MINERAL AND FLUID INCLUSIONS IN THE DIAMONDS FROM THE URAL PLACERS, RUSSIA

Abstract:

A study of compositions of mineral inclusions from representative collection (more than 150 samples) of diamonds from the placers of Ural Mountains was performed. Overwhelming majority of rounded octahedral and dodecahedral stones typical for placers contain eclogitic (E-type) mineral inclusions (up to 80%) represented by garnets with Mg# 40-75 and Ca# 10-56 including a unique high calcic “grospydite” composition, omphacitic pyroxenes containing up to 65% of jadeite as well as kyanite, coesite, sulphides and rutile. U/P type inclusions are represented by olivine, Cr-pyrope, chromediopside, enstatite and chromite typical for diamonds worldwide. One typical rounded dodecahedral diamond was found to contain abundant primary oriented submicrometer (<0.5 mm) octahedral fluid inclusions identified by TEM, caused a milky color of the whole diamond crystal. EEL spectrum of an inclusion has a peak at about 405 eV indicating the presence of nitrogen. Raman spectrum with a peak at 2348 cm^-1 confirms the solid state of the nitrogen. This means that fossilized pressure inside fluid inclusions may be within the range of 4.0-4.5 GPa at room T. Volatile components of inclusions were analyzed by combined gas chromatography mass-spectrometry. They are represented by nitrogen (40%), water (26%), carbon dioxide (3,0%) and heavy hydrocarbons (C_nH_2n+2) which represent 30% rel. of all detected hydrocarbons. Equilibration PT conditions were estimated mainly from chemical composition of coexisting garnets and clinopyroxenes (35 pairs) as 950-1250^oC at 5.0 GPa. However, the identification of unique fluid inclusions in one typical placer diamond allows to expand pressure limit at least up to 6.0 GPa. We conclude that Ural placer diamonds are of kimberlitic origin and are comparable in their high E-type/P-type inclusion ratios with those from north-eastern part of the Siberian Craton and in part with diamonds of Arkhangelsk kimberlite province.