Chronologie et évolution des fluides hydrothermaux dans le district à fluorine de Santa Catarina,Brésil: Données de l'analyse des terres rares et de l'étude des inclusions fluides

RésumRé Dans le district de Santa Catarina, la teneur en terres rares des fluorines filoniennes caractérise chacune des neuf générations successives mises en évidence (concentrations en Sm, rapports La/Sm ou La/Yb, anomalie en Europium). Deux cycles hydrothermaux majeurs, rapportés au rifting Sud-Atlantique (130-115 m.a.) ont été distingués à partir de l'évolution des rapports La/Sm et La/Yb et des températures d'homogénisation des inclusions aqueuses primaires. Dans cette évolution, une coupure brutale sépare les générations I–IV et V–VI. Les fluorines plus tardives sont rapportées à un évènement néocrétacé-éotertiaire. Une signature géochimique constante est notée pour les terres lourdes, quelle que soit la génération considérée (ex: Yb/Tb = 5; Gd/Yb = 1) suggérant l'existence d'une source unique pour les terres rares et probablement le fluor. A titre d'hypothèse, on pense qu'il s'agit de fluorine disséminée dans les granitoïdes altérés du socle.

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In the Santa Catarina district, REE contents of vein fluorites characterize each one of the nine successive generations (Sm concentrations, La/Sm or La/Yb ratio, Eu anomaly). Two main hydrothermal cycles, related to the South Atlantic rifting (130-115 m.y.), are distinguished through the evolutions of the La/Sm and La/Yb ratios and of the homogenization temperatures of primary aqueous inclusions. In this evolution, a sharp break separates generations I–IV and V–VI. Later fluorite deposits are related to a Neo-Cretaceous-to-Eo-Tertiary event. A constant geochemical signature for heavy REE is noticed, whatever the fluorite generation (e.g. Yb/ Tb = 5; Gd/Yb = 1) suggesting an unique source for REE and possibly fluorine. This source is tentatively determined as disseminated fluorite in altered granitoids from the basement.

     

Genetic and metamorphic conditions constrained by fluid inclusions from Paleoproterozoic (c. 1.8 Ga) magnesite ore deposits, NE Brazil

Precambrian magnesite occurrences hosted by metadolomites from the Orós belt, Ceará, Brazil, are part of a greenschist–amphibolite, metavolcano-sedimentary terrain, dated at 1.8 Ga, cut by Meso- to Neoproterozoic Brasiliano granites and Neoproterozoic basic sills. These rocks were affected by a shear zone between 580 and 500 Ma. The magnesite-bearing marbles can be grouped as medium-grained (1–9 mm) at the Riacho Fundo ore deposit or sparry magnesite (1–15 cm) at the Cabeça de Negro ore deposit. The sparry magnesite shows textural characteristics related to original sedimentary structures. Both types of magnesite-bearing marbles contain aqueous and aqueous-carbonic fluid inclusions that yield homogenization temperatures between 170 and 370 °C. Applying a pressure correction, these temperatures are compatible with the evolution from greenschist to amphibolite facies metamorphic conditions, as described in previous work on the Orós region. It also agrees with data in specialized literature on the metamorphism of carbonate rocks. Fluid inclusion distribution, composition, and physical-chemical characteristics suggest temperature increase, probably related to metamorphism on these rocks. The medium-grained magnesite records partial contamination of CO₂-rich inclusions by relict carbonaceous material (bitumen, hydrocarbons?) that favors, but does not confirm, a syngenetic sedimentary origin and could have caused the lowering of CO₂ melting point in these inclusions. Therefore, though textural evidence points to a sedimentary-diagenetic model, fluid inclusions record conditions of a metamorphic event.     

UV observations of the symbiotic star EG And

EG And is a S-type symbiotic star. Here is present UV observations made by IUE from 1978 to 1991. From these observations the electronic density and the electronic temperature were obtained. The size of He II emitting region was estimated as 7.3R . Line and continuum variations confirm the binary nature of EG And.     

Aridity indices to assess desertification susceptibility: a methodological approach using gridded climate data and cartographic modeling

Natural Hazards - Desertification is a land degradation phenomenon with dire and irreversible consequences, affecting different regions of the world. Assessment of spatial climate susceptibility to...     

Fish bile as a biomarker for metal exposure

Fish accumulate several trace elements in muscle, gills and liver, however studies also indicate that metals can be excreted through bile. Since metal contamination leads to modifications in bile composition, biliary excretion offers an alternative way to evaluate the presence of trace-elements. Bile is easier to obtain than other organs and presents a simpler matrix, making it easier for chemical pre-treatment. To verify if bile can be useful as a biomonitoring tool for metal contamination, liver and bile trace element concentrations were determined and correlated. The Artificial Neural Networks statistical technique was used to verify if liver trace-element quantification could be substituted by bile analysis. Results show that significant correlations were obtained between trace elements in bile and liver and the ANN validated the hypothesis that certain trace-elements in bile could be utilized instead of liver trace-elements. Further studies in this field are of interest to further validate this biomarker.     

Generalized extreme wind speed distributions in South America over the Atlantic Ocean region

Statistical analysis of extreme values is applied to wind data from National Centers for Environmental Prediction and National Center for Atmospheric Research reanalysis grid points over the ocean region bounded at 23°S and 40°W and 42°W towards the south and southeastern Brazilian coast. The period of analysis goes from 1975 to 2006. The generalized extreme value and generalized Pareto distributions are employed for annual and daily maxima, respectively. The Pareto?CPoisson point process characterization is also used to analyze peaks over threshold. Return levels for 10, 25, 50, and 100?years are calculated at each grid point. However, most of the reanalysis data fall within 1?C10-year return periods, suggesting that hazardous wind speed with low probability (return periods of 50?C100) have rarely measured in this period. Wide confidence intervals on these levels show that there is not enough information to make predictions with any degree of certainty to return periods over 100?years. Low extremal index (??) values are found for excess wind speeds over a high threshold, indicating the occurrence of consecutively high peaks. In order to obtain realistic uncertainty information concerning inferences associated with threshold excesses, a declustering method is performed, which separates the excesses into clusters, thereby rendering the extreme values more independent.     

Field observations of SPM using ADV, ADP, and OBS in a shallow estuarine system with low SPM concentration—Vitória Bay, SE Brazil

The present study investigated direct and indirect methods using optical and acoustic instruments for the acquisition of information required to estimate the concentration of suspended particulate matter (SPM) in Vitória Bay, a shallow estuarine system in SE Brazil. The aim was to calibrate and compare the use of different instruments (OBS, ADP, and ADV) to estimate SPM concentration in the water column and near the bed. Concentration was determined by correlating filtered water samples with the optical and acoustic measurements. In general, the methodology proved tenable for the chosen shallow estuarine environment with low SPM concentration (<60?mg/L). Pearson’s coefficient ranged from 0.75 to 0.85, when correlating measurements taken at three sampled depths. Differences in the correlation coefficient values showed that calibration at three depths (near the bed, mid-water column, and near the surface) was more effective than for surface samples alone, even in shallow (???m deep) water. When calibration was attempted for concentration in the entire water column with samples at just one elevation, the correlation value was very low, thus increasing the error in estimating the SPM concentration.