The Morphology and Surface Features of Olivine in Kimberlite Lava

Home , Igwisi Hills

Open Access Discussions Solid Earth Solid 3 , and R. J. Brown 1,2 2284 2283 , L. A. Porritt 1 erences in the surface features and morphologies of the three crystal , J. K. Russell ff 1,2 This discussion paper is/has beenPlease under refer review to for the the corresponding journal final Solid paper Earth in (SE). SE if available. Department of Earth, Ocean and Atmospheric Sciences, University ofSchool British of Columbia, Earth Sciences, UniversityDepartment of of Bristol, Earth Wills Sciences, Memorial Science Building, Labs, Bristol, Durham BS8 University, 1RJ, Durham, UK DH1 3LE, UK as single crystals (e.g. Mitchell,and 1986). physical They windows are into consequently the important inner geochemical Earth. Abundant, sub-rounded to rounded, ovoid that produced impact cavities superimposednation on decompression of structures. these The processes combi- duringfor the the rapid distinctive ellipsoidal ascent shape of of kimberlite olivine magmas xenocrysts is found in responsible kimberlites worldwide. 1 Introduction Kimberlite magmas, derived from veryerupt low partial significant melts amounts of of the crystalline mantle, lithospheric erode, carry mantle and as whole rock nodules and and surfaces of xenocrystic olivinecesses grains attending resulted their from rapid threepenetrative transport distinct flaking from mechanical from their micro-tensile pro- failure sourcetained induced in abrasion by the and rapid mantle decompression; attrition (2)a lithosphere: arising sus- turbulent, (1) from volatile-rich particle-particle flow collisions regime, between and; grains (3) in higher energy particle-particle collisions olivine, liberated from a fresh peridotiteification xenolith, of in order olivine to examinewithin surface the kimberlite potential textures magmas. mod- Image due analysis,significant SEM to imagery di transport and laser from microscopypopulations. reveals the Xenocrystic olivine mantle grains are toshapes characterised the by and rough impact surface surfaces, ellipsoidal pits.dented Mantle shapes olivines consistent are with characterisedare growth smooth-surfaced by as and flaked a exhibit surfaces flat crystal and crystal aggregates. in- faces. Phenocrystic We olivines infer that the distinctive shapes Many kimberlite rocksolivine grains contain that are derived large mainlyolivine from proportions grains the disaggregation from of of a peridotite.zania, Xenocrystic ellipsoidal-shaped lava are erupted compared xenocrystic to from phenocrystic the olivine, Quaternary liberated Igwisi from Hills picritic kimberlites, lavas, and Tan- mantle Abstract Solid Earth Discuss., 5, 2283–2312,www.solid-earth-discuss.net/5/2283/2013/ 2013 doi:10.5194/sed-5-2283-2013 © Author(s) 2013. CC Attribution 3.0 License. The morphology and surface featuresolivine of in kimberlite lava: implicationsascent for and emplacement mechanisms T. J. Jones 1 Vancouver, V6T 1 Z4,2 Canada 3 Received: 26 November 2013 –2013 Accepted: 29 November 2013 – Published: 13Correspondence December to: T. J. Jones ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 O 2 C (Davis and 2 km NE–SW ◦ ∼ 10 ka IHV are the 1000 ective viscosities at ∼ usion of lava. Brown ff ff ∼ erent physical signatures on nities to low alkali, calcite-rich ff ffi release following digestion of orthopyroxene 2286 2285 2 Pas. 3 Pas); these are high compared to the range assumed for 6 1 Pa s, e.g. Sparks et al., 2006). Higher viscosities are ascribed to 10 ∼ 2 10 > of kimberlite magma, now mostly preserved as three small volcanic edi- 3 30 Ma (Brown et al., 2012). As a result of their young age, the deposits show m ects of shallow magma degassing and partial crystallization of the ground- ∼ ff 7 10 × Dawson (1994) showed that the IHV showed strong a Brown et al. (2012) proposed three eruptive phases for the IHV: initial phreatomag- Here, we semi-quantitatively describe and analyse the shapes and surfaces of fresh Mg-Al chromite, low Al enstatite,ogopite low (Reid Al magnesian et chromelite al., diopside field; and 1975). high the The Mg olivinephases olivine phl- itself present composition is in plots forsteriteture the chemically rich at micro-xenoliths and in formation. has provide the Under apyroxene, the estimates kimber- thermobarometry assumption low for places that Ca pressure the the signature.Boyd, and clinopyroxene source Other 1966). coexisted temperatures Additionally, tempera- mantle assuming with at chemical low equilibrium, Ca enstatite the with coexistence of pyrope low results Al in pressures of 50–60 kbar (Boyd, 1970). Igwisi material is characterisedcarbonate-apatite-spinel-perovskite-serpentine by matrix. ellipsoidal These forsterite xenocrysts olivine and hostedcrystalline the poly- in xenoliths a are fine ovoid grained 1994) and In a range few upand cases to Mg-Al olivine 3 spinel. ellipsoids cm Some are (i.e. rimmed olivine “micro-xenoliths”; contains by Dawson, or mineral partially inclusions including rimed chrome by pervoskite pyrope, emplacement ( kimberlite magmas ( to the e mass. Brown et al. (2012)olivine also settling, provided a which lower suggested limitstrength a on and partially magma viscosities viscosity crystallised of by Bingham-like 10 modelling fluid with a yield kimberlites such as the Benfontein sills, South Africa. Reid et al. (1975), noted that the remarkably low degreestrending of fissure alteration. (Fig. 1). The IHV are locatedmatic along explosions were a followed by weaket eruptions al. columns and (2012) e estimated that the lava flows had relatively high e noes by fices: the NE, SWyoungest and kimberlite volcanoes central on volcano Earth; (Brown postdating et the next al., youngest 2012). kimberlite volca- The are two-fold: firstly,shapes to and surfaces