Petrology of kamafugites and kimberlites from the Alto Paranaíba Alkaline Province, Minas Gerais, Brazil     

Ana Araujo  Richard W. Carlson  José C. Gaspar  Luiz A. Bizzi 《Contributions to Mineralogy and Petrology》2001,142(2):163-177

Mafic rocks representative of the alkaline magmatism of the Alto Paranaíba Province in southwestern Minas Gerais, Brazil were studied by means of petrography, mineral, whole-rock and isotope geochemistry with the objective of better understanding this Cretaceous magmatism and the characteristics of the magma sources. Because of the variety and complexity of lithotypes examined in this research and the paucity of detailed studies of these Brazilian rocks in the literature, this study also attempts to establish parameters that allow for a clear distinction between kimberlite and kamafugite. Fifty-two occurrences are described and classified as kimberlite or kamafugite. Among the kamafugites, both ugandite (characterized by the presence of leucite) and mafurite (with kalsilite) end members have been characterized. Mineral compositions were found to be efficient in distinguishing between kimberlites, mafurites and ugandites in the province, primarily on the basis of phlogopite composition. The Re-Os isotope systematics permitted a better understanding of the relation of the sublithospheric mantle source to the magmatism in the region. Kimberlites, mafurites and ugandites have different ¹⁸⁷Os/¹⁸⁸Os ratios (0.117 to 0.129, 0.127 to 0.145 and 0.142 to 0.147, respectively). The Rb-Sr and Sm-Nd isotope systematics failed to indicate first-order differences between kamafugites and kimberlites, whilst ²⁰⁶Pb/²⁰⁴Pb ratios for the kimberlites are higher than those for the other rock types. Kimberlite and kamafugite isotopic compositions appear to be related to the mixture of at least two dominant mantle components: one with an isotopic signature similar to that of lithospheric peridotites, i.e., with ¹⁸⁷Os/¹⁸⁸Os ratios of the order of 0.118, similar to those observed in mantle-derived xenoliths entrained in kimberlites intruded in the Kaapvaal, Wyoming, and Siberian cratons; another with higher ¹⁸⁷Os/¹⁸⁸Os ratios of the order of 0.135, within the range of ratios reported for pyroxenite veins in alpine-type peridotites and ocean island basalts. Different melting depths of heterogeneous lithospheric sources by a mantle plume are suggested to explain the isotopic characteristics of the Alto Paranaíba Alkaline Province alkaline rocks.  相似文献   

The Rio Grande fracture zone in the western South Atlantic and its tectonic implications     

Luiz Antonio Pierantoni Gamboa  Philip D. Rabinowitz 《Earth and Planetary Science Letters》1981,52(2):410-418

A broad zone of linear, mappable basement structures is observed north and northeast of the Rio Grande Rise in the South Atlantic Ocean. These structures lie along the same flow line as the Sa?o Paulo Ridge, the Florianopolis High, and onshore lineaments, suggesting that they all comprise the same tectonic trend: the Rio Grande fracture zone. The morphology developed along this fracture zone during the early opening of the South Atlantic Ocean formed a barrier to open ocean circulation during the Aptian and allowed the formation of extensive evaporite deposits to the north of it.  相似文献   

Statistical estimation of the relative efficiency of natural attenuation mechanisms in contaminated aquifers     

Ole F.?ChristensenEmail author  G.?Cassiani  P. J.?Diggle  P. Ribeiro?Jr  G.?Andreotti 《Stochastic Environmental Research and Risk Assessment (SERRA)》2004,18(5):339-350

Using monitored natural attenuation is an increasingly popular strategy for dealing with contaminated aquifers. This paper provides a statistical methodology for the estimation of the relative efficiency of natural attenuation mechanisms. The methodology provides estimates, with associated measures of uncertainty, of the relative efficiency of four types of bio-degradation (oxidation using oxygen as the electron-acceptor, denitrification, iron reduction and sulfate reduction). A data set from Trecate, Italy, is analysed using the methodology. The analysis shows that sulfate is the main cause of hydrocarbon removal on this site. It is also seen that oxidation using oxygen seems to be more preferential than the other reactions, in the sense that this reaction is relatively more efficient than other reactions at locations where the hydrocarbon concentration is low.  相似文献   

Isotope measurements and ground water flow modeling using MODFLOW for understanding environmental changes caused by a well field in semiarid Brazil     

Luiz Alberto Ribeiro Mendonça  Horst Frischkorn  Marlúcia Freitas Santiago  Josué Mendes Filho 《Environmental Geology》2005,47(8):1045-1053

Measurements of the environmental isotopes carbon-14 and oxygen-18 and of the electrical conductivity in ground water, together with MODFLOW-88 (DOS) simulations of pumping tests, were used to achieve a detailed understanding of the functioning of an aquifer in the area of a well field. The methodology was applied in the sedimentary basin of the Cariri region, in the south of Ceará State in Brazil. Localized hydraulic connections between adjacent aquifer units could be detected, and mixing ratios for the contributions from the units involved were determined. Cone of depression simulations for a three-year drought period revealed well interference and a drastic lowering of the piezometric level, thereby explaining the hydrogeologic changes and transformations in vegetation that were observed in the area of the well field. The combined use of the modeling tools and the geochemical field observations is shown to provide more detailed insight into the conceptual model of the groundwater flow system.  相似文献   

