Canadian Minerologist Vol. 25, pp. 9l-96 (1987)
LAMELLAEOF BADDELEYITEAND Fe-CI-SPINELlN ILMENITE FROM THE BASISTOPPENSILL, EAST GREENLAND
H. RICHARD NASLTIND Dewrtmentof Earth sr,iencr.,s',Darttnouth College, Honover, New Hampshire 03755, U.S.A.
ABSTRACT Mo*-c:16:baddel€yite, ilm€nit€, spinelle-Fe-Cr, orydation, r6duction,exsolution, Basistoppen (filon-couche de), Ilnenite inthe Gabbro Picrite Zone of the Basistoppen Gro€nlandoriental, gabbro tlol€itique. rill, Ea$t Greenland, contain; exsolution lamellae of bad- proportions &leyite and Fe-Cr*pinel. The relative of bad- INTRoDUCTIoN deleyite and Fe-Cr-spinel in isolated ilmenite grains indi- cate that the ilmenite was reduced during zubsolidus re-equilibration, although the sill in general was oxidized Baddeleyiteis a conlmon ascessoryphase in lunar during cooling. Calculatedpre-exsolutioncompositions for basalts (Agrell et al. 1970, Ramdohr & El-Goresy the ilrnenite contain up1o2.4Vo Cr2O3and up to 7100ppm 1970,Keil et al. 1971,Brown et al. 1971,Haggerty Zr. The calsulated Zr-content is higher than tlat of any 193) and in undersaturatedterrestrial igueousrocls previously reportd lgnar or terrctrid ilmenite. The (Widenfalk & Gorbatschst 1971, Keil & Fricker reduction-exsolutionof Fe-Cr*pinel is consideredthe result 1974,Fieremans & Ottenburgs 199, Raber & Hag- of the incorporatiou of Cr in the ilmenite dudng crystalli- gerty 1979, Clarke 1981, Scatena-Wachel& Jones zation at concentrations that exceeded limit the solubility 1984), and has been reported as a rare constituent of Cr in ilmenite at the temperatures of re-equitbration. The Zr content of the host ilnenite is well below the solu- in teltites (King 1966,Clarke & Wosinski 1967)and bility limit of Zr in ilmenite, indicating that oversaturation in shock_alteredsediments surrounding meteorite_ was not ttre driving force for the exsolution ofbaddeleyite. impact structures@l-Goresy 1965,Kleinmann 1969). The exsolution of baddeleyite in tle ilmenite may have Baddeleyiteis not a cotnmon constituent of subalka- partly compensatedfortle overallreduction of tle ilmenite line terrestrial basaltsor gabbros, but has beenprevi- resulting from the exsolution of Fe-Cr-spinel. ously reported as a late-stagemagmatic mineral in two gabbro dykes fromAxel Heiberg island (Keil & ilmsldls, Keywords: baddeleyite, Fe-Cr-spinel, oxidation, Fricker 1974)and in ultramafic cumulates from the reduction, exsolution, Basistoppensill, East Greenland, Rhum intrusive complex (Williams 198). Analyses tloleiitic gabbro. of lunar and terrestrial baddeleyite indicate that it is an important reservoir for the incompatible ele- Solvntarnn ments Zr, Hf, and U in igneous rocks (Lovering & Wark 1971, Frondel 1975, Williams 1978); as a L'ilmenite de la zone Gabbro Piqite du filon-couche de result, an understanding of the formation and dis- Basistoppen, au Groenland oriental, contient des lamelles tribution of baddeleyite in igneous rocks is impor- d'exsolution de baddeleyite et spinelle-Fe-Cr. Les propor- tant for modeling the chanetng concentrations of tions relatives de baddeleyite et spinelle-Fe-Cr dans des incompatible elements during tle evolution of grains d'iln6nite isol6 indiqueirt que I'ilm6nite fut r€duite magmas. au cours de la r€-€quilibration au dessousdu solidus, bien Althougb baddeleyiteis commonly associatedwith que le sill, en g€n€ral, soit oryd6 