understand of olivine the crystalsto characteristic constrain processes of ideas kimberlite on that rocks; the and ascent create secondly, of kimberlite the magma. highly distinctive 2 Igwisi Hills volcanoes The Quaternary Igwisi Hills3.4 monogenetic volcanoes (IHV), Tanzania, erupted at least microscopy. Such features cantransport provide from insights the into mantlesell upwards: the et a nature al., topic 2012; of still Sparks, relatively kimberlite 2013; poorly magma Sparks understood et (e.g. al., Rus- xenocrystic 2006). olivine extracted from a Quaternary kimberlite lava in Tanzania. Our goals tion of the grains1975; during Russell ascent, et or milling al.,was (Brett recently 2012). proposed et Rapid as al., a CO 2009;surface mechanism over for Brett, short propelling 2009; timescales kimberlite (Russell magmas Reid etnificant rapidly et al., to factor al., 2012). the in Such a increasing processIntuitively, chemical the is potentially corrosion erosive a and potential sig- millingthe of should exteriors the leave di of ascending mantle-derived magmasgrowth crystals, at which, rims, depth. if or not removed overprinted by by alteration, late stage should crystal be discernible with scanning electron sions, lavas and pyroclastic rocks1985; (e.g. Dawson Brett and et Hawthorne, al.,Mitchell, 1973; 2009; 1986, Clement Gernon 2008; and et Moss Skinner,morphologies al., 1979, and remains 2012; Russell, Kamenetsky somewhat 2011) et enigmatic althoughcrystalline al., given the rocks. 2008; that origin Explanations they of include represent their rounding ellipsoidal disaggregated by magmatic corrosion and dissolu- to elliptical grains of xenocrystic mantle olivine are characteristic of kimberlite intru- 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 45 vol. %) of ∼ cult to resolve well un- ffi 20 %) subhedral to euhedral > 1 mm) inclusions of spinel, clinopyroxene, < 2288 2287 1 mm rounded olivine grains occur along with ∼ 0.4–0.8 cm and then olivine grains were hand-picked under ∼ ¯ ahoehoe-type lava flow from the NE volcano at Igwisi Hills (Fig. 2a; 0.2 mm sub-grains, which represent recrystallization from an original ∼ EDS analysis shows the IHV olivine crystals to be close to the forsterite end / The groundmass of the host lava is very fine grained, and di SEM to variable degrees. Replacement occursperiphery by strain-free and elongate is neoblasts focused aroundSeveral larger the at olivine the grains contain grain smallgarnet ends ( and/or with apatite. maximum curvature (Fig. 2b andder c). a petrographic microscope. Small spinel and perovskite. Carbonate comprises up to 40 % of the groundmass material. member of the olivinecontain smaller, solid solution. Somesingle olivine margins crystal of (Fig. 2c). these Thestill peripheral rounded retain olivine olivine the grains are overall grains randomlyconforms rounded orientated but shape, to suggesting the that rounded recrystallization structure. post-dates It and is observed that recrystallization has occurred the lower parts of a p Brown et al., 2012). Theytios are of rounded 1.5. to The sub-rounded olivinelava in with grains shape the were olivine with enclosed carefully typical by removedily a aspect by thin disaggregated ra- layer cutting on of out groundmass. rinsing The a withgroundmass fine small groundmass water. with read- The volume EDS of sample prior surfaceetching was to features checked observed analysis. for were This attached solely the ensuredrather result than any of the dissolution, the experimental volcanic mechanical method. or or magmatic processes diameter, were handpicked under a binocular microscope. 3.3 Igwisi Hills volcanoes Large (1–10 mm) spheroidal olivine crystals make up a large proportion ( xenolith chosen was friable enough to disaggregate by hand. Olivine grains, 2–4 mm in reference material for the morphology of unadulterated mantle olivine. The peridotite 3.2 Mantle olivine These samples comprise pristinelected lithospheric from mantle-derived a peridotitic basanitic xenoliths dike(Peterson, col- from 2010). Mt. Preston, Detailed westernrecord British study mantle Columbia equilibration of (BC), conditions Canada their andically have during mineralogy not or and been post-emplacement modified textures (Peterson, texturally 2010). shows or As chem- that such, they they provide a suitable The picritic lava flowforsteritic from olivine Iceland phenocrysts contains with abundantwas a ( coarsely grain crushed to size ofa approximately binocular 2.5 microscope. mm. This Thegrains allowed sample with the unnatural crystals breakage surfaces. to be selected individually to avoid phenocrysts from the Icelandicfrom lava magma illustrate stored in surface the texturesmaterial, crust. of Olivine was crystals from used the crystallising in IHtive lava, this as of study opposed the to to transport the ensureeruption pyroclastic processes that processes. within the The the surface secondary texturestherefore magma be influence were rather ignored. representa- of than, subaerial for example, eruption explosive mechanisms can 3.1 Phenocrystic olivine In this study threewisi sample Hills sets (IH) lava were are analysed. comparedolivine with Individual phenocrysts olivines olivine disaggregated from from crystals an mantle from xenoliths,liths Icelandic and the can lava. Crystals Ig- be disaggregated used from as mantle a xeno- proxy for the original material prior to kimberlite ascent, whilst 3 Sample suite 5 5 25 15 20 10 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (2) (3) 1.6 mm. > ), a measure of C ), by Eq. (3): E ) for geometric parameters including 2290 2289 1. As the value tends to 0, an increasingly elongated < http://rsbweb.nih.gov/ij/ ) as defined by: ϕ σ are the area and the perimeter of individual olivine grains recovered 2 (1) / ) Trace 16 P 2 Trace ϕ − Trace and A/P 84 /P A ϕ R π ( P 4 = Lastly, we define a new parameter of ellipticity ( = = ϕ E C where by image analysis via ImageJ.all A other circularity shapes circularities value are ofpolygon 1 is indicates formed. Figure a 3c perfect showsall circle a IHV sharp and peak olivine for in grains olivine showquantifies circularity circularity at the ca. values highly 0.85, within rounded nearly a nature narrow to range be of well rounded 0.7 and to close 0.95. to This a circle. and Wright, 1987)sizes presumably in reflecting the the cratonic relativelyattending mantle transport narrow lithosphere and range and eruption. ofroundness, the We for olivine also small the calculated grain olivine degree the grains of using: circularity mechanical ( attrition 5.1 Image analysis results The cumulative frequency plotted (Fig. as 3b) a shows function thestandard of deviation cumulative sphere-equivalent ( area grain percent. size We plot- have computed theσ Inman graphical to be 0.595 (Inman, 1952). On this basis, the olivine grains are very well sorted (Cas sample suites. The modelssurface were properties. then used to make a quantitative comparison of the 5 Results models for the micron-scale topographic features of olivines from each of the three Jerram et al., 2009; Moss etfor al., the 2010). IHV The lava is corresponding shown olivinebased in grain on Fig. size distribution 3b. a The high grain sizedigital resolution and image (1200 shape was distributions dpi) of manually scan olivineof traced of are the using a slab Adobe polished outline Illustrator slab and10 (Fig. to mm each 2a). in provide olivine diameter. The a grain. Manual resulting representation Olivine tracingThe grains captured digital in all representation the olivine of grains IHVusing with the rocks long vary ImageJ olivine axes from grains software 1– within ( the slab (Fig. 3a) was analysed 4.1 Image analysis Olivine is the dominant phasedistributions in are kimberlite commonly used and to therefore characterise its kimberlite abundance units and (e.g. grain Field size et al., 2009; 4 Methodology Columbia, to collect data oncalibrated the surface in topography the of same thehowever olivine way it grains. as This uses standard device contactless stylus is create measurement instruments a with for surface an topographic measurements automatic map line at stitching a function high to resolution. These data sets were used to create 4.3 3-D laser measuring microscopy An Olympus LEXT 3-Dvanced Measuring Materials Laser and Process Microscope Engineering OLS4000 Laboratory was (AMPEL), used University at of the British Ad- circularity, axis length and area. 4.2 Scanning electron microscopy Hand-picked olivine grains fromand all studied three under sample theversity sets Philips of XL30 were British Scanning mounted, Columbia Electron carbonsuites. Microscope to coated (SEM) document at surface the textures Uni- observed across all sample 5 5 20 15 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | er- (4) ff , and Trace a   1 represent other < erent in shape. In this 2 Trace ff b 3 + 2 Trace a 2 Trace b + 2292 2291 2 Trace a 3 s erent magnifications: it is very smooth, and flakes and pits − ff 50 µm superimposed on the flaked surfaces indicating a syn- to 2 Trace ∼ b 1 is true for a perfect ellipse and all other values + = erence can be observed between the flaking layer and the smoother E ff 2 Trace 3 µm) topography relief. Indeed, 98 % of the topography ranges between , axes as those measured on the olivine outline. The model ellipse is cal- a ≤ is the perimeter of a model ellipse which has the same major, 3 R Trace P b   π = The exterior surfaces of the IHV olivine grains also feature hemispherical impact R The contour map (Fig.negligible 7a ( and b) for the phenocrystic olivine reveals a surface having image you are alsois able not to just observe a thatis superficial layering surface dominant exists feature. below on Figure the thehemispherical 6d surface, shows cavities Igwisi the a similar flaking Hills larger to lavasurface scale that overprinting olivine feature, the shown which surface; irregular in flakes it Fig. and is ridges. 6c. comprised It5.3 of creates a several semi- relatively 3-D smooth laser measuring microscopy cavities or pits (Fig.rounding 6b). the The pits interiors isaverage diameters of rough of and the flaky. pits Thepost-flaking are series cavities smooth, of are events. while near-perfect FigureFig. the hemispheres, 6c 6b shows surface with in a terms sur- semi-hemispherical of structure cavity, identical and to overprinting relationships, but di 5.2.3 Igwisi Hills lava olivine The distinctive surfaces of the IHVand olivine rough crystals are compared highly to flaked,are the exfoliated, irregular characterised phenocrystic as and a mantleences series olivine. in of These relief. meandering irregular This surfaces ridges surfacerather feature and is it arc-like not occurs steps, confined over creating to the di a entire small surface proportion of each of the IHV sample; olivine analysed. olivine crystals within the peridotiteolivine forming is the marked observed by moulds little (Fig. surfacesome 5b). topographic places The relief polygonal despite mantle flakes common are flake peelingtopographic structures. away di In from the crystal (Fig.olivine 5c surface. and d). A clear (Fig. 5a). The overall morphology is governed by the interlocking of adjacent mantle P minor, culated by themanujan, Ramanujan’s 1962). Approximation shown in Eq. (4) (Campbell, 2012; Ra- Where 5.2.2 Mantle olivine SEM imaging confirms that these samplesnot represent experienced primary alteration olivine or surfaces that unnatural have fracturing during extraction from the host rock of the olivine crystals whichThe crystallised overall as crystal a morphology phenocrystfeatureless phase