Petrogenesis of Oxide Minerals in Kimberlite, Elliott County, Kentucky

American Mineralogist, Volume 67, pages 2842, 1982

Petrogenesisof oxide minerals in kimberlite, Elliott County, Kentucky

JenrnBv J. Acne, Jeur,s R. GennrsoN, JR. eNo LewnnNcr, A. Tevron Department of Geologicql Sciences The University of Tennessee Knoxville. Tennessee379 16

Abstract

Two kimberlitepipes in Elliott County, Kentucky contain inclusionsof alteredcrustal rocks, ultramaficxenoliths, and a diversemegacryst suite in a groundmassof olivine, Mg- rich orthopyroxene,Mg-rich clinopyroxene,ilmenite, Cr-spinel,pleonaste, titanomagnetite, perovskite,calcite, phologopite,vermiculite, pectolite, and septechlorite.The megacryst assemblageconsists ofolivine, garnet,ilmenite, clinopyroxene, orthopyroxene, and phlogopite. Although rare, intergrowths of clinopyroxene + ilmenite and rutile + ilmenite also occur. Ilmenitemegacrysts have reverse-zoned, Mg-rich rims at least75-1fi) pm wide. Ilmenite megacryst cores show a correlation of increasing Fe2O3 with decreasing MgO and Mg/(Mg+Fe), which is interpretedas a fractionationtrend. Ilmenitesfrom clinopyroxene- ilmenite intergrowths are similar in composition to the more magnesianof the megacryst core compositions.Clinopyroxenes from the intergrowthshave compositionssimilar to clinopyroxene megacrysts; these clinopyroxenes from the intergrowths have estimated equilibration temperatures of 1295"-1335C. Groundmassilmenites are more magnesian than the ilmenite megacryst cores. Groundmassilmenites can be grouped into two compositionallydistinct populations:(l) a high-MgO,low-Cr2O3 population (<0.73 wt.Vo Cr2O3)which is compositionallysimilar to the Mg-rich megacrystrims, and (2) a hieh-MgO, high-Cr2O3population (1.43-3.76 wt.%o Cr2Ol which is compositionallysimilar to small ilmenite inclusionsin groundmassolivines. The former group of groundmassilmenites is interpreted to be MgO-enrichedfragments of disaggregated,recrystallized megacrysts;the latter group is interpreted to be primary ilmenites that crystallized in equilibrium with the groundmassassemblage from an MgO-enriched kimberlitic liquid. Estimated equilibration temperaturesfor the groundrnassassemblage are from 965'to 1085'C; at thesetempera- tures, the position of the kimberlite solidus restricts the depth at which this crystallization event occurredto be less than 100km. Both megacrystand groundmassilmenites have well-developedreaction rims of perovskite + titanomagnetite+ Mn-ilmenite. The Mn-ilmenite (17.9 mol% MnTiO:) is quite distinct from either megacryst or groundmassilmenites and is clearly of secondaryorigin. Discreteperovskites and titanomagnetiteswithin the groundmassare similar in composition to the phaseswithin the reaction assemblage.Cr-spinel (63.6-68.1molVo (Fe,Mg) Cr2O) also occursin the groundmass;these Cr-spinels commonly contain overgrowths of pleonaste. Ilmenite megacrysts and clinopyroxene-ilmenite intergrowths are interpreted to have formed, along with the other members of the silicate megacryst assemblage,from a fractionating, mantle-derivedliquid at about 150km depth within the temperatureinterval of 1165'-1390"C (StageI of kimberlite development).Subsequently, the mineralsexperi- enced a sudden increase of the MgO/FeO ratio in the kimberlitic liquid (Stage II of kimberliteevolution). This may havebeen due to upwardmovement of the megacrystsand xenocrystsinto a diferent melt regime, possibly associatedwith assimilationof mantle wall-rocksuch as a carbonatedperidotite. From this MgO-enrichedliquid, a compositionally distinct suite of Mg-rich groundmassminerals precipitated and earlier formed olivine and ilmenite megacrystsreacted with this liquid and formed high-MgO rims (StageIII). The final recorded stageof kimberlite evolution (StageIV) at Elliott County is the "autometasomatic" reaction of ilmenite with a CaO-enriched fluid to form the reaction assemblage perovskite + titanomagnetite+ Mn-ilmenite. 0003-.09x/82/0102-002E$02. 00 2E AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

Introduction material.One pipe (Pipe 1), located0.5 km southof Ison Creek, has abundant megacrystsand xeno- Important constituents of kimberlite are oxide liths, both mantle and crustal, while the other (Pipe minerals. Since compositionsof oxide phasesare 2), located along Ison Creek, is almost devoid of very susceptibleto changesin temperature,bulk xenolithic material. Mantle xenoliths are quite composition, zr:rdfsr, they provide a record of a small, rarely exceeding 1 cm in diameter and form large portion of the crystallizationhistory and the volumetrically the smallest fraction of included evolutionof kimberlite. material(Garrison and Taylor, 1980).In Pipe l, the Ilmenite is the most abundantoxide phasefound abundance of highly metasomatized fragments of in kimberlite;it is presentboth as a megacryst(i.e., supracrustallimestones, siltstones, and shales(up a )l cm subroundedgrain) and as a groundmass to 20 cm in diameter) approaches20% of the rock. mineral. A diagnosticfeature of kimberlitic ilmen- This metasomatismof the sedimentary and crystal- ites is their distinctive, high-MgO composition line crustal materials suggeststhat the kimberlite (Mitchell, 1973;1977).In addition to rheir distinc- must have been hot at the time of xenolith incorpo- tive chemicalsystematics, they exhibit diversetex- ration during the final emplacement(Garrison and tural relationships.Megacrysts occur as largesingle Taylor, 1980;Garrison et al., 1980). crystalsor may be totally recrystallizedto mosaic This report summarizes petrographic and elec- polycrystallineaggregates. Ilmenite also commonly tron microprobe data for a wide variety of oxide occurs in lamellar or graphic intergrowths with mineralsin the kimberlite pipes. Detailed chemical clinopyroxeneand/or orthopyroxene(e.g., Gurney and textural studies of these kimberlites have al- et al., 1973;'Dawsonand Reid, 1970;Frick, 1973: lowed us to reconstruct a large portion of their Garrisonand Taylor, 1981).Ilmenite can alsooccur crystallization history, from the early megacryst as small sphericalinclusions in silicatephases such stagethrough the groundmasscrystallization stage as olivineor garnet(e.9., Boyd and Nixon, 1973). and the autometasomatic event associated with Spinels are restricted to the kimberlite ground- final emplacement. mass,although they may occur as exsolutionlamel- lae in ilmenite (Danchin and D'Orey, 1972)or in Petrography late-stagereaction zones around ilmenite. Ground- Megacrystsof olivine, ilmenite, garnet, clinopy- mass spinels show a wide variety of chemical roxene, phlogopite, and orthopyroxene occur with- compositions,ranging from Cr-spinel to pleonaste in the fine-grainedkimberlitic groundmass.These to titanomagnetite(Haggerty, 1976). megacrystsaverage 0.5-l .0 cm in diameter, al- Perovskiteis most commonly found in reaction though discrete crystals of garnet, clinopyroxene, zonesassociated with titanomagnetite,around both ilmenite, and phlogopiteup to 1.5 cm in diameter megacrystand groundmassilmenites. Perovskite are not uncommon.Pipe I containsmegacrysts in may also occur as discretegrains in the kimberlite the following relative abundance:olivine > ilmenite groundmass(Boctor and Meyer, 1979;Boctor and ) garnet > phlogopite > clinopyroxene > ortho- Boyd,1980). pyroxene (Garrison and Taylor, 1980).Pipe 2 has The two kimberlite pipes examinedin this study the relative abundance:olivine > ilmenite > phlog- are located in Elliott County in easternKentucky opite > garnet > orthopyroxene > clinopyroxene. along the eastern edge of the Appalachian Plateau. Olivine and phlogopite have corroded, fritted rims These kimberlites contain a variety of mantle-de- reflectingreaction with the kimberlitic magma.Gar- rived megacrystsand xenoliths, as well as frag- net is always surroundedby a "kelyphitic" rim of ments of highly metasomatizedlower crustal and dark microcrystalline material, probably spinel + supracrustalmaterials. Bolivar (1972)documented phyllosilicates.Olivine megacrystscommonly ex- the kimberliticnature of thesebodies and described hibit fracturing and kink banding; many have been their mineralogy. Zartman et al. (1972) reported a totally recrystallized to mosaic polycrystalline ag- Rb-Sr ageof 257-+22 m.y. for thesebodies, indicat- gregates (dunites; Garrison and Taylor, 1980). ing an early Permian emplacementage. Three graphic clinopyroxene-ilmenite intergrowths The Elliott County kimberlites are unique in (Garrisonand Taylor, 1981)and one rutile-ilmenite severalaspects. The kimberlite is extremelyfresh, intergrowths were recovered from the Elliott Coun- providingan excellentopportunity to examineboth ty kimberlites. the megacrystsand the fine-grainedgroundmass The groundmassof the kimberlite consists of 30 AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

