Morphology and surface features of olivine in kimberlite: implications for ascent processes

Jones, T.J.; Russell, J.K.; Porritt, L.A.; Brown, R.J.

Abstract

Most kimberlite rocks contain large proportions of ellipsoidal-shaped xenocrystic olivine grains that are derived mainly from disaggregation of peridotite. Here, we describe the shapes, sizes and surfaces of olivine grains recovered from kimberlite lavas erupted from the Quaternary Igwisi Hills volcano, Tanzania. The Igwisi Hills kimberlitic olivine grains are compared to phenocrystic olivine, liberated from picritic lavas, and mantle olivine, liberated from a fresh peridotite xenolith. Image analysis, scanning electron microscopy imagery and laser microscopy reveal significant differences in the morphologies and surface features of the three crystal populations. The kimberlitic olivine grains form smooth, rounded to ellipsoidal shapes and have rough flaky micro-surfaces that are populated by impact pits. Mantle olivine grains are characterised by flaked surfaces and indented shapes consistent with growth as a crystal aggregate. Phenocrystic olivine exhibit faceted, smooth-surfaced crystal faces. We suggest that the unique shape and surface properties of the Igwisi Hills kimberlitic olivine grains are products of the transport processes attending kimberlite ascent from mantle source to surface. We infer that the unique shapes and surfaces of kimberlitic olivine grains result from three distinct mechanical processes attending their rapid transport through the thick cratonic mantle lithosphere: (1) penetrative flaking from micro-tensile failure induced by rapid decompression; (2) sustained abrasion and attrition driven by particle–particle collisions between grains within a turbulent, volatile-rich flow regime; and (3) higher-energy particle–particle collisions producing impact cavities superimposed on decompression structures. The combination of these processes during the rapid ascent of kimberlite magmas is responsible for the distinctive ellipsoidal shape of olivine xenocrysts found in kimberlites worldwide.

Citation

Jones, T., Russell, J., Porritt, L., & Brown, R. (2014). Morphology and surface features of olivine in kimberlite: implications for ascent processes. Solid Earth, 5(1), 313-326. https://doi.org/10.5194/se-5-313-2014

• Journal Article Type: Article
• Acceptance Date: Apr 2, 2014
• Online Publication Date: May 16, 2014
• Publication Date: May 16, 2014
• Deposit Date: Oct 1, 2014
• Publicly Available Date: Jan 14, 2015
• Journal: Solid Earth and Discussions
• Print ISSN: 1869-9510
• Electronic ISSN: 1869-9529
• Publisher: European Geosciences Union
• Peer Reviewed: Peer Reviewed
• Volume: 5
• Issue: 1
• Pages: 313-326
• DOI: https://doi.org/10.5194/se-5-313-2014
• Public URL: https://durham-repository.worktribe.com/output/1453471