is surface at from shown two the di in basalticare Fig. melt. not 4a. present Figure 4b and c shows a typical described in 2-D asellipticity an can ellipse be found rather in than Appendix a A. circle.5.2 Tables of values of Scanning circularity electron microscopy and 5.2.1 Phenocrystic olivine Samples used for SEM imaging showthe little extraction evidence of of fractured the surfaces crystals resulting from from the parent rock. They represent the primary surfaces Figure 3d shows the distribution ofthis ellipticity parameter values for the digitalisedshapes slab which (Fig. deviate 3a). from For aof 0.935 perfect and ellipse. c. The 97 olivineand % grains of 1.0. have the When a measured olivine mean comparing grains ellipticity Fig. have ellipticity 3c values and between 0.9 d it is clear that the olivine grains are better 5 5 25 15 20 10 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 − erential, an expansion rate, faster ff 2294 2293 10–25 µm. ∼ erent to the phenocrysts and the mantle nodules. We observe a highly ff erential expansion of the clast rim vs. its interior. This produces tangential ff Therefore it is hypothesised that the exfoliation features (Fig. 6) are a result of the Similarities are expected considering these features are believed to be related to The IHV lava olivine grains display shapes and surface morphologies which are sig- The IHV samples have a greater surface roughness than both the phenocrystic and 2 µm of the median surface. The contour map (Fig. 7c and d) for the mantle olivine surface in a similar manner to thermal exfoliation (Preston and White, 1934; Thirumalai, thought to occur in kimberlitesource. deposits, This crystals initial crack cracking approximately event 17will releases km continually the above crack maximum their from energy, although this the point crystals to surface levels (Brett,outer 2009). surfaces of the crystal experiencingthan a the stress timescales di needed forcause viscous di relaxation. Rapid decompression ofcompression within solids the can exterior rim and1934). radial tension We in the hypothesise interior (Preston that and White, the stresses are partially released by exfoliation of the expansion (Brett, 2009). Tensileduring cracks, rapid within ascent; kimberlitic they often olivine, occurrai are in et a al., known parallel 2008; to formation Tingle, along 1988). occur to Tensile cleavage failure ascent planes occurs rate, (Hu- when experienced the by changedefining the in the stress, olivine tensile coupled crystal strength is for greater kimberlite than olivine it (Brett, can 2009). viscously Given relax, the ascent rates on the IH samplesand flaking it is is pervasive, a much dominant more feature on intense, every occurs IHV as olivinerapid multiple analysed. decompression, layers both groups ofcurrently olivine out have of been equilibrium brought toberlitic with melts the formation at surface conditions. great and Olivine depths are as(Sparks is peridotitic et incorporated mantle al., into 2006) xenoliths. Rapid in kim- ascent thethe of kimberlite surrounding 1–20 system ms pressure drives the and xenoliths temperature out conditions of equilibrium resulting with in decompression and nificantly di flaked, irregular and rougha surface superficial with flakes single penetrating layerflaking internally; feature to (Fig. they are 6a). be not We astructure just interpret feature has this some akin penetrative similarities to multiple to exfoliation layer the and flaking related observed to in the decompression. mantle The olivine. flaking However between the crystal and theand melt. reaction For during example, transport there within is the no mantle evidence nodules, of themselves melt (Peterson, 2010). infiltration display flaked surfaces and athe stepped mantle topography olivine (Fig. grains 5c). have However,their sharply at textural a faceted, subhedral larger equilibrium to scale developed euhedalditions under shapes they reflecting the formed stable at highpost-dates, in temperature these pressure sharply the con- faceted mantle.to crystal The form faces. micron-scale during These flaking decompressionascent. flaking is when structures The developed the are flakes on, crystal inferred are(Fig. and 5c). experiences identical Therefore, they lower in are pressures chemistry related to during to decompression rather that than to of chemical the reaction surrounding fresh olivine 6.1 Origin of IHV olivine surfaceAll features analysed samples ofwith phenocrystic zero olivine topography. These show surfaces smooth, areThey near attributed show no to featureless similarities a surfaces to free the growth Igwisi from Hills a samples. liquid Mantle melt. olivine samples commonly mantle olivine grains (Fig.circular 7f). depressions Contour of maps variable for diameter;served the these under IHV are the interpreted SEM olivine (Fig. asdiameters (Fig. 6b). of the 7e) The craters/pits also features ob- record are approximately 8–9 µm in depth with 6 Discussion shows a stepped appearance;crease all in the surface elevation measuredand along mantle generate the olivine a x-axis. surfaces decrease Thebelieved show in that steps these a topography are steps of de- represent sub-parallel aboutflaking surfaces to parallel of 15 the to µm the crystal olivine y-axis across faces crystals the exposed exterior by field (cf. minor SEM of images view; in it Fig. 5). is ± 5 5 25 15 20 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 25–45 % volume loss (Campbell ected by the composition of the ∼ ff 2296 2295 ¯ ahoehoe-type lava. We propose a three part model to ex- used p ff Secondly, decompression makes the olivine more susceptible for abrasion through We expect olivine to be subjected to both chemical and mechanical processes dur- Etch-pit formation by chemical dissolution is a Figure 8a