Pyroxene-Ilmenite Intergrowths in Garnet Pyroxenite Xenoliths from A

American Mineralogist, Volume 63, pages 258-265, 1978

Pyroxene-ilmeniteintergrowths in garnetpyroxenite xenoliths from a New York kimberliteand Arizona latites

DaNIEI-J. ScHur-2n,1Hnnwent HBI-usre.gnr Department of Geological Sciences Queen' s Uniu e rs ity, Kings ton, Ont ario

lNo Rospnr M. Ce,ssrr Department of the Earth Sciences SUNY, Brockport, New York 14420

Abstract

Lamellar pyroxene-ilmeniteintergrowths in garnet pyroxenitexenoliths from a kimberlite in lthaca, New York, and from Tertiary latitesnear Prescott,Arizona, may representa link between exsolution intergrowths in pyroxenes from layered basic intrusions and intergrowths in pyroxene xenocrysts from kimberlites presently interpreted to result from eutectic crystallization.

Introduction vored (see referencesabove) for four reasons:(l) textural similaritiesto alloys formed by eutecticcrys- Nodules of pyroxene-ilmenite intergrowths from tallization (Boyd, 197l); (2) restricted compositional kimberlites have received much attention because range of the intergrown phases;(3) an apparent low many investigatorsbelieve them to be unique to these solubility of TiO, in pyroxenes at high pressures rocks. They have also been thought to be possible (Akella and Boyd, 1973);and (4) experimentalpro- indicators of the depth of origin of kimberlites.Such duction of texturally similar intergrowthsby eutectic intergrowthsare best known from African pipes (for crystallization(Wyatt et al., 1975;Wyatt, 1977). reviews, see Gurney et al., 1973; Boyd and Nixon, Natural terrestrialpyroxene-ilmenite intergrowths 1973), but have also been described from the Mir are not restrictedto kimberlites,but are alsocommon pipe in Yakutia (Ilupin et al., 1973),and two North in pyroxenesfrom cumulatesin layered basic intru- American localities: the Stockdale pipe, Kansas sions(Morse, 1969:Deer and Abbott, 1965;Haselton (Brookins, 1971;Gurney et al., 1973;McCallister el and Nash, 1975).As noted by Gurney et al. (1973), al., 1975), and kimberlites in the southern Laramie theseintergrowths have a strong textural resemblance Range, Wyoming (McCallum et al., 1975;Smith el to the kimberlitic intergrowths, yet their origin by al.,1976). subsolidusexsolution is generallyaccepted. The origin of theseintergrowths is still uncertain. In this paper we intend to demonstratethat pyrox- The proposedmechanisms of formation are eutectoid ene-ilmenite intergrowths in kimberlites may also transformation of garnet (Ringwood and Lovering, originate by exsolution.Two new intergrowth occur- 1970),exsolution from an ilmenite-structuredpyrox- rencesare described,one from a kimberlite dike in ene(Dawson and Reid, 1970),eutectic crystallization Ithaca, New York, and the other from Tertiary latites (Williams, 19321'MacGregor and Wittkop, 1970; near Prescott,Arizona. As in other kimberlitic inter- Gurney et al., 1973;Boyd and Nixon, 1973;Smith er growths, textural evidencefor an origin by exsolution al., 1976;Wyatt, 1977'and others),and cotecticcrys- is ambiguous in the Ithaca sample.However, unlike tallization (Frick, 1973). Although the intergrowth most kimberlitic occurrences,the intergrowth-bear- texturesare compatiblewith all of thesemechanisms, ing clinopyroxenefrom Ithaca is not a xenocryst,but an origin by eutectic crystallization is currently fa- occursas grains in a garnetpyroxenite xenolith which ' Present address: Department of Geological Sciences,Uni- compositionally resemblesgarnet pyroxenite xeno- versity of Texas at Dallas, Richardson.Texas 75080. liths from the Arizona latites. Ilmenite lamellae in 0003-0o4x/78/0304_0258$02.00 258 SCHULZE ET AL.,, PYROXENE-ILMENITE INTERGROWTHS

