Weathering geochronology by (U-Th)/He dating of goethite   总被引：3，自引：0，他引：3  

David L. Shuster  Paulo M. Vasconcelos  Kenneth A. Farley 《Geochimica et cosmochimica acta》2005,69(3):659-673

Nine samples of supergene goethite (FeOOH) from Brazil and Australia were selected to test the suitability of this mineral for (U-Th)/He dating. Measured He ages ranged from 61 to 8 Ma and were reproducible to better than a few percent despite very large variations in [U] and [Th]. In all samples with internal stratigraphy or independent age constraints, the He ages corroborated the expected relationships. These data demonstrate that internally consistent He ages can be obtained on goethite, but do not prove quantitative ⁴He retention. To assess possible diffusive He loss, stepped-heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic ³He. The ³He release pattern indicates the presence of at least two diffusion domains, one with high helium retentivity and the other with very low retentivity at Earth surface conditions. The low retentivity domain, which accounts for ∼ 5% of ³He, contains no natural ⁴He and may represent poorly crystalline or intergranular material which has lost all radiogenic ⁴He by diffusion in nature. Diffusive loss of ³He from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate, so probably represents diffusion from individual micrometer-size goethite crystals. The ⁴He/³He evolution during the incremental heating experiments shows that the high retentivity domain has retained 90%-95% of its radiogenic helium. This degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to Earth surface temperature and held for the appropriate duration. Considering both the high and low retentivity domains, these data indicate that one of the samples retained 90% of its radiogenic ⁴He over 47.5 Ma and the other retained 86% over 12.3 Ma. Thus while diffusive-loss corrections to supergene goethite He ages are required, these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton-irradiation ⁴He/³He method.  相似文献   

Hypsometric Curves as a Tool for Paleosurface Mapping     

Paulo Cesar Soares  Silvana Bressan Riffel 《Mathematical Geology》2006,38(6):679-695

A procedural paleosurface mapping tool, using hypsometric curves and digital elevation models, was developed and applied to three hydrographic basins that erode common areas in the coastal ranges of Brazil, in southeastern South America. The method consists of identifying areas favorable for the occurrence of paleosurfaces and their corresponding surficial formations as correlated to specific erosion events. Attributes from hypsometric curves, and logarithmic functions fitted to proper curve segments, are combined with common morphometric properties of paleosurfaces given a set of criteria. Zones satisfying the criteria are mapped as pertaining to specific paleosurfaces. Field survey confirms the agreement between predicted and observed occurrence and absence of paleosurfaces.  相似文献   

Bayesian and multivariate methods applied to favorability quantification in Recôncavo Basin and Ribeira Belt, Brazil     

Sidnei Pires Rostirolla  Paulo César Soares  Hung Kiang Chang 《Natural Resources Research》1998,7(1):7-24

A methology to define favorable areas in petroleum and mineral exploration is applied, which consists in weighting the exploratory variables, in order to characterize their importance as exploration guides. The exploration data are spatially integrated in the selected area to establish the association between variables and deposits, and the relationships among distribution, topology, and indicator pattern of all variables. Two methods of statistical analysis were compared. The first one is the Weights of Evidence Modeling, a conditional probability approach (Agterberg, 1989a), and the second one is the Principal Components Analysis (Pan, 1993). In the conditional method, the favorability estimation is based on the probability of deposit and variable joint occurrence, with the weights being defined as natural logarithms of likelihood ratios. In the multivariate analysis, the cells which contain deposits are selected as control cells and the weights are determined by eigendecomposition, being represented by the coefficients of the eigenvector related to the system’s largest eigenvalue. The two techniques of weighting and complementary procedures were tested on two case studies: 1. Recôncavo Basin, Northeast Brazil (for Petroleum) and 2. Itaiacoca Formation of Ribeira Belt, Southeast Brazil (for Pb-Zn Mississippi Valley Type deposits). The applied methdology proved to be easy to use and of great assistance to predict the favorability in large areas, particularly in the initial phase of exploration programs.  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   

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   

Timing and duration of supergene mineralization at the Xinrong manganese deposit,western Guangdong Province,South China: cryptomelane <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating     

Jian-Wei Li  Paulo Vasconcelos  Wei Zhang  Xiao-Dong Deng  Nurdan Duzgoren-Aydin  Dai-Rong Yan  Jian-Qiang Zhang  Ming-An Hu 《Mineralium Deposita》2007,42(4):361-383

