Canadian MineralogistVol. 21, pp. 4l-58 (1983)

SPINEL ZONATION IN THE DE BEERS KIMBERTITE, SOUTH AFRICA:POSSIBLE ROLE OF PHLOGOPITE

JILL DILL PASTERISDepartnrent of Earth and Planetary Sciences & McDonnell Center for the Space Sciences,

Washington University, St. Louis, Missouri 63130, U.S,A.

ABSTRAcT

Grains of zoned spinels from the well-mapped,multi-intrusion De Beers kimberlite pipe, Kimber-ley, South Africar were studied optically andanalyzed with an electron microprobe. Strict sam-pling control made it possible to show that spinelcompositions and zoning trends are specific toindividual kimberlite intrusions. In general, thecompositions of kimberlitic spinel define a particular trend in the spinel prism, from the Al-richto the Cr-rich regions at the base of the prism,then up toward the Fe3+- or Ti-rich upper edge ofthe prism. commonly without great change in theFe2+71Fs:+ * Mg) ratio. No single kimberliteexpresses the entire trend. The main root-zoneintrusions of the De Beers kimberlite possessabundant grains of atoll spinel, with a core ofchromite or titanomagnetite. The gaps in atollspinels were once filled with Mg-pleonaste (Mg-Fe-Al-spinel), remnants of which are found onlyin contact zones adjacent to wallrock, Late-stageprecipitation of high-Al, refractory spinel mayhave been triggered by the cessation of phlogopiteprecipitation during rise and fractionation of thekimberlite. Phlogopite is abundant in several of theroot-zone intrusions, and commonly demonstratesmultiple episodes of precipitation. An increase in/(Og) and other changes in the kimberlite meltsubsequently caused breakdown and resorption ofpleonaste everywhere in the pipe except in thecontact zones, where the reaction did not go tocompletion.

Keywords: kimberlite, South Africa, root zone,spinel, phlogopite. atoll texture.

SoMMAIRE

On a 6tudi6 optiquement et analyse i la mi-crosonde 6lectronique les spinelles zon6s du pipebien cartographi6 de De Beers (Kimberley, Afriquedu Sud), i multiples venues kimberlitiques. Uncontr6le strict de l'6chantillonnage perrnet de prou-ver que composition et type de zonation sont carac-t6ristiques de chaque venue. En g6n6ral, de telsspinelles ddfinissent une lign6e caract6ristique dansle. prisme triangulaire des compositions qui part dup6le Al, va vers le p6le Cr i la base, et ensuitevers l'ardte sup6rieure du prisme, dans une direc-

tion d'enrichissement en Fe3+ ou Ti, sans grandchangement dans le rapport Fe2+/(Ire!+ * Mg).Aucune kimberlite ne montre d elle seule la lign6ecomplite. Les phases intrusives principales de lazone profonde du complexe De Beers contiennenten abondance des spinelles i texture en atoll, avecnoyau de chromite ou de titanomagn6tite. Leslacunes de I'atoll 6taient originellement remplies depl6onaste-Mg (spinelle Mg-Fe-Al), dont on trouveles vestiges dans les cristaux de spinelle au contactdes roches encaissantes. La cristallisation tardived'un spinelle alumineux r6fractaire pourrait r6sulterde la fin de la cristallisation de la phlogopite lorsde la mise-en-place et cristallisation fractionn6e dela kimberlite. La phlogopite est abondante dansplusieurs de ces phas€s intrusives profondes, etmontre dans plusieurs cas des stades multiples decristallisation. C'est surtoui i une augmentationde la fugacit6 d'oxygdne dans le bain kimberlitiquequ'on devrait attribuer Ia d6stabilisation et la 16-sorption du pl€onaste partout sauf dans les zonesde contact, oir la r6action est rest6e incompldte,

(Traduit par la R6daction)

Mots-clis: kimberlite, Afrique du Sud, zoDe pro-fonde, spinelle phlogopite, texture en atoll.

INtnopucrtoN

The mineralogy and petrology of numerouskimberlite intrusions are now documented inthe literature (e.9., Haggerty L975, Mitchell1978a, b, 1979, Mitchell & Clarke 1976, Mit-chell & Meyer 1980, Boctor & Meyer 1979).The present study concerns the root zone ofthe well-mapped De Beers kimberlite diatremein South Africa. It currently is being mined, andtherefore was accessible for extensive horizontaland vertical sampling, in contrast to the morelimited ac@ss available in many previousstudies of kimberlites. The De Beers diatremeconsists of several intrusions, which are recog-nizable by their field relations and their p€trog-raphy.

The availability of fully documented samples(drill core and personally collected specimens)from several intrusions (facies) within a single

41

42 TIIE CANADIAN MINERALOGIST

vERTEALsfcTroil

CRATER ZONE-rc!;TIJ EROTED

DNTR€TC ZONE

N

0In

62On LEVEL

SSTA€E

A,VERTICAL SECTIOI{F-At{ vtEws

diatreme made it possible to compare petro-logical relationships among the kimberlite facies.

C,D'o l @. SCALE

\I\

'fif;'EAST PlPE,7zOm LR/EL

OATA @URTESY OF'DE BEERS CONSO-DATEDMINES.

Enlorg€rnsnl ofEo3l Pipo inFigurc D

PERPT€RAL KIMBERLM

\ - - - - - - - ? \ - _ - / -

BRECCIA ZONE

AAB NOTTO SCALE

Frc. l. Diagrammatic vertical section (A) and cross sections (B,C,D) ofthe De Beers pipe, Kimberley, South Africa. Each of the three majorregions of the diatreme (C,D) probably represents at least one separateintrusion. Fig. lB (enlargement of East Pipe in Fig. tD): according tothe De Beers geologists. the East Pipe consists of an earlier PeripheralKimberlite. a later Core Kimberlite, and the Intrusion Breccia (part ofthe "Breccia Zone"). The Intrusion Breccia probably is not a separateintrusion. but rather Peripheral Kimberlite that has undergone extensivephysical (and to some degree. chemical) interaction with the walls of thediatremc. Figure from Pasteris (1981), copyrighted by American Geo-physical Union.

The opaque oxide phases were chosen for parti-cular study because they are the best-preserved

SPINEL ZONATION IN KIMBERLITE, SOUTIT AFRICA 43

minerals of t}le rocks, and because their com-positional variability makes them good indi-cators of changing conditions in the parentmelt.

A major goal of this study was to determinethe extent to which the opaque phases in kim-berlite can be used as indicators of l) differen"tiation processes within a kimberlite facies, and2) genetic relationships among the facies.

Gnorocy oF THE Dn Bmns KrMsrnrrreCor"rpLEx

The De Beers kimberlite pipe is one of fivemajor diatremes in the Kimberley district, SouthAfrica. The diatremes afe Cretaceous in age,about 90 Ma old (e.g., U-Pb and pb--Fb datingby Allsopp & Kramers 1977, Davis 1977). Thebasement rocks in the region are precambriangranite and amphibolite gneiss. They are locallyoverlain unconformably by Ventersdorp vol-canic rocks (-23M Ma) and by more than300 m of the sedimentary and volcanis rocksof the Karroo System (Carboniferous to Jurassicin age). The Precambrian and Ventersdorprocks are the dominant wallrocks in the under-ground workings of the De Beers mine, wheremo-st of the samples examined in this study werecollected.

-The De Beers pipe has a geometry typicalof many kimberlite pipes. It is dyke-[G atdepth, but expands abruptly into a funnel-shaped diatreme at about 20OO m below theoriginal Crelaceous land surface (Fig. l). Most

samples analyzed in the present study comefrom the dyke-like root zone, which has thebest-preserved igneous features of the intrusion.