clino- and orthopyroxenesof the Arizona xenoliths been describedby Martens (1924), Sheldon (1927), resemblethose found in pyroxenesfrom layered in- and Foster (1970). The host rocks and a variety of trusions and can be shown to have originated by xenocrysts,including garnets,clinopyroxenes, chro- exsolution.From the compositionalsimilarity of the mites,and ilmeniteswere studiedby Foster (1970). Ithaca and Arizona xenoliths we propose that an Foster and Reitan (1972) concluded that the host exsolution origin of the intergrowth from the Ithaca rocks are kimberliteson the basisof textures,miner- kimberliteis a distinctpossibility. alogy, and chemical composition. Ultramafic xenoliths have not been reported previously, though Garnet clinopyroxenite from lthaca, New York Foster (1970) mentionedthe rare occurrenceof a A small garnet clinopyroxenitexenolith containing dark-greendiopside attached to garnets. clinopyroxene-ilmeniteintergrowths was collectedin The intergrowth-bearing garnet clinopyroxenite October 1976 from a 3-foot wide kimberlite dike occurs in a kimberlite sample containing several outcroppingin the CascadillaGorge on the campus smaller phlogopite-bearingand garnet-bearingxeno- of Cornell University, Ithaca, New York. Eighty-two liths as well as inclusionsof a chrome-richspinel with such dikes and severalsmall diatremesare known diopside. The ovoid xenolith has the approximate from 24 localitiesin the central Finger Lakes region, dimensionsof 1.5 X 0.75 X 1.0cm, and consistsof a New York. They are thought to be of Late Jurassicor relatively large almandine-rich garnet with several Early Cretaceousage, and their geologic setting has equantclinopyroxene grains approximately 3 mm in diameter. Chemical analysesof the mineral phases are presentedin Table l. The clinopyroxenecontains Table I Mineral analysesfrom Ithacagarnet clinopyroxenite fine discontinuousilmenite lamellae (Fig. la) which (62-5\ have partly altered to sphene. Grains of ilmenite which differ in composition AverageL234567 from the lamellae occur interstitially and as in- of n n=I4 n=l n=l n=4 n=5 n=7 n=5 clusionsin the garnet. The garnet also has small inclusions of clinopyroxenecontaining somewhat tta2OX 0.1 0.3 0.4 0.4 1.3 r.b 2.3 more TiO, than the clinopyroxenehost to the ilme- M90 9.2 2.2 L.7 5.6 14.0 12.9 L2.2 nite lamellae(Table l). Minor granulargarnet occurs Al203 zL.a 0.3 0.5 0.8 2.2 4.4 L2.9 in the lamellae-bearingclinopyroxene, and (?) secon- sio2 39.0 0.2 0.5 u.r Jz.) 5L.2 4L.2 dary amphiboleis also presentin the xenolith. K2o 0.0 0,0 0.o 0.o 0.0 o.o 1.8 CaO 5-7 0.0 0.0 0.1 22.r 2L.O II.5 Garnet pyroxenitesfrom Chino Valley, PauldenQuad- r r TiO2 o.I 41.3 37.9 90.1 v-z 0.5 2-6 rangle, Arizona Cr203 0.1 0.3 0.3 0.3 0.1 0.1 0.1 Tertiary latites of the Sullivan Buttes Latite de- ltnO 0.6 0.0 0.o o-7 0.I v.a u.r scribed by Krieger (1965) and Krieger et al. (1971) FeO* 24.2 54.2 57.9 45.5 7.9 8.3 L2.6 contain a rich variety of ultramafic xenoliths including metamorphic eclogites,garnet clinopyroxenites, flflf'""= roo.e es.s se.2 roo.o roo.4 roo.2 e7.3 garnet websterites, phlogopite-bearing websterite, and garnet-amphibolerocks. Most of the xenoliths X the effective detectability linit of Na2O is belongingto the garnet pyroxenitefamily come from apploxitrBtely 0.5 wt c two of the latite units sampled.The xenoliths in this * electron microprobe analyses, total Fe reported group have angular to rounded shapesand range in as FeO diameter from a few mm to 5 cm. They are very fresh ** TiO2 values are not accurate to more than 5 relative wt t and appear to have reactedslightly or not at all with garnet pyroxenite I. garnet the host magma. Of more than l5 specimensexamined in thin section,4 garnetclinopy- 2. Iarge interstitial granular ilmenite in garnet roxenitesand 2 garnet websteriteswere found to con- 3. tiny ilmenite as rods andl dots in garnet tain pyroxeneswith "ilmenite" intergrowths. 4. il-nenite lamellae in clinopyroxene 5. clinopyroxene host to ilnenite lanellae Ga rneI clino py rox enit es 6. clinopyroxene inclusion in garnet These xenoliths consist of relatively coarse clino- 7. anrphibole (chlorine detected but not analysed) pyroxeneporphyroclasts (up to 2 cm in diameter)set 260 SCHULZE ET AL.: PYROXENE.ILMENITE INTERGROWTHS

