As brechas de origem ígnea: revisão e proposta para uma classificação geológica

Com base em observações no Planalto de Poços de Caldas, e numa revisão bibliográfica, apresenta-se proposta para a classificação das brechas subaéreas e intratelúricas de origem ígnea. Engloba-se no termo "brecha ígnea" todas as rochas clásticas de origem ígnea, cujos clastos (de angulosos até arredondados) sejam maiores do que 2 mm. A classificação proposta divide as brechas em vulcânicas subaéreas, vulcânicas Intratelúricas (de conduto) e plutônicas. As brechas vulcânicas subaéreas são as autoclásticas de derrame (incluindo as de ajuste, de talus e de colapso externo e interno de domos vulcânicos), as piroclásticas aerotransportadas (tufitos, lapillitos), os depósitos piroclásticos de fluxo (entre os quais se encontram os ignimbritos e os depósitos freato-magmáticos de "maare") e as brechas epigenéticas associadas ao vulcanismo (entre as quais os lahars são as mais importantes). As brechas subvulcânicas de conduto compreendem as autoclásticas (de fricção, etc.), e xenolíticas de conduto (com matriz de rocha vulcânica) e o importante grupo de brechas de conduto que resultam da atividade explosiva. Estas ultimas dividem-se principalmente em brechas nãointrusivas (de colapso, do tipo "shatter" ou de fragmentação, e as que se encontram como brechas "quebradas" ou zonas "craquelées") e intrusivas de conduto (do tipo tufisítico, geralmente com matriz predominante ou pelo menos abundante, e os aglomerados e brechas intrusivas, geralmente com pouca matriz); utilizando características estruturais, apresenta-se um esquema de geração destes vários tipos, baseado em modelos da literatura. As brechas plutônicas são divididas em brechas protoclásticas de contato e fricção (cataclasitos e milonitos), agmatitos, brechas de substituição e vários tipos de diques e condutos de brecha (estruturalmente idênticas às brechas subvulcânicas de conduto, porém associadas a corpos plutônicos). Revisam-se os termos texturais e estruturais utilizados para descrever as brechas e propõe-se adicionalmente que se enfatizem na descrição tanto as características dos clastos (tipo, seleção, arredondamento, tamanho, etc.) como também as relações clastos vs matriz; identificam-se assim três tipos de brechas: clasto-sustentada, sustentada por clastos e matriz, sustentada por matriz ("clast-supported", "clast-matrix supported" e "matrix-supported"). Em todos os casos, revisam-se os possíveis mecanismos que levam à formação do material brechóide.The nomenclature and classification of breccias of igneous origin has been, and still is, a subject of considerable debate, which reflects the ongoing discussions about their formation and emplacement. This is especially valid for those breccias emplaced in plutonic or subvolcanic environments. During field studies in the large alkaline massif of Poços de Caldas, southern Brazil, the author was confronted by a seemingly bewildering variety of breccias and other pyroclastic rocks, thus providing the impetus for a revision of the subject and the proposal for simple ways of describing and classifying such breccias. Special emphasis was placed on the important group of breccias emplaced at depth ("subsurface breccias") which are notoriously difficult to catalogue and to interpret; subaqueous types are non-existent, or at least very rare, in Poços de Caldas, and are not dealt with in the present discussion. Breccias should be clearly characterized borrowing terminology and methods from the field of study of fragmental supracrustal (epiclastic and pyroclastic) rocks, both for descriptive purposes (e.g., description of form, sorting, roundness, petrographic variety, etc.) as well as for size ranges; in addition, other textural terms, such as "clast-supported", "clast-matrix supported", and "matrix-supported", should be used to describe the clast-to-matrix relationship. A breccia of igneous origin is defined as a clastic rock with suitable amounts (>;25%) of (angular to rounded) fragments greater than 2 mm which originates as a direct response to igneous phenomena. Genetic aspects of breccia formation are not considered a satisfactory guideline for classification purposes, mostly because breccia-forming processes are still very poorly undeir stood. The mechanisms, which have been proposed in the literature, range from well- researched and qualified hypothesis (such as the "implosion-fluidization" model of Biondi, 1979) to highly unlikely speculations. Still lacking is the ability to take the guesswork out of interpretations of texture and structure of breccias. In the proposed classification, emphasis is placed on descriptive and geologic aspects, in order to define the geologic environment in which the breccia was emplaced. On the one hand, this procedure introduces a standard nomenclature, independent of any genetic meaning (or misconception); additionally, emphasis on detailed description of the geologic environment may ultimately lead to a sounder basis for interpretation. The simplest classification of breccias is therefore threefold, recognizing only "volcanic", " subvolcanic" and "plutonic breccias". The first group comprises all deposits related to volcanic supracrustal activity, including various types of autoclastic flow breccias (including rubble found on advancing block lavas, and talus deposits and collapse fragments shed during cooling and growth of subaerially exposed volcanic domes), tephra deposits, pyroclastic flows and epigenetic sheets formed as a direct response to volcanism (e.g., lahars). The second group, associated with subvolcanic phenomena, includes types found in small, more or less confined bodies (pipes, necks, dikes, sills, irregular bodies: "conduits" in the broadest sense). Among these are autoclastic (e.g.,"friction" breccias) and xenolithic varieties (fragments enclosed within a groundmass of non-fragmented volcanic rocks). The most significant types, the economically important conduit breccias associated with explosive activities, are subdivided, merely for descriptive purposes, into non-intrusive varieties ("shatter" breccias, with angular fragments; "cracked" or "craquelée" rock; collapse breccias, with fragments derived from nearby wall-rocks) and the clearly intrusive types (including both conduit breccias with more or less angular fragments, accompanied by varying but usually important amounts of matrix, and conduit agglomerates, with rounded fragments and little or no matrix). There has been wide speculation as to the proper generating mechanism, fluidization usually being advanced as the main emplacement cause (probably triggered by an initial "implosion"). In well-defined subvolcanic conduits, it is observed that shatter breccias, "cracked" rock, and conduit breccias are distributed in a roughly zoned pattern; the first two types are restricted to wall-rock areas and are probably formed as a response to explosion-related com pressions and decompressions, while the conduit breccias appear as centrally located bodies. The group of "plutonic breccias" are divided into border types (protoclastic and friction breccias marginal to plutons, which are, structurally, cataclasites or milonites), agmatites (fragments in a groundmass of plutonic rock), "metasomatic" substitution varieties, and plutonic conduit breccias (with features similar to those encountered in the equivalent subvolcanic types)

