1,721,176 research outputs found
Electrical conductivity of a phonotephrite from Mt Vesuvius: The importance of chemical composition on the electrical conductivity of silicate melts
No full textThe bulk electrical conductivity of the phonotephritic lava from the 1944 eruption of Mt Vesuvius was measured using complex impedance spectroscopy in a multianvil apparatus at 1 GPa and temperatures up to
700 °C. Melting experiments prior to the electrical measurements were also performed on this sample in a
piston cylinder apparatus in order to gauge how bulk conductivity varies as a function of its melt fraction.
Unlike the behaviour found in basaltic rocks in which conductivity increases with increasing melt fraction,
we observe a conductivity decrease of the order of a factor of ten for samples at 700 °C ranging in melt
fraction from 32 vol.% to completely molten.We attribute this anomalous behaviour to the progressive loss of highly conductive leucite upon melting. The addition of potassium to the melt phase, however, does not
result in an increase of the total alkali concentration due to the melting of other mineral components. We
also present an empirical model to predict the electrical conductivity of fully molten silicate liquids as a function of temperature and chemical composition, based on conductivity data for natural silicate liquids
found in the literature. The inclusion of compositional terms reduces the error by more than a factor of four
with respect to a composition independent, temperature-only parameterization
Clinopyroxene and titanomagnetite cation redistributions at Mt. Etna volcano (Sicily, Italy): Footprints of the final solidification history of lava fountains and lava flows
No full textFor a better understanding of the final solidification history of eruptions at Mt. Etna volcano (Sicily, Italy), we have investigated cation redistributions at the interface between sub-millimetre-sized clinopyroxene and titanomagnetite crystal rims and coexisting melts. The studied products were scoria clasts from lava fountains and rock samples from pahoehoe and aa lava flows. Our data indicate that scoria clasts from lava fountaining were rapidly quenched at the contact with the atmosphere, preserving the original crystal textures and compositions inherited during magma dynamics within the plumbing system. Kinetics and energetics of crystallization were instantaneously frozen-in and post-eruptive effects on mineral chemistry were negligible. The near-equilibrium compositions of clinopyroxene and titanomagnetite indicate that lava fountain episodes were supplied by high-temperature, H2O-rich magmas ascending with velocities of 0.01-0.31 m/s. In contrast, magmas feeding lava flow eruptions underwent a more complex solidification history where the final stage of the crystal growth was mostly influenced by volatile loss and heat dissipation at syn- and post-eruptive conditions. Due to kinetic effects associated with magma undercooling, clinopyroxenes and titanomagnetites formed by crystal attachment and agglomeration mechanisms leading to intricate intergrowth textures. The final compositions of these minerals testify to closure temperatures and melt-water concentrations remarkably lower than those estimated for lava fountains. Kinetically-controlled cation redistributions at the crystal-melt interface suggest that the solidification of magma was driven by degassing and cooling processes proceeding from the uppermost part of the volcanic conduit to the surface. (C) 2015 Elsevier B.V. All rights reserved
Heterogeneous nucleation mechanisms and formation of metastable phase assemblages induced by different crystalline seeds in a rapidly cooled andesitic melt
No full textPt-capsules loaded with a Pt-coil and two crystalline seeds immersed into an andesitic melt were rapidly
cooled from 1300 to 1100 °C with a rate of 3 °C/min at atmospheric pressure and air oxygen fugacity. Results
show that the Pt-coil does not induce any heterogeneous crystallization process as well as iron diffusion process
from the melt into the platinum substrate. In contrast, the presence of crystalline seeds in a solidifying
andesitic melt significantly alters the phase assemblage, composition and texture of the new-forming crystals
in response to a heterogeneous nucleation mechanism and the formation of metastable phases
The role of cooling rate in the origin of high temperature phases at the chilled margin of magmatic intrusions
No full textBoth large (i.e. from hundreds to thousands of metres thick) and small (i.e. from centimetres to a few metres
thick) magmatic intrusions are characterized by mineral compositional variations proceeding from the outermost
to the innermost part of the intrusive body. However, in the case of large intrusions, mineral compositions
become progressively more primitive (e.g. An-rich plagioclases and En-rich pyroxenes) from the
chilled margin towards the interior; whereas, the opposite occurs for small intrusive bodies.
Since it is unclear to what extent variable cooling rate conditions may alter the phase compositions, we have
performed isothermal and dynamic experiments within a temperature interval of 1250–1100 °C using four
different cooling rates of 150, 50, 10 and 2.5 °C/h. Numerical simulations of thermal regimes in and around
small and large magmatic intrusions have also been performed and compared with phase compositional variations
observed in our laboratory experiments.