Supergene Mn-oxide deposits are widely distributed in Guangxi, Guangdong, Yunnan, and Hunan Provinces, South China, accounting for 18% of the total Mn reserves in the country. Direct dating of supergene Mn enrichment, however, is lacking. In this paper, we present high-resolution ⁴⁰Ar/³⁹Ar ages of Mn oxides from the Xinrong Mn deposit, western Guangdong, to place numerical constraints on the timing and duration of supergene Mn enrichment. A total of ten cryptomelane samples, spanning a vertical extent of 67 m, were dated using the ⁴⁰Ar/³⁹Ar laser incremental heating technique, with seven samples yielding well-defined plateau or pseudo-plateau ages ranging from 23.48 ± 0.91 to 2.06 ± 0.05 Ma (2σ). One sample yields a staircase spectrum that does not reach a plateau; the spectrum, however, indicates the presence of two or more generations of Mn oxides in the sample, whose ages are best estimated at 22.34 ± 0.31 and 10.2 ± 0.86 Ma, respectively. The remaining two samples gave meaningless or uninterpretable results due to significant ³⁹Ar recoil and contamination by old phases. The ⁴⁰Ar/³⁹Ar data thus reveal a protracted history of weathering and supergene Mn enrichment that started at least in the end of the Oligocene or beginning of Miocene and extending into the latest Pliocene. Staircase-apparent age spectra, resulting from banded or botryoidal samples, yield an average growth rate of Mn oxides at 0.6–0.7 × 10⁻³ mm kyr⁻¹. The values indicate that a 1-mm grain of Mn oxides may host minerals precipitated during a time span of ca. 1.5 m.y., and accumulation of Mn oxides to form economic deposits under weathering environments may take millions of years. The distribution of weathering ages shows that the oldest Mn oxides occur on the top of the profile, whereas the youngest minerals are found at the bottom, suggesting downward propagation of weathering fronts. However, two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations. Such a complexity of age distribution is interpreted in terms of preferential penetration of Mn-rich weathering solutions along more permeable fault zones, or as a result of multi stages of dissolution and re-precipitation of Mn oxides. A synthesis of geochronological and geological data suggests that formation of the Xinrong deposit was a consequence of a combination of favorable lithological, climatic, and structural conditions. Because the climatic and structural conditions are similar among the provinces of South China during the Cenozoic, the geochronological results obtained at Xinrong may also have implications for the timing of supergene Mn enrichment throughout South China.  相似文献   

Understanding Geological Data Distribution and Orientation via Correspondence Analysis     

Manuel Francisco Pereira  Paulo Sérgio Lucio 《Mathematical Geology》2007,39(7):673-695

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Sandstone dissolution landforms in the Furnas Formation,southern Brazil     

Mário Sérgio de Melo  Paulo César Fonseca Giannini 《地球表面变化过程与地形》2007,32(14):2149-2164

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A Matrix Approach Coupled with Monte Carlo Techniques for Solving the Net Radiative Balance of the Urban Block   总被引：2，自引：0，他引：2  

Marta J. N. Oliveira Panão  Helder J. P. Gonçalves  Paulo M. C. Ferrão 《Boundary-Layer Meteorology》2007,122(1):217-241

A new method is developed for solving the shortwave and longwave net radiative balance of a three-dimensional urban structure, represented by parallelepiped blocks uniformly distributed in each direction. The method is based on a novel approach to determine the shape factors among surfaces, which are estimated by Monte Carlo techniques due to the complex geometry associated with the three-dimensional urban structure. Then, a set of linear equations is solved to quantify the radiative balance, in order to obtain their exact solution, considering all the inter-reflections among surfaces. The comparison between the new and the ray-tracing tracking methods resulted in a Pearson correlation coefficient of 0.996. However, by integrating the linear equations’ exact solution with Monte Carlo techniques, the new method reduces by a factor of 36 the central processing unit (CPU) time used to perform the calculations of the ray-tracing tracking method. The use of the model for a sensitivity study allows us to verify the effective absorptance and emittance increases with the canyon aspect ratio of the urban layout. An urban structure formed by square cross-sectional blocks absorbs more solar radiation than an urban structure formed by rectangular cross-sectional blocks. The approximation of a specific geometry for an equivalent bi-dimensional infinite street can be applied for rectangular cross-sectional blocks, where the width is 11 times or more greater than the depth dimension.  相似文献   

Natural to anthropogenic forcing in the Holocene evolution of three coastal lagoons (Caldas da Rainha valley, western Portugal)   总被引：1，自引：0，他引：1  

J.L. Dinis  V. Henriques  M.C. Freitas  C. Andrade  P. Costa   《Quaternary International》2006,150(1):41

In the coast of Central Portugal three lagoons were created by the Holocene flooding of diapiric-related depressions but experienced afterwards a significant sediment accumulation. Fast environmental and morphological changes after the Middle Holocene were clearly forced by anthropogenic activities since the Middle Ages and show a strong feedback on the human communities. Erosion in the studied watersheds depends on climatic and anthropic changes; especially, demographic rises increase agriculture and deforestation in the watersheds, and sedimentation in the lagoons. The region was successively occupied by ethnic groups since the Neolithic (including Romans, Sueves, Visigoths and Muslims), but the main changes were largely due to anthropic forcing following the Christian Reconquest by the Kingdom of Portugal. In fact, during the Middle Ages and Renaissance the area had intense nautical, fishing and agricultural activities, even if reduced during the 14th century crisis. Later, due to severe sediment accumulation and shoaling, sailing was drastically reduced and most of the area drowned in the maximum transgression was claimed to farming. It is also noteworthy that the social evolution and sediment entrainment in the watersheds appear to be in tune with climatic trends deduced after regional and global data. In synthesis, we conclude that the human activities during the last millennium greatly accelerated the natural silting trend of the lagoons.  相似文献