The De Beers root zone has three major parts(Figs. I A,D,C) called the East Pipe, the WestPipe, and a Neck, each comprised of one ormore separate intrusions. The East Pipe consistsof two major intrusions, the Peripheral (ear-lier) and Core (later) kimberlites, plus severalkimberlite dykes, a kimberlite sill, and a peri-dotite dyke. Field relations in the root zonesuggest there were tlree or four major episodesof intrusion: Peripheral, followed by Core, fol-lowed by the Neck and the West Pipe (DeBeers Geology Department). The IntrusionBreccia ("Breccia Znne" of Fig. lB), whichoccurs in both the Neck and the F.ast pipe, isnot considered a separate injection, liut ratheras kimberlite melt that has been affected byinteraction with large volumes of wallrock. Oneof the reasons for comparing the compositionalpatterns of spinel among the different parts ofthe root zone is to determine if they too definethe same number and physical extent of separateintrusions.

The East Pipe and Neck of the De Beers roorzone were sampled by horizontal and verticaltraverses across the 105-m level to the 729-mlevel (depths below the level of the presentsurface in the mine). Contact zones between thekimberlite and the wallrock were as well sam-pled as the middle of the intrusions. Drill-coresamples (24) were studied from the 562-m levelthrough the 720-m level of the West Pipe.

TABLE 'I.

SUI,{MARY OF PETROGRAPHIC FEA'IURES OF ]ltE DE BEERS ROOT ZONE

Region 'I Features 'ee of Serpen-ti nlzatl on

t r i x Ph logo. . Ph logo. inel Abund. (A)Preservatlon (P)

inel Zonlng

|cestPi pe

rell consolldated.:oarse phlog. v.rbund. Resernbles;ore K.

tepth-depend., usu.t t least part ia l .

v . abund., usu.color less; i fcolored, R pleo

abund. i oftena l te red r ims;zoned (R core) orunzoned (N or R)

-ow A. Poor P I ype 2 ; a to l ltexture.

CoreKinberl I

more {a l l rk . lnc l .& serpentinlzationthan Peripheral K.

/ar iable, usu.noderate

abund. , R p leo iome, altered Low A. Poor P. Type 2 ; a to l ltexture.

PeripheraKlmber'li t

fell consolldated,/. serpent. Feter{a l l rk . inc l . than;ore K.

rsu. part ia l . some: paleco lo r .

rbund. at wa] l rk.lontactsi zonediR core) or un-zoned (N or R).

Low A. P betterthan in Core.Grains largest atflal lfk. contacts.

Type 2; ato l ltexture; sonep l eonasteremnants.

NeckKimberl ite

least altered &friable mck ofbody. Homogeneous& flne-qralned.

var iable, usu. in somesamples.

abund. at v{allrkcontacts;al teredel seuhere.

\: l-39. P betterlhan elsewhere..argest grains in)ipe at contact.

Type l, often inperovskite. Type 2;pleonaste abund. at| {a l l rk ; a to l l s .

IntrusionBreccia

f lne-grai ned,basaltlc. Someflov bandlng.FeH wa l l rk lnc l .

variable, fronnone to complete.

abund. , o f tenwel l preserved.

l : l -2%. P as in{eck. Largesty ra lns a t wa l l rk:ontact.

Type I, often lnperovskite. Type 2ipleonaste abund. atm l l r k i a t o l l s .

Dlkes &5iil

v. friablei extrenehydration & carbon-ation. Coarse phlog

usu. coltDlete. matrix t phenosforn contlnuurn.

abund. i zoned(R core) or un-zoned (N or R) .

Moderate A;srnl l .

usu. unzoned titanomag &Type

'1. I lmenlte

'lncl. in t itanomag.

. nonna= inclusions €ochr0ism R = reverse pleochrolsm serpent. = serpentinized

44 THE CANADIAN MINERALOGIST

Pnrnocnerry oF TrrE Dn Bnnns Krtvrsenl-lre

The different intrusions (facies) of the DeBeers kimberlite can be distinguished on thebasis both of their silicate and opaque oxidephases Clable l). Throughout, olivine and phlog-opite are the dominant phenocrysts, but theytypically also occur in the matrix, which maycomprise more than SAVo of the rock volume.The matrix also contains calcite, serpentine,apatite, monticellite and opaque oxide phases.The major opaque oxide-minerals are spinel,rutile, perovskite and ilmenite, which togetherrepresent at most a few percent of the rock. Aminor portion of the kimberlite consists ofmantle-derived xenoliths and xenocrysts, partic-ulary of garnet lhetzolite and ilmenite.

In hand specimen, different kimberlite faciescan be distinguished by color (hues of blue andgreen), friability, amount of crustal xenoliths,and degree of serpentinization and carbonation.All are porphyritic; some are obvious breccias.In thin section, fufther distinguishing featuresare the abundance and preservation of phlogo-

pite, the abundance of segregations of calciteand serpentine, and zonation in the grains ofspinel (Table 1).

ANALYTICAL PROCEDURES

Analysis of spinel and phlogopite grains wascarried out using the automated wavelength-dispersion MAC 40O electron microprobe at theGeophysical Laboratory, Washington, D.C.(Tables 2,3,4). The operating conditions forspinel analysis were 15 kV accelerating voltagen5b nA beam current, l-2'p,m diameter beam-size, and 30 seconds counting time or 50,000counts per element. The standards used arenatural minerals, synthetic oxide phases andsynthetic glasses. Because of the small size ofthe grains being analyzed, the elements Ca andSi were carefully monitored to ensure that matrixcalcite and serpentine were not being excitedby the beam. The data-reduction program isbased on Bence-Albee correction procedures(Bence & Albee 1968, Albee & Ray 1970).Fet* and Fe'* in the spinel (Tables 2, 3) were

TABLE 2. REPRE5EI'IIATIVE ZONED SPINELS FROM TltE DE BEERS PIPE

1 -----2 3 ---) 4 5 ----i 6 7 ---) I 9 -_t 10

e.00 38.43 4.14 6.13 5. ]8 7.3? -9.q1 ^9.1! ? ' l l ,9 '9 : 5^ '22i:ii I:ii qi'.ii ;'.b, :i'.ii i'.41 y.e? ?7.!1 .1':: !?'12 "t'ig

' l l - - - - - - t l2-+I3

Mqo .10.46 4,23 13.83 ]s.96 1.1.6.1 13.14 11.56 ]?.q9 !Z 'q8 19.89 21.42 13.26 1?.74

;$ f i :d; zd.s5 is.go l l .3e 1e.55 zt ' .oi i i : te zl.ag v.?1 e.64 e.67 15.23 3r.02

!h0 0.94 . l .or 0.70 0.46 0.e6 o.si -0:64 -0.99

q.gr 0.80 0.67 0.57 0.77

Nlo 0 .z r 0 .37 0 ,2 ' t 0 .00 0 .03 o . i i o : t i o . lC q '14 0 ' t7 0 '21 o ' le 0 '16c .o o .o r o . ra o . :o o .so o .ao o . i i ' o .z i o . i i o 'z+ o 'gs o 'zg t ' t+ o ' tg

A1203Cr203Fe20 3

1 1 . 3 241.848 .87

0.54'10.5945.35 d: ; : z \ ' .s i io :G zr i .oa tq. ts 30.q9 t t . t t ' za 'su q l ' l .46

