Opaque Oxide Mineral Crystallization in Lunar High-Titanium Mare Basalts
American Mineralogist, Volume63, pages1209-1224, 1978
Opaqueoxide mineral crystallization in lunarhigh-titanium mare basalts
RrcsnnoD. WenNnn Departmentof Geologyand Institute of Meteoritics Uniuersityof NewMexico, Albuquerque, New Mexico 87131
C. E. Nrsnu Departmentof Geology Brooklyn Collegeof the Cily Uniuersityof New York Brooklvn.New York I1210
lNo Kr-eusKutr- Departmentof Geologyand Institute of Meteoritics Uniuersityof New Mexico,Albuquerque, New Mexico 87131
Abstract
Lunar high-Ti(TiOr, 8-14 weight percent) mare basalts include low-K (KrO ( 0.1weight percent)Apollo ll and 17 and high-K (K,O - 0.3 weightpercent) Apollo ll varieties. Subdivisionof low-K Apollo ll and 17 basaltsis made,primarily on the basisof trace- elementdata. Magmatic crystallization of opaqueoxide minerals commences early, with the following characteristicassemblages:
Low-K l-Apollo l7 Cr-ulvdspinell* armalcolite* ilmenite l_Apolloll Cr-ulvdspinel* ilmenite High-K Apollo I I Armalcolite* ilmenite(Cr-ulvdspinel rare or absent) Compositionsof the aboveminerals vary accordingto their texturalsetting. Late-stage, nearly pure ulvdspinelis alsooften present,especially in coarse-grainedbasalts. Subsolidus assem- blages(l) Cr-ulvdspinel(host) * ilmenite* nativeFe and (2) ilmenite(host) + rutile * aluminianchromite * nativeFe and,/ortroilite are common in high-Timare basalts, particu- larlyApollo 17.Systematic lamellar orientation of thesubsolidus oxide minerals suggests, for the mostpart, an originby exsolutionrather than by reduction.Compositional data indicate that host mineralsundergo only minor changesin MgO and Fel(Fe+Mg) as a resultof subsolidus crystallization. Composition of the magma, its cooling rate, and oxygen fugacity affect the stability, abundance,and order of crystallizationof opaqueoxides in high-Ti mare basalts.Cr- ulvdspinelis restrictedto basaltswith >0.3 weight percentCrzOs. Armalcolite crystallizes from meltswith >10 weightpercent TiO, and ceasesto crystallizewhen Fel(Fe+Mg) of armalcolitereaches -0.70 and a reactionrelation occurs. Slower cooling enhances reaction of armalcolitewith themelt and results in decreasedmodal abundance. The modal abundance of Cr-ulvdspinelalso decreaseswith slower cooling, probably becauseof increasedCr sub- stitutionin pyroxeneand earliernucleation of plagioclase.Ilmenite modal abundanceis directly relatedto TiO, contentof the magmaand is independentof coolingrate. However, slowercooling tends to produceilmenite grains of more magnesiancomposition through increasedresorption of earlyolivine crystals. Absence of armalcolitein Apollo I I low-K basaltsand rarity of Cr-ulvdspinelin Apollo I I high-K basaltsmay bedue to crystallizationat oxygenfugacities, respectively, above -10-13 or below-10-14. Cr-ulvdspinel,armalcolite, and ilmeniteare enriched in TiOzand CrrOsrelative to high-Ti marebasalt magmas. Gravitational crystal settling of one,two, or all threeof theseminerals
1Cr-ulvdspinel is used throughout this paper as an abbreviation for chromian ulviispinel 0003-004x/78/ll 12-1209$02.00 1209 I2IO WARNERET AL: MAREBASALTS
will producederivative liquids increasingly poorer in TiO, andCr2oa. Simultaneous fractiona- tion of olivine depletesthe derivativeliquids in MgO. Compositionalvariations observed in high-Timare basalts can be accountedfor by moderate(Apollo 17) to small(Apollo 1l) amountsof fractionationinvolving early-crystallizing opaque oxide minerals and olivine.
Introduction Ray (1970).Details as to analyticaltechniques and microprobe standardsemployed are pre- Mare basalts returned by the Apollo ll and 17 electron sentedin Warner et al. (1976a) and Nehru el a/. lunar missions are unusually rich in TiO, (8-14 (1976).Modes of Apollo l7 basaltswere determined weight percent) and hence are referred to as high- in reflectedlight and are reportedin Warner et al. titanium (high-Ti) mare basalts.The high Ti content (1e78). clearly distinguishesthese basalts from terrestrialba- salticrocks and from other lunar mare basaltspoorer in TiOz. Lunar high-Ti mare basaltsalso contain 0.2- Classificationof high-Ti mare basalts 0.6 weight percent CrrOr, which is somewhat less High-Ti marebasalts may be dividedon the basis CrrO, than in low-Ti mare basaltsbut an order of of KzO into low-K (KrO ( 0.1 weightpercent) and magnitude greaterthan in terrestrialbasalts. Due to high-K (KrO - 0.3 weightpercent) varieties. All the this unusual composition, crystallization of one or Apollo 17 and about half of the Apollo 1l high-Ti more opaque oxide minerals commencesat or near marebasalts belong to the low-K group.The remain- the liquidus of the rocks and continuesin abundance ing Apollo I I basaltsall are of the high-K variety. throughout the entire crystallizationsequence. Possi- Rhodeset al. (1976)subdivided the Apollo 17 bilities thus exist for fractionation of early-crystalliz- high-Timare basalts into threetypes (A, B, andC) on ing opaque oxide minerals.This hasimportant impli- the basisof trace-element(particularly REE) abun- cations for the differentiationof high-Ti mare basalt dancesand certain distinctive trace-elementratios magmas. (e.g., Ba/Rb; LalSm). Thesetypes were definedon In order to assessthe role of early opaque oxide the basisof compositionaldata obtainedon fine- mineral crystallizationin the differentiationof lunar grainedbasalts. While somecoarse-grained basalts high-Ti mare basalt magmas,we have undertaken a clearly may be assignedto one or another of these detailed study of the opaque oxide mineralogy of threetypes, many of themhave trace-element abun- Apollo ll and l7 high-Ti mare basalts.Within this dancesthat fall outsidethe rangesdefined by the fine- suite there are fairly major variations in composition grained samples. Such basalts were assignedby and texture. As a result, we can evaluate effects of Rhodeset al. to a fourth type (type U). Although magma composition (including oxygen fugacity) and theserocks possiblyconstitute a separatemagma cooling rate upon the stability, order of appearance, type,they more probablybelong to one of the three abundance,and composition of opaque oxide miner- above types and owe their chemicaldivergence to als. Subsoliduscrystallization of theseminerals has sampling problems associatedwith their coarse- in many high-Ti mare In to occurred basalts. order grainednature. In thispaper we followthe Rhodesel