A correção do drift instrumental em ICP-AES com espectrômetro seqüencial e a análise de elementos maiores, menores e traços em rochas

Analytical results of major, minor and trace elements are presented, which were obtained on international rock reference materiais during the development of work routines in the ICP-AES Laboratory, Mineralogy and Petrology Department, Instituto de Geociências, São Paulo University. Analyses were performed on an ARL-3410 model with sequential spectrometer using solutions obtained by alkaline fiision of 0.25 g of rock powder with 0.75 g of lithium tetra and metaborate (eutectic mixture), with a 1:1000 sample dilution (method used at the Imperial College, London). In order to minimize the effects of intensity drift, caused mostly by variations in the uptake conditions of the solution, strict procedures of drift controi and off-line correction were adopted. The results show good repeatability and maintain long-term precision, demonstrating that reliable analyses can be obtained by careful correction procedures. However, as it is not possible to controi short-term drift in sequential spectrometers, most of the analyses add up to between 98 and 101%; key major element ratios are nevertheless maintained, ensuring the quality of the analysis. The accuracy is reliable for the analyzed elements, which, besides major and minor elements, also include Zn, Ni, Cr, V, Zr, Sc, Y, La, Ba and Sr, and even for those notoriously difficult to analyze by ICP-AES, such as K, P and La.Apresentam-se resultados de análises químicas de elementos maiores e traços em materiais de referência de rochas, obtidos durante o desenvolvimento de rotina analítica no Laboratório de Química e ICP-AES do Departamento de Mineralogia e Petrologia do Instituto de Geociências da USP. As análises, realizadas em equipamento ARL-3410 dotado de espectrômetro seqüencial, foram efetuadas em soluções obtidas por fusão alcalina, com misturas de 0,75 g de tetra e metaborato de lítio e de 0,25 g de pó de rocha, e diluição final dos analitos de 1:1000 (metodologia do Imperial College, Londres). Para mmimizar os efeitos de drift, provenientes especialmente de variações nas condições de introdução da amostra no plasma, adotam-se procedimentos rígidos de controle e correção off-line da flutuação do sinal. Os resultados apresentam boa repetibilidade e precisão a longo intervalo, refletindo a eficiência usualmente alcançada pelo procedimento adotado. Em vista da dificuldade de corrigir o drift de pequeno intervalo em plasmas dotados de espectrômetro seqüencial, os fechamentos das análises são variáveis entre 98 e 101%, mas as relações entre os elementos maiores se mantêm constantes, garantindo a qualidade dos resultados. A exatidão é satisfatória para todo o conjunto de elementos por ora estudado (além dos maiores e menores, ainda Zn, Ni, Cr, V, Zr, Sc, Y, La, Sr e Ba), mesmo para aqueles de determinação mais difícil no ICP-AES, como K, P e La

