8,835 research outputs found
New geochemical and isotopic constraints on the genesis of the Oliveira Azeméis granitoid melts (Porto-Tomar Shear Zone, Iberian Variscan Chain, Central-Western Portugal).
The Porto-Tomar Shear Zone (PTSZ) is a very important tectonic structure that separates, in central-western
Portugal, two of the major tectonic units of the Iberian Variscan Chain: the Ossa-Morena Zone, to the west,
and the Central Iberian Zone, to the east. The Oliveira de Azeméis area lies in the northern sector of the PTZC
and it is characterized by the occurrence of strongly deformed granitoids. Country rocks are dominantly pelitic
metasediments which, according to recent geological mapping (Pereira et al., 2007), belong to the Precambrian
Lourosa Formation and the Ordovician São João de Ver Formation. Using Rb-Sr whole-rock isotopic data, Pinto
(1979) proposed an age of 379 12 Ma for the Oliveira de Azeméis granitoids.
In this work, new results were obtained on these granitoids in the area between the villages of Travanca and Curval,
especially in the Sacramento quarry. In this critical outcrop, strongly deformed two-mica granite (displaying S-C
structures, with dextral NNW-SSE shear planes) pass into diatexites and metatexites with garnet, cordierite and
sillimanite-bearing melanosomes. Leucosomes seem to have mainly granitic s.s. compositions, but cm-thick bands
of leucotonalite were also found.
Major element geochemistry of granite samples shows the following ranges: 71.4% SiO2 74.2%; 0.74%
Fe2O3t 2.48%; 0.35% MgO 0.60%; 0.49% CaO 1.32%; 2.90% Na2O 3.11%; 4.70% K2O
5.47%; 1.17 ASI 1.36. Trace element data reveal a strong fractionation between highly incompatible LILE
and less incompatible HFSE (248 PM normalized Rb/Y 671) and between LREE and HREE (18.6 PM
normalized La/Lu 54.7). These features, in particular the peraluminous composition, the high K contents and
the distinct rare-earth fractionation suggest that the Oliveira de Azeméis granites are mostly the result of partial
melting of metasediments with a large pelitic component and that garnet is a likely residual phase.
Isotope geochemistry data show that the previously reported isochron should not correspond to a true age since
the 87Sr/86Sr(380Ma) obtained in the granite samples analysed in the present work are very low, varying from
0.6978 to 0.7063, with an average value of 0.7023, which are unrealistic in S-type granitic melts. Probably, the
380 Ma date is the consequence of mixing of different melt source components in the samples used in its
calculation. Using the granite whole-rock samples collected in this work, a 328 28 Ma errorchron (MSWD=4.0;
initial 87Sr/86Sr=0,7106 0.0045) is now obtained.
Assuming a typical syn-tectonic Variscan age of 320 Ma for the studied granites, 87Sr/86Sr and "Nd range
from 0.7100 to 0.7133 and from -6.5 to -7.9, respectively. A micaschist sample collected in this area displays
87Sr/86Sr(320Ma) = 0.7146 and "Nd(320Ma) = -9.2. Therefore, the Sr and Nd isotope composition agrees with
the clearly dominance of a melt component derived by anatexis of a metapelitic source.
Two samples of a garnet-bearing (and comparatively zircon-rich) diatexite show 87Sr/86Sr(320Ma) values (0.7120
and 0.7102) similar to those found in granites, but have higher "Nd(320Ma): -2.0 and -1.6. This may be explained
by either (a) the involvement of a different source in the genesis of this diatexite or (b) the occurrence of Nd
isotope disequilibrium during the melting process, with the preservation of high 143Nd/144Nd ratios in refractory
phases such as garnet and/or zircon.
A Rb-Sr wr-feldspar-biotite-muscovite isochron of 301.2 5.6 Ma (MSWD=0.42; initial
87Sr/86Sr=0,71516 0.00074) in a granite sample is interpreted as recording the final stage of the operation
of the shear zone, which was accompanied by mica recrystallization.
Funding: projects Petrochron (PTDC/CTE-GIX/112561/2009) and Geobiotec (PEst-C/CTE/UI4035/2011).
References
Pereira E. et allia (2007) – Carta Geológica 1/50000 de Oliveira de Azeméis. INETI, Lisboa.