18.24'17.56 8 .93

'10.23

0 .975 . 0 1 20,70

0 . I 7Tl0zSl0r

4 .910 .09 u . c q

17 . 210 .26

J . v 50 .39

7 .710 . 0 1

8 .65n t o

8.090 .240 0

l'lqF;2+l.hNiCa

0 .5140.5760.0260.0060.002

0.2330.9210 .0320 . 0 1 10 .007

0.682o.7170.0200.0060 . 0 1 3

0.8000,2690 . 0 1 10.0000 . 0 1 5

n (oq

0 .56 I0 .0280 .001

0.6580 .7590.0?60 .0030 . 015

0 .5820 .6100 . 0 1 80,006

0.6440.7770.0200.0040

0.8410 .3220.0240.004

0.928 0.983 0,652 0.6300.252 0.252 0.4m 0.8600 .021 0 .017 0 .016 0 .0220.004 0.005 0,005 0.004

5

A1 0.440 0.024 0.351 'r.28r 0.168 0.243 0.?0q q.?9q 9'?19 9')?) g'lx? l '?ig R'?lgt i i : t f i ; : :5 ; 6.0:o 0.052 1.166 o. i ie 0: t8t 0.03, 1.11e 0.686 0.121 r . r r4 0 ' r r5r"r* o.zzo t.zoo o.z+s o.qzz o,zos o.idi d.i60 0.7i0 o.zoq o.sss 0.996 0.284 0.626

Ti o.1zz o. la7 o.4zs 0.084 o. lee 0.461 0.220 0.11? 9. \92 o. i :o 0 '237 9 '1?! 0 '515si 0.003 0.016 o:d6d 0:0ii 0.000 0.d00 0:006 4.008 0.008 0.008 . 0.030 . 0.00e 0.006

T0TAL 3.00 3.00 3.00 3

1-2t Core Kimber'lite; 720m 'level, neaf contact with Peripheral Kimber'llte and basenent gneissi dark core (l)

and l lght-colored r im (2) .3-4: iteck firnber'litei OZOm

'i6vef, near contact with Ventersdorp wal.lrock;.tan-core (3) and black.rim (4).

5-6: Intrusion Breccia i 720n level near Necki chromite core-(5) and tan r ln (b) '7-8: l , le i t p ipe;720m ievel ; b lue-gray core (7) and tan r lm (8)- . .9-ll: per.ipheral finoertrte;'Z2orn ievll near'c6nta"t tiitir'.doie'fiinler]ite; gray core (9), edge of grav core (10)'

and outef r im (11). High Mg nainta ined throughout.zoning.'12- '13: KJrnber l i te s i I i , b lue ior 'e (12) and edge of core (13).

Note: colors refer to appearance in r€flected. light with oil-inmersion objectives;-Ferr calculated--assumingiiiiir'tiriii 'v or ibinii'(FrnitC-progrimt; in all the core*im pairs and trJplets above, the Ms and Alcontents remaln essentially constant or rise'

SPINEL ZONATION IN KIMBERLITE, SOUTI{ AFRICA 45

?0.t2 20.4010.76 9.980.29 0 .280.02 0.000.18 0.34

46.10 45.801.62 2.86

L6.22 L6.20

3.46 2 .720. r7 0 .23

L2.66 3,18 2.36 3.090.40 0.00 0.00 0.25

L 2 3

Mso 19.94 13.36 13.8:lFeo 9.51 24.75 25.90lilno 0.84 o,tz 0.70rflo 0.09 0.16 0.2ICao 0.59 0.30 0.36

Al,0r 8.95 6.76 9.(mcr ;0 ;21 .15 9 .09 l .16Fe20i 21.62 27.6 2s.93

Tl0, 9.83 14.86 t7.21sro i 0 .14 0 .71 0 .26

0.815 0,8180.2x 0.2450.007 0.0070.002 0.0000.009 0.005

r.524 1.4810.025 0.0350,313 0.333

0.2670.24r0.697

IABLE 3. I!|ICRoPR0BE AMIYSES S m BEERS SPINELS fmt PLEOMSTE RIMS Trxrunel eNo CovposITIoNAL ReLertoNsnlpsAvroxc rnp Spruel MlNntels

Textural and zoning featuresSpinel grains in the De Beers kimberlite occur

in the following forms: individual grains in thegroundmass, inclusions in perovskite, inter-growths with perovskite and ilmenite (but withno apparent reaction relationship), mantles onilmenite macrocrysts, both as discrete grainsand as massive reaction rims, and symplectic(fingerprint-like) intergrowths with silicatephases (Figs. 2-8). Spinel very rarely is seenas inclusions in olivine.

The spinel grains are relatively small ( ( 35&m ) and were studied both in reflected andtransmitted light with high-power oil-immersionobjectives. They usually occur individualtyrather than in clusters, and they commonlyexhibit resorption features.

Both unzoned titanomagnetite and zoned Cr-bearing spinel grains occur at De Beers, butzoned grains are more common, The two typicalpatterns of zoning from core to rim are: (1)chromite + (ilmenite) + titanomagnetite (Fig. 4),and (2) chromite/titanomagnetite + pleonaste(Mg-Fe-Al-spinel) -> magnetite (Figs. 5,6).Grains of type 1 are found as inclusions inperovskite and individually in the matrix. How-ever. type-2 grains always occur separately. Thissuggests that type-l zoning represents an earliersequence of precipitation, distinct from type-2zoning.

In type-2 grains, the spinel core (euhedralto anhedral) commonly is separated from itslighter-colored outer rim(s) by a gap of severalmicrometres (Fig. 7). The term "atoll spinel'lwas applied by Mitchell & Clarke (1976) tosuch features in a Canadian kimberlite. In theDe Beers atoll spinels, the core phase commonlyis euhedral chromite or anhedral titanomagnetite.The outermost rim is so narrow ( = 5 /,m) astq make electron-microprobe analysis difficult,brit it clearly is rich in magnetite. The composi-tion and optical properties of the "gap" resemblethose of the fine-grained, calcite-serpentine mix-ture that comprises most of the kimberlitegroundmass (Fig. 7).

The abundance, texture and composition ofthe opaque oxide phases are as characteristicof the different facies of kimberlite as are theprimary and secondary nonopaque phases (Tablel). In general, type-l and type-2 spinels occurin the Neck and Intrusion Breccia. whereas theWest Pipe and East Pipe (Core and Peripheralkimberlites) contain only type-2 spinels. TheWest Pipe and Core kimberlites are recognized

L4.2722.&0.7x0.29

15.932.30

29.9

5 0 1

20.08 m.32 m.3910.36 9.74 10.540.30 0.29 0.320.00 0.03 0.070.43 0.30 0.30

47.L6 49.32 48.222.03 2 .56 1 .17

15.80 14.38 15.51

At 0.335Cr2- 0.530Fe" 0.65a

lq21 0' 2 0.667 0.682 0.679 0.811 0.813Fe-' 0.252 0.69X 9.7L7 0.603 0.235 0.219ltr 0.023 0.023 0.020 0.019 0.C07 0.007Nr 0.002 0.004 0,006 0.007 0.000 o.(X)lCa 0.020 0.0X1 0.013 0.@8 0.012 0.009

0.351 0.599 1.505 1.5600.030 0.058 0.043 0,0540.745 0.710 0.322 0.2N

M.23! 0.374 0.428 0.304 0.065 0.048 0.062 0.071 0.056sr 0.004 0.0?4 0.009 0.013 0.000 0.000 0.007 0.005 0.006

IoTAL 3.00 3.00 3.00 3.00 3.00 3.00 3.oo 3.00 3.oo

Analj6es f1-4: to tltilmagnetlte cores of sp'inels rlth black pleonaster'lns. {1r cent.al region of Perlpheral Klrberlite, 720 nlevel; *2-3: [ec& Klnber]lte, 620 d lryel, near contact rlthVentersdorp rallmck; t4: tleck Klderlite, 620 o level,about 4.5 m fr@ aontact.

Analyses *5-9: bl6l, I'19-p'lenaste spJnel fron 720 m leyel of IntrusjonBrccld. Elack splnels r ln te cores l l*e those ln AnalJ6esf, t-4.