ascertain to what extent this may modify original a/. classificationof the Apollo l7 high-Ti mareba- magmatic compositions,we have attemptedto char- salts,but with the provisothat we do not at thistime actefize the compositional changes associated with recognizetheir type U categoryas forming a distinct subsoliduscrystallization. basalttype. Beatyand Albee(1978) subdivided the Apollo I I Methods of study low-K basaltsinto four groups(Br, Br, Br,and Bo) on Polished thin sections of 62 Apollo 17 and 17 the basisof mineralogy,texture, and age.Trace-ele- Apollo I I sampleswere studied optically in reflected ment data supporttheir grouping:group Bt basalts and transmitted light to determine textural relations havehigh, groups B, and Br intermediate,and group of the opaqueoxide mineralsand to selectrepresenta- Bs low REE abundances.REE abundancesin group tive grains for analysis with an Anl Evrx-stvtelectron B, basaltsare comparable to thosein typeA Apollo microprobe. Mineral analyseswere correctedfor in- l7 basalts.For the purposesof this paperwe shall strumental effects following the procedure of Keil follow the Beaty and Albee classificationof the (1967) and for differential matrix effects using the Apollo 1l low-K basalts.However, we notethat the method of Bence and Albee (1968) and Albee and very limited sampling at the Apollo I I site makes WARNERET AL,: MARE BASALTS t2lr subdivisionhere statistically much less valid than for TiO2, I1.3; Al2O3,8.9; CrzOg, 0.37; FeO, 18.7;MnO, the Apollo 17high-Ti basalts. 0.17;MgO, 7.3; CaO, 10.8;NarO, 0.58;KzO, 0.31). The Apollo I I high-K basaltsexhibit a muchnar- rower rangein compositionthan the Apollo I I and Opaqueoxide mineralogy l7 low-K basaltsand appearto havebeen derived from a singlelava flow. Consequently,they are not General subdivided. Early-crystallizingopaque oxide minerals in high- Table I showsthe averageand range in major Ti marebasalts include ilmenite. armalcolite. and Cr- elementcomposition for the high-Ti mare basalt ulv6spinel.Armalcolite, previously unknown before typesenumerated above, based on a compilationof its discoveryin Apollo I I samples,is essentiallyinter- all analysesreported in the literature.Differences in mediate in composition between FeTi2Ou and major elementcomposition among the Apollo l7 MgTizOu(Anderson et al., 1970),and also contains basalttypes, particularly between A and B, are mi- minor amountsof (Cr,Al,Ti8+)rTios(Kesson and nor. Type C basalts(represented only by threesam- Lindsley,1975; Wechsler et al., 1976).Cr-ulvdspinel ples collectedfrom the samelocality) contain the is intermediatein compositionbetween FerTiO, (ul- highestamounts of MgO and CrrOr. Comparedto vcispinel)and FeCrrOo(chromite), with substantial the Apollo l7 basalts,Apollo I I low-K basaltsare, substitutionof Mg for Fe and Cr for Al (Agrellet al., on the average,lower in TiOr, MgO, and CrrOr. 1970).Also presentin high-Timare basalts are late- Individualsamples overlap much of the Apollo l7 stageminerals such as nearlypure ulvdspinel,tran- range,however. The group B, Apollo I I low-K ba- quillityite,zirconolite, and baddeleyite.These occur saltshave the lowestabundances ofthese oxides. The in cristobalite-richor glassymatrix areas,particularly Apollo I I high-K basaltsare distinguished by signifi- in coarse-grainedbasalts. cantly higher KrO (average0.31 weight percent). The Apollo 17,Apollo ll low-K, and Apollo-ll They have similar averageTiO, and slightlylower high-K basaltsare eachcharacterized by a different averageMgO and CrrO, than the Apollo l7 high-Ti assemblage,as shownbelow: marebasalts. Compared to the Apollo I I low-K ba- salts,the Apollo I I high-K basaltsare, on the aver- l7 Cr-ulvrispinel/ulvdspinel age,richer in TiOr, MgO, and CrrOr. fApollo Low-K | + armalcolite-f ilmenite Table 2 lists all the studiedsamples according to Apolloll Cr-ulvdspinel/ulvdspinel the classificationpresented above. Fine- and coarse- I L + ilmenite grainedvarieties within eachtype are distinguished. Note that: (l) all Apollo 17 type A and C basalts High-K Apollo 1l Armalcolite* ilmenite(Cr- studied are fine-grained,although coarse-grained ulvdspinel/ulvdspinelrare or membershave been reported by Rhodeset al. (1976); absent) (2) all group B, Apollo I I low-K basaltsare coarse- Modalabundance, compositional variation, and text- grainedand all group B, basaltsfine-grained; (3) no ural settingof the opaqueoxides from the different group B, Apollo I I low-K basaltswere studied; (4) basalttypes are summarizedin Table 3 and the fol- group B, consistsof only one sample(10050-see lowing sections. Compositional data are from Beatyand Albee,1978); and (5) all Apollo I I high-K Warneret al. (1976a)and Nehru et al. (1976),supple- basaltsare fine-grained(James and Jackson,1970). mentedby additionalunpublished data. Note that We haveincluded two mare basaltlithic fragments the classificationfor Apollo 17 basaltsused by from soil sample10085 (10085,125 and 10085,666) Warner et al. (1976a)is basedsolely on textural cri- with groupB, on the basisof similarityin textureand teria, and thus differs somewhatfrom that adopted mineralogy.Likewise, we havegrouped mare basalt here. lithic fragments10085,122 and 10085,251with the Apollo I I high-K basalts.In addition,we havein- cluded with the latter a clast in brecciasample Cr-ulubspinel and ulubspinel 10060,71that Jamesand Jackson(1970) previously Cr-ulvdspinelis most common in low-K basalts classedseparately as a vitrophyric basalt.We have (especiallyApollo 17) and more abundantin fine- done so as the result of a defocussedbeam analysis than in coarse-grainedvarieties. It is absentin low-K (Prinz et al., l97l) of the clastwhich showedit to be basaltsthat contain (0.3 weightpercent CrrO, (that of high-K composition(weight percent: SiOz, 41.5; is, group B, Apollo I I low-K basaltsand a few com- t2l2 WARNERET.AL.: MARE BASALTS
Table 1. Averageand rangein compositionof high-Ti mare basalts
Apollo 17
Type A Type B Type C tut (2e) (37) (3) (20)
s102 3e.6 (38.4-41.9) 38.7 (37.8-40.3) 38.s (38.2-38.5) 38.4 (37. V40.2) Tio2 1r.8 (8.6-13.2) 12.1 (9.8-13.7) r2.3 (LL.9-L2.5) rr.9 (7.8-14.s) A1203 e.0 (8.4-10.0) 9.r. (7.8-10.6) 8.7 (8.G8.7) 9.1 (7.3-1r..7) CrZO3 0.39(0.17-0.53) 0. 39(0.26-0.61) 0.s8(0.53-0.61) 0. 45(0. 23-0.6s) Fe0 19.4 (r8.0-20.9) r.9.s (r.7.0-20.8) 18.2 (18.1-r.8.3) r.8.9 (15..1-20.6) ltrO 0.25(0.22-O.28) 0.25(0.22-0.29) 0.26(0.2s-0.28) 0.25(0.22-0.28) Mgo 8.1 (s.2-10.0) 7.9 (6.8-9.4) 10.5 (r.0.4-10. 6) 8.7 (6.3-10.3) Ca0 r0.7 (9.Tr2.4) 10.7 (9.3-13.3) 10.4 (10.3-10.5) 10.7 (9.s-13.6) Na2O 0. 41(0 . 37-0.48) 0.38(0.33-0.46) 0.36(0.35-0. 37) 0.40(0.31-0.50) Kzo 0.07(0.06-0.08) 0. 0s(0.03-0.0 7) 0.08(0.07-0.09) 0. 06(0. 05-0. 09 )