shows an experimentally etched Igwisi Hills lava olivine, at this scale it re- Additionally the IHV olivine grains also have discrete depressions and impact pits. mixture of solid, gas and melt2012). driven by This the assimilation gives of rise orthopyroxene (Russell to et al., the final highly elliptical, near spherical morphologies docu- become incorporated intoxenoliths a then kimberlitic undergo disaggregation melt facilitateddriven as by continuous mineral whole decompression. expansion This rock duringthe imposes ascent- mantle olivine a crystal. xenoliths. build-up The imposed of These timescalescously internal of relax stresses ascent are and within faster the than2009). the stresses This olivine are can results vis- in relieved the by penetrative surface flaked parallel surfaces micro-exfoliation observed. (Brett, particle-particle collisions sustained within a rapidly ascending, volatile rich, turbulent Olivines from IHV lavas havepact elliptical features morphologies that, and to surfacesthe our result which knowledge, of preserve transport are im- within unique. theoccur Importantly, subsurface in prior these to a features eruption, gently must becauseplain the be e the olivine grains distinctive attributes of the IHV olivine grains (Fig. 9). Firstly, olivine crystals studied are clearly dominatedcially by etched mechanical samples processes; are no identified.dominate similarities We at hypothesise with that greater the the depth artifi- chemicalfluids (in processes and the may becomes mantle more lithosphere).rich buoyant, Then, mixture its as traveling at ascent the velocitiesprocesses velocity magma that begin will exsolves support to rise. dominate. turbulent This flow and results in in which a6.3 mechanical gas- Implications for kimberlite ascent etch pit the originaland exhibits olivine a surface honeycomb-like texture; has theexhibit also natural these surfaces been textures. of modified IHV olivine and grains is do not now moreing porous transport. However the shapes and surfaces recorded within the erupted products they do not resemble anything observed in the natural samples. Away from the main features become apparent onstructures the strongly olivine controlled surface. by The the etched crystallographic olivine structure shows of elongate the olivine (Fig. 8c); etchant, the crystalpurities chemistry, crystallographic (“poisons”) in orientation,etching the and surface etchant. the (Wegner This and presence Christie,to “poison” of 1974). replicate may im- exactly Thus, magmatic enhance our dissolution experimentsfor the processes; comparative are rather selectivity purposes. not they intended of serve as the an analogue sembles all other IH olivinetures in shown the in sample Fig. suite 6. However displaying at numerous higher characteristic magnifications fea- (Fig. 8b) previously unidentified 1990; Edwards and Russell, 1996, 1998).olivine We crystals therefore question have been to what modifiedprocesses extent by the described chemical IHV in processes this rather study. thanwith To the 10 investigate % mechanical this, hydrochloric IHV acid olivine onwere crystals timescales then varying were analysed from etched under minutes the to SEM days. Their and surfaces compared to the natural samples. higher energies (Campbell et al., 2013;olivine Dufek morphologies et to al., 2009, an 2012). experimentalesise When abrasion that comparing data the IHV set IHV (Kuenen, olivineet 1960) are al., we resultant 2013). hypoth- from at least 6.2 Mechanical or chemical shaping Olivine is well known to interact with kimberlitic melt during transport (e.g. Donaldson, turbulent transport, thereby enhancing2013). the overall rates of attrition (CampbellFigure 6b et and al., cduced shows by (semi) particle-particle hemispherical collisions, cavities, similar they to represent the impact abrasion pits, process pro- however involving 1969). These partial spalls would be easily removed via crystal-crystal collisions during 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | er- ff -driven turbulent ascent of the magma from 2 2298 2297 -driven magma ascent, decompression results in parallel 2 We would like to thank AMPEL for support with the Laser Measuring Mi- . er significantly from both olivine phenocrysts and from olivine disaggregated from Xenoliths and xenocrysts of minerals other than olivine are rare within the IHV rocks. The textures on the olivine grains at the Igwisi Hills volcanoes are unique; to our Thirdly, higher energy particle-particle impacts create the hemispherical and semi- ff ical processes operating during rapiddepth. CO mantle nodules in the laboratory. Aences. three (1) part model During is rapid proposedmicro-exfoliation CO to on explain the these surface di ofsion the of xenocrystic solid, olivine gas grains. and (2)highly Turbulent melt suspen- elliptical phases and during sub-spherical ascentimpacts promotes grain result abrasion morphology. in that (3) impact results High pittingdissolution in of velocity and the the particle-particle melt surface. reabsorption Synthetic as experimentsogy the rule and causative out surface process chemical features for of rounding. the The morphol- xenocrystic olivine grains are consistent with mechan- The shapes and surfacekimberlite textures lava of have xenocrystic been described olivine andellipsoidal, grains interpreted. exhibit extracted The rough from xenocrystic surfaces, a olivine covered grains with young di are flakes and pits. These surface features 7 Conclusions magmas may result in moreparticles. viscous magmas that dampen the energy of colliding solid This can be explained bydiamond the (in decreasing high order: abrasion diamond-pyrope-olivine-picroilmenite-apatite-kimberlite, stabilityAfanas’ev of et olivine, al., which 2008). isphases second Therefore are only during