Noter colors refer to appear@q ln ref lsted l lqht r l th oi l -lmerslon obJect ives; Fe$ catcul ated ass!6lng stolchl@etryof spinel (FERRIC proqrd).

I0TAL 98.67 98.49 98.56 98.93 99.34 99.30 99,85 98,95 98.41

0.8170.2290.0060.0000.010

1.4790.0620.334

TABLE 4. MICROPROBE AMLYSES OF DE BEERS PHLOEOPITE GRAII{S

L 2 3 4 5

Ia ,o 0 .03 0 .00 0 .06 0 .26 t .17Kzo 9 .34 I0 .24 10 .46 9 .89 9 .45

!490 24.43 26.55 24.83 24.17 19.64Feo 2 .96 2 .42 6 .48 6 .? t 4 .67I' lno 0.09 O,02 0.01 0.01 0.04l l io 0 .06 0 .05 0 .08 n .d . n .o .Cao 0.71 0.29 0.02 0.92 0.86

Al r01 15 .79 12 .54 10 .73 9 .49 14 .27c r ; o ; 0 . 0 2 0 . 0 r 0 . 1 1 0 . 2 s 1 . 6 5

T io , 1 .63 0 .52 0 .61 0 .83 3 .99sr0 ;38 .04 42 .03 43 .19 4r .98 40 .38

rotlt g:.tt gg,oe gs.5s gc.os go.12.

l la 0.002 0.000 0.008 0.037 0.162K 0.861 0.924 0.943 0.919 0.854

t49r.2,632 2,7& 2.613 2.623 2.074Fe- '0 .178 0 .141 0 .381 0 .378 0 .277lln 0.004 0.C00 0.000 0.001 0.003llt 0.002 0.002 0.004 n.d. n.o.Ca 0.053 0.021 0.000 0.077 0.069

At 1.344 1.044 0,893 0.815 1.192Cr 0.(80 0.@ 0.006 0.01.7 0.093

Ti 0.087 0,027 0.031 0.046 0.213sl 2.741t 2.97t 3.050 3.056 2,861

Totll 7.911 7,9L0 7.929 7.96L 7.792

Cation values nomallzed to11 oxjgens; ldeal fomula is8 cat lons rer 1l ornens.Anatses f t -3 frm i iesentstudy of De B€ers kimberl.ite.Analyses f4 and 5 fr@Boettcher et al . ,s (1979)data on the 0e Beersk l n b e r l l t e , p , 1 7 5 , s m p l eKb-5-1-A. i l l : larqe, matr ixphlogoptte; sdple DB19; NeckK i m b e r l l t e l 7 2 t u . l e v e l . f 2 :soal l , natr i r phlogoplte;smple DB3l i Core Klmberl i te:742 n. level. f3i reersepl@chrolc core of zonedphlogoplte; smple D8428;k l m b e r l i t e d l k e ; 1 2 5 m .level. d4: reverse Dlschrolccore of zoned phloqoDlte.f5: nomal olmchr6i i : r tm onIraln f,4.

calculated by assuming stoichiometry (FERRICprogram, Geophysical Laboratory). Reconnais-sance microprobe analysis was performed onphlogopite grains, using a broader beam (> 5pm) and a beam current of 40 nA. Many phlog-opite grains are at least partly chloritized orserpentinized, as reflected in the oxide totalsin Table 4.

46 THE CANADIAN MINERALOGIST

Fro. 2. Dark grey, subhedral titanomagnetite inclusions in light grey perovskite grain. Peripheral Kimber-

lite. Reflected light, oil-immersion objective. Scale bar 20 y.m'

: : . : :. . : : l :

: : : : : t l

SPINEL ZONATION IN KIMBERLITE. SoUTH AFRICA

Frc. 3. Major phase (light grey), ilmenite, has been fractured and altered. Dark grey, highly titaniferousmagnetite forms part of symplectic replacement zone. Small, highly reflecting inclusions in the ilme-nite are sulfides. Neck Kimberlite. Reflected light, oil-immersion objective. Scale bar 20 pm.

Ftc. 44. Zoned atoll spinels attached to or partly enclosed in perovskite. 4B. Sketch. p perovskite, Ttilanomagnetite, M magnetite-rich spinel, C chromite. A anatase or perovskite. Neck Kimberlite. Re-flected light, oil-immersion objective. Scale bar 10 3r,m.

Ftc. 5. Zoned atoll spinel. Medium grey titanomagnetite. black (see arrow) Mg-pleonaste, white rims ofmagnetite-rich spinel. Core Kimberlite. Reflected light, oil-immersion objective. S"ate bar 6 p,m.

Fto. 6. Multirimmed grains of atoll spinel (left) and zoned spinel (right). In zoned grain, tan titano-magnetite core' Mg-pleonaste (black) inner rim, magnetite-rich (white) outer rim. Note partial re-sorption of both titanomagnetite and pleonaste. Note also juxtaposition of highly resorbed (left)and. slightly resorbed (right) grains of atoll spinels. Neck Kintberlite. Reflected Jight,-oil-immersion ob-jective. Scale bar 20 p,m.

otj-Ftc. 7. Atoll spinel with wide "gap" filled with kimberlite groundmass of serpentine and calcite. Note

the small, euhedral chromite core and magnetite-rich spinel rim. Peripheral Kimberlite. Reflected light,oil-immersion objective. Scale bar 6 p,m.

Frc. 8. Sympletic intergrowth of zoned spinel (chromite cores and magnetite-rich rims on the digits) andaltered silicate. Such symplectite usually forms part of what appeir to be mantle xenoliths. CoreKimberlite. Reflected light. oil-immersion objective..scale bar 50 um.

petrographically by the low abundance and poorpreservation of their spinel grains; atoll grains(type-2 spinel with no pleonaste zone) are themost common. The kimberlite dykes and sillcontain small single-phase euhedra of titano-magnetite and type-l zoned spinel. Type-2 znn-ing and atoll spinel grains aie absent in thedvkes.

C ompositional patterns

A principal feature of spinel grains in theDe Beers and other kimberlites is a composi-tional discontinuity betiveen the core phases,which are usually Cr-rich, and the rim phases,which are Ti-rich. This gap occurs both withingrains and within kimberlite suites (e.g., Hag-

48 THE CANADIAN MINERALOGIST

gerty 1975); it also is apparent in magmaticore deposits, in igneous complexes and, to acertain extent, in lunar rocks (e.9., Haggerty1976). The discontinuity may be due to aperitectic relationship between spinel andpyroxene (e.g., Irvine & Smith 1969), to parti-cular l(Or) conditions during crystallization ofthe melt (e.g., Ulmer 1969, Hill & Roeder 1974),or to gaps in solid solution in the spinel system(e.s., Muan et al. 1972).

Unfortunately, equilibrium coexistence ofspecific silicate and spinel phases cannot beinferred for the De Beers kimberlite nor, prob-ably, for most kimberlitic pipes. Standard geo-thermometry and geobarometry techniquestherefore, are not applicable. It is also difficultto prove equilibrium between specific grains ofilmenite and magnetite. Where the associationdoes occur in kimberlites, tle phases usually

ln l rus ion Brecc io

are too Mg-rich for the Buddington & Lindsley(1964) geothermometer - oxygen barometer tobe applicable (Pasteris 1980a). One must there-fore regard the zoned grains of kimberliticspinel in themselves as indicators of magmatictrends. In the following paragraphs, spinel com-positional patterns in the De Beers root zoneare compared among the various facies andwith patterns observed in other kimberlites.