pendant le refroidisse- ilmenite previously pr6-exsolution pour in lunar rocks, it has not been ment. La composition calculde l'ilm6- phase nite contient jusqu'i 2.490 Cr2O3et jusqu'l 7100ppm Zr. reported as an exsolutiou in either lunar orter- Le contenu de Zr calcul€ est plus €lev6 que celui de restrial ilmenite. Cr-rich spinel has b€en reportd as n'importe quelle ilmdnite, lunaire ou terrestre. an exsolution phaseof ilnenite in lunar rocks (Hag- L'exsolution-r6duction du spinelle-Fe-Cr est considdrde eerty et aL ln0, Dence e/ sl. lyTl), in meteorites commer&ultat del'incorporation de Cr dansl'ilm€nitepen- (Buseck&Keil 1960 and in kimberlites (Danchin & dant la cristallisatiotr, e concentration exceda[t la limite d'Orey 1972,Haggerty& Tompkins 1984),but has de solubilitd de Cr dans l'ilm6nite aux tempdraturesde 16- not been previously reported in other terrestrial Quilibration. La teneur en Zr de I'iln€nite hdte est nette- occwrencesof ihnenite (Randobr &El4oresy 190, ment infdrieure d sa limite de solubilit6, d'orl l'on d6duit Danchin & d'Orey 1972). que la sursaturation n'est pas la force motrice de I'exsolu- tion de la baddeleyite. L'exsolution de la baddeleyite dans l'ilmenite peut avoir compensdpartiellement la rdduction GeoLocIcAL SerrNc. totale de I'ilmdnite r6sultant de I'exsolution du spineUe-Fe-Cr. The tholeiitic Basistoppensill, in East Greenland' (Iraduit par la R6daction) intruded the upper part of the Skaergaardcomplex 9t 92 TIIB CANADIAN MINERALOGIST shortly after the solidification of the main Skaer- exsolution phasein all of tle ilnenite grains contain- gaard magma@ouglas 1964,Naslund 1984a,b, c). ing exsolution lamellaeof baddeleyite.Thrce textuml The 60&m-thick sill is zonedi a magnesium-richini- relationships were observed:(A) isolated intercumu- tial magma differentiated to an iron-rish residual late ilmenite grains containing exsolution lamellae magma. Zone boundaries are based on changesin of baddeleyite and Fe-Cr-spinel, (B) baddeleyite the assemblageof sumulateminerals during diff€r€Nr- exsolutioulamellae within ilmenite lamellaein cumu- tiation. The Gabbro Picrite Zone (GPZ), of interest late Fe-Cr-spinel, and (C) baddeleyite exsolution here, is a discontinuous unit located at tle base of lamellae in intercumulate ilrnenite that precipitated the sill. The GPZ contains cumulate otvine adjacent to Fe-Cr-spinel (Fig. 1). The relative abun- (Forr-zs),orthopyroxene (Br",+t), and Fe-Cr-spinel, dancesof itnenite, baddeleyite and spinel in tlese and intercumulate plagioclase (Anrr-.), augite samples were calculated from the areas of these @na-roFg-11Woao+J,and ilnenite. Biotite may be phasesin back-scattered electron images and were a minor cumulate phase. corrected for mineral densities. The approximate relative abundancesof ilmenite:baddeleyite:spinelare ll0:1:11 in textureA, 580:l:3750in textnreB, and Tnxruner, RsI,erroNsgrps 45:l:130 in textureC. The observedgrain-size ofthe baddeleyite rangesup to l0 x 3 pm in texture A, up Baddeleyite is a common exsolution phase in to I x I pm in texture B, and up to 20 x 5 pm in tex- ilmenite from the GPZ, but is less common in ture C. Cumulate Fe-Cr-spinel in the GPZ is up to ilmenite from the other zonesof tle Basistoppensill. 1.0 mm across, and intercumulate ilmenite in the Fe-Cr-spinel is found eitler adjacent to or as an GPZ is up to 0.5 mm in maximum dimension.
