Canadian Mineralogist Vol. 25, pp.2sQ,6a Q987)

Nb-GT. IN THE ORAPA , BOT$WANA

RICHARD P. TOLLO Deportnent of Geologlt,George Washington University, Woshington, D,C. 20052,U.S,A.

STEPHEN E. HAGGERTY Departnentof Geologlt,University of Massachasetts,Amherst, Massachusetts 01003, U,S.A.

ABSTRACT Les composbde la forme R2+TisOzpouvaient 6tre pris en consideration,mais nul composdde ce type n'a &6 Disqete nodules(0.5 - I cm in diameter)of M-Cr-rutile, d€couvertdans un environnemente hautes pression et tem- containing maximum concentrations of 20.9 wt.9o M2O5 pdrature.Les structures cristallines i cisaillement,du type and 8.2 wt.9o Cr2O3,occur in tle Orapa kimberlile in Bot- cv-PbO2,seraient les pr6curseursles plus probablespour swana. Our data expand the compositional range ofrutile affronterles liautes concentrations en Nb, Cr, Z etFe dans from the upper mantle, and provide tle first direct evidence lescondilions de pression et temp6raturedu manteausup& for metasomalism at this locality. The nodules have rieur. equilibrated to coa$e (l-2 nm) lamellar intergrowths of Mg-Cr- in host M-Cr-rutile. The composition of (Iraduit par la R€daction) this rutile, characterizedby high s6a19a1s6f chromium and zirconium (the latter of which is strongly partitioned into Mots-clds:kimberlite, nodules, Orapa, Botswana, rutile, this phase), differs from that of rutile in alkaline rocks, ilndnite,armalcolite, structures e cisaillement,manteau' carbonatites,meteorites and lunar samples.Decomposition mdtasomatisme. of armalcolite- and crichtonite-seriesprogenitors, replace- ment of rutile by ilmenite, and reaction of rutile with fluids INtnolucrroN or melts are unlikely mechanismsfor generatingsuch inter- growths. Compoundsof the form R2+TisOT would be pos- Ilmenite is a chaxacteristicconstituent of kimber- sible precursors, but they have not been reported in high- P and high-T environments. Crystallographic shear-based lite grorindmass and kimberlitic nodule suites, but structures related to o-PbO2 are the most probable pro- rutile is uncommon in such assemblages.Rutile has genitors to accomodate ttre extraordinarily large concen- beenreported, however,in minor amounts from a trations of Nb, Cr, Zr and Fe at tle P-T conditions of the variety of kimberlite-related associations: (l) kim- upper mantle. berlite groundmass (Clarke & Mitchell 1975, Hag- gerty 1975, Elthon & Ridley 1979); Q) kimberlite Keywords: kimberlite, nodules, Orapa, Botswana, rutile, autoliths (Ferguson et al. 1973); (3) MARID suite ilmenite, armalcolite, crystallographic shear, mantle, xenoliths in kimberlite & Smith l97T; melasomatism. @awson @) peridotite xenoliths in kimberlite @oullier & Nicholas 1973);(5) grospyditeand eclogitexenoliths in kim- berlite (Nixon & Boyd 1973,Sobolev 1977), and (O SOMMAIRE inclusionsin diamonds(Pinzet ol. 1975,Meyer & Tsai 1976).The composition of rutile in such sam- Dans la kimberlite d'Orapa, on trouve du rutile i forte ples is characteristically close to stoichiometric poids teneur en Nb et Cr fiusqu'd 20,9 et 8.2t/o en de TiO2, with Fe, Cr, Al and Nb presentin minor con- M2O5 et Crp3, respectivement)sous forme de nodules de 5 i l0 mm de diambtre. Nos observations dlargissent le centrations only. domaine de composition du rutile du manteau sup6rieur Intergrowths of ilmenite and rutile have been et fournissent le premier t6moignagede m€tasomatismeen reported from a number of kimberlite localities in celte r6gion. Les nodules ont atteint l'6quilibre en tant recent years, either as discrete nodules (Haggerty qu'intercroissanceslamell€c d'ilndnite i Mg et Cr et de 1979, 1983, Tollo e/ al. l98l), "xenocrysts" (Shee rudle e Nb et Cr; les lamellesd'ihn6nite, dpaissesde l-2 1979),part of the assemblageof kimberlite ground- mm, sont intercaldesdans cellesde rutile. La composition mass(Haggerty 1975,Boctor & Meyer 1979),or in de ce rutile, caract6risdepar les teneurs 6lev6esen Cr et Zr association with zircon (Raber & Hag$erty L919, (ce y particulibrementconcentrd), diffbre de celle dernier est Haggerty Gurney 1984).The mode of origin most du rutile trouv6 dans les roches alcalines, carbonatites, & m6t6orites et dchantillons lunaires. La ddcomposition de commonly proposed for such ilnenite-rutile inter- progdniteurs d'armalcolite ou de crichtonite, le rempla- grofihs involves the high-pressurebreakdown of a cement du rutile par I'iln{a!1e, la rdaction du rutile avec precursor -type phase similar to des fluids eu des lains fondus, sont autant de m6canis- armalcolite @e,Mg)Ti2O5in composition (e.9., mes improbables pour produire pareilles intercroissances. Haggerty 1975,Boctor & Meyer 1979,Raber & Hag- 251 252 THE CANADIAN MINERALOGIST gerty 1979).The composition of rutile in theseinter- Jagersfontein, expands the available data-base for growths, however, differs markedly from stoichio- theseassemblages and is usedto evaluatemodels for metric TiO2; combined CrrO3 + M2O5 contents the genesisof such intergrowths. may be aslarge as 29 wt.tlo (Tollo et al. 1981,Tollo 1982,Haggerty 1983),making armalcolite an unli- kely candidate as a precursor phase. An alternative DETERMTNATIvSMETHoos proposal is 1[31 high-pressure crystalloeraphically sheared (CS) rutile structures axe present at upper Transmitted light and high-resolution oil- mantle P-T conditions and that tlese decompose immersion microscopy were performed on all speci- upon kimberlite eruption (Haggerty 1983).Replace- mens using standard techniques. X-ray-diffraction ment of rutile by ilmenite is another possibility (Pas- determinations were made on duplicate subsplits of teris 1980). selectedpowdered sample using a 57.3-mm diameter Characterization of the petrography and qurera with^ Ni-filtered copper Kcu radiation chemistry of ilmenite-rutile intergrowths from the O = 1.5,()50A) and filn mounted accordingto the Orapa kimberlite, Botswana, with new data from Straumanis method.