Fig. I (a)llmenitelamellae(highlyreflective)intergrownwithclinopyroxene,fromlthaca,sample62-5.Darkirregularspotsareholes in the section Reflectedlight. (b) "llmenite" lamellaein clinopyroxene,showing two phasesdiffering in reflectivity Sample l-2, Arizona garnet clinopyroxenite.Reflected light. in a matrix of fine-grainedgranular clinopyroxene analysesshow that they are Fe-Ti oxides with TiO, and garnet. Most samplescontain secondaryamphi- contentsranging from lessthan 5 to slightly above40 boles. Clinopyroxene porphyroclasts with lamellar weight percent. Under reflected light, most of the intergrowths of one or all of the minerals garnet, lamellae are seento consistof two phases(Fig. lb), "ilmenite," and orthopyroxenehave been recognized. presumablyone Fe-rich, the other Ti-rich, that prob- Due to their narrow width, we have not beenable to ably originated from the breakdown of an originally obtain good chemical analysesof the "ilmenite" la- homogeneousFe-Ti oxide. Two-phaselamellae were mellae. but calculations from silicate-contaminated also noted from kimberlite xenocrystsby MacGregor and Wittkop (1970)and Mitchell et al. (1973).Apart from occasional fine orthopyroxene exsolution la- garnet Table 2. Mineral analysesfromArizona clinopyroxenite mellae, the fine-grainedgranular clinopyroxenesare lamellae-free.

AverageI234 5 6 Four types of intergrowth relationshipscan be dis- of n n=8 n=10 n=17 n=14 n=16 n=16 tinguished(for chemicalanalyses of different phases, seeTable 2): Na2OX o.2 0.2 1.3 nq ( l ) Clinopyroxenewith planar lamellaeor bead- M90 14.6 15.r 9.3 9.2 L4.8 like stringsof garnetand fine lamellaeof "ilmenite" A12O3 2.7 1.9 2!.A 2L.7 3.0 r.7 parallelto (100)(determined on the universalstage).

