1,721,411 research outputs found
Pb isotope composition of recycled mantle pyroxenites: insights into the the HIMU source of oceanic basalts
No full textMelt-rock reactions in the mantle peridotites from Tuscany ophiolites
No full textHere we present mineralogical and geochemical data on mantle peridotites from the “Monti Rognosi” Jurassic
ophiolite from southern Tuscany. The ophiolites are included in Cretaceous-Eocene flysch formations and are mainly
composed of serpentinitized peridotites containing small gabbroic bodies. Locally the serpentinites preserve lenses of
relatively fresh mantle assemblages.
The samples selected for the present study are clinopyroxene-poor porphyroclastic lherzolites (Cpx = 5-7 vol%).
The clinopyroxene mainly occurs as large exsolved porphyroclasts, frequently corroded most likely in response to
reaction with a melt that crystallised thin gabbroic (Pl+Opx) veins and patches. The orthopyroxene occurs as large
deformed porphyroclasts characterized by undulose extinction and irregular outlines. Olivine typically forms corrosion
loops on orthopyroxene. Small amount of dark brown Cr-rich spinel with irregular shapes are generally rimmed by
plagioclase.
Olivine has forsterite contents between 88.7-89.6 mol%. The cores of (exsolved) orthopyroxene porphyroclasts have
Al2O3 and CaO contents up to 2.6 and 1.6 wt% respectively. Neoblasts have lower Al2O3 and CaO contents. Ca-in-Opx
thermometry (Brey & Kohler, 1990) provides T of 1070 ± 50°C and 960 ± 40°C for the porphyoclastic and neoblastic
Opx, respectively. The clinopyroxene (Mg# 88-90) has relatively high Al2O3 (about 7 wt%), TiO2 (0.5-0.8 wt%) and
Na2O (0.2-0.5 wt%) contents. Plagioclase is highly calcic (An = 81-90 mol%). Mg# and Cr# composition of olivine and
spinel plot off the mantle array. The highest Cr# values of the spinel (43-46) are associated with high TiO2 contents
(0.5-0.8 wt%). Similar chemical variations are commonly observed in plagioclase peridotites that experienced by
melt/rock reactions.
Major element whole-rock compositions depart from the trends of residual peridotites, being characterized by higher
Al2O3 and FeOtot, and lower CaO and SiO2. Hence, these chemical compositions are not compatible with products of
simple partial melting. Bulk rock REE compositions apparently record small degrees (2-4%) of fractional melting in the
spinel stability field, thereby arguing against clinopyroxene compositions. In fact, clinopyroxenes show a wide REE
heterogeneity, ranging from markedly to slightly LREE-depleted (CeN/SmN = 0.02-0.40), with nearly flat and variable
HREE contents (YbN = 10-22). We propose that the clinopyroxene compositional variations were related to infiltration
and impregnation of the Monti Rognosi residual peridotites by a MORB-type (Opx-saturated) melt in the plagioclase stability
field
Subduction-related ultradepleted melts in a nascent arc: geochemical and isotopic evidence from the intrusive sequence of the new caledonia ophiolite
Full text linkThe obducted sections of intraoceanic arc lithosphere represent a favourable setting to investigate the crust generation in nascent arcs. The New Caledonia ophiolite hosts a mafic-ultramafic igneous sequence overlying harzburgitic rocks that underwent very high degrees of partial melting in a forearc setting (Marchesi et al., 2009; Secchiari, 2016). This sequence has been interpreted as a crust-mantle section in a nascent arc (Pirard et al., 2013). The igneous rocks include dunites, wehrlites and mafic rocks (mainly gabbronorites). Dunites and wehrlites derive from melt/peridotite interactions involving primitive arc tholeiites and boninitic liquids, whereas the mafic rocks are the result of cumulus processes (Marchesi et al., 2009; Pirard et al., 2013). The samples of this study are olivine gabbronorites occurring in the upper part of the sequence as metre-sized sills. Subhedral Ca-rich plagioclase (An = 90-96 mol. %) is the dominant phase (50-80 vol %). Mg-rich olivine (5-15 vol %, Fo = 87-89 mol %) occurs as irregularly shaped crystals. Cpx (15-20 vol%) is generally rimmed by interstitial to poikilitic Opx (5-15 vol %). Cpx shows high Mg# (88-92), low Al 2 O 3 (1.5-2.4 wt%) and negligible TiO 2 and Na 2 O contents. Bulk rock compositions are characterized by high Mg# (86-93) and very low incompatible trace element contents (e.g. LREE concentrations mostly below 0.1 times chondrite). They show marked LREE depletion, nearly flat and low HREE (Yb N = 0.2-0.9) and positive Eu anomalies. The highly depleted