less to rapid likely to turbulent survive, ascent relative other to olivine. mantle mineral sources (60–80 km) ofleneite), other which mafic lead volcanic to systemsstresses lower within rates (e.g., of crystals bassalt, decompression during basantie,lower and transport. average nephe- the solid Additionally, particle build-up these abundances, of magmas whichactions lower reduces in within tensile the general frequency a have of turbulent particle fluid. inter- Lastly, the lower volatile component of non-kimberlitic exfoliated olivine. These eventsgrains collide are at of high velocities. lower However, there– frequency is an they and upper must limit only to be2012). the occur collisional lower when energy than olivine the threshold required forknowledge crystal there is breakage only one (Dufek othering et study, Brett have (2009), al., been where briefly similar described. textures The andother round- features mafic have eruptions. not This been may observed result on from crystals the from slower ascent rates and shallower melt shapes is supported by experimentalexisting studies (Kuenen, weaknesses 1960). and Abrasion will preferentially exploithashi, erode pre- 1981). weaker compounds (Suzuki and Taka- hemispherical impact pits (Fig. 6b and d) that are superimposed on the rounded and mented. The hypothesis of abrasion producing rounding of angular clasts to spherical and Exploration awarded to RJB. Acknowledgements. croscope. JKR is supported byport the from NSERC a Discovery Marie Grants CurieHills program. International Volcanoes LP Outgoing was Postdoctoral acknowledges Fellowship. supported sup- Fieldwork by on the a Igwisi grant from National Geographic Committee for Research pdf Supplementary material related to thishttp://www.solid-earth-discuss.net/5/2283/2013/sed-5-2283-2013-supplement. article is available online at 5 5 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erentiation in ff , Mineral. Soc. Amer. 3 O 2 at 30 kilobars pressure and its applica- 6 O 2 2300 2299 -CaMgSi 6 O 2 Si 2 , 49, 91–97, 2008. + 1986. J. Volcanol. Geoth. Res., 174, 1–8, 2008. tion, B. Volcanol., 73, 983–1003, 2011. 125–145, 1952. a case study on quantifying olivine populations in kimberlites,Sharygin, Lithos, V. 112, V., 223–235, and 2009. Kuzmin,sia): D. types, V.: compositions Olivine and in origins, the J. 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Symposium II, Cambridge, in: Proceedings Extended Abstracts, 1979. periments and models of2009. substrate-flow interaction, J. Geophys. Res.-Sol. Ea., 114, 1–13, Stuart, F.: Eruption of kimberlitethe magmas: youngest physical kimberlitic volcanology, volcanoes geomorphologyHills known and volcanoes, on age Tanzania), earth B. of Volcanol., (the 74, Upper 7, Pleistocene/Holocene 1621–1643, Igwisi 2012. of British Columbia, Columbia, 2012. in volcanic conduits, Earth Planet. Sc. Lett., 377/378, 276–286, 2013. Lithos, 112, 201–212, 2009. Spec. Pap., 3, 63–75, 1970. Sobolev, N. V.: Mechanical abrasion oftions, kimberlite Russ. Geol. indicator Geophys minerals: experimental investiga- Donaldson, C.: Forsterite dissolution in superheated basaltic, andesitic and rhyolitic melts, Min- Dawson, J. B. and Hawthorne, J. B.: Magmatic sedimentation and carbonatitic di Dawson, J.: Quaternary kimberlitic volcanism on the Tanzania Craton, Contrib. Miner. Petrol., Davis, B. and Boyd, F.: The join Mg Clement, C. and Skinner, E.: : A textural-genetic classification of kimberlites, S. Afr. J. Geol., Clement, C. and Skinner, E.: A textural genetic classification of kimberlite rocks, Kimberlite Cas, R. A. and Wright, J. V.: Volcanic Successions, Modern and Ancient: a Geological Approach Campbell, M. E., Russell, J. K., and Porritt, L. A.: Thermomechanical milling of accessory lithics Brown, R. J., Manya, S., Buisman, I., Fontana, G., Field, M., Mac Niocaill, C., Sparks, R., and Campbell, M.: Thermomechanical milling of lithics in volcanic conduits, M.S. thesis, University Brett, R. C., Russell, J. K., and Moss, S.: Origin of olivine in kimberlite: phenocryst or impostor?, Brett, R. C.: Kimberlitic olivine, M.S. thesis, University of British Columbia, Columbia, 2009. Boyd, F.: Garnet peridotites and the system CaSiO 3-MgSiO 3-Al Afanas’ev, V. P., Nikolenko, E. I., Tychkov, N. S., Titov, A. T., Tolstov, A. V., Kornilova, V. P., and References Moss, S. and Russell, J.: Fragmentation in kimberlite: products and intensity of explosive erup- Mitchell, R. H.: Petrology of hypabyssal kimberlites: relevance to primary magma compositions, Mitchell, R. H.: Kimberlites: Mineralogy, Geochemistry, and Petrology, Plenum Press, New York, Kuenen, P. H.: Experimental abrasion 4: eolian action, J. Geol., 68, 427–449, 1960. Jerram, D. A., Mock, A., Davis, G. R., Field, M.,Kamenetsky, and V. S., Brown, Kamenetsky, R. M. J.: B., 3-D Sobolev, crystal A. size V., Golovin, distributions: A. V., Demouchy, S., Faure, K., Inman, D. L.: Measures for describing the size distribution of sediments, J. Sediment. Res., 22, Hurai, V., Prochaska, W., Lexa, O., Schulmann, K., Thomas, R., and Ivan, P.: High-density Gernon, T., Field, M., and Sparks, R.: Geology of the Snap Lake kimberlite intrusion, Northwest Field, M., Gernon, T. M., Mock, A., Walters, A., Sparks, R. S. J., and Jerram, D. A.: Variations of Edwards, B. and Russell, J.: A review andEdwards, analysis of B. silicate R. mineral and dissolution experiments Russell, J. K.: Time scales of magmatic processes: new insights from Dufek, J., Manga, M., and Patel, A.: Granular disruption during explosive volcanic eruptions, Dufek, J., Wexler, J., and Manga, M.: Transport capacity of pyroclastic density currents: Ex- 5 5 30 25 20 30 15 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | N KENYA MOZAMBIQUE Dar es Salaam TANZANIA inferred extent of lava ow of lava extent inferred IHV 1090 UGANDA 400 km

