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Approaches and best practices for dating orogenic processes using <sup>40</sup>Ar/<sup>39</sup>Ar geochronology
No full textThe accumulation and transfer of argon (Ar) within minerals, rocks, and fluids in response to diverse metamorphic, deformation, and fluid histories makes 40Ar/39Ar geochronology attractive for investigating and constraining orogenic (mountain building) processes. There are many 40Ar/39Ar analytical approaches that can be used to explore these processes. The objective of this chapter is to provide an overview of Ar sources and mobility in rocks, lay out best practices for designing a 40Ar/39Ar study in an orogenic setting, and describe some of the most commonly used methods and interpretational tools available. Brief case studies are used to illustrate these concepts
The complexities in interpreting Argon isotopes:Chock-full of components
No full textThe 40K/40Ar and derived 40Ar/39Ar isotopic dating methods are used extensively for age determination of a wide variety of rock types. 40K/40Ar is primarily used in terrestrial samples that have experienced relatively simple geological histories. 40Ar/39Ar is used for both terrestrial and extraterrestrial samples, the latter including meteorites and those collected by planetary missions. The versatility of 40Ar/39Ar geochronology lies in the range of gas extraction techniques used (lasers, furnaces, and crushing release) and the number of different Ar components (i.e., of different origins) present. Often, the Ar present in a sample is a heterogeneous mix of two or more of these components making it more challenging to disentangle the 40K-derived radiogenic 40Ar. Moreover, the application of this method to lunar and martian meteorites can provide information about the planetary surface including surface exposure, impact, and alteration. This chapter describes the wealth of Ar components present in extraterrestrial samples, terrestrial basalts, and tektites. Selected examples are used to demonstrate how a combination of different Ar extraction techniques and graphical approaches, supported by textural and chemical data, are used to deconvolve some if not all of the Ar components that may be present in a sample.</p
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
Zircon Lu-Hf isotopic compositions of metaluminous and peralkaline A-type granitic plutons of the Emeishan large igneous province (SW China): Constraints on the mantle source
No full textVariations 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
The Emeishan large igneous province: A synthesis
No full textAbstractThe late Permian Emeishan large igneous province (ELIP) covers ∼0.3 × 106 km2 of the western margin of the Yangtze Block and Tibetan Plateau with displaced, correlative units in northern Vietnam (Song Da zone). The ELIP is of particular interest because it contains numerous world-class base metal deposits and is contemporaneous with the late Capitanian (∼260 Ma) mass extinction. The flood basalts are the signature feature of the ELIP but there are also ultramafic and silicic volcanic rocks and layered mafic-ultramafic and silicic plutonic rocks exposed. The ELIP is divided into three nearly concentric zones (i.e. inner, middle and outer) which correspond to progressively thicker crust from the inner to the outer zone. The eruptive age of the ELIP is constrained by geological, paleomagnetic and geochronological evidence to an interval of ≤3 Ma. The presence of picritic rocks and thick piles of flood basalts testifies to high temperature thermal regime however there is uncertainty as to whether these magmas were derived from the subcontinental lithospheric mantle or sub-lithospheric mantle (i.e. asthenosphere or mantle plume) sources or both. The range of Sr (ISr ≈ 0.7040–0.7132), Nd (ɛNd(t) ≈ −14 to +8), Pb (206Pb/204Pb1 ≈ 17.9–20.6) and Os (γOs ≈ −5 to +11) isotope values of the ultramafic and mafic rocks does not permit a conclusive answer to ultimate source origin of the primitive rocks but it is clear that some rocks were affected by crustal contamination and the presence of near-depleted isotope compositions suggests that there is a sub-lithospheric mantle component in the system. The silicic rocks are derived by basaltic magmas/rocks through fractional crystallization or partial melting, crustal melting or by interactions between mafic and crustal melts. The formation of the Fe-Ti-V oxide-ore deposits is probably due to a combination of fractional crystallization of Ti-rich basalt and fluxing of CO2-rich fluids whereas the Ni-Cu-(PGE) deposits are related to crystallization and crustal contamination of mafic or ultramafic magmas with subsequent segregation of a sulphide-rich portion. The ELIP is considered to be a mantle plume-derived LIP however the primary evidence for such a model is less convincing (e.g. uplift and geochemistry) and is far more complicated than previously suggested but is likely to be derived from a relatively short-lived, plume-like upwelling of mantle-derived magmas. The emplacement of the ELIP may have adversely affected the short-term environmental conditions and contributed to the decline in biota during the late Capitanian
Igneous Rock Associations 16. The Late Permian Emeishan Large Igneous Province
