The solubility of [4H]Si defects in α-quartz and their role in the formation of molecular water and related weakening on heating

The nature of the solubility of water as [4H]_Si defects in quartz, and their role in providing a source of molecular water on heating, is investigated. Existing ab inito energy calculations on the incorporation of water in quartz are used to show that energetically 4H for Si substitution is likely to constitute the most prevalent mode of water uptake on the atomic scale in quartz under equilibrium conditions, and that the planar defects previously observed by a number of different authors by electron microscopy in wet quartz are likely to be planar rafts of aggregated [4H]_Si defects which are formed on supersaturation. These new conclusions call into question the previous identification of the planar defects as high pressure water clusters and require that their role in the production of molecular water in the context of recent theories of hydrolytic weakening be re-assessed. Accordingly the existing ab initio results have been used to establish the characteristics of the phase diagram for the system quartz-water in the temperature and pressure range of interest in hydrolytic weakening. Additional electron-optical experiments on wet quartz show that, on annealing at temperature in the electron microscope, similar planar defects develop in wet quartz by a diffusion process. In the context of existing theories of hydrolytic weakening it is now proposed that the conversion of [4H]_Si defects to molecular water, where this is dictated by the equilibrium phase diagram, leads to a relatively large increase in volume and to the appearance of the bubbles of free water and the nucleation of associated prismatic dislocation loops of Burgers vector b=1/3 a $\langle 11\bar 20\rangle $ as previously observed. Ultimately the development of these loops leads to dislocation-induced plasticity.     

Plate and intraplate processes of Hercynian Europe during the late paleozoic

It is speculated that until Late Carboniferous time the region of Hercynian Europe was occupied by an elongated island arc system underlain by a segment of continental crust. In the Upper Carboniferous, two subduction zones are assumed to have extended from the north and south beneath Hercynian Europe. An extensive zone of hot, partially molten upper mantle lay above and between these, and diapiric uprise of portions of this material led to separation of mafic magmas, widespread partial melting in the lower and middle crust, high temperature-low pressure metamorphism in crustal rocks, and regional uplift and extension of the crust, as indicated by intermontane troughs and their associated volcanic rocks.In Visean to Westphalian time Hercynian Europe collided with both the large neighbouring plates North America-Europe and Africa. During these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust, the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of Hercynian Europe. After the collision processes had been terminated, processes of upper mantle activity continued, causing further crustal uplift and even, enhanced crustal extension for several tens of million years into the Lower Permian. Decline of the upper mantle activity beneath Hercynian Europe is indicated by crustal subsidence and formation of a peneplain in Permian time followed by the Upper Permian transgression of both the Zechstein sea and the Tethys sea which mark the end of the Hercynian geodynamic cycle.     

The observational signature of the first H ii regions

We use three-dimensional smoothed particle hydrodynamics simulations together with a dynamical ray-tracing scheme to investigate the build-up of the first H  ii regions around massive Population III stars in minihaloes. We trace the highly anisotropic breakout of the ionizing radiation into the intergalactic medium, allowing us to predict the resulting recombination radiation with greatly increased realism. Our simulations, together with Press–Schechter type arguments, allow us to predict the Population III contribution to the radio background at  ∼100 MHz  via bremsstrahlung and 21-cm emission. We find a global bremsstrahlung signal of around  1 mK  , and a combined 21-cm signature which is an order of magnitude larger. Both might be within the reach of the planned Square Kilometer Array experiment, although detection of the free–free emission is only marginal. The imprint of the first stars on the cosmic radio background might provide us with one of the few diagnostics to test the otherwise elusive minihalo star formation site.     

Synchronous uplift of the lower crust of the Ivrea Zone and of Southern Calabria and its possible consequences for the Hercynian orogeny in Southern Europe

Along the border of the Adriatic microplate, pre-Alpine granulite-facies rocks from the deepest crust are outcropping at only two places: in the Ivrea Zone of the Southern Alps and in Southern Calabria. In these two areas the main features of the present crustal structures, i.e. overlapping of large continental crustal and upper mantle segments, are interpreted as resulting from their Hercynian geodynamic evolutions.The tilted, nearly complete crustal sections in both areas display very similar lithological sequences and experienced a common geological evolution, as deduced from petrological and radiometric dates. At the end of Hercynian time (～295 m.y.), the Ivrea body and the lower crustal rocks of Southern Calabria were contemporaneously sheared off from the upper mantle and uplifted into intermediate crustal levels, where they slowly cooled during Mesozoic time. The tectonic uplift was accompanied by granitoid plutonism and andesitic to rhyolitic volcanism, which continued after the Hercynian uplift.Considering the presently similar crustal structures and the Upper Carboniferous and Permian geological evolutions along the whole Adriatic plate boundary, the Ivrea Zone and Southern Calabria are used to resolve the pre-Alpine history of the boundary zone between the Adriatic and the Central European block: the uplift of the lower crustal/upper mantle flakes of the Adriatic block was due to flat overthrusting of these flakes on the continental crust of “Central Europe”. The material of the Central European crust underthrust (subducted) thereby melted during the re-equilibration of the geotherms which had been disturbed by the subduction process; this led to an extensive calc-alkaline plutonism and volcanism of crustal origin along the Adria boundary. In this boundary region, the overlying uppermost crustal levels (“Schiefergebirgsstockwerk”) were synchronously folded (“Asturian phase”) in response to the overlapping of the deeper crustal levels. Subsequently to the orogeny, the mountain chain was eroded and molasse basins developed on the overthrust Adriatic crustal segment during the Lower Permian.In this model, the granulite-facies flakes of the Ivrea Zone and of Southern Calabria are interpreted as pre-Hercynian lower crustal segments which were thrust into the middle crust during the Hercynian orogeny, thus giving rise to wave velocity inversions in the crust. Further, it is proposed that similar geodynamic processes have played a role in the genesis of the Conrad discontinuity which is present in many parts of the Hercynian fold belt. But only in the Ivrea Zone and in Southern Calabria the crustal discontinuities formed in Hercynian time were uplifted to the surface as a result of Alpine reactivation of the Adriatic boundary zone and due to their special positions in the bends of the Alpine-Apennine-Maghrebide mountain system.According to the present knowledge of the Carboniferous paleogeography and of the orogenic evolution on both sides of the Adria sufure zone, this fault zone was located within the European continent. Its role during the Hercynian orogeny is discussed envisaging two possibilities: an A-subduction zone or a subfluence zone (in the sense of Behr and Weber).