Geochemistry of magnesian ilmenites from kimbedites in South Africa and Lesotho ROGER H. MITCHELL

LITHOS Mitchell, R. H. 1977: Geochemistry of n,agnesian ilmenites from kimbcrlites in South Africa and Lesotho. Lia..u~ I"3, 29--37. Oslo. ISSN 00244937.

Magnesium ilmenite from discrete nodules and lamellar intergrowths with pyroxene from the Kao, SekameTag, Frank Smith, and Monastery kimberlites has been analysed for Ti, Fe, Me, Nb, Zr, Cr, Cu, Zn, Mn, Co, and Ni. Each kimberlite contains discrete ilmenites which exhibit a wide compositional range within the ilmenite-geikeilit¢ series. Lamellar ilmenltes from Frank ;~,,,~th and Monastery differ in composition but both show a limited range in composition which lies within the :ompositional range skown by discrete ilmenites from these pipes. The ilmenites are enrich.'d in Nb, Zr, Cr, Co, an;t Ni and deple,cd in Mn, Cu, and Zn relative to Me-poor ilmenites from ~,azlc intrusions. Nb, Zr, and Ni correlate with Fe, Ti and Mg variations but the abundances of the other trace elements are independent of major element variation. R-mode factor analysis is interpreted t.o imMy that the geochemistry cannot be interpret, d in terms of a differentiation hypott:esis in which trace elements are removed from or concentrated in residua. Factor scores and major element abundances indicate that each pipe is characterized by a particular suite of discrete ilmenite nodulus, which are considered to be phenocrysts in a proto-kimbcr,Jte magma. Lamellar ilmenite-pyrux,¢ne intergzowths are unlikely to have had a eutecti: origin, and show no simple relationship to the discrete ilmenites.

R. H. Mitchell, Department of Gpology, Lakehead UniversJty, Thunder Bay, Cmtario. Canada.

Magnesian ilmenite is one of the most charac- teristic minetaL~ of kimberlite, yet at the same time it is one of the most enigmatic particular- ly with regard to its origin and relationship to kimberlite. Several recent studies summarized below, for example, have fa~led to determine unequivocably whether magres ian ilmenite is a mantle xenocryst or a kimberli te phenocryst. Magnesian ilmenite has several modes of oc- currence ':n ' :imberlite, e.g. discrete nodules (monominera',ic megacrysts), l amenar inter- growths with pyroxenes and granular inter- growths with pyroxenes, garnets and olivines, all of' which shrew no obvious relationship to each other.

Discrete nodules (100/~-10 cm) are the com- monest mode of occurrence c~f ilmenite. These ate essentially monomineral;ic aggregates of ilmenite of widely varying c,,mposition which have been interpreted by Mitchell (!973) to be a cumulate formed during the high pressure differentiation of a proto-kimbedi te mz~gma. 1"he cumulate ilmenit,- was sheared, broken up arJd randomly mixed with the parent magma as it began its ascent from the mantle. Garnet and zircor,~ are ra~,ely inter~rown with tbese ilmenites (Nixon & Boyd 1973b).

L amellar intergrowths consist of ilmenite intergrown with single crystals of clino-, or or thopyroxene in a graphic texture. Gurney et al. (1973) "and Wyatt et al. (1975) have inter- preted the intergrowths to be cumulate:, el a eutecti¢ composition crystallizing at high pres- sures from a kimberlite magma.

Granu la r intergrowths (Nixon & Boyd 1973b) consist of non-oriented intergrowths with silicates. This mode of occurrence of ilmenite has been the least studied and ma~y include iilmenites which are the result of "['i metasomat ism of garnet lherzolite (Harte & Gurney 1975).

Boyd & Nixon (1975) believe that the dis- crete nodules and lamellar ilmenites are al,~. xenocrysts derived by crystallization of melts in the low velocity zone of the mantle. The hypothesis is based upon the discre'te nodule assemblage in kimberlite (Boyd & Nison 1975) being formed in equilibrium with silicates simi- lar to those found in intergrowths and that they are a part of the assemblage ilmenite- clinopyroxenc-orthopyro:Lene-garnet, this as- semblage having been f ragmented during pas- sage of the kimberlite magma througla the low velocity z~ne. Boyd & Nixo~ ¢i975} consider

30 R, H. M i t c h e l l LITHOS I0 (19771

Table I. Composition of discrete ilmenites from the Kao, Frank Sm,th and Sekameng and lamellar ilmenites from Monastery.