nature of these rocks is mirrored by Cpx trace element composition. In contrast, fluid-mobile trace elements are relatively enriched and distinct Pb and Sr positive anomalies are observed in both whole rocks and Cpx. The calculated compositions of putative liquids in equilibrium with the most primitive gabbronorites are consistent with melting of a refractory peridotite source in the spinel stability field. The inferred melts have high Mg# of 75-76 and very low REE, Nb, Zr, Hf contents. On the other hand, Ba, Pb and Sr spikes in the calculated liquids argue for the involvment of a subduction-related component. Overall, the liquids show geochemical signatures akin to olivine boninite melts with lower absolute concentrations of LREE and other incompatible trace elements (Ba, Pb, Sr, Zr, Hf). We propose that the New Caledonia gabbronorites reflect injection of primitive, ultradepleted (“boninitelike”) magma batches in the lower forearc crust, possibly before aggregation, during the first phases of arc formation. The investigated gabbronorites provide homogeneous initial Nd isotope compositions (e Nd ranging between +8.2 and +9.4), coupled with high 208Pb/207Pb and 207 Pb /Pb ratios, falling in a domain intermediate between DMM and Pacific sediments. The heterogeneous Pb isotopic signature of the gabbronorites most likely reflect a subarc asthenospheric mantle source variably modified by subduction-related fluids
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
Evidence from the New Caledonia peridotites for contrasting behavior of highly siderophile and chalcophile elements in supra-subduction zone and normal upper mantle
No full textThe New Caledonia Peridotitic Nappe represents one of the largest exposures of mantle rocks worldwide and
provides unique insight on upper mantle processes. The Peridotitic Nappe is dominated by a harzburgite-dunite
association forming the Massif du Sud, but also includes minor lherzolites in the northern part of the island. The
geochemical features of the harzburgites require a complex history, including episodes of melting and local overprint by
fluids or hydrous melts in a supra-subduction zone setting, while lherzolites are relatively homogeneous, abyssal-type
peridotites (Secchiari, 2016). In order to examine how these mantle processes affected the behavior of highly
siderophile (HSE: Os, Ir, Ru, Rh, Pt, Pd, Au, Re) and chalcophile elements (S, Se, Te), a set of fully characterized (i.e.,
major and trace elements; Sr, Nd and Pb isotopes) peridotites has been studied. The lherzolites display homogeneous
187Os/188Osi (0.1273-0.1329), slightly subchondritic to chondritic Re/Os (0.04-0.11) and PGE, Au, Se and Te
abundances, similar to other lherzolites from normal upper mantle. Sulfur contents are variable (202-1268 mg/g) and
were likely increased by serpentinization. Harzburgites can be grouped in two sub-types. Type-A harzburgites
(epsilonNdi=9.3-13.3) have subchondritic 187Os/188Osi (0.1203-0.1254) coupled with low Os (0.554-1.513 ppb) and Re
contents (0.017-0.021 ppb) and somewhat fractionated HSE patterns. The HSE display lower concentrations than in
lherzolites, with Os-Ru enrichments over Ir (OsN/IrN = 1.6-1.9; RuN/IrN = 1.5-1.7) and positive anomalies of Pt and Au.
S-Se-Te have low contents (< 0.1 x primitive mantle) and approximately flat patterns, likely reflecting the presence of
small amounts of sulphides. These features are consistent with high melting degrees that led to sulphide exhaustion and
Os-Ru and Pt alloy stabilization. Type-B harzburgites (-0.8 ≤ εNdi ≤ +4.0) are fresh rocks showing chondritic to
suprachondritic measured 187Os/188Os (0.1273-0.1524) coupled with very low Os concentrations (0.003-0.277 ppb) and
high 187Re/188Os ratios of up to 30. The HSE have melt-like patterns with low concentrations of Os and Ir, strong
negative Pt anomalies and positive Au spikes. Sulfur, Se and Te concentrations are remarkably low (e.g., a few mg/g S).
HSE patterns are interpreted to reflect localized interaction and modification of type A harzburgites with oxidized (?)
subduction-related fluids or melts, which destabilized residual mantle sulphides and PGE alloys, causing strong
depletion of compatible PGE and S, Se, Te. The complex element patterns of these peridotites are unknown from
mantle rocks that were exhumed in extensional environments. The data suggest that HSE fractionations (e.g., Pt
anomalies and A
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
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