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Magnetic characterization of the Venda Nova pluton (ES): tectono-structural implications for post-collisional plutonism in the Araçuaí Orogen (BR) and paleomagnetic contributions to Western Gondwana
No full textO Orógeno Araçuaí (AO) faz parte de um dos sistemas de orogenias Neoprotezóicas do Brasiliano que edificam o Gondwana. Sua fase final é marcada pela intrusão de plutons pós-colisionais formados pelo colapso-extensional do corpo orogênico. Estes corpos são excelentes candidatos para se estudar o estado de esforços no AO durante sua intrusão e, por serem gerados pelo último evento termotectônico relevante desde a consolidação do Gondwana nesta região, são ótimos alvos para estudos paleomagnéticos. O pluton Venda Nova (Sul do Espírito Santo, BR) consiste numa intrusão elíptica composta, basicamente, por um núcleo gabróico e um envelope sienomonzonítico bordejado a oeste por um estreito anel charnockítico, que intrude rochas orto e paraderivadas sin-colisionais do AO. Apesar de interpretado como um corpo pós-colisional, o mesmo não possui dados estruturais decisivos e datação geocronológica. O objetivo deste trabalho é caracterizar o pluton Venda Nova quanto a sua trama magnética, interpretando-a em função de seu alojamento e do estado de esforços no Orógeno, além de gerar um polo paleomagnético de qualidade que reflita a posição do Gondwana no final do Cambriano. Para isso, usufruiu-se de uma robusta caraterização magnética com curvas termomagnéticas, histereses magnéticas, magnetização remanente isotermal (IRM) e curvas reversas de primeira ordem (FORCs First Order Reversal Curves). Os dados obtidos apontam para uma distribuição de fases magnéticas, na maioria composta por magnetitas multidomínio. A investigação da lineação e foliação magnética foi efetuada por meio da análise de anisotropia de suscetibilidade magnética (ASM) e anisotropia de magnetização remanente anisterética (AARM). Os dados mostram um padrão majoritariamente concêntrico com maiores mergulhos da foliação nas bordas e lineações mais horizontalizadas na porção central do corpo indicando que forças gravitacionais dominavam o estado de esforços durante sua intrusão. As desmagnetizações a campos alternados (AF Alternating Field) e mista (AF até 10 mT seguido de térmica) foram eficientes para isolar a magnetização remanente característica das amostras (ChRM) sendo que, o tratamento misto indicou a presença de uma estreita faixa de temperaturas de bloqueio, com queda em torno de 560°C, indicando que magnetitas de domínio PSD/SD devem portar a magnetização estável das rochas do Venda Nova. O polo paleomagnético gerado por 198 amostras distribuídas em 22 sítios, localizado em 330,5°E; 3,2°N e (A95 = 3.80°), pode ser estatisticamente explicado pelo registro completo da variação paleosecular. Sua posição em comparação com curvas de deriva polar aparente do Gondwana insere-se próximo de polos com intervalo entre 510 Ma e 500 Ma, sendo essas idades coerentes com o período pós-colisional do Orógeno Araçuaí.The Araçuaí Orogen (AO) is part of one of the Brasilianos Neoprotezoic orogenic systems that formed Gondwana. Its final phase is marked by the intrusion of post-collisional plutons formed by the extensional collapse of the orogenic body. These bodies are excellent candidates to study the stress state in the AO during their intrusions and, also excellent targets for paleomagnetic studies as they were generated by the last relevant thermotectonic event since the consolidation of Gondwana in this area, becoming unaffected by later tectonic events. The Venda Nova pluton (Southern Espírito Santo, Brazil) consists of an elliptical intrusion, basically composed of a gabbroic core and a syenomonzonitic envelope bordered to the west by a narrow charnockitic ring, which intrudes ortho and syn-collisional rocks of the AO. Despite being interpreted as a post-collisional body, decisive structural data and geochronological data are yet lacking for the Venda Nova pluton. The objective of this work is to characterize the Venda Nova pluton in terms of its magnetic fabric, interpreting it according to its emplacement and the state of stress in the AO, and in addition to generate a reference paleomagnetic pole that can reflect the position of Gondwana at the end of the Cambrian. A robust magnetic characterization was accomplished through thermomagnetic curves, magnetic hysteresis, isothermal remanent magnetization (IRM) and first order reversal curves (FORCs). These data point to a distribution of magnetic phases mainly composed of multidomain magnetites. Investigation of magnetic lineation and foliation was carried out through the analysis of anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The data shows, majorly, a concentric pattern with greater dip in foliation at the edges and more horizontal lineations in the central portion of the body - indicating that gravitational forces dominated the state of stress during its intrusion. AF and mixed (AF up to 10 mT followed by thermal) demagnetization procedures were efficient to isolate the characteristic remanent magnetization (ChRM) of the samples - the mixed treatment showing a hard-shoulder-like intensity decay pattern close to 580°C indicating that PSD/SD magnetites must carry the stable magnetization of the Venda Nova rocks. The paleomagnetic pole, generated by 198 samples distributed in 22 sites, plots at 330.5° E; 3.2° N (A95 = 3.8°), and can be statistically explained by the complete record of paleosecular variation. Compared with the Gondwana\'s apparent polar wander path it is inserted near poles with an age interval between 510 Ma and 500 Ma, which is consistent with the post-collisional period of the Araçuaí Orogen
Structure and magnetic stability of natural iron oxides in the vortex-state