D 7)"/

B ,, . €tLt E a-4, . ,\'/\, r.anur ,..'\ z { .{r./1 t ' ' //\ ' t. / t\/' .,/\/ /\ "t"'''/ / \/ /\

c

SCALE: F--{ lcm

FIc. l. Generalizedclassification of ilmenite-rutile intergrowth textures as viewed in " polished section. (a) Coarse lamellar intergrowth with one lamellar direction dominant. (b) Fine lamellar intergrowth with individual lamellae both sigmoidal and straight in form. (c) Patchy intergrowth with fine lamellae locally concen- trated around patches. (d) Polygranular atoll intergrowth consisting of ilnenite concentratedalong polygranular boundaries in host rutile. (e) Symplectitic inter- growth of rutile with silicate.Note patchy and fine lamellaeof ilnenite within rutile. M-CT-RUTI-LE IN KIMBERLITE FROM BOTSWANA 253

All electron-microbeamanalyses were performed lae generally occupying the interior regions. The rim on the ETEC Autoprobe at the University of Mas- and lamellae ilmenite are locally continuous, with sachusetts. Full automation of this instrument is individual lamellus 6nnghing into the interior from achieved ttrrough an on-line Interdata computer the peripheries. Electron-microbeam analysesreveal using a quantitative analysis progmm developed no significant chemical differences betweenthe two jointly by the University of Massachusettsand the rypes. ETEC Corporation. Matrix corrections were The fine lamellar texture consistsof numerousvery employed according to the procedures of Bence & fine larnellae of ilmenite oriented within tle host Albee (1%8) and Albee & Ray (1970).Operaring con- rutile in a manner similar to that described above. ditions generally included an acceleratingvoltage of Some of thesesamples approach the "blitz" texfure 15 kV, an aperture current of 0.3 pA, l5-second described by Ramdohr (1969). These fine lamellae counting intervals for each element, and an average are typicaily less than 50 pm in length and may be beam-diameter of approximately 5 pm. Ferric iron either straight or sigmoidal in form. Many nodules contents were calculated for specific oxide phases from Jagersfontein consist entirely of this texture, using a correction proceduremodified from the pro- and severalcoarse lamellar nodules fromboth pipes gram FERRIC witten by J. Berg & W. Kelley, which show a population of very fine lamellae oriented assumes:(1) perfect stoichiometry, (2) iron as the parallel to the coarseilmenite. only elementin multiple valencestates, and (3) a per- Patchy intergrowths characterizeseveral nodules fect analysisof the other elementspresent. These cal- from Jagersfonteinin which the ilmenite is typically culationswere made using theuncertainty restrictions oriented in an irregular pattern only locally related of Finger (1972). Geochemical standards used for to obvious microfractures or possiblecleavage in the analysis included a variety of both natural and syn- host rutile. Numerous very fine lamellae of ilnenite thetic silicate and oxide materials from the Analyti characteristicallyoccur in the immediate vicinity sur- cal Geochemistry Facility collection at the Univer- rounding such irregular patches. sity of Massachusetts. Polygranular atoll textures, on the other hand, are characterizedby inegular soncentrationsof ilmenile PeTRocRAPFTY along intergranular boundariesin the numerousvar- ieties of polycrystalline rutile host. Morphologically, The suite consistsof elevendiscrete nodules reco- atoll ilmenite appearsto be similar to the "external vered from the mine concentrateat the Orapa A./K-l granule" texture described for itnenite - titanifer- kimberlite pipe in Botswana and more than one ous magnetite intergrowths by Buddingfon & Linds- dozen similar nodules collected from the Jagersfon- ley (1964)and may in fact also have an origin by sub- tein pipe in South Africa. The nodules from Orapa solidus diffusion. The patchy ilrnenite texture are generally black to steel-greyin color, round to appearsless likely to have resulted from the subsoli- ovoid in shape, with characteristically smooth sur- dus diffusion of fine lamellae and may, especially faces, and averageless than I cm in diameter. The where oriented along microfractures, preserve evi- nodules from Jagersfontein are generally similar in dence of in situ formation of ilmenite. Areas of appearanc€,but typically smaller, averagingless than patchy and polygranular ilmenite coexist with fine 0.5 cm in diameter. The variety of intergrowth tex- lamellaein severalpolycrystalline nodules from both tures observed in polished sestion is illustrated in pipes. Figure l. Symplectitic intergrowths of rutile and silicate are The coarse lamellar texture appears to be more relatively common in the Jagersfontein suite (Hag- common at Orapa than at Jagersfontein and typi- gerty 1983,Schulze 1983), typicaly occurring as part cally consists of numerous distinct lamellae of of a nodule otherwisecharacterized by the fine lamel- ilmenite in regular geometric orientation within the lar, patchy, or polygranular atoll texture. Similar host rutile. Ramdohr (1969,p. 987) describeda simi- intefgrowths are also reported from the Mbuji-Mayi lar, altlough presumably nonkimberlitic, inter- kimberlite, Zaire (Ottenburgs& Fieremans1979). growth in which he consideredthe orientation ofthe The interface between coexisting silicate and rutile lamellae to be limited to crystallographically equiva- in such symplectites is characteristically sharp and lent directions within the rutile. Lamellae within the variable in form. Very fine-scale ilmenite lamellae kimberlitic nodules describedhere are typically lens- and patchy intergrowths occur locally within tlte sym- shaped,marked by distinct reflection-anisotropy, plectitic rutile in most specimens.The original sili- and range from 0.5 to 2.0 mm in length and from cate phase is typically altered beyond recognition; 0.1 to 0.2 mm in width. As shown in Figure 2, however, olivine and greenpyroxene have beeniden- ilnenite occurs in two distinct modesin thesecoarse tified. lamellar intergrowths: (l) rim itnenite situated Examined in polished section, the host rutile in locally along the periphery of individual nodules, or thesenodules displays characteristic red-orange inter- around serpentineinclusions, and (2) coherentlamel- nal reflections that are typically visible along promi- 254 THB CANADIAN MINERALOGIST

nent cleavages.Electron-microbeam Clable l) and nio-bian rutile according to the nomenclature X-raydiffriction Clable2) analysesindicate thit this ofternf et at, (19(l.l).Teiturally, the host rutile of host phasemay be mostappropriately classified as these nodules varies from single, apparently