Plots of microprobe-derived compositionsshow three distinct compositional groups on thespinel compositional prism (Figs. 9,lO), namely(l) chromite (base of prism), (2) titano-magnetite (toward top of prism), and (3) Mg-pleonaste (almost exactly on the spinel-hercy-nite join). There is a wide spread in compositionin groups I and 2, but not in grouP 3.

Two major compositional trends are evidentin Figures 9 and 10. The first is a normal mag-

o.oFRE 3708OFRB 3?OCAFRBs?O8.OFRB 37 7ocnc t coorrrOcnc t Rnr

Fctr%

l"o.MgA

')/-,

MgCr.O.

Frc. 9. Compositional plot for the spinels of the Intrusion Breccia; Fe2+ only plotted (Pasteris 1982).The lines ind arrows represent core-to-rim zonation. Each type of symbol (e.g., circle, square, trian-gle) represents a particular sample. All samples came from the 720-m level of the mine, except forFRB377, which came from the 620-m level (Fig. 1). Letter notations on the diagram refer to colorsin reflected light and to unusual (and perhaps xenocrystic) grains (l,h,Q). Note the distinctivezonation pattern (type 2): chromite/titanomagnetite + Mg-pleonaste + magnetite; in some casesn not allthree separate zones are present.

SPINEL Z)NATION IN KIMBERLITE, SOUTH AFRICA

West Pipeo DBW 42Ao oBw47AD8W55AoDBW/C58

hc1o.

Me Cr.O.

FIc. 10. Plor of compositions of spinel from the West Pipe. Tie lines and arrows connect core andrim of a zoned grain of spinel; each symbol tlpe represents a particular drill-core sample. The speci-mens represent the following depth zones: DBW42B (620 m), DBW47 (660 m), DBW5sA (7lO m),DBW/C58 (72o m).

49

matic pattern, extending from chromite coresto titanomagnetite rims (Fig. lO). Betweenthese end-members, data points are few, a fea-ture that is common to many kimberliles andmafic complexes. The second trend appeamunique to the De Beers pipe. In this case, titano-magnetite-rich cores are directly mantled bypleonaste rims (Fig. 9), and many of the titano-magnetite compositions appear to lie on amixing line with pleonaste. Ifowever, the latterfeature is not a function of integration of ad-jacent spinel zones by the electron-microprobebeam; many of the titanomagnetites plotted areatoll cores with no pleonaste rims present.

Although trend I (chromite -r titanomag-netite) is represented throughout the pipe, trend2 (titanomagnetite -> pleonaste) is recordedonly in the Intrusion Breccia and Neck. (Rem-nant pleonaste was optically identified in theCore and Peripheral kimberlites, but in grainstoo small for electron-microprobe analysis.)

Thus, compositional differences are evidentin the spinel grains from different parts of theDe Beers pipe (Table 2). The major distinctionsare as follows: the presence or absence of trend2, the extent (length in composition space) ofthe spinel compositional trends, the values ofthe Fe2"/(Fe" + Mg) ratios (FFM ratios),and the slope of the chromite-Jitanomagnetitetielines in compositional space. The typicalrange in FFM ratios for each part of the rootzone is as follows: Core 0.20-0.45, Peripheral0.20-0.55, Neck O.20-0.55, Intrusion Breccia0.20-0.80, West Pipe 0.4(H).55, and kimberliteDykes and Sill O.35-0.9O. It is also clear fromthe data that spinel grains in close proximityto the wallrock commonly have compositionsand textures distinctive from those in their hostintrusions.

The apparently'aberrant data points on Fig-ures 9 and l0 (see captions) represent sym-plectites and other spinel grains derived from

50 THE CANADTAN

mantle xenoliths. They are usually richer in Aland Mg than groundmass spinel in the kimber-lite. Additional compositionally distinctive spinelgrains in the kimberlite are those that mantlelarge crystals of ilmenite; they usually are richerin Ti and Fe than all other grains. There alsois an optically distinct, salmon-colored (in re-flected light), Ti-enriched spinel that is com-monly present in veins between fragments of anilmenite grain, or between an ilmenite and itsmassive perovskite rim (Fig. 3).

MINERALOGIST

Spinel data were plotted on Fe'+/(Fe!+ *Fe'*) versas Fe"+/(Fe'* * Mg) or FFF ver,rasFFM diagrams (not shown; see Pasteris l980a)to monitor how the increase in Fes+ correlateswith the change in total Fe. Because of the con-straints that logically can be placed on thesystem under investigation, the FFF ratio prob-ably can be used as an index of relative oxida-tion within the pipe. The iron ratios showdistinctions among pipe facies and changesthroughout differentiation. As FFM increases,

REPRESEN TAT I VE SP/ruELS

.6.4 .8 t.o t.2

E cotions = 3F2+re

Frc. I l. Cr-Fe2+ plot of representative spinel compositions from each majorregion or intrusion in the De Beers pipe. For the sake of clarity' Mg-pfeonaste compositions (0.15<Cr<0.85; 0.23(Fet+<0.27) were notplotted on the graph. The vertical dashed line divides the main-stageCore and Peripheral kimberlites (Fe!+(0.5) from the other facies in theDe Beers pipe (Fe!+>0.5). This method of plotting is described inPasteris (1982). Solid circle: Core and Peripheral kimberlircs, open circle:West Pipe, asterisk: peridotite dyke in the De Beers pipe, star: NeckKimberlite, kimberlite dykes and sill.

SPINEL ZONATION IN KIMBERLITE. SOUTH AFRICA 5 1

FFF decreases in almost all cases. Thus, al-though Fe'* increases with reference to Mgduring fractionation, it increases more slowlythan Fe3+. Note here that Fes+ and Fee+ werecalculated by assuming stoichiometry. Assumingthat groundmass ilmenite was precipitated asspinel crystallized (Pasteris 1980b) and thatthe melt was oxygen-buffered, the negative slopeof the spinel differentiation trend on an FFF-FFM plot may represent a decrease in tempera-ture along a single l(CI) buffer.

In an effort to compare spinel compositionsamong the several kimberlite facies at approxi-mately the same degree of fractionation, theFFF values were determined at selected FFMratios. Chromite grains from the Core andPeripheral kimberlites have a higher content ofFet* than chromite from other parts of thepipe, an effect probably due to bul\-compositiondifferences or perhaps to high l(O,).

As a first approximation, one can assume thatall the De Beers intrusions have the same or asimilar parent kimberlite melt. If this is true,the degrees of fractionation of the facies areindicated by the relative Fe-enrichment in theirspinel compositions (Fig. I I ). A plot of Crversus Fez+ (Fig. 1l) in spinel indicates subtledifferences between some facies, and shows aclear distinction between the Core and Peri-pheral kimberlites and all the other facies. Thediagram indicates that the apparent sequencefrom least to most differentiated is PeripheralKimberlite, Core Kimberlite, Neck, IntrusionBreccia, West Pipe, Kimberlite Dykes and Sill.This interpretation is in accord with the fieldevidence suggesting that the major episodes ofintrusion at De Beers were the Peripheral, fol-lowed by the Core, followed by the Neck and theWest Pipe. Spinel compositions also support thefield interpretation that the contact kimberlitecalled the Intrusion Breccia was not a separateintrusion. It probably is Neck Kimberlite witha distinct cooling history. For instance, rapid,nonequilibrium cooling may have produced thestrong Fe-enrichment recorded in IntrusionBreccia spinel grains.

Sppcler RErarroxsHrps rN THE Coxtecr ZoNns

In the case of both phlogopite and spinel,not only do compositional and textural rela-tionships differ among the kimberlite facies, butthey also are strongly dependent upon theproximity to the wallrock. For instance, kimber-lite in the contact region contains type-2 spinelgrains with an intermediate zone of Mg-pleo-naste, which, in turn, usually is rimmed by a

thin magnetite-rich zone (Figs. 5,6). Awayfrom contact regions in the same kimberlitefacies, the intermediate zone of Mg-pleonasteusually does not exist. Instead there is a gapthat gives rise to the atoll spinel grains de-scribed previously as a variation of type-2 zon-ing (see Fig. 7; cl. Mitchell & Clarke 1976).

