1,720,991 research outputs found
Recharge flux to ocean-ridge black smoker systems: a geochemical estimate from ODP Hole 504b
No full textWe use the Sr-isotopic composition of rock and anhydrite, as a monitor of fluid composition, in DSDP/ODP Hole 504B to calculate the recharge flux to the axial high-temperature hydrothermal circulation. The fluid and rock Sr-isotope profiles are well fit by a tracer transport mass-balance model that approximates fluid-solid exchange by linear kinetics. The calculated time-integrated flux of 1.7+/-0.2 x 106 kg m-2 is significantly less than the 5 x 106 kg m-2 calculated from thermal models that assume all magma is intruded into a high-level magma chamber at the base of the sheeted dykes. Our low flux is consistent with the observed thermal structure as recorded by secondary alteration minerals in Hole 504B and the intrusion of magma as lenses distributed through the lower oceanic crust. It leaves open the questions as to how the lower oceanic crust cools and how seawater geochemical budgets balanc
Melt generation at very slow-spreading oceanic ridges: constraints from geochemical and geophysical data
No full textWe show that there is a strong and consistent correlation between geochemical and geophysical estimates of the amount of melt generated in the mantle beneath oceanic ridges. This correlation holds across all spreading rates and on scales down to the size of individual ridge segments. There is an abrupt decrease in the amount of melt generated at full spreading rates below ~20 mm/a. Our observations are consistent with the conclusion that <10% of the melt is frozen in the mantle before it reaches the crust and that serpentine probably represents only a small percentage of the material above the Moho. The melt is well mixed on a ridge segment scale, probably in high level magma chambers, but the melts remain distinct between segments. The rare earth element concentrations of basalts from very slow-spreading ridges are higher than those from normal oceanic ridges, which is directly indicative of reduced mantle melting, and they show characteristic light rare earth element enrichment, interpreted as caused by a deep tail of small percentage wet melting. The decrease in melt production at rates below ~20 mm/a points to the importance of conductive cooling inhibiting melting of the upwelling mantle at very slow-spreading centres
Ridge-flank hydrothermal circulation is not a cure for the imbalance in oceanic Sr-isotope budgets (abstracts of papers presented at AGU Fall Meeting, San Francisco, CA, 10-14 Dec 2001)
No full textConstraining hydrothermal fluxes: insights from the northern Oman ophiolite (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)
No full textLow degree melting under the Southwest Indian Ridge: the roles of mantle temperature, conductive cooling and wet melting
No full textBoth low mantle temperatures and conductive cooling have been suggested as the cause of the atypically thin oceanic crust and the incompatible element enrichment characteristic of very slow-spreading ridges. Here we present a model of melting under the Southwest Indian Ridge, which takes into account mantle temperature, conductive cooling, source composition and wet melting. The model parameters are constrained by oceanic crustal thickness, lava chemistry and isotopic composition and water content. The results suggest that conductive cooling to a depth of around 20 km, expected in areas with a full spreading rate of 15 mm/yr, is necessary to generate the Southwest Indian Ridge lava chemistry, but not that from faster spreading rate ridges at 23°N on the Mid Atlantic Ridge or 45°N on the Juan de Fuca Ridge. The mantle potential temperatures of ~1280°C, estimated for the Southwest Indian Ridge lavas are close to the global average of the upper mantle. Mantle water contents of 150-300 ppm can explain the observed melt water contents and allow sufficient melting at depth to explain the observed heavy rare earth element depletions in the melts
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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