1,720,973 research outputs found
Whole rock analyses of mafic volcanic samples from western North America and from Parnaíba Basin, Brazil
No full textThis datset contains whole rock XRF and ICP-MS analyses of 255 mafic volcanic samples from western North America and one sample of the basaltic Mosquito Formation in the Parnaíba Basin, NE Brazil. Data are published in Klöcking et al. (2018, G-cubed) and Klöcking et al. (2018, GSL Special Publication), respectively. This work was funded by BP Exploration as part of the Parnaíba Basin Analysis Project
Cenozoic volcanism in Northeast Brazil and its links to the Cameroon Line
Get PDFCenozoic volcanism in Northeast Brazil occurs in three distinct
regions: the Macau-Queimadas volcanic lineament (MQVL) and the
Mecejana volcanic field, both in the Borborema Province, and the Fernando
de Noronha archipelago. They are the only significant occurrences of
Cenozoic magmatism along the South American passive margin and their
location matches the position of the Cameroon Line in Africa, to which they
were adjacent before the opening of the South Atlantic. The Cameroon Line
is one of the few and the largest occurrence of Cenozoic volcanism along the
African Atlantic passive margin. Volcanic activity in the two conjugate
margins postdate continental separation by 10s of millions of years. Here, the
Brazilian occurrences are compared with one another and with the
Cameroon Line using sixteen new 40Ar/39Ar ages obtained for Borborema,
which for the first time include Mecejana, major and trace element data and
87Sr/86Sr and 140Nd/139Nd isotopic ratios. Activity in Mecejana dates to the late
Eocene to early Oligocene (35.03 ± 0.28 to 30.80 ± 0.22 Ma). Volcanism in
the MQVL is synchronous with Mecejana but also continues intermittently to
the Miocene (12.39 ± 0.34 Ma), with a peak in the late Oligocene to the early
Miocene marked by activity in the local basins. Comparisons with other
published ages show that volcanism in the MQVL also continues into the late
Miocene and is synchronous with activity in Fernando de Noronha. There are
no age progression patterns in the region, and a deep mantle plume is not a
suitable model to explain local magmatism. Geochemical results show a
compositional continuum in Northeast Brazil, with variability controlled by
depth and degrees of partial melting of a stratified sublithospheric source that
includes both spinel and garnet lherzolite and extends across both
continental and oceanic lithospheres. Northeast Brazil volcanic occurrences
result from the same magmatic processes and should be treated as a single
volcanic province. They are also synchronous and geochemically virtually
identical to the Cameroon Line, indicating a continental lithospheric control in
their genesis. We propose a model that relies on edge-driven convection and
the rheological characteristics of the lithospheric thermal boundary layer
(TBL) to explain magmatism in both conjugate margins, where edge-driven
convection triggered by the local continental lithospheric morphology affects
the continental TBL and cause it to flow laterally towards the oceanic sector.
The intermediate rheology of the TBL explains the mainly asthenospheric
characteristics of the magmatic source in both conjugate margins, but with
the presence of subtle geochemical enrichment that must be caused by
relatively recent enrichment events. The model finds strong support in the
distribution of volcanism along the Atlantic passive margins, including
seamounts and other volcanic provinces that also straddle continental and
oceanic lithospheres such as the Canaries and the adjacent Moroccan
volcanism, and the New England seamount chain
Geochemical fingerprinting of Icelandic silicic Holocene tephra layers
Get PDFThe overall aim of this research project has been to develop a reference dataset of 19 Holocene silicic Icelandic tephra layers sourced from the Torfajökull, Askja, Katla, Öræfajökull and Hekla volcanic systems. The dataset comprises geochemical data (including major, trace and rare earth element data for bulk and glass phases collected by XRF, electron microprobe, ion probe and laser ablation ICP-MS) and physical data (including sedimentary logs, field photographs, distribution maps and GPS localities of reference sections).
