1,853 research outputs found
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
-band at , {\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 between 3E-17 s and 1E-16 s.
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
Recommended from our members
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
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