1,721,089 research outputs found
Water in minerals of the continental lithospheric mantle and overlying lower crust: A comparative study of peridotite and granulite xenoliths from the North China Craton
No full textWater contents of clinopyroxene, orthopyroxene and olivine in mantle peridotite xenoliths from the Hannuoba and Nushan basaltic volcanoes (North China Craton) were determined by Fourier transform infrared spectroscopy (FTIR). Results show that contents vary from 50 to 155 ppm (H(2)O wt.) for clinopyroxene and from 20 to 55 ppm for orthopyroxene in Hannuoba peridotites, and from 5 to 355 ppm for clinopyroxene and from 5 to 140 ppm for orthopyroxene in Nushan peridotites. Water contents in clinopyroxene and orthopyroxene define a good positive correlation with a ratio of similar to 2.1. This, together with the roughly homogeneous distribution of hydrogen from core to rim region in pyroxene grains, suggests that they have preserved the initial water content of the mantle source. By contrast olivine in these peridotites contain very little, if any, water, which were probably caused by significant loss of H during their ascent due to the faster H diffusion in olivine relative to pyroxenes and by the relatively lower water content in the mantle source of Hannuoba and Nushan. Previous studies [Xia, Q.K., Yang, X.Z., Deloule, E., Sheng, Y.M., Hao, Y.T., 2006. Water in the lower crustal granulite xenoliths from Nushan, eastern China. journal of Geophysical Research, 111(B11202) doi: 10.1029/2006JB004296; Yang, XZ, Deloule, E., Xia, Q.K., Fan, Q.C., Feng, M., in press. Water contrast between Precambrian and Phanerozoic continental lower crust of eastern China.Journal of Geophysical Research doi:10.1029/2007JB005541] have shown that the water content ranges from 275 to 720 pprn for clinopyroxene, from 60 to 185 pprn for orthopyroxene and from 65 to 205 ppm for plagioclase in Hannuoba mafic granulite xenoliths, and from 200 to 2330 ppm for clinopyroxene, from 140 to 1270 pprn for orthopyroxene and from 145 to 900 pprn in plagioclase in Nushan maficgranulite xenoliths.The significant difference between the water content of peridotite and that of coexisting mafic granulite xenoliths, e.g. 0-100 ppm (mostly 200 ppm) for the estimated bulk concentration, demonstrates that the distribution of water in the deep continental lithosphere is vertically heterogeneous. This may be due to the different genesis of peridotites and granulites (the residue of partial melting vs. the product of melt crystallization). Based on these water data, we define the theological structure of the lowermost crust and uppermost mantle below the Hannuoba and Nushan regions. The viscosity (strength) of the underlying shallow mantle is similar, whereas the deep crust is relatively stronger at Hannuoba and weaker at Nushan. Difference in the viscosity structure may further influence the dynamical processes between different zones in this craton. (C) 2008 Elsevier B.V. All rights reserved
Oxygen and hydrogen isotope heterogeneity of clinopyroxene megacrysts from Nushan Volcano, SE China
No full textClinopyroxene megacrysts hosted by alkali basalts are a common form of mantle material found in Nushan volcano, Anhui Province, SE China. Compositionally, the clinopyroxene megacrysts are Al-rich augites with no intra-mineral chemical zoning. Oxygen and hydrogen isotopic ratios are constant within single clinopyroxene megacrysts, but display large variation among different samples (delta(18)O = 4.9 similar to 6.5parts per thousand; deltaD = -45 similar to -111parts per thousand). Regular covariations among major elements and REEs and similar Sr-87/Sr-86 (0.70309 to 0.70346) and Nd-143/Nd-144 (0.512846 to 0.513010) ratios indicate that these clinopyroxene megacrysts were derived from the same source and crystallized. in a parent magma similar in composition to their host basalts. Oxygen isotope data constrain the mantle source to be slightly O-18-depleted, likely resulting from interaction with low delta(18)O oceanic crust. Oxygen and hydrogen isotope data define two different compositional groups for the Nushan clinopyroxene megacrysts, suggesting that the parent magma experienced different petrogenetic processes in the course of its evolution to two groups (group I and group II) and crystallized in two different magma chambers possibly located at different depths. The slight increase of delta(18)O and H2O, and constant deltaD with Mg# values of group I megacrysts imply a simple fractional crystallization process; the relative large increase of delta(18)O and H2O and decrease of deltaD with Mg# values of group II megacrysts suggest possible lower continental crust contamination and magma degassing during their crystallization. High water contents of some group II clinopyroxenes decoupled from the others possibly resulted from interaction with host basaltic magmas during their ascent to the surface. Therefore, major element, REE content and O-H-Sr-Nd isotopic compositions suggest the following petrogenetic stages: (1) mantle source contamination due to interaction with low delta(18)O material likely being subducted oceanic crust; (2) crystallization of group I clinopyroxene by simple crystal fractionation; (3) crystallization of group II megacrysts during magmatic contamination by lower continental crustal material with high delta(18)O and water degassing from the evolving magma; (4) increase of water content and hydrogen isotopic composition re-equilibration of some group II megacrysts likely due to the interaction with host basaltic magmas during their ascent to the surface. (C) 2004 Elsevier B.V. All rights reserved