TLOr/r{gO t.46 1.53 r.37 Fe/ (Fe+Mg) 0.57 0.58 o.49 0.55
'Ut = coarse-gralned Apollo 17 basalts of mcertaifl affinity (see text) Number of basalts averaged in each type or group indicated ln ( )
positionally-evolved Apollo l7 basalts,designated in inclusions shows a positive correlation with the Fa Table 2 by asterisks).It is extremelyrare in Apollo I I content of the host olivines.Cr-ulvdspinel grains near high-K basalts. the margins of olivine grains and elsewherein the The compositionalvariation exhibited by Cr-ul_ basaltsare generallymore Fe-rich. The latter tend to vdspinel is summarizedin Table3. In most basalts. be larger than the inclusionsin olivine, rangingin size greatest the variation from grain-to-grainis in Fel to upwards of 100 pm (microphenocrystic in finest- (Fe+Mg) (FFM). Within theranges indicated, varia- grained Apollo l7 basalts-Fig. lA). tion in FFM is continuous.Concomitant variations Ulvdspinel is most abundant in coarse-grained of other major oxidesare small,with Ti showinga low-K basalts and is found primarily in late-stage tendency to increaseand Cr and Al to decreasewith mesostasis.It occurs both as discretegrains or inter- increasing FFM. Individualgrains are for the most grown with ilmenite (Fig. lB); metal and troilite are part homogeneous.A few grains are zoned; these sometimesalso present. It is nearly pure FqTiOo, increase in Ti and Fe and decreasein Cr and Al from with a narrow range in FFM (0.96-1.0). In fine- coreto rim. Two additionalobservations are impor_ grained low-K basalts,ulvdspinel contains ( I weight tant. First, the rangein FFM of Cr-ulvdspinelin percent Cr2Or, and a gap thus existsin CrrO, between Apollo I I low-K basaltsis lessthan that in Apollo l7 ulvdspineland the most Fe-rich Cr-ulvdspinelgrains. basalts, the differencebeing that Fe-richgrains (FFM In coarse-grainedlow-K basalts,the amount of CrrO, < 0.91)are lackingin the Apollo ll low-K basalts. contained by ulvdspinel is much more variable, and Second,in somehalf-dozen type B Apollo l7 basalts the above-mentionedgap is smaller,and evenbridged (0.37 with weightpercent Cr2O3, the Mg-rich grains in a few Apollo 17 basalts. (FFM < 0.85)are absent. Compositionsof individual grainsvary according Armalcolite to their texturalsetting. The most Mg-ricn gralns generallyoccur as inclusionsin olivine,or in tita- Armalcoliteis presentin most Apollo I I high-K naugitein the caseof olivine-poorcoarse-grained and Apollo l7 basalts,but is not foundin epotlo tt basalts.The inclusionsin olivine are usuallysmall low-K or in the most compositionally-evotvea (only 1i.e., a few tens of microns)idiomorphic crystals, TiO, < l0 weightpercentj Apollo 17 basalts.It is and sometimesoccur in clusters.The FFM of the more abundantin fine- than in courre-g.ainedba- WARNERET AL,: MARE BASALTS l2l3
Table l. (continued)
Apol1-o 11 Low-K Apo11o 11 High-K
Br Group B, GrouD B^ Group B, Group t (3) (2) (4) (1) (8)
4L.9 (4r.0-42.5) 38.3 (37.9-38.5) 39.0 (38.1-39.9) 4L.I 40.s (39.6-41.o) 9.9 (9.1-10.7) r.1.8 (11.4-12.2) 1r.3 (10.5-12.7) L2.T 11.8 (11.3-12.5) 10.8 (10.2-11.3) 10.4 (10.3-10.5) 10.2 (9.5-11.3) 10.0 8.6 (7.9-9.s) 0.22(0.r9-0.24) o.2s(0.2vo.27) 0. 36(0. 24-0.44) 0. 33 0.36(0.32-O.37) 18.5 (17.9-19.2) 2O.2 (L9.8-20.5) 19.0 (L8.4-19.4) 18.1 19.3 (18.7-r9.6) 0.27(0.25-0,29) 0.25(0.2T0.27) o.26<0.2TO.27) o.27 0.23(0.zL-O.25) 6.1 (s.9-6.3) 7.4 (7.3-7,s) 7.e (7.L-8.e) 8.6 7.5 (7.O-7.8) r2.r (rr.7-r2.3) 10.9 (10.5-11.2) rr.6 (11.3-12.2) 11.3 10.6 (10.1-L1.0) 0.44(0.4r--0.47) 0.39(0.38-0.39) 0. 37(0. 33-0. 42) 0.38 0.50(0.4&-0.s3) 0.11(0.10-0.12) 0.04(0.03-0.0s) 0.0s (0.02-0. 07) 0.05 0.31(0.28-0.33)
1.62 r. ), I. +J L.4L r.57 0.53 0. 60 0.57 0.54 0.59
Table 2. Classification of samples studied in this investigation
Apollo 11 Apollo Apollo 11 Lorr-K 17 Hlgh-K Type A Type B Type C tut Group B.' Group 81 Group B, Flne Fine Coarse Fine Coarae Coarse- ! ane Coarse' Flne
71508 71507 7L557 74245 71509 10044 10020 10050 LOO22 7L527 7L525 71566 74275 71535't 10047 10045 10024 71528 7L526 7L597 7L549 10058 10062 10057 71529 71535 78575 71556 10085,125# 10071 71538 7t537 78575 71559!t 10085,666# LOOl2
71539* 7L545 /r)b) roo85,t22tl 7t546 7L547 7L567 10085,251/i 71548 71558 71568* loo6o, 7lll 71555 7L576 77535 (clast) / r)ov 7r579 77536
7r575 71585 78505 7L577 7L586 78578 7r578 TL)61 78569 71588 78579 71589
78588 71595 T 6JYJ 7L596 78596 732L9 78598* 775L6 78599 78585
r6)6t 78589 78597
rur = coarse-gralned Apollo 17 basalts of uncertaln affinlty *Apoll-o 17 basalt8 \rlth textural and nlneralogical afflnr.tles to Apollo 16t-K basalts (see text) ilsEinples renunbered fron Janes and Jackson (1970, Table 1) as follors: 10060,71 = 10050,21-1 10085,r22= 10085132-2 10085,125=10085,12-4 10085,251= l-0085,32-11 10085,655=10085,12-2 t2t4 IYARNERET AL: MARE BASALTS
Table3. Modal abundanceand compositionalrange of opaqueoxide minerals in high-Ti mare basalts
Apollo 11 Apo1lo 17 Apollo 11 L@-K Hlgh-K Type A Type B Type C 'ul Group Bl Group Ba Group B/ Fine ' Fine Coarse Flne Coarse Coarge- Flne Coarse- Flne Cr-ulviispinel Modal abundance 0,2 O.2 <0.1 0.5 Tr (0-0.5) Tr Tr Fe/(Fe+ue) O.73-O.gj O,j4-O.99 o.72-O.95 0.72-O.93 o.72-0.99 o.72-O.9L 0.76-0,80 0.75,0,89 crl (cr+Al) O. 59-0.80 O.6r-0.79 0.56-0.84 0.69-0.71 0.63-0.83 0.61-0.79 0,66-0.68 O.74,O.76 2Til(Cr+Al+2Ti) 0.39-0.73 0.40-0.69 o.27-0.65 0.42-0.s5 o.25-O.7r o.45-O.64 0.4r-0.56 0.56,0.58 Ar@lcolite Modal abundance 0.1 0.2 <0,1 1.3 <0.1 (0-0.s) Fel(Fe+Ms) 0.51-0.74 0,47-0.70 0.50-0.59 0.46-0.63 0. 49-0.68 crro:. (i4) 0.4vo.67 1.0-2.s r.r-2.6 1.0-2.0 r.4-2,4 L.2-2.2 Ilren1te Modal abundance 16 L6 16 15 10 L4 L4 (r3-r7) Fs/(Fe+Mg) 0.70-0.99 0.70_1.00 0.69-0.98 0.81-0.98 0.68-1.00 0,95-0.99 0.74-O.99 0.86-0.95 crro, (7") 0.79-0.99 0.1-1.7 0.1_1.3 0.4-1.0 0.3-r.2 0-1.0 0-0.6 0.2-L.7 0.5-0.7 0.1-1.0
Modal-abundances for Apollo 17 basalts - are averagea of data repolted ln wamer et al. (1978); those for Apollo Il basalts are fron Janes and Jackson (1970, Table 2) comPositl@al taoges for Apollo 17 basalts are fron aoalyses given in wamer d aL. (Lg76a) supplerented by addltlooal @published data; those - for Apol1o 1l basalts are froo analyses given In ttehiu et aI. (Ig7O7