Get PDFThe Late Permian Emeishan large igneous province (ELIP) covers ~0.3x106 km2 of the western margin of the Yangtze Block and Tibetan plateau of SW China with displaced, correlative units in northern Vietnam (Song Da zone). The ELIP is of particular interest because it contains numerous world-class base metal deposits and is contemporaneous with the Late Capitanian mass extinction. The flood basalts are the signature feature of the ELIP but there are also picritic and silicic volcanic rocks and layered mafic–ultramafic and silicic plutonic rocks exposed. The ELIP is divided into three zones (i.e. inner, middle and outer) which correspond to a decrease in crustal thickness from the inner to the outer zone. The eruptive age of the ELIP is ~260 Ma and is constrained by paleomagnetic observations to an interval of ≤ 3 m.y. The presence of picritic and basaltic volcanic rocks is evidence for a high temperature regime; however, it is uncertain if these magmas were derived from sub-continental lithospheric mantle or sub-lithospheric mantle (i.e. asthenosphere or mantle plume) sources or both. The range of Sr (ISr ≈ 0.7040 to 0.7132), Nd (eNd(T) ≈ -14 to +8), Pb (206Pb/204PbPb1 ≈ 17.9 to 20.6) and Os (gOs ≈ -5 to +11) isotope values of the ultramafic and mafic rocks does not permit a conclusive answer to source origin but it is clear that some rocks were affected by crustal contamination. However, the identification of depleted isotope compositions suggests that there is a sub-lithospheric mantle component in the system. The ELIP is considered to be a mantle plume-derived large igneous province and may have contributed to ecosystem collapse during the latest Capitanian.SOMMAIRELa grande province ignée d’Emeishan de la fin du Permien (ELIP) s’étend sur environ 0,3 x 106 km2 à la marge ouest du bloc Yangtze et du plateau tibétain du sud-ouest de la Chine, avec des unités corrélatives déplacées dans le nord du Vietnam (zone de Song Da). L’ELIP est intéressant parce qu’il renferme de nombreux gisements de métaux de base de classe mondiale et qu’il est contemporain de l’extinction de masse de la fin du Capitanien. Les basaltes de plateau sont la signature géologique de l’ELIP, bien qu’on y rencontre aussi des roches volcaniques picritiques et siliciques ainsi que des formations stratifiées de roches mafiques à ultramafiques et plutoniques acides. L’ELIP est divisé en trois zones (interne, médiane et externe) correspondant à une diminution de l’épaisseur crustale de la zone interne vers la zone externe. L’éruption de l’ELIP date d’environ 260 Ma mais les observations paléomagnétiques limitent sa durée à ≤ 3 m.a. La présence de roches volcaniques picritiques et basaltiques indique un régime à haute température mais on ne sait pas si ces magmas proviennent de sources mantelliques lithosphériques sous-continentales ou sous-continentales mantelliques (c.-à-d. asthénosphère ou panache mantellique) ou des deux. La gamme des valeurs isotopiques Sr (ISr ≈ 0,7040 à 0,7132), Nd (eNd(T) ≈ –14 à +8), Pb (206Pb/204PbPb1 ≈ 17,9 à 20,6) et Os (gOs ≈ –5 à +11) des roches ultramafiques et mafiques ne permet pas de décider de l’origine de la source mais il est clair que certaines roches ont subis de contaminations crustales. Cependant l’existence de compositions isotopiques appauvries indique la présence dans le système d’une composante mantellique sous-lithosphérique. L’ELIP est considéré comme une grande province ignée dérivée d’un panache mantellique qui pourrait bien avoir contribué à l'effondrement de l’écosystème à la toute fin du Capitanien.
Igneous Rock Associations 21. The Early Permian Panjal Traps of the Western Himalaya
Get PDFThe Early Permian (290 Ma) Panjal Traps are the largest contiguous outcropping of volcanic rocks associated with the Himalayan Magmatic Province (HMP). The eruptions of HMP-related lava were contemporaneous with the initial break-up of Pangea. The Panjal Traps are primarily basalt but volumetrically minor intermediate and felsic volcanic rocks also occur. The basaltic rocks range in composition from continental tholeiite to ocean-floor basalt and nearly all have experienced, to varying extent, crustal contamination. Uncontaminated basaltic rocks have Sr–Nd isotopes similar to a chondritic source (ISr = 0.7043 to 0.7073; eNd(t) = 0 ± 1), whereas the remaining basaltic rocks have a wide range of Nd (eNd(t) = –6.1 to +4.3) and Sr (ISr = 0.7051 to 0.7185) isotopic values. The calculated primary melt compositions of basalt are picritic and their mantle potential temperatures (TP ≤ 1450°C) are similar to ambient mantle rather than anomalously hot mantle. The silicic volcanic rocks were likely derived by partial melting of the crust whereas the andesitic rocks were derived by mixing between crustal and mantle melts. The Traps erupted within a continental rift setting that developed into a shallow sea. Sustained rifting created a nascent ocean basin that led to sea-floor spreading and the rifting of microcontinents from Gondwana to form the ribbon-like continent Cimmeria and the Neotethys Ocean.RÉSUMÉLes Panjal Traps du début Permien (290 Ma) constituent le plus grand affleurement contigu de roches volcaniques associées à la province magmatique de himalayienne (HMP). Les éruptions de lave de type HMP étaient contemporaines de la rupture initiale de la Pangée. Les Panjal Traps sont essentiellement des basaltes, mais on y trouve aussi des roches volcaniques intermédiaires et felsiques en quantités mineures. La composition de ces roches basaltiques varie de tholéiite continentale à basalte de plancher océanique, et presque toutes ont subi, à des degrés divers, une contamination de matériaux crustaux. Les roches basaltiques non contaminées ont des contenus isotopiques Sr–Nd similaires à une source chondritique (Isr = 0,7043 à 0,7073; eNd (t) = 0 ± 1), alors que les roches basaltiques autres montrent une large gamme de valeurs isotopiques en Nd (eNd (t) = –6,1 à +4,3) et Sr (Isr = de 0,7051 à 0,7185). Les compositions de fusion primaire calculées des basaltes sont picritiques et leurs températures potentielles mantelliques (TP de ≤ 1450°C) sont similaires à la température ambiante du manteau plutôt que celle d’un manteau anormalement chaud. Les roches volcaniques siliciques dérivent probablement de la fusion partielle de la croûte alors que les roches andésitiques proviennent du mélange entre des matériaux de fusion crustaux et mantelliques. Les Traps ont fait irruption dans un contexte de rift continental qui s’est développé dans une mer peu profonde. Un rifting soutenu a créé un début de bassin océanique lequel conduit à une expansion du fond océanique et au rifting de microcontinents tirés du Gondwana pour former le continent rubané de Cimméria et l'océan Néotéthys
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