Diamonds from the Ural Mountains: their characteristics and
the mineralogy and geochemistry of their inclusions.
PhD thesis, University of Glasgow.
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This thesis has investigated the geological origin of diamonds from the Ural Mountains. A set of inclusion-bearing diamonds from alluvial deposits in the western part of the Urals was characterised on the basis of their morphological features, nitrogen contents and nitrogen aggregation states, carbon and nitrogen stable isotopes, mineral inclusion geochemistry and radiometric isotopic ages of the inclusions.
The vast majority of the studied diamonds are rounded dodecahedra, which indicates that the diamond population has experienced major resorption after crystallisation. The majority of the diamonds are affected by radiation damage and display evidence of transportation. Non-abraded diamonds exhibit similar surface features to those abraded, so they are probably of similar origin. The studied inclusion-bearing set of diamonds shares some characteristics with the overall, mostly inclusion-free, diamond population from the Ural Mountains. This similarity in physical characteristics strongly suggests that the Ural diamonds are all part of a single population.
A Fourier Transform Infra-Red (FTIR) spectroscopy study allowed both the concentration of nitrogen and the aggregation states of this element to be quantified. Diamonds from other known primary deposits in the East European Craton (EEC) have FTIR signatures that do not match that of the studied population. Nitrogen thermometry results suggest that the Ural diamonds probably crystallised under similar pressure-temperature conditions. If a similar overall regime of formation for the Urals alluvial diamonds is considered, then a single primary diamond source or a spatial proximity between primary contributory sources seems likely.
The variations in δ15N – δ13C measured in the Ural diamonds of the peridotitic and eclogitic paragenesis suggest derivation from a similar, initially homogenous, mantle carbon source which has been subjected to metasomatic-induced isotopic fractionation. However, for some δ15N-enriched – δ13C-depleted eclogitic diamonds, the possibility of crystallisation from subduction-related metasomatic fluids/melts cannot be excluded.
Based on the chemical composition of syngenetic mineral inclusions recovered from the Ural diamonds, the eclogitic paragenesis (60%) dominates over the peridotitic (26%), with a minor websteritic assemblage also present (2%). The remaining 12% are diamonds with sulphide inclusions of unknown paragenesis. The chemistry of the mineral inclusions almost completely overlaps that of previous electron microprobe studies of inclusions in diamonds from worldwide localities. Geothermobarometric calculations show an overall agreement between the equilibration conditions of the three inclusion parageneses. The Ural diamonds crystallised at temperatures of 1050-1300°C, at minimum depths of about 165 km, within a diamondiferous lithosphere extending to at least 230 km at the time of diamond formation.
The Re-Os isotope genesis age data for syngenetic sulphide inclusions and the 40Ar/39Ar laser probe eruption ages of syngenetic clinopyroxene inclusions were determined. Six eclogitic sulphide inclusions, two of which coexist in the same diamond, gave an isochron age of 1280 ± 310 Ma which may be associated with rift-related magmatism that affected the EEC at ca. 1.3 Ga. The determined genesis age is also similar to genesis ages reported for eclogitic diamonds from a number of mines in southern Africa, and this is probably indicative of a global diamond formation event at that time. 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 indicates that the Ural diamonds are not derived either from the diamond-bearing kimberlites of the Siberian craton, nor from presently known Russian and Finnish kimberlite provinces on the EEC.
An integrated model for the genesis, eruption and accumulation of the Ural diamonds in the context of the evolution of the EEC is proposed. The Urals placer deposits are mainly confined to 407-397 Ma sedimentary rocks along the western side of these mountains, with diamond size distribution indicating sediment transportation at that time generally from the north-west. The diamondiferous sedimentary accumulation in the Urals is envisaged as being analogous to that presently found along the Namaqualand / Namibian coastal belt in the western margin of southern Africa. During the construction of the Ural Mountains, the diamondiferous sediments became part of the western accretion zone when the EEC united with the Kazakhstan and Siberia plates during late Devonian through to late Triassic times. The evidence presented in this thesis suggests the existence of an undiscovered kimberlite/lamproite primary source, probably on the Volgo-Uralia crustal segment of the EEC, which gave rise to the Urals diamond deposits.
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