Synthetic fluid inclusions. XV. TEM investigation of plastic flow associated with reequilibration of fluid inclusions in natural quartz

The nature and abundance of dislocations in quartz surrounding fluid inclusions were studied to obtain a better understanding of processes associated with fluid inclusion reequilibration. Synthetic fluid inclusions containing 10 wt% NaCl aqueous solution were formed in three samples at 700 °C and 5 kbar. One of the samples was quenched along an isochore to serve as a reference sample. The other two samples were quenched along a P-T path that generated internal pressures in excess of the confining pressure. The two samples were held at the final reequilibration P-T conditions of 625 °C and 2 kbar for 30 and 180 days, respectively. Following the experiments, microstructures associated with fluid inclusions were examined with the TEM. Quartz in healed fractures in the reference sample that was quenched isochorically shows a moderate dislocation activity. Quartz adjacent to reequilibrated fluid inclusions in the other two samples, however, showed a marked increase in dislocation activity compared to the un-reequilibrated sample. Deformation of the inclusion walls occurred anisotropically by expansion of mobile dislocations in their slip systems. Dislocation expansion was controlled by glide in the rhombohedral planes {1 0 1 1} that was restricted to narrow zones (≤3 μm) in the immediate vicinity of the fluid inclusion walls outside of the healed fracture plane. These plastic zones were observed after both short term (30 days) and long term (180 days) experiments and are attributed to hydrolytic weakening of quartz around fluid inclusions owing to diffusion of water into the quartz matrix during the experiment. The close spatial association of submicroscopic water bubbles with dislocations, and the rarity of water bubbles in the reference sample, show clearly that in both the 30 and 180 day experiments reequilibration involves water loss from the fluid inclusions. Our results indicate that synthetic fluid inclusions in this study recover (chemically and volumetrically), even at relatively fast experimental loading rates, such that internal stresses never reach the point of brittle failure. The driving force for fluid inclusion deformation involves two related mechanisms: plastic deformation of hydrolytically weakened wet quartz in the healed fracture, and water leakage associated with preexisting and strain-induced dislocations. Received: 5 May 1998 / Accepted: 10 February 2000     

Stratospheric circumpolar vortex as a link between solar activity and circulation of the lower atmosphere

Possible reasons for the temporal variability of solar activity (SA) and galactic cosmic ray (GCR) effects on the tropospheric circulation are studied. Long-term variations in the amplitude and sign of SA/GCR effects are shown to be closely related to the state of the stratospheric circumpolar vortex. A ??60-year periodicity was detected the vortex strength which affects the evolution of the large-scale atmospheric circulation. It is shown that the correlation coefficients between pressure in the troposphere and SA/GCR characteristics change the sign in the periods of transformations of the large-scale circulation caused by changes in the state of the vortex. The obtained results suggest an important part of the circumpolar vortex in the mechanism of solar-climate links.     

An experimental study of solid matrix weakening in water‐saturated Savonnières limestone

Petrophysical properties of carbonate reservoirs are less predictable than that of siliciclastic reservoirs. One of the main reasons for this is the physical and chemical interactions of carbonate rocks with pore fluids. Such interactions can significantly change the elastic properties of the rock matrix and grains, making the applicability of Gassmann's fluid substitution procedure debatable. This study is an attempt to understand the mechanisms of fluid‐rock interactions and the influence of these interactions on elastic parameters of carbonates. We performed precise indentation tests on Savonnières limestone at a microscale level under dry, distilled water, and n‐Decane saturated conditions. Our experiments display softening of the rock matrix after water saturation. We have found that mainly the ooid cortices, peloid nuclei and prismatic intergranular cement are affected by water flooding. We also observed a shear modulus reduction in Savonnières limestone in an experiment performed at ultrasonic frequencies. One of the most important results obtained in our experimental study is that the Gassmann fluid substitution theory might not always be applicable to predict the elastic moduli of fluid‐saturated limestones.     

The Hallstatt solar cycle from radiocarbon data

The data on the ¹⁴C concentration in the Earth’s atmosphere are studied on a time span of 50000 years. It is shown that the Hallstatt cycle (a temporal variation with a period of 1500–2000 years) has been present in this series for at least 30000 years. However, this cycle is not purely of solar origin; nonsolar (supposedly climatic) factors contribute into it in certain epochs.     

Control Volume Method for the Dynamo Problem in the Sphere with the Free Rotating Inner Core

A model of thermally driven dynamo in the Boussinesq approximation in the spherical shell with the free rotating inner core is considered. To solve equations we use a new in dynamo modeling control volume technique (for details of this method for hydrodynamics see Patankar, 1980). The main advantage of this method over previous attempts to solve magnetohydrodynamics equations in the spherical grids is that no filtering of high harmonics in the pole regions is needed. We present the results of simulations for the self-consistent dynamo system evolution over the diffusion time and longer periods. Different ways of stabilizations of magnetohydrodynamics equations, when convective terms are of the same order (or larger) as conductive ones, are considered.     

Gamma-ray bursts from unstable Poynting-dominated outflows

Poynting-flux driven outflows from magnetized rotators are a plausible explanation for gamma-ray burst engines. We suggest a new possibility for how such outflows might transfer energy into radiating particles. We argue that, in a region near the rotation axis, the Poynting flux drives non-linearly unstable large-amplitude electromagnetic waves (LAEMW) that 'break' at radii     where the MHD approximation becomes inapplicable. In the 'foaming' (relativistically reconnecting) regions formed during the wave breaks, the random electric fields stochastically accelerate particles to ultrarelativistic energies which then radiate in turbulent electromagnetic fields. The typical energy of the emitted photons is a fraction of the fundamental Compton energy     with     plus additional boosting due to the bulk motion of the medium. The emission properties are similar to synchrotron radiation, with a typical cooling time ∼10⁻³ s. During the wave break, the plasma is also bulk accelerated in the outward radial direction and at larger radii can produce afterglows due to interactions with the external medium. The near equipartition fields required by afterglow models may be due to magnetic field regeneration in the outflowing plasma (similar to field generation by LAEMW in laser–plasma interactions) and mixing with the upstream plasma.     

On the Possibility of Forecasting the Sun's Activity Using Radiocarbon Solar Proxy

Group sunspot number (GSN) values, averaged over decades, were reconstructed for a time interval 8505 BC–AD 1945 using data on the concentration of radiocarbon in tree rings. The prediction of an average level of solar activity was made for the future four decades by means of a nonlinear forecasting method. It was shown that the average activity of the Sun during 2005–2045 would be lower than at present. The given result is compared to the long-term forecasts by other authors. The importance of a paleoastrophysical approach for obtaining a long-term solar prognosis and for revealing the basic characteristics of solar activity change was confirmed.     

Modern progress in solar paleoastrophysics and long-range solar-activity forecasts

Decade-averaged Wolf numbers are reconstructed for the time interval 8005 B.C.–1945 A.D. using radiocarbon data derived from tree rings. Comparisons of other paleoastrophysical reconstructions of solar activity with this temporal series verify its validity and reliability. A prediction of the mean solar activity for the next forty years is made using these reconstructed Wolf numbers. It is likely that the mean solar activity during 2005–2045 will be lower than the activity of recent decades. This conclusion is compared with the long-term predictions proposed by other researchers. The prospects for paleoastrophysical predictions for the long-term variations of solar activity in the future are discussed.