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Geochemical and Sr-Nd-Pb isotope investigation of the New Caledonia harzburgite: unravelling the evolution of a sub-arc mantle source
No full textThe New Caledonia Ophiolite hosts one of the largest obducted mantle sections worldwide, offering a unique opportunity to investigate key mantle processes. The mantle section is dominated by a harzburgite-dunite sequence but it also includes minor spl and pl lherzolites. Geochemical data indicate that the harzburgites suffered multiple melting episodes followed by localized interaction with fluids in a supra-subduction zone setting, while the lherzolites are akin to abyssal-type peridotites (Secchiari et al., 2016).
In order to constrain how these processes affected the behaviour of highly siderophile (HSE: PGE=Os-Ir-Ru-Rh-Pt-Pd+Au-Re) and chalcophile elements (S-Se-Te), a set of fully characterised peridotites (major, trace element, Sr-Nd-Pb isotopes) has been studied.
The lherzolites are slightly serpentinized and display chondritic to slightly suprachondritic 187Os/188Osi (0.1273-0.1329 at 53 Ma). The gently sloping HSE patterns with increasing depletion towards Au are similar to other oceanic or continental lherzolites. These features were inherited from sulphide melt-silicate partitioning during partial melting, melt infiltration and mixing of different generation of sulphides. S contents (202-1268 ppm) were likely increased by serpentinization, whereas Se/Te are similar to other lherzolites.
The harzburgites can be grouped in two sub-types. Type-A (+9.3≤Ndi≤+13.3) have subchondritic 187Os/188Osi (0.1203-0.1254), low Os (0.55-1.51 ppb) and very low Re/Os. Their HSE patterns display strong fractionations, enriched Os-Ir-Ru segments and Pt-Au positive spikes. S-Se-Te are often below the detection limit. These patterns can be ascribed to high melting degrees, leading to sulphide exhaustion and PGE alloys stabilization.
Type-B harzburgites (-0.8≤Ndi≤+4.0) show chondritic to suprachondritic measured 187Os/188Os (0.1273-0.1524), notably low Os-Ir contents (0.003-0.277 ppb) and highly variable 187Re/188Os (2-30). The “melt-like” HSE patterns exhibit strongly fractionated Os-Ir-Ru (OsN/RuN=0.02-0.46), negative Pt anomalies and positive Au spikes. S-Se-Te are close to or below the detection limit. We interpret these compositions as reflecting localized modification of type-A harzburgites by subduction-related fluids and/or hydrous melts, leading to partial destabilization of Os-Ir rich alloys due to high fO2.
Our work suggests that some of the features shown by arc lavas (e.g., positive Pt spikes) may mirror the geochemical signature of the sub-arc mantle.
REFERENCES
Secchiari, A., Montanini, A., Bosch, D., Macera, P., Cluzel, D. (2016): Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr–Nd isotope data. Lithos, 260, 28-4
Highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle: constraints from the New Caledonia ophiolite
No full textExposures of arc-related mantle sections preserved in ophiolites constitute a natural laboratory for investigating subduction zone processes. The New Caledonia ophiolite consists primarily of harzburgites, locally overlain by mafic-ultramafic cumulates, and minor spinel and plagioclase lherzolites (Secchiari et al., 2016). Recent studies have shown that the harzburgites and the associated cumulates represent a crust-mantle transect formed in a nascent arc environment (Marchesi et al., 2009; Pirard et al., 2013; Secchiari et al., submitted). However, scarce geochemical data are available for the harzburgites, so that the processes recorded by this mantle section are still poorly constrained.
In order to unravel the evolution of the New Caledonia harzburgite, a comprehensive geochemical dataset (major and trace element, Sr-Nd-Pb isotopes) has been obtained on a new set of fresh samples unaffected by serpentinization.
The studied harzburgites are low-strain tectonites showing porphyroclastic textures, locally grading into promylonitic textures. They exhibit a refractory nature, as attested by the notable absence of primary clinopyroxene, very high Fo content of olivine (90.9-92.9 mol.%), high Mg# of orthopyroxene (89.8-94.2) and Cr# of spinel (39-71). Thin films of undeformed clinopyroxene with very low Na2O (0.03-0.13 wt.%) and TiO2 (0.04-0.10 wt.%) contents have been also recognized in association with Al2O3- (0.88-1.53 wt.%) and CaO- (0.37-0.97 wt.%) poor secondary orthopyroxene.
The harzbugites display U-shaped REE profiles (GdN/YbN=0.06-0.48) with remarkably low REE concentrations (< 0.1 chondritic values), in the range of modern subduction zone peridotites. Geochemical models shows that the HREE composition of the harzburgites can be reproduced with high degrees (up to ~25%) of fractional melting of a DMM source in the spinel stability field. Extended trace element diagrams highlights depleted compositions coupled with strong positive anomalies for Pb and FME (i.e. Cs, Ba, Sr).
Nd isotopic ratios range from poorly to slightly radiogenic (-0.8≤Ndi≤+13.3) and negatively correlate with Sr isotopes (0.70257≤87Sr/86Sr≤0.70770). Pb isotopes cover a wide range, trending from DMM toward enriched, sediment-like, compositions.
We interpret the geochemical signature displayed by the New Caledonia harzburgite as reflecting a highly depleted sub-arc mantle source variably modified by fluid input and/or hydrous melts percolation during Eocene subduction.
REFERENCES
Marchesi, C., Garrido, C.J., Godard, M., Belley, F., Ferré, E. (2009): Migration and accumulation of ultra-depleted subduction-related melts in the Massif du Sud ophiolite (New Caledonia). Chem Geol, 266, 3-4, 171-186
Pirard, C., Hermann, J., O’ Neill H. (2013): Petrology and Geochemistry of the Crust–Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific. J Petrol, 54, 9, 1759–1792
Secchiari, A., Montanini, A., Bosch, D., Macera, P., Cluzel, D. (2016): Melt extraction and enrichment processes in the New Caledonia lherzolites: Evidence from geochemical and Sr–Nd isotope data. Lithos, 260, 28-43
Secchiari A., Montanini, A., Bosch, D., Macera, P., Cluzel, D. (2018): The contrasting geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting. Submitt. to Contrib Mineral Pet
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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