Results indicate that, over rapid cooling rate conditions, the crystal compositions faithfully reproduce those of
high-temperature formations, i.e. An-rich plagioclases, En-rich pyroxenes and Usp-poor spinels. However,
such a process is limited to a maximum distance of 2–3 m from the margin of the intrusion. Moreover, in active
volcanic systems, heat fluxes are released from the main regions of magma storage into host rocks; therefore,
only magmas solidifying at the contact of cold wall rocks may develop chilled margins with features
related to rapid cooling rate conditions. In the presence of hot host rocks, thermal gradients are significantly
reduced and the role played by cooling dynamics on textural and compositional variations of minerals is
practically negligible
Petrological constrains on the coarse-grained, high-Mg basaltic enclaves of Capo Marargiu (Sardinia, Italy)
No full textWe present results from a textural and geochemical study conducted on calc-alkaline volcanic and hypoabyssal rocks from the Oligo-Miocene Capo Marargiu Volcanic District (CMVD; Sardinia, Italy).
Stratigraphic units of CMVD consist of lava domes, a pyroclastic breccia interbedded with lava flows, and dikes. The pyroclastic breccia is in lateral contact with a low crystallinity (∼15 vol.% phenocrysts), massive hypoabyssal body hosting decimetre-sized, coarse-grained enclaves with porphyritic textures (∼50 vol.% phenocrysts).
These crystal-rich enclaves and the host rock exhibit phase assemblages of clinopyroxene + plagioclase + amphibole + olivine + magnetite + low-Ca pyroxene, and plagioclase + clinopyroxene + magnetite + low-Ca pyroxene, respectively. Clinopyroxene phenocrysts (≤5 mm in size) in the crystal-rich enclaves show two compositionally distinct populations: type 1 clinopyroxenes are diopsides (Mg#85-90), whereas type 2 clinopyroxenes are augites (Mg#74-82). Plagioclase phenocrysts (≤1 mm in size) from crystal-rich enclaves and the host rock are normally zoned with An75-96 cores grading to An50-75 rims. The composition of amphibole phenocrysts (≤20 mm in size) is Mg-hastingsite. The mineral texture is poikilitic suggesting early crystallization of amphibole with respect to plagioclase. In fact, the primitive (∼Mg#76), high-T amphiboles include clinopyroxene, whereas the more evolved (∼Mg#65), low-T phenocrysts host plagioclase. Amphiboles are also surrounded by characteristic reaction coronas, consisting of tiny microlites (<5 μm in size) of clinopyroxene, low-Ca pyroxene, plagioclase, magnetite and ilmenite. Olivine occurs as phenocrysts (≤1 mm in size) with Fo79-87 cores surrounded by Fo66-79 rims.
The bulk-rocks of crystal-rich enclaves are high-Mg basalts (i.e., 10 wt.% MgO), whereas the host rock is a more differentiated basaltic andesite (i.e., 5 wt.% MgO). Major oxides and compatible trace element modelling suggest that the basaltic andesitic magma originates by crystal fractionation of olivine + clinopyroxene from a high-Mg basalt [1]. In turn, compatible trace elements in the high-Mg basalt are low (330 ppm Cr, 130 ppm Ni) relative to picritic arc magmas, as the result of crystal fractionation of olivine ± Cr-spinel from a primary magma at depth [1; 2].
Thermobarometric calculations on phenocrysts from the crystal-rich enclaves in equilibrium with the high-Mg basalt yield pressures (600-400 MPa) and temperatures (1200-950 °C) consistent with phase diagrams derived by experiments conducted on primitive arc liquids [3]. Nevertheless, (i) the breakdown of the opacitic amphibole rim [4], (ii) the late appearance of plagioclase, and (iii) the correspondence between ∼Mg#65 natural amphiboles and mineral compositions experimentally-derived at 200 MPa [5], indicate that the crystal-rich enclaves experienced a decompression path started at higher crustal depths. In this view, the high-Mg basalt and the basaltic andesite represent two different regions of a chemically zoned magma chamber formed by crystal fractionation of a primary magma ponding at ∼500 MPa. Subsequently, buoyancy forces associated with density gradients caused upward migration of the basaltic andesite carrying portions of the adjacent high-Mg basalt to shallower crustal levels.
References:
[1] Yamamoto M (1988) Contrib Mineral Petrol 99:352-259
[2] Eggins SM (1993) Contrib Mineral Petrol 114:79-100
[3] Foden JD and Green DH (1992) Contrib Mineral Petrol 109:479-493
[4] Reagan MK et al. (1983) Bull Volcanol 49:415-434
[5] Sisson TW and Grove TL (1993) Contrib Mineral Petrol 113:143-16
A new test for equilibrium based on clinopyroxene–melt pairs: Clues on the solidification temperatures of Etnean alkaline melts at post-eruptive conditions
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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
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