Zoned spinel grains with Mg-pleonaste rimssurrounding titanomagnetite cores occur almostexclusively in the Neck and the Intrusion Brec-cia. where the kimberlite is in direct contactwith wallrock. Although Mg-Al-spinel rims arecommon here, one can see, in any given thinsection from the two regions, not only fullypreserved rims of pleonaste, but also atoll spinelgrains that have only a partial pleonaste rim(Fie. 6) .

The following textural evidence strongly sug-gests that Mg-Al-spinel rims originally werepresent in all the major root-zone facies. Inthe Intrusion Breccia and Neck, some grainswith chromite or titanomagnotite cores havethick rims of pleonaste. In these cases, thepleonaste is subhedral and has no outer mag-netite rim. More common in this region of thepipe are fragmentary patches of pleonastearound a chromite or titanomagnetite core. Insuch cases, a discontinuous, poorly developedouter rim of magnetite partly encloses the grain(Fig. 6). Only very rarely is (fragmentary)pleonaste identified in the Core and Peripheralkimberlites (Fig. 5). Such fragments usuallyare too small for quantitative analysis. On theother hand, in those samples (from the EastPipe, Neck, and West Pipe) in which pleonasteis not found, a black, translucent material oc-curs in some cases between the chromite coreand magnetite rim. Microprobe analyses of thetranslucent material indicate variable quantitiesof Si, Ca and Ti (not titanite), but almost noAl. Much more commonly, this zone is filledwith the same calcite-serpentine mixture thatcomprises the kimberlite matrix (supported bymicroprobe analysis). It is the latter grains ofspinel that possess an atoll texture (Fig. 7).

Textural and compositional relationships thussuggest that in the main intrusions of the rootzone, Mg-Al-spinel rims originally were present.Resorption of pleonaste is believed to have oc-curred in all but the contact kimberlite of theNeck and Intrusion Breccia.

The tgxtural features of phlogopite pheno-crysts also are related to their proximity to thewallrock. At increasing distances from thediatreme wall, phlogopite phenocrysts show en-hanced development of thick, black rims (opaqueto transmitted ligho of a fine-grained, inhomo-

52 THE CANADIAN MINERALOGIST

Frc. 12. Pleochroically zoned phlogopite that ispossibly a mantle x€nocryst (owing to large size).It is rounded (possibly due to physical attrition)and chemically altered to a black rind that extendsinward along some cleavage planes. Matrix is cal-cite, serpentine and serpentinized olivine. Speci-men D844, kimberlite sill: transmitted light;scale bar 200 g,m.

geneous product (Fig. l2). However, phlogo-pite phenocrysts are well preserved in the kim-berlite nearest to the wallrock contacts.

GuNErrc CoNsrDERATroNs: PossrBLE OnrctusoF SPINEL ZoNTNc

Spinels grains with a chromite core and atitanomagnetite rim can be explained by crystal-lization during normal magmatic differentiation.However, several questions arise concerning thepleonaste zones around titanomagnetite cores ofspinel grains from certain regions of the kimber-lite pipe: l) Is the observed zonation in Al inspinel produced by magmatic precipitation? 2)How did late-stage enrichment occur in bothMg and Al" which is a reverse of the normalmagmatic trend? 3) Why are pleonaste-rimmedspinel grains confined to certains regions of thepipe? 4) If resorption of pleonaste occurredelsewhere in the pipe, what was the reactionmechanism?

Several mechanisms were considered thatmight have produced the zoning sequence oftitanomagnetite core, Mg-pleonaste inner rim,magnetite outer rim. Atomic-absorption spec-troscopy, performed on bulk samples of kim-berlite throughout the pipe, indicate no signif-icant Al-enrichment due to wallrock contami-nation. Thus, the formation or preservation ofpleonaste cannot be linked to the observablebulk-chemistry of the present rock.

The possibility was also considered that thezoning could reflect magma mixing [cl. Mit-chell & Meyer (1980) for the Jos kimberlitedykel, i.e., the introduction of a separate, some-what Al-richer kimberlite melt caused precipita-tion of the pleonaste. One problem with thishypothesis is that the contaminant melt wouldhave to be injected into only certain intrusionsin the pipe. lt also seems unlikely that the pleo-naste-rimmed grains of spinel represent anentirely foreign suite of opaque phases; their sizeand the composition of their central chromiteand titanomagnetite fractions match those ofthe other populations of spinel at De Beers.

A complex process of exsolution and recrystal-lization of an original aluminous titanomag-netite was considered as a possible origin forthe zonation sequence titanomagnetite -+ pleo-nastc -> magnetite. A ternary solvus exists inthe two titanium-bearing systems FeCrrO+-FetTiOr-FeAlsOa and MgCr:On-MgeTiOn-MgAl:Or,at least up to l300oC (Muan et al. 1972); thesolvus configuration is similar to compositionalpatterns in the De Beers spinel grains, as in-dicated in the triangular (Al, Cr, Fe'* + 2Ti)plot in Figure 13. Although the compositions ofthe grains of zoned spinel at De Beers (Table3) are compatible with a solvus relationship,the rimming textures are not. Furthermore, iftitanomagnetite and pleonaste represent two limbsof a solvus, one would expect the two phases tobe equally restricted in composition. However,Figure 13 shows this not to be true.

Although the widespread serpentinization inthe De Beers kimberlite probably did notaffect either the precipitation or the resorptionof pleonaste, it was considered as a means ofproducing the thin outer rims of magnetite onthe spinel, However, electron-microprobe anal-yses show that. the pseudomorphic serpentine isapproximately as iron-rich as unaltered olivine,obviating the need for expulsion of a magnetitecomponent duling serpentinization.

Owing to the relative geochemical immobilityof Al, it appears to be a magmatic process thatproduced the zonation in Al. Because Al con-tents of the De Beers population bf spinel not

SPINEL ZONATION IN KIMBERLITE. SOUTH AFRICA

ZTi r FcSi

53

CrA I

O FRB 37OB

O FRF 37OAI

AFRB 37OB'

OFRB 377

O cRc I corae

OcRc 1 Fiue

phlogopite, the relative coefficients of absorp-tion are o<ll-y; .for

'reverse pleochroism,

q> {) -y. The reverse-pleochroic core- comprisesmost of the phlogopite blade and usually isirregular to elliptical in outline, as thoughrounded by chemical reaction or physical abra-sion. In the best preserved grains, the outlineof each zoned crystal is euhedral despite theanhedral core (Fig. 14). Most of the bladeshave undergone alteration, the initial stages ofwhich produce black rims opaque to trans-mitted light (Fig. 12). Electron-microprobeanalyses show the black rims to be inhomo-geneous. Some of them contain only 4-6 wt.VoKzO and only about 36 wt.Vo SiO, (cl. ap-proximately lO wt.Vo and 42 wt.7o , respectively,in unaltered phlogopite), but more than I wt.VoNarO. Representative compositions of the re-verse-pleochroic core and normal-pleochroic rimof a large phlogopite blade appear in Table 4.

