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Morphology and Surface Features of Olivine in Kimberlite : Implications for Ascent Processes.', Solid Earth., 5 (1) Durham Research Online Deposited in DRO: 14 January 2015 Version of attached le: Published Version Peer-review status of attached le: Peer-reviewed Citation for published item: Jones, T.J. and Russell, J.K. and Porritt, L.A. and Brown, R.J. (2014) 'Morphology and surface features of olivine in kimberlite : implications for ascent processes.', Solid earth., 5 (1). pp. 313-326. Further information on publisher's website: https://doi.org/10.5194/se-5-313-2014 Publisher's copyright statement: c Author(s) 2014. This work is distributed under the Creative Commons Attribution 3.0 License. Additional information: Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full DRO policy for further details. Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 https://dro.dur.ac.uk Solid Earth, 5, 313–326, 2014 www.solid-earth.net/5/313/2014/ doi:10.5194/se-5-313-2014 © Author(s) 2014. CC Attribution 3.0 License. Morphology and surface features of olivine in kimberlite: implications for ascent processes T. J. Jones1,2, J. K. Russell1, L. A. Porritt1,2, and R. J. Brown3 1Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, V6T 1 Z4, Canada 2School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, UK 3Department of Earth Sciences, Science Labs, Durham University, Durham, DH1 3LE, UK Correspondence to: T. J. Jones ([email protected]) Abstract. Most kimberlite rocks contain large proportions 1 Introduction of ellipsoidal-shaped xenocrystic olivine grains that are de- rived mainly from disaggregation of peridotite. Here, we de- scribe the shapes, sizes and surfaces of olivine grains re- Kimberlite magmas, derived from low degrees of partial covered from kimberlite lavas erupted from the Quaternary melting of the mantle, erode, carry and erupt significant Igwisi Hills volcano, Tanzania. The Igwisi Hills kimberlitic amounts of crystalline lithospheric mantle as whole rock olivine grains are compared to phenocrystic olivine, liber- xenoliths and as single crystals or xenocrysts (e.g. Mitchell, ated from picritic lavas, and mantle olivine, liberated from 1986). They are consequently important geochemical and a fresh peridotite xenolith. Image analysis, scanning elec- physical windows into the inner Earth. Abundant, sub- tron microscopy imagery and laser microscopy reveal signif- rounded to rounded, ovoid to elliptical grains of xenocrys- icant differences in the morphologies and surface features of tic mantle olivine are characteristic of kimberlite intrusions, the three crystal populations. The kimberlitic olivine grains lavas and pyroclastic rocks (e.g. Brett et al., 2009; Clement form smooth, rounded to ellipsoidal shapes and have rough and Skinner, 1979, 1985; Dawson and Hawthorne, 1973; flaky micro-surfaces that are populated by impact pits. Man- Gernon et al., 2012; Kamenetsky et al., 2008; Mitchell, 1986, tle olivine grains are characterised by flaked surfaces and 2008; Moss and Russell, 2011). indented shapes consistent with growth as a crystal aggre- However, the origin of their ellipsoidal morphologies re- gate. Phenocrystic olivine exhibit faceted, smooth-surfaced mains somewhat enigmatic given that they represent disag- crystal faces. We suggest that the unique shape and sur- gregated crystalline rocks. Explanations include rounding by face properties of the Igwisi Hills kimberlitic olivine grains magmatic corrosion and dissolution of the grains during as- are products of the transport processes attending kimber- cent (Kamenetsky et al., 2008; Mitchell, 1986; Moore, 2012; lite ascent from mantle source to surface. We infer that the Pilbeam et al., 2013) or mechanical milling (Arndt et al., unique shapes and surfaces of kimberlitic olivine grains re- 2006, 2010; Brett et al., 2009; Brett, 2009; Kamenetsky et sult from three distinct mechanical processes attending their al., 2008; Reid et al., 1975; Russell et al., 2012). Rapid rapid transport through the thick cratonic mantle lithosphere: CO2 release following digestion of orthopyroxene was re- (1) penetrative flaking from micro-tensile failure induced cently proposed as a mechanism for propelling kimberlite by rapid decompression; (2) sustained abrasion and attrition magmas rapidly to the surface over short timescales (Rus- driven by particle–particle collisions between grains within sell et al., 2012). Such a process is potentially a significant a turbulent, volatile-rich flow regime; and (3) higher-energy factor