Get PDFOs minerais magnéticos guardam registros de campos magnéticos antigos, proporcionando pistas sobre a história geológica dos planetas. Vários processos físicos e químicos geram minerais magnéticos com diferentes tamanhos de grão e morfologias, afetando diretamente suas propriedades físicas e sua capacidade de registrar campos magnéticos. Enquanto os grãos mono-domínio uniformemente magnetizados (single domain, SD) são bem compreendidos, as propriedades dos grãos de pseudo-mono-domínio (pseudo-single domain, PSD), que são ligeiramente maiores e magnetizados de maneira não uniforme, permanecem pouco estudadas. Este trabalho combinou modelagem numérica, análises clássicas de magnetismo de rocha e técnicas químicas/tomográficas a partir de luz síncrotron para explorar como as características dos grãos em escala nanométrica influenciam suas propriedades magnéticas em escala macroscópica. O foco foi dado aos carbonatos de capa neoproterozoicos da América do Sul, com o intuito de estudar o crescimento de minerais magnéticos em ambientes sedimentares. Essas rochas exibem magnetização natural secundária com propriedades magnéticas únicas, como ciclos de histerese distorcidos e razões de histerese anômalas. Para abordar as histereses distorcidas, foi proposta uma função Gamma-Cauchy modificada para desmisturar componentes de coercividade, correlacionando-as com diferentes populações magnéticas nas amostras. Medições magnéticas e análises químicas in-situ utilizando luz síncrotron revelaram magnetita submicrométrica associada a minerais argilosos, indicando a formação de grãos PSD devido a processos desencadeados termicamente que ocorreram durante a formação do supercontinente Gondwana, e um resfriamento subsequente que bloqueou sua remanência. Uma vez identificados e caracterizados adequadamente os grãos PSD nas amostras estudadas, a investigação transitou da análise macroscópica para a nanoscópica. O potencial da Nano-tomografia Pticográfica Computadorizada de Raios-X (PXCT) e da modelagem micromagnética foi então explorado ao analisar as propriedades magnéticas de grãos de maghemita micrométrica, modelando magnetofósseis gigantes do Paleoproterozoico alinhados em uma estrutura semelhante a um magnetossomo. Este modelo demonstrou que cadeias de grãos multivórtices podem produzir magnetotaxia eficiente mesmo em intensidades de campo magnético baixas. Aplicando esses métodos às rochas carbonáticas, a PXCT identificou centenas de grãos de óxido de ferro com tamanhos e formas variadas. Alguns grãos, particularmente aqueles próximos à transição de SD para o estado de vórtice único (SV), exibiram estados de domínio múltiplos que eram instáveis para o registro de campo a longo prazo. Além disso, certos grãos mostraram probabilidades de ocupação de estados de domínio dependentes da temperatura, complicando a recuperação das intensidades de campos magnéticos passados. Em resumo, mostramos que técnicas avançadas de imagem e análise química, combinadas com modelos numéricos, podem desvendar uma ampla gama de propriedades magnéticas macroscópicas. Esta metodologia versátil tem o potencial de ser aplicada a diversos materiais terrestres e extraterrestres, oferecendo novas perspectivas sobre as histórias magnéticas dos planetas e de outros corpos celestesMagnetic minerals hold crucial records of ancient magnetic fields, providing insights into the geological history of planets. Various physical and chemical processes generate magnetic minerals with differing grain sizes and morphologies, directly affecting their physical properties and ability to record magnetic fields. While uniformly magnetized single domain (SD) grains are well understood, the properties of pseudo-single domain (PSD) grains, which are slightly larger and non-uniformly magnetized, remain poorly studied. This research combined numerical modeling, classical rock magnetic analysis, and synchrotron-based chemical and tomographic techniques to explore how the characteristics of nanoscale grains influence their macroscale magnetic properties. The focus was on Neoproterozoic cap carbonates from South America to study magnetic mineral growth in sedimentary environments. These rocks exhibit secondary natural magnetization with unique magnetic properties, such as distorted hysteresis loops and anomalous hysteresis ratios. To address distorted hysteresis, a modified Gamma-Cauchy function was proposed to unmix coercivity components, correlating them with different magnetic populations in the samples. Magnetic measurements and in-situ synchrotron-based chemical analysis revealed sub-micron magnetite associated with clay minerals, indicating the formation of PSD grains due to thermally triggered processes that occurred during the Gondwana supercontinent assembly, and a subsequent cooling that blocked their remanence. Once samples with targeted PSD grains were identified and properly characterized, the study transitioned from macroscale to nanoscale analysis. The potential of Ptychographic Xray Computed Nano-tomography (PXCT) and micromagnetic modeling was then investigated by analyzing the magnetic properties of micrometric maghemite grains, modeled after giant Paleoproterozoic magnetofossils aligned in a magnetosome-like structure. This model demonstrated that chains of multivortex grains can produce efficient magnetotaxis even at low magnetic field intensities. Applying these methods to the carbonate rocks, PXCT identified hundreds of iron oxide grains with varied sizes and shapes. Some grains, particularly those near the SD to single-vortex (SV) transition, exhibited multiple domain states that were unstable for long-term field recording. Additionally, certain grains showed temperature-dependent domain state occupancy probabilities, complicating the recovery of past magnetic field intensities. In summary, we show that advanced chemical and imaging techniques, combined with numerical models, can unravel a wide range of macroscopic magnetic properties. This versatile methodology has the potential to be applied to various terrestrial and extraterrestrial materials, offering new perspectives on the magnetic histories of planets and other celestial bodies
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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