TiO2 FeO FezO3* MgO Gkl Ilmt Hmt Nb Z~" Zr/Nb Cr Cu Zn Mn Co Ni

i(ao discrete nodules

K i 48.34 29.57 11.15 7.66 28.09 60.87 10.33 1907 1:;04 0.66 2454 21 162 1998 217 356 K 2 47.74 27.35 13.26 8.60 31.28 55.83 12.18 1894 1269 0.67 2714 26 150 1990 287 337 K 3 47.91 27.44 12.97 8.62 31.36 56.00 11.91 1938 1~'75 0.66 2360 21 168 2114 229 325 K 4 47.32 26.87 14.15 8.67 31.58 54.90 13.01 971 933 0.96 774 21 174 1729 254 350 K 5 49.67 25.55 11.34 10.59 37.82 51.20 J0.23 1264 880 0.69 3910 35 173 1899 248 675 K 6 48.10 28.87 11.89 7.92 28.99 59.30 10.99 1876 1261 0.67 2360 21 163 2065 243 348 K 7 48.40 26.03 12.98 9.67 34.84 52.65 11.81 1609 IG85 0.67 2200 21 219 2090 243 427 K 8 48.09 28.09 12.34 8.36 39.47 57.48 11.36 1917 1264 0.65 2175 23 161 2022 228 361 K 9 50.56 22.98 1 1 . 4 2 12.48 43.74 45.19 10.10 1267 754 0.63 7349 35 166 1910 272 1069 K 10 .49.08 27.32 11,.26 9,29 33.59 55.43 10.28 1450 996 0.69 2463 45 167 2021 260 544

Fralrlt Smith ifi.wrete noihdes

F I 47.73 22.29 10 .93 1 7 . 8 3 59.84 20.59 18.89 1259 759 0.60 4524 35 133 1649 261 928 F 2 50.31 24.9¢ 19 .73 11.26 40.07 49.80 9.64 1044 723 0.69 187 16 146 t836 236 371 F 3 51.37 24.02 9.61 1 7 . 3 3 43.33 47.37 8.53 477 385 0 . 8 1 6024 34 134 I508 273 1285 F 4 5'0.21 26.52 9.98 .0.~2 36.97 53.31 9.03 1129 772 0.68 2 9 6 1 III 151 J870 298 IP22 F 5 51.12 25.59 9.15 ;I.30 40.20 51.08 8.22 877 617 0.7 219 16 158 1872 286 335 F 6 49.39 27.4~ 10.71 9.?6 33.87 55.82 9.78 1521 1005 0.66 273 17 160 1904 284 481 F 7 5086 20.26 12 .45 14.18 49.04 39.31 10.87 484 429 0.88 5632 17'3 137 1619 340 182 F 8 50.71 23.93 1 0 . 6 9 12 .01 42.45 47.46 9.54 889 657 0.74 316 35 163 2060 366 501 F 9 51.29 21.38 i1.18 13.76 47.77 41.66 9.8 496 408 0.82 5508 3.¢ 152 1676 317 1359 F I0 51 .81 22.71 9.67 13.28 45.35 44.47 8.52 506 420 0.83 3455 45 145 1683 307 1227

Sekameng discrete nmhth,s

S I 45.85 29.40 14.98 6.50 24.13 61.25 14..04 2099 1635 0.78 994 f., 154 1879 235 180 S 2 48.87 29.53 11.87 7.44 27.38 60.98 11 .03 1 9 2 8 1349 0.70 905 5 16(1 2002 252 202 $ 3 42.02 29.~6 20.73 4.61 17.42 62.27 19.78 2574 1950 0.76 1112 3 155 ~1612 237 175 $ 4 49.17 28.70 10.38 8.55 31.15 58.6 '7 9.55 1853 1068 0.58 981 9 175 2134 273 227 S 5 4'7.19 28. '77 13.32 7.52 27.66 59.3'7 12.36 2052 1367 0.67 872 5 15(3 204't 258 214 $ 6 4,:1..04 29.11 17.85 5.7(~ 21.52 61.07 16.85 2608 1819 0.70 983 I 156 11811 2711 173 S 7 47.57 28.31 13.()2 7.95 29.11 58.18 12.04 2~142 1180 1").58 882 3 175 2282 314 189 S ~ 4,'~.41 29.63 1(1.96 7.f~8 28.20 61.05 10.16 7'(q,17 1306 0.65 1669 8 142 1784 278 378 $ 9 47.67 29.61 12.15 7.27 2,..79 61.22 11.30 1941 1131 0.58 917 3 155 2328 345 207 S 10 45.15 28.76 16.38 6.51 24.17 5~).93 15.35 1720 1287 0.75 747 4 163 1813 286 207