in increasing the erosive potential of the ascending particle–particle collisions producing impact cavities super- magmas at depth. Intuitively, chemical corrosion and milling imposed on decompression structures. The combination of should leave different physical signatures on the exteriors of these processes during the rapid ascent of kimberlite magmas mantle-derived crystals, which, if not overprinted by late- is responsible for the distinctive ellipsoidal shape of olivine stage crystal growth rims, or removed by alteration, should xenocrysts found in kimberlites worldwide. be discernible with scanning electron microscopy (SEM). Such features can provide insights into the nature of kim- berlite magma transport from the mantle upwards: a topic Published by Copernicus Publications on behalf of the European Geosciences Union. 314 T. J. Jones et al.: Morphology and surface features of olivine in kimberlite still relatively poorly understood (e.g. Russell et al., 2012; ing emplacement, the kimberlite reaches olivine saturation Sparks, 2013; Sparks et al., 2006). and crystallises rims on the xenocrystic olivine and minor Here, we semi-quantitatively describe and analyse the groundmass phases (Kamenetsky et al., 2008). shapes and surfaces of fresh xenocrystic olivine extracted Brett et al. (2009) used textural evidence combined with from a Quaternary kimberlite lava in Tanzania. Our goals electron microprobe analysis of Ni contents in olivine to doc- are two-fold: firstly, to understand the processes that create ument the xenocrystic origins of kimberlitic olivine, regard- the highly distinctive shapes and surfaces of olivine crystals less of grain size or shape. Their work corroborates the re- characteristic of kimberlite rocks and, secondly, to constrain ported chemical trends observed in olivine from the Igwisi ideas on the ascent of kimberlite magma. Hills kimberlite (Fig. 8; Dawson, 1994). Brett et al. (2009) demonstrate small volumes (≤ 5 %) of heterogeneous olivine crystallisation from the kimberlitic melt during ascent. This 2 Olivine in kimberlite occurs on cores of pre-existing rounded xenocrystic olivines (at all sizes). The crystallisation is caused by orthopyroxene This study focusses on the physical and morphological fea- dissolution which drives the melt towards olivine saturation. tures present on the surfaces of kimberlitic olivine. Our work The small volume of crystallisation only affects the overall builds on an extensive literature of petrographic and geo- shapes of the smallest xenocrysts to create the euhedral (ap- chemical studies of kimberlitic olivine (Table 1) aimed at parent) “phenocryst” population of olivine grains. constraining the origins of kimberlite magmas (e.g. Arndt et Other studies have proposed a complex interplay be- al., 2006; Table 1; Arndt et al., 2010; Brett et al., 2009; Daw- tween reabsorption and crystallisation during kimberlitic as- son, 1994; Kamenetsky et al., 2008; Mitchell, 1970, 1986, cent (Moore, 2012). In Moore’s model, rounding is caused 1995, 2012; Pilbeam et al., 2013). The large number of de- by melt resorption, predicted to occur at multiple times tailed studies of olivine in kimberlite reflects the pervasively and linked to superheating events during ascent. Pilbeam et high abundance of olivine in these enigmatic rocks. al. (2013) also suggest resorption processes during ascent. Mitchell (1986) separated olivine within kimberlite Here, the digestion of a xenocrystic cargo coupled with the into phenocrystic and xenocrystic populations. Xenocrys- fractional crystallisation is thought to explain the marginal tic olivine, termed “macrocrysts”, are rounded in shape and profiles on olivine. During reactive transport magmatic rims the most abundant volumetrically. “Phenocrystic” olivine are crystallised on xenocrystic cores (Pilbeam et al., 2013). was distinguished in the groundmass on the basis of a Our study here aims to explore the evidence for mechan- finer-grain size (< 0.5 mm) and its (sub-)euhedral charac- ical processes which operate on olivine crystal cargo (i.e. ter. However, in recent years, many workers have ascribed xenocrysts) during kimberlite ascent. Our results, in part, an- a xenocrystic origin to both olivine populations. Both pop- swer some of the questions concerning the mechanisms of ulations are observed to have xenocrystic cores and mag- kimberlite ascent and reconcile some of the disparate obser- matic rims (e.g. Arndt et al., 2010; Brett et al., 2009). Work vations and ideas found in the literature
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