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.