beenidentified: (I) subroundedmegacrysts (0.5-1.5 cm in diameter) which exhibit various degrees of recrystallization (Fig. la); (II) graphic intergrowths of ilmenite in clinopyroxene (Fig. 1b); (III) thin lamellaeof ilmenite in rutile "phenocrysts" (Fig. 1c); (N) anhedral, single, groundmass crystals (0.05-1.0mm) (Fig. 1d);(V) round inclusions(up to 25 p,m) within euhedral to subhedral groundmass olivines (Fie. 2a); (VI) small ilmenite grains associated with titanomagnetiteand perovskite in reaction rims around megacryst and groundmass ilmenites (Fie.2b). Ilmenite megacrysts have rounded anhedral to subhedral shapeswhich are attributed to abrasion during emplacement of the kimberlite (Mitchell, 1973). Textures indicate a continuous variation from single crystals with slightly undulose extinction to totally recrystallized polycrystalline mosaics with sharp extinction. The following textural fea- tures provide evidence that the mosaic aggregates of ilmenite within the Elliott County kimberlites are in fact recrystallizedmegacrysts: (1) twin lamellae are occasionally present near the edges of megacrysts(Fig. 3a),(2) bandsof fine mosaicilmenite cut Fig. l. Reflected light photomicrographsof ilmenite textural large grains (Fig. 3b), (3) long bands or tapering types. (a) TypeI megacrystilmenite; scale bar : I mm. (b) Type wedgeswithin grains have slightly different extinc- 11ilmenite in clinopyroxene-ilmenite intergrowth; il : ilmenite; tion anglefrom the host (Fig. 3c), and (4) foliation in : = (c) cpx clinopyroxene; scale bar 2 mm. TypeIII ilmenite in by alignment elon- rutile-ilmenite intergrowth; il : ilmenite; ru : rutile; scalebar: some megacrystswas formed of 100pm. (d) TypeIV groundmassilmenite; il = ilmenite; mt = gatesutured grains (Fig. 3d). titanomagnetite;pf = perovskite;scale bar = 75 pm. Three nodular clinopyroxene-ilmenite intergrowths were collected from the Elliott County euhedral phenocrysts of olivine and subhedral kimberlite Pipe I (Garrison and Taylor, 1981). grains of olivine, ilmenite, Cr-spinel, pleonaste, Thesenodular intergrowths from Elliott County are Mg-rich orthopyroxene, Mg-rich clinopyroxene, texturally similar to other such intergrowths from perovskite,titano-magnetite, phlogopite, vermiculite, pectolite, calcite, and septechlorite.Locally, calcite occurs in veins and fractures. Possiblede- watering of sedimentaryinclusions has produced extensive serpentinization of the surrounding groundmassolivines (to Al-poor lizardite). Ultramafic xenoliths included in the kimberlite are small and usually representincomplete peridotite assemblages(Garrison and Taylor, 1980).Pipe 1 containsthe greatestabundance of ultramafic xenoliths; theseinclude garnetperidotite, spinelperido- tite, spinel-ilmenitewebsterite, and dunite. Only two xenolithswere recoveredfrom Pipe 2: a dunite and a serpentinizedgarnet peridotite. Fig. 2. Reflected light photomicrographsof ilmenite textural Ilmenite types. (a) Type V ilmenite inclusion in olivine; il : ilmenite; ol : olivine; pf : perovskite; scale bar : 250 pm. (b) Type VI Ilmenite is the most abundantoxide phasein the secondaryMn-ilmenite; il : ilmenite; pf = perovskite; m-il : Elliott County kimberlites. Six textural types have Mn-ilmenite; scale bar : 100 u.m. AGEE ET AL,: OXIDE MINERALS IN KIMBERLITE 3I

kimberlites in South Africa (e.g., Gurney et al., 1973)and the United States (e.g., Eggler et al., 1979).The intergrowths from Pipe I range in size from 0.5 cm to 2.0 cm in diameter and consist of large single crystals of pale-green clinopyroxene with graphic intergrowths of ilmenite (Fig. lb). These graphic ilmenites (100-400 pm thick) form one optically continuous crystal (up to 30 modal Vo of nodule), although occasionally they exhibit mi- nor recrystallizationto polygonalmosaics or show unduloseextinction. The host clinopyroxenecom- monly showsundulose extinction but rarely recrystallization. The grain boundaries between the graphic ilmenite and host clinopyroxene are generally free of alteration, although in some cases, tracesof phlogopitecan be observed.The ilmenites frequently extend out into the host kimberlite be- yond the clinopyroxene; this is probably due to fragmentation and abrasion during transport through the kimberlite conduit. The rutile-ilmenite intergrowth recovered from Elliott County Pipe 2 consistsof small 5 pm x 10 pm sigmoidallenses of ilmenitein a rutile host. The rutile is mantled by a rim of polycrystalline ilmenite (Fig. 1c). This texture is very similar to that de- Fig. 3. Deformation and recrystallization textures in ilmenite scribedfrom South African kimberlites by Haggerty megacrysts. (a) Twin lamellae near the edge of a megacryst; (1975, 1976)and Mitchell (1973).Haggerty (1975) scalebar = I mm. (b) Bands of mosaic polycrystalline ilmenite = describedtwo types of rutile-ilmenite intergrowths: crossing ilmenite megacryst; scale bar I mm. (c) Undulose crystal ilmenite megacryst;scale bar = (1) those in which rutile extinction in large single and ilmenite are equally I mm. (d) Foliation in ilmenite megacryst outlined by elongate abundant, and (2) those in which ilmenite forms sutured grain boundaries; scale bar : I mm. All are reflected only a few percent of the intergrowth. The Elliott light photomicrographs. County intergrowth belongs to the latter type. Anhedral single crystals of ilmenite, ranging from naste (Fig. 4a). Type 11 pleonastes also occur as 0.05 to 1.0 mm in diameter, occur throughout the discrete single grains (up to 30 pm in diameter) in groundmassof the kimberlite and in this study are the groundmass.Type lll titanomagnetitesoccur as texturally referred to as groundmassilmenites (Fig. discrete grains (up to 75 pm in diameter) in the 1d). These groundmassilmenites are never poly- groundrnassand associatedwith perovskite in reac- crystalline and show uniform extinction. They tion rims around ilmenite (Figs. ld and 4a). Type IV could have two possibleorigins (Pasteris,1980): (l) spinel occurs as small (<1 prm thick) rodlets in disaggregatedrecrystallized megacrysts, and/or (2) ilmenite (Fig. 4c). This texture is similar to that a compositionally distinct second generation of describedfrom the Premier Mine, South Africa by ilmenite that crystallized with the other groundmass Danchin and D'Orey (1972).These spinel rodlets phases. are found in three of the six textural types of Spinel ilmenite in the Elliott County kimberlites; they occur in megacrysts,groundmass ilmenites, and in Four different types of spinel can be distinguished the ilmenite inclusionsin olivines. in the Elliott County kimberlites:(I) pale-bluechro- mite, (II) gray-bluepleonaste, (III) brown titano- Perovskite magnetite,and (IV) brown spinel rodlets in ilmen- Perovskite occurs as discrete, equant, anhedral ite. Types I, II, and III occur in the groundmassof blebs (up to 30 pm across) scatteredthroughout the the kimberlites. Type 1 chromites (up to 50 p,m in groundmassand in reaction rims around ground- diameter) are commonly rimmed by Type 11 pleo- massand megacryst ilmenites (Fig. 4a, b). It is most 32 AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

