GROUNDMASS OLIVINE IN KIMBERLITES VS OLIVINE NEOBLASTS FROM SHEARED PERIDOTITE XENOLITHS: MORPHOLOGY AND COMPOSITION

Olivine is the main rock-forming mineral in kimberlites, and the results of numerous studies of kimberlitic olivine are widely used in most models of kimberlite petrogenesis. Kimberlitic olivine investigations also provide constraints for models 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, we demonstrate that although the morphology of these olivines can be similar, they can be distinguished using compositional and internal structural criteria. The study is based on an extensive analytical dataset (more than 500 olivine grains up to 5 mm in size were analyzed from a single volcaniclastic kimberlite phase of the Udachnaya-East pipe, Siberian craton) and on a summary of the previously obtained data on the compositions of olivine from 158 xenoliths of sheared peridotites from the same kimberlite pipe. New data on the composition of olivine from 20 sheared peridotite microxenoliths (≤0.5 cm) are also presented in this work. Microxenoliths of sheared peridotites can be divided into two groups according to the Mg# of their olivine: 89.5–92.5 (50% of the samples) and 84.5–87.5 (45% of the samples). In contrast, olivine from sheared peridotite xenoliths (≥5 cm) shows a unimodal Mg# distribution, with most olivine compositions (80% of the samples) falling within the Mg# range 89.5–92.5. The paper presents a series of well-documented examples where olivine neoblasts derived from disaggregated sheared peridotite xenoliths acted as nuclei for the growth of magmatic olivine. In addition, a unique example of the disaggregation of a microxenolith of sheared peridotite in situ during kimberlite emplacement is provided. This example is the compact occurrence of nonresorbed and unzoned euhedral neoblasts within a 1 mm field of view in the kimberlite rock. The largest portion of true magmatic kimberlitic olivine occurs in the groundmass size fraction (<0.25 mm) of unaltered rocks; however, olivine of this size fraction in kimberlites worldwide remains de facto unstudied.