Radiogenic argon distribution within a mineral grain: implications for dating of hydrothermal mineral-forming event in Sludyanka complex,Siberia, Russia

Usually, in age-determination procedures, the experimenter is interested in the mean concentrations of parent and daughter substances. However, the structure of a solution of the Fick’s diffusion equation enables one to specify a procedure, allowing, through macroscopic experiments, one to determine the distribution of radiogenic argon (as well as any of other isotopes) within a mineral grain. Argon, influenced in the geological past by a metamorphic event, has a half-sinusoidal distribution, whereas argon preserved in geological time has a rectangular distribution. The ratio of the quantity of argon of the first type to the total argon is called an α-factor. It is believed that this coefficient can be of great interest for geochronological studies, being a symptom of virtual Ar losses and the time instant, at which they happened. A laboratory procedure and a mathematical algorithm to determine the α-factor are developed. As an example, a determination of the α-factor on a vein-phlogopite from the well-studied Sludyanka deposit, Southern Baikal, Siberia, Russia, is described. Rb–Sr isochron age on phlogopite–calcite–apatite paragenetic assemblage yielded 460±7 Ma and is constrained by U–Pb zircon ages of 471±1 and 447±2 Ma, respectively, on an emplacement of early syenites and monzonites and later ‘post-phlogopitic’ pegmatites [E.B. Salnikova, S.A. Sergeev, A.B. Kotov, S.Z. Yakovleva, R.H. Steiger, L.Z. Reznitskiy, E.P. Vasil’ev. U–Pb zircon dating of granulite metamorphism in the Sludyanskiy complex, Eastern Siberia. Gondwana Res., 1, 195–205 (1998). L.Z. Reznitskii, G.P. Sandimirova, Y.A. Pakhol’chenko, S.V. Kuznetsova. The Rb–Sr age of phlogopite deposits in Sludyanka, southern Baikal region. Dokl. Earth Sci., 367, 711–713 (1999). L.Z. Reznitskii, A.B. Kotov, E.B. Salnikova, E.P. Vasil’ev, S.Z. Yakovleva, V.P. Kovach, A.M. Fedoseenko. The age and time span of the origin of phlogopite and lazurite deposits in the Southwestern Baikal area: U–Pb geochronology. Petrology, 8, 66–76 (2000).]. In this study, the phlogopite gave an apparent K–Ar age of 365.6±10.4 Ma which has no geological meaning. The α-factor of 0.27 was derived from a laboratory kinetic experiment. The age determined by the rectangularly distributed argon corresponded to α×0.129 nmm³/g (total radiogenic argon in the sample)=94.17 nmm³/g, and an age of ～274 Ma (e.g. a period without any argon losses). Here, at the first sight, we deal with common argon losses not conjugated with any geological event, because after 447 Ma only lower temperature (in a range of 50–300 ^°C) hydrothermal processes were recorded. A paragenetic phlogopite-bearing vein mineral, hyalophane, having a disturbed Rb–Sr isotopic system, however, yielded ⁴⁰Ar/³⁹Ar subplateau age of 271±2 Ma [V.V. Ivanenko, M.I. Karpenko, M.A. Litsarev. Age of the Sludyanka phlogopite deposits (in Russian; data of the ³⁹Ar–⁴⁰Ar method). In Geological Series 5, pp. 92–98, Izvestiya Akademii Nauk SSSR, Moskva (1990).], marking similar timing of the K–Ar isotopic system closure. This indicates that K–Ar isotopic system in the phlogopite was not closed (or was partially reset) long after its crystallisation and closing of the Rb–Sr system, in spite of the fact that the crystal was not affected by processes having temperatures above the accepted closure temperatures for micas. There seems to have acted an exchange mechanism of OH-groups between surrounding fluid and hydrocrystal structure. Thus the argon loss did not occur, spontaneously, but argon particles seem to have been captured and carried to the mineral surface by vacancies formed during displacements of OH-groups.     

Integrable boundary interaction in 3D target space: The “pillow-brane” model

We propose a model of boundary interaction, with three-dimensional target space, and the boundary values of the field X∈R³$\mathbf{X}\in {\mathbb{R}}^3$ constrained to lay on a two-dimensional surface of the “pillow” shape. We argue that the model is integrable, and suggest that its exact solution is described in terms of certain linear ordinary differential equation.     