Fto. 2. Photomicrographs of ilnenite-rutile intergrowths in reflected light and under oil-immersion objectives. (a) - @) Coarse lamellar ilmenite in orthogonal or slightly offset orthogonal aneJesin rutile. Variations in the color of the lamellae are due to reflection anisotropy. Granular patchesof itnenite are presentat the edgerof tle photomicro- graph in (b), with either straight or slightly sinuous contacts with the host rutile. Sample OR5-1. (c) Coarse nonor- thogonal ilmenite lamellaein rurile and fine-grained ilmenite in approximately orthogonal relationship. SampleOR5-2. (d) Extremely coarse patchy iLnenite enclosing irregular blebs of host rutile. Note that the rutile has a fine set of second-generationilmenite lamellaeand that theseare subparallelamong rutile grains. SampleOR5-3. (e) Dominantly coarse lamellar ilmenite (white) and associatedpatchy ibnenite (dark grey) in rutile (hght erey). This is a lower- magnification view of OR5-3 shown in (d). (f) Diffusion-controlled migration of coarseatoll ilmenite around a ser- pentinized (presumed olivine) inclusion in rutile. Fine lamellae of ilnenite parallel the lengh of the photomicro- graph. Scale:width of field for (d) is 0.12 mm, and for tie remainder, 0.34 mm. run 1. cEm6 comslmds or rmrE m f,mru n IUENTE - mEl ]mmm, 255 troDms mx owA AnDJA@smn stel@n! oR-4058 oR-409B OR-12.14 OR-il36A ntll6 li.@nit€ rutll€ ll@olto rrtlla iloenlto ntlle il@nlte .,2.58 Tlor 73.9{ 52.97 78.81 52.6L 85.26 76.39 51.33 cra6: 7.75 3.50 6.15 4.83 5.97 2.6A 7.92 3.71 Ar;o; o. 03 o. oo o. os o. 09- 0.01 0.01 0.02 0.00^ - - 5.64r roloi-5.24+-s.98+- 4.91+ re6 2,391 24.13 1.831 19.86 1.?1' 28.20 2..3Sr 26.11 l{qo 0.24 13.09 0.18 1s.71 0.00 10.48 0.10 ll.16 llno nA 0.51 NA 0.{4 trA 0.48 trA 0.49 ztg2 0.95 0.09 0.76 0.21 0.86 0.12 0.81 0.12 trbzos 13.38 0.5? 10./t6 0.86 6,94 0.24 12.73 0.52 NtO NA 0.23 NA 0.03 NA 0.21 trA 0.26 Ta'OR r.50 rA 1.25 U 0.10 m o.42 NA rc€aI 1d6:TE', t06:23 100.-T3 106:tZ rdi.ni ':n:tI 160-77- gr:fr

catlons based on t@ lrutllel and three (Il@nlt€) oty96n3 0.796 0.913 0.836 0.890 0.881 0.927 0.011 0.906 0.088 0.06! 0.076 0.086 0.065 0.01!9 0.080 0.069 AI 0.001 0.000 0.001 0.002 0.001 0.000 0.000 0.000 Fell - 0.09t - 0.101 - 0.08? - 0.100 0.026 0.'163 0.ot9 0.374 o.ot0 0.553 0.029 0.5!3 !r9 0.005 0.4i10 0,00{ 0.527 o.ooo 0.366 0.002 0.391 lln - 0.010 - 0.008 - 0.009 - 0.010 2t 0.007 0.00! 0.005 0.002 0.006 0.001 0.006 0.001 ub 0.087 0.006 0.057 0.009 o.ot3 0.003 0.09t 0'006 ||t - 0.00{ - 0.001 - 0.005 - 0.005 ta 0.006 - 0.005 o-ool - 0.002 Tolal t;TI? -060 I-o-I5 2.t66 iSit ,:ririr r:rTt 2-o-or n ttfpor cra l@llar ln le/patchy cra la@llar cr6 l@llat @dal I 66 3{ 79 2L 75 25

t{A: not analyzed * cal@lateal r total lron exPresdd aa F6o

stehens oR5-l oR5-2 oR5-3 0R5-4 ruttle il@nlte etLla il@nlt€ atlle llnetrlte lutile ilnenite Tlo2 64.30 49.59 70.46 52.6a 73.72 53.59 73.2A 52.56 CIZOr 8.21 2.95 7.29 2.93 7.15 4.50 6.7A 2.27 AI2o3 0.01 0,03 0.04 0.04 0.08 0.12 0.02 0.03 Fe2O3 I .00+ 6.06+ - 2.68+ - 6.93+ FeO 4.10 25.08 2.94' 24.10 3.171 24.89 3.861 24.i.3 uso 0.4r 11.64 0.64 13.41 1.12 13.28 0.43 13.21 UnO 0.05 0.49 0.01 0.50 0.05 0.51 0.03 0.54 ZtO2 1.35 0.17 1.12 0.20 1.15 0.22 L.25 0.21 llbzog 20.90 1.51 !5.74 0.94 L2.79 0.72 14.80 0.78 NtO NA NA NA NA NA IIA l|A NA Ta2Oq 1.54 0.00 l.2l 0.03 0.84 0.00 0.04 0.00 roEal roo-:37 t .i6 59.-a9 l6b:ll r06:i7 100.s1 100.49 100-;66-

Catlons baBed on tro (rutile) antl threa (ilmenite) oryq€n€ rt 0.708 0.876 0.769 0.904 0,792 0.921 0.791 0.905 cr 0.095 0.055 0.084 0.053 0,081 0.081 0.075 0.0!1 A1^- 0.000 0,001 0.001 0.001 0.001 0.003 0.000 0.010 P€::-0,111-0.104 - 0.046 - 0.119 Fez' 0.050 0.193 0.036 0.460 0.038 0.4?6 0.036 0.462 Itg 0.009 0.407 0.014 0.456 0.024 0.453 0.010 0.4s1 l.ln 0.001 0.010 0.000 0.010 0.001 0.010 0.001 0.001 tt 0.010 0.002 0.008 0.002 0.008 0.002 0.009 0.002 Nb 0.138 0.016 0.103 0.010 0.083 0.007 0.091 0.008 NT ra 0.001 0.000 0.005 0.000 0.003 0.000 0.002 0.000 roral llTf Z:TaI' :-0-2d t:T6b t.031- z.-nn T:0lT z.-n6 n type, cra 1@11a! cts l6Eel1ar crs lanellar cr6 leella! @dal I NA NA NA NA

