Processos de fusão parcial no Complexo Migmatítico de Mundão (Centro Norte de Portugal): Evidências geoquímicas

As formações de idade ante-ordovícica estão amplamente representadas na região centro norte de Portugal, onde constituem um espesso conjunto metassedimentar, conhecido na literatura como “Complexo Xisto-Grauváquico ante-Ordovícico” (CXG), ao qual tem sido atribuída uma idade neoproterozóica a câmbrica inferior. Durante a orogenia varisca, as rochas do CXG foram afectadas por deformação polifásica e metamorfismo regional, de intensidade variável. Na área de Mundão, o metamorfismo regional atingiu as condições de fusão parcial, dando origem a um complexo migmatítico (CMM), composto por metatexitos estromáticos, diatexitos “schollen” e diatexitos nebulíticos leucocráticos. Do ponto de vista das relações com a deformação, o processo de migmatização parece ter-se iniciado no final da 1ª fase de deformação (D1), associada ao espessamento crustal do orógeno, prolongando-se durante todo o episódio extensional (D2) e, pelo menos, grande parte da tectónica transcorrente com que se relaciona a 3ª fase de deformação (D3). Os dados de geoquímica de elementos maiores e traço (em particular os padrões de terras raras) mostram que os metapelitos e metagrauvaques não migmatizados e os mesossomas dos metatexitos estromáticos apresentam assinaturas químicas muito semelhantes, caracterizadas por teores baixos de SiO2 e CaO, K2O > Na2O, concentrações altas de Al2O3, Fe2O3, TiO2, padrões de REE pouco fraccionados (LaN/YbN = 6.55 – 7.05) e anomalias negativas de Eu (Eu/Eu* = 0.52-0.56). A semelhança encontrada sugere que os mesossomas não sofreram extracção significativa de “melt”, correspondendo a protólitos relativamente pouco modificados. Em contrapartida, os leucossomas dos metatexitos estromáticos estão enriquecidos em SiO2 (74-75%), CaO (0.62-0.77%) e Na2O (2.11-2.79%), empobrecidos em Al2O3, Fe2O3, TiO2 e REE e mostram anomalias positivas de Eu de amplitude variável (Eu/Eu* = 1.05-1.58). As suas composições são típicas de fundidos gerados por pequenos graus de fusão parcial em condições de desequilíbrio, embora também possam ser atribuídas à precipitação precoce de feldspato a partir dos fundidos primários (cumulados). Por fim, os diatexitos nebulíticos leucocráticos têm conteúdos em SiO2 variando entre 70-72%, exibem um marcado enriquecimento em K2O (5.20-6.67%), padrões de terras raras mais fraccionados (LaN/YbN = 11.88 – 22.75) e pronunciadas anomalias negativas de Eu (Eu/Eu* = 0.34-0.47), pelo que são interpretados como líquidos mais fraccionados resultantes de taxas de fusão superiores. O cálculo das temperaturas de saturação em Zr para as amostras de leucossomas e de diatexitos leucocráticos apontam para temperaturas de fusão entre 740-780C, o que é consistente com processos de anatexia controlados essencialmente por reacções de desidratação de moscovite

Anatexis of metasedimentary rocks in the Iberian Variscan Belt: the example of the Mundão migmatites (Northern Central Portugal)