si02 53.3 53.7 39.2 39.1 52.7 f{.1 Although most clinopyroxenegrains show only one Kzo 0.o 0.0 0.0 0.0 0.0 0.0 set of "ilmenite" lamellae, a second set was recog- CaO 2I.3 22.L 5.6 5 .7 20.3 0.8 nized in rare casesunder reflectedlight. Some grains Ti02 o.2 0.t 0.1 0.1 0.2 0.1 show, in addition, very thin orthopyroxene lamellae which are also parallel to (100). Garnet lamellaeare Cr2O3 o.2 0.2 o.2 0.2 0.2 many timesthicker than the "ilmenite" lamellae(Fig. llnO 0.1 0.1 0.8 0.8 0.1 o.2 2b) which, when viewed perpendicularto the plane of FeOt 6.3 s.5 23.6 22.9 7.2 the thin section,have a width of up to l0 microns. Anhydlous Most of the lamellaeconsist of discontinuousplates, 1oo,4 r oo- 1 roo.B 99.9 99.8 roo. l totat some of which are elongated(Figs. 2a, 2c). As most sectionsare oblique to the plane of the X the effective aletectability lindt of Nazo is lamellae,the proportion of lamellar"ilmenite" can approxinately 0.5 wt t easily be over-estimated.From sectionsnormal to the * electron microprobe analyses, total Fe lepolted as FeO lamellae it can be seen. however. that the modal

1. la.mellae-free clinopyroxene content of the intergrown phaserarely exceeds5 per-

2. Ianellae-bearing clLnopyroxene cent. 3. granular garnet Host pyroxenesand lamellaehave been deformed

4. lanella! garnet together, as seen by bent lamellae, undulatory ex- in host grains.All stages 5. clinopyroxene intelgrordn with orthopyroxene tinction,and kinkbands the between undulatory extinction and complete recrys- 6. orthopyroxene intergrorn with clinopyroxene tallization of the clinopyroxene hosts can be ob- SCHULZE ET AL,: PYROXENE.ILMENITE INTERGROWTHS 261

Fig. 2 (a) Sample l-2, showing granular nature of the xenolith. Clinopyroxeneis presentwith no exsolution lamellae,"ilmenite" lamellaeonly, and both "ilmenite" and garnet lamellae.Granular garnet,oxides and minor amphibole are also present PIanepolarized transmittedlight (b) Clinopyroxeneporphyroclast with "ilmenite" lamellae(narrow and almost vertical)surrounded by granulargarnet and lamellae-freeclinopyroxene. From clinopyroxene-richportion of Arizona garnetwebsterite 54-17. Transmitted light, crossednicols (c) Bent exsolutionlamellae of "ilmenite" (black parallelograms)and garnet (grey to black, dependrngon thickness)in orthopyroxene porphyroclast from orthopyroxene-richportion of Arizona garnet websterite54-17 Transmitted light, crossednicols (d) Lamellar intergrowth of orthopyroxeneand clinopyroxenefrom Arizona xenolith 50-l Orthopyroxeneis lighter than clinopyroxene,and shows fracturing approximatelyperpendicular to length of the lamellae.Transmitted light, crossednicols served.In the latter case,remnants of garnetlamellae phibole.In one section(not yet analyzed),numerous are the only indication that larger clinopyroxenes grains of this type of pyroxene occur between rem- existedpreviously. Some xenoliths show evidenceof nants of garnet exsolutionlamellae (Fig. 3), suggest- complete recrystallizationof all minerals. Chemical ing that these grains recrystallizedfrom an earlier compositions of host grains and recrystallizedcli- large clinopyroxenewhich had exsolvedgarnet. Ex- nopyroxenesare compared in Table 2. solution of orthopyroxeneoccurred after the recrys- (2) Clinopyroxene with garnet lamellae only. Ex- tallization of this earlier clinopyroxene.Locally, the cept for the absence of "ilmenite" lamellae, host present intergrowth has recrystallized,in turn, to grains and garnet lamellaeresemble Type I in terms form a fine-grained mosaic of clino- and ortho- of chemical composition and all aspectsof deforma- pyroxene. A rough reconstruction of the chemical tion and recrystallization. composition, using the composition of the present (3) Clinopyroxene with "ilmenite" lamellaeonly. intergrowth (Table 2) and a clino- to orthopyroxene Thesegrains are smallerthan Types I and2, but may ratio of 50:50 (basedon 27 120-secelectron micro- also be strained and partly recrystallized.Lamellae probe traversesapproximately perpendicularto the are generally confined to the centers of grains and lamellae),indicates that the CaO content of the ear- resemble in all aspectsthe "ilmenite" lamellae of lier phasewas approximately 10.8weight percentaf- Type L Under transmitted light some obliquely-cut ter garnet exsolution. Considering that the exsolved grains give the impressionthat two sets of lamellae garnet contains about 5 weight percent CaO (Table are common (Fig. 2a). However, all such grains 2), this suggeststhat the CaO content of the original which wereexamined under the universalstage and in phasemust have been somewhatless than 10 weight reflected light revealed only one set. percent. (4) Clinopyroxene with coarse orthopyroxene lamellae(Fig. 2d). Clinopyroxenegrains with up to 50 Garnet websterites percent exsolvedorthopyroxene lamellaeare smaller Garnet websteriteshave textures similar to thoseof than Types I and2. They occur in separatedomains the garnet clinopyroxenitesand contain clinopyrox- interspersedwith granular garnet and secondaryam- eneswith all intersrowth tvnes describedfrom them. 262 SCHULZE ET AL : PYROXENE.ILMEN|TE INTERGROWTHS

