Bader’s topological analysis of the electron density in the pressure-induced phase transitions/amorphization in α-quartz from the catastrophe theory viewpoint

In this work, the Bader’s topological analysis of the electron density, coupled with Thom’s catastrophe theory, was used to characterize the pressure-induced transformations in α-quartz. In particular, ab initio calculations of the α-quartz structures in the range 0–105 Gpa have been performed at the HF/DFT exchange–correlation terms level, using Hamiltonians based on a WC1LYP hybrid scheme. The electron densities calculated throughout the ab initio wave functions have been analysed by means of the Bader’s theory, seeking for some catastrophic mechanism in the sense of Thom’s theory. The analysis mainly showed that there is a typical fold catastrophe feature involving an O–O interaction at the quartz–coesite transition pressure, while the amorphization of α-quartz is coincident with an average distribution of the gradient field of the electron density around the oxygen atom which is typically observed in the free atoms. This approach is addressed to depict a phase transition from a novel viewpoint, particularly useful in predicting the stability of a compound at extreme conditions, especially in the absence of experimental data.     

Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba,Borborema Province,Northeastern Brazil: Evidence of marble melting within a collisional suture

Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India.In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture.A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low ⁸⁷Sr/⁸⁶Sr ratios and highly positive δ¹³C values. Available geochemical data for the upper Paleoproterozoic indicate that the ⁸⁷Sr/⁸⁶Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks.     

Rotational properties of ten main belt asteroids: Analysis of the results obtained by photoelectric photometry

V photoelectric lightcurves of ten main belt asteroids (11 Partenope, 20 Massalia, 31 Euphrosyne, 41 Daphne, 55 Pandora, 71 Niobe, 79 Eurynome, 129 Antigone, 344 Desiderata, and 387 Aquitania), obtained during the 1981–1983 oppositions, are reported. The rotation period of 11 Partenope is P = 7.83 hr and that of 344 Desiderata P = 10.53 hr. The shape and the pole coordinates of 20 Massalia, 31 Euphrosyne, and 129 Antigone were also derived and those of 41 Daphne confirmed. The lightcurves of the remaining objects are presented: a preliminary discussion of their possible rotational properties and their morphological features is given.     

Petrogenesis of Al–silicate-bearing trondhjemitic migmatites from NE Sardinia, Italy

The migmatites from Punta Sirenella (NE Sardinia) are layered rocks containing 3–5 vol.% of centimeter-sized stromatic leucosomes which are mainly trondhjemitic and only rarely granitic in composition. They underwent three deformation phases, from D₁ to D₃. The D₁ deformation shows a top to the NW shear component followed by a top to the NE/SE component along the XZ plane of the S₂ schistosity. Migmatization started early, during the compressional and crustal thickening stage of Variscan orogeny and was still in progress during the following extensional stage of unroofing and exhumation.

The trondhjemitic leucosomes, mainly consisting of quartz, plagioclase, biotite ± garnet ± kyanite ± fibrolite, retrograde muscovite and rare K-feldspar, are locally bordered by millimeter-sized biotite-rich melanosomes. The rare granitic leucosomes differ from trondhjemitic ones only in the increase in modal content of K-feldspar, up to 25%. Partial melting started in the kyanite field at about 700–720 °C and 0.8–0.9 GPa, and was followed by re-equilibration at 650–670 °C and 0.4–0.6 GPa, producing fibrolite–biotite intergrowth and coarse-grained muscovite.

The leucosomes have higher SiO₂, CaO, Na₂O, Sr and lower Al₂O₃, Fe₂O₃, MgO, TiO₂, K₂O, P₂O₅, Rb, Ba, Cr, V, Zr, Nb, Zn and REE content with respect to proximal hosts and pelitic metagreywackes. Sporadic anomalous high content of calcium and ferromagnesian elements in some leucosomes is due to entrainment of significant amounts of restitic plagioclase, biotite and accessory phases. The rare granitic leucosomes reveal peritectic K-feldspar produced by muscovite-dehydration melting. Most leucosomes show low REE content, moderately fractionated REE patterns and marked positive Eu anomaly. Proximal hosts and pelitic metagraywackes are characterized by higher REE content, more fractionated REE patterns and slightly negative Eu anomaly.

The trondhjemitic leucosomes were generated by H₂O-fluxed melting at 700 °C of a greywacke to pelitic–greywacke metasedimentary source-rock. The disequilibrium melting process is the most reliable melting model for Punta Sirenella leucosomes.     

High flow events and sediment transport in small streams in the ‘tertiary basin’ area in piedmont (Northwest Italy)

Rainfall, peak discharges, and suspended sediment transport were surveyed for 280 events in three small (0.8 to 10 km²) catchments in a hilly area derived from Neogene marls, silts, and sands. Under similar hydrological input conditions, stream flow behaviour and sediment delivery differed considerably from one catchment to another, depending on topography, lithology, land use, and especially sediment availability. Analytical treatment of data showed a good fit between sediment yield and peak flow discharge. Less good, although still significant, was the correlation between sediment concentration and discharge values for different flow stages. Rainfall peak/basin lag time and rainfall/discharge showed poor or no correlation, mainly due to strong variations in rainfall distribution. Sediment concentration in the catchments varied enormously according to season, from zero up to 334 g 1^?1; sediment yield was 160-900 tonnes km^?2 yr^?1 in the two major catchments, and over 5200 tonnes km^?2 yr^?1 in the headwater catchment, stressing the importance of small tributaries not only in inducing floods in downstream channels, but also in sediment supply.     

