Olivine inclusions in Siberian diamonds: high-precision approach to minor elements

The geochemistry of mineral inclusions in diamonds is an important source of information on the composition of continental lithospheric mantle at depths exceeding 120-150 km. At these depths two main types of geological environment support diamond formation: they are ultramafic (or peridotitic) (U-type) and eclogitic (E-type) environments as shown by minerals that occur as inclusions in diamonds and compose xenoliths of diamondiferous peridotites and eclogites in kimberlites. In primary, diamond-bearing kimberlite and lamproite rocks the ratio of diamonds from these two geological environments varies widely between localities. However, U-type diamonds dominate in the overwhelming majority of diamond Occurrences worldwide. Olivine is the most typical inclusion in U-type diamonds and coexists both as touching or non touching inclusions with enstatite, pyrope, chromite in harzburgitic or dunitic (without enstatite) assemblages and Cr-rich diopside (Iherzolitic and wehrlitic assemblages). More than 260 olivine inclusions sometimes associated with enstatite, Cr-rich diopside, pyrope and chromite were Studied from diamonds of major Siberian mines, including Aikhal, Yubileinaya, Internatsionalnaya, Komsomolskaya, Sytykanskaya and alluvials from the north-eastern part of the Siberian Craton. Most olivine inclusions were prepared for analysis on a single polished Surface with diamond. Olivine from diamonds of the Snap Lake dyke system (Canada) were studied for comparison. The olivine composition in eight xenoliths of diamondiferous peridotites from Udachnaya pipe, representing the rarest mantle samples, was also re-examined. Inclusions were analyzed for major and minor elements with an electron microprobe so as to obtain high precision and accuracy, especially for Ni, Ca. Mn, Cr, Co and Al. Minor-element abundances of the overwhelming majority of analyzed olivine vary within the following ranges in wt.% (NiO 0.320-0.408; CaO 0.005-0.045; MnO 0.079-0.131; Cr2O3 0.013-0.115; CoO 0.009-0.022 and Al2O3 0.007-0.039). About 70 % of all studied olivines demonstrate very low CaO (<0.02 wt.%), which reflects a relatively low temperature of equilibration for the Iherzolitic paragenesis, or lack of clinopyroxene associated with olivine. Some olivines of anomalous composition are detected in microdiamonds only. They contain relatively low forsterite (Fo) [100 Mg/(Mg+Fe)] along with very low MnO and unusually high NiO (86.6-90.0; 0.063-0.076 wt.% and 0.461-0.556 wt.%, respectively). The grain of such composition located in the inner diamond growth zone is associated with an olivine grain of normal composition in the outer zone of one microdiamond sample. Such a difference in olivine compositions may indicate an enrichment of the diamond growing Source in garnet at the initial stages of diamond growth. The high-precision approach to trace elements by electron probe opens new possibilities for more accurate estimation of olivine assemblages, the temperature and pressure of diamond formation, and identification of clearly anomalous olivine compositions which may represent a crustal signature.