1,720,986 research outputs found
Geochemistry, mineralogy and petrology of the eclogitized manganese deposit of Praborna (Valle d’Aosta, Western Italian Alps).
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The crystal structure of dissakisite-(La) and structural variations after annealing of radiation damage.
No full textWetting angles of hydrous carbonatitic liquids and reversal in wettability for silicate and carbonatitic magmas in the mantle
No full textCarbon-bearing solids, fluids, and melts in the Earth’s deep interior play an important role in the long-term carbon cycle. Carbonatite magmas have been suggested as important agents of mantle metasomatism and yet, their physical features are expected to control the mobility from the source region to shallow Earth. The mobility and infiltration rates of carbonatitic melts, together with their influence on the annealing of mantle peridotites are poorly constrained processes. Although natural carbonatitic melts are complex chemical systems with C-O-H species as a major component, previous work has been performed in anhydrous model systems. Here we present a quantitative laboratory simulation of variables and processes controlling the ascent, mobility and connectivity of carbonatites in a model mantle material investigating the dihedral angle of hydrous carbonatitic liquids. We aim at comparing the texturally equilibrated volume proportions of volatile-rich carbonatitic melts with silicate melts in a partially molten peridotite, and we examine whether carbonatitic liquids are always more wetting than silicate melts. The infiltration experiments were performed employing an end loaded piston-cylinder apparatus, at T= 1200°C and P = 2.5 GPa to investigate the percolation of carbonatitic liquids and interconnectivity of melt pockets in a peridotitic matrix. Hydrous carbonatitic melt pockets were found along olivine grain boundaries; image analysis on electron back scattered and X-ray maps allow us quantifying the apparent dihedral angles between the liquid and olivine and to calculate the grain boundary wetness. Experiments performed at 5 wt.% of water contents result in dihedral angles evolving from ~31° to ~41° with a volume of liquids from 2 to 10 vol.%, while experiments carried out at 30wt. % of water content show a dihedral angle values of almost 50° with a range of volume infiltrated melts between 4 to 9vol.%. These results indicate that dihedral angles progressively increase with increasing water dissolved from 25°-28° in anhydrous carbonatitic liquids up to 50° in water-rich carbonatitic liquids, and, as expected, the volume of interstitial liquid decreases with water increasing. The increase of wetting angles is representative of a sintering process of the solid matrix, which evolves with time in the development of channels of pores, as highlighted relating the grain boundary wetness with fraction of liquid infiltrated. We suggest that the low grain boundary wetness measured may be due to a relatively low melt-rock interfaces which develop with channelized liquid, and that channelization is promoted by chemical gradient, as established by a carbonatitic segregate in the silicate matrix. If H2O is available, we expect that H2O strongly partitions into carbonatitic liquids. As a result, their dihedral angle may evolve up to 50°, a value which is significantly higher than that characterizing silicate melts at similar mantle conditions
Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)
No full textCharacterization of the eve verda in the St.-Marcel valley, Aosta (italian Western Alps)”
No full textApproccio multi-integrato alla caratterizzazione delle colonne in marmo dei Musei Civici di Como
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The Cogne magnetite deposit (Western Alps, Italy): A Late Jurassic seafloor ultramafic-hosted hydrothermal system?
Full text linkThe Cogne magnetite deposit (Western Alps, Italy) is the largest in a series of apatite and sulphide-free magnetite orebodies that are hosted in serpentinites belonging to western Alpine ophiolitic units. The nearly endmember composition of magnetite, which is unusual for an ultramafic setting, and the relatively high tonnage of the deposit (18*10^6 tonnes at 45–50 wt% Fe) make Cogne an intriguing case study to explore magnetite-forming processes in ophiolites. The Cogne magnetite shows variable textures, including nodular ores, veins and fine-grained disseminations in serpentinites after mantle peridotites and totally serpentinized melt-impregnated peridotites (troctolites). An increase in Co/Ni ratio from magnetite-poor serpentinized peridotites (0.05) to nodular magnetite ores (>1) is observed. Trace element analyses of magnetite from different sites and lithologies by laser-ablation inductively-coupled mass spectrometry indicate that magnetites have typical hydrothermal compositions, characterized by high Mg and Mn (median values up to 24,100 and 5000 ppm, respectively), and low Cr, Ti and V (median values up to 30, 570 and 60 ppm, respectively). Moreover, the variations in trace element compositions distinguish magnetite that is hydrothermal fluid-controlled [highest (Mg, Mn, Co, Zn)/Ni ratios] from magnetite whose composition is affected by host-rock chemistry (highest Ni ± Ti ± V). U-Th-Pb dating of magnetite-associated uraninite constrains the formation of the deposit to the Late Jurassic (ca. 150 Ma), during an advanced stage of the opening of the Alpine Tethys. Thermodynamic modelling of fluid-rock interactions indicates that fluids produced by seawater–peridotite or seawater–Fe-gabbro are not sufficiently Fe-rich to account for the formation of the Cogne deposit. This suggests that fractionation processes such as phase separation were critical to generate hydrothermal fluids capable to precipitate large amounts of magnetite in various types of ultramafic host-rocks. The oceanic setting and geochemical and mineralogical similarities with some modern ultramafic-hosted volcanogenic massive sulphide deposits on mid-ocean ridges suggest that the exposed mineralized section at Cogne may represent the deep segment of a seafloor, high-temperature (300–400 C) hydrothermal system. The occurrence of similar magnetite enrichments in present-day oceanic settings could contribute to explain the presence of significant magnetic anomalies centred on active and inactive ultramafic-hosted hydrothermal fields
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
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