ZAMBIA 1100 500 *

E. Adapted from Brown et al. (2012).

100 1 0

IH53 55

◦ 1090 NE volcano central volcano

2301 2302

120 1 metres

S, and longitude 31

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0 1

110 1 51 ◦ 0

inferred extent of lava ow of lava extent inferred 1090

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1 fs/lapillistones f Map of Igwisi Hills volcanoes, showing the relative positions of the two northernmost eusive rocks eusive pyroclastic rocks pyroclastic alus fault (inferred) soil lacustrine mud and silt t bedded and stratified scoria-rich and and pelletal clast-rich tuffs and lapillistones (bpeltT/bpelL bT/bL) masive to bedded juvenile clast-rich lapillistone (mbL) massive to stratified lithic-bearing tu (mlT/mlL) dense, lithic-bearing lava olivine-bearing dense to poorly vesicular lava t solid drif contours in 2 m intervals trib. Miner. Petrol., 43, 195–212, 1974. ceedings The 11th US Symposium on Rock Mechanics (USRMS),ratus, 1969. Am. Mineral., 73, 1195–1197, 1988. ical constraints on kimberlite volcanism, J. Volcanol. Geoth. Res., 155,1981. 18–48, 2006. “kimberlites”, Phys. Chem. Earth, 9, 199–218, 1975. fuelled buoyancy, Nature, 481, 352–356, 2012. sis, University of British Columbia, Columbia, 2010. kimberlite, Am. Mineral., 95, 527–536, 2010. Wegner, M. and Christie, J.: Preferential chemical etching of terrestrial and lunar olivines, Con- Tingle, T. N.: Retrieval of uncracked single crystals from high pressure in piston-cylinder appa- Thirumalai, K.: Process Of Thermal Spalling Behavior In Rocks An Exploratory Study, in: Pro- volcanic ediﬁces and the location ofnoes the are lava situated (IH53) at sampled latitude in 4 this study. The Igwisi Hills volca- Suzuki, T. and Takahashi, K. I.: An experimental study of wind abrasion, J. Geol., 89, 509–522, Fig. 1. Sparks, R., Baker, L., Brown, R., Field, M., Schumacher, J., Stripp, G., and Walters, A.: Dynam- Sparks, R.: Kimberlite volcanism, Annu. Rev. Earth Pl. Sc., 41, 497–528, 2013. Russell, J. K., Porritt, L. A., Lavallée, Y., and Dingwell, D. B.: Kimberlite ascent by assimilation- Reid, A. M., Donaldson, C., Dawson, J., Brown, R., and Ridley, W.: The Igwisi Hills extrusive Ramanujan, S.: Ramanujan’s Collected Works, New York, Chelsea, 1962. Peterson, N. D.: Carbonated mantle lithosphere in the western Canadian Cordillera, M. S. the- Preston, F. W. and White, H. E.: Observations on spalling*, J. Ceram. Soc., 17, 137–144, 1934. Moss, S., Russell, J. K., Smith, B. H. S., and Brett, R. C.: Olivine crystal size distributions in 5 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | High resolution scan (a)

