Petrogenetic significance of minor elements in olivines from diamonds and peridotite xenoliths from kimberlites of Yakutia

Abstract

Peridotite xenoliths and diamonds from kimberlites represent an important source of information about the composition of the continental lithosphere at depths exceeding 120–150 km. Ultramafic (or peridotitic) U(P)-type of geological environment is dominating at these depths compared to eclogitic (E-type). Olivine is the most typical mineral both of peridotite xenoliths and as diamond inclusions in most kimberlites worldwide. In spite of its simple chemical composition it contains a number of petrogenetically significant minor elements such as Ti, Al, Mn, Ca, Cr, Ni, Co in low concentrations, mostly below 0.1 wt.% of oxide except of NiO. More than 500 industrial quality diamonds of size range between 0.8 and 3 mm containing olivine inclusions sometimes associated with enstatite, pyrope, chrome diopside and chromite were selected from current diamond production of nine major Siberian diamond mines. This collection also includes revised olivine diamond inclusions from Arkhangelsk (Russia), Majhgawan (India) diamond mines and Urals (Russia) alluvial mines. More than 30% of studied samples were prepared for analysis of olivines on a single polished surface with diamond. More than 300 peridotite xenoliths were selected for olivine studies from a representative collection from unaltered kimberlites of Udachnaya diamond mine. These xenoliths include low-temperature coarse lherzolites, harzburgites and dunites which are represented by spinel, garnet–spinel and garnet varieties. More than 70 xenoliths of high-temperature porphyroclastic (sheared) lherzolites from Udachnaya are also included in the examined collection. Olivines were analyzed for major and minor elements with a JEOL JXA 8200 electron microprobe at the Max-Planck Institute of Chemistry, Mainz. Special efforts were made to obtain high precision and accuracy in electron microprobe analyses, especially, for Ti, Al, Ni, Co, Ca, Mn and Cr. These elements were analyzed by using long counting time and high beam current yielding detection limits of around 10–15 ppm and errors of 20–30 ppm (2 standard errors). Minor element abundance of the overwhelming majority of studied forsteritic (Fo) olivines with Fo [100 Mg/(Mg + Fe)] between 91 and 94 varies within the following ranges in wt.% (NiO 0.310–0.420; CaO 0.005–0.045; MnO 0.070–0.131; Cr 2O 3 0.012–0.117; CoO 0.008–0.022; Al 2O 3 0.007– 0.039; TiO 2 0.005–0.042). It is concluded that all examined olivines share similar compositional features and are formed at high pressures. A significant positive correlation of Fe and Mn was observed as well as a trend to positive correlation of Cr and Al for a number of samples due to a possible dependence on T values and Cr/Al in bulk peridotites and a trend to positive correlation of Ca and Al due to the effect of temperature.