Paragenesis and complex zoning of olivine macrocrysts from unaltered kimberlite of the Udachnaya-East pipe, Yakutia: relationship with the kimberlite formation conditions and evolution

Abstract

Unaltered Mg-olivine (Fo=85-94) is a predominant mineral of the kimberlite block (serpentine-free) of the Udachnaya-East pipe, and it prevails in peridotite xenoliths and as inclusions in diamonds. The kimberlite of this pipe, like a series of hypabyssal kimberlites in other regions, contains two main types of olivine macrocrysts according to their size and morphology: those rounded or irregularly shaped (olivine I) and euhedral phenocrysts (olivine II), which are usually no larger than 0.5mm and very seldom reach 1mm in size. This study was focused on several thousand olivine samples assigned both to olivines I and to olivines II, with a gradual transition between them. Particular attention was paid to the search for mineral inclusions in olivine and for phenocrysts with a clear zoning. In the phenocryst cores of homogeneous composition, we have revealed orthopyroxene inclusions as well as clinopyroxene (chrome-diopside and chrome-omphacite) inclusions with wide variations in the Na2O and Cr2O3 contents, significantly higher than the previously established ones: up to 6.00wt.% Na2O and 4.23wt.% Cr2O3. Convincing evidence for the high-pressure origin of the olivine macrocryst cores is the presence of pyrope inclusions with 1.41-9.14wt.% &2O3, 4.64-6.61wt.% CaO, and Mg#=75.6-83.7 in six samples, which testifies to the high-pressure lherzolite paragenesis of the phenocrysts cores. The cores of the studied olivine phenocrysts are identical in the contents of Ni, Co, Ca, Cr, and Mn to olivines from diamonds and peridotite xenoliths. However, they differ significantly in the steady elevated Ti content, equal to 100-300ppm for the majority of the phenocrysts, including those containing pyroxene and pyrope inclusions.