Isotopes in Environmental and Health Studies Vol. 42, No. 2, June 2006, 189–201 Radiogenic argon distribution within a mineral grain: implications for dating of hydrothermal mineral-forming event in Sludyanka complex, Siberia, Russia IVAN S. BRANDT, SERGEI V. RASSKAZOV, ALEXEI V. IVANOV, LEONID Z. REZNITSKII and SERGEI B. BRANDT* Institute of the Earth’s Crust SB RAS, Lermontov St. 128, Irkutsk 664033, Russia (Received 7 December 2004; in final form 1 September 2005) Usually, in age-determination procedures, the experimenter is interested in the mean concentra- tions 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 distri- bution, 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 coef- ficient 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 gran- ulite 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 labo- ratory kinetic experiment. The age determined by the rectangularly distributed argon corresponded to α × 0.129 nmm 3 /g (total radiogenic argon in the sample) = 94.17 nmm 3 /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 40 Ar/ 39 Ar 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 39 Ar– 40 Ar 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 indi- cates that K–Ar isotopic system in the phlogopite was not closed (or was partially reset) long after *Corresponding author. Tel.: +7-39-5242-8990; Fax: +7-39-5242-6900; Email: bra@crust.irk.ru Isotopes in Environmental and Health Studies ISSN 1025-6016 print/ISSN 1477-2639 online © 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/10256010500502686