SIEci@n 3 oR5-5 oRs-6 JF-126 JF-115 rutile ilmenlb ntil.€ il@nit6 ntil€ ll@nlte ntlle ll@nito TIO2 68.68 51.30 66.26 50.79 84.74 52.09 82.74 51.36 Ct203 5.06 L.52 7.74 3.07 6.77 6.84 7.r3 7.00 A12O3 0.04 0.01 0.06 0.00 0.13 0.21 o.23 0.22, re2O3 - 5.64+ - s.50+ 6.31- FaO 5.01r 30.32 3.76' 27.76 0.881 19,15 1.08' 17.54 tiso 0.90 9.4S 0.47 10.44 0.04 15.37 0.04 16.22 ho 0.10 0.67 0.09 0.58 NA 0.26 0.00 0.32 ZtO2 1.35 0.21 r.r8 0.23 o.92 0.13 1.19 0.36 Nb2O5 19.11 1.53 t8.28 r.08 5,54 0.12 6.a2 .0.44 Nto NA NA NA NA NA O,24 0.1.0 0.19 Ta2Os 0.68 0.00 1.82 0.00 0.13 NA 0.73 0.15 lotal 163:33 16b;6E' 9e .70- 10n:Tf 1m-:T5 99.-0T 16t:t6 16b:Tr

catLong baseal od trc (nti.lg) anal th.ee (llmnlte) oay96tr6

0.746 0.909 0.733 0.885 0.880 0.S85 0.860 0.859 0.0s8 0.028 0.091 0.056 0.07{ 0.122 0.079 0.L25 AI 0.00r 0.000 0.002 0.000 0.002 0.005 0.004 0.006 - 0.r00 - 0.134 - 0.094 - 0.107 ;;2* 0.06r 0.598 0.046 0.538 0.009 0.362 0.011 0.330 ug 0.019 0.333 0.oro 0.361 0.001 0.518 0.001 0.5d{ h 0.001 0.011 0.001 0.011 - 0.00s 0.000 0.006 Zt 0.009 0,002 o.oo8 0.003 0.006 0.002 0.008 0.004 Nb 0.125 0.016 o.L22 0.011 0.041 0.001 0.048 0.004 NT - 0.005 0.001 0'003 ta o.io: o.ioo o.oo? o.ooo 0.001 - 0.08! 0.001 rotal 1.023 2.000 I-oro tffi i.Ti? I:See= 1.615 1.33t

n

eyp€ ! cr5 I@eLld crd l@Ilat crs la@116t crs l@LLar @dal I NA m 68 32 NA 256 THE CANADIAN MINERALOGIST

TABI,E 1 (CONIINT'D)

SP€cfrn3 JG80-25 JG80-31 ttc80-33 JAC26-t-r rutiLa i1@nlte lutlle llEeni.te ntil€ llnenlte lutlle ilenite Tto2 86.06 52.09 92.88 56.62 86.88 52.77 78.83 55.13 Cr2Oj 6.51 1.27 4.36 3.20 6.49 5.97 4.99 2.59 AI2o: 0.09 0.3r 0.01 0.24 0.07 0.37 0.08 0.06 F62O3 - 5.54+ - 2.83+ - 5.61. - 4.66+ F€O 0.811 18.78 0.07r 19.39 1.01* 19.28 t.47. 20.82 uqo 0.00 15.60 0.0M.44 0.00 1s.69 0.40 15.95 ho NA 0.31 NA 0.38 NA 0.31 0.06 0.26 2tO2 0.74 0.16 1.08 0.21 0.?5 0.20 r.r9 0.21 Nb2O5 5.49 O.24 2.O2 0.05 4.79 0.21 12.06 0.00 N10 NA 0.29 NA 0.21 NA O.24 0.00 0.18 Taroc 0.23 M 0.14 NA 0.34 NA NA I{A foral 99.93 100.59 lo0-;37 1!i:57 164.3f 10b:T? 93-0-6 99.37

Cationa baaed on tro (ntil€l and th!€€ (ll@nitel oxygons

ai 0.892 0.879 0.942 0.942 0.897 0.889 0.839 0.934 0.071 0.129 0.046 0.056 0.071 0.106 0.056 0.046 0.002 0.008 0.000 0.006 0.001 0.010 0.001 0.002 F63t 0.094 - 0.047 - 0.095 - 0.0?9 Fa2+ 0.008 0,353 0.001 0.359 0.010 0.362 0.017 0.393 xg 0.000 0.522 0.000 0.576 0.000 0.524 0.008 0.536 Mn 0.006 - 0.007 - 0.006 0.001 0.005 zt o.6os 0.002 0.007 0.002 0.005 0.002 0.008 0.002 Nb 0.034 0.002 0.012 0.001. 0.030 0.002 0.077 0.000 Nt 0.005 - 0.004 - 0.00,1 0.000 0.003 Ta o.ior -:_ 0. 001 0.001 total I-o-Il 2.000 ITTE 2.TOT l.Tr5 z.Tad I:T0? z;00T 5 5

type ! crs 1@e11a! cla le€lla! crB la@llat DodaL $ NA NA M

slel@n! itc80-34B ,tG80-4{A JG80-'l4C oR-406 rutila ntlle rutll€ dtile !1O2 9r.56 91.61 91.03 84.53 cE203 a,4a 4.85 5.24 N2o3 0.02 0.00 0.01 0.31 F62O3 P60 olor. olrr. 0,r0r rlez. u90 0.00 0.00 0.00 o.22 ho m M NA NA zto2 1.46 0.99 1.03 0.93 lbZOS r.93 2.70 6.49 Nio NA NA NA NA Ta205 0.02 0.0? 0.0r 0.53 totel E't:E 6r.a 10o-:Tt tb6:zT Cations hB€d on ts oxy96ns