Monastery lamellar ilmeuite~'

M 6 . . . . . . . . . 449 14 171 2312 236 1075 M 7 50.72 28.17 11:14 9.65 33.72 55.2'l 1 0 . 5 3 . . . . 357 8 150 1888 198 580 M 8 51.15 27.97 12.,~L8 9.98 34.44 54.17 10.88 . . . . . 577 4 160 1873 165; 6'77 M 9 4.8.97 25.94 13 .98 10.06 35.62 51.54 12.50 - -- 712 17 142 !895 193 604 M 10 4~.197 27.08 13.9,b 9.82 34.16 52.85 12.26 -- - 359 I 140 2755 215 556 M II 50.58 27.28 12.2t In.09 35.25 53.48 10.79 679 9 167 1736 197 725 M 12 51.24 28.29 12.2;~ 9.86 34.04 54.80 10.70 -- - - -- 662 5 144 1656 227 614 M 13 50.91 28.87 12.15 9.'~6 32.54 56.33 10.66 . . . . 265 II 170 1765 218 649 M 14 50.54 27.37 1 2 . 6 6 10 .01 34.86 53.50 11.14 . . . . 716 6 163 1803 215 797 H 15 50..66 29.57 11.96 8.83 3".).87 58.01 t0.55 - - - 124 47 167 1901 221 637

':' Fe,-,O3 calcula~ied I'rt,m total iron by Carmichad 's (19671 methq~d. i Gk, IIm and Itm = I~ol. % MgTiO4, FeTtO~ and Fe20~ respectively. ~a jo r elemerlts and Nb and Zr ,tetermlned by X-ray fluorescer ce, olher trace elements by alomic absorption. Major element ddla ",.Jr IVlonastery lamelllar ilmenite determired by Cambridge Mk V microl~robe at Dolhot *',ie ILl Mli,,,'erslly.

L~,THOS !0 (1977) Magnesian ilmenites 31

~'Tble 2. Compositions of dis, fete ilmeniles - Monastery Kimberlite.*

TiOz AlzO3 Cr203 Fe,,20~l FeOt MnO MgO G[,': z IlmZ Hm 2

I 46,,20 0.65 0.12 19.15 27.2 0.16 7.96 28 4 54.4 17.2 2 52 70 0.40 0.87 9.61 25.54 0.27 12.11 41.9 49.7 8.4 3 50.92 0.38 0.31 13.35 26,08 0.16 10.97 37.9 50.5 11.6 4 49,,42 0.70 0.08 15.46 27.72 0.20 9.27 32.3 54.2 13.6 5 48 84 0.45 0.0 16.61 28.14 0.22 8.73 30.4 55.0 14.6 6 50,33 0.56 0.0 12.34 28,54 0.22 9.26 32.6 56.4 I 1.0 7 48.82 0.45 0.04 15.52 26.83 0.23 9.45 33.1 52.8 14.1 8 45.36 0.30 0.88 18.99 28.48 0.23 6.78 24.6 58.0 17.4 9 45.89 0.96 0.34 14.19 26.13 0.13 10.44 ~6 4 51.1 12.5

l0 50.10 0.45 0.28 14.07 26 22 0.16 10.48 36.5 51.2 12.3 II 49.8c.'~ 0.63 0.2[ 15.15 26.51 0.13 10.18 35.3 51.5 13.2 12 45.20 1.00 0.36 16.43 24,95 0.21 10.TI 37.1 48.5 14.4 3 47.62 0.66 0.03 16.57 27.42 0.13 8.57 30.5 .~4.7 14.9

14 53.51 0.32 0.75 7.96 25.41 0.31 12.57 43.6 49.4 7.0 5 48,18 0.78 0.03 17.05 27.37 0.20 8.84 31.0 53.9 15.1

16 50.16 0.51 0.0 13.44 27.96 0.23 9.49 33.2 549 i 1.9 17 50,,59 0.68 0.27 13.49 26.43 0.22 10.57 36.1 51.5 I 1.8 18 47.47 0.68 0.06 P7.07 27.67 0.15 8.34 29.6 55.1 15.3 19 47 89 0.60 0.0 16.76 27.37 0.Z~ 8.69 30.7 54.3 15.0

* Analysed at Purdue Univers t3'. MAC 500 automated microprobe. I Calculated by Carmichael's (1967) method. z Gk, lira and Hm = tool. % MgTiO.~, FeTiO~ and Fe2Oa rt:spectively.

that the temperatures of formation of the dis- crete nodules of ilmenite are the same as those determined by pyroxene geothermometry for the discrete pyroxenes; these latter exhibit a wide range of equilibration temperatures which Boyd & Nixon (197.'i) con:~ider to be incom- patible ' ith a phen(K:rysta~ origin, it should be noted that equilibration temperatures of ilmenite-pyroxene intergrowths are distinctly lower (1000-1300"C) than those of discrete pyroxenes ( 1300-140()"' C).