Recognition criteria,characteristics and implications of the fluvial to marine transition zone in ancient deltaic deposits (Lajas Formation,Argentina)

The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone, which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the fluvial to marine transition zone are preserved in the rock record, large‐scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated. Fluvial–tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward‐oriented cross‐stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the fluvial to marine transition zone. Heterolithic deposits with decimetre‐scale interbedding of coarser‐grained and finer‐grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial–tidal interactions with overall fluvial dominance or of purely fluvial processes. No purely tidal or tide‐dominated facies were recognized in the studied deposits. Moreover, fluvial–tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off‐axis (interdistributary) parts of the system. Along major channel axes, the interpreted fluvial to marine transition zone is mainly represented by the fluvial‐dominated section, whereas little or no tide‐dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial‐dominated, tide‐influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient fluvial to marine transition zones, is fundamental to refining interpretations of ancient deltaic successions.     

Dealing with double vagueness in DEM morphometric analysis

A method is presented to explicitly incorporate spatial and scale vagueness – double vagueness – into geomorphometric analyses. Known limitations of usual practices include using a single fixed set of crisp thresholds for morphometric classification and the imposition of a single arbitrary number of scales of analysis to the entire digital elevation model (DEM). Among the advantages of the proposed method are: fuzzification of morphometric classification rules, scale-dependent adaptive fuzzy set parametrization and an objective definition of maximum scale of analysis on a cell-by-cell basis. The method was applied to several DEMs ranging from the ocean floor to surface landscapes of both Earth and Mars. The result was evaluated with respect to modal morphometric features and to characteristic scales, suggesting a more robust method for deriving both morphometric classifications and terrain attributes. We argue that the method would be preferable to any single-scale crisp approach, at least in the context of preliminary hands-off morphometric analyses of DEMs.     

Magnesiochloritoid: Compressibility and high pressure structure refinement

The response of magnesiochloritoid to pressure has been studied by single crystal X-ray diffraction in a diamond anvil cell, using crystals with composition Mg_1.3Fe_0.7Al₄Si₂O₁₀(OH)₄. The unit cell parameters decrease from a = 9.434 (3), b = 5.452 (2), c = 18.136 (5) Å, β = 101.42° (2) (1 bar pressure) to a = 9.370 (7), b = 5.419 (5), c = 17.88 (1) Å, β = 101.5° (1) (42 kbar pressure), following a slightly anisotropic compression pattern (linear compressibilities parallel to unit cell edges: β _a = 1.85, β _b = 1.74, β_c = 3.05 × 10^?4 kbar^?1) with a bulk modulus of 1480 kbar. Perpendicular to c, the most compressible direction, the crystal structure (space group C2/c) consists of two kinds of alternating octahedral layers connected via isolated SiO₄ tetrahedra. With increasing pressure the slightly wavy layer [Mg_1.3Fe_0.7AlO₂(OH)₄] tends to flatten. Furthermore, the octahedra in this layer, with all cations underbonded, are more compressible than the octahedra in the (A1₃O₈) layer with slightly overbonded aluminum. Comparison between high-pressure and high-temperature data yields the following equations: $$\begin{gathered} a_{P,T} = 9.434{\text{ }}{\AA} - 174 \cdot 10^{ - 5} {\text{ }}{\AA}{\text{kb}}^{{\text{ - 1}}} \cdot P \hfill \\ {\text{ }} + 9 \cdot 10^{ - 5} {\text{ }}{\AA}^\circ C^{ - 1} \cdot (T - 25^\circ C) \hfill \\ b_{P,T} = 5.452{\text{ }}{\AA} - 95 \cdot 10^{ - 5} {\text{ }}{\AA}{\text{kb}}^{{\text{ - 1}}} \cdot P \hfill \\ {\text{ }} + 5 \cdot 65 \cdot 10^{ - 5} {\text{ }}{\AA}^\circ C^{ - 1} \cdot (T - 25^\circ C) \hfill \\ c_{P,T} = 18.136{\text{ }}{\AA} - 549 \cdot 10^{ - 5} {\text{ }}{\AA}{\text{kb}}^{{\text{ - 1}}} \cdot P \hfill \\ {\text{ }} + 16 \cdot 2^{ - 5} {\text{ }}{\AA}^\circ C^{ - 1} \cdot (T - 25^\circ C) \hfill \\ \end{gathered} $$ with P in kbar and T in °C. These equations indicate that the unit cell and bond geometry of magnesiochloritoid at formation conditions do not differ greatly from those at the outcrop conditions, e.g. the calculated unitcell volume is 917.3 ų at P = 16 kbar and T=500 °C, whereas the observed volume at room conditions is 914.4 ų. In addition, they show that the specific gravity increases from formation at depth to outcrop at surface conditions.     

The transition between ‘orogenic’ and ‘anorogenic’ magmatism in the western Mediterranean area: the Middle Miocene volcanic rocks of Isola del Toro (SW Sardinia,Italy)

The Isola del Toro is a small island (∼1 km²) about 5 nautical miles SW of Sardinia entirely made up of mildly alkaline sodic evolved rocks (benmoreites and trachytes) related to each other by fractionation processes. ⁴⁰Ar/³⁹Ar dating of an anorthoclase mineral separate from a trachyte gave an age of 11.83 ± 0.10 Ma. This age estimate falls within an interval previously considered a‐magmatic in Sardinia. The results of this study shift the beginning of the Middle Miocene–Quaternary anorogenic volcanism of Sardinia back to the Late Serravalian. The transition between orogenic and anorogenic activity in Sardinia is markedly different from that recorded in the rest of the circum‐Mediterranean area, where this transition is much more gradual, both in time and in terms of magma composition. On the other hand, the transition between orogenic and anorogenic magmatism in Sardinia is rather sharp and lacks hybrid compositions. The Isola del Toro volcanic rocks represent the first anorogenic products produced during the opening of the western Mediterranean Sea.