The oldest rocks exposed in the axial zone of the Iberian massif of Central Northern Portugal (Central Iberian Zone) belong to a thick turbiditic metasedimentary sequence of Neoproterozoic-Lower Cambrian age, consisting of metagreywackes and metapelites with minor calc-silicates and conglomerates. In the Mundão area, this sequence was extensively migmatized during the Variscan orogenic event. Field and structural evidence show that the partial melting conditions were reached in the last stages of crustal thickening (D1), continued during subsequent extensional deformation (D2) and culminated with the emplacement of large volumes of S-type granite magmas in a transcurrent tectonic regime (D3). The Mundão anatectic complex comprises two main migmatite types: metatexites and diatexites, showing in general transitional contacts. The metatexites are composed of dark fine-grained layered hosts and small volumes of quartz-feldspathic veins (leucosomes), whilst the diatexites have igneous looking appearance, larger amounts of the lighter portions and schollen or nebulitic structures. In the latter, the foliation is defined by the presence of biotite rich schlieren and/or the alignment of feldspar crystals in the leucocratic bands. Petrographically, the Mundão migmatite types are distinguished by variations of the modal proportions of the same mineral assemblage (quartz + plagioclase + biotite K- feldspar + apatite + zircon + monazite + opaques). Fibrolitic sillimanite occurs either as small clots of fibres or as needles included within the other mineral phases (plagioclase and muscovite). Tourmaline is a common accessory phase. Large flakes of retrograde muscovite overgrowing the syn-migmatization structures are generally present. The lack of garnet and orthopyroxene in these rocks suggests that melting was dominantly controlled by muscovite breakdown. Whole rock major and trace element geochemistry reveals that the unmigmatized schists and metagreywackes and the darker portions of the metatexites have similar geochemical signatures, characterized by low SiO2 and CaO contents, K2O > Na2O, high Al2O3, Fe2O3T, TiO2, poorly fractionated REE patterns (LaN/YbN = 6.55 – 7.05) and negative Eu anomalies (Eu/Eu* = 0.52-0.56). The similarities encountered appear to indicate that the dark metatexite layers represent partially melted metasediments from which little or no melt extraction occurred. The leucosomes of the metatexites are enriched in SiO2 (74-75%), CaO (0.62-0.77%) and Na2O (2.11-2.79%), depleted in Al2O3, Fe2O3T, TiO2 and REE and have positive Eu anomalies (Eu/Eu* = 1.05-1.58). Their compositions are typical of melts produced either by small amounts of partial melting in disequilibrium conditions and/or by early removal of feldspars from the primary melts (cumulates). In contrast, the leucocratic nebulitic diatexites display SiO2 values varying between 70-72%, K2O enrichment (5.20-6.67%), fractionated REE patterns (LaN/YbN = 11.88 – 22.75) and distinct negative Eu anomalies (Eu/Eu* = 0.34-0.47) and may therefore correspond to more fractionated melts derived from higher melt fractions. The zircon saturation temperatures for samples from the leucosomes of the metatexites and the leucocratic diatexites range between 740 and 780C, suggesting that migmatization occurred mainly via muscovite dehydration reactions. Apparently, the amount of melt generated was enough to flow, implying melt fractions above the melt escape threshold of 20% vol. melt

A sedimentação carbonífera na Bacia do Buçaco (Centro de Portugal)

Os aspectos gerais das ocorrências do Carbónico continental em Portugal são descritos por Ribeiro et al. (1979), Teixeira & Gonçalves (1980) e Teixeira (1981). A revisão da cronoestratigrafia da flora fóssil destas formações e uma síntese da evolução tectónica de cada uma das bacias podem ser encontradas no volume editado por Sousa & Oliveira em 1983, dedicado ao Carbónico de Portugal. (...

The puzzling interpretation of NIR indices: The case of NaI2.21

We present a detailed study of the Na I line strength index centered in the $K$-band at $22100$, {\AA} (NaI2.21 hereafter) relying on different samples of early-type galaxies. Consistent with previous studies, we find that the observed line strength indices cannot be fit by state-of-art scaled-solar stellar population models, even using our newly developed models in the NIR. The models clearly underestimate the large NaI2.21 values measured for most early-type galaxies. However, we develop a Na-enhanced version of our newly developed models in the NIR, which - together with the effect of a bottom-heavy initial mass function - yield NaI2.21 indices in the range of the observations. Therefore, we suggest a scenario in which the combined effect of [Na/Fe] enhancement and a bottom-heavy initial mass function are mainly responsible for the large NaI2.21 indices observed for most early-type galaxies. To a smaller extent, also [C/Fe] enhancement might contribute to the large observed NaI2.21 values.Comment: 13 pages, 4 figures, accepted for publication in MNRA

IMF and [Na/Fe] abundance ratios from optical and NIR Spectral Features in Early-type Galaxies

We present a joint analysis of the four most prominent sodium-sensitive features (NaD, NaI8190, NaI1.14, and NaI2.21), in the optical and Near-Infrared spectral range, of two nearby, massive (sigma~300km/s), early-type galaxies (named XSG1 and XSG2). Our analysis relies on deep VLT/X-Shooter long-slit spectra, along with newly developed stellar population models, allowing for [Na/Fe] variations, up to 1.2dex, over a wide range of age, total metallicity, and IMF slope. The new models show that the response of the Na-dependent spectral indices to [Na/Fe] is stronger when the IMF is bottom heavier. For the first time, we are able to match all four Na features in the central regions of massive early-type galaxies, finding an overabundance of [Na/Fe], in the range 0.5-0.7dex, and a bottom-heavy IMF. Therefore, individual abundance variations cannot be fully responsible for the trends of gravity-sensitive indices, strengthening the case towards a non-universal IMF. Given current limitations of theoretical atmosphere models, our [Na/Fe] estimates should be taken as upper limits. For XSG1, where line strengths are measured out to 0.8Re, the radial trend of [Na/Fe] is similar to [Mg/Fe] and [C/Fe], being constant out to 0.5Re, and decreasing by 0.2-0.3dex at 0.8Re, without any clear correlation with local metallicity. Such a result seems to be in contrast with the predicted increase of Na nucleosynthetic yields from AGB stars and TypeII SNe. For XSG1, the Na-inferred IMF radial profile is consistent, within the errors, with that derived from TiO features and the Wing-Ford band, presented in a recent paper.Comment: 22 pages, 8 figure, accepted for publication in MNRAS. The new Na-enhanced models will be available soon at http://miles.iac.es