ous fragments of the host ilmenite occur; these islands of ilmenite indicate that extensive reaction must have occurred. Between the spinel and perovskite zones are rare occurrences of secondary Type VI ilmenite (Fig. 2b).

Mineral chemistry Minerals were analyzedwith a MAC 400 S electron microprobe utilizing the correction procedures of Bence and Albee (1968)and the data of Albee and Ray (1970). Except for the rims of ilmenite megacrysts, all minerals were found to be extremely homogeneous.When grain size permitted, four to ten analyses were made on gach grain and an averageanalysis and 1o standarddeviation comput- ed. Representativeaverage analyses of all minerals are included in Tables I through 3. Ilmenite Representativeaverage analyses of ilmenite megacrysts from the Elliott County kimberlites are presentedin Table 1. The compositionalrange of ilmenite megacrystsis shown in an expanded portion of the MgTiO3-FeTiO3-Fe2O3ternary in Figure 5. The compositionsof the megacrystcores (31.6- 46.3molVo MgTiO3 and 6.3-14.9 molVoFe2O) are similar to ilmenites reported from kimberlite pipes in West Africa and Lesotho (Fig. 6; Haggerty, 1975).The ilmenite megacrystsshow a correlation of increasingFe2O3 Q.14-16.4 wt.%) with decreasing Mg/(Mg+Fe) (50.6-36.7)(Fig. 7) and decreasing MgO (13.2-8.63wt.%). This correlationof decreasing MgO and Mg/(Mg+Fe) with increasing Fe2O3 suggeststhat the ilmenite megacrystsare related by crystal fractionation. As crystallizing silicates and Fig. 4. Reflected light photomicrographsof spinel types and ilmenites deplete the melt in MgO and decrease perovskite. (a) Composite spinel grain consisting of Type I Mg/(Mg+Fe), the preferential incorporation of chromite rimmed by TypeII pleonaste;mt = titanomagnetite;sp = pleonaste;cr : chromite;scale bar : 100pm. (b) Reaction Fe*2 into the silicate phases increases the selvageon ilmenite megacryst. Note islands of ilmenite within Fe+3/Fe+2ratio in the remainingmelt. Sinceilmen- the zone of perovskite and titanomagnetite;il : ilmenite; pf : ite is the only major crystallizing phase that can perovskite; scale bar : 100 p,m. (c) Type 1V spinel rodlets in incorporatesubstantial Fe+3, the ilmenitesmay be = : : ilmenite; sp spinel; il ilmenite; scale bar 40 U,m. expectedto show an increase in Fe2O3with differ- entiation(i.e., decreasingMg/(Mg+Fe)). common in the reaction rims around ilmenite. This The rims of ilmenite megacrysts have higher reactionrim of perovskite is developedas a 50-75 MgTiO3 content (44.3-53.1molVo MgTiOr) than the pm wide zone with a fritted, digitate boundary that megacrystcores (Table 1; Fig. 5); however, the projects into the ilmenite. The outer boundary of rims have a similar hematite content (4.4-13.3 the perovskitezone (i.e., nearestthe host kimber- molVoFe2Or). The megacryst rims exhibit a strong lite) is in contact with a discontinuous zone of correlation of increasing Fe2O3(5.15-15.0 wt.Vo) subhedral(up to 50 pm wide) Type III titanomagne- with decreasingMgO (15.6-12.6wt.%); there ap- tite (Fig. 1d).Within thesezones, optically continu- pears to be no correlation between Mg/(Mg+Fe) AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE JJ

Table 1. Representativeaverage analyses ofilmenite megacrysts

rl Ptoe z Cpx-ilm megacryst regacrys t megacryst megacryst I ntergrowth rtm rt m rim core rim f io| 52.3 $r" 53.\ Q) 48.2(2) 48.9 (5) 53.0 (4) 53.7 (2) 52.0 (2) fi.s (5) 54.2 (3) 41203 0.63(4) 0.62(2) 0.39(2) 0.52(4) 0.65(3) 0.55(3) 0.61(l) 0.53(r) 0.57(4) crzo3 0.07(1) 0.07(4) 0.27Q) 0.30(1) o.09(2) o.09(1) n.d.no 0.il51j\ o.r(zt Fe203*** 9.44 8.95 r5.4 15.0 8.86 9.29 9.22 7.58 6.71 FeO 24.6 (6) 23.5 (81 25.7 rc, 21.5 (9) 25.2 (\) 21.5 (7) 25.9(8) 21.8(5) 25.7Q) Mno 0.28(3) 0.43(5) g.3l(3) 0.41(1) o.29o) 0.39(3) o.26(2) 0.44(2) o.2\(3) Mso 12.5(1) 13.5Ql 9.33Q\ 12.5(6) 12.5(1) 15.0(3) 11.7(4) 14.6(1) 13.0(r) Ni 0 n.d. 0.04(3) n.d. 0.10(1) n.d. 0.04(2) n.d. 0.07(6) 0.09(4) x 99.82 t00.5r 100 .50 99.33 t00.69 100 .67 qq (q 98.87 loo.84 Cation ? ? 3 55 Basis ) AI 0.017 0.017 0.011 0.0r4 0.018 0.018 0.0t7 0.014 0.015 Cr 0.001 0.00t 0.005 0 .005 0.002 0.002 0.006 0.005 tl 0.908 0.9t3 0.853 o .856 0.910 0.9r3 0.909 0.923 0.927

Fe- 0.154 0.153 o.z9t o.251 0.152 0.155 0.15r 0.r3r 0.115 M9 o .430 0.459 0.327 0.437 o.43r 0.501 0.404 0.4r8 0.\42 Ni 0,001 0.00r 0.001 0.001 0.001 _+z fe o -475 0.448 0.506 0.417 0.480 0.404 0.504 0.418 0.490 HN 0.005 0.008 0.006 0.008 0 .006 0.007 0.005 0.009 0.005

2.000 2.000 2,000 2.000 2.000 2.000 2.000 2.000 2.000

* units in 0 represent 1q standard deviation of replicate analyses i n terms of I east uni ts ci ted. ** n.d. = not detected; less than 0.03 ?. *** Fe determined as Feo; oxides based on crystal chemistry.