Intermittency and the Exchange of Scalars in the Nocturnal Surface Layer     

Otávio C. Acevedo  Osvaldo L. L. Moraes  Gervásio A. Degrazia  Luiz E. Medeiros 《Boundary-Layer Meteorology》2006,119(1):41-55

The determination of nocturnal surface fluxes in low wind conditions is a major problem for micrometeorological studies. The eddy correlation technique, extensively used in field measurements, becomes inappropriate if not enough turbulent activity exists. At the same time, the phenomenon of turbulence intermittency is responsible for the existence of localized events of short duration within which a large fraction of the total nighttime scalar exchange occurs. The scalar flux within a certain intermittent event varies considerably depending on the window used for the flux calculation. In many cases, events with very different time durations occur in the same night, and therefore, the proper determination of the surface flux would require averaging within data windows of different sizes for each event. In this work, the surface exchanges of temperature, moisture and carbon dioxide are analysed at a micrometeorological tower at southern Brazil. Intermittent turbulence is a common occurrence at the location. The analysis shows that the fluxes vary with turbulence intensity and the estimation technique. A variable-window size method for flux estimation is suggested and shown to cause an increase in the magnitude of the nocturnal surface fluxes  相似文献   

Development and erosion in the Brazilian Amazon: A geochronological case study     

Dr. Bruce Forsberg  Dr. Jose Marcus Godoy  Dr. Reynaldo Victoria  Dr. Luiz Antonio Martinelli 《GeoJournal》1989,19(4):399-405

A ²¹⁰Pb chronology of sedimentation in a small floodplain lake in the Jamari river basin (Rondonia, Brazil) provided insight into the relationship between development and erosion in the Brazilian Amazon. Positive correlations between increased sedimentation in the lake and levels of deforestation and mining in the Jamari basin suggested a major increase in erosion associated with these activities. This increase in erosion could diminish agricultural yields in the basin and significantly reduce the useful life of a hydro-electric plant recently built on the river.  相似文献   

Spatial and temporal variations in soil chemistry on the Amazon floodplain     

Reynaldo L. Victoria Dr.  Luiz A. Martinelli  Jeffrey E. Richey Dr.  Allan H. Devol Dr.  Bruce R. Forsberg Dr.  Maria N. G. Ribeiro 《GeoJournal》1989,19(1):45-52

Soil samples were collected on the floodplains of the Amazon River and its principal Brazilian tributaries during dry, early rising water, and early falling water periods. The concentrations of basic cations and pH in these alluvial soils were always higher than those in the more common terra firme soils while the concentrations of aluminum were generally lower. Among the alluvial soils, those from the main channel floodplain were generally higher in basic cations and pH, and lower in aluminum than those from the tribuary floodplains. The concentrations of basic cations in soils along the main channel floodplain decreased downstream. No significant difference was found in the levels of basic cations, pH, or aluminum between sampling periods.  相似文献   

The use of GPR for characterizing underground weathered profiles in the sub-humid tropics     

Paulo Roberto Antunes Aranha  Cristina Helena Ribeiro Rocha Augustin  Frederico Garcia Sobreira   《Journal of Applied Geophysics》2002,49(4)

Ground penetrating radar (GPR) has been used as a tool to access information about ground subsurface features. Such information is very important for different types of studies, varying from those related to archeological research to those studying geological elements of bedrock. More recently, however, GPR has been increasingly applied to environmental studies, especially for soil research. This paper presents the results of an application of GPR for the study of weathered profiles. GPR was used to discover the degree of trustworthiness of the information on the ground subsurface through the interpretation of the results of the radar sections as well as the data collected from boreholes, which reached until 21 m. The results show a relatively high degree of details obtained by GPR, indicating the possibility of speeding up ground subsurface surveys related to geomorphological, geological, and pedological studies.  相似文献   

A Possible Mechanism to Form Low Mass Objects at Low Redshifts     

André Ribeiro  Patricio Letelier 《Astrophysics and Space Science》2005,298(4):509-518

We develop a quantitative criterion for the collapse of matter within a region where a global texture just collapsed and a non-zero peculiar velocity field is produced. We also describe the evolution of the density contrast in this region according to the non-linear spherical collapse model. In particular, we find that the overdensity needed to virialisation is reached at z ≤ 2.2, covering a mass range of about 10⁴–10⁷ M_⊙ in the context of an Einstein-de Sitter universe.  相似文献   

HT-fabrics in a garnet-bearing quartzite from Western Portugal: geodynamic implications for the Iberian Variscan Belt     

Francisco J. Fernández  Helder I. Chaminé  Paulo E. Fonseca  José M. Munhá  António Ribeiro  Jesus Aller  Mercedes Fuertes-Fuentes  Frederico S. Borges 《地学学报》2003,15(2):96-103

ABSTRACT The study of a garnet-bearing quartzite from a major suture zone in Iberia reports an unusual high-T fabric. Quartz c -axis patterns were plotted using shaped garnet as reference axis for the finite stretch ( X -axis). The pole figures are characterized by a dominant single maximum around X together with other point maxima along the XY plane (mylonitic foliation). These patterns suggest that dominant < c > prism slip and subordinated < a > prism slip operated during quartz plastic deformation in agreement with P–T conditions for syntectonic garnet growth (4–5 kbar and 700 ± 50 °C) and, pre-dating the well-known (late) Variscan D1 event (<6 kb and 600 ± 30 °C). The geotectonic framework suggests that the fabrics were formed along the western shear margin of the Ossa-Morena Zone during the early stages of the Variscan orogeny.  相似文献