0.92S 0.9 28 0.920 0.882 0.048 0.052 0.056 0.057 Al-. 0.000 0.000 0.000 0.005 Ferr Fe2t 0.001 0.o02 o.ior o.irs t{s 0.000 0.000 0.000 0.004 llD zt o.o-r o o.ioo o.ior 0.006 Nb 0.013 0.012 0 .016 0.Oil1 lrt ta o.ioo 0.000 o.ioo o.ioz TOTAI ITTO T.ooo- 1:td-6 IT16

n

typer poly atoll/ poLy atoll/ IDly atoll/ h@g€n@u6 fn le fn ld/ alEtt fn 1e rutlle rcdal $

homogeneouscrystals characteristicof the coarse MINERAL Cgnursrnv lamellar intergrowths to severalpolycrystalline tlpes. A single nodule (OR-40O composedentirely of Chemical compositions determined by electron- rutile has beenrecovered from the Orapa site. It is microbeam analysisfor coexistingilmenite and rutile ovoid in shape, approximately I cm in elliptical from individual nodules are presentedin Table l. diameter, and characterized by an overall smooth Only the compositionof the host rutile is given for exterior surface and dark reddish black color. the fine lamellar intergrofihs becauseof the proba- Although microbeam analysesindicate a M,/Ta bility of overlap by the microbeam during the anal- atomic ratio of approximately 20:1, detailed ysis of individual fine-scalelamellae of itnenite. The microscopy and X-ray-diffraction analysis have host rutile of the intergrowths shows a broad range failed to delineatethe presenceof fine-scaleM-rich of compositionalvariation and is typically charac- exsolution lamellae similar to those identified terized by sienificant substitution of niobium (6.5 by Cernf et al. (1964) in rutile from a variety of - 20.9 wt.oloMzOs) and chromium (5.2 - 8.2 wt.t/o nonkimberlitic locations worldwide. CrrO). Some of the values recorded from Orapa M-CI-RUTILE IN KIMBERLITE FROM BOTSWANA 257 are the highest yet reported from . Iron TAAI]E 2. CO,IPASAEIVE X-RAY DATA TOR is generally present in concentrations ofless than I RIXIIIA NODI'XTES 406 and JCPDS + L1-396 wt.9o FeO in rutile from Jagersfontein (Haggerty calqLat€d d-values (A) 1983), consistentwith the new results in Table l. | !' oR-406 JCPpS tll-396 Similar rutile from Orapa is characterized by FeO htl r00 3,245 3.23 contentsin the range of l-3 wt.9o. In all cases,areas rlo 1.686 1.687 for analysis were selectedmicroscopically and are 2lt 30 2 .482 z.Aa lo1 free of fine lamellae of ilnenite. Beam overlap with t5 1.625 r.627 220 subsurface precluded ilmenite is by the consistency 10 2.183 2.190 111 ofthe FeO valuesobtained in 3 to 5 spot analyses. 10 1.360 1.361 3ol Note that the total iron content is expressedin the 5 1.347 1.348 112 ferrous state in Table l, although stoichiometric 2.051 210 charge-balancecalculations suggesttlat a portion of 1.480 1.48r 002 the total iron may be presentas Fe3*. In general, L .452 1.4s4 310 evidence from electron paramagnetic resonance studies (Carter & Okaya 1960)appears to substanti- r estimted visualJ.y; I d-value data reported as the Eee of, !ou! indivld.ual deteminatLons, cuK( radl-a- ate the likelihood of such substitution by ferric iron; tlon (^E 1.540.50 A), Nl flLter however, the limited experimental data presently available(Webster & Bright 1961,Wittke 1967)sug- values of less than I to 5 wt.qo FeO. The CrrO, gest that a significant ferrous component may be contents,on the other hand, vary widely for the com- presentas well. The actual Fd+,/Fe3+ rado cannot bined suite and show no obvious trend separating be determined from the data presentedin this study the two individual localities. Atomic ratios of M,/Ta becauseof the possiblevariability in valenceof some for rutile from both localities show strong enrich- other cations present in rutile, notably niobium. ment in niobium and indicate that the host phaseof Iron contents for the combined Orapa- theseintergrowths should be classified-asniobian Jagersfonteinsuite define a rather limited range, with rutile accordingto the nomenclature of tern! et at.

30 F c) Alkolineigneous suites E 25 o F Kimberliticnodules *- ?o ro F ,t5 ++ fo l- (J + 10 + rO ! z 5 o 50 60 70 80 90 roo Ti (aroutc"zo) Ftc. 3. Compositions of rutile from intergrowths in kimberlitic nodules (this study), lunar surface materials, meteorites + and terrestrial igneous rocks plotted in terms of atomic percent Ti versas (Nb5+ + cf + Ta5+). Data for the nonkimberlitic fields are taken from the literature (seetext for specific references).The open circle representsOrapa nodule OR-406. 2s8 THE CANADIAN MINERALOGIST

A.o

Alkolineigneous suites

Kimberlitic nodules

Nb 'to 30 50 70 90 Fe ATOMIC o/o Frc.4. Compositions of niobian rutile from kimberlitic nodules (this study) and various alkaling iggsus rocks (seetexl for literaiure sources)plotted in terms of atomic percentM-Fe-Ta. The open circle representsOrapa nodule OR-406. Hem

Ilm

Oropodiscrete nodules Jogersfontein discretenodules

Oropo/Jogersfontein ilm-rut intergrowths

30 50 7A 90 Geik MOLE olo Frc.5. Compositions of itnenite from coarselamellar, patchy and atoll-type intergrowths with rutile plotted in the ter- nary system ilmenite--hematite. The compositions of individual discrete nodules of ilmenite from Orapa and Jagersfonteinare plotted for comparison. Nb-CT-RUTILE IN KMBERLITE FROM BOTSWANA 259

(1964). X-ray-diffraction analysis of a single crystal Hlavaet al. 1972)are also plotted and show an over- of rutile from Orapa (Iable 2) supDortsthis classifi- lap with pafr of the kimberlitic field. It should be cation. noted that t}te field of alkaline igneous rutile is in Rutile compositions obtained from nodules from fact composed of two slightly overlapping popula- both Orapa and Jagersfonteinare plotted in Figures tions: rutile from granitic pegmatites and related 3 and 4. In Figure 3, the samplqsof kimberlitic rutile rocks at low TiO2 values and rutile from carbona- from this study define a narrow compositional field tites and associatedintrusive bodies at higher values dictinct from those of similar rutile from meteorites of TiO2. In general, rutile from granites and gra- @l Goresy l97l) and terrestrial alkaline igneous nitic pegmatite complexesis characterizedby a lower suites @alacheet al. l944,Cern! et al. 1964,Siivola Nb/Ta value and may be contrasted with the rela- 1970). The compositions of two rutile samples tively M-enriched carbonatitic rutile. In Figure 4, obtained from lunar surfacematerials (Marvin 1971, kimberlitic rutile plots in a distinct, relatively res-