Table 2. Representativeanalyses of Type III, IV, V, and VI ilmenites

Pipe 1 Pipe 2 inclusion lamellae rim in in of TyPe Vl groundmass groundmass g roundmass groundmass ol ivine ruti le ruti le

Ti0^ 55.U >5.5 51.2 Ea 58.7 57.9 48.3 Ar o.29 0.55 0.32 0 .95 0.89 n.d. 0. o.4r a5^0^ 34 Lr-u- 1.69 0.13 2.71 0.34 \.\2 0.06 o.21 0.95 FeZO3* 5.72 8.72 5.15 I' E 7.86 r.85 r1.3 Fe0 19.8 23.6 20.6 21.5 18.4 28.1 2\.4 27.o Mn0 nqh 0.34 0.51 0.tt5 0.26 n?q 0.50 8.69

Hgo 16.8 tJ,o 16.3 tJ.o l6.0 I 3.0 15.4 4.\7 Ni0 0.07 n.d. 0.lq o.12 0.r6 n.d. n.d. n,d.

t r00.71 100.35 101.13 100.67 r00.29 1O0.25 r00.7r l0l .13 Cat I on Basis AI 0.008 0.018 0.008 o.026 0.024 n nnq n nlt 0.030 0.002 0.048 0.006 o.078 0.001 0.004 0.018

tl 0.932 0.913 o.927 0.874 0 .880 1.005 0.975 0.879 r2 Fe'' o .096 o.t5o 0 .086 o.21\ 0.133 0.031 0.207

l"lg u,))o 0.461 0.538 o.462 o.533 U.)'T 0.513 0.151 Ni 0.001 0.002 0.002 0.002 _+2 tse 0.367 0.449 0.382 0.408 n ?!q 0.440 0.\56 0.545 l,tn 0.010 0.007 0.0r0 0 .009 0.005 0.007 0.011 0.177

x 2.000 2.000 2.000 2.000 2.000 r.990 2,000 2.000

* Fe determined as Feo; oxides based on crystal chemistry. t* n.d. = not detected; less than 0.03 Z. 34 AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

Table 3. Representativeanalyses of spinels, rutile, and perovskite

Type I spinel Type ll spinel Type lll spinel perovskite rutlle

sl0^ n.a, n,a. n,a. n.a. 0.09 Ti02 | .52 3.85 2.05 3.27 ?1 E 2l .o >I.o 97.5 Al q6.r z5^0- 8.02 8.70 50.5 8.48 10.0 o.33 0.il Cr203 >t.5 49.8 5.78 6.o7 2.07 0 .46 0.16 n.q, Fe^0-** z5 9.55 8.24 9.78 12.1 27.8 25.5 n.d. Fe0 17.0 t8.2 11.0 10.1 17.\ 22.7 I .40 0.05 lln0 o.52 o.53 0.21 0.11 0.87 0.89 0.13 n ,d, M9o 11.0 12.2 19.8 20.4 22.6 |8.4 0.23 n.d. Ni0 n.d. n.o. n.d. n.d. n.d. n.d. n.d. n.d.

Ca0 n.a. n.a. J6.O

t 98.9| 10r.52 99,12 98.15 100.72 98,95 qn arr 97.76 Cat i on Basis c c 4 4 4 { sl 0.00r AI 0.317 o.333 1.596 1.488 0 .305 o.376 0.008 0.00t LT 1.363 1.273 o.123 0.131 0 .050 0.012 0.002 Ti 0.038 0.095 0.041 0.065 o.502 0.498 0,994 0.999 +? 0.2\1 0 .200 0.t97 0,2\9 0,6t+2 0.5t0 l'lg 0.549 0 .590 0.n3 o.832 1.034 '_l1u 0.007 Ni _+2 te 0.478 0.493 o,245 o.231 0.443 0.505 0.025 I'ln 0.0r5 0.015 0.005 ':ll' 0,023 '-liu 0.002 Ca 0.95r

t 3.000 3.000 3.000 3.000 3.000 3.000 t.991 1.000

* n.a. = not €nalyzed. ** Fe determlned as Fe0; oxldes based on crystal chemlstry. *** n.d. = not detected'

and Fe2O3such as that observed for the megacryst I Megacryst core O Cpx-llm int€rgrowth cores(Fig. 7). D Groundmass There occurs a regular, well-developed composi- i llmenite in Olivine tional gradient O M€gacryst Rirns between the megacryst core and the high-MgO rim (Fig. 8). This profile is developed over a distanceof at least 75-100 pm, althoughthe true extent cannot be determined due to the extensive reaction rim of perovskite + titanomagnetite * Mn ilmenite. In many cases, the profiles can be

FeIiO3 50 MgIi03 70 50 30 Fig. 6. Ternary FeTiO3-MgTiO3-Fe2O3 plot showing the Fig. 5. Compositionsof the ilmenite textural types plotted in compositionsof the Eiliott County megacryst and groundmass an expandedportion of the FeTiO3-MgTiO3-Fe2O3temary. All ilmenites relative to ilmenites from Africa (Ha€gerty, 1975; data plotted as rnolVo. Mitchell, 1977)., AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE 35

