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Petrography and geochemistry of the Eugui magnesite deposit (Western Pyrenees, Spain): Evidence for the development of a peculiar zebra banding by dolomite replacement
No full textThe Eugui-Asturreta magnesite deposit (Western Pyrenees, Spain) forms a discoidal body with a maximum thickness of 130 m, located within a folded Namurian carbonate sequence. The magnesite rocks are composed of lens-shaped crystals (mas 8 cm) arranged in black and white bands (zebra structure). The morphology and textural characteristics of magnesite and the structural relationships between magnesite and dolostone host rocks indicate that magnesite replaced the host dolostone. The magnesite crystals replaced the host dolostones, growing perpendicularly from stylolites and meeting between two adjacent stylolite sets. The portions of the crystals close to the stylolites are black colored because incorporated carbonaceous matter and clay laminations of the replaced dolostone (incorporative growth), whereas the crystal terminations are white because impurities have been displaced and lie at an intercrystalline position and into the meeting zone (displacive growth). These texture differentiations could be interpreted by changes in the ratios of dolomite dissolution versus magnesite precipitation due to increasing amounts of Ca released during the replacement process. Late dolomite is commonly pseudomorphous after magnesite crystals. Dolostone host rocks and magnesite show similar Mn, Al, and K contents. The host dolostones have a lower average FeO content (0.24 wt %) than magnesite (1.82 wt %) and than later dolomite after magnesite (1.04 wt %). The black magnesite bands with a low percentage of impurities show REE contents and patterns similar to those of the relatively pure dolostone host rocks, suggesting an origin by metasomatic replacement. The white magnesite bands display REE contents similar, but slightly lower in LREE to those of the host dolostones, as would be expected by magnesite replacing dolomite. REE patterns of dolomite after magnesite are very similar to those of magnesite, suggesting that REE behaved conservatively during the replacement of magnesite by dolomite and that the process has been induced by fluids impoverished in REE and equilibrated with the carbonate sequence. The petrography and geochemistry of Eugui magnesite indicate that the deposit originated by metasomatic replacement of a dolostone precursor promoted by fluids moving through stylolite and bedding planes. Sedimentary, structural, textural, and geochemical relics of the replaced dolostones are preserved in the magnesite rocks. The peculiar textural characteristics of the magnesite represent replacement features and cannot be interpreted by diagenetic or metamorphic recrystallization of an original marine microcrystalline magnesite deposit
Palladium and gold minerals from the Baronskoe-Kluevsky ore deposit (Volkovsky complex, Central Urals, Russia)
No full textDrill cores from the newly discovered Baronskoe-Kluevsky Pd-Au deposit (Volkovsky massif, Central Urals) have been investigated by reflected-light and electron microscopy, and the ore minerals were analyzed by electron microprobe. The most abundant Platinum-group mineral (PGM) is vysotskite, ideally PdS, characterized by an unusual Pt,Ni-poor composition. Palladium also occurs in kotulskite (PdTe), stillwaterite (Pd8As3 and unknown Pd-As-Te compounds with vincentite-type Pd-3(As,Te), stillwaterite-type Pd-8(As,Te)(3), and Pd-7(As,Te)(2) stoichiometries. The main carrier of Au is Pd-rich electrum, approaching the composition Au75Ag15Pd10, with minor Fe, Cu, Ni and Pt. The precious minerals are closely associated with minute blebs of chalcopyrite + magnetite disseminated throughout serpentinized olivine-apatite host rock. Paragenetic relationships among the ore minerals define a succession of crystallization events in the order: 1) Cu-Pd sulfides + electrum, 2) replacement by Pd-Te-As and late Pd-As PGM, 3) final replacement by magnetite. The paragenesis is tentatively related with cooling of a fluid phase in the late-to post-magmatic stage
Chromite composition and platinum-group mineral assemblage in the Uktus Uralian-Alaskan-type complex (Central Urals, Russia)
No full textChromitite segregations in dunites of the Uktus Uralian-Alaskan-type complex (Central Urals, Russia) display large variation of the chromite composition: Cr/(Cr + Al) - 0.46-0.77, Fe2+/(Fe2+ + Mg) = 0.28-0.66, and Fe3+ /(Fe3+ + Fe2+) =0.23-0.59. Three types of PGM assemblages have been recognized, varying in accordance with chromite composition: type I, dominated by Ru-Os-Ir (sulfides), is associated with magnesiochromite having Fe3+ /(Fe3+ + Fe2+) < 0.30, in the southern dunite body. Type 11, containing abundant Pt-Ir (alloys, minor sulfides), is found in magnesiochromite with Fe3+ /(Fe3+ + Fe2+) = 0.40-0.44; type III, consisting of Ir-Rh-Pt-Pd (alloys, sulfarsenides, antimonides) in Fe-rich chromite having Fe2+ /(Fe2+ + Mg) = 0.66 and Fe3+ /(Fe3+ + Fe2+) = 0.59. Positive anomalies of Ir and Pt, and a negative peak of Ru characterize the PGE patterns of chromitites with type II and III PGM assemblages, whereas a positive Pt anomaly is observed in their dunite host. Intensive fractionation of Pt-Fe alloys in the Uktus chromitites reflects the anomalous behavior of Pt which is decoupled from Rh and Pd. Among other factors, the high iron activity and oxygen fugacity in the parent melt appear to exert a major control on precipitation of Pt-Fe alloys, below sulfur saturation. The strong Pt anomaly in chromitites from Uktus may indicate that Uralian-Alaskan-type magmas were derived from a Pt-rich mantle source
Age, geochemistry and petrogenesis of the ultramafic pipes in the Ivrea Zone, NW Italy