Little is known ¢~f the trace element geo- chemistry of magnesian ilmenite beyond the prelimiinary studies c,l' Mit~.hell et al. (1973) and Gurney el. al. (!97,]) who determined that magnesian flmcnite i~ enriched in Cr, Zr, Nb, l 'a and Hf relative to magnesium-poor ilmenite from common basic igneous rocks. Trace ele- ,,-nent studies are potentially useful in assessing '~he relationship between discrete and lamellar ilmenites and whether or not each k imberlite is characterized by a particular suite of mag- P.esian ilmenites.

This paper presents some new data (Tables i and 2) for major and trace elements for lamei- la.r ilmenites (Monastery, South Africa) and discLrete iimenites (Monastery, Frank Smith; South Africa; Kao, Sekameng; Lesotho) from South African kimberlites.

Major element geochemistry A. Discrete nodules

Fig. ! illustrates the compositional range of discrete nodules from [;ekameng, Kao, Monas- tery, and Frank Smit'l kimberlites expressed as the ternary percentages of the ilmenite (Fe2+TiO3), geikeilite (MgTiO 3) and hematite (Fe23+O 3) molecules. Fig. I and recent ~,tudies noted below confirm Mitchell's (1973) proposal that every kimberHte pipe contains iimemte of widely varying chemical composition, e.g. Seka- meng (MGO=4.6-8.6%; this work); Kao (MGO=7.7-12.5%; this work); Monastery (MGO=6.8-12.6%; this work); Frank Smith (MgOffi8.5-17.8%; this work, Boyd, pers. comm.), Wesselton (MgO = 6.1-19.0%; Mitchell 1973); Premier ¢MgO=9.4-23.1%; Gurney et al. 1973); Arturo de Paiva (MGO=39-9.6%; Boyd & Danchin 1974).

Although overlap occurs between the compo- sitions of ilmenites from some kimi~edites, it is evidenL from Fig. I that the Fra,~k Smith ilmenites are distinctly richer in MgO than ilmenites from Sel.:ameng. The ~-ange,; in MgO noted above may be extended by further sampling, but it is possible tl',at some kir:- be.rlites su~ch as Sekameng and Arturo de Paiva are characterized by ilmenites poorer in MgO

""~ R. H.. Mitchell I,ITHOS 10 (1977)

MgTiOa

( J ,

,,, %

A MONASTER~

• KoO

m FRANK SMITH

• $EKAMENG

,m ' i i ,,

• b m ' ~ ~

\

• ,J • '

' \

,":ee O3 "' '"' "' FeTiO=

,.ig. i . Compositions of discrete ilmeniles from the I.rank Smith, Mo| |aslery, Sekameng and Kao kim- P)crlites. Dala are from Tables I aad 2.

than are found in other pipes, e.g. Frank Smith, Premier. Insufficient data i. ~ as yet available to und,ertake a meaninglul statistical analysis of ilm~nite compositi,,>ns, but the extant data possibly indicate that each pipe may possess nts own population of discrete ilmenites.

B. Lanwflar intergro~vths

Fig. 2 :shows that ilmenite from lamellar inler- growths (of both ~.~.rtho- and clinopyroxene) has a rest,~"i,,:ted rarJ;.,.,e ,of compos i t i on wh ich flies within tht, range of compositions of the discrete nodules. Lamellar ilmenites from the Frank Smith kimberlite (MgO= 10.2-12.9%; Nixon & Boyd 1973a. Boyd, pers. comm.) are di~:inctly more magw.:s.i,tn than lameilar ilmenitzs from Monastery (MgO=8.5-10.1%; this wo:k, Gur'- ncy el al. 1(.)7. ',,, Boyd. pers. comm.).

The major element data are of relevance to possible eutectic origins for the lamellar inter- ,3rowths because:

(I) The lamellar ilmenites show a small but definite range in composition. Such a range would be possible only if a eutectic composition were changing in response to, perhaps, pressure or volatile composition.

(2) Lamellar ilmemtes from Frank Smith and Monastery do not terminate the trend of compositions of the discrete nodules as might be expected if a eutcctic were reached after a period of cr)stallization of ilmenite alone, assuming that the discrete and lamellar il- menites are co-genetic as postulated by Boyd & Nixon (1975).