Results indicate that Icelandic volcanic systems show unique geochemical signatures which result from the systems proximity to the active rifting zone and the proposed upwelling mantle plume that underlies the island. Within individual volcanic systems, eruptions produce tephra with distinct geochemical characteristics, which allow for the independent confirmation of tephra identity. The identification and discrimination of tephra layers can in some cases be achieved using major element chemistry (e.g. Hekla, H1104 – H5) while other tephra layers can only be discriminated using trace element chemistry (e.g. Torfajökull, Landnám and Gràkolla). Certain tephra layers however show near-identical geochemistry and therefore discrimination is not possible (e.g. Hekla, HA, HB, HC, HM, HN, HX, HY, HZ) without the incorporation of other proxy data.
Icelandic micro-tephra horizons are identified in soil, lacustrine and marine sedimentary sequences and are used for dating and correlation in Quaternary studies. Data collected for this project will facilitate reliable data comparison and tephra identification between proximal and distal localities across the North Atlantic region. The data may also contribute to the debate regarding the formation of silicic rocks within Iceland, particularly with regard to the Hekla central volcano. The geochemical data collected for this thesis shows distinct age-dependant geochemical sub-groups suggesting temporal sub-surface relocation of the Hekla magma source
Cosmos greenstone terrane: Insights into an Archaean volcanic arc, associated with komatiite-hosted nickel sulphide mineralisation, from U-Pb dating, volcanic stratigraphy and geochemistry
Get PDFThe Neoarchaean Agnew-Wiluna greenstone belt (AWB) of the Kalgoorlie
Terrane, within the Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton,
Western Australia, contains several world-class, komatiite-hosted, nickel-sulphide ore
bodies. These are commonly associated with felsic volcanic successions, many of
which are considered to have a tonalite-trondhjemite-dacite (TTD) affinity. The
Cosmos greenstone sequence lies on the western edge of the AWB and this previously
unstudied mineralised volcanic succession contrasts markedly in age, geochemistry,
emplacement mechanisms and probable tectonic setting to that of the majority of the
AWB and wider EGS. Detailed subsurface mapping has shown that the footwall to the
Cosmos mineralised ultramafic sequence consists of an intricate succession of both
fragmental and coherent extrusive lithologies, ranging from basaltic andesites through
to rhyolites, plus later-formed felsic and basaltic intrusions. The occurrence of thick
sequences of amygdaloidal intermediate lavas intercalated with extensive sequences
of dacite lapilli tuff, coupled with the absence of marine sediments or hydrovolcanic
products, indicates the succession was formed in a subaerial environment. Chemical
composition of the non-ultramafic lithologies is typified by a high-K calc-alkaline to
shoshonite signature, indicative of formation in a volcanic arc setting.
Assimilation-fractional
crystallisation modelling has shown that at least two compositionally
distinct sources must be invoked to explain the observed basaltic andesite to rhyolite
magma suite. High resolution U-Pb dating of several units within the succession
underpins stratigraphic relationships established in the field and indicates that the
emplacement of the Cosmos succession took place between ~2736 Ma and ~2653 Ma,
making it significantly older and longer-lived than most other greenstone successions
within the Kalgoorlie Terrane. Extrusive periodic volcanism spanned ~50 Myrs with
three cycles of bimodal intermediate/felsic and ultramafic volcanism occurring
between ~2736 Ma and ~2685 Ma. Periodic intrusive activity, related to the local
granite plutonism, lasted for a further ~32 Myrs or until ~2653 Ma. The Cosmos
succession either represents a separate, older terrane in its own right or it has an
autochthonous relationship with the AWB but volcanism initiated much earlier in this
region than currently considered. Dating of the Cosmos succession has demonstrated
that high-resolution geochronology within individual greenstone successions can be
achieved and provides more robust platforms for interpreting the evolution of ancient
mineralised volcanic successions. The geochemical affinity of the Cosmos succession
indicates a subduction zone was operating in the Kalgoorlie Terrane by ~2736 Ma,
much earlier than considered in current regional geodynamic models.
The Cosmos
volcanic succession provides further evidence that plate tectonics was in operation
during the Neoarchaean, contrary to some recently proposed tectonic models
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
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