Latest Precambrian to Early Cambrian U-Pb zircon ages of augen gneisses from Calabria (Italy), with inference to the Alboran microplate in the evolution of the peri-Gondwana terranes
No full textIn situ U–Pb dating of zircons from five
samples of Calabrian augen gneisses shows that their
protoliths are Latest Precambrian to Early Cambrian
in age (562 ± 15, 547 ± 7, 540 ± 4, 539 ± 16 and
526 ± 10 Ma), and contain Archaean (3.1 Ga), Palaeoproterozoic
(1.7–2.4 Ga) and Neoproterozoic
(0.6–0.9 Ga) inheritance. Geochemical signature of
augen gneisses is typical of high-K calc-alkaline
post-collisional magmatism. Their Sr–Nd isotopic
compositions [0.7093 < (87Sr/86Sr)i < 0.7139; –3.2 <
eNd(t) < –5.4; 1.5 < TDM < 1.7 Ga] indicate the
involvement of a crustal component in significant
proportions. The Calabrian augen gneisses have,
therefore, to be distinguished from the orthogneisses of
Sardinia and northern Algeria, and from the porphyroids
of Sicily, which are Middle Ordovician. By contrast,
the Calabrian augen gneisses show a close
similarity to the Pan-African post-collisional granitoids
of the northern edge of the West African craton
(e.g. the Moroccan Anti-Atlas). This suggests a peri-
Gondwana origin and corroborates previous palaeogeodynamic
reconstructions attributing the Alboran
microplate to the northern margin of the West African
craton
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
Low water content of the Cenozoic lithospheric mantle beneath the eastern part of the North China Craton
Full text linkNominally anhydrous minerals in 46 peridotite xenoliths hosted by Cenozoic basalts
from five localities (Fangshan, Penglai, Qixia, Changle, and Hebi) of the eastern part of
the North China Craton (NCC) have been investigated by Fourier transform infrared
spectrometry (FTIR). The water contents (H2O wt %) of clinopyroxene (cpx),
orthopyroxene (opx), and olivine (ol) range from 27 to 223 ppm, 8 to 94 ppm, and ∼0 ppm,
respectively. On the basis of (1) the homogenous H2O content within single pyroxene
grains and (2) the equilibrium partitioning of H2O between cpx and opx, it is suggested
that the pyroxenes largely preserve theH2Ocontent of their mantle source, although possible
H loss during xenolith ascent cannot be excluded for ol. The recalculated whole‐rock
H2O contents, using mineral modes and assuming a partition coefficient of 10 for water
between cpx and ol, range from 6 to 56 ppm (average of 23 ± 13 ppm). In combination with
previously reported data, the recalculated whole‐rock water contents of peridotite xenoliths
(105 samples from 9 localities) hosted by Cenozoic basalts from the eastern part of the
NCC range from 6 to 85 ppm (average of 25 ± 18 ppm). The Cenozoic lithospheric mantle of
the eastern part of the NCC is therefore characterized by a low water content compared to
continental lithospheric mantle worldwide represented by typical cratonic and off‐cratonic
peridotites (normally 40–180 ppm, with average values of 119 ± 54 ppm and 78 ± 45,
respectively) and to oceanic mantle values (>50 ppm) inferred from MORB and OIB.
Peridotite xenoliths have low‐to‐moderate spinel Fe3+/SFe (0.02–0.34) and whole rock
DFMQ values (from −4.2 to 2.2, normally between −2.5 and 1.5), which are not correlated
with pyroxene H2O contents. Therefore, the low water contents cannot have resulted from
oxidation of the mantle xenoliths and may have been caused instead by heating from an
upwelling asthenosphere flow that acted in concert with NCC lithospheric thinning during
the late Mesozoic to early Cenozoic. If so, the present eastern NCC lithospheric mantle
represents essentially relict ancient lithospheric mantle after the thinning event, rather than
newly accreted and cooled asthenospheric mantle
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
Quartz-bearing rhyolitic melts in the Earth’s mantle
No full textThe occurrence of rhyolite melts in the mantle has been predicted by high
pressure-high temperature experiments but never observed in nature. Here we
report natural quartz-bearing rhyolitic melt inclusions and interstitial glass
within peridotite xenoliths. The oxygen isotope composition of quartz crystals
shows the unequivocal continental crustal derivation of these melts, which
approximate the minimum composition in the quartz-albite-orthoclase sys tem. Thermodynamic modelling suggests rhyolite was originated from partial
melting of near-anhydrous garnet-bearing metapelites at temperatures
~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite
with olivine converted lherzolite rocks into orthopyroxene-domains and
orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the
mantle provides direct evidence for element cycling through earth’s reser voirs, accommodated by dehydration and melting of crustal material, brought
into the mantle by subduction, chemically modifying the mantle source, and
ultimately returning to surface by arc magmatism
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