Re-Os and 40Ar/39Ar isotope measurements of inclusions in alluvial diamonds from the Ural Mountains: Constraints on diamond genesis and eruption ages

The Re-Os isotope data for 20 syngenetic sulphide inclusions, recovered from 15 diamonds and the 40Ar/39Ar laser probe eruption ages of 7 syngenetic clinopyroxenes recovered from 5 diamonds, all from alluvial placer deposits in the Ural Mountains, have been determined. Six eclogitic sulphide inclusions, two of which coexist in the same diamond, yield an isochron age of 1280 +/- 310 Ma (2 sigma), with an unusually high initial Os-187/Os-188 ratio of 2.10 +/- 0.58 (2 sigma). The age is interpreted to date remobilisation of carbon and sulphur, and homogenisation of Ors, during rift-related thermal-magmatic events that affected the East European Craton (EEC) at ca. 1.3 Ga. The high initial Os ratio suggests Re-Os evolution over a 100 to 500 Ma period within previously metasomatised lithosphere, most likely the EEC. Five eclogitic clinopyroxenes recovered from four diamonds yielded similar 40Ar/39Ar ages averaging 472 +/- 28 Ma, which likely approximate the time of source kimberlite/lamproite eruption. This age suggests that the Ural diamonds are not likely to have derived either from the well known diamond-bearing kimberlites of the Siberian craton, nor from presently known Russian and Finnish kimberlite provinces on the EEC. The Urals placer deposits are mainly confined to 397-407 Ma sedimentary rocks along the western side of these mountains, with sediment transportation at that time generally from the north-west. Present evidence suggests the existence of an undiscovered kimberlite/ lamproite source, probably on the Volgo-Uralia crustal segment of the EEC, which gave rise to the Urals diamond deposits. (C) 2009 Elsevier B.V. All rights reserved.