Matrix grains of phlogopite typically rangein length from 0.02 to 0.04 mm. It is difficultto find individual, unaltered matrix laths ofphlogopite suitable for microprobe analysis. Thedata in Table 4 indicate a lack of coherentcompositional distinctions among the optically

Ftc. 13. Plot of spinel analyses from the De Beers pipe Intrusion Breccia.Tie lines connect core (Cr-, Ti-, Fe3+-rich) and rim (Al-rich) in a grain.Symbols are the same as in Fig. 9. Note that Al contents are low andfairly constant except in the late-stage pleonaste, in which Al is high andrclatively constant.

only are very low, but also almost invariant(Fig. 13) throughout their precipitation (pleo-naste excepted), it seems reasonable that the Alcontent may have been controlled by coprecip-itation of spinel and another aluminous phase.

Tnxrunal eNn CouposrrloNAl FEATURESOF PHLOGOPITE

Phlogopite occurs as phenocrysts, in thegroundmass, as fine-grained rims on serpentinepseudomorphs after olivine, - as part of garnetkelyphites, as mantle-derived inclusions (mo-nomineralic or polymineralic), and as possibleproducts of metasomatism (Table l).

The largest phlogopite grains exceed I cmin length, but most grains classified as pheno-crysts are in the range 0.3 to 2.0 mm. (Someof the largest grains are probably xenocrysts,but because phlogopite xenocrysts and pheno-crysts cannot be distinguished optically, all largegrains will be referred to as phenocrysts.) Manyof the grains are zoned, consistently displayingreverse-pleochroic cores and normal-pleochroicrims (cl. Boettcher er a/. 1979, Farmer &Boettcher 1981). For normal pleochroism in

oooo

1 0 l l |

54 THE CANADIAN MINERALOGIST

Frc. 14A,B: Pleochroically zoned phlogopite phenocryst, which is a com-mon feature in the De Beers kimberlite. Polarizers rotated 90o betweenphotographs A and B. Reverse-pleochroic core, normal-pleochroic rim.Transmitted light; specimen DB35B, kimberlite dyke; scale bar 100 pm.

distinguishable types of phlogopite. However,at least in some cases, groundmass phlogopitetends to be lower in Fe and Ti, and higher inAl and (in some cases) Mg than either reverse-or normal-pleochroic phlogopite phenocrysts.This represents a reversal of the normal mag-matic trends toward late-stage enrichment inFe and Ti. Groundmass blades in the majorroot-zone kimberlite facies characteristically arepale to colorless and, where colored, they ex-hibit reverse pleochroism and appear unzoned.The typical chronological sequence for phlog-opite precipitation in the main root-zone (asdistinct from the kimberlite dykes) appears tobe (1) reverse-pleochroic phenocryst, (2) nor-mal-pleochroic phenocryst, and (3) reverse-pleochroic or colorless groundmass phlogopite.The reversal in the pleochroic scheme and thecompositional differences Clable 4) between thethree defined types of phlogopite suggest ahiatus in crystallization between core and rimphenocrysts (cores commonly rounded) andbetween phenocryst and groundmass precipi-tation (cl. Farmer & Boettcher 1981).

In the kimberlite dykes and the sill that post-date emplacement of the main diatreme, phlogo-pite is very abundant. The grain:size distri-bution is not bimodal, however. Matrix andphenocryst grains form a sontinuum. The phlogo-

pite grains may be zoned and have reverse-pleochroic cores, or they may be unzoned, inwhich case they have either reverse or normalpleochroism (Table 1).

PnoposEp RslerroNsHrp BrrwErN SplNneNn PHLocoPttr

Relationships observecl in the De Beerskimberlite

Both spinel and phlogopite occur as zonedgrains in the kimberlite groundmass. They arealso the only major aluminous phases in therock. Thus, possible correlations were investigatedbetween the composition and abundance of allavailable generations of phlogopite and spinel(cl. Rimsaite l97l). Compositional variationsin phenocrysr phlogopite (for instance, its Crcontent) do not appear to correlate with thosein spinel. Furthermore, no correlation was foundbetween the presence of groundmass phlogopiteand the presence of rims of Mg-Al-spinel. Thus,the incorporation of Mg and Al into one ofthese two matrix phases did not preclude theformation of the other phase. As described in aprevious section, however, there is a strongpositive correlation between the preservation ofMg-pleonaste rims on zoned grains and the

SPINEL ZONATION IN KIMBERLITE, SOUTH AFRICA 55

preservation of phlogopite phenocrysts, bothbeing enhanced at wallrock contacts. Phlogo-pite phenocrysts show increasing alterationaway from the contact, but their serpentinizedand chloritized pseudomorphs are readily recog-nized (and recorded in Table l).

The positive correlation of the presence ofthese two phases appears to rule out late-stageMg-Al-enrichment in the melt as primarilybeing due to thebreakdown of phlogopite pheno-crysts. However, another type of reaction rela-tionship is possible between phlogopite andspinel. Precipitation of phlogopite ddpletes themelt in K, Mg, Al, Si and HrO. The activityof one of these components may have de-creased sufficiently to eliminate phlogopite pre-cipitation temporarily, as demonstraied by thedistinct generations of phlogopite in mostregions of the root zone. If the activity of Kor H:O were the limiting factor, phases suchas Fe{i-oxides could continue to crystallize.The concentration of Mg and Al eventuallywould rise in the melt, causing precipitation ofother silicates and oxides by reactions such asthe following:

4KMgsAlSiso,o(OH), -> 5Mg:Sior +phlogopite forsterite

2MgALOr + (2K,O + 4H,O 1 7 SiO,)spinel melt

The simplified reaction indicates that decreasesin KzO and SiO, in the kimberlite melt couldstabilize the assemblage forsterite * spinel *melt species over phlogopite.

As explained above, both the phlogopite andspinel relationships differ between the dykeand the main root-zone kimberlites (Table l).These distinctions could be a function of dif-ferences in bulk composition, in pressure ofemplacement, and in reactions with volatiles.Some mechanism that caused discontinuous pre-cipitation of spinel and phlogopite apparentlywas active (perhaps episodically) in the root-zone intrusions, but not in the dykes. Texturalevidence (Pasteris 1980a) indicates that, unlikethe main intrusions in the root zone, the dykekimberites did not undergo fluidization or anysimilar rapid evolution of a gas phase (separablefrom the melt). For instance, the spinel grainsin the dyke are not rounded (cl. Mitchell| 978b) as are the titanomagnetite cores ofthe root-zone spinel (Fig. 6); large segregationsof calcite and serpentine are more common inthe dyke rocks, and much less rounding oflarge grains is evident in the dykes.

The histories of phlogopite and spinel devel-opment in the dyke and root-zone kimberlitesapparently began to diverge after the precipita-

tion of normal-pleochroic rims on reverse-pleo-chroic phlogopite phenocrysts. The followingmodel is a possible history of subsequent pro-cesses. In the root-zone intrusions. volatile re-lease or simple depressurization (both decreas-ing activity of HsO) interrupted the precipita-tion of titanornagnetite and phlogopite. Mg andAl increased in concentration in the melt duringthe phlogopite hiatus, permitting pleonaste toprecipitate. In the dykes, however, this disrup-tion mechanism was not active. Continuousprecipitation of titanomagnetite kept Fee+ andTi concentrations low in the melt. and normal-pleochroic (presumably low-Fe3*) phlogopitecontinued to precipitate. As a consequence ofcontinuous phlogopite precipitation, Mg and Alconcentrations in the dyke melt never reachedthe saturation point of pleonaste,

Subsequent.resorption of Mg-pleonaste in theroot-zone intrusions is difficult to explain owingto its refractory nature. The ambient l(Or) mayhave risen during late-stage precipitation ofspinel, causing oxidation of the initially ferrousplbonaste. The presence of at least a moderatel(Or) is supported by the magnetite componentcletermined in the pleonaste (about ll wt.VoFeO and 15 wt.Vo Fe:Or in preserved grains).An increase in the Fe'* component could haveshifted the pleonaste to a composition withinthe ternary extension of the known magnetite-hercynite solvus (Turnock & Eugster 1962),causing segregation of a magnetite* and aspineL" component. The Fe'* component, whichwas oxidized to magnetiteeal w&S precipitated"epitactically", whereas Mg and Al were re-sorbed by the remaining melt. This gave rise tothe common atoll texture, and explains why theouter rim of magnetite occurs only where pleo-naste is partly resorbed. The mechanism may beanalogons to that documented by Price & Putnis(1979) in low-temperature, apparently non-equilibrium oxidation-exsolution of pleonastegranules associated with titanomagnetite.