salts, and is most abundant in type C Apollo 17 also occurs as tiny lathlike or needle-shapedin- basalts. clusionsin olivine; thesegrains have FFM ) 0.5 and Armalcolite occurs as discrete single grains and thus crystallizedlaIer. In type A basalts,grains with with rims of ilmenite (Fig. lD). The latter haveorigi_ FFM < 0.5 are absent,and the most Mg-rich grains nated from reactionof armalcolitewith the surround_ are inclusions in olivine. Tabular grains only partly ing melt (cl Andersonet aI.,1970;James and Jack_ mantled by ilmenite (Fig. lC) are common in type A son, 1970). Preservation of discrete grains or of basaltsand have higher FFM. euhedralfaces on grains that are only partly mantled It has been suggestedby El Goresy et al. (1974) by ilmenite (Fig. I C) is the result of thosefaces being that a compositional bimodality exists between ar- armored against reaction, usually by contact with malcolites from fine-grained and coarse-grained earfy-formed olivine or titanaugite (papike et ql., Apollo 17 basalts,and that, consequently,there are t974). two different varietiesof armalcolite,as proposedby Discrete armalcolite grains are usually homoge_ Haggerty (1973).Our data (Fig. 3) show no evidence neous in composition. In contrast, ilmenite_mantled for such bimodality. We thus concur with Smyth grains, particularly if they occur as micropheno_ (1974), Williams and Taylor (1974), andPapike et al. crysts, are often zoned. In zoned grains Fe increases (1974)that there is no compositionalbasis for distin- and Mg and Cr decreasefrom core to rim. Most of guishing two varietiesof armalcolite. the zoning occurs in the outer few microns adjacent to ilmenite. A large part of the compositional range Ilmenite exhibited by armalcolite in a given rock may be prei_ Ilmenite is by far the most abundant opaqueoxide ent in a single zoned microphenocryst. in all high-Ti mare basalts (Table 3). The amount The overall range in FFM and CrrO, displayedby correlatesdirectly with bulk-rock TiO, (Table l). The armalcolite is similar in Apollo I I high-K and Apollo average modal abundance is greatest in Apollo l7 l7 basalts(Table 3). Within the Apollo l7 suireihere basalts,whereas group B, Apollo I I low-K basalts is a small but real compositional differencebetween and Apollo l7 basaltswith (10 weight percentTiO, the most Mg-rich grains presentin B (lower FFM) as have the lowest modal abundances. opposed to type A basalts (Fig. 2). The observed Modal abundanceis independentof averagegrain differencesmay be correlatedwith the habit and oc_ size of the basalts. However, there are significant currenceof grains. the In type B basaltsarmalcolite differencesin ilmenite morphology betweenfine- and typically occurs as microphenocrysts completely coarse-grainedrocks. In the former, ilmenite typi- mantled by ilmenite (Fig. lD). Grains with FFM ( cally occurs as lathlike or acicular grains, often with 0.5 are, without exception,of this habit. Armalcolite skeletalsawtooth-bladed margins (Fig. lE). Acicular l2l5
Fig. l.Reflected light photomicrographs of opaque oxide minerals in lunar high-Ti mare basalts. (A) Cr-Ulvtispinel (Cr-Uv) microphenocryst in fine-grained Apollo l7 basalt 78587. Note presenceof octahedrally-oriented ilmenite (l) lamellae and small grain of native Fe (M). Horizontal dimension -200 pm. (B) Ulvdspinel (Uv)-ilmenite intergrowth in late-stagemesostasis in coarse-grained Apollo l I low-K basalt 10047.T : troilite. Horizontal dimension -200 pm. (C) and (D) Armalcolite (A)-ilmenite relationshipsin fine- grainedtypes A (78596)and B (71595)Apollo l7 basalts,respectively. Horizontal dimensions- lmm. (E) and (F) Representativeilmenite crystal habits in fine-grained (71596) and coarse-grained(78576) Apollo l7 basalts, respectively. Horizontal dimension -2 mm. (G) Intersecting rutile (R) and aluminian chromite (Cm) lamellae in ilmenite in coarse-grainedApollo l7 basalt 78576.Horizontal dimension -200 pm. (H) Assemblagealuminian chromite * rutile at contact between ilmenite and composite metal-troilite grain in coarse-grained Apollo 17 basalt77536. Horizontal dimension -200 pm. WARNERET AL.: MARE BASALTS
.=o al., 1974)morphology, with numerousholes or mar- o o ginalembayments (Fig. lF). o E Individual grains of ilmenite are compositionally o homogeneousin all samples.The overallrange in -c FFM is approximately .9 A 0.7-1.0in low-K basaltsand I 0.8-1.0in Apollo ll high-K basalts(Table 3). In CD ^^^ jA ^a group 81 Apollo I I low-K basaltsand composition- at ally-similarApollo l7 basaltsthe rangeis narrower o . J^^^. E O1 (-0.9-1.0). An interestingfeature of the Apollo l7 .= suiteis that Mg-rich ilmenitegrains (i.e., those with (tl FFM < 0.8)are much more common in coarse-than E in fine-grainedbasalts, despite the fact + that the fine- |E and coarse-grainedvarieties are essentiallyisoche- A=TYPEA mical. \ O=TYPEB tL In general,the compositionof ilmeniteis dictated by textural setting, irrespectiveof grain size. The e.oo _o.55 0.60 0.65 Fe / Fe + Mg) in iock most Mg-rich ilmenitesoccur within olivine(either as / discretecrystals or associatedwith Fig. 2. Fe,/(Fe+Mg)in most Mg-rich armalcoliteus. Fel(Fe* melt inclusions) Mg) in rock in fine-grainedtypes A and B Apollo l7 basalts. or, in olivine-poorcoarse-grained Apollo l7 basalts, within sector-zonedtitanaugites. Ilmenite mantling armalcolite skeletalmicrophenocrysts up Io 2-5 mm long are also tendsto be Mg-rich.Micropheno- crysts, commonin Apollo l7 basalts(particularly type B). In matrix laths, etc., are more Fe-rich,and the most Fe-rich grains coarse-grainedbasalts, grains tend to be moreequant are those associatedwith late- and are characterizedby "amoeboidal" (Papike el stageminerals such as cristobalite,pyroxferroite, ul- vdspinel,and tranquillityite. The CrrO,content of ilmeniteranges from 0 to 1.7 o =fine-groined bosolts weightpercent and is more or lesssimilar in all basalt . =coorse-groined bosolts types(Table 3). Usselman(1975) reported that ilme- nitesfrom Apollo I I low-K and high-K basaltsex- o o hibit differenttrends on a MgO-CrrO3variation dia- gram. Our ilmenite data indicate oo that the fields for 8o Apollo I I high-Kand low-K basaltsvirtually overlap P (Fig. 4), and do not support the separatetrends shownby Usselman(1975, Fig. 2). However,MgO us. CrrOavariation in ilmenitesfrom Apollo l7 ba- o o.o5 salts contrastswith that from Apollo I I basalts. Here,CrrOs is relativelyinsensitive to the amountof MgO in ilmenite(Fig. aC). A possibleexplanation is discussedlater in the paper. Fe-Mg partitioning betweencoexisting minerals Our dataon the compositionsof coexistingminer- als in high-Ti mare basaltsmay be usedto calculate Kp valuesfor variousmineral pairs. Using the param- eter Fel(Fef Mg), or FFM, we have computedFe- Mg partition coefficientsbetween coexisting opaque oxideminerals andlor (Table general, o.5 06 01 olivine 4). In Fe/Fe +Msl thecalculated Kp's arethe same for Apollo I I and l7 basalts.An exceptionis the pair armalcolite-ilmenite, Fig. 3. Cr (cationproportion on basis of5 oxygens)us. Fel(Fe+ whereApollo I I high-K basaltsyield a lowervalue Mg) in armalcolitesin fine-grainedand coarse-grainedApollo l7 basalts.Solid line indicatescore-to-rim variation present in single (0.646)than Apollo l7 basalts(0.764). Part or all of zonedmicrophenocryst. this discrepancymay be due to analyticaldifficulties WARNERET AL,: MARE BASALTS
