1,721,005 research outputs found
The Iceland plume in space and time: a Sr-Nd-Pb-Hf study of the North Atlantic rifted margin
No full textNew Sr–Nd–Pb–Hf data require the existence of at least four mantle components in the genesis of basalts from the North Atlantic Igneous Province (NAIP): (1) one (or more likely a small range of) enriched component(s) within the Iceland plume, (2) a depleted component within the Iceland plume (distinct from the shallow N-MORB source), (3) a depleted sheath surrounding the plume and (4) shallow N-MORB source mantle. These components have been available since the major phase of igneous activity associated with plume head impact during Paleogene times. In Hf–Nd isotope space, samples from Iceland, DSDP Leg 49 (Sites 407, 408 and 409), ODP Legs 152 and 163 (southeast Greenland margin), the Reykjanes Ridge, Kolbeinsey Ridge and DSDP Leg 38 (Site 348) define fields that are oblique to the main ocean island basalt array and extend toward a component with higher 176Hf/177Hf than the N-MORB source available prior to arrival of the plume, as indicated by the compositions of Cretaceous basalts from Goban Spur (~95 Ma). Aside from Goban Spur, only basalts from Hatton Bank on the oceanward side of the Rockall Plateau (DSDP Leg 81) lie consistently within the field of N-MORB, which indicates that the compositional influence of the plume did not reach this far south and east ~55 Ma ago. Thus, Hf–Nd isotope systematics are consistent with previous studies which indicate that shallow MORB-source mantle does not represent the depleted component within the Iceland plume [Thirlwall, J. Geol. Soc. London 152 (1995) 991–996; Hards et al., J. Geol. Soc. London 152 (1995) 1003–1009; Fitton et al., Earth Planet. Sci. Lett. 153 (1997) 197–208]. They also indicate that the depleted component is a long-lived and intrinsic feature of the Iceland plume, generated during an ancient melting event in which a mineral (such as garnet) with a high Lu/Hf was a residual phase. Collectively, these data suggest a model for the Iceland plume in which a heterogeneous core, derived from the lower mantle, consists of ‘enriched’ streaks or blobs dispersed in a more depleted matrix. A distinguishing feature of both the enriched and depleted components is high Nb/Y for a given Zr/Y (i.e. positive ΔNb), but the enriched component has higher Sr and Pb isotope ratios, combined with lower εNd and εHf. This heterogeneous core is surrounded by a sheath of depleted material, similar to the depleted component of the Iceland plume in its εNd and εHf, but with lower 87Sr/86Sr, 208Pb/204Pb and negative ΔNb; this material was probably entrained from near the 670 km discontinuity when the plume stalled at the boundary between the upper and lower mantle. The plume sheath displaced more normal MORB asthenosphere (distinguished by its lower Hf for a given Nd or Zr/Nb ratio), which existed in the North Atlantic prior to plume impact. Preliminary data on MORBs from near the Azores plume suggest that much of the North Atlantic may be ‘polluted’ not only by enriched plume material but also by depleted material similar to the Iceland plume sheath. If this hypothesis is correct, it may provide a general explanation for some of the compositional diversity and variations in inferred depth of melting [Klein and Langmuir, J. Geophys. Res. 92 (1987) 8089–8115] along the MAR in the North Atlantic
Mantle source evolution along the South Atlantic transect (31°S) records a transition from HIMU plume component to depleted MORB
No full textInteractions between mantle plumes and mid-ocean ridges create considerable spatial variation in composition along ridge axes. What is less well known is the temporal variation in MORB compositions along single mantle flow lines. IODP Expeditions 390/393/390C/395E recovered basaltic basement from seven sites along a flow line, the South Atlantic Transect (SAT), on the western flank of the mid-Atlantic Ridge (MAR) at ∼31°S. SAT basalts ≤49 Ma are tholeiitic with isotopic compositions similar to MORBs from the MAR between 25° and 28°S. Basement from the oldest SAT site (U1556; 61.2 Ma) is more complex, consisting of three stratigraphic sequences (SSA, SSB and SSC) ranging from MORB-like at the bottom (SSC) to Ocean Island Basalt (OIB)-like at the top (SSA); their isotopic compositions are distinct relative to both younger SAT basalts and the EM1-type Tristan–Gough plume that dominates the region, being more akin to HIMU. The presence of previously unrecognized HIMU mantle in this region is due to one or more ridge jumps that occurred west of the Walvis Ridge at ∼65 Ma. These ridge jumps relocated the spreading axis over a portion of the HIMU plume that had previously given rise to late-stage, off-axis HIMU magmatism adjacent to the Walvis Ridge. Upwelling beneath the spreading center progressively tapped a variably depleted source, reproducing it in reverse in the volcanic stratigraphy at Site U1556. Continued upwelling beneath the spreading center removed most of the HIMU plume material within ∼12 Myr, the time of Site U1558 (49.2 Ma).</p
High precision HF isotope measurements of MORB and OIB by thermal ionisation mass spectrometry: insights into the depleted mangle
No full textThe existing Hf isotope database for mid-ocean ridge basalts (MORB) is limited in both quantity and precision. Nevertheless, in Hf–Nd isotope space, MORBs show a wide variation in / over a relatively restricted range in /. The highest / ratios (?0.283355) within the MORB range are restricted to just four samples (<6.5% of total). Of these high / MORBs, three are from ridge segments adjacent to known active plumes and one is from a ridge segment for which a plume influence has been suggested. By comparison, MORBs from ‘normal' ridge segments show a more limited range in / ratios (0.283040 to 0.283311). We suggest that NMORB and the depleted MORB mantle reservoir (DMM) are characterised by a similarly limited range in / ratios. Furthermore, we suggest that the high / MORB-like basalts may ultimately be related to mantle plumes and represent melts of a depleted component entrained by the plumes before they traverse the shallow MORB mantle. We illustrate our preferred model with new hafnium isotope data on 11 MORB samples from the Atlantic and Pacific Oceans, two oceanic gabbros from the Indian Ocean (all collected away from known plume localities) and basalts associated with the Iceland and Azores plumes obtained using a new high precision thermal ionisation mass spectrometry (TIMS) technique. The new TIMS technique routinely yields beam intensities of 150–700 mV (total Hf beam of 2.8–13.5 V), allowing a routine internal precision on the measured / ratio of 0.002–0.006%, to be achieved using just 1–3 g of Hf separate. This represents a considerable improvement over the 0.008–0.056% internal precision quoted as typical for conventional single or triple filament TIMS analysis of Hf. The external reproducibility for the international Hf standard JMC 475 has also been significantly improved over conventional TIMS and is currently ~0.002%. This is comparable with the 0.003% external reproducibility currently obtained on the new Fisons Instruments Plasma 54 at the Ecole Normale Superieure de Lyon
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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