Identification of storm surge events over the German Bight from atmospheric reanalysis and climate model data

A new procedure for the identification of storm surge situations for the German Bight is developed and applied to reanalysis and global climate model data. This method is based on the empirical approach for estimating storm surge heights using information about wind speed and wind direction. Here, we hypothesize that storm surge events are caused by high wind speeds from north- westerly direction in combination with a large-scale wind storm event affecting the North Sea region. The method is calibrated for ERA-40 data, using the data from the storm surge atlas for Cuxhaven. It is shown that using information of both wind speed and direction as well as large-scale wind storm events improves the identification of storm surge events. To estimate possible future changes of potential storm surge events, we apply the new identification approach to an ensemble of three transient climate change simulations performed with the ECHAM5/MPIOM model under A1B greenhouse gas scenario forcing. We find an increase in the total number of potential storm surge events of about 12 % [(2001–2100)–(1901–2000)], mainly based on changes of moderate events. Yearly numbers of storm surge relevant events show high interannual and decadal variability and only one of three simulations shows a statistical significant increase in the yearly number of potential storm surge events between 1900 and 2100. However, no changes in the maximum intensity and duration of all potential events is determined. Extreme value statistic analysis confirms no frequency change of the most severe events

On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs

The electronic properties of shallow acceptors in p-doped GaAs{110} are investigated with scanning tunneling microscopy at low temperature. Shallow acceptors are known to exhibit distinct triangular contrasts in STM images for certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to identify their energetic origin and behavior. A crucial parameter - the STM tip's work function - is determined experimentally. The voltage dependent potential configuration and band bending situation is derived. Ways to validate the calculations with the experiment are discussed. Differential conductivity maps reveal that the triangular contrasts are only observed with a depletion layer present under the STM tip. The tunnel process leading to the anisotropic contrasts calls for electrons to tunnel through vacuum gap and a finite region in the semiconductor.Comment: 11 pages, 8 figure

Upper limits for a narrow resonance in the reaction p + p -> K^+ + (Lambda p)

The reaction pp -> K^+ + (Lambda p) has been measured at T_p = 1.953 GeV and \Theta = 0 deg with a high missing mass resolution in order to study the Lambda p final state interaction. Narrow S = -1 resonances predicted by bag model calculations are not visible in the missing mass spectrum. Small structures observed in a previous experiment are not confirmed. Upper limits for the production cross section of a narrow resonance are deduced for missing masses between 2058 and 2105 MeV/c^2.Comment: 8 pages, 5 figure

Study of the production mechanism of the eta meson by means of analysing power measurements

Information about the production mechanism of the eta meson in proton-proton collisions can be inferred by confronting the experimental studies on the analysing power for the p(pol)p --> pp eta reaction with the theoretical predictions of this observable. Results show that the predictions of pure pseudoscalar- or vector meson exchange model are insufficient to describe the analysing powers.Comment: 4 pages, 4 figures, Presented at MESON 2006: 9th International Workshop on Meson Production, Properties and Interactions, Cracow, Poland, 9-13 Jun 200