o=fine-groined bosolts o=coorse-groinedbosolts ot o atao o taooo .oft o .!o o be -€1," g l#1" o " oD lSo o /vO oq'o o
CrrOo (wt oA)
Fig. 4. Variation of MgO us. CrzOsin ilmenites in (A) Apollo I I high-K, (B) Apollo I I low-K and (C) Apollo l7 basalts' Solid-dashed line representsMgO us. CrrO, variation in ilmenite in equilibrium with synthetic high-Ti basaltic liquid (from Usselman, 1975). in determiningcompositions of the marginsof zoned cation).In someApollo l7 basaltslarger, positively armalcolitegrains. The computedKo's show that identifiablegrains of metal are present. FFM is lower in armalcolitethan in coexistingCr- Assemblage(2) is characterizedby intergrown ulvdspinelor mantlingilmenite, is approximatelythe blebsand lamellaeof rutileand aluminianchromite samein ilmenitecoexisting with Cr-ulvdspinelor ul- in ilmenite(Fig. lG). The relativeproportions of vdspinel,and is muchhigher in opaqueoxide miner- rutile and aluminianchromite are more or lesssub- alsthan in olivine. equal,except in the caseof ilmeniterims on armalco- lite. Here rutile is significantlymore abundant,often to 5 percentor more by volumeof the Subsoliduscrystallization of opaqueoxide minerals amounting ilmenite.The intergrowths consist of basally-oriented Assemblages aluminianchromite intersected by rhombohedrally- rutile(Cameron, 1970). The lamellae usually Subsoliduscrystallization is a commonfeature of oriented l-3 pm thick,and taper or pinchout opaqueoxide minerals in high-Timare basalts (Hag- arewell-defined, places intersection.Occasionally, ilmenite grains gerty, 1973;El Goresy and Ramdohr, 1975).The at of intergrowthsof rutile and alumi- resultingassemblages are as follows: containirregular in blebsup to 20-30 thick. Native (l) Cr-ulvdspinel(host) * ilmenitet nativeFe nian chromite rrm (2) Ilmenite(host) * rutile * aluminianchromite t nativeFe andlor troilite. Table 4. Kp values of coexisting minerals in high-Ti mare basalts These assemblagesare very common in Apollo 17 lessso in Apollo 1l high-K and group Bg basalts, FW Apollo I I low-K basalts,and rareor absentin group Mlneral pair )
B, Apollo I I low-K and compositionally-similar 0.646 0.059 8 Arc1co11te/il@nlte* , Apollo l7 basalts. \ 0.764 0.038 4J Ar@lcolite/cr-u1v6sptne1 o.7L6 0.029 !2 Assemblage(l) is characterizedby octahedrally- Ildenlte/Cr-u1v6splne1 or ulviisplnel 0,973 0.014 27 Atualcolite/olivlne 1.935 0.121 22 oriented(Haggerty, 197 I ) lamellaeof ilmenitewithin Cr-u1v6splnel/olivine 2.748 0.218 65 Cr-ulvdspinel(Fig. lA). The ilmenitelamellae are o - standard deviatlon usually Fe is sometimesassociated with this type of inter- playeda major role. The bestcandidates for this area growthin Apollo l7 basalts,but suchoccurrences are few coarse-grainedApollo l7 basaltsin whichilme- rare exceptin a few coarse-grainedrocks. nite commonlycontains bleblike intergrowths of ru- A closely-relatedassemblage is that of troilite-f tile andaluminian chromite together with nativeFe. ilmenite*aluminianchromite*rutile, which is quite common in coarse-grainedApollo 17 basalts.This Compos i t ional considerat i ons assemblageoccurs where troilite and ilmeniteare in An importantaspect of subsoliduscrystallization is contact(El Goresyand Ramdohr,1975)and consists the extent (if any) to which the original magmatic of a bandof aluminianchromite which is within the compositionsof the opaqueoxides are modified. This originalgrain boundaryof ilmenitebut followsthe is difficult to ascertainfor Cr-ulvdspinelgrains, be- contoursof the neighboringtroilite (Fig. lH). Rutile causethe ilmenitelamellae are much too narrow for is usuallyassociated with the aluminianchromite, quantitativeanalysis except in a few instances.These and the ratio of the two varieswidely. Both phases show that MgO partitionsin favor of the ilmenite often connectwith lamellaeinside the ilmenite.The lamellae,but that FFM of the host Cr-ulvdspinelis sameassemblage is also found at contactsof ilmenite only slightlydifferent (Table 5, cols.I and 3). Thus, with native Fe or compositemetal-troilite grains. exsolutionof ilmenitehas a very smalleffect on alter- ing FFM in Cr-ulvdspinel. Origin Within the host Cr-ul- vdspinelthere is commdnlyan enrichmentin CrrO, Two principalmechanisms have been proposed for and AlrOsin a narrowzone adjacent to the ilmenite the origin of the subsolidusassemblages in high-Ti (Table5, col. 2). However,away from the lamellae mare basalts.Haggerty (1971, 1973) and El Goresy there is probablylittle compositionalchange, since and Ramdohr(1975) argue that these are the result of we find no discernibledifference in the composition subsolidusreduction, while Haselton and Nash of Cr-ulvdspinelsbetween samples in which abun- (1975)and Kessonand Lindsley(1975), among oth- dantexsolutions occur and those with littleor none. ers,suggest that they originatedula isochemicalreac- For the most part, aluminianchromite and rutile tionsleading to exsolution.Our observationslead us lamellaein ilmenite are also too small for quan- to prefer,in general,an exsolutionmechanism. The titative analysis,except in some coarse-grained ubiquitous octahedralorientation of ilmenite la- Apollo 17basalts. Here, analyses show that rutileis mellaein Cr-ulvdspineland the systematiclamellar nearlypure TiO2 with 0.5-l weightpercent FeO and orientation of rutile and aluminian chromite in (0.2 weightpercent MgO, and that aluminianchro- ilmenitestrongly suggest the operationof an inter- mitescontain 40-50 weightpercent