(3) 'Eutectic textures' are observed in inter- growths with clino- and orthopyr<)xene. It is not conceivable that two eutectics involving ilmenite of similar compositions be present in the same environment.

Wyatt et al's. (1975) study of the crystalliza- tion of an ilmenite-c)linopyroxene mixture does not actually demonstrate a eutectic. The bulk composition chosen for study is considered to be a eutectic composition by inference, yet thi,, composition does not crystallize eutecticall'/, although the crystallization sequence observed results in an apparent binary eutectic a/ter clinopyroxene crystallizes alone. Further ex- periments are required to define the phase rela- tionships. An alternative explanation of the data is that bulk compositions on the join ilnlenite-clinopyroxen~ are pseudobinary and that the intergrowth ]s the result of cotectic crystallization of skeletal ilmenite surrounded by faster growing pyroxene plates (Frick 1973).

The problem of the origin of lamellar il- menites is analogous to the origins of graphic granite (quartz-feldspar intergrowths) which are texturally and crystallographically similar, i.e. a trigonal mineral enclosed in a monoclinic single crystal h('~t. Smith (1974) has reviewed the evidence per,aining to the origin of graphic granite and concluded that the intergrowth is not the result of either eutectic or cotectic crystallization but 'is the product of simultane- ous crystallization ,~f quartz and feldspar under vapour rich cond;tions und depending upon epitaxial kinetic factors' (Smith 1974:608). A similar origin mus~ be considered I or the lamel- lar intergrowlhs but cannot be ass~:ssed because of the lack of experimental data on relevant synthetic systems.

LITHOS 10 (1977) Magnesian ilmenih;s 33

MgTi03

0 FRANK SMITH MI_.__NE

\ - - - - DISCRETE " ~ . . . . . LAMELLAR

"" 60

~ , 90

\ V V V % ~4

Fe203 3o 2o ,o Fer i03 Fe=03

gTi03

MONASTERY

DISCRETE LAMELLAR

50

i", 6 0 , "4"%

I • • lh I

%

v v v \ X 30 20 I0 r e / I U r ' - "I": t'~ 3

Fig. 2. Composition of lamellar ilmenites from the Frank Smith an~t Monastery l:imberlhes compa-ed with compositional fields of discrete ilmenites from the same kimberlites. Lamellar ilme~lite data art. from Table I, Nixon & Boyd 1973a, and Boyd, unpublished data.

Trace element geochemistry The matrix of interelement correl,'ttion coeffi- cients (r) for major and trace elements for discrete nodules from Sekameng, Kao and Frank Smith is given in Table 3. Discrete and lamellar ilmenites from Monastery are not included in the matrix because oi incomplete trace element data.

Niobium and zirconium

Nb and Zr both increa:;e with increasing FeO and decreasing MgO and TiOz as might be expected if the nodules were formed by crystal- liquid differentiation. The present data con- firm that magnesian ilmenites are unusually rich in Nb and Zr but do not support Mitchell et ars. (1973) hypothesis that each pipe rr, ight possess ilmenite with a charactt:ristic Zr/Nb ratio.

Chromium and nickel

The data confirm that the ilmenites are richer in Cr than ilmenites from basic rocks and that

wide variations in Cr content occur within a single pipe. Correlations with MgO are poor (r=0.37) for Cr and moderate ;r=0.79) for Ni. LameUar ilmenites from the Frank '..Smith and Monastery pipes are not noticeablly I~oorer in Cr than discrete nodules from these pipes.

Ni conte,lts agree well with the ranges given by Nixon & Kresten (1973; 200-2000 ppm Ni) and Ilupin & Nagaeva (1971; 280~-2330 ppm Ni). Ni correlates strongly wi*h MgO (r=0.79) and shows a strong negative correlation with Nb and Zr (r=0.79). LameUar and discrete iimenites do not differ in their Ni contents. Ni is enriched in magnesian ilmenites ='clative to ilmenitcs from gabbro and dacite (lvlathison 1975, Wilkinson 1971).

Nixon & Kresten (1973) determined that the Cr/Ni ratios of kimberlite ilmenites ranged mainly from 2-10. The present data for Kao and Sekarneng agree with Nixon & b:resten's (1973) data with Cr/Ni ratios of c. 5Z despite a wide range in FeO/MgO ratio.~ (Fig. 3). The Frank Smith ilmenites in contrast show a wide variation in Cr/Ni ratio, the variation being essenti',dly due to Cr variation. Lamellar ii-

3 -- Lnthos !/'/7

34 R. A'. Mitchell LITHOS 10 (1977)

Tahh, 3. Matrix of inter-element correlation coefficients for 30 discrete magnesian ilmenites.

