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Jacopo Baima Matteo Ferrabone Roberto Orlando Alessandro Erba Roberto Dovesi 《Physics and Chemistry of Minerals》2016,43(2):137-149
The phonon dispersion and thermodynamic properties of pyrope (\(\hbox {Mg}_3\hbox {Al}_2\hbox {Si}_3\hbox {O}_{12}\)) and grossular (\(\hbox {Ca}_3\hbox {Al}_2\hbox {Si}_3\hbox {O}_{12}\) ) have been computed by using an ab initio quantum mechanical approach, an all-electron variational Gaussian-type basis set and the B3LYP hybrid functional, as implemented in the Crystal program. Dispersion effects in the phonon bands have been simulated by using supercells of increasing size, containing 80, 160, 320, 640, 1280 and 2160 atoms, corresponding to 1, 2, 4, 8, 16 and 27 \(\mathbf {k}\) points in the first Brillouin zone. Phonon band structures, density of states and corresponding inelastic neutron scattering spectra are reported. Full convergence of the various thermodynamic properties, in particular entropy (S) and specific heat at constant volume (\(C_\mathrm{{V}}\)), with the number of \(\mathbf {k}\) points is achieved with 27 \(\mathbf {k}\) points. The very regular behavior of the S(T) and \(C_\mathrm{{V}}(T)\) curves as a function of the number of \(\mathbf {k}\) points, determined by high numerical stability of the code, permits extrapolation to an infinite number of \(\mathbf {k}\) points. The limiting value differs from the 27-\(\mathbf {k}\) case by only 0.40 % at 100 K for S (the difference decreasing to 0.11 % at 1000 K) and by 0.29 % (0.05 % at 1000 K) for \(C_\mathrm{{V}}\). The agreement with the experimental data is rather satisfactory. We also address the problem of the relative entropy of pyrope and grossular, a still debated question. Our lattice dynamical calculations correctly describe the larger entropy of pyrope than grossular by taking into account merely vibrational contributions and without invoking “static disorder” of the Mg ions in dodecahedral sites. However, as the computed entropy difference is found to be larger than the experimental one by a factor of 2–3, present calculations cannot exclude possible thermally induced structural changes, which could lead to further conformational contributions to the entropy. 相似文献
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Elastic properties of six silicate garnet end members from accurate ab initio simulations 总被引：1，自引：1，他引：0
Alessandro Erba Agnes Mahmoud Roberto Orlando Roberto Dovesi 《Physics and Chemistry of Minerals》2014,41(2):151-160
The elastic properties of six silicate garnet end members, among the most important rock-forming minerals, are investigated here for the first time via accurate ab initio theoretical simulations. The Crystal program is used, which works within periodic boundary conditions and allows for all-electron basis sets to be adopted. From the computed elastic tensor, Christoffel’s equation is solved along a set of crystallographic directions in order to fully characterize the seismic wave velocity anisotropy in such materials. Polycrystalline isotropic aggregate elastic properties are derived from the computed single-crystal data via the Voigt-Reuss-Hill averaging procedure. Transferability of the elastic properties from end members to their solid solutions with different chemical compositions is also addressed. 相似文献
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A. Costa F. Dell’Erba M. A. Di Vito R. Isaia G. Macedonio G. Orsi T. Pfeiffer 《Bulletin of Volcanology》2009,71(3):259-273
Tephra fallout associated with renewal of volcanism at the Campi Flegrei caldera is a serious threat to the Neapolitan area.
