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Petrogenesis of Oxide Minerals in Kimberlite, Elliott County, Kentucky

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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, titanomagne- tite, perovskite,calcite, phologopite,vermiculite, pectolite, and septechlorite.The mega- cryst 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 perov- skite + 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 composi- tion 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 compositional- ly 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 "autometaso- matic" 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 associ- ated 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 extinc- tion to totally
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