TiO2 FeO Fe203 MgO N b Zr Cr Cu Zn M n Co N i

Tit~ 2 1.00 - 0 , 4 4 - 0 . 7 8 0.73 - 0 . 8 4 -0 .91 0.27 0.24 -0' .13 - 0 . 0 8 0.38 0.66 Fe ' ) - 0 . 4 4 1.00 -0 .21 - 0.94 0.05 0.66 - 0 . 3 4 - 0 . 3 2 (],.34 0.49 - 0 . 2 5 - 0 . 7 0 Fe:03 -0 .78 -0.21 1.00 - 0 . 1 4 0.46 0.53 - 0 . 0 6 - 0 . 0 4 - 0 . 0 9 -0 .25 - 0 . 2 4 - 0 . 2 4 M~O 0.73 - 0 . 9 4 - 0 . 1 4 1.00 -0 .82 - 0 . 8 6 0.37 0.34 - 0 . 3 2 -0.41 0.34 0.79 NF -0.84 0.65 0.46 - 0 . 8 2 1.06 0.97 -0 .38 - 0 . 3 6 0.24 0.47 - 0 . 4 3 - 0 . 7 9 Zr - 0.91 0.66 0.53 - 0 . 8 6 0.97 1.00 -0 .35 - 0 . 3 2 0.22 0.31 - 0 . 4 8 - 0 . 7 8 Cr 0.27 - 0 . 3 4 - 0 . 0 6 0.37 - 0.38 - 0 . 3 5 1.00 0.92 - 0 . 2 3 -0 .27 028 0.61 Cu 0.24 -0 .32 - 0 . 0 4 0.34 -0.,36 - 0 . 3 2 0.92 1.00 - 0 . 2 6 - 0 . 2 9 0.35 0.58 Zn - 0.13 0.34 0.09 -0 .32 0.24 0.22 -0 .23 - 0 . 2 6 1.00 0.55 - 0 . 2 3 - 0 . 4 0 Mn -0 .08 0.49 - 0 . 2 5 -0.41 0.47 0.31 -0 .27 - 0 . 2 9 0.55 1.00 0.03 - 0 . 5 8 Co 0.38 -0 .25 - 0 . 2 4 0.34 -0 .43 - 0 . 4 8 0.29 0.35 - 0 . 2 3 0.03 1.00 0.39 Ni 0.66 - 0 . 7 0 - 0 . 2 4 0.79 - 0 . 7 9 - 0 . 7 8 0.61 0.58 - 0 . 0 4 -0 .58 0.39 1.00

menites from Monastery are characterized by low Cr/Ni ratios. Some: clustering of data for individual pipes is evident in Fig. 3, but indi- vidual pipes cannot be characterized by il- menite Cr/Ni ratios as suggested by Nixon & Kresten (1973).

content of ilmenite from common igneous rocks, which increases with increasing SiP 2 content of rock, e.g. gabbro, 100-470 ppm; dacites, 300-400 pprn; granite, 699 ppm (Lya- kovitch & Balanova 1969, Wilkinson 1971, and Matlhison 1975).

Copper

No previous data exist for Cu in kimberlite ilraenites. Cu shows a strong correlation with C: (r=0.92). The ilme,,ites are poor in Cu relative to ilmenites from basic intrusions, e.g. 35-220 ppm (Mathison 1975).

Zi,"IC

No previous data exist for zinc in magnesian ilmenites. Zn contents are similar in all samples and show no correlation with other elements. The Zn contents are low relative to the Zn

0~, 0 ,/t') h0 " ~(~0'

E I ML3NtASTERY ( NJxon and g r e . , t e n , 1 9 7 3 I .... Q. .... ,..,. ill ..

!

~,~/,~

v, ' KAO"' N~o, and K r e s l e n , 1 9 7 3 )

L ~ I I I I I 1 I I 1 i i I i I ~ ~ ~ 1 ~ t l L I I 1 L I

'bl: p~i~ '~(Itl i11~ ~ ")ll(ILI I()(')t)l ~ h()(.)OL'

p p ~ Cr

F~g. 3. Chromium and nic[cl contents of magnesian ihnenites.

Manganese

Mn levels are similar in all samples and show a slight tendency to incrt.ase with FeO (r= 0.49). The Mn contents are low relative to those of ilmenites from basic rocks which typically contain over ().5t~ MnO (Gasperini & Naldrett 1972, Mathison 1975).