CrrO, and l0-20 nally-drivenprocess (i.e., exsolution) rather than an weightpercent Al2Os (e.g., Table 5, cols.4 and 5). externally-imposedprocess (i.e., reduction). Further- MgO and FeO are lowerin aluminianchromite than more,we notethat oftennative Fe is not evenpres- in the hostilmenite (Table 5) but FFM is onlyslightly ent, particularlyin the rutile-aluminianchromite- lower. In fact,K$FM (ilmenite/aluminian chromite) is host ilmeniteassemblage. Even where it doesoccur, nearunity (analysesof 14coexisting pairs yield a Ko native Fe does not constitutea priori evidencein of 1.036with o of 0.039).The net effectof exsolution favor of subsolidusreduction, because oxygen fuga- is thusa markeddecrease in CrzOgand Al2O,and a cities at the time of magmaticcrystallizatlon were slightincrease in MgO and FeOin the hostilmenite, sufficientlylow to permit precipitationof metal di- with FFM remainingessentially unchanged. TiO, in rectly from the melt. Rather,as suggestedby Hasel- the hostilmenite decreases only whenthe amountof ton and Nash (1975) and Kesson and Lindsley exsolvedrutile is significantlygreater than that of (1975),native Fe in subsolidusassemblages may sim- aluminianchromite, as is the case,for example,in ply be due to an isochemicalbreakdown reaction TiOr-enriched ilmenites formed as reaction rims betweenferrous iron and exsolvedtrivalent titanium aroundarmalcolite (cl ElGoresyet al.,1974;Papike (the existenceof trivalenttitanium in lunar minerals et al., 1974). hasbeen confirmed by Sunget al., 1974).Inilmenite El Goresy and Ramdohr (1975)and Usselman grainssusceptible to exsolution,contact with troilite (1975)have cautionedthat analyzedilmenites from and/or nativeFe (Fig. lH) apparentlypromotes ex- high-Ti mare basaltsmay not be representativeof solutionof aluminianchromite and rutile, however magmaticilmenite compositions. From our composi- (seeHaggerty et al., 1970).Nevertheless, in some tional data we concur that this is valid as regards high-Ti mare basaltssubsolidus reduction may have CrrOr and Al2Oo,but not necessarilyfor MgO and WARNERET AL,: MARE BASALTS t2t9 per' certainlynot for FFM. The MgO us.CrzOg variation Table 5. Representativeelectron microprobe analyses(weight minerals in subsolidus assemblages of ilmenite in equilibrium with high-Ti basalticliq- cent) of uid, as experimentally determined by Usselman (1975),is shown in Figure 4. The trend indicatesa positivecorrelation between MgO and CrzOg.Ilme- Tio2 53.8 L7.4 22.4 9t.L O.4 )4.) nites from Apollo I I low-K and high-K basaltsgen- A12O3 .07 9.2 7.O .o4 11.9 .06 erally have undergonerather limited subsolidus ctzo3 1 11 )Ot '41 .22 47.O .76 ' reequilibration,and tend to follow the experimen- FeO 35.3 36.3 39.4 .57 27.8 37.2 tally-derivedtrend. Our ilmenitedata do not support MnO .42 .40 .4L * .34 .4L the separatetrends shown by Usselman(1975, Fig. Mgo 8.0 7.L 6.7 .18 6.0 7.2 2), and, therefore,his proposal that ilmenitesfrom Total E9.3 99.6 99.t 98.1 99.4 100.1 Apollo I I low-K and high-K basaltshave undergone substantiallydifferent post-magmatic reequilibration Fe/(Fe+Mg) 0.71 0.74 0.77 0.64 0.72 0.74 historiesis unwarranted.Ilmenites from Apollo 17 basalts,especially those from coarse-grainedrocks, Colum (1): Analysls of ilrenite 1are11a in cr- uLv6spinel (Apollo 17 basalt 71588). (2): Analysis of deviatesignificantly from the trend, however.The spinel imediately adjacent to ilmenite lanella. (3): deviationis one of CrzOsdeficiency, and increasesin Analysis of host Cr-u1v6splne1. (4): Analysls of rutile bleb lri ll-menlte (Apollo l7 basalt 77536). (5): Analvals magnitudewith increasingMgO; this in turn corre- of aluolnian chromite associateil wlth rutlle. (6): Analy- with abundanceof exsolvedaluminian chro- sls of host Ll@nite. lates the * = less than 0.01 wt.Z mile (cf. El Goresyand Ramdohr, 1975).Therefore, prior to exsolution,these ilmenite compositions probablyfollowed the experimentaltrend. Assuming mite.As reportedby El Goresyand Ramdohr (1975)' this to be the case,simple calculations show that and supportedby our observations,a direct correla- exsolutionshould change MgO in the host ilmenite tion existsin Apollo l7 basaltsbetween MgO content by at most a few tenthsweight percent. For example, of the host ilmeniteand the amount of aluminian an ilmenitethat originallycontained 7 weightpercent chromiteand rutilevisibly present. Furthermore, we MgO shouldhave -2 weightpercent Cr2Os according note that Apollo 17 basalts(especially the coarse- to Usselman(1975, Fig. 2). If this ilmeniteexsolved grainedsamples) contain the most Mg-richilmenite 1.5 weightpercent CrzOg in the form of aluminian grains(Table 3), and alsoshow the greatest amounts chromite lamellae,and an approximatelyequal of subsoliduscrystallization in ilmenite.The group B1 amountof rutile wasexsolved, MgO in the ilmenite Apollo I I low-K basalts(and a few compositionally- would changeonly to -7.3 weight percent.For a similarApollo l7 basalts)contain the leastMg-rich significantlygreater change in MgO to occur would ilmenitegrains, and ilmenites in theserocks are virtu- requirethe original ilmeniteto havecontained an ally free of subsolidusreaction. Such a correlation implausiblyhigh amount of CrzOs.Thus, even in would be fortuitous if the primary mechanismfor ilmenitesthat haveundergone extensive exsolution, subsoliduscrystallization were an externally-imposed theirMgO shoulddepart from the originalmagmatic reduction process,as suggestedby El Goresy and valuesby at most a few tenthsweight percent,and Ramdohr. FFM shouldbe unchanged.We thus disagreewith the conclusionof El Goresyand Ramdohr(1975, p. Factors affecting magmatic crystallization of opaque 737) that "high-MgO contentcannot be usedas an 4 oxide minerals prioi evidencefor earlycrystallization (of ilmenite)." Magma compositionand oxygen The operationof an internally-drivenmechanism, fugacity triggeredby CrrO3supersaturation of the original Overall compositionalvariation in high-Ti mare ilmenites(and/or TiO, supersaturationin the caseof basaltsis fairly significant,except in the Apollo I I ilmenitereaction rims on armalcolite),provides a high-K suite.This affordsan opportunityto examine simpleexplanation for the relativeamounts of sub- effectsof magmacomposition (and oxygenfugacity) soliduscrystallization products found in ilmeniteon on which opaqueoxide minerals may crystallize,their a grain-to-grainas well as a rock-to-rockbasis. As