. However, only since 1999 have taphonomical criteria been developed during Brazilian zooarchaeological investigations. Wide ecosystem diversity results in the differential preservation of faunal remains. Until the end of the 20th century, many zooarchaeological studies presented insufficient information on complex predator–prey interaction systems in many regions, mostly from the Amazonian lowlands and the highlands of the Central Plateau. Animal remains might have been the result of natural predation, and they are often found mixed within anthropic material that could distort interpretation of deposits. Thus, there are major problems in defining diagnostic criteria to distinguish natural from cultural marks.Taphonomic analyses are needed to explain faunal remains in order to understand the particularities of human–animal relationships in Brazilian prehistory. Zooarchaeological and taphonomical data are presented from five human occupation sites dating from 9000 to 1000 BP to verify human–fauna relationships and natural deposits in three different regions of Brazil. These sites illustrate both natural and anthropic contexts. 相似文献