ELECTRONMICROBEAM TRAVERSE OF A RUTIL-E- ILMENITE LAMELLAR INTERGROWTH

d'g 5o t5

80

70

60

50 ro 20 30 40 50 60 70 80 90 {oo ANALYSIS NUMBER Ftc. 6. Electron-microbeam traverse of an ilmenite-rutile lamellar intergrowth. 260 THE CANADIAN MINERALOCIST tricted field showing considerableenrichment in nio- pipes are, in general, more chromium-rich than the binm, limited substitution of iron, and only a slight associated iknenite in discrete nodules, thereby amount of tantalum. On the other hand, rutile from reflecting differences in bulk composition resulting alkaline igneous suites appears to be characterized from different modesof origin. The data presented by a greater extent of substitution of both iron and in Table I representaverage compositions obtained tantalum, and by lower concentrationsof niobium. from multiple analyses of several ilrnenite spots Within this field, rutile from carbonatitic complexes within individual intergrowths. Detailed microbeam- tends to cluster closer to the M-Fe sideline than analyses of numerous lamellae of ilmenite from rutile from granitic pegmatites, which characteristi- individual coarselamellar intergrowths demonstrate cally show a gxeater extent of Ta substitution. a typical variability of up to 2 wt,alo MgO. These Ilmenite compositions presentedin Table I from data, however, fail to delineate any consistent spa- coarselamellar, patchy, and polygranular atoll-type tial patterns characterizingthis inhomogeneitywithin intergrowthswith rutile from both Orapa and Jagers- individual lamellae. Data for selectedelements from fontein are clearly kimb€rlitic in charaster,with MeO a detailed microbeam-traverseacross such an inter- and Cr2O3values ranging from 9.5 to 17.4 wt.tlo growth are presentedin Figure 6, demonstrating in and from 1.5 to 7.3 wt.Vo, respectively.The Jagers- addition that rutile compositionsmay also vary, with fontein ilmenite is, on average,more magnesianand values of FeO and TiO, typically fluctuating within more chromian than the ilmenite from Orapa. a range somewhatgreater than 2 wt.Vo. The traverse Individual compositions, plotted in terms of the ter- is also instmctive with regard to diffusion mechan- nary solid-solution end-members isms in that compositionalcontacts have a moder- FeTiO3-MgTiOr-FqO3 in Figure 5, define a distinct ate slope, but M-shaped profiles are only weakly linear grouping oriented nearly parallel to the developed. ilmenite-geikielite sideline at approximately 5 mole The data in Table I serve to illustrate the parti- 9o hematite. The main compositional fields of the tioning of elements observed between coexisting ilmenite in discrete nodules from both Orapa (this ilmenite and rutile in the intergrowths from Orapa study) and Jagersfontein (Haggerty, unpubl. data) and Jagersfontein. Magnesium is almost exclusively are also plotted for comparison. The three fields incorporated in ilmenite, reflecting the strong mutual show very little overlap,with the Jagersfonteindis- substitution of Mgz+ for Fd* in octahedrallyco- crete nodules typically more magnesian than their ordinated structural positions in the ilnenite. counterparts from Orapa, thereby mirroring the Chrome values indicate that Cr3+ is present in sig- same relationship shown by the respectivepopula- nificant proportions in both . For tle Orapa tions of lamellae.These data suggestthat the ilmenite samples, chromium is consistently concentrated in lamellae may have equilibrated under conditions of the rutile, with an averagerelative enrichment-factor generally similar, but possibly somewhat lower, (wt.9o CrrO, in rutile/wt.9o Cr2O3in ilmenite) of fugacities of oxygen relative to the associateddis- approximately2.4 for the ten samples.This factor, cretenodules. The compositionsof lamellaeat both however, is quite close to unity for the Jagersfon-

'ABIA 3. CAI'AI, TED BUIN COUPOSXTTONS FOR SELE(ITED IITItsRCROI{TES .rAC26-l-1 ocrall Lhbeilit!c glEt@r OR-4OsB On-4244 0B-436A .tf-126 tra@re 8€rage arul@llte (1, TLor 66.78 ?8.40 70.12 71.29 62.A6 70.1t 76.92 crz6t 6.30 5.28 6.87 6.79 l.o0 5.05 1.6{ erioi o.o2 0.03 0.01 0.16 0.10 0.06 0.02 ee60- 11.23 8.0? 9.41 8'25 16.21 10.61 13.4t !w !.61 2.20 2.87 {.95 9.45 1.92 7.08 ;io 0.17 o.l0 o.t2 0.08 o.t9 0.13 0.54 zto, 0.66 0.72 0.64 0.6? 0.61 0.66 nR rbr6<- 9.02 5.53 9.68 1.19 '1.t8 6.72 NR f,rd o.0B o.os 0.06 0.08 0.00 0.07 rR rb2o3 0.99 -!-.2! 0.32 0.09 ---!4.' 0.'!1 NR mEaI 55.66- r55:3U 166:16 o][Eg 9sr-s- ii']Tji cao o.05 slo, 0.28 Eodr 166:TT

€ti.ons baagd @ fic ffyEona t.921 0.166 0.004 0.323 0.262 0.00{ tr;r0.01l. 0.109 0.002 $:l0.00r! zJ09-