nite" lamellaehave not been recognized.The different pyroxenesoccur in domains which appear to occupy the former positions of coarserporphyro- clasts.

Textural history and origin of lamellae in Arizona samples The subsolidushistory outlined on Figure 3 sug- gests that the pyroxene-"ilmenite" intergrowths formed by exsolution.All calcicclinopyroxene porphyroclasts contain garnet with or without "ilmenite" lamellae,but a number of smallerclinopyrox- ene grains contain "ilmenite" lamellae only, indicating that garnet exsolutionwas followed by partial recrystallizationof the pyroxenebefore "ilmenite" exsolution commenced. These recrystallized clinopyroxenegrains exsolved"ilmenite" only. No orthopyroxene-"ilmenite"intergrowths lacking garnet lamellae were observed, suggestingthat ortho- pyroxenedid not recrystallizeprior to exsolutionof Fig 3. Schematicdrawing of textural evolution of pyroxenesin "ilmenite." Suchbehavior conforms with commonly garnet websterite from Arizona. (Cluster of grains 2-3 mm in observed pyroxene deformation textures indicating diameter.) that orthopyroxenehas a greaterresistance to recrys- A Three coexistingaluminous pyroxenes: (l) orthopyroxene; tallizationthan clinopyroxene. (2) calcic clinopyroxene;(3) subcalcicclinopyroxene. "ilmenite" B Exsolutionof lamellargarnet in all pyroxenes Following exsolution,both ortho- and C Furtherexsolution and partial recrystallization clinopyroxene were intensely deformed and recrys- Orthopyroxene:continuation of garnet exsolution, formation of tallized, leaving only remnants of porphyroclasts. "ilmenite" lamellaeparallel to garnetlamellae Planeoi drawing The "ilmenite" probably formed separategrains and is perpendicularto lamellae Lamellaebisect prismatic cleavage. most of the garnet lamellaetransformed into granu- Calcic clinopyroxenes:continuation of garnet exsolution,partial garnet recrystallizationfollowed by "ilmenite" exsolution Plane oi lar at this stage.Secondary amphibole formed drawing somewhat oblique 1o lamellae in original last. The subcalcicclinopyroxene also exsolvedgar- clinopyroxene,but highly oblique to blade-likelamellae in net first, was deformed, recrystallized,and ortho- recrystallizedgrains pyroxeneexsolved from the recrystallizedgrains. As Subcalcic clinopyroxene: continuation of garnet exsolution, no remnantsof the original subcalcicclinopyroxene recrystallizationof pyroxene betweengarnet lamellaefollowed by formation of clino- and orthopyroxene intergrowth. Note have beenobserved, it is impossibleto definitely de- cross fractures in orthopyroxene. Granular opaques may be termine whether "ilmenite" exsolution ever took recrystallizedexsolution lamellae. place in the subcalcicpyroxene. However, granular D. Deformation and recrystallization ilmenite in the matrix may be the result of recrys- Orthopyroxene: recrystallization into granular orthopyroxene, tallizationof lamellar"ilmenite." garnet, and "ilmenite " garnet "ilmenite" lamellae, Clinopyroxene: bending of and Discussion recrystalliz-ationinto granular clinopyroxene, garnet, and "ilmenite" Considering their derivation from different host Subcalcicclinopyroxene: recrystallization of ortho-clinopyroxene rocks at widely separatedlocalities, the ilmenite-bear- intergrowth into fine-grainedmosaic of ortho- and clinopy- ing clinopyroxenesand accompanyinggarnets in the roxene Some garnet lamellae survive in recrystallizedmosaic. garnet clinopyroxenitesfrom lthaca and Arizona havesurprisingly similar chemical compositions (Ta- The chemical compositions of thesephases are also bles l-3). The Arizona sampleshave strong textural reasonablysimilar (Table 3). Additional phasesare resemblancesto garnet pyroxenitexenoliths from ba- deformed orthopyroxene porphyroclasts (Fig. 2c) salts in Hawaii (Beesonand Jackson,1970) and at with garnet and "ilmenite" lamellaeparallel to (100) Dish Hill, California (Shervaiset al., 1973),but they aswell asrecrystallized orthopyroxene grains without differ from theserocks by the presenceof "ilmenite" lamellae(Table 3). Orthopyroxenewith only "ilme- lamellae, an inferred additional original pyroxene SCH ULZE ET AL.: PYROXENE-ILMENITE INTERGROWTHS 263