Possible broader implications forkimberlite melts

Examination of these and other availabledata on spinel in kimberlites, especially thoseof Mitchell (1978a, b, 1979), leads to the con-clusion that, broadly speaking, there is onegeneral trend with several variations for kim-berlitic spinel compositions (cf. Fig. 9). Theinitial trend is along the base of the spinelprism from the MgAlzOe-rich corner towardmore Fe-rich chromite, with the addition of asmall amount of Ti [seen in the Peuyuk and

56 THE CANADIAN MINERALOGIST

Elwin Bay kimberlites, Northwest Territories(Mitchell & Clarke 1976, Mitchell 1978a) andin the Bellsbank kimberlite, South Africa @octor& Boyd 1979)1. Subsequently, Ti (and Fe"*)increase, commonly without significant increasein the Fe2+/(Fe'* + Mg) ratio. Compositionsgreatly enriched in the ulviispinel-magnetitecomponent can be attained in late-stage spinelor in rims on earlier Cr-rich spinel.

The following are several of the most obviousmodifications to this general statement: l) En-tire compositional clusters of spinel data fromone pipe commonly are displaced along the FFMaxis of the prism diagram with respect to datapoints from other pipes. 2) In most cases, spinelcompositions from a single pipe will not de-lineate the entire trend, e.g., a) the initialphase may be an Al-poor chromite, as a Tun-rae, N.W.T. and Kirkland Lake, OntariolMitchell 1978b, 1979) and at De Beers (thisstudy), or b) the final Ti-enrichment may notbe attained, as at Elwin Bay. N.W.T. (Mitchell1978a). or c) only the final part of the trendmay be represented, as at the Benfontein sill,South Africa (Dawson & Hawthorne 1973).Green Mountain, Colorado (Boctor & Meyer1979), and in the Jos dyke, N.W.T. (Mitchell &Meyer l98O). 3) The late-stage spinel maybecome very rich in Fet* without much Ti en-richment, as in the massive micaceous kimber-lite facies at Tunraq (Mitchell 1979) and insome atoll spinel rims at De Beerr.

Of particular interest are the exceptions tothe above trend, such as late-stage enrichmentin Mg and Al in the pleonaste rim on spinelgrains in some facies of De Beers (this sfudy)and late-stage enrichment in Al over Cr in thespinel grains of the Kao kimberlite (Haggerry1975), By a reaction process, aluminous spinelcompositions developed in garnet kelyphitesin the Monastery pipe (Haggerty 1975). Thesevariations attest to the sensitivity of spinel tochanges in its environment during both primaryprecipitation and subsequent re-equilibration,and to the need to assess_ the extent of spinel-silicate interaction.

The relationships described above suggest thatcompositions of groundmass kimberlitic spinelcan be used to distinguish different kimberliteintrusions of a pipe and perhaps to order themwith respect to their age of emplacement. Invery complex multi-intrusion diatremes likethe Premier pipe, South Africa, this wouldbe no small feat. The spinel compositions alsooffer a means of comparing and contrastingkimberlite pipes with respect to degree of frac-tionation.

Survruanv oF CeNctusloNs

From this study of ,ftimberlitic spinel, it isclear that spinel compositions and textures differsystematically within h single pipe. With respectto the De Beers. pipe, field relations and com-positional patterns of the spinel are compatiblewith the hypothesis that the main kimberlitefacies of the root zone represent successiveintrusions from a single, fractionating parentmeh at depth. The obviously later-injected kim-berlite dykes and sill are more fractionatedthan the main root-zone facies. The dykes andsill may have arisen from a different parentthan the main root-zone rocks, or may rep-resent the late-stage residual liquid of the sameparent kimberlite. Textural and compositionalfeatures of the kimberlite dykes and sill suggestthat they, unlike the main root-zone intrllsions,did not undergo fluidization.

It also appears, from the data in this studyand others, that there may be a single trenddefined by spinel compositions precipitating dur-ing normal differentiation of kimberlite. If so,it may be possible to place in sequential orderthe separate intrusions of multi-injection kim-berlite pipes. It also may be possible to bettercompare kimberlites from different districts withrespect to their degrees of fractionation. A betterunderstanding of spinel compositional patternsis especially important in highly carbonated(secondarily replaced) kimberlite-like intrusions,.such as those from Kansas and Missouri (Pasteris,unpubl. data), in which zoned spinel is the onlyprimary phase remaining.

Evidence in the De Beers kimberlite suggeststhat the aluminum content of the melt was con-trolled over much of the interval of crystalliza-tion by the coexistence of phlogopite and low-aluminum spinel. During the rise of the kim-berlite melt, phlogopite is believed to have be-come unstable, and much of the available alum-inum partitioned into an aluminous spinel phaselpleonaste). Subsequent increase in l(Or) in thelate-stage kimberlite melt caused breakdown ofthe pleonaste and precipitation of a magnetite-rich rim on the grains of residual spinel. Thelatter process produced the atoll texture char-acteristic of the spinel. In the De Beers pipe,Mg-pleonaste zones in the spinel are inferred tohave formed initially throughout the body,wherever the atoll texture now is observed. How-ever, pleonaste was preseryed only in kimberlitedirectly adjacent to the wallrock, which is alsothe region of best preservation of phlogopitephenocrysts. Presumably the pleonaste andphlogopite breakdown reactions did not go to

€

SPINEL ZONATION IN KIMBERLITE, SOUTTI AFRICA 57

completion here owing to rapid cooling.The mechanism described above for the devel-

opment of grains of atoll spinel is proposed spe-cifically for the De Beers kimberlite. Cursorymicroscope observations of about 25 other kim-berlites from North America and southern Afri-ca suggest that the atoll texture in spinel is verycommon. However, it could be due to a varietyof mechanisms. The only other kimberlite-likesuite reported to have spinel with a titanomag-netite core and a Mg-pleonaste rim are the un-usual extrusive rocks of the Igwisi Hills, Tan-zania (Reid et al.1975, Pasteris unpubl. data).At Igwisi, the spinel grains are much larger thanthose of the De Beers kimberlites. The pleonasterims usually are preserved, but apparent resorp-tion effects, including atoll textures, are ob-served. The above resorption mechanism is pro-posed for atoll spinel at De Beers because of theoptically traceable, progressive disapparance ofMg-pleonaste throughout the kimberlite root-zone.

AcKNowT.EDGEMENTS

Much of this paper represents work done fora Ph.D. thesis at Yale University under the ad-visorship of Brian J. Skinnern Philip M. Orville,John V. Walther, and Robert J. Tracy. Electron-microprobe analyses were carried out at the Geo-physical Laboratory, and a travel grant for fieldwork was provided by the Carnegie Institutionof Washington, D.C. Kimberlite sampleq wereobtained personally by the author through thecourtesy of the De Beers Geology f,bpartment,Kimberley, South Africa. To the individuals in-volved in the above organizations and to RogerMitchell and an anonymous reviewer, who readearlier versions of the manuscript, the authorexpresses her sincere appreciation. However, sheretains full responsibility for all opinions andinterpretations expressed.

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Received April 6, 1982, revised manuscript accepted'l u l y 5 , 1 9 8 2 .