Cobalt

Co ah,,ndances do not correlate well wit.h the other elements. Co is enriched relative to ilmenites from ba:fic rocks, e.g. Somerset Dam, 85-215 ppm (Mathison 1975).

Factor analysis Several elements behave in the manner one might expect in a series of crysr, tls related to each tPther by a diffe~-entiation process, the most notable being the increase in Nb ."rid Zr and decrease in Ni wii~h increasing iron enrich- ment. Other elemen~ts, i.e. Cr and Zn show no correlation with tl~e major elements, a phenomenon not to be e~pected if the increases in Zr a..~d Nb are ~he result of concentration of these elements in the liquid as cry~,~.allization proceeds.

L I T H O S i 0 ( 1 9 7 7 ) Magnesian iimenites 35

Table 4. Promax obl ique pr imary pattern matr ix ( K M I N = 3) for 30 discrete magnesian i lmenites.

F a c t o r ! 2 3 4 5 6

T i O 2 0 .81 - 0 . 4 6 - 0 . 0 - 0 . 0 6 - 0 . 0 7 0 . 1 4

F e O 0 . 3 2 I . 13 0 . 0 0 . 0 3 0 . 0 3 - 0 . q 7

F e 2 0 3 - . 10 - 0 . 2 3 0 . 0 0 0 . 0 4 0 . 0 5 - O. :~ I M g O () .07 - 1 . 04 - 0 . 0 t - 0 . 0 4 - 0 . 0 5 0 . 1 0

N b . - 0 . 4 2 0 . 4 5 0.0.'.. - 0 . 1 2 - 0 . I I 0 . ~6

Z r -- 0.48 0.60 0.04 - 0.05 - 0.1 i 0.C8 Cr 0.01 - 0.,04 I..90 0.01 - 0.06 0.~5 C u -- 0.0¢, 0 . 4 0 0 . 9 8 - 0 .01 0 . 0 6 0 . 0 ~

Zn (b.07 0.03 0.00 0.94 - 0.04 0. I, ] M n (:,. 17 - 0.02 0.04 0.10 0.08 0.9'~ C o -. ¢.07 0 n2 0.01 - 0 . 0 3 1.04 O.O'J N i 0 . 1 9 - 0 .41 0 . 3 0 - 0 . 0 2 0 . 0 3 - 0.32.

The geochemical data was subjected to R- mode factor analysb; in the hope of further understanding the ge3chemistry of the discrete nodules, the result~ being presented as a promax oblique prim;Lry pattern matrix (Came- ron 1967) in Table 4.

Six non-orthogon.d factors, account for 97.5°/'o of the total variance of the data, each of these factors, having a significant loading for at least one trace or major element. Factors 1 and 2 (Table 4) are factors which represent elemental st:~titutions at ilrrenite lattice sites and might be regarded as 'differentiation fac- tors'. I:actor I represents th,.' substitution Ti, Fea*~Nb , Z~" and factor 2 .effects the il- menite-geikeilite solid solution. Factor 3 has strong positive Ioadings for only Cr and Cu and the remaining factors are specific for indi- vidual trace elements. The physical meaning of factors 3--6 cannot be determined but must represent some physical parameter not mea- sured, e.g. temperature, pressure, fo z, silica activity, etc. The factor analysis, whilst beir~g inconclusive, does serve to indicate that 'di~- ferentiation' plus some other 'factor', possibly one of those indicated above, is required t j account for the geochemistry of the ilmenit¢.

Factor scores for factors I and 2 which account for 68.1% of the total variance are plotted on non-orthogonai axes in Fig. 4 an~! show that these factors alone will discriminat., = between the suites of discrete nodules. Plots incorporating other factors do not result in such discrimination. Fig. 4 is considered to indicate that each pipe is charaeter~Ted by its own suite of discrete nodules.

Discussion Some aspects of the geoch.~mistry of the dis- crete nodules appear at fir:;t sight to be con- sistent with crystal-liquid differentiatiJn of a kimberlite magma; other as'~ects, in purticul~:r the lack of correlation of .rost trace elements with major elements, are y~ot compatible with this hypothesis.