. total lro e:preseeil as Feot NAr not ealyzEtlt llRt Dot re[Drteilt (1) atata f,r@ sagg€rty (1975' Nb-Cr-RUTILE IN KIMBERLITE FROM BOTSWANA 261 tein intergowths. The larger cations Z/* and Nb5" here; (2) armalcolite typically occurs at the interface are clearly incorporated preferentially in the rutile betweenrutile and a silicate host and appears,there- from botl localities, with averagefactors of relative fore, to be a reaction product of TiO2 + liquid or enrichment of approximately 6 and 23, respectively. fluid (Haggerty 1983); (3) attempts to homogenize On the basisof similaritiesin ionic radii (Shannon an ilmenite-rutile intergrowth from Jagersfontein, & Prewitt 1969)and the variety of charge-balancing over a range of temperatures(1000 - 1400'C) and mechanismsof substitution possible, these patterns oxygen fugacities (10-5to 1013 atm) at I atm total of element partitioning are predicted. pressure,produced armalcolite, but rutile was always Bulk compositions were obtained on five coarse an additional phase (Tollo et al. l98l), and (a) lamellar ilmenite-rutile intergrowths from Orapa (3) experimental data on the stability of armalcolite at and Jagersfontein(2) by either the method of modal high pressures(Lindsley e/ a/. 1974,Kesson & Linds- and compositional recombination (4 assemblages), ley 1975,Friel el al. 1977)show that armalcoliteis or from t}te traverseaverage (Fig. O of 100electron- unstable above - 20 kbar, the stable assemblage microbeam analyses at 20 pm intervals (l assem- being rutile + ilmenite, so that a prograde reaction blage).These data are presentedin Table 3 along with is required for the assemblageswe describe. an overall averagebulk-composition. The most ex- Other possibilitiesinclude the decompositionof treme variation is in Ti content (62.9 - 78.4 w.tlo membersof the crichtonite family of minerals, linds- TiOj, followed by Mg Q.2 - 9.5 fi.90 MgO). Nio- leyite and mathiasite (Haggerty et al. 1983), whtch bium and Fe each vary by a factor of 2, but Cr (4.0 contain approximately 60 wt.9o TiOr. However, t}te - 6.3 wt.9o Cr2O) and the minor elementsare more largecations @a, K, Sr, Ca, REQ, which are essen- restricted. tial componentsofthese phases,are not presentin the ilmenite-rutile intergrowths. Replacementof GENESIS rutile by itnenite or interaction of rutile with Fe-rich fluids or melts appearsunlikely becauseatoll textures The ilmenite-rutile intergrowths have an appear- of the type illustrated in Figure I are relatively rare. ance suggestive of an origin by a subsolidus, Bulk compositions are variable, but may be broadly exsolutionlike process,and a range of bulk compo- modeledon R2*Tiror-type compounds(Grey et al. sitions indicative of armalcolite as a possibleprecur- 1973),satisfying high concentrationsof Ti, Cr, Zr sor phase. However, although armalcolite is rela- and Fe. However, the lack of confrmation regard- tively abundant as an early liquidus phasein certain ing the substitution of Nb in the structure, the sta- high- lunar mare (e.g., Haggerty bility relations of thesecompounds at high pressures, 1973), kimberlitic examplesdescribed to date occur and the existenceof such phasesin kimberlites make exclusively in small amounts in the DuToitspan this possibility somewhat speculative. (Haegergy195), Jagersfontein(Haggerty 1983),and Doped rutile of demonstrated high-pressure ori- Bultfontein (Haggerty et al. 1983) kimberlites in gin is structurally related to o-PbO2 by crystallo- South Africa, the Mothae kimberlite in Lesotho graphic shear (-Greyet al. 1973,Yagi et al. 1979, @aber & Haggerty 1979),and a kimberlite in Yaku- Mrng et ol, 1980).Rutile structures characterizedby tia, USSR (Tatarintsev et al. 1983).The closeassoci- crystallographic shear(CS) dominate in the range of ation of armalsolite with intergrowths of iknenite 5 - 15 wt.9o Cr2O3 (Gibb & Anderson 1972), and rutile at both Mothae and DuToitspan led to the encompassingthe compositions of most rutile and suggestion that the latter phases were formed by the bulk compositionsof ilmenite-rutile intogrowths some variant of the generalizedreaction: from Orapa and Jagersfonstein(Iables l, 3). Rutile having 5 wt.Vo or lower CrrO, contents is structur- ally homogeneous,and anion-deficient rutile exists armalcolite = rutile + ilmenite with 15-17 wt.9o CrrO3. Niobium content in the @e,Mg)Ti2O5 = TiOz + (Fe,Mg)TiOt kimberlitic intergrowths langes from 6.5 to 20.9 wt.9o M2O5, but whether or not substitution of Nb is CS-related is uncertain. Stoichiometric M-rutile which is supported by experimental data (Lindsley is shown experimentally to exist at low fugacities of et al. 1974). Rutile compositions in ilmenite-rutile oxygen (< 10-12atm), but nonstoichiometriccom- intergrowths, however, differ markedly from pounds are presentat high"f(O, (> l0i2 atm) at stoichiometric TiO2, and bulk compositions are l000oc according to the data of Dirstine & Rosa characterizedby substantialconcentrations of Cr and (1979a,b).In the context of high fugacitiesof oxy- Nb. At least four additional lines of evidence gen and nonstoichiometry, note that disordered preclude armalcolite as a possibleprecursor: (l) kim- columbite in a symplectitic intergrowth in M-rutile berlitic armalcolite enriched in Cr + Nb also con- (19 - 47.wt.s/oMzOs) has beendescribed in a peg- tains substantial calcium (- 4 wt.Vo CaO), but Ca matite (Cerny et al. l98t). Preliminary data on one is below detectionlimits in the assemblagesdescribed Jagersfontein ilmenite-rutile assemblageshow that 262 THE CANADIAN MINERALOGIST