Dixon (1976)with molVoDi : 2Cal(Ca+Mg+Fe) (Garrisonand Taylor, 1980;1981). Using the clino- .6 too pyroxene-ilmenite geothermometer of Bishop * 2/(Fe*2 *2 *3) ."ffi. ( 1980),with X|" r,o, : Fe + Mg + 0' 5Fe .5 e. .rl:_. andXffi: [2Cal(Ca+Mg+Fe)][Mg/(Mg+ Fe)], Gar- ile " 1... . ' Itg. fe rison and Taylor (1981)estimated the equilibration .l tt ' at temperatures(atP : 50 kbar) ofthe clinopyroxene- I glrndilN ilmenite pairs at 1210'-1245'C; these equilibration .3 . n4rcrrst cqls o ctr-iln!ilb temperaturesare generally consistentwith the diop- o ilmdh h olirir! estimatesconsid- .2 side-cnstatite solvus temperature o 2 4 6 t 11 16 18 20 ering the uncertainties associated with applying ,llort' thesegeothermometers. The compositional similar- Fig. 7. A plot of Mg/(Mg*Fe) vs. wt.VoFe2O3for the Elliott ities betweenthe clinopyroxenesand ilmenites from County ilmenites. the intergrowths and the discrete ilmenite and clinopyroxene megacrystsfrom Elliott County sug- extended into the reaction zone by analyzing the gest that the clinopyroxene-ilmenite intergrowths remnant islands of ilmenite within the reaction and the clinopyroxene and ilmenite megacrystsare zones(Fig. 4b). Haggertyet al. (1979)noted similar geneticallyrelated (Garrisonand Taylor, 1981). compositional gradients in megacrysts from the The Elliott County Type IV groundmass ilmen- Monastery Mine, South Africa; they referred to it ites aredistinctly richer in [email protected] .3 mol% as the magmatic reaction trend. Elthon and Ridley MgTiOl) than the ilmenite megacryst cores, al- (1979)also found an increasein MgO near the rims though quite similar in MgTiO3 content to the MgO- of ilmenitesfrom the Premier Mine, South Africa. enriched rims of the ilmenite megacrysts(Fig. 5; Boctor and Boyd (1980)reported similar composi- Table 2). The groundmassilmenites show a wide tional gradientsdeveloped within the outer 300-400 rangeof Fe2O3Q.9-13.9 molVoFe2O3). Analogous pm of discreteilmenite megacrystsfrom the Liqho- to the MgO-enrichedmegacryst rims, the ground- bong kimberlite, Lesotho. Since the perovskite + mass ilmenites exhibit a correlation of increasing titanomagnetite + Mn-ilmenite reaction zone is Fe2O3Q.29-15.7 wt.Vo) with decreasingMgO (15.7- rarely over 100pm wide, it appearsthat the compo- 13.5 wt.Vo), but no correlation between Fe2O3and sitional gradientsnear the rims of the Elliott County Mg/(Mg+Fe). The groundmassilmenites exhibit a ilmenite megacrystsprobably cannot be extended wider range of CrzOr (0.12-3.76wt.%) than either over 200pm inward, a distancesomewhat less than the megacrystcores (<0.03-1.30 wt.Vo CrzO) or observedfor the Liqhobong ilmenite megacrysts. the megacrystrims (0.04-2.22 wt.Vo Cr2O). The These compositional gradients to the high-MgO similarity in chemical systematics between the rims probably developedas the result of reaction of MgO-enrichedrims of the megacrystsand the Type the ilmenitemegacrysts with a MgO-enrichedmelt; IV groundmass ilmenites suggests that both may therefore, they representa diffusion profile between have equilibrated with the same MgO-enriched the megacrystcore and an overgrowth with a higher MgO content. Ilmenites from the clinopyroxene-ilmenite intergrowths (44.7-44,6 mol% MgTiO3 and 5.4-5.8 molVoFe2O)have compositions similar to the more o MgO-richmegacryst cores (Table 1; Figs. 5 andT). o Although lower in Cr2O3 (0.06-0.38 wt.%o),the ;s clinopyroxenesfrom the intergrowths have compositions similar to the clinopyroxene megacrysts from Elliott County (0.56-0.88wt.Vo); the clinopyroxenes from the intergrowths have estimated equilibrationtemperatures 100 of 1295'-1335'C which microns from rim overlap the range of temperatures estimatedfor the Fig. 8. Electron microprobe traverse from rim to core of a Pipe I clinopyroxenemegacrysts (1305"-1390' C), Type I ilmenite megacryst showing the smooth MgO usingthe diopside-enstatitesolvus of Lindsley and compositionalgradient. AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE melt. The core-rim-groundmass pattern of the El- exsolution from the rutile. Energy Dispersive Anal- liott County ilmenitesis analogousto the chemical ysis (EDA) of the rutile indicates the presence of systematicsdisplayed by the Elliott County olivine substantialNb: this is also reflectedin the low sum megacrystsand groundmassolivines; the rims of of the rutile analysisin Table 3. olivine megacrysts have Mg/(Mg+Fe) ratios of The high-MnO Type Vlilmenites found within the 0.89-0.90, regardless of the core compositions perovskite + titanomagnetite * Mn-ilmenite reac- which rangein Mg/(Mg+Fe) from 0.85to 0.93. The tion rims around the ilmenite megacrysts and Type rims of these olivine megacrysts are similar to the N groundmass ilmenites contain 17.9 molVo low-NiO groundmass olivines in Mg/(Mg+Fe), MnTiO3, 16.3molVo MgTiO3, and 10.5molVoFe2O3. CaO, and NiO, reflectinga later-stagereaction of They are quite distinct from either megacryst or the olivine megacrysts with a liquid more enriched groundmassilmenite (Table 2). Their distinctive in MgO than the liquid in equilibrium with the most composition and textural relationship to perovs- fractionated megacrysts (i.e., with the lowest kites and titanomagnetitesindicate that these MnO- Me/(Me+Fe)). rich Type VI ilmenites are clearly of secondary The ilmenitesoccurring as inclusionsin ground- origin. High-MnO ilmenites'havebeen described in massolivines (50.9-56.4molVoMgTiO3 and 3.9-9.1 kimberlite by Haggerty et al. (1979), Hsu and molVo Fe2O3)have compositions that overlap the Taylor (1978),Boctor and Meyer (1979),and Wyatt more MgO-rich groundmass ilmenites, although (1979).In each case,they are associatedwith late- they are slightly higher in [email protected] wt.%) stagereaction rims or occur in cracks within ilmen- (Figs.5 andT;Table 2). This similarity suggeststhat ite megacrysts. these inclusions are genetically related to at least some of the Type IV groundmass ilmenites. Fur- Spinel thermore,this also suggeststhat at leastsome of the Representativeanalyses of spinels from the El- groundmassilmenites belong to a compositionally liott County kimberlites are presented in Table 3. distinct population of ilmenites formed during a The Type 1 chromites contain 63.6-68.1 mol% later stage of ilmenite crystallization and are, in (Fe,Mg)CrzO 4, 10.0-12.1 molVo(F e,Mg)FezO+, 3 .8- fact, not MgO-enriched fragments of disaggregated ll.0 mol% (Fe,Mg)2TiOa, &rd 15.9-16.7 mol% recrystallizedmegacrysts. These high-Cr2O3 ilmen- (Fe,Mg)Al2Oa.The TypeII pleonastesthat occur as ites actually approach the composition of the small overgrowthson Type I chromites and as discrete idiomorphic groundmass ilmenites reported from groundmass grains contain 74.3-:79.8 molVo the Liqhobong kimberlite by Boctor and Boyd (Fe,Mg)Al2O4, 6.2-6.6 molVo (Fe,Mg)Cr2Oa,8.9- (1980),although somewhat more variablein Fe2O3. 12.5 molVo (Fe,Mg)Fe2Oa, and 4.1-6.6 molVo The Fe-Mg equilibria between groundmassolivine (Fe,Mg)TiOc. Type III titanomagnetitesthat occur (Foss.o)and ilmenite inclusions,using the geother- within the perovskite * titanomagnetite + Mn- mometerof Andersenand Lindsley (1979),suggest ilmenitereaction zones and as discretegrains in the equilibration of these groundmassphases at 965" C groundmassare distinctly higher in TiO2 than the (at p : 50 kbar). A 5 kbar change in the assumed other spinels(38.0-53.0 molVo (Fe,Mg)2TiOa, 17.5- pressurewill change the estimated temperature by 26.3 molVo (Fe,Mg)Al2Oa, and 3.0-6.I molVo only9.5'C. (Fe,Mg)CrzO+).The compositionalranges for spin- The ilmenite from the ilmenite-rutile intergrowth els from the Elliott County kimberlites are plotted (55.0mol% MgTiOs) containsno Fe2O3Gable 2). in the spinel prism of Figure 9 (Irvine, 1965).The The ilmenite that rims the rutile-ilmenite inter- typical kimberlite trend (Haggerty, 1976)of early growth is similar in composition (52.0 molVo Cr- and Al-rich spinels followed by late-stagecrys- MgTiO3 and 1.5 molVo Fe2Or) to the low-Fe2O3 tallization of spinels enriched in Fe*3 and Ti is groundmassilmenites. The absenceof Fe2O3in the readily seenat Elliott County. This compositional ilmenite lamellae in the rutile suggeststhat they are changefrom groundmass Ty p e 1 Cr-spinel and Typ e not related to the rim ilmenite. The rim ilmenite is // pleonaste crystallization to the precipitation of probably related to the Type IV groundmassilmen- Type III titanomagnetite reflects the breakdown of ites and the MgO-enriched rims of the ilmenite ilmenite in the presenceof a CaO-enrichedfluid. megacrysts,reflecting reaction of the rutile pheno- From the Elliott County data, it is not clear whether cryst with the MgO-enrichedliquid. The ilmenite the changefrom Cr-spinel to pleonaste crystalliza- lamellaewithin the rutile were probably formed by tion is continuousor discontinuous. AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE 37

