1,721,008 research outputs found
Hydrothermal alteration in the Lowermost Ocean Crust: Unique constraints from Oman Drilling Project Hole CM1A
No full textThe hydrothermal alteration of the ocean crust at mid-ocean ridges and on ridge flanks contributes to global geochemical budgets by modifying the chemical and isotopic compositions of seawater and the lithosphere. In this research study, layered gabbros from the lowermost ocean crust preserved in the Cretaceous Samail ophiolite of Oman are investigated to characterise and quantify the types, modes and extents of hydrothermal fluid-rock interactions between fast-spreading lower ocean crust and seawater. Oman Drilling Project Hole CM1A recovered a 160 m-thick section of layered gabbros directly above the crust-mantle transition zone in Wadi Zeeb in the Wadi Tayin massif. A multi-stage hydrothermal alteration occurred predominantly from greenschist (550-350 °C), to prehnite – actinolite (350-200 °C) and zeolite (200-100°C) facies conditions. The Layered Gabbro Sequence is moderately to extensively altered by pervasive wall-rock alteration and cross-cut by five vein generations: 1) chlorite + serpentine and chlorite ± amphibole microcracks and veins; 2) epidote and clinozoisite veins; 3) prehnite veins and microcracks; 4) prehnite + calcite veins; 5) calcite veins and microcracks. Alteration halos have secondary mineral assemblages dominated by either chlorite or prehnite. The highest extents of alteration are within and around major zones of prehnite-rich cataclasis and hydrothermal brecciation. The great majority of this alteration occurred whilst the Wadi Tayin Massif remained submerged in the Tethyan ocean.Hydrothermal fluid-rock exchanges in the lowermost ocean crust are quantified through mass balance calculations using whole rock geochemical data. Gabbroic rocks display generally slight to substantial (10-200% change) additions of Al, Mg, Ca, H2O, C, Li, Co, Zn, Cs, Pb, Ta and Th relative to initial compositions, owing to extensive background and halo alterations. Prehnite-rich cataclasites exhibit moderate to extensive gains of CaO, Al2O3, H2O, CO2, Li, Cs, LREE, Ta, Th and U, but moderate losses of FeO, MnO, MgO, Na2O, K2O, Cu, Rb, Sr and Ba. This suggests enduring and intense fluid-rock exchanges with seawater-derived hydrothermal fluids downwelling in the lowermost ocean crust through such fault zones at 320-200 °C. The contributions of alteration in 1 km-thick lower ocean crust to global geochemical cycle were estimated from elemental mass changes, assuming a production rate of 3 km2/yr. Calculated annual chemical fluxes are more significant to the lowermost crust than to the oceans, with important contributions for Si, Al, Mg, Ca, H2O, C, Li, Co and Zn by faults, veins and halo alterations.The lowermost ocean crust hydrothermal alteration during oceanic spreading produced substantial modifications of initial gabbroic rocks 87Sr/86Sr up to altered 0.7055 – 0.7065 values. The final δ18O signatures of altered rocks were variably 18O-enriched or 18O-depleted compared to fresh magmatic value, with -1 to +1.6 ‰ exhibited by epidote minerals. These isotopic signatures reveal extensive exchange between rocks and evolved seawater-derived hydrothermal fluids at least between 400-150 °C. Seawater recharge conduits are likely represented by prehnite-rich veins and fault zones that form at 320-200 °C, whereas possible pathways for discharge of hot hydrothermal fluids at 350-400 °C correspond to epidote vein networks
Dataset supporting the thesis 'Hydrothermal alteration in the Lowermost Ocean Crust: Unique constraints from Oman Drilling Project Hole CM1A'
No full textWhole-rock and mineral separates analyses of OmanDP Hole CM1A and Wadi Zeeb samples
This geochemical dataset includes major (hydrous and anhydrous) and trace element compositions, volatiles contents, Sr and stable O and C isotopic compositions.</span
Affrontare la crisi nel settore "total look uomo". Il caso dell'Industria Adriatica Confezioni
No full textLa contabilità per il governo dell'economia agricola della nazione: Arrigo Serpieri e la riforma dei consorzi di bonifica
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
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