CSis probably present(P. Self & p.R. Buseck,pers. ilrnenite-rutile intergrowths in kimberlites. Replace- comm. 1983),but in another it is absent(I. Grey, ment or interaction with fluids or melts is also con- pers. comm. 1985),leaving the most likely possibil- sidereil unlikely. Compounds of the form R2+Ti.O" ity in a state of flux. The assemblagesare lessthan are possibleprogenitors, but these have not been ideal to waluate the prsence of CS becauseof exten- identified in high-pressureassociations. Equilibrated sive equilibration. Rutile grains most likely to exhibit intergrowths of rutile and ilmenite are possibly CS are those free of "exsolved" ilmenite and con- related to high-pressurecrystallographic-shear struc- tainiag large concentrationsof M, Cr, Zr and.Fe. tures of the o-PbO2 type, and this is supported spe- All but one of the ilmenite-rutile intergrowths are cifically by a correspondencein Cr content$between free of included or attached silicates or other oxide kimberlitic rutile and that derived experimentally. minerals. Serpentine,which is assumedto be derived Nb-Cr-rutile from the Orapa kimberlite is inter- from olivine, is the sole exceptionand is surrounded preted to be a product of metasomatismin coarse by iknenite in an atoll texture (Fig. 2). Clinopyrox- silicate veinlets that were disrupted because of ene, and olivine are presentin other rutile mechanical instability. Intergrowths of the type assemblagesfrom Jagersfontein,and a metasomatic described in this study offer potential new insights origin hasbeen ascribed, through fluid transport and to exotic high-pressure structural forms, provide interaction with depletedharzburgite in the subcon- unique examples for diffusion-rate studies in the tinental lithosphere (Haggerty 1983).The fluids are upper mantle, serveas guidesto metasomatism,and enrichedin titanium, alkalis and silicate-incompatible represent identifiable repositories for silisate- elements@a, Sr, Zr, Nb, REE); at Bultfontein, vein- incompatible elements. lets of K-richterite, phlogopite, , lindsleyite, armalcolite and M-Cr-rutile develop in a harzbur- Acrxowr,socsMENTs gitic substrate of olivine + enstatite (Jones el a/. 1982,Haggerty et ol. 1983).Chromium is a restite It is a pleasureto gratefully acknowledgethe per- elementin depletedharzburgite and is presentlargely mission granted by the De Beers Mining Corpora- in chromian spinel. Harzburgite and metasomatized tion that has enabledmultiple visits to Jagersfontein harzburgites are recognized at Jagersfontein, but and a single collecting trip to Orapa, without which eclogites are the only upper-mantle nodules found this study could not have been made. The electron- at Orapa (Shee& Gurney 1979, Tollo 1982).The microbeam facility was maintained by M.D. Nb-Cr-rutile and ilrnenite assemblages,therefore, Leonard, and the analytical study was supported by provide the first data documenting that NSF through grants EAR78-02539 and metasomatism has also been in effect at this local- EAR83*08297(S.E.H., principal investigator). Valu- rE. We interpret the extreme rarity of included or able discussion$were held with J.J. Gurney, A.J. attachedsilicate mineralsto growth in coarse-grained Erlank, P.R. Buseckand I. Grey, on mattersrelat- veinletsthat disaggregatedupon kimberlite eruption. ing to Orapa, metasomatism,and CS-basedstruc- Phlogopite in particular is mechanically unstable, tures. We thank F.R. Boyd and one anonymous and the preservation of metasomatized nodules is reviewer for helpful comments. Robin Taylor con- generallyvery poor. genially performed the typing.

REFERENcES CoNcLUsIoNs Ar.sEE,A.L. & Rev, L. (1970):Correction factors for High-pressure M-Cr-rutile containing lamellar electon probemicroanalysis of silicates,oxides, car- ilmenite is consideredto result from an exsolution- bonates,phosphates, and sulphates.Anal. Chem. like processwith strong partitioning of Nb + Zr in 42, 14A8-,414. rutile and Mg in ilnenite, and with Cr selectivelypar- BBNcB,A.E. & Arsns, A.L. (1968):Empirical conec- titioned to a lesserdegree in rutile. Values of MrO5 tion factors for the electron microanalysisof sili- (up to 20.9 wt.9o) and Cr2O, (8.2 wt.go maximum) catesand oxides."L Geol. 76,382403. are higher than auy previously recorded for rutile from a kimberlite. The chemicallydistinguishing fea- Bocron,N.Z. & MeyEn,H.O.A. (1979):Oxide and sul- tures betweenkimberlitic M-rutile and that present fide minerals in kimberlite from GreenMountain, in alkaline suites, pegmatites and carbonatites are Colorado. In Kimberlites, Diatremes, and Dia- high Cr and low Ta concentrations ia high-pressure monds: their Geology,Petrology, and Geochemis- (Proc. regimes. Meteoritic and lunar rutile is moderately try 2nd Int. Kimb. Conf., Vol. l; F.R. Boyd & H.O.A. Meyer, enrichedin Cr and overlapswitl somecompositions eds.).Amer. Geophys.Union, Washington,D.C., 217-228. described from kimberlires. Our data indicate that neither armalcolite- nor Bourlrcn, A. & Nrcor.as,A. (1973):Texture and fabric crishtonite-seriesminerals are possibleprecursors for of peridotite trod,rler from kiinberlite at Mothae, M-Cr-RUTILE IN KIMBERLITE FROM BOTSWANA 263

Thaba Putsoa and Kimberley. In Lesotho Kimber- ferric iron content of a microprobe analysis.Car- lites @.H. Nixon, ed.). LesothoNat. Dev. Corp., negieInst. Wash. YearBook 71,600-603. Maseru,Lesotho. Fnrrr, J.J., Hanren, R.I. & Ut,rrasn,G.C. (1977): BuoorNoroN,A.F. & LtrDsr.ey,D.H. (1964):Iron- Armalcolite stability as a function of pressureand titanium oxide minerals and syntheticequivalents. oxyger fugacity. Geochim. Cosmochim,Acta 41, J, Petrologlt5, 310-357. 4034tO.

Canmn,D.L. & Oraye, A. (1960):Electron paramag- Grss, R.M. & A\rpsRsoN,J.S. (1972): The system netic resonanceof Fe3+in Tio2 (rutile). Phys. Rev, TiO2-Cr2O3:electron microscopy of solid solutions 118,1485-1490. andlrystatlographic shearstructures. J, Solid State Chern.4,379-390. 6nnrr, l.,6ecH, F. & Povowone,P. (1964):Review of ilmenorutile-struverite minerals. NeuesJahrb. Gnrv,I.E., IGro,A.F. & Atlpnrss,J.G. (1973):Com- Mineral. Abh. l0l, 142-172. pounds in the system Cr2O3-FgO3-TiO2-ZrO2 basedon intergrowth of a-PbO2 and V3O5struc- tural types. J. Solid State Chem. 8, 8699. -, Peu,,8.J., HewrHonNe,F.C. & Crnpuar, R. (1981): - A niobian rutile disorderedcolumbite inter- (1973):Armalcolite and genetically growth pegmatite, Haccnnrv, S.E. from the Huron Claim opaqueminerals in the lunar samples. southeasternManitoba. Can.Mineral. 19, associated 541-548. Proc. 4th Lunar Sci.Conf,, Geochim.Cosmochim. Suppl. 4, 1,777-797, O.arurB,D.B. & Mrrcnet.r.,R.H. (195): Mineralogy Acta, petrology and of the kimberlite from Somerset (1975):The chemistryand genesisof opaque Island, N.W.T., Canada.Phys. Chem. Earth 9, in kimberlites. Phys. Chem, Earth 9' t23-t35. minerals 295-307. Dewson, J.B. & Sumr, J.V. (1977): The MARID (1979):The Jagersfonteinkimberlite, South (mica-amphibole-rutile-ilmenite-diopside)suite of Africa: an emporium of exotic mineral reactions xenolithsin kimberlite. Geochim.Cosmochim. Acta from the upper mantle. Kimberlite SymposiumII 41,3W-323. (Cambridge), (abstr.).

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