1,721,051 research outputs found
Phase composition in volcanic rocks from South Tuscany magmatic area (Central Italy): inferences on conditions of magma crystallization
No full textMicroprobe analyses of olivine, pyroxene, mica and feldspar phenocrysts in trachybasalts, olivine latites and latites from Radicofani, Mts. Cimini and in minettes from Montecatini val di Cecina and Orciatico (Tuscan magmatic area) are reported. Olivines are more strongly magnesian in Mts. Cimini samples (Fo = 90-93 mol%) than in the Radicofani samples (Fo = 68-84 mol%) while in Montecatini val di Cecina and Orciatico rocks olivine is completely transformed. Clinopyroxenes have variable compositions ranging from endiopside to diopsidesalite and augite in the Radicofani trachybasalts and from endiopside to augite in the Mts. Cimini rocks. Minette cpx have strongly different compositions being low-iron endiopside in the Orciatico samples, and augite in the Montecatini val di Cecina rocks. All the analyzed cpx have high Al contents and invariably low calculated ferric iron. Ca-tschermak molecule is the main component of all cpx whereas Ca-Ti-tschermak molecule is important only in Orciatico cpx. Mts. Cimini micas are biotites, while minette micas are phlogopites. The latter are strongly zoned in Ti, Mg and Cr and these elements display significantly different abundances in the micas of the two minette outcrops. Such bimodalism has been recognized to be common in micas from world-wide distributed minettes. Substitution mechanism 2 Mg(VI) = Ti(VI)-(VI) appear to dominate with respect to Mg2Si = Ti2Al. In Radicofani and Mts. Cimini samples feldspar phenocrysts are both plagioclase (An = 86-52 mol%) and K-feldspar (Or = 85-73 mol%), while in the minettes only K-feldspar (Or = 79-77 mol%) is present. The obtained mineral chemistry data joined with experimental studies allow to elucidate equilibrium/disequilibrium relationships among phases and to di scuss evolutionary processes of South Tuscany volcanics. In general, crystal fractionation accompanied either by mixing between differently evolved magmas at Radicofani or by assimilation of crustal material at Mrs. Cimini appears to be the main evolutionary processes. For minettes different conditions in their crystallization history are recognized: the Orciatico rocks have mica compositions indicating crystallization at low pressure under subvolcanic conditions, whereas for Montecatini in val di Cecina rocks a polybaric crystallization is evidenced
Entropy Determination of Magmatic Enclaves Uusing X-ray Element Maps: Petrological Implications
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Replenishment of felsic magma chambers by continuous inputs of mafic magmas: field evidence and fluid-mechanics experiments
No full textChaotic Dynamics and Fractal Patterns During Mixing of Magmas: New Petrological and Volcanological Tools
No full textPetrology of the late-Carboniferous Punta Falcone gabbroic complex, nothern Sardinia, Italy
No full textThe Punta Falcone gabbroic complex represents an evolved high-alumina basalt which rose from the mantle through the lower crust, and subsequently intruded a granite magma in middle crustal levels, during the calc-alkaline magmatic activity which took place in the Sardinian and Corsican islands in the Carboniferous. The gabbroic complex has a stratified sub-vertical structure, and consists of three zones developing from the bottom to the top of the magma chamber. An interaction zone can be recognized along contacts with the surrounding granite stock, and it is characterized by finer-grained and more evolved rocks than the interior of the gabbroic complex. Processes occurring in its interior zone have been substantially different from those occurring in its marginal interaction zone. Petrographical and geochemical features indicate that the differentiation of the interior of the gabbroic complex can be accounted for by low pressure, closed-system in-situ crystallization. The different gabbroic units represent mixtures between cumulus phases and trapped liquid. Plagioclase + pyroxenes, and successively plagioclase + calcic amphibole + oxides nucleated and grew in-situ on the floor and walls of the chamber. Floating of plagioclase towards the top of the magma chamber resulted in the accumulation of the denser liquid at the bottom. Compaction phenomena and convective fractionation processes permitted the development of the pile of cumulus crystals with their trapped liquid, and the migration of part of this evolved liquid towards the top of the magma chamber. On the basis of major and trace element modelling a mathematical artifice has been developed to evaluate cumulus-intercumulus processes that occurred in the interior of the gabbroic complex. Accordingly, the formation of the different units can be modelled by mixtures between the parental magma and different percentages of minerals formed during the first stages of crystallization. Contemporaneously with the differentiation of the interior zone, the envelope of fine-grained rocks enclosing and grading into the coarser inner part of the gabbroic complex experienced both chemical and physical processes. Chemical processes resulted in the evolution of the marginal interaction zone by crystal fractionation plus contamination by the acid magma. Physical processes were closely related to the thermodynamic instability of this marginal zone, and consisted of mingling and back veining phenomena which developed interdigitations of granite veins along contacts. In addition, an increase of the melt fraction of the granite magma, superheated by the latent heat of crystallization of the mafic magma, caused the occurrence of tilting of the mafic magma chamber, and resulted in the development of the sub-vertical structure of the gabbroic complex
Magma Genesis Versus Geodynamics: a Tale of the Miocene.Pliocene Tuscan Magmaic Province
No full textDetermination of Initial Percentages of Magmas and Fluid-dynamics of mixing by Fractal and Image Analysis of Mixed Pumices
No full textTourmaline Nodules from Capo Bianco Aplite (Elba Island, Italy): an Example of Diffusion Limited Aggregation Growth in a Magmatic System
No full textThe morphology of tourmaline nodules occurring in the Capo Bianco aplite (Elba Island, Italy) is studied. Outcrop features indicate that tourmaline nodules are the product of magmatic crystallization, as they are aligned along flow fields developed within the magmatic hosting mass. Mesoscopic observations indicate that nodule morphologies are very variable, from rounded to dendritic. Morphometric analyses show that tourmaline nodules are fractals and that fractal dimension quantifies their degree of irregularity. Numerical simulations of nodule growth are performed by using a Diffusion-Limited Aggregation process. The presence in natural samples of nodules with different morphologies is explained by considering a chaotic magmatic system characterized by a complex interplay between growth rate in different dynamical regions, latent heat of crystallization, and local convection dynamics. It is suggested that higher growth rates correspond to growth of tourmaline nodules in dynamical regions where the transfer of nutrients is very efficient. In such conditions, the latent heat released by the growing nodule is high, inducing strong local convection dynamics, destabilizing the nodule interface, and promoting the formation of dendritic morphologies. On the contrary, the growth of nodules in dynamical regions characterized by weak transfer of nutrients is inhibited leading to weak local convection dynamics and, consequently, to the formation of rounded morphologies
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