Table3. Mineralanalyses from Arizonagarnet websterite (54-17)

Average I 2 3 45 67 I 9roLl ofn n=6 n=I0 n=14 n=5 n=6 n=9 n=6 n=10 n=6 n=9 n=9

t'la2OX 0.5 u-f, 0.4 1.3 1.3 o.2 2.7 1.1 0.5 !t9o 27.8 27.A ro.3 ln a r5.5 15.I 10. 3 r0.4 15.7 15.0 30.3 A1203 1.1 r.2 2r.8 2I.9 1.6 2.O 2r.a 2L.8 11.5 4.0 t.8 sio2 54.4 54.3 39.4 39.5 54.3

FeO* r). / 16.2 22.2 22.L 5.2 ).q 22.L 22.L 9.4 6.5 11.4

Anhydrous 100.0 100 .6 100.4 101.O 100,7 IO0.2 100.5 100.3 96.6 99.9 101.3 total

x the effective detectabitity llmit of Na2o is approximately 0.5 wt * * electron rnicroprobe analyses, total Fe reported as Feo 1. recrystallized lamellae-free orttropyroxene 2. lamellae-bearing orthopyroxene 3. galnet lanellae in orthopyroxene 4. granular garnet around orthoPyroxene 5. lamellae-bearing clinopyroxene 6. recrystallized la.nellae-free cLj.nopyroxene 7. girnet lanellae in clinopyroxene 8. granula! garnet aroutd clinopyroxene 9. anphibole 10. clinopyroxene intergrown wittt orthoPyroxene 11. orthopyroxene intergrd'rn witlt clinopyroxene phase,and lower whole-rock Mg/(Mg * Fe) values. which have pyroxene-ilmenite ratios ranging from The intergrowth-bearingpyroxenes from the lth- 10:l to l:1 (Haggertyet a|.,1977).However, ignor- aca and Arizona samples are more calcic and slightly ing the differencesin scale,a definite textural similar- more Fe-rich than intergrowth-bearing kimberlite ity betweenkimberlitic and the intergrowths in this xenocrysts,which have a relativelyrestricted compo- study exists(see Plate 67A of Gurney et al., 1973). sitional range (Fig. 4). The ilmenite lamellae of the Haselton and Nash (1975) pointed out that inter- Ithaca clinopyroxene (Table l) are poorer in MgO growth textures which are ambiguous by themselves than lamellae from most previously-describedkim- can be properly interpreted only if the geological berlite xenocrysts,but contain more MgO than ilme- environment of formation is known. Therefore, it is nite intergrowths in orthopyroxenes from gabbros perhaps not surprising that "ilmenite" lamellae in (Haselton and Nash, 1975).As the MgO content of pyroxenesfrom gabbroic rocks are explainedby ex- the "ilmenite" lamellaeappears to be systematically solution (Morse, 1969;Haselton and Nash, 1975), related to that of the orthopyroxene (Haselton and becauseenough geological constraints exist to make Nash, 1975),we expectthat the lamellaeof the Ari- this explanation most plausible.On the other hand, zona sampleshave an MgO content similar to that of most intergrowth-bearing nodules from kimberlites the Ithaca sample. Aside from these differences,our are single crystalsof clino- or orthopyroxene whose pyroxene-"ilmenite" intergrowths are much smaller original mineral assemblageand textural history are than those found as nodulesfrom kimberlites,where unknown. Compelling evidencefor exsolution does they reach several centimeters in diameter. The not exist, leaving other modes of origin as viable weight percent ilmenite in our samplesis generally alternatives(see referencesin "Introduction"). The lower than for those from other kimberlite nodules. wide acceptance of an origin by eutectic recrys- 264 SCHULZE ET AL.: PYROXENE.ILMENITE INTERGROWTHE