Full text linkPipe-like ultramafic bodies, hosting Ni-Cu-PGE sulphide deposits, intrude the Main Gabble and the roof metasediments of the Ivrea Zone, NW Italy. These bodies were emplaced at 287 +/- 3 Ma and represent the last mantle-derived melts associated with an underplating event that largely drove the crustal evolution of this area during the late Carboniferous (similar to 300-290 Ma). nle Pipes are composed of volatile-rich ultramafic locks and gabbros with an alkaline signature simultaneously enriched in both incompatible and the most compatible elements but depleted in elements of intermediate compatibility. The isotope composition of these pipe rocks is E-290Ma(Nd) similar to3.7 to -1.9 and E-290Ma(Sr) similar to0.8-26. In a E-290Ma((Nd)) vs E290(Ma)(Sr) diagram they define a linear array between the unmetasomatized and metasomatized peridotites of Finero, but distinctlj oblique with respect to the trend defined by Balmuccia Peidotites. irhe delta S-34 ranges from 0.0 to + 0.9%(0) and is indicative of a mantle source. We suggest that the pipes represent infiltration of melts derived from a depleted mantle protolith flushed with alkaline metasomatic fluids, probably of juvenile mantle origin, which underwent partial melting as a consequence of the depression of the solidus owing to the increased activity of water and other volatiles. The similarity in age, trace-element, and isotopic signatures indicates that the Pipes were probably produced in the course of the same metasomatic event that affected the Finero ultramafic body. The overall geochemical characteristics of the pipes ale more consistent with magmatism related to a mantle plume than with a subduction setting
Platinum group minerals in ophiolitic chromitites from Tehuitzingo (Acatlan complex, southern Mexico): implications for post-magmatic modification
No full textPodiform chromitite bodies occur in serpentinites at Tehuitzingo (Acatlan complex, southern Mexico). Serpentinite and chromitite are believed to represent a fragment of Paleozoic ophiolitic mantle formed in a supra-subduction zone setting. The ophiolitic mantle sequence is associated with eclogitic rocks, enclosed in a metasedimentary sequence. This association suggests that serpentinites, chromitites and eclogitic rocks underwent a common metamorphic evolution, starting from high pressure (eclogite facies) followed by retrogression (epidote-amphibolite and greenschist facies). The chromitites are strongly altered so that chromite grains are transformed to ferrian chromite; no primary silicates (i.e. of magmatic origin) have been preserved. The chromitites are Al-rich, and contain up to 303 ppb platinum group elements (PGE), with a marked predominance of Os+Ir+Ru over Rh+Pd+Pt, resulting in a characteristic negative-slope of the chondrite-normalized PGE pattern. Consistent with the geochemical data the platinum group minerals (PGM) assemblage is dominated by Ru-Os-Ir minerals, occurring both as single-phase or as composite grains generally less than 10 mu m in size. The PGM mineralogy includes laurite, osmium, irarsite and Ru-Fe oxide or hydroxide. Based on textural relations, paragenesis and composition, it was possible to establish that Os-rich laurite and irarsite were early liquidus phases, which now occur as inclusions in unaltered chromite. However, most of the PGM are found in the alteration assemblages of the chromitites in close association with ferrian chromite, chlorite, and heazlewoodite. Laurite from the secondary assemblage is Os-poor and commonly shows overgrowths of Os - Ir alloys. Internal zoning of some laurite grains indicates that Os-poor laurite formed from a Os-rich laurite by release of Os and some Ir, that are readily incorporated in the Os - Ir alloys. Such process requires a decrease of sulfur fugacity with decreasing temperature; this is not consistent with the fS(2)-T trend in magmatic systems. It is proposed, therefore, that the magmatic PGM assemblage underwent mineralogical reworking starting from relatively high temperature during metamorphism. Temperatures, estimated from chlorite geothermometry ( 399 - 210 degrees C), possibly reflect effects of low-grade metamorphism. After that the PGM and the associated sulfides started to be oxidized. Although it is difficult to determine the extent of PGE mobilization on the basis of mineralogical observations, our data suggest that the metamorphism affecting the Tehuitzingo chromitites caused only re-distribution of PGE on a small scale. Thus, we conclude that metamorphism modified the primary PGM assemblage without having changed the whole-rock PGE concentration
Composition and mineralogy of PGE-rich chromitites in the Nurali Lherzolite-Gabbro complex, southern urals, Russia
No full textWe have investigated two subeconomic bodies of chromitite in the Nurali Iherzolite-gabbro complex, in the southern Urals, Russia, with regard to the composition of the chromian spinel and the distribution and mineralogy of the platinum-group elements (PGE). The bodies of chromitite, referred to as CHR-I and CHR-II, occur as small concordant lenses located at two stratigraphic levels within layered wehrlite and clinopyroxenite, overlying the Iherzolitie mantle tectonite. The chromian spinel is Al-rich, showing an increase of Cr/(Cr + Al), Fe2+/(Fe2+ + Mg) and TiO2 from CHR-I to CHR-II. The total PGE contents vary from 1.26 to 11.61 ppm, and show increase in (Pt + Pd)/(Os + Ir + Ru) from 0.1 to 52.2 as a result of the appearance of magmatic sulfides in the upper chromitite. The PGM assemblage shows a drastic change from laurite-erlichmanite-dominated to enriched in Pt Pd sulfides and alloys. Laurite is the first PGM to crystallize, and its composition typically reflects the Ru/Os ratio of the primitive mantle, indicating that the parent melt of the chromitite did not undergo fractionation during ascent. The Nurali chromitites are rather unusual as they have characteristics in common with chromitites associated with ophiolitic cumulates, layered intrusions, Alaskan-type complexes, and the subcontinental orogenic mantle
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
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