Perovskite The compositionof perovskiteshows little variation whether in the reaction rim around ilmenite or as discretegroundmass grains (Table 3). Perovskite hasonly minor FeTiO3(2.6 nol%) and MgTiO3(0.7 mol%) in solid solution.Energy dispersiveanalysis (EDA) indicates the presenceof substantialNb and REEs. Boctor and Meyer (1979) considered the abundanceof Nb and REEs in perovskiteto be the result of carbonate complexing of the REEs; this complexing stabilizes the REEs in the late-stage kimberlitic fluids.

Discussion Previous investigations of oxide minerals in kim- Fig. 9. Compositional fields of the different spinel types berlite (e.9., Haggerty, 1973;1975; Danchin et al., plotted in the spinel prism (Irvine, 1965).The range of molVo 1975)have dealt with the mineralogy of the mega- Fe2TiOais given for each spinel type. l: Type1 chromite; II : crysts and/or groundmass, as well as compilations TypeII pleonaste;III : Type III titanomagnetite. of data from many localities. The Elliott County kimberlite is well-preserved and permits a detailed investigation of both megacrysts and groundmass phasesfrom the same kimberlite. The relationship and to a lesser extent, the orthopyroxenesof the between these different generations of minerals is megacrystassemblage exhibit compositional trends complex; however, when the oxide data are com- generallyattributed to precipitation from a fraction- bined with the data on megacryst and groundmass ating liquid. silicatesby Garrison and Taylor (1980),the evolu- The silicate megacrysts from Elliott County ex- tion of the Elliott County kimberlite can be divided hibit well-defined chemical trends which Garrison into four distinct stages:(I) the crystallization of the and Taylor (1980)attribute to fractional crystalliza- megacrystphases, (II) an increasein the MgO/FeO tion from a "proto-kimberlitic" liquid. Garnet meg- ratio in the kimberlitic liquid, (III) the crystalliza- acrysts show correlation of Mg/(Mg+Fe) (0.79- tion of the groundmassphases with the accompany- 0.86)and CaO (4.54J.10 wt.Vo)with CrzOg(0.21- ing back-reaction of the megacrystswith the MgO- 9.07 wt.%); the more magnesiangarnets have more enriched liquid, and (IV) a late-stage"autometaso- uvarovite in solution. Olivine megacrystsshow a matic" reaction of ilmenite with a CaO-enriched decreasein NiO (0.5-0.05 wt.Vo)with decreasing fluid to form perovskite + titanomagnetite * Mn- Mgi(Mg+ Fe) (0.93-0. 85). The clinopyroxenemega- ilmenite. crysts show increasingTiO2 (0.38-0.56wt.Vo) and The ilmenite, as well as the silicate, megacrysts decreasingCr2O3 (0.88-0.56 wt.%) with decreasing couldhave formed as the resultof(1) disaggregation Mg/(Mg+Fe) (0.91-0.87).The high-AlzO:, high Z of peridotite to form discrete mineral grains, or (2) (1165'-1255" C) orthopyroxenemegacrysts exhibit direct precipitation from a "proto-kimberlitic" liq- a general trend of decreasingTiO2 and Cr2O3with uid (Eggleret al.,19791'Garrison and Taylor, 1980). decreasingMe/(Mg+Fe). The latter origin was suggestedfor the Elliott Coun- An origin by precipitation from a "proto-kimber- ty silicate megacrysts (Garrison and Taylor, 1980) litic" liquid is also preferred for the Elliott County because:(1) the largegrain sizeof the megacrystsis ilmenite megacrysts because: (1) ilmenite is an generally larger than the grain size expected from extremely rare minor constituent of mantle perido- disaggregatedperidotite; (2) the common inter- tite, (2) the sizeof the ilmenite megacrysts(0.5-1.5 growths of the minerals is typical for phases co- cm) is much larger than the grain size expectedfrom precipitatedfrom a liquid; (3) the megacrystsdo not disaggregatedperidotite, and (3) the ilmenite mega- show complete or nearly complete compositional crysts display a correlation of increasingFe2O3 with overlapwith the phasesof includedperidotite xeno- decreasingMgO and Mg/(Mg+Fe), which can be liths; and (4) the garnets,clinopyroxenes, olivines, attributed to fractional crvstallization. AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