Co Co

Mg Fe

Fig. 4. Lower portion ofa Ca-Mg-Fe ternary plot showing distribution ofgarnets, clinopyroxenes,and orthopyroxenes. Open triangle representsall garnets from 54-17 (lamellar and granular, associatedwith both opx and cpx); filled triangle representsgarnet from 62-5 and both lamellar and granular garnets from l-2; open square representsthe orthopyroxene in the lamellar opx-cpx intergrowth from 54-17; the filled square representsthe orthopyroxenes with and without lamellae from 54-17 and from the lamellar opx-cpx intergrowth from l-2; the open circle representsthe clinopyroxene from 62-5 (both inside the garnet and as host to ilmenite lamellae); the X representsthe lamellae-bearingclinopyroxene from l-2, and the cross representsthe lamellae-freeclinopyroxene from l-2; the open diamond represents both lamellae-free and lamellae-bearing clinopyroxene ftom 54-17;the filled circle representsthe clinopyroxene from the lamellar cpx- opx intergrowthsin l-2 and 54-17. The solid lines enclosethe fields of opx, cpx, and garnet from garnet pyroxenite xenoliths from Oahu, Hawaii (Beeson and Jackson, 1970) and Dish Hill, California (Shervais et al., 1973). The dotted line enclosesthe field of clinopyroxenes intergrown with ilmenite lamellae from kimberlites in Yakutia (Ilupin el al., 1973), South Africa (Boyd, l97l), and Wyoming (Smith et al., 1976). 62-5'.lthaca, New York garnet clinopyroxenite l-2'. Arizona garnet clinopyroxenite 54-17: Arizona sarnet websterite. tallization for kimberlitic intergrowths (see "In- MgO contentof the ilmeniteis systematicallyrelated troduction") has led to the impressionthat ex- to that of the orthopyroxene.The intergrowthsde- solution of ilmenite is restrictedto pyroxenesfrom scribedhere conform to this systematicdistribution, shallow intrusions, whereaseutectic crystallization and are intermediatebetween the moreiron-rich in- leadingto pyroxene-ilmeniteintergrowth is a high- tergrowthsfrom the Skaergaardintrusion and the pressurephenomenon. more magnesianpyroxene-ilmenite pairs from a lu- Although we have not yet obtained satisfactory nar breccia and kimberlites.Whereas estimates of P-Z estimates,our Arizona samplescertainly show equilibrationtemperatures based on pyroxenecom- that exsolutionof ilmeniteoccurs in pyroxenesfrom positionsyield values of I 150-l165'Cfor kimberlitic deep-seatedrocks. Lacking definite textural con- intergrowths(Smith et al., 1976),estimates based on straints,we cannotprove an exsolutionorigin of the the distributionof magnesiumbetween coexisting il- lamellaein the kimberlitexenolith from Ithaca.Such meniteand pyroxene(Anderson et al., 1972)are 930- an origin is neverthelesssuggested by the similarityin 950'C and720"Cfor the kimberliticand Skaergaard mineral compositionsof the Arizona and Ithaca intergrowths, respectively(Haselton and Nash, xenoliths. 1975).These latter temperaturesare consistently Exsolutionshould not be rejectedas a possible lower than expectedcrystallization temperatures of mechanismof formation of other kimberlitic inter- the pyroxenes,implying that the intergrowthsmay growths.As notedbefore, Haselton and Nash (1975) not be the result of eutecticcrystallization, but of found that in pyroxene-ilmeniteintergrowths the subsolidusequilibration. SCH ULZ E ET AL, : P YRO X ENE-I L M EN ITE I NTERGROWTH S 265

