Vol 24, No 1 (2016)

The presence of condensate glass was established for the first time in natural terrestrial impact glasses. Its existence was previously predicted on the basis of shock evaporation experiments. Condensate glass was found in the irghizites of the Zhamanshin crater. Its characteristic features were described, including a combination of high silica content with the presence of nanoscale crystalline nuclei and typical globular morphology under TEM.

The paper presents data on the production of the ³He nuclide in rocks under the effect of cosmicray particles. The origin of the nuclide in the ground in neutronand proton-induced spallation reactions, reactions induced by high-energy muons, and negative muon capture reactions is analyzed. The cross sections of reactions producing ³He and ³H are calculated by means of numerical simulations with the GEANT4 simulation toolkit. The production rate of the ³He nuclide in the ground is evaluated for the average level of solar activity at high geomagnetic latitudes and at sea level. It is proved that the production of ³He in nearsurface ground layers by spallation reactions induced by cosmic-ray protons may be approximately 20% of the total production rate of cosmogenic ³He. At depths of 10–50 m.w.e., the accumulation of ³He is significantly contributed by reactions induced by cosmic-ray muons. Data presented in the paper make it possible to calculate the accumulation rate of ³He in a rock depending on depth that is necessary for the evaluation of the exposure time of the magmatic or metamorphic complex on the Earth’s surface (³He dating).

This paper reports experimental data on the investigation of the chemical composition of condensed silicate matter produced during the high-temperature pulse vaporization of feldspars. The experiments simulated the conditions of vaporization accompanying a high-velocity impact. Samples of albite, bytownite, calcic and sodic labradorite, and sanidine were used in the experiments. The investigation of the condensate layers obtained in the experiments included the determination of element distribution and structural characteristics of the materials using layer-by-layer X-ray photoelectron spectroscopy. It was shown that the vaporization of the samples occurred mainly through the release of complex atom–molecule groups referred to as clusters. “Nepheline,” “wollastonite,” and “sillimanite” clusters were identified as characteristic groups. The thermodynamic evaluation of melt composition at temperatures up to 5000 K performed using the Magma program confirmed high activities of these components in feldspar melts.

In experimental studies, water decomposition under reducing conditions results in the high fugacity of hydrogen, which is able to form interstitial solid solutions with wüsite. New experiments demonstrate that the hydrogen fugacities attained in the presence of high water pressure and at the oxygen fugacities imposed by the buffer equilibria magnetite–wüstite (MW) and wüstite-iron (WI) are sufficiently high for the occurrence of hydrogen–oxygen interactions and for the formation of H₂-saturated compound. Instead of buffer equilibria, hydrogen fugacity was directly controlled in the experiments at 700–1200°C and 200 MPa using the improved Shaw membrane technique in the form of a specially designed cell with Ar–H₂ mixtures; H₂ mole fraction in the mixtures ranged from 0.0 to 0.8. According to the phase rule for systems with perfectly mobile independent components, buffer reactions in the presence of water-bearing phases change into divariant equilibria. The X-ray study of phase composition along Mag–Wus divariant field shows an increase of wüstite content relative to magnetite with increase of hydrogen mole fraction in a fluid. Mass-spectrometric study showed that the relative bulk solubility of hydrogen in buffer phases at 950°C decreases with an increase of iron content in wüstite owing to the change in wüstite stoichiometry under more reducing conditions. Experimental data indicate that the hydrogen solubility in wüstite results in a shift of the magnetite stability field toward reducing region for \(0.65\log f_{O_2 } \) at 700°C and for \(1.2\log f_{O_2 } \) at 1200°C, whereas the stability of wüstite with metallic iron practically coincides with anhydrous equilibrium, which is related to the close values of hydrogen solubility in wüstite and in the metallic iron at temperatures below 950°C. However, the hydrogen solubility in metallic iron significantly increases with increasing temperature and its divariant field is shifted to a reducing region.