Gurney et al. (1979)studied a large number of val. Data from the Elliott County high-Z orthopyr- coexisting ilmenite-silicate pairs from the Monas- oxenemegacrysts (i.e., with estimatedequilibration tery Mine and demonstrateda co-variance of MgO temperaturesof 1165'-1255'C) suggestthat StageI in ilmenite and Mg/(Mg+Fe) in the coexistingsili- megacryst crystallization continued to about cate phases.They suggestedthat the Fe2O3varia- 1165"C at pressuresof 51-53 kbar. It is not clear tion of the ilmenitescould be due to increasesin/p" whether ilmenite megacrystscrystallized through- as the kimberlitic liquid evolves. As discussed' out this interval, alongwith orthopyroxene,garnet, earlier, we believe the increase in Fe2O3 with phlogopite,and olivine. decreasing MgO and Mg/(Mg+Fe) in the Elliott County ilrnenitemegacrysts can be produced,as a Crystallization of groundmass phase s resultof crystallizingsilicates and ilmenites,deplet- ing the liquid in MgO and decreasingMg/(Mg+Fe) The high-MgO Type 1V groundmassilmenites slowly with fractionation. The preferential incorpo- from the Elliott County kimberlite pipes could have ration of Fe+2 into the abundant silicate phaseswill two possibleorigins: (1) they may representMgO- produce an increase in Fe+3/Fe*2 ratio in the re- enriched fragments of disaggregatedrecryst allized maining melt. Since both mechanismswould pro- ilmenite megacrysts,and/or (2) they may represent gressivelyenrich ilmenite in Fe2O3,the MgO-Fe2O3 a compositionally distinct second generation of and Mg/(Mg+Fe)-Fe2O3 systematics may prove ilmenite that crystallized with the other groundmass valuable indicators of the T-X trajectory of the phases.The similarity in chemical systematicsbe- evolvingkimberlite liquid. tweenthe MgO-enrichedrims of the ilmenitemega- The silicate megacrysts from Elliott County are crysts and the Type IV groundmassilmenites sug- consideredto be derived from a fractionating proto- geststhat, regardlessof their initial mode of origin, kimberlitic liquid, representingthe earliest stagesof both probably equilibrated wirh the same MgO- kimberlitedevelopment (Garrison and Taylor, l9g0; enrichedliquid. 1981);the data presentedabove suggestthat the By examining the Cr2O3content of the Type IV Elliott County ilmenite megacrystsprobably crys- groundmassilmenites, it is possibleto delineatethe tallized.from the same liquid. The compositional secondgeneration groundmass ilmenites from the similarities of ilmenite and clinopyroxene in the MgO-enriched fragments of disaggregatedmega- nodular graphic intergrowths to megacryst clino- crysts. Since the Type V ilmenite inclusions in pyroxene and ilmenite in the host kimberlite sup- groundmassolivines are high in [email protected] port co-precipitation of clinopyroxene and ilmenite, wt.Vo),it is most probable that the ilmenites that at least during a portion of kimberlite crystalliza- crystallized in equilibrium with the groundmass tion. The clinopyroxene megacrystsgenerally have assemblageare high in Cr2O3. Over 90Voof the higher estimatedequilibration temperatures(l 305.- analyzed,megacryst cores have between 0.03 and 1390'C), by as much as 50oC, than the clinopyrox- 0.40 wt.Vo Cr2O3;over 86Voof the analyzed mega- enesfrom the nodular graphic intergrowths (1295'- cryst rims have between0.04 and 0.73 wt.VoCr2O3. 1335"C) suggestingthat clinopyroxenecrystallized This suggeststhat the megacrystilmenites, as well over a short temperature interval before being as their MgO-enriched rims, are dominantly of the joined by ilmenite. This sequence of events is low-Cr2O3 variety. The low-Cr2O3 nature of the consistentwith phase relations outlined by Wyatt MgO-enrichedmegacryst rims probably reflects in- (1977).The high Mg/(Mg+Fe) ratios (0.47-0.50)and complete equilibration with the MgO- and Cr2O3- MgO content(13.0-13.5 wt.%) and low Fe2O3con- enriched liquid. Only 54% of the analyzed Type IV tent (6.14-6.71 wt.%) of the ilmenites from the groundmassilmenites have Cr2O3 contents less graphicintergrowths is consistentwith the ilmenites than 0.73 wt.Vo;the remaining46Vo have between from the intergrowths representing some of the 1.43and 3.76 wt.% Cr2O3.These high-Cr2O3Type earliestformed ilmenites (j.e., least fractionated). /Vgroundmassilmenites are similarin Fe2O3(4.58- The clinopyroxenes from the intergrowths have 8.58 wt.%) to the Type V ilmenite inclusions in chemical systematicsthat suggestthey represent groundmassolivines (4.43-10.0wt.Vo Fe2O), fur- some of the more fractionated members of the ther substantiatinga genetic relationshipwith the clinopyroxene population (Garrison and Taylor, groundmass mineral assemblage.As discussed 1980).Ilmenite and clinopyroxene probably crystal- above, thesehigh-Cr2O3 Type V and Type 1V ilmen- lized together over only a short temperature inter- ites have compositionsthat approachthe composi- AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE tions of the small idiomorphic groundmassilmenites membersof the ilmenite megacrysts.If the carbon- reported by Boctor and Boyd (1980)from the Liq- ated peridotite contains substantial MgO, in the hobong kimberlite. In contrast to the Liqhobong form of easily assimilateddolomite or magnesite, groundmass ilmenites, the high-Cr2O3 Type IV assimilation would also add substantial MgO and groundmassilmenites from Elliott County do have a subsequentlyincrease MgO/FeO. This increasein well-developed reaction assemblageof titanomag- the availability of divalent speciesmay explain the netite + perovskite + Mn-ilmenite. This may sim- observedlow-Fe2O3 content of the high-Cr2O3Type ply reflect the evolution of the late-stagekimberlitic IV and V ilmenites. It should be realized that any fluid in time and P-T-X space. We suggest that such model invoked to explain the increasein MgO kimberlite groundmassilmenites sensu stricto can in the kimberlitic liquid at Elliott County should be distinguished from small MgO-enriched frag- also explain the apparently ubiquitious MgO-en- mentsof disaggregatedmegacrysts by their distinct- richment observed in kimberlite in other localities. ly higher Cr2O3content (i.e., generallygreater than Temperatureestimates for various pairs of Stage 1.00wt.% CrzOr). III groundmassminerals suggestequilibration of the In the Elliott County kimberlites, the groundmass groundmassassemblage at temperatures as low as phases orthopyroxene (i.e., the low-T, high-mg 965"C. The Fe-Mg equilibriabetween groundmass Group II orthopyroxenes of Garrison and Taylor, olivine (Foas.o)and an ilmenite inclusion, using the 1980),olivine (Mg/(Mg+Fe) : 0.88-0.90),and high- geothermometerof Andersen and Lindsley (1979), Cr2O3Type IV ilmenite are generally more magne- suggestsequilibration at 965'C (at P : 50 kbar). sian than the more fractionated members of the This is consistentwith the temperature estimatesof correspondingmegacryst phases (i.e., those with 970'-1020'C reported by Garrison and Taylor the lowest Me/(Mg+Fe) ratios). Both ilmenite and (1980)for the low-?, high-mg (Group II) orthopy- olivine megacrystsexhibit MgO-enriched rims simi- roxenes in the Elliott County kimberlite. Using the lar in compositionto the groundmassphases. Both olivine-spinel geothermometer of Fabries (1979), groundmassilmenite and orthopyroxene in the El- the Fe-Mg equilibria between the Type l chromites liott County groundmassassemblage are higher in and groundmassolivines (Fosz.sto Foeq.6),repre- Cr2O3than megacryst ilmenite and orthopyroxene senting the entire range of observed groundmass suggestingthat the MgO-enrichedliquid from which olivine compositions, suggestequilibration at tem- the groundmass assemblagecrystallized was also peraturesof 990"-1085'C; this range is consistent enrichedin Cr2O3.These chemical systematics indi- with both of the previous temperature estimatesfor cate that the crystallization of groundmassphases groundmassphases. and the back-reactionof megacryst phases (i.e., If the Stage III groundmassmineral assemblage StageIII) was preceded by an effective increase in indeed crystallized from the MgO-enriched kimber- the MgOiFeO ratio (and CrzOr) in the kimberlitic litic liquid at 965"-1085oC, it is possibleto place a liquid (i.e., StageII). constraint on the depth at which this event oc- Garrison and Taylor (1980) have suggestedthat curred. From the position of the univariant solidus the Stage II increase in MgO/FeO ratio of the for kimberlite, as outlined by Eggler and Wendlandt kimberlitic liquid was related to an increase in /6, (1979),it is clear that StageIII groundmasscrystal- and/or assimilation of carbonated peridotite. They lization probably occurred at pressuresnot greater suggested that the assimilation of a carbonated than 25-35 kbar. This suggests that Stage III peridotite such as dolomite or magnesiteperidotite groundmasscrystallization occurred at depths shal- would have the effect of adding the gaseousspecies lower than about 100km, 50 km shallower than the COz to the liquid. In the absence of graphite or depth at which Stage I megacryst crystallization diamond(Eggler et al.,1980), this CO2would be an occurred. Assuming that the Group II low-? ortho- oxidizing agent and would subsequently increase pyroxenes at Elliott County were in equilibrium the /6, and the Fe3+/Fe2+ratio of the liquid. This with garnet, Garrison and Taylor estimatedthat the increasein Fe3* would have the effect of increasing Group II orthopyroxenes crystallized at pressures the MgO/FeO ratio. This mechanism seemsincon- of 47-50 kbar using the geobarometer of Wood sistent with the observation that the high-Cr2O3 (1974).Since garnet does not belong to the StageIII TypeIV and V ilmenites have Fe2O3contents which groundmassmineral assemblage,we feel that the are generally lower than the Fe2O3content of the geobarometerof Wood (1974)is not applicable and more fractionated(i.e., lowest Mg/(Mg+Fe) ratio) that the above maximum pressureestimate of 25-35 AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE

