Electron microprobe U–Th–Pb monazite dating of the Transamazonian metamorphic overprint on Archean rocks from the Amapá Block, southeastern Guiana Shield, Northern Brazil

The Amapá Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U–Th–Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb–Pb zircon dating was also performed on one sample.Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 ± 6, 2093 ± 8, 2088 ± 8, 2087 ± 3 and 2086 ± 8 Ma, and by the zircon age of 2091 ± 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapá Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10–2.08 Ga.The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 ± 7 and 2038 ± 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones.     

Biogeochemical behavior of Ampelozizyphus amazonicus Ducke in the Pitinga mining district, Amazon, Brazil

The vegetal species Ampelozizyphus amazonicus Ducke (Rhamnaceae Family) was chosen as a sampling medium for the lateritic surfaces of the Pitinga Mine in the Amazon region, in order to study the biogeochemical behavior of this species and compare it with the chemical composition of a reference plant. The Pitinga mining district is one of the largest producers of tin in the world. This district contains unique deposits of cryolite and rare metals such as Zr, Nb, Ta, Y and REEs related to granitic bodies that intrude into the volcanic and acid pyroclastic rocks. The results showed that the species A. amazonicus predominantly concentrates significant levels of Zr, Nb, Ta, Th, Be, Sc over U, Hf, Ga and In. These elements are characteristic of the mineral paragenesis for the region, suggesting that this plant can provide a representative sampling medium future geochemical exploration programs in the region.     

Facies analyses and volcanic setting of the giant Neves Corvo massive sulfide deposit,Iberian Pyrite Belt,Portugal

In the Iberian Pyrite Belt, volcanic rocks are relatively scarce, accounting for approximately only 25% of the geologic record, with the remaining 75% consisting of sedimentary units. This association is very clear in the host succession to the Neves Corvo massive sulfide deposit in Portugal. The Neves Corvo host succession comprises the products of explosive and effusive rhyolitic eruptions intercalated with mudstone that records a submarine below-wave-base environment and provides precise biostratigraphic age constraints. The first and second volcanic events involved eruptions at local intrabasinal vents. The first event generated thick beds of fiamme breccia that are late Famennian in age. The fiamme were originally pumice clasts produced by explosive eruptions and were subsequently compacted. The second event was the late Strunian (latest Famennian) effusion of rhyolitic lava that was pervasively quench-fragmented. The third and final event is younger than the massive sulfide deposits poorly represented in the mine area and minor compared with the two other events. The integration of biostratigraphic data with the volcanic facies architecture indicates that the Neves Corvo ore deposits are similar in age to the late Strunian rhyolitic lava. Although regionally the Iberian Pyrite Belt is essentially a sedimentary succession, ore formation at Neves Corvo can be closely linked to discrete volcanic events that produced a relatively narrow range of volcanic facies.     

Vitellogenin in winter flounder (Pleuronectes americanus) from Long Island Sound and Boston Harbor

Vitellogenin is an egg-yolk precursor protein in teleosts which is crucial to the survival of larvae. Manufactured in the liver, where pollutants are known to accumulate, and transported to the ovary by the blood, its synthesis by the liver or uptake by the gonad can be compromised by accumulation of xenobiotics. In three studies, winter flounder (Pleuronectes americanus) blood samples were taken to determine normal levels of vitellogenin during the reproductive cycle, and to learn how its production might be affected in degraded environments. Specifically, these studies followed the seasonal cycle of vitellogenin production in winter flounder through monthly sampling at relatively clean (Shoreham, New York) and degraded areas (Black Rock and New Haven harbors, Connecticut) in Long Island Sound; examined the relationship between parental vitellogenin levels and survival of offspring by sampling fish that had been spawned at the Milford Laboratory for a reproductive success study; and determined the effect of gross liver lesions on vitellogenin production by sampling flounder from Boston Harbor, Massachusetts, which have been reported to have a high prevalence of liver tumors. Blood vitellogenin levels were determined by measuring alkali-labile phosphate (ALP). Large fish (>30 cm) from the two degraded sites had elevated serum ALP levels relative to those from the clean area. Lowered total ovarian lipid levels in large fish from Black Rock Harbor suggested impaired vitellogenin uptake. There were no significant differences in serum ALP among the small (≤30 cm) fish from the three sampling sites. Boston Harbor flounder with gross liver lesions had lower ALP values than fish without such lesions. There were no significant differences in ALP values among the spawned fish.     

