Озеро Смердячее

THE SMERDYACHEYE LAKE: NEW EVIDENCE FOR IMPACT ORIGIN AND FORMATION AGE. L. L. Kashkarov, D. D. Badjukov, A. I. Ivliev, G. V. Kalinina, and M. A. Nazarov, Vernadsky Institute of Geochem-istry and Analytical Chemistry, Kosygin St., 19, Moscow, 119991, Russia, ugeochem@geochem.home.chg.ru.

Introduction: The Smerdyacheye Lake of about 350 m in diameter is located in the Moscow district approximately 140 km east of Moscow. This lake has a circular shape and a well-developed rim. Based on the features, it has been suggested [1] that the lake is a possible meteorite crater. The suggestion was con-firmed by a fragment of a possible impactite found in the rim [2]. In this paper we report on a glass bead discovered recently in the vicinity of the lake. In com-position and texture the glass is similar to im-pactglasses and supports strongly impact origin of the lake. First data on fision track age and TL characteris-tics of this glass bead are reported.

Occurrence and chemistry: The glass bead was found 3 km north of the lake in a tranch made in a pinewood. The bead of a rounded shape and a dark green color is about 2.5x1.5x1.5 cm in size and cov-ered partly with a white shell of a few mm in thick (Fig 1). The glass is totally isotropic, shows a fluidal texture and contains rare bubbles. No very significant compositional variations were found in the glass. The compositional range is (wt%):

Fission track dating: The main problem of fission track dating of young glasses is related with precise determination of ρ/ρ0 and d/d0 [3], where: ρ, ρ0 and d, d0 are track densities and track pit diameters of fossil and neutron-induced tracks, respectively. In the Smer-dyacheye glass two fossil track groups with very different d-values were recognized. . After prolonged chemical etching in 20% HF (up to 5 min, at +20^{oC) d-values of the neutron induced tracks were found to be in the interval of 5 - 17 μm. Note, it is not correct to take into account the average diameter (dAV) of the neutron-induced tracks because such tracks with d < 12 μm correspond to new appearing tracks during the prolonged etching.}

The U concentration (CU) was measured by (INAA and thermal neutron-induced 235U fission track meth-ods. The flux of thermal neutrons (Fth n)was (1.50 ± 0.05) × 1015 cm-2. The U contents obtained by INAA and the track method are 4.5 and 4.14 ± 0.12 ppm, respectively. The study of neutron-induced track den-sities demonstrates inhomogeneous U distribution in the glass. Measurements of CU in the shell material using the autoradiographic fission track method give: (1) an average CU value of 3.78 ± 0.11 ppm; (2) a CU range in areas of 4 × 10-4 cm2 from 2.0 to 6.3 ppm ; and (3) a CU range in areas of less than 100 × 100 μm from 10 to 50 ppm. The scatter of U concentrations should be related with incomplete homogenezation of the parent melt during the short-time high-temperature

I-st and II-nd groups are fossil tracks of different di-ameters; III-rd group is neutron-induced tracks de-tected on the area of 1.20 Ч 10^{-2 cm2; ** N_{TR – number of fossil tracks on the area of 4.27 cm2,; *** dAV is the average diameter; dq is a quartil from the maximum of the diameter distridution of neutron-induced tracks}}

Table 2. Fission track ages of the Smerdyacheye glass

ρ^{’ is a measured track density; ** ρ_{SP is a corrected track density; *** T = 6Ч10-8ЧFth n Ч( ρSP / ρIND )GL.}}

Thermoluminescence analyses: Measurements of the natural TLNAT, saved from the moment of last heatin-gof the glass, and TLREN induced by X-ray irradiation were carried out using a high-sensitivity equipment [3]. The following parameters of TL glow-curves were measured: (1) the TL-intensity in the low-and high-temperature intervals; (2) the minimum temperature of heating of the glass, at which a very faint TL-luminosity occurs; 3) the peak position on the glow-curves, (TPEAK , oC), the height of a peak (ITL, rel. units) and the full width of a peak at a half of its maximum (FWHM, oC). Some of the parameters are given in Table 3 and 4.

TLNAT
TPEAKoC	INAT	FWHM,oC
190 320	0.003 0.01	60 80

TLREN
TPEAKoC	IREN	FWHM, oC
110 320	0.10 0.065	50 80

Conclusion: The study shows that the age of the Smerdyacheye crater should be about a few tens of Ka. The date does not conflict with the geological con-strains on the crater age [3].

References: [1] Y. V. Kestlane and K. H. Melle (1987) XX All-Union Meteoritic Conference, Tallin, abstracts, part I, 47-48. [2] Badjukov D. D. et al. (2003) LPSC XXXIV, #1556. [3] D. Storzer and G.A.Wagner (1969) EPSL, V.5, 463-468.