Point defects and pre-dose requirements for sensitization of the 300°C TL peak in natural quartz

This paper discusses the structural features required to stimulate a strong thermoluminescence (TL) glow peak near 300°C in clear natural quartz. For that reason, fresh TL data taken from several specimens prepared from five single crystals with known impurity content are shown. The TL emission was measured with a test dose of 10 mGy of γ-rays in the readout intervals 50–160 and 160–320°C. The readings were carried out prior and after the administration of a pre-dose of 175 kGy of γ-rays followed by heat-treatments at 400°C. For each single specimen, the OH content and the population of inclusions were evaluated by infrared spectroscopy and optical microscopy, respectively. The darkening induced by high γ dose was evaluated by optical spectroscopy. It was observed that the absorption at 475 nm and TL responses decrease with increase of the OH. It was shown that both smoky darkening and TL signals were better explained in terms of Li/Al and Li/OH content ratios rather than the absolute values of aluminum and alkali concentrations. The sensitization with high γ dose and heating is essential to create and stabilize a class of defects sites with Li⁺ ions dislodged from [AlO₄/Li]⁰ and Li-dependent OH centers. It is suggested that the defect sites formed with Li⁺ act as electron traps during test dose irradiation, whereas electron-hole recombination occurs essentially at [AlO₄]⁰ centers during the TL output near 300°C.     

Mechanics of thick-skinned Variscan overprinting of Cadomian basement (Iberian Variscides)

Remnants of the Cadomian basement can be found in the Iberian Variscides (IBVA) in several key sectors of its autochthonous units (composed of Neoproterozoic to Lower Palaeozoic metasedimentary sequences) and within the Continental Allochthonous Terrane (CAT). Comprehensive characterization of these critical exposures shows that the prevailing features are related to major geological events dated within the age range of 620–540 Ma. Indeed, near the Cambrian–Ordovician boundary, the IBVA Internal Zones experienced pervasive basement thinning and cover thickening, reflecting diffusive displacement of intracratonic rifting that continued until Lower Devonian times. In the thick-skinned Internal Zones, Helvetic/Penninic style nappes were generated, whereas flower upright axial structures developed along transpressive, intraplate shear zones. These features contrast with those preserved in the thin-skinned IBVA External Zones, dominated by décollements above (un-)deformed Palaeozoic and Cadomian basement. The inferred attenuation of rheological contrast between Cadomian basement and Palaeozoic cover can be explained by inherited fabrics due to thermal softening operated during the Cambrian–Lower Devonian extensional regime. Deeper décollements (and subsequent strain partitioning) are also expected to develop at the upper-lower crust (and at the Moho?) transition, as imaged by the available seismic profiling and MT surveys. The whole data implies a significant discontinuity between Cadomian and Variscan Cycles that should have constrained subsequent lithospheric evolution.     

A geochemical approach to distinguishing competing tectono-magmatic processes preserved in small eruptive centres

Small eruptive centres (SECs) representing short-lived, isolated eruptions are effective samples of mantle heterogeneity over a given area, as they are generally of basaltic composition and show evidence of little magmatic processing. This is particularly powerful in volcanic arcs where the original melting process generating stratovolcanoes is often obscured by additions from the down-going slab (fluids and sediments) and the overlying crust. The Pucón area of southern Chile contains active and dormant stratovolcanoes, Holocene, basaltic SECs and an arc-scale strike-slip fault (the Liquiñe Ofqui Fault System: LOFS). The SECs show unexpected compositional heterogeneity considering their spatial proximity. We present a detailed study of these SECs combining whole rock major and trace element concentrations, U-Th isotopes and olivine-hosted melt inclusion major element and volatile contents to highlight the complex inter-relations in this small but active area. We show that heterogeneity preserved at individual SECs relates to different processes: some start in the melting region with the input of slab-derived fluids, whilst others occur later in a centre’s magmatic history with the influence of crustal contamination prior to olivine crystallisation. These signals are deduced through the combination of the different geochemical tools used in this study. We show that there is no correlation between composition and distance from the arc front, whilst the local tectonic regime has an effect on melt composition: SECs aligned along the LOFS have either equilibrium U-Th ratios or small Th-excesses instead of the large—fluid influenced—U-excesses displayed by SECs situated away from this feature. One of the SECs is modelled as being generated from fluid-enriched depleted mantle, a source which it may share with the stratovolcano Villarrica, whilst another SEC with abundant evidence of crustal contamination may share its plumbing system with its neighbouring stratovolcano Quetrupillán, showing that polygenetic–monogenetic connections are unpredictable. Such marked preservation of individual magmatic histories highlights the isolation of individual melting events even in complex and highly volcanically active areas.     

