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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
Geochemical and Sr-Nd-Pb isotope investigation of the New Caledonia peridotite nappe: unravelling the history of a poorly known mantle section.
No full textPlum-cake subcontinental mantle beneath SE Alps as resulting from the geochemistry of mantle xenoliths.
No full textThe New Caledonia mantle section: tracking source depletion and contamination processes in a suprasubduction setting.
No full textSubduction-related ultradepleted melts in a nascent arc: geochemical and isotopic evidence from the intrusive sequence of the New Caledonia ophiolite.
Full text linkNew insights on highly siderophile and chalcophile element behaviour in abyssal-type and supra-subduction zone mantle sections of the New Caledonia ophiolite
No full textThe geochemical message from the New Caledonia gabbronorites: insights on depletion and contamination processes of the sub-arc mantle in a nascent arc setting.
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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
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