Electromechanically active pair dynamics in a Gd-doped ceria single crystal

Oxygen-defective ceria, e.g. Gd-doped ceria, shows giant electromechanical properties related to a complex local rearrangement of its lattice. Although they are not entirely identified, the electroactive mechanisms arise from cation and oxygen vacancy (V-O) pairs (i.e. Ce-V-O), and the local structural elastic distortion in their surroundings. Here, we study the geometry and behaviour of Ce-V-O pairs in a grain boundary-free bulk Ce0.9Gd0.1O1.95 single crystal under an AC electric field of ca. 11 kV cm(-1). The analysis was carried out through X-ray absorption spectroscopy (XAS) techniques at the Ce L-III edge. Using Density Functional Theory (DFT) calculations, we investigated the effects of the strain on density of states and orbitals at the valence band edge. Our research indicates that electrostriction increases at low temperatures. The electromechanical strain has a structural nature and can rise by one order of magnitude, i.e., from 5 x 10(-4) at room temperature to 5 x 10(-3) at -193 degrees C, due to an increase in the population of the electrically active pairs. At a constant V-O concentration, the material can thus configure heterogeneous pairs and elastic nanodomains that are either mechanically responsive or not

Changing Northern Hemisphere storm tracks in an ensemble of IPCC climate change simulations

Winter storm-track activity over the Northern Hemisphere and its changes in a greenhouse gas scenario (the Special Report on Emission Scenarios A1B forcing) are computed from an ensemble of 23 single runs from 16 coupled global climate models (CGCMs). All models reproduce the general structures of the observed climatological storm-track pattern under present-day forcing conditions. Ensemble mean changes resulting from anthropogenic forcing include an increase of baroclinic wave activity over the eastern North Atlantic, amounting to 5%–8% by the end of the twenty-first century. Enhanced activity is also found over the Asian continent and over the North Pacific near the Aleutian Islands. At high latitudes and over parts of the subtropics, activity is reduced. Variations of the individual models around the ensemble average signal are not small, with a median of the pattern correlation near r = 0.5. There is, however, no evidence for a link between deviations in present-day climatology and deviations with respect to climate change

Wind gust estimation for Mid-European winter storms: towards a probabilistic view

Three wind gust estimation (WGE) methods implemented in the numerical weather prediction (NWP) model COSMO-CLM are evaluated with respect to their forecast quality using skill scores. Two methods estimate gusts locally from mean wind speed and the turbulence state of the atmosphere, while the third one considers the mixing-down of high momentum within the planetary boundary layer (WGE Brasseur). One hundred and fifty-eight windstorms from the last four decades are simulated and results are compared with gust observations at 37 stations in Germany. Skill scores reveal that the local WGE methods show an overall better behaviour, whilst WGE Brasseur performs less well except for mountain regions. The here introduced WGE turbulent kinetic energy (TKE) permits a probabilistic interpretation using statistical characteristics of gusts at observational sites for an assessment of uncertainty. The WGE TKE formulation has the advantage of a ‘native’ interpretation of wind gusts as result of local appearance of TKE. The inclusion of a probabilistic WGE TKE approach in NWP models has, thus, several advantages over other methods, as it has the potential for an estimation of uncertainties of gusts at observational sites

Determination of Deuteron Beam Polarizations at COSY

The vector and tensor polarizations of a deuteron beam have been measured using elastic deuteron-carbon scattering at 75.6 MeV and deuteron-proton scattering at 270 MeV. After acceleration to 1170 MeV inside the COSY ring, the polarizations of the deuterons were checked by studying a variety of nuclear reactions using a cluster target at the ANKE magnet spectrometer placed at an internal target position of the storage ring. All these measurements were consistent with the absence of depolarization during acceleration and provide a number of secondary standards that can be used in subsequent experiments at the facility.Comment: 12 pages, 13 figure