kbar for the StageIII groundmasscrystallization is T-X space.Thus, it appearsthat the metasomatic more reasonable. reaction of ilmenite to form the assemblageperov- The presence of rutile in kimberlite has been skite + titanomagnetite * Mn-ilmenite may be a attributed to the breakdown of armalcolite natural consequenceof the evolution of the late- ((Fe,Mg)Ti2O5)when it is found in intergrowths stagekimberlitic fluid in time and P-T-X space. containing approximately equal amounts of rutile The discrete grains of perovskite and titanomag- and ilmenite (Haggerty, 1975).The rutile-ilmenite netite fouhd throughout the groundmass of the intergrowth from Elliott County contains less than Elliott County kimberlite could be: (1) primary 2Voilmenite lamellae and cannot be explained by phasesprecipitated from the late-stage kimberlitic this mechanism.This intergrowthis best explained fluid, (2) disaggregatedfragments of reaction zone as the result of exsolution of ilmenite from a rutile assemblages,and/or (3) the completelyreacted rem- that contained a small amount of FeO and MgO in nants of very small groundmassilmenite grains. solution. Haggerty (1976)and Mitchell (1973 1979) Thesediscrete groundmass grains are composition- have noted similar intergrowths in kimberlites in ally similar to the phases within the reaction zone South Africa and Canada: as in the case of the assemblagesuggesting that, regardlessof mode of Elliott County kimberlite, these intergrowths were origin, both textural types probably equilibrated also rimmed by high-MgO ilmenite. This ilmenite with the same fluid. rim probably formed at the time of StageIII ground- Since Mn-ilmenite is present in the reaction as- mass crystallization as the rutile reacted with the semblage around ilmenite, the activity of Mn*2 MgO-enrichedliquid. The rutile host could be (1) a must have also increasedin the late-stagekimberlit- xenocryst, or (2) a primary phaseprecipitated from ic fluid. Wyatt (1979)suggested that Mn-rich ilmen- the kimberlitic liquid. ites can form by interaction of kimberlite with local meteoric waters. In a study of the Premier kimber- Formation of the late-stagereaction assemblage lite, Wyatt (1979) found the MnTiO3 content of At Elliott County, the final recorded stage of ilmenites to increase toward the grain boundaries; kimberlite evolution (Stage IV) is the reaction of these zoned grains were found only in the margins ilmenite to form the assemblageperovskite + titan- of the kimberlite pipe and small kimberlite dikes. omagnetite * Mn-ilmenite. This reaction suggests Wyatt concluded that these Mn-ilmenites formed the presenceof a late-stageCaO-enriched fluid; the during post-magmatic interaction of the Premier presenceof Mn-ilmenite also suggestsan increase kimberlite with local meteoric waters, the Mn in the availability of MnO in the fluid. The late-stage source being the local country rock. The intimate reaction of ilmenite with a CaO-enriched fluid to textural relationship of the Elliott County Type VI form perovskite and titanomagnetite may occur Mn-ilmenites to the minerals of the reaction zone according to the following reactions: assemblageindicates a genetic relationship of Mn- 2CaCO3= 2Caf,"]6+ Oz + CO2 ilmenite to the reaction zone perovskite and titanomagnetite. The ubiquitious occurrence of the per- 2Ca*2 + 02 + [3(Fe,Mg)TiO3* Fe2O3]."- ovskite * titanomagnetite reaction assemblagein kimberlites from other localities 2CaTiO3+ [(Fe,Mg)2TiO4+ (Fe,Mg)Fe2Oa].. argues against a country rock controlled metasomatic event as a Thesereactions explain the major elementsystem- mechanismfor the formation of the Mn-ilmenite. as atics of the phasesinvolved; minor elementssuch well as perovskite and titanomagnetite. as Al2O3,Ct2O3, and MnO are easily accommodat- ed into the reaction scheme. The availability of Acknowledgments Ca*2 is controlled by the breakdown of calcite. This study was supportedin part by NSF Grant EAR-7906318, Garrison (1981)has suggestedthat the breakdown a University of Tennessee Faculty Research Grant, and the of earlier-formed groundmasscalcite may have in- Department of Geological Sciencesdiscretionary fund. Critical reviews by D. H. Eggler and N. Z. Boctor are gratefully ac- creasedthe availability of Ca*2 in the kimberlitic knowledged. fluid; textural evidence suggests that the breakdown of groundmasscalcite may have been exten- References sive in the Elliott County kimberlite. The early- Albee, A. L. and Ray, L. (1970)Correction groundmass factors for electron formed calcite probably becameunsta- probe microanalysis of silicates, oxides, carbonates, phos- ble as the late-stagekimberlitic fluid evolved in p- phates,and sulfates. Analytical Chemistry, 42, l4lg-1414. AGEE ET AL.: OXIDE MINERALS IN KIMBERLITE 4l

Andersen,D. J. and Lindsley, D. H. (1979)The olivine-ilmenite from Frank Smith Mine, near Rarkly West, South Africa. thermometer.Proceedings of the Lunar and Planetary Science Transactions of the Geological Society of South Africa,76, Conferencel0th. 493-507. 195-200. Bence, A. E. and Albee, A. L. (1968) Empirical correction Garrison,J. R., Jr. and Taylor, L. A. (1980)Megacrysts and factors for the electron microanalysis of silicates and oxides. xenolithsin kimberlite, Elliott County, Kentucky: A mantle Joumal of Geology, 76,382403. samplefrom beneaththe Permian Appalachian Plateau. Con- Bishop, F. C. (1980)The distribution of Fe2* and Mg between tributionsto Mineralogyand Petrology,75,27-42. coexistingilmenite and pyroxene with applicationsto geother- Garrison,J. R., Jr. and Taylor, L. A. (1981)Petrogenesis of mometry. American Journal of Science, 28O,46-:77. pyroxene-oxide intergrowths from kimberlite and cumulate Boctor, N. Z. and Boyd, F. R. 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Their geology, petrology, and geochemistry, p. 16l-171. Wyatt, B. A. (1979)Mangoan ilmenite from the Premier kimber- American GeophysicalUnion, Washington, D. C. lite. Geological Society of South Africa, Geokongres, 456- Pasteris, J. D. (1980) The significanceof groundmassilmenite 4ffi. and megacryst ilmenite in kimberlites. Contributions to Miner- Zafiman,R. E., Brock, M. R., Heyl, A. V., and Thomas,H. H. alogyand Petrology,75,315-325. (1972)K-Ar and Rb-Sr ages of some alkalic intrusive rocks Wood, B. J. (1974) The solubility of alumina in orthopyroxene from central and eastern United States. American Journal of coexisting with garnet. Contributions to Mineralogy and pe- Science.265.8E4-870. trology, 46, l-15. Wyatt, B. A. (1977)The melting and crystallization behavior of a natural clinopyroxene-ilmeniteintergrowth. Contributions to Manuscript received,August 12, 19E0; Mineralogy and Petrology, 61, l-9. acceptedfor publication,August 27, 1981.