Abstract
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.
Highlights
Kimberlite magmas, derived from low degrees of partial melting of the mantle, erode, carry and erupt significant amounts of crystalline lithospheric mantle as whole rock xenoliths and as single crystals or xenocrysts (e.g. Mitchell, 1986)
Contour maps for the Igwisi Hills monogenetic volcanoes (IHV) olivine (Fig. 8e) record circular depressions of variable diameter; these are interpreted as the craters/pits observed under the scanning electron microscopy (SEM) (Fig. 7c)
We propose a three-part model to explain the distinctive attributes of the IHV olivine grains (Fig. 11)
Summary
Kimberlite magmas, derived from low degrees of partial melting of the mantle, erode, carry and erupt significant amounts of crystalline lithospheric mantle as whole rock xenoliths and as single crystals or xenocrysts (e.g. Mitchell, 1986). Rapid CO2 release following digestion of orthopyroxene was recently proposed as a mechanism for propelling kimberlite magmas rapidly to the surface over short timescales (Russell et al, 2012). Chemical corrosion and milling should leave different physical signatures on the exteriors of mantle-derived crystals, which, if not overprinted by latestage crystal growth rims, or removed by alteration, should be discernible with scanning electron microscopy (SEM). Such features can provide insights into the nature of kimberlite magma transport from the mantle upwards: a topic
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