Acknowledgments intergrowthsfrom the moon and the Skaergaardintrusion. Financialsupport for this researchfrom NationalResearch Earth Planet.Sci. Lett , 26,28'l-291. pyroxene- Council Grant A8375 is gratefully acknowledged. We also thank Ilupin,I. P.,F. V. Kaminskiyand N. V. Troneva(1973) ilmenitegraphic inclusions Ian H. Campbell for stimulating discussionson pyroxenes.A from the Mir kimberlitepipe (yakutia) critical review by David Eggler helped to improve the manuscrrpt. and theirorigin. Int. Geol.Reu., 16,1298-1305. Krieger,M. H. (1965)Geology of the Prescottand pauldeneuad- References rangles,Arizona. U.S. Geol.Suru. Prof. pap.,467. -, S. C. Creaseyand R. F. Marvin (1971)Ages of some Akella,J. and R. F. Boyd(1973) Effect of pressureon the composi- Tertiary andesiticand latitic volcanic rocks in the prescott- tion of coexisting pyroxenes and garnet in the system CaSiOr- Jeromearea, north-central Arizona. (J.5.Geol. Suru. prof. pap., MgSiOs-FeSiOr-CaAlTi,Or. Carnegie Inst lI/ash. Year Book, 750-8,BtsT-160. 7) 5)1-\)6 MacGregor,I D. and R. W. Wittkopp (1970)Diopside-ilmenite Anderson,A. T., T. F. Braziunas,J. Jacobyand J. V. Smith ( 1972) xenolithsfrom the MonasteryMine, OrangeFree State,S A. Thermal and mechanical history of breccias 14306, 14063, Geol Soc. Am. Abstacts with Programs,2, ll3. 142'10,and 14321. Proc. Third Lunar Sci. Conf.,819-835. Martens,J. H. C. (1924)Igneous rocks of Ithacaand vicinity, New Beeson,M. H. and E. D. Jackson (1970) Origin of the garnet York. Geol.Soc. Am. Bull. 35.305-320. pyroxenite xenoliths at Salt Lake Crater, Oahu. Mineral. So: McCallister,R. H., H. O. A. Meyerand D. G. Brookins(1975) Am Spec. Publ., 3, 95-112. "Pyroxene"-ilmenitexenoliths from theStockdale pipe, Kansas: Boyd, F. R. (1971)Enstatite-ilmenite and diopside-ilmeniteinter- chemistry,crystallography, and origin. phys. Chem.Earth,9, growths from the Monastery Mine. Carnegie Inst. Vltash. year 287-293. Book.70. 134-138. McCallum,M. E.,C. B. Smith,L. K. Burns,D. H. 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Manuscript receiued, June 20, 1977; accepted Haselton, J. D. and W. P. Nash (1975) Ilmenite-orthopyroxene for publication, Nouember 29, 1977.