abundance,order of appearance,etc. shown by Usselman(1975), high CrrO' correlates Cr-ulvdspinelis presentonly in high-Timare ba- with high MgO. High-MgOilmenites would thusbe saltswith )0.3 weightpercent CrrOe, indicating that expectedto preferentiallyexsolve aluminian chro- this valueis more or lesscritical for magmaticcrystal- 1220 WARNERET AL,: MARE BASALTS lizationof that phase.Neglecting variations caused falling temperature, and thus magmatic crystalliza- by differencesin rate of cooling (seefollowing sec- tion of armalcoliteis further constrainedin that FFM tion), modal abundanceof Cr-ulvdspinelcorrelates of the melt must be below a certain value. Papike el positivelywith bulk-rockCrrOr. Thus, type C Apollo al. (1974) have suggestedthat armalcolite ceasesto l7 basaltsare the most CrrO3-rich(Table l), andin crystallize in high-Ti mare basalts when its FFM theseCr-ulvdspinel is most abundant(Table 3). An- reaches0.66 (our data suggesta value -0.70). As other critical factor that may control magmaticcrys- with Cr-ulv vicinity of reactingolivine grains should reflectthis the melt in these components,since both TiO, and enrichmentby beingmore Mg-richthan would nor- CrrO, are enriched in armalcolite relative to high-Ti mallybe expectedon the basisof the bulk rock com- basaltic liquids by factors )5. Similarly, TiO, and position.This wouldresult in a greaterpopulation of CrrOr partition strongly in favor of ilmenite as op- Mg-rich ilmenitegrains in the more slowlycooled posed to high-Ti mare basaltic magmas. Clearly, coarse-grainedbasalts (as observed), and con- then, the net effect of early crystallization of one, two, sequentlyalso in a narrowingof the gap in FFM or all three of the above opaque oxide mineralsis to betweenarmalcolite and ilmenitein the rock.A simi- rapidly depletethe melt in TiO, and CrrOr. lar effect should hold for pyroxeneas well, that is, Partitioning of MgO between Cr-ulvdspinel, ar- pyroxenegrains in coarse-grainedApollo l7 basalts malcolite, or ilmenite and high-Ti basaltic liquids is shouldbe more Mg-rich than in fine-grainedrocks. near unity in eachcase, so that the amount of MgO in Compositionaldata of Warneret al. (1975a,1976b) the melt is largely unaffected by their crystallization. indicatethat this is the case,lending further support FFM of armalcolite is slightly lower than that of the to the aboveinterpretation. coexistingmelt, whereasFFM of Cr-ulvdspinel and Analysesof ilmenitesin Apollo I I low-K basalts ilmenite is much higher than in the melt. Con- show an oppositerelation to that observedin the sequently,early opaque oxide mineral crystallization Apollo l7 basalts.Here, ilmenite grains in thecoarse- has a tendency to slightly lower FFM in high-Ti grainedgroup B, rockstend to be moreFe-rich than basalticliquids. However, MgO is continuouslybeing thosein the more rapidly cooledgroup B, basalts depleted in the melt and FFM raised, becauseof (Table3). In this casethe two groupsare not isoche- simultaneouscrystallization of first olivine and later mical, and it appearsthat the differencesin bulk pyroxene. It is this effect which causes the opaque compositionare the controllingvariables. The group oxide minerals to become increasinglymore Fe-rich B, basaltshave lower TiO, and higherFFM, and as crystallizationproceeds. ilmenite is correspondinglyboth later crystallizing Gravitational crystal settling of early-formed and more Fe-rich.We note also that the group B, opaque oxidesshould produce derivativeliquids that basaltsdo not contain olivine, and so an olivine becomepoorer in TiO, and CrrOs.Simultaneous oli- resorptioneffect was not possible.The most evolved vine fractionation will also result in the derivative Apollo l7 basaltsare compositionallysimilar to the liquids becoming increasingly MgO-poor. An ex- group B, Apollo I I low-K basaltsand they also ample of the general differentiation trend to be ex- contain no olivine and only Fe-rich ilmenites pected is given by the equilibrium liquid-line-of-de- (Warneret al., 1976a). scentdetermined for an Apollo l7 basalt (70215)by In summary,slower cooling enhances chances of Kesson (1975). The original composition contained armalcolitereaction with the melt, and thereforere- (in weight percent)TiO2, 13.2;CrrOr,0.46; MgO, 8.7; sultsin a decreasedmodal abundance of armalcolite. FeO, 19.9(FFM : 0.56).At I l35oc, the liquid com- The modal abundanceof Cr-ulvrispinelalso de- position changed to (in weight percent) TiOr, l0.l; creaseswith slowercooling becauseof increasedCr CrrOr,0.l8; MgO, 6.4; FeO, 19.5(FFM : 0.64). substitutionin pyroxeneand becauseof earlier The compositional variations observedin Apollo plagioclasenucleation (depleting the melt in Al that l7 basaltsreasonably match this experimentallyde- otherwisemight go into Cr-ulvdspinel).Cooling rate termined liquid-line-of-descent.For types A and B greatlyaffects ilmenite morphology but not its abun- basalts, mixing calculations (Wright and Doherty, dance.Ilmenite compositions are cooling-rate-depen- 1970) indicate that deriving the most fractionated dent only insofar as the relativecooling rate affects membersfrom the most primitive requiresseparation the degreeof olivineresorption. of -12 percent armalcolite * ilmenite, -l percent Cr-ulvdspineland - l2 percentolivine (Warner et al., Effect of early opaqueoxide mineral crystallizationon 1975b). If the most primitive compositions are the the differentiationof high-Ti mare basalticmagmas result of addition of the above minerals, even less Crystallization of Cr-ulvdspinelfrom a high-Ti fractionation may have beeninvolved. Compositions mare basalticmagma depletes the melt in CrrO, and of Apollo I I low-K basaltsfall along the equilibrium TiOr, becausethe amountsof CrrO, and TiO, incor- liquid-line-of-descentdetermined for Apollo l7 high- poratedby Cr-ulv 1975).To account for the compositional variations probe microanalysesof silicates,oxides, carbonates,phosphates 1408-1414' among the various types of Apollo I I low-K basalt and sulfates.Anal. Chem.,42, Anderson, A. T., T. E. Bunch, E. N. Cameron, S. E' Haggerty, and olivine requires significantly lesseropaque oxide F. R. Boyd, L. W. Finger, O. B. James,K. Keil, M. Prinz, P. fractionation than is necessary for the Apollo l7 Ramdohr and A. El Goresy (1970)Armalcolite: a new mineral suite.The high KrO content of the Apollo 1l high-K from the Apollo I I samples. PLC, 1, 55-63. basaltsmakes it unlikely