1,721,012 research outputs found
Studying metamorphic microstructures: a brief insight on modern methodological approaches
No full textMicrostructures evolve in response to processes that rocks undergo during their residence time in the Earth crust and mantle. For a thourogh understanding of microstructurs all their critical aspects need to be analyzed. Much of the recent advances in the comprehension of microstructures and the interrelated processes is linked to the advent of modern instrumental equipments as electron and proton microprobes (EMPA, mPIXE) and to novel analytical techniques as electron backscatter diffraction (EBSD) and orientation contrast imaging (OC). In this review we highlight the basic concepts and the potential applications of such modern analytical tools in microstructural analysis of metamorphic rocks
Further data on the pre-Alpine metamorphic pressure conditions of the Austridic phyllitic complexes in the Eastern Alps.
No full textSpecial Volume of IGCP Project No 27
Dissolution-precipitation creep of K-feldspar in mid-crustal granite mylonites
No full textThe deformation of K-feldspar within lower amphibolite facies granite mylonites from the Gran Paradiso nappe (North-Western Alps, Italy) is primarily accomplished by dissolution, replacement and precipitation processes, with little or no evidence for dislocation glide or creep. New elongate grains of K-feldspar precipitate in dilational domains within and around K-feldspar porphyroclasts as a part of the process of myrmekite formation that progressively consumes the porphyroclasts. The new grains grow in epitaxial continuity with the parent porphyroclasts, which
hinders the development of a bulk crystallographic preferred orientation (CPO) in the aggregates of new grains when large amounts of relict K-feldspar are still present. In mylonites, the magmatic K-feldspar is eventually transformed into 100-300 μm thick, nearly monomineralic, fine-grained (20-50 μm in size) aggregates of new grains showing an oblique shape fabric and a CPO. The observed CPO is not consistent with the activity of any slip system in K-feldspar. Instead, it is interpreted to result from dissolution-precipitation creep, with a slow reaction rate parallel to [010] and [001] crystallographic axes and a fast reaction rate parallel the [100] axis. Consistent with such a dissolution-precipitation mechanism, boundaries of new K-feldspar grains are highly corroded when oriented approximately parallel to the extensional instantaneous stretching axis, whereas boundaries approximately orthogonal to the same stretching axis show well-developed crystal facets. The CPO developed is weak, which suggests that the anisotropy in the dissolution/growth rate of K-feldspar is also weak
Depressurized Cavities within High-strain Shear Zones: their Role in the Segregation and Flow of SiO2-rich Melt in Feldspar-dominated Rocks
No full textWe observe void growth and coalescence into cavity-bearing
shear bands during deformation of wet synthetic anorthite aggregates
containing53 vol. % silica-enriched melt. Samples were deformed
in the Newtonian creep regime to high strain during torsion
experiments at 11008C and 400MPa confining pressure. Localized
cavity-bearing shear bands show an S^C’-geometry: the bands
(C’) are oriented at about 308 to the compression direction of the
imposed simple shear and the internal foliation (S) of the bands is
rotated towards the horizontal external shear plane. Cavity-bearing
shear bands started to nucleate in the sample periphery above
a shear strain threshold of 2. Quartz crystallized from the
water-saturated SiO2-rich melt within large cavities inside these
bands, which requires that the melt is decompressed by4200MPa
during their formation. The dynamically evolving cavities are
sites of locally reduced pressure that collect the melt distributed in
the adjacent matrix.Therefore, cavitation damage under ductile conditions
may result in the development of an efficient melt channelling
system controlling SiO2-rich melt flow in the lower crust. Electron
backscatter diffraction analysis shows that the quartz inside the
cavity bands has a crystallographic preferred orientation (CPO).
The development of the CPOis explained by the preferred dissolution
of crystals oriented with the rhombohedra and trigonal dipyramids
orthogonal to the compression direction and by preferential growth of
crystals aligned with their500014axis in the extension direction
of the externally applied simple shear deformation
Towards a better understanding of the fibrolite problem: the effect of reaction overstepping and surface energy anisotropy
No full textTrachytes from the Euganean Hills District (Italy) contain metapelitic xenoliths that have been pyrometamorphosed during incorporation in the melt. In xenoliths containing sillimanite crystallized during a previous regional HT/LP metamorphism, fibrolite systematically nucleates at the grain boundaries of sillimanite prisms and within plagioclase crystals. Ternary feldspar thermometry shows that plagioclase in contact with sillimanite plots along the 750degreesC solvus that reflects near-equilibrium conditions of regional metamorphism. Plagioclase containing fibrolite plots closer towards the 950degreesC solvus, reflecting the tendency of plagioclase to re-equilibrate at high temperature during pyrometamorphism by a fibrolite-forming reaction:
K-feldspar (1) + plagioclase (1)
-->K-feldspar (2) + plagioclase(2) + fibrolite.
In sillimanite-free xenoliths, fibrolite did not form during pyrometamorphism, because these xenoliths do not contain plagioclase. In these xenoliths, andalusite prisms are replaced by oriented fibrous corundum needles and K-feldspar. The petrographic evidence suggests that when reaction rate is high as a result of reaction temperature overstepping, sillimanite grows as fibrolite because the surface energy of {110} faces is low and their growth rate is rapid. The same explanation may hold also for the growth of acicular corundum pseudomorphing andalusite prisms in sillimanite-free xenoliths
Deformation Mechanisms at the Hypersolidus-Subsolidus Boundary Recorded in the Cumulate Gabbro Nodules of Stromboli (Southern Italy): A Microstructural Indicator of High-T Shear Zones in the Basement of Active Island arc Volcanoes
No full textWhen epitaxy controls garnet growth
No full textWithin a mica schist from the coesite-bearing Brossasco-Isasca Unit (Western Alps), microstructural
analysis shows that Alpine garnet grains are aligned with the crenulated foliation. Garnet crystallographic
orientation was analysed with electron backscatter diffraction (EBSD): the obtained
crystallographic dispersion patterns and distribution patterns of misorientation axes suggest a strong
parallelism of {110} garnet planes with a 56W-dipping foliation. The data are interpreted as evidence
for an epitaxial growth of garnet upon (001) biotite planes, sometime during and/or after dispersion of
the biotite/garnet crystals from their initially foliation-parallel orientation by rotation about the Alpine
crenulation axis. This interpretation is based on the comparison of the measured EBSD data with: (i)
theoretical dispersion trajectories of garnet crystallographic data, (ii) numerically modelled pole figures,
and (iii) numerically modelled misorientation axis distribution patterns. Our data suggest that epitaxial
growth of garnet upon biotite is allowed by distortion of the pseudohexagonal basal oxygen ring
structure on (001) biotite surfaces, and that distortion is driven by introduction of missing ions. Our data
further suggest that the spatial distribution of precursor phases influences the distribution patterns of
garnet within mica schists
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
The crystalline basements of the Italian eastern Alps: a review of the metamorphic features
No full textWe briefly review the state of the art on the metamorphic evolution of the crystalline basements of the Eastern Alps, with emphasis on the Italian sector. For each of the main structural units,
Penninic, Austroalpine and Southalpine, we present a short outline followed by the most recent results obtained by the research team of metamorphic petrology operating at the University of Padova
- …