FIc. l. Textural relationships in ilmenite-baddeley{te-spinel assemblagesfrom the Basistoppensill, traced from photo- graphs of back-scatteredelectron images. llmenite white, baddeleyiteblack, Fe-Cr-spinel diagonalty ruled; silicatas stippled. Al and A2. Isolated intercumulate ilmeNritewitl exsolution lamellae of baddeleyite and Fe-Cr-spinel (Al sample2l2-5, A2 sample 208-5). B. Isolated cumulate Fe-Cr-spinel witi exsolution lamellae of itnenite containing blebsoJ baddeleyite(sample 641-3). C. CumulateFe:Cr-spinel with adjacentintocumulate ilmenite containing exsolved baddeleyite (sample 208-13). ILMEMTE FROM TITE BASISTOPPEN SILL, GREENLAND 93
MII{ERAL CoMPosnIoNs t'rl t4l m 0,60Wttp6€t Owing to the small grain-size, baddeleyite in tex- 7JA, g2Jg 98.03 97.80 96.40 9r.9tt 92.83 04J0 - _ fm 1.30 't.81 0.93 1.72 1.95 1.86 turs A and B could only be analyzed qualitatively: lEA 2.62 0.E8 0,$0, 4.89 2.11 1.42 W O.gil 0,0r 0,08 0.0"02 0.00 0.u 0.18 energydispersion spectra indicate that it is alnost AEG' O.I8 0.01 0.0.0.1 0.31 0.5i1 pure Zra2. Baddeleyite in texture C was analyzed Cr2G 0.17 : 0.00 o.t6 0.13 itd 0.00 0.+t3 0,49 quantitatively on an electron microprobe witl rc 2.18 o.is r.io 0.00 o.ie z.ir 0.45 l*o 0.09 0.12 0.03 0.17 wavelengthdispersion spectrometersusing synthetic t@ 021 o.ri: 0.14 0.14 0.14 Total 09J5 roo"i roo.iz t(x'30 9820 99.38 10027 ZrSiOn and HfSiOa as standards for 7-r and Hf ,1 246 86 26 t 7n$ a2 - t05 48 -48 a7 Clable D. The Basistoppenbaddeleyite is similar in frl Beppendl Fl Khbseo Fabs & Hamslt lsrs) composition to that reported from lunar rocks, and tzl Em ffiubr! OmsGlgTS) [8]Apolo 12 (t
TAEIEACHEMNAL@MPO$NONOFII.,IiIENIIEAl\lDFE{R€PINE. 208-5lf 208-5 m [email protected] ll 2@13 rnl 212-8ll 212€ rfit 041€ ll et!13 mt Ortr€Fbhtperaeri' llc'2 47.86 6.08 48.84 5.r7 49.76 7.27 43.84 9.58 o2(B 0.88 19.9r O.74 17.24 0.34 9.95 r.20 11.74 v2(! 0.19 1.23 0.00 0.85 0.00 r.35 0.00 0.87 Ae@ 0,18 9.74 0.12 3.'t0 0.08 1.64 o.il 2.68 fte@ 8.,r1 3,r.81 7.U 38.06 5.52 42.06 3.48 U.62 FOr 40.10 30.39 40.93 35.35 4'1.40 37.5t1 41.88 39.49 lip 1.57 0.79 .t.40 0.55 1.23 0.47 2,74 1.08 ueo 0.87 028 0.87 O.72 1.20 0.s2 0.t4 0.05 Eral 99.15 99.40 100.50 100.84 99.53 t00.59 99.01 99.8'l CallnBF lbnulaudr ll/b 0.910t1 0.1700 0.9192 0.1371 0.9423 0.204t 0.9511 0.2694 Cr3+ b.0137 0.5686 0.0148 0.5fi7 0.0068 0.29t9 0.0242 0.3/t0 Vgr 0.001i9 0.@87 0.0@0 0.0145 0.0000 0.0400 0.0000 0.m0r A3+ 0.0054 o.1u2 0.0035 0.1392 0.0024 0.072, 0.0033 0.118r FoS+ 0.1551 0.8882 0.1420 r,0576 0.1017 t.tE7 0.0888 0.9718 Fs2+ 0.8519 1.1334 0.8585 1.0812 0.8718 1.1727 0.8878 1.&8 i&t2+ 0.0338 0.0249 0.0309 0,0184 0.0282 0.01.19 0.0800 0.0342 W2+ 0.0254 0.0139 0.0325 0.0389 0.0450 0.0178 0.00s3 0.0028 C€&u&ro xln 0.9191 0.s259 0.94tr 0.9849 ,Cusp 0.4098 0.294s 0.3fi'l r 0,4447 ldrF. 8i0 C. 74 C. @c. &t0 c. bg(@l -1/10 -16.0 -17.7 -17:7 lexlIo - A C B ' Cao,8IO2,z@,NEB<0.10%:analts€saoalre@osdalloart3 nncre|oboMrB. t Fe2+r'FsSrcahulabd ftom silolddomsfyi- brdjrl ttPoo aro o.CanEd h fie M. X flrnardXuspdsr$omsr(rg8t);bmp. ad bg(@) nqnqrroootSper@rE Lh.lslot(lS1), 94 TIIE CANADIAN MINBRALOCIST ao2
OXIDATION