that they could be relatedto Beaty, D. W. and A. L. Albee (1978) A textural, modal and of the Apollo 1l low-K basalts(abstr.). either the Apollo l7 or Apollo ll low-K basaltsby chemical classification Lunar Planetary Sci , IX, 6l-63. (Jamesand Jackson, 1970; fractional crystallization Bence, A. E. and A. L. Albee (1968) Empirical correction factors Walker et al.,1975). Within this suite,compositional for the electron microanalysis of silicates and oxides. J. Geol', variation is very limited (Table l), suggestingthat 76,382-403 little or no fractionation has occurred. Cameron, E. N. (1970) Opaque minerals in certain lunar rocks 221-245. The concurrenceof observedcompositional varia- from Apollo ll. PLC, I, El Goresy, A. and P. Ramdohr (1975) Subsolidusreduction of basalts with those predicted tions in high-Ti mare lunar opaque oxides: textures, assemblages,geochemistry, and from gravitational crystal settling of early opaque evidence for a late-stage endogenic gaseous mixture. PLC' 6' oxide minerals (and olivine) strongly suggeststhat 729-745. modest fractionation of this type hasoccurred. How- O. Medenbachand H. J. Bernhardt(1974) Taurus- opaque mineralogy and geochemistry. ever, aside from one olivine-enrichedApollo 17 ba- Littrow TiO.-rich basalts: PLC.5.627-652. (1977), evidencefor salt describedby Warner et al. Friel, J. J., R. I. Harker and G. C. Ulmer (1977) Armalcolite the existence of corresponding cumulate-enriched stability as a function of pressureand oxygen fugacity Geochim. samplesis lacking. Possiblysuch rocks existand sim- Cosmochim.Acta, 4 l, 403-410. ply were not sampled.Alternatively, if fractionation Green, D. H , A. E. Ringwood, W. O. Hibbersonand N. G. Ware petrologyof Apollo l7 mare basalts.PlC, took place during emplacementof the flows, early- (1975) Experimental 6 87 t-893. may have been frozen into , settling opaque oxides Haggerty,S. E. (1971)Subsolidus reduction of lunar spinels.Na- place before they could form an obvious cumulate ture Phys. Sci., 234, I l3-l 17. layer and, instead,the "cumulates" are represented - (1973)Apollo l7: Armaliolite paragenesisand subsolidus by clustersof early crystalsdispersed throughout the reduction of chromian-ulvdspinel and chromian-picroilmenite Am. Geophys. Union, 54, 593-594. interior of the flows. Such partial cumulatescould be (abstr.) EOS, Trans. -, F. R. Boyd, P M. Bell, L. W. Finger and W G. Bryan may be represented among difficult to identify and (1970)Opaque mineralsand olivine in lavasand brecciasfrom the returned samples by the more TiOz-, CrrO.-, Mare Tranquillilatis. PLC, 1, 513-538. and MgO-rich basalts. Haselton,J. D. and W. P. Nash (1975)A model for the evolution of opaquesin mare lavas.PLC,6,747-755. Acknowledgments James,O. B. and E. D. Jackson(1970) Petrology of the Apollo I I Geophys. Res.,75, 5793-5824' We thank Dave Lange for preparing polished thin sections of ilmenite basalts "/. (1967) The electron microprobe X-ray analyzer and its Apollo l7 basaltsand implementingcomputer programs for cor- Keil, K. in mineralogy. Fortschr. Mineral',44' 4-66. rection of mineral analyses; George Conrad for maintenance of application pet- electron microprobe facilities; Bill Mansker, John Berkley, and Kesson,S. E. (1975) Mare basalts:melting experimentsand Richard Warren for obtaining mineral analyses;Steve Willcut and rogenetic interpretations. PLC, 6, 921-944. Sandy O'Kelley for assistance in data compilation; and Mary - 26fl D. H. Lindsley (1975) The effectsof Al3+, Cr3+, and Fillmon for typing the manuscript. The manuscript has benefited Ti'+ on the stability of armalcolite.PLC' 6,911-920. from critical reviews by Jeff Warner and Eugene Cameron. We are Lindsley, D. H., S. E. Kesson,M. J. Hartzman and M K' Cush- especially appreciative of the diligent efforts of Associate Editor man ( 1974) The stability of armalcolite: experimental studies in CornelisKlein. This work was supportedby NASA grant NGL 32- the system MgO-Fe-TiJO. PLC, 5, 521-534. 004-063 (Klaus Keil, Principal Investigator). Longhi, J., D. Walker, T. L. Grove, E. M. Stolperand J' F. Hays (1974) The petrology of the Apollo 17 mare basalts.PIC, J, References' 447469. Nehru. C. E., R. D. Warner and K. Keil (1976) Electron Micto' Agrell, S. O., A. Peckett,F. R. Boyd, S. E. Haggerty,T. E. Bunch, probe Analyses of Opaque Mineral Phasesfrom Apollo I I Basalts E. N. Cameron, M. K. Dence, J A. V. Douglas, A. G. Plant, Spec. Publ. No. 17, University of New Mexico-Institute of R. J. Traill, O. B. James,K. Keil and M. Prinz (1970)Titanian Meteoritics. chromite, aluminian chromite and chromian ulvdspinel from and S. Waterston(1975) Petroge- Apollo ll rocks, PlC, 1, 8l-86. O'Hara, M. J., D. J. Humphries for chemical, mineralogical, Albee, A. L. and L. Ray (1970) Correction factors for electron nesis of mare basalts: implications and thermal models for the moon. PLC' 6' 1043-1055' Papike,J J., A E. Benceand D. H Lindsley(1974) Mare basalts Taurus-Littrow region of the m oon. PLC, 5' 471-504. 2 PLC, I, 2, etc. : ProceedingsLunar Science Conference, 1st, from the Znd, etc. Prinz. M., T. E. Bunch and K, Keil (1971)Composition and origin 1224 ,YARNERET AL : MARE BASALTS of lithic fragmentsand glassesin Apollo I I samples.Contrib. -, J. L. Berkley,W. L. Mansker,R. G. Warrenand K. Keil Mineral. Petrol., 32, 2ll-230. (1976a)Electron Microprobe Analysesof Spinel, Fe-Ti Oxides Rhodes,J. M., N. J. Hubbard,H. Wiesmann,K. V. Rodgers,J. C. and Metal from Apollo l7 Rake SampleMare Basalts.Spec. Pub. Brannonand B. M. Bansal(1976) Chemistry, classification and No. 16,University of New Mexico-Instituteof Meteoritics. petrogenesisof Apollo l7 mare basalts.pac, Z, 1467-1499. -, R. G. Warren,W. L. Mansker,J. L. Berkleyand K. Keil Smyth, J. R. (1974)The crystalchemistry of armalcolitesfrom (1976b)Electron MicroprobeAnalyses of Oliuine, Pyroxeneand Apolfo 17.Earth Planet Sci.Leu., 24, 262-2'10. Plagioclasefrom Apollo 17 Rake Sample Mare Basalts. Spec. Sung,C.-M., R. M. Abu-Eidand R. G. Burns(1974) Ti3+/Ti1+ Publ. No. 15, Universityof New Mexico-Instituteof Meteor- ratiosin lunarpyroxenes: implications to depthoforigin ofmare i tics. basaltmagma. PLC, 5,717-726 -, K. Keil and G. J. Taylor (1977)Coarse-grained basalt Usselman,T. M. (1975)Ilmenite chemistry in marebasalts, an 71597:a productofpartial olivineaccumulalion. PLC,E,1429- experimentalstudy (abstr.). ln Originsof Mare Basaltsand Their 1442. Implicationsfor Lunar Euolution,