Ionization fraction and the enhanced sulfur chemistry in Barnard 1

Barnard B1b has revealed as one of the most interesting globules from the chemical and dynamical point of view. It presents a rich molecular chemistry characterized by large abundances of deuterated and complex molecules. Furthermore, it hosts an extremely young Class 0 object and one candidate to First Hydrostatic Core (FHSC). Our aim was to determine the cosmic ray ionization rate and the depletion factors in this extremely young star forming region. We carried out a spectral survey towards Barnard 1b as part of the IRAM Large program ASAI using the IRAM 30-m telescope at Pico Veleta (Spain). This provided a very complete inventory of neutral and ionic C-, N- and S- bearing species with, up to our knowledge, the first secure detections of the deuterated ions DCS+ and DOCO+. We used a state-of-the-art pseudo-time-dependent gas-phase chemical model to determine the value of the cosmic ray ionization rate and the depletion factors. The observational data were well fitted with $\zeta_{H_2}$ between 3E-17 s$^{-1}$ and 1E-16 s$^{-1}$. Elemental depletions were estimated to be ~10 for C and O, ~1 for N and ~25 for S. Barnard B1b presents similar depletions of C and O than those measured in pre-stellar cores. The depletion of sulfur is higher than that of C and O but not as extreme as in cold cores. In fact, it is similar to the values found in some bipolar outflows, hot cores and photon-dominated regions. Several scenarios are discussed to account for these peculiar abundances. We propose that it is the consequence of the initial conditions (important outflows and enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that permits to maintain most S- and N-bearing species in gas phase to great optical depths. The interaction of the compact outflow associated with B1b-S with the surrounding material could enhance the abundances of S-bearing molecules, as well.Comment: Paper accepted in Astronomy and Astrophysics; 28 pags, 21 figure

Two-dimensional electron gas at the PbTi O3/SrTi O3 interface: An ab initio study

In the polar catastrophe scenario, polar discontinuity accounts for the driving force of the formation of a two-dimensional electron gas (2DEG) at the interface between polar and nonpolar insulators. In this paper, we substitute the usual, nonferroelectric, polar material with a ferroelectric thin film and use the ferroelectric polarization as the source for polar discontinuity. We use ab initio simulations to systematically investigate the stability, formation, and properties of the two-dimensional free-carrier gases formed in PbTiO3/SrTiO3 heterostructures under realistic mechanical and electrical boundary conditions. Above a critical thickness, the ferroelectric layers can be stabilized in the out-of-plane monodomain configuration due to the electrostatic screening provided by the free carriers. Our simulations also predict that the system can be switched between three stable configurations (polarization up, down, or zero), allowing the nonvolatile manipulation of the free-charge density and sign at the interface. Furthermore, the link between ferroelectric polarization and free-charge density demonstrated by our analysis constitutes compelling support for the polar catastrophe model that is used to rationalize the formation of 2DEG at oxide interfaces

Maternal immunization: A call to accelerate progress

Maternal immunization provides an excellent evidence-based strategy for preventing severe disease and decreasing neonatal and infant mortality. A substantial proportion of these deaths are due to infectious diseases, most of them vaccine-preventable, then, there is a real opportunity for intervention. Maternal immunization has been an underexploited area for many years, with the exception of neonatal tetanus. There are now programs for influenza and acellular pertussis vaccination in many countries and two maternal vaccine targets under development are focused on decreasing the burden of infant respiratory syncytial virus (RSV) and Group B Streptococcus (GBS). Bodies like the Strategic Advisory Group of Experts (SAGE) on Immunization established by the WHO, the Global Vaccine Action Plan (GVAP) and Gavi, The Vaccine Alliance, have recognized the relevance of maternal immunization on several occasions. However, why is the field not moving faster, as one might expect? Major initiatives and programs should consider spelling out more clearly the role and benefits of this intervention and calling for specific actions, including future strategic approaches for the post 2020 immunization strategy following the GVAP; and single out the area as one of its priorities as a key component of immunization across the life course. While waiting for the new vaccines like RSV and GBS and optimizing the use of influenza and pertussis there is momentum now to coordinate efforts, address the missing information and action gaps, and call to accelerate progress

Analyticity in theta and infinite volume limit of the topological susceptibility in SU(3) gauge theory

The large volume behaviour of the topological susceptibility in SU(3) gauge theory is investigated on the lattice to establish an upper limit on the parity violating terms.Comment: 2 pages, 1 figure, poster presented at Lattice 2004(topology