Combined hydrochemical,isotopic, and multivariate statistics techniques to assess the effects of discharges from a uranium mine on water quality in neighboring streams

The Caldas Uranium Mine (CUM), located on the Poços de Caldas Plateau in the southeastern region of Brazil, is presently undergoing a decommissioning process. The aim of the present investigation is to identify and characterize the effects of acid mine drainage (AMD) originating from the CUM on surface water quality. To achieve these aims, sampling stations were located at two AMD sources: the retention pond at the foot of waste rock pile#4 (WRP#4) and the settling pond that receives effluents from the tailings dam (TD). Ten additional sampling stations were located along watercourses in the vicinity, both downstream and upstream of the mine. Sampling was performed during the rainy and dry seasons in 2010 and 2011. The water analysis detected significant changes in pH, electrical conductivity, F^?, Cd, U, Zn, Al, Mn, As, Ca, SO₄ ^2?, Pb, ²³⁸U, ²²⁶Ra, ²¹⁰Pb, ²³²Th, ²²⁸Ra, and Mo in waters downstream of both pond discharge sites. It was demonstrated that the disequilibrium between ²²⁶Ra and ²³⁸U can be used to trace the extent of AMD impacts in nearby streams. Variations in ¹⁸O and ²H enabled the flow of mining-impacted water to be traced from the ponds to nearby streams. Multivariate analysis yielded a three-factor model: Factor 1 was interpreted as being associated with AMD (from WRP#4) and Factor 2 with a Ca–Mo relationship associated with the chemical constitution of the ore and with the treatment of tailings wastes in the area (from TD); Factor 3 was interpreted as being associated with the natural influence of geogenic processes on water quality in the area. The results of this study provide a scientific basis for recommending appropriate remedial actions during mine decommissioning.     

The role of the Río de la Plata bottom salinity front in accumulating debris

The Río de la Plata, one of the most important South American estuarine environments, is characterized by a bottom salinity front that generates an ecotone between the river and the estuary. Based on bottom trawls and costal sampling we describe the distribution, types, and amount of debris found in the bottom and shoreline across this front. Plastics and plastic bags were the main debris types in both areas. Concentrations of total debris upriver the front were always significantly higher than downriver the front showing that the front acts as a barrier accumulating debris. Moreover, a large part of debris end ups accumulated in the coastal area upriver the frontal position. This area is particularly sensitive because the coastline encompasses an UNESCO Man and the Biosphere Reserve and a Ramsar site, and due to the ecological significance of the front for many valuable species.     

Volume 49

Volume Contents

Volume 49     

Protection areas of the São Pedro do Sul Spa, Portugal

This paper presents a case study of the protection areas of one of the Portugal's most important spas. According to its chemical composition this thermal mineral water is rich in bicarbonate, sodium, carbonate, fluor and sulfate. The therapeutic indications are mainly illnesses related to rheumatism and respiratory system. As a complement the thermal mineral water is also used for geothermal purposes, as in the heating of greenhouses for the production of Bananas and Ananas. The study presents geological and hydrogeological characterization of the area and its vulnerability to pollution and the physical and chemical characterization of the thermal mineral water. Finally, in accordance with the Portuguese legislation, the protection areas are defined: for immediate, intermediate and distant zones. The criteria and other aspects that led to their definition are also presented.