Groundmass Olivine in Kimberlites vs Olivine Neoblasts from Xenoliths of sheared Peridotites: A Comparison of Morphology and Composition
A.V. Golovin¹, A.A. Tarasov¹
1Sobolev Institute of
Geology and Mineralogy Siberian Branch Russian Academy of Sciences, Novosibirsk, Russia
Keywords: kimberlites; olivine; xenoliths; sheared peridotites; morphology and variations of olivine composition, Udachnaya-East pipe; Siberian Craton.
Abstract
Olivine is the major rock-forming mineral in kimberlites, and the results of numerous studies of kimberlitic olivine are widely used in most models of kimberlite petrogenesis. The composition of olivine from kimberlites is also applied in reconstructions of the composition, structure, and evolution of the lithospheric mantle beneath ancient cratons. However, a persistent problem is the identification of magmatic olivine in kimberlites and olivine derived from disaggregated mantle xenoliths, as both may display very similar morphologies. In this study, based on extensive empirical material (more than 500 olivine grains up to 5 mm in size from a single phase of volcaniclastic kimberlite of the Udachnaya-East pipe, Siberian craton) and on a compilation of previously obtained data on olivine compositions from 158 xenoliths of sheared peridotites from the same kimberlite body, we demonstrate that although the morphology of these olivines may be similar, they can be distinguished using compositional and internal structural criteria. This study also presents new data on olivine compositions from microxenoliths (20 samples) of sheared peridotites. The Mg# distribution of olivine in microxenoliths is bimodal, defining two compositional groups with Mg# ranges of 89.5–92.5 (50% of samples) and 84.5–87.5 (45% of samples), whereas olivine from xenoliths of deformed peridotites shows a unimodal Mg# distribution in which the majority of compositions (80% of samples) fall within the Mg# range of 89.5–92.5. The study presents a series of well-documented examples where olivine neoblast grains derived from disaggregated mantle xenoliths of sheared peridotites clearly acted as seeds for the growth of magmatic olivine. In addition, a unique example is described of the destruction of a microxenolith of sheared peridotite during kimberlite crystallization, manifested by the compact occurrence of non-resorbed and unzoned euhedral neoblast crystals within a 1 mm field of view in the kimberlite matrix. The largest proportion of true kimberlitic olivine occurs in the groundmass size fraction (<0.25 mm) of unaltered rocks; however, olivine of this size fraction in kimberlites worldwide remains essentially unstudied.
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