) c b) c) d) 2304 2303 Photomicrograph under crossed polarized light of same thin (c) Photomicrograph under plane polarized light of Igwisi Hills lava sample (b)

) a a)

) b

, showing the development of sub grains whilst retaining an ellipsoidal shape. (b) Xenocrystic olivine grains within Igwisi Hills lava sample IH53. Caption on next page. Fig. 3. Fig. 2. section of polished slab of lava samplexenocrystic showing olivine. abundance, distribution and highly ellipsoidalstudied shapes of by Dawson (1994). Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | b and 2 Olivine (a) 1 for a per- is the olivine A distribution = d C (d) Area). · ), where π d ( (4 2 / 2 False colour digitised image of log − (a) = ϕ

2306 2305 Histogram showing the olivine percentage area as (b)

c) b) a) Carbon Tape 1 provide a relative quantiﬁcation of roundness. ) of a grain is calculated as (Perimeter) C < C <

A distribution curve, showing the number of olivine grains having a speciﬁc circu- 27 % xenocrystic olivine. ∼ (c) Grain size analysis of olivine grains in the IH53 lava. SEM imagery for an olivine phenocryst from an Icelandic picritic basalt lava. ) Detailed images of primary surfaces (unfractured) typical of phenocrystic olivine which c polished slab of IH53comprises lava; olivine are coloured dark green. Total slab area is 7138 mm Fig. 3. a function of graindiameter. size. The grain sizelarity, where is circularity expressed ( as grain bounded by mountingand tape showing overallshows morphology minimal topography. of crystal grown from melt. ( Fig. 4. fect circle, and values 0 curve, showing the number of olivine grains having a speciﬁc ellipticity. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Several semi- ) Olivine grain (a (c) Detailed image of primary

(b)

0µm

50µm 50µm 20

) ) ) c d b Large scale image showing distinctive common small scale surface ﬂakes on the (a)

(d)

b) d) 2308 2307

A common semi-hemispherical impact pit, again having , 2013] (d)

Hemispherical impact cavity featuring a smooth interior. 200µm (b) Polygonal textured surface ﬂaking Earth 5 [JonesFig. Solid al., et

(c)

)

) a a) c) b

00 SEM imagery for an olivine grain from mantle-derived peridotite. SEM imagery for Igwisi Hills lava olivine. Fig. 6. surface texture. Fig. 5. bounded by mounting tape (top) showing overall shape of grain. surface (unfractured) of olivineperidotite. typical of grains forming an interlocking mosaic within mantle mantle olivine surface. Highertopography. relief is observed at the edge of the ﬂake creating a stepped a smooth interior. hemispherical excavations, the edge oflayer ﬂaking this of attrition the feature olivine demonstrates surface. penetrative multiple Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | c ) Phenocrystic olivine, ( b and a b) d) f) 2310 2309 ) Igwisi Hills lava olivine. f and e c) a) e) MATLAB generated contour maps and topographic nets based on data from laser- Caption on next page. ) Mantle olivine and ( d and Fig. 7. scanning of grain surfaces undera microscope. common Topographic colour-scale variations used (z-axis) for areelevation, all illustrated positive images. by z-axis Data values sets therefore arerepresent correspond normalised a to to relief 256 the µm greater mean than by topographic the 256 µm mean. area All maps on the olivine surface. ( Fig. 7. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | An etch pit (c) tion tion a ega por r or asion and disagg (Stage 2) (Stage (Stage 3) (Stage (Stage 1) Towards Towards of xenoliths tinued abr Mantle source of exfoliated surfaces of exfoliated Sustained low-energy Sustained low-energy impacts cause pitting Rapid ascent promotes on Xenolith decompression-induced particle-particle impacts Occassional high-energyOccassional C emplacment/eruption milling of olivine creates ellipsoidal-shaped grains Mechanical inc continuously abrade olivine abrade continuously micro-exfoliation of olivine Detailed image of etched olivine surface; white 2312 2311 (b)

)

c) a) b Ascent path Ascent

SEM imagery of olivine surfaces subjected to dissolution by weak acids (see text). Summary model for the evolution of olivine during ascent of kimberlite. Kimberlite as- Overall morphology of etched grain. cent processes causetrainment brittle of deformation mantle of xenoliths.particle-particle mantle Peridotitic xenoliths collisions peridotite undergo and leading mechanicaland decompression. to disaggregation The promotes due production decompression to tensile and islel failure en- driven micro-exfoliation. in by High liberated rapid velocitymixture olivine ascent (i.e. of crystals turbulent) kimberlite transport and magmaparticles of is supports leading to olivine expressed continual abrasion in and as high-frequency roundingate a low surface of impact solids-rich olivine energy pits paral- grains. on melt-gas impacts Periodic the higher-energy between in impacts olivine’s a cre- exfoliated high surfaces. The proportion combination ofIH of ellipsoidal-shaped, lavas. these abraded processes and results decompressed olivine grains within the (a) Fig. 8. developed on original surface ofby the Igwisi crystallographic Hills structure olivine; of dissolution the pit olivine. shape is strongly controlled arrows highlight etch pits. The white box shows the area represented by part c. Fig. 9.