Solvntarnn LAMELLAE of BADDELEYITE and Fe-CI-SPINEL

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Solvntarnn LAMELLAE of BADDELEYITE and Fe-CI-SPINEL 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<e0 ggligZt ) FlAel Hrb! 0Ebtr6 (l(dl & F,U@r19741 t4 tm 20 (BEtgillolts) is generallypoorer in ZrO2 and richer in TiO2, SiO2, Fl Kinboes€cdrwbclslt JqE lgEl) t-runDqot@lFshMsge and FeO than baddeleyite previously reported from terrestrial gabbroic and ultramafic rocks. FeO and TiO2 contents in the Basistoppen baddeleyite, however, may be high dus to secondaryfluorescence DIscussIoN effects in the host ilmenite resulting from the small size of the analyzed gxains. The presenceof ilmenite lamellae in a host spinel In the ilnenite and Fe-Cr-spinel coexisting witl is a common feature of slowly cooled igneousrocks. baddeleyite, the trivalent cations Fd+, Cr3+, 41r+ Suchtextures are the result of oxidation oftle spinel and \r3+ are concentrated in spinel, whereas Tia+ during cooling and are sometimes referred to as and the divalent cations Fe?+, Mn2+ and MgF are oxidation-exsolution (tlaggerty l9O. Althoueh lss concentrated in ilnenite (Table 2). Zy'" was not sommon, tle oxidation of ilmenite during cooling detectedin either phase(detection limit - 750 ppm). can result in the oxidation-exsolution of rutile or Calculations based on the recalculation schemeof pseudobrookite (Haegerty 197Q. Similarly, lamel- Stormer (1983)and the temperafure{O, curves of lae of baddeleyitecould form in an ilmenite host dur- Spencer& Lindsley (1981) sueeestthat the ilmenite ing cooling under oxidizing conditions (Fig. 2). The and spinel in theserocks last equilibrated at temper- development of spinel lamellae in an ilmenite host, atures betwe€n @ and 850oC and orygen fugaci- however, requires a reduction of the ilmenite-spinel ties berween1O13 and 1O2rbars.
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