In order to assess the hazards related with tephra loading, we have considered three different eruption scenarios representative
of past activity: a high-magnitude event similar to the 4.1 ka Agnano-Monte Spina eruption, a medium-magnitude event, similar
to the ∼3.8 ka Astroni 6 eruption, and a low-magnitude event similar to the Averno 2 eruption. The fallout deposits were reconstructed
using the HAZMAP computational model, which is based on a semi-analytical solution of the two-dimensional advection–diffusion–sedimentation
equation for volcanic tephra. The input parameters into the model, such as total erupted mass, eruption column height, and
bulk grain-size and components distribution, were obtained by best-fitting field data. We carried out tens of thousands simulations
using a statistical set of wind profiles, obtained from NOAA re-analysis. Probability maps, relative to the considered scenarios,
were constructed for several tephra loads, such as 200, 300 and 400 kg/m2. These provide a hazard assessment for roof collapses due to tephra loading that can be used for risk mitigation plans in
the area. 相似文献
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Calcareous nannofossil assemblages at Site 641C (Galicia Margin, North Atlantic) were investigated in order to determine changes in fertility and temperature of surface waters. Taxa such as Zeughrabdotus spp. <3.5 μm, Biscutum constans, Discorhabdus rotatorius and Diazomatolithus lehmanii, which thrived in higher fertility conditions, are particularly abundant across the CM0 interval as opposed to those with oligotrophic affinities such as Watznaueria spp. and Nannoconus spp., which are generally reduced in abundance. The abundances of nannoconids are much lower than those observed in Tethyan sections, indicating higher fertility conditions. Slumpings and low recovery prevent the identification of the onset of the “nannoconid crisis”, but a sharp drop in nannoconid abundances, observed prior to the CM0 interval, correlates with the “nannoconid decline” observed in several Tethys sections.The normalized ratio between low and high fertility taxa (Fertility Index) was used to characterize the nannofossil assemblages in terms of productivity changes. The highest values of the Fertility Index were observed across magnetic chron CM0. The paucity of cold water taxa such as Seribiscutum spp. and Repagulum parvidentatum suggests warm water conditions throughout the deposition of upper Barremian–lower Aptian sediments on the Galicia Margin. 相似文献
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Elisabetta Erba 《Marine Geology》1991,100(1-4):83-101
Peculiar pellicles have been recorded in the extant anoxic hypersaline basins of the Bannock area and the Western Strabo Trench (Eastern Mediterranean). Discrete layers of pellicles were found in 29 cores. They are oriented parallel to bedding planes or are folded within the sandy base of turbidites and within slumped sediments. In addition, similar pellicles occur at the surface of or interlocked within gypsum crystals recovered from the Bannock area.
The pellicles are 0.5–3 mm thick and dark greenish-grey in colour. They foliate into very small (<1 mm) undulating and anastomosing microlaminae entrapping abundant biogenic and inorganic particles. The amorphous organic matter of the pellicles is most probably produced by bacterial masses linked to mineralization processes of the organic debris. Anaerobic sulphate-reducing bacteria (Desulphovibrio) are abundant and the presence of methane bacteria is likely.
Detailed analyses with the light microscope and scanning electron microscope yielded differences and similarities concerning ultrastructure, content, abundance and morphotypes of the bacterial pellicles from the Bannock area and the Western Strabo Trench. A consistently well-developed ultrastructure comprising thin microlaminae characterizes the Bannock pellicles, whereas the ultrastructure of the Tyro and Poseidon pellicles is usually not organized. Siliceous microfossils are abundant in samples from both sites, but centrate diatoms prevail in the Bannock pellicles whereas pennate diatoms are overwhelmingly dominant in the Tyro and Poseidon pellicles. Calcareous micro- and nannofossils, together with pyrite framboids, are more abundant in samples from the Bannock area. Bacteria are extremely abundant in samples from the Bannock area and three different morphological types have been distinguished. Only one type of bacteria was commonly observed in pellicles from the Western Strabo Trench basins.
Pellicles form at the normal seawater/brine interface, as substantiated by transmittance profiles and bacteriological investigations. Therefore, the term “deep mid-water bacterial mats” is proposed herein for these peculiar layers.