MT,CHR HM no n2o,3 Flo. 2. .RO-RO2-IrQ triangular plot of ilneldte, baddele,yite,and Fe-Cr-spinel com- positions from the Basistopp€nsi[. End-membercompositions are itrdicated: BAD baddele,yite,RUT rutile, II;[4 ilm€rdte, USP ulvdspinel, MT magnetite,CIIR chro- mite, and HM hematite. Dotted lines indicate solid-solution series;solid lines con- nect co€xisting phases.The diagram is plotted in cation percent. The upper left insert showsthe effect of oxidation-exsolution on ilmenite and spinel. The upper right insert shorpsthe effect of reduction-exsolution on ilmenite. texture A is approximately 1l:1, indicating that the (Fe0.A{n0rTiO) containing up to 1900ppm Zr have ilmenite-baddeleyite-spinel assemblagemust have been reported from the groundmass of kimberlite undergone redustion during so6ling. Texture types dykes Cfompkins & Haggerty 1985). Ilmenite in B and C contain primary Fe-Cr-spinel, so tlat the ilmenite-baddeleyite-spinel clusters from the ratio of spinel:baddeleyitein tlese assembfuesis not Basistoppen sill contains less than 750 ppm Zr, useful for determining whether oxidation or reduc- indicating that the driving force for the exsolution tion occurred during formation. of baddeleyite was not oversaturation of Zr in Samplesof terrestrial ilmenite have syst€matically ilnenite. A solubiliry limit for Zr in ilnenite below lower Zr conteirts(average 300 ppm) than do samplc 750 ppm would require that prwiously described of lunar ilmenite (average7(D-1100 ppm) when com- examplesof ilnenite with high6r Zr contents either parisons are made between rocks with similar bulk are metastableor contain unrecopized Zr-rich inclu- Zr-contents (Arrhenius et al. l97l), Lunar ilnenite sions. containing up ts 4?fi Wm Zr and terrestrial ilnenite The distribution coefficient for 7'r between containing up to ll00 ppm Zr have been reported ilmenite and silicate melt (Kpilm,/lq) has been cal- (Arrhenius et al. lVll, Bl-Goresy et al.,I91la). culated to be approximately 5.4 for lunar rocks crys- Experimental results suggest,however, that eventle ta[izine at l2COoC,and approximately 2.2-3.0 for highest concentrations of Zr observed do not the terrestrial Palisades sill crystallizing at representtle solubiliry limit of Zr in ilmenite, which 1m0- I I mo C (Arrhenius et al. 197l\. A value of 3.0 is in excss of 6@ ppm (Arrhenius et al. l/71). for Kpiln/lq was obtained for Zr in ilnenite Geikielite-iltnenite"(Mg;1.fq.4TiO3) containing up megacrystsfrom a kimberlite (Fugimati et a/. 1984). to 7600 ppm Zr and ilmenite-pyropianite"" Calsulated pre-exsolution compositions of ilmenite ILMEMT]S FROM TIIE BASISTOPPEN SILL, CREENLAND 95 from tle Basistoppen sill contain up to 71m ppm Watson, S.E. Ha€gerty, and R.F. Martin sipifr- Zr. Using the chilled margin composition (175 ppm cantly improved'his manuscript. I am indebtedto Zr) for the initial Basistoppen magma and assum- the Deparment of Geology of Harvard University ing a Kpilm/lq between 3.0 and 5.4, the interstitial for the use of their electron-microprobe facility. ilmenite in the GPZ is inferred to have crystallized from a calculated residual liquid enriched tnZrby a factor of 7.5 to 13.5. lf Zr behaved as a perfectly excluded element during the crystallization of tle