Pinto M.S. (1979) – PhD Thesis. Univ. Leed
Spin torque ferromagnetic resonance with magnetic field modulation
We demonstrate a technique of broadband spin torque ferromagnetic resonance
(ST-FMR) with magnetic field modulation for measurements of spin wave
properties in magnetic nanostructures. This technique gives great improvement
in sensitivity over the conventional ST-FMR measurements, and application of
this technique to nanoscale magnetic tunnel junctions (MTJs) reveals a rich
spectrum of standing spin wave eigenmodes. Comparison of the ST-FMR
measurements with micromagnetic simulations of the spin wave spectrum allows us
to explain the character of low-frequency magnetic excitations in nanoscale
MTJs.Comment: Also see: http://faculty.sites.uci.edu/krivorotovgroup
Gluon saturation and the Froissart bound: a simple approach
At very high energies we expect that the hadronic cross sections satisfy the
Froissart bound, which is a well-established property of the strong
interactions. In this energy regime we also expect the formation of the Color
Glass Condensate, characterized by gluon saturation and a typical momentum
scale: the saturation scale . In this paper we show that if a saturation
window exists between the nonperturbative and perturbative regimes of Quantum
Chromodynamics (QCD), the total cross sections satisfy the Froissart bound.
Furthermore, we show that our approach allows us to describe the high energy
experimental data on total cross sections.Comment: 6 pages, 5 figures. Includes additional figures, discussion and
reference
The Safe-Port project: an approach to port surveillance and protection
SAFE-PORT is a recently started project addressing the complex issue of determining the best configurations of resources for harbour and port surveillance and protection. More specifically, the main goal is to find, for any given scenario, an adequate set of configuration solutions — i.e., number and type of sensors and equipments, their locations and operating modes, the corresponding personnel and other support resources — that maximize protection over a specific area.
The project includes research and development of sensors models, novel algorithms for optimization and decision support, and a computer-based decision support system (DSS) to assist decision makers in that task. It includes also the development of a simulation environment for modelling relevant aspects of the scenario (including sensors used for surveillance, platforms, threats and the environment), capable to incorporate data from field-trials, used to test and validate solutions proposed by the DSS. Test cases will consider the use of intelligent agents to model the behaviour of threats and of NATO forces in a realistic way, following experts’ definitions and parameters
Cosmic homogeneity: a spectroscopic and model-independent measurement
Cosmology relies on the Cosmological Principle, i.e., the hypothesis that the
Universe is homogeneous and isotropic on large scales. This implies in
particular that the counts of galaxies should approach a homogeneous scaling
with volume at sufficiently large scales. Testing homogeneity is crucial to
obtain a correct interpretation of the physical assumptions underlying the
current cosmic acceleration and structure formation of the Universe. In this
Letter, we use the Baryon Oscillation Spectroscopic Survey to make the first
spectroscopic and model-independent measurements of the angular homogeneity
scale . Applying four statistical estimators, we show that the
angular distribution of galaxies in the range 0.46 < z < 0.62 is consistent
with homogeneity at large scales, and that varies with
redshift, indicating a smoother Universe in the past. These results are in
agreement with the foundations of the standard cosmological paradigm.Comment: 5 pages, 2 figures, Version accepted by MNRA
Different faces of the phantom
The SNe type Ia data admit that the Universe today may be dominated by some
exotic matter with negative pressure violating all energy conditions. Such
exotic matter is called {\it phantom matter} due to the anomalies connected
with violation of the energy conditions. If a phantom matter dominates the
matter content of the universe, it can develop a singularity in a finite future
proper time. Here we show that, under certain conditions, the evolution of
perturbations of this matter may lead to avoidance of this future singularity
(the Big Rip). At the same time, we show that local concentrations of a phantom
field may form, among other regular configurations, black holes with
asymptotically flat static regions, separated by an event horizon from an
expanding, singularity-free, asymptotically de Sitter universe.Comment: 6 pages, presented at IRGAC 2006, Barcelona, 11-15 July 200
Phase diagram of a random-anisotropy mixed-spin Ising model
We investigate the phase diagram of a mixed spin-1/2--spin-1 Ising system in
the presence of quenched disordered anisotropy. We carry out a mean-field and a
standard self-consistent Bethe--Peierls calculation. Depending on the amount of
disorder, there appear novel transition lines and multicritical points. Also,
we report some connections with a percolation problem and an exact result in
one dimension.Comment: 8 pages, 4 figures, accepted for publication in Physical Review
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