Both features ,:an be accounted for within a differentiation hypothesis by assuming that the change in Fe/Mg ratio of ilmenite doe:; not reflect the Fe/Mg ratio of the liquid fro:n which it formed and that the amount of il-

Foc tot I

Q FRANI~ bVIITH o 5rI(AM[/~(., / _ t KAO t / ~ E

. / ,

Fig. ,I. Factor scores for discre.te nodules f rom F rank Smith, SeP~:ameng, and Kao, plot ted on non-orth3gonal axes (i'~.ctc,rs ! and 2). The corre la t ion ( r e - 0 4 ) be- tween the factors is the cosine of the interaxial angle.

36 R . H . M i w h e l l L1THOS 10 (1977)

menite crystallizing is small relative to the total amount of magma available. An example of this kind of crystallization has been de- scribed by Arcu~us et ,-t.I. (1974), who showed that only minor changes in temperature and liquid composition induce wide variations in the composition of spinels crystallizing in the system MgO-iron oxide-Cr2OySiO z. Coexist- ing silicates in contrast showed little composi- tional variation, t h i s kind of plocess may pro- vide an explanation of the occurrence of spinels of wideiy varying composition in the groundmass of kimberlite (Mitchell & Clarke 1976) and as inclusions in diamond (Meyer & Boyd 1972), together with olivine of almost constant compos,,tion, if other oxides .occurring as liquidus phas,::s behave in a similar manner to spinel, one might expect that the liquid would not be enriched or depleted in trace elements if that oxide did not strongly selec- tively incorporate trace elements, as only a limited amount of crystaflization would not cause significant changes in the bulk composi- tion of the parent magma. Hence major a:~d trace elements would show no correlation. This, would be true also if no other phase we,',:': crystallizing in abundance. For example, garnet (which under mantle conditions, might selec- tively incorporate Cr) crystallizing with the il- menite would remove Cr and a positive corr,'- lation with MgO ~;hould be expected. It is cor- sidered here, therefore, that discrete nodu=e magnesian ilmenite is a single early liquidus please (phenocryst) and that the amount formed is small relative to the available magma. The variable Fe/Mg ralios do not reflect large variations in the Fe/Mg .ratio of the liquid but are due to variation in some parameter such as lemperalure, oxygen fugaeity or silica acclivity. The positive correlation of Fc and Zr and Nb may r,.~suit no~ from concentration of Zr and Nb in residual liquid but from increasing pre- ference: of these elements for substitution in Fe'FiO.3 rather than MgT'iO 3 as the ilmenites become richer" in iron.

Discrele iln'eaile nodules are here con- sidered h> be phenocrysts because of the indica- lions that eac,h pipe possesses its own suite ot" discrete nodcles and because the differences in composition between [amellar ilmenltes from Frank Sznith and Monastery imply some 'ge- netic' relationship to the kimberlite in which they c;ccur. 'Fhis in~.erpretation is admittedly sub.[lectiv~.; ar~ alternative hypothesis is to con-

side." tha~ kimberlites simply sample different parts of a heterogeneous low velocity zone as advocated by Boyd & Nixon (1975). In this case ea~.h kimberlite would 'also contain its own suite of ilmenites. Boyd & Nixon's (1975) hypothe.,is is based upun a large range in equilibration temperatures for discrete nodules, which in turn is based uaon the assumption that all nodules were a pm't of the assemblage ilmenite-clinopyroy, ene-orfl.qopyroxene-garnct. A major problem with the hypothesis is that it is not known how applicable solvus geo- thermometry based upon i3-on-titanium free systems is to magmas which c~stal l ize ilraenite. A secondary problem is that monomineralic crystals are typical, pyroxene-ilmenite inter- growths are rare and three and i~ur phase assemblages very rare, as aa'e orthopyroxene- garnet and clinopyroxene-garnet (not cclogite) intergrowths. In favour of Boyd & Nixon's (1975) hypothesis is the apparent absence of olivine-ilmenite intergrowths.

in summary the present data do not conclu- sively prove that ilmenite is a phenocryst in kimberlite magmas, nor do the major and trace element data indicate any simple relationship between lameUar and discrete ilmenites. Much further work is required on the major and trace element geochemistry and nature of the co- existing silicates of discrete, lamellar and gran- ular ilmenites from several kimberlites before we can objectively assess the relationship (if any) between the various paragenetic types of ilmenite.

Acknowledgements. - F. R. Boyd is thanked for access to his unpublished data and for critical comments on an earlier version of this paper. D. B. Clarke (Dal- housie University) and H. O. A. Meyer (Purdue Uni- versity) arc thanked for use of their microprobes and H. Poulsea for assistance with the factor analysis. Some of Ihe samples were provided by P. H. Nixon and D. G. Rolfe. This work is supported by the National Research Council of Canada.

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Accepted for publicalion July 1976 Printed January 1977