REIERENCES Basistoppensill, this liquid would representthe final 13.3 to 7.4s/oof crystallization. AcnBr,r.,S.O., ScooN,J.H., Mun, I.D., LoNo,J.V.P.' Ilneldte in equilibrium with Fe-Cr-spinel from the McCoNNeLL,J.D.C. & Pscxrnrr,A. (19?0):Obser- Basistoppensill contains O.3to L2tlo Cr2O3.Calcu- vations on the chemistry, mineralsgy, and petrol' lated preexolution ilmenite from tlese samplescon- ogy of someApollo 1l lunar samples.Proc. Apollo tains up to 2.4t/o Cr2O3.The reduction-exsolution 11 Lunar Sci.Conf. t'93-128. of Fe-Cr-spinel in these erains is considered to be the reult of the concentration of Cr in ilnenite dur- ARRr{B{rus,G., Ewnsox, J.E., Frucrner.o, R.W. & ing crystallization at magmatic temperatures, fol- Furrre, H. (1971): Zirconium fraqtionation in lowed by a decreasein the solubiliW limit of Cr in Apollo ll and Apollo 12 rocks. Proc, 2nd Lunar itnenite during cooling and re-equilibration. The Sci.Conf. l,169-t76. ilmenite was reduced during srcrsling,in spite of the Bnrrr, R.,. Goowt, R.C., Dowrv, E., Pnryz,M. & faclthat the rocks as a whole were oxidized during Kur,, K. (1973):Oxide mineralsin lithic fragments cooling, becauseFe-Cr-spinel was the only stable from Luna 20 fines. Geochim. Cosmochim.Acta phasethat could act as a sink for excessCr. The exso- 37,761:773. lution of baddeleyitein the ilmenite may have partly compensated for the overall reduction of the BnowN,G.M., Evelrus, C.M., HolLeNo, J.G., ilnenite-baddeleyite-spinel asstnblage. Pecrcrr, A. & Pnnups, R. (1971):Picrite basalts, The stableZr-bearing nineral in magrnassatuated ferrobasalts,feldspathic norites, and rhyolites in a fractionated lunar crust. Proc. 2nd Lunar or nearly saturated witl silica is zircon strongly @utterman Sci.Conf.1, 583-6(P. & Foster 1!b7, Watson 199). In the Basistoppensill baddeleyite is believed to have formed during sub- Busrcx, P.R. & Knn, K. (196O: Meteoritic rutile. solidus re-equilibration, and therefore was never in Amer. Mineral. 51, l5(tr-1515. contact with silica-saturatedmagma. Baddeleyitein the Rhum inrusive complexMIIiams 198)' the Axel Burre,RMAN,W.C. & Fosrsn,W.R. (1967):Zircon sta- Heiberg gabbros (Keil & Fricker 1974), and lunar bilif and the ZrO2-SiO2 phase diagram. Amer, basalts (Acrell et al. 1970, Lovering & Wark 1971, Mineral.52,880-885. Simpson & Bowie 1971, Keil et al. 1971, El-Goresy G. (1981):The Palobora complex.Indastrial et al. l97lb\ has been interpreted to be a late-stage Cr.arrn, Minerals 169,45-62, mineral that crystallizedfrom interstitial Zr-enriched magm:l The residual magmas in these rocks, CrarurqR.S., Jr. &WosnrsKr,J.F. (1967):Baddeleyite however, are likely to have been nearly saturated in inclusionin the Martha's Vineyardtelsite. Geehim, silica, and tlerefore under equilibrium c.opditions Cosmochim.Actq 31, 397#, should have precipitated zircon rattrer than bad- deleyite. In sone of these samples,the baddeleyite DeNcnnr,R.V. & n'Onev,F; (1972)zChromian spinel may have formed during subsolidusre-equilibration exsolutioninilmenite fromthe Premiermine, Trans- of Zr-rich ilmenite or spinel. In some lunar basalts vaal, South Africa. Contr. Minerol. 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