On gravitational instability of the interstellar gas

In the galaxy, Jeans' critical length for the interstellar gas is appreciably smaller than the critical length for the stars, a necessary condition for the gravitational instability of the former to have a local character. An accurate discussion of the orders of magnitude involved leads to the establishment of a well defined limiting procedure and to simplified equations in which the effect of stars occurs only through the equilibrium, but disappears from the perturbations. The equations are spatially local, but their coefficients are time-dependent, in that they describe the evolution of a small wave packet dragged along by the supersonic gas motion. They have been solved in several interesting cases by the introduction of an effective, time-dependent wave vector, which describes the deformation of a wave profile due to the velocity gradients. The ordinary Jeans' instability is recovered only when the velocity gradient is a skew tensor; otherwise we find a stabilizing effect in accelerated and sheared flows, a destabilizing effect in a decelerated flow. Possible connections of this model with the observed turbulent structure of the interstellar gas are discussed.     

The density of Red Sea brines

The densities of artificial and real Red Sea brines have been measured at 25°C with a vibrating tube densimeter. Measurements were also made on mixtures of Red Sea brines with seawater and pure water. The results have been used to characterize the density—composition relations of waters across the interface of brine and average Red Sea waters. The results for the real and artificial brines are in reasonable agreement.The composition of the brines and mixtures with seawater have been characterized by conductivity measurements of weight diluted samples. The conductivity salinities were found to be conservative to within $\text{±0.07‰}$ for the mixtures.The densities of brines and mixtures of brines and seawater were estimated from apparent molal volume data using Young's rule. The calculated densities for all of the solutions were found to be in good agreement with the measured values, demonstrating the applicability of Young's rule to concentrated natural waters.     

Diffuse Baryons in Cosmic Structures

I review the processes that govern the amount and the thermal state of the hot plasma pervading clusters and groups of galaxies: the gravitational heating driven by the DM merging histories, the radiative cooling of baryons, and the energy fed back by SNe and by AGNs or quasars. I argue that the X-ray emissions and the entropy levels now observed from clusters to groups require the AGNs to contribute substantially to preheat the gas before it falls into clusters, and the quasars to blow some plasma out of groups and galaxies.     

Undrained instability detection under general stress conditions

Acta Geotechnica - This work proposes a criterion for the detection of undrained instability under multiaxial stress conditions at elemental level. To do so, it develops the application of the...     

Nonlinear dynamic response of r.c. framed structures subjected to near-fault ground motions

The nonlinear dynamic response of reinforced concrete (r.c.) framed buildings subjected to near-fault ground motions is studied to check the effectiveness of current code provisions with reference to study cases. Three-, six- and twelve-storey r.c. plane frames, representative of symmetric framed buildings, are designed according to the European seismic code (EC8), assuming medium and high ductility classes and stratigraphic profiles A (rock) and D (soft soil) in a high-risk seismic region. The nonlinear seismic analysis is performed using a step-by-step procedure; a bilinear model idealizes the behaviour of the r.c. frame members. Artificially generated motions (matching EC8 response spectra for subsoil classes A and D) and horizontal motions (recorded on rock- and soft soil-site at near-fault areas) are considered. The results indicate that near-fault ground motions may require a special consideration in the code, in particular when designing r.c. framed structures placed on a soft soil-site; particular attention should be paid to the design of the frame members of the lower storeys.     

Mineralogy, geochemistry and geological significance of tourmaline-rich rocks from the Paleozoic Cinco Villas massif (western Pyrenees, Spain)

Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif, western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks, which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2 tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning. Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying heavier δ¹⁸O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines show intermediate values of 11.3 to 14.6‰ for δ¹⁸O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales (≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids, and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism. Received: 18 November 1996 / Accepted: 25 April 1997     

Late Neoproterozoic crustal growth in the European Variscides: Nd isotope and geochemical evidence from the Sierra de Córdoba Andesites (Ossa-Morena Zone, Southern Spain)

Because the Hercynian overprint was extremely weak, the Sierra de Córdoba (southeastern Ossa-Morena Zone, OMZ) provides an excellent opportunity to study the tectonic evolution of sequences deposited close to the Late Neoproterozoic–Early Palaeozoic boundary. In order to put constraints on the sources and geodynamic significance of the Late Proterozoic magmatism, a representative set of 18 igneous rocks, and 3 interbedded sedimentary rocks from the San Jerónimo Formation have been studied for major and trace element geochemistry and for the Sm–Nd isotopic systematics. The igneous rocks are generally porphyritic to microporphyritic andesites, with abundant plagioclase (±amphibole) phenocrysts. With the exception of two intrusive rocks, possibly not related to the Late Proterozoic episode, all the samples display positive _{Nd550 Ma} values, ranging from +2.9 to +7.6. Most of them, with +4<_{Nd550 Ma}<+6, exhibit LREE enrichment, high La/Nb ratios, and elevated Zr/Nb ratios ranging from 21 to 32. There is no obvious correlation between the shape of REE patterns, La/Nb ratios and _{Nd550 Ma} values, precluding simple models of late-stage interaction with typical crustal components having low _Nd and high LREE/HREE and La/Nb ratios. Based on their major element composition and enriched, continental crust-like trace element characteristics, combined with distinctly positive _Nd initial values, the Córdoba andesites document an episode of crustal growth through the addition of calc-alkaline magmas, extracted from a mantle reservoir which was strongly depleted in LREE on a time-integrated basis. The occurrence of interlayered sediments of continental provenance (negative _Nd values) does not favour a purely ensimatic arc setting, remote from continental land masses, for this subduction-related magmatism, but the geochemical data suggest an active margin environment located on relatively juvenile crust. In any case, the Córdoba andesites document the addition of materials chemically similar to the bulk continental crust which were extracted from mantle sources with strong time-integrated LREE depletion. Therefore, they provide evidence for crustal growth related to Cadomian orogenic events during Late Proterozoic times.     

Criterion for flow liquefaction instability

This study describes a general liquefaction flow instability criterion for elastoplastic soils based on the concept of loss of uniqueness. We apply the criterion to the general case of axisymmetric loading and invoke the concepts of effective stresses and loss of controllability to arrive at a general criterion for the onset of liquefaction flow. The criterion is used in conjunction with an elastoplastic model for sands to generate numerical simulations. The numerical results are compared with experimental evidence to give the following insights into predicting liquefaction. (1) The onset of liquefaction flow is a state of instability occurring under both monotonic and cyclic tests, and coincides with loss of controllability. (2) The criterion proposed herein clearly and naturally differentiates between liquefaction flow (instability) and cyclic mobility. (3) Flow liquefaction not only depends on the potential of the material to generate positive excess pore pressures, but more importantly, it also depends on the current state of the material, which is rarely predicted by phenomenology.