After bacterial mats grow at the interface, they are included within the sediments by following one of two different paths: (a) when pelagic sedimentation prevails, pellicles sink to the basin floor due to the increasing load of biogenic and inorganic detritus, and (b) under a tectonically active regime, turbidity currents and slumps can disrupt the interface and destroy the floating mats. In the latter case, fragments of pellicles are transported to the bottom within the gravity flows. 相似文献
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The Valanginian carbon isotope event: a first episode of greenhouse climate conditions during the Cretaceous 总被引：1，自引：0，他引：1
Lower Cretaceous pelagic carbonates outcropping along the Southern Alps of northern Italy provide a record of Tethyan palaeoceanography as well as of low frequency fluctuations in the global carbon cycle. The carbonate C-isotope stratigraphy established at five selected localities in the Southern Alps allows an accurate picture to be drawn of the duration and amplitude of the Valanginian C-isotope event. δ13 C values near 1.25–1.50% determined in Berriasian and lower Valanginian sediments are replaced by more pdsitive δ13 C values near 3% in the late Valanginian. The carbonate C-isotope excursion ends in the early Hauterivian with values fluctuating between 1.5% and 2%. The carbonate C-isotope excursion is accompanied by a positive excursion in the total organic carbon C-isotope curve. The Valanginian C-isotope excursion identified in Tethyan sediments correlates with a C-isotope excursion recorded in the western North Atlantic, in the Gulf of Mexico, and in the Central Pacific (DSDP Sites 534,391,535 and 167). By analogy with the Aptian stage, also marked by a significant positive C-isotope excursion, the time of positive δ13 C values is regarded as a time of accelerated carbon cycling coupled with increased burial rates of organic carbon and detrital material in oceanic sediments. A warm and humid climate, possiblycoupled with a high atmospheric CO2 content and a high global sea-level, may have triggered the acceleration of the global carbon cycling. In this case the Valanginian C-isotope event would reflect a first episode of Greenhouse Earth conditions during the Cretaceous. 相似文献
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Sixto R. Fernández-López Giulio Pavia Elisabetta Erba Myett Guiomar Maria H. Henriques Roberto Lanza Charles Mangold Davide Olivero Daniele Tiraboschi 《Swiss Journal of Geoscience》2009,102(2):271-295
The Bathonian Global Stratotype Section and Point (GSSP) is proposed at the base of limestone bed RB071 (bed 23 in Sturani
1967) in the Ravin du Bès Section (43° 57′ 38′′ N, 6° 18′ 55′′ E), Bas-Auran area, “Alpes de Haute Provence” French department.
The Ravin du Bès Section, as formal candidate GSSP for the base of the Bathonian Stage, satisfies most of the requirements
recommended by the International Commission on Stratigraphy: 1) The exposure extends over 13 m in thickness. At the Bajocian-Bathonian
transition, no vertical (bio-, ichno- or tapho-) facies changes, condensation, stratigraphic gaps or hiatuses have been recorded.
Structural complexity, synsedimentary and tectonic disturbances, or important alterations by metamorphism are not relevant
constraints. 2) There is a well-preserved, abundant and diverse fossil record across the boundary interval, with key markers
(ammonites and nannofossils) for worldwide correlation. The base of the Bathonian Stage and Zigzag Zone in Bas-Auran corresponds
to the first occurrence level of Gonolkites convergens Buckman, which coincides with the first occurrence of Morphoceras parvum Wetzel. Calcareous nannofossils, as secondary global marker, are present in all beds and allow characterizing the Bajocian-Bathonian
transition. 3) Regional analyses of sequence stratigraphy and manganese chemostratigraphy are available. Spectral gamma-ray
data corroborate an Early Bathonian deepening half-cycle of second order. 4) The criteria of accessibility, conservation and
protection are assured by the “Réserve Naturelle Géologique de Haute Provence”. The Cabo Mondego Section (Portugal) is suggested
as the Bathonian auxiliary section and point (ASSP) within this GSSP proposal. 相似文献
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