UV observations of the symbiotic star EG And

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

A two-surface plasticity model for stiff clay

This paper presents a constitutive model for describing some important features of the behavior of natural stiff clay evidenced experimentally such as the limited elastic zone, the presence of strain hardening and softening, and the smooth transition from elastic behavior to a plastic one. The model, namely ACC-2, is an adapted Modified Cam Clay model with two yield surfaces: similarly to bounding surface plasticity theory, an additional yield surface—namely Inner yield surface—was adopted to account for the plastic behavior inside the conventional yield surface. A progressive plastic hardening mechanism was introduced with a combined volumetric-deviatoric hardening law associated with the Inner yield surface, enabling the plastic modulus to vary smoothly during loading paths. The main feature of the proposed model is that its constitutive equations can be simply formulated based on the consistency condition for the Inner yield surface, so that it can be efficiently implemented in a finite element code using a stress integration scheme similar to that of the Modified Cam Clay model. Furthermore, it is proved to be an appropriate model for natural stiff clay: the simulations of a set of tests along different mechanical loading paths on natural Boom Clay show good agreement with the experimental results.     

Least-cost adaptation options for global climate change impacts on the Brazilian electric power system

Global climate change induced by the emission of greenhouse gases may pose challenges to energy security. The vulnerability of energy sources, in particular of renewable sources, to climate change raises the need to identify adaptation measures. This paper applies an integrated resource planning approach to calculate least-cost adaptation measures to a set of projected climate impacts on the Brazilian power sector. The methodology used has the advantage of finding optimal solutions that take into consideration the whole energy chain and the interactions between energy supply and demand. Results point in the direction of an increased installed capacity based, mostly, on natural gas, but also sugarcane bagasse, wind power and coal/nuclear plants, to compensate for a lower reliability of hydroelectric production, amongst other impacts. The indirect effect of these results is the displacement of natural gas from other consuming sectors, such as industry, in favor of its use for power generation. Results obtained are, however, based on the techno-economic premises used in the simulation, which may vary in the long term.     

Influence of damage on pore size distribution and permeability of rocks

The model proposed in this article relates permeability to porosity measurements that can easily be performed in the laboratory. The pore size distribution (PSD) curve is updated with strains and damage. The updated volumetric fractions of natural pores and cracks are introduced in the expression of permeability. Contrary to classical permeability models based on PSD integrations, the model proposed in this article accounts for possible changes in the porosity modes: one mode for undamaged samples and two modes for cracked samples. The proposed approach also accounts for varying states of damage, as opposed to classical fracture network models, in which the cracks pattern is fixed. The only material parameters that are required to describe the microstructure are the lower and upper bounds of the pores size for both natural pores and cracks. All the other PSD parameters involved in the model are related to macroscopic parameters that can easily be determined in the laboratory, such as the initial void ratio. The framework proposed in this article can be used in any damage constitutive model to determine the permeability of a brittle porous medium. Drained triaxial compression tests have been simulated. Before cracks initiation, permeability decreases while the larger natural pores are getting squeezed. After the occurrence of damage, permeability grows due to the increase of cracks density. The model performs well to represent the influence of the confining pressure on damage evolution and permeability variations. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterising groundwater and surface-water interconnections using hydrogeology,hydrochemistry and stable isotopes in the Ouémé Delta,southern Benin

The Mio-Pliocene aquifer of the coastal sedimentary basin of Benin is the most exploited aquifer for water supply to the urbanised region in the southern part of the country. The population explosion is putting increasing pressure on quantitative and qualitative aspects of the groundwater resources. Preventing groundwater contamination caused by surface waters requires a thorough understanding of surface-water/groundwater interactions, especially the interactions between the Mio-Pliocene aquifer and surface waters. This study aimed to investigate the interactions between groundwater and surface waters along the major rivers (Sô River and Ouémé Stream) and brooks in the Ouémé Delta. Field campaigns identified 75 springs located in the valleys which feed the rivers, and thus maintain their base flow. The piezometric results indicated, through flow direction assessment, that the Mio-Pliocene aquifer feeds Ouémé Stream and Sô River. Chemical analyses of groundwater and surface waters show similar chemical facies, and changes in the chemical composition in groundwater are also observed in the surface waters. Moreover, the isotopic signatures of surface waters are similar to those of the groundwater and springs, which led to the identification of potential groundwater discharge areas. As a result of groundwater discharge into surface waters, the fraction of groundwater in the surface water is more than 66% in the brooks, regardless of the season. In the Ouémé Stream and Sô River, the fraction of groundwater is 0–21% between June and September, while from October to March it is 47–100%.