An efficient numerical tool for dose deposition prediction applied to synchrotron medical imaging and radiation therapy

Medical imaging and radiation therapy are widely used synchrotron‐based techniques which have one thing in common: a significant dose delivery to typically biological samples. Among the ways to provide the experimenters with image guidance techniques indicating optimization strategies, Monte Carlo simulation has become the gold standard for accurately predicting radiation dose levels under specific irradiation conditions. A highly important hampering factor of this method is, however, its slow statistical convergence. A track length estimator (TLE) module has been coded and implemented for the first time in the open‐source Monte Carlo code GATE/Geant4. Results obtained with the module and the procedures used to validate them are presented. A database of energy‐absorption coefficients was also generated, which is used by the TLE calculations and is now also included in GATE/Geant4. The validation was carried out by comparing the TLE‐simulated doses with experimental data in a synchrotron radiation computed tomography experiment. The TLE technique shows good agreement versus both experimental measurements and the results of a classical Monte Carlo simulation. Compared with the latter, it is possible to reach a pre‐defined statistical uncertainty in about two to three orders of magnitude less time for complex geometries without loss of accuracy.     

Metastable carbon allotropes in picosecond-laser-modified diamond

In this paper, we report on the bulk modifications of type IIa single-crystal diamond with visible 10-ps pulses (at λ = 532 nm) and microstructural changes characterized by the appearance of several ‘unidentifiable’ vibrational modes in the frequency range of 1000–1400 cm^?1 in the Raman spectra of laser-modified diamond. It is found that the new Raman modes are strongly pronounced in the spectra of high-stress regions in immediate proximity to the bulk microstructures in the absence of the G mode at ~1580 cm^?1 characteristic of the sp² phase. The high internal stresses are determined from the splitting of the triply degenerate diamond Raman line. The revealed structure transformation is localized within a narrow bulk layer near the bulk microstructures formed, and the stress relaxation is found to result in disappearance of the detected vibrational modes in the spectra. It is suggested that the formation of bulk regions with a sp³ carbon structure consisting of Z-carbon and hexagonal diamond is responsible for the appearance of new Raman modes in the spectra of laser-modified diamond. These findings evidence that the stress-assisted formation of novel metastable carbon phases or defect structures occur in the course of bulk modification of diamond with ps-laser pulses. In addition, we report the results of simulations of internal stresses in the system ‘graphitized cylinder-in-diamond’ to show (1) the effect of the mechanical properties of laser-modified diamond on the resulting stresses and (2) formation of bulk microscopic regions with high stresses of >10 GPa, i.e., the conditions at which various sp³ carbon allotropes and defect structures become more stable than graphite.     

Multicyclic fatigue of stainless steel treated by a high-intensity electron beam: surface layer structure

It is shown that an electron-beam treatment of the 08X18H10T steel specimens under the condition of melting of the ~5 μm surface layer (electron beam energy density 25 J/cm²) increases their fatigue life by a factor of 3.5. The structural-phase states and defect substructure of this steel are studied by the methods of optical, scanning and transmission electron microscopy, and the factors responsible for its increased fatigue life are revealed.     

On isotropic turbulence in the dark fluid universe

As a first part of this work, experimental information about the decay of isotropic turbulence in ordinary hydrodynamics, [`(u²(t))] μ t^-6/5\overline{\mathbf{u}^{2}(t)}\propto t^{-6/5}, is used as input in FRW equations in order to investigate how an initial fraction f of turbulent kinetic energy in the cosmic fluid influences the cosmological development in the late, quintessence/phantom, universe. First order perturbative theory to the first order in f is employed. It turns out that both in the Hubble factor and in the energy density, the influence from the turbulence fades away at late times. The divergences in these quantities near the Big Rip behave essentially as in a non-turbulent fluid. However, for the scale factor, the turbulence modification turns out to diverge logarithmically. As a second part of our work, we consider the full FRW equation in which the turbulent part of the dark energy is accounted for by a separate term. It is demonstrated that turbulence occurrence may change the future universe evolution due to dissipation of dark energy. For instance, the phantom-dominated universe becomes asymptotically a de Sitter one in the future, thus avoiding the Big Rip singularity.     

Selective One-Pot N-Monomethylation of 2-Nitroanilines Under Ptc Conditions

Direct PTC methylation of 2-nitroanilines with dimethyl sulfate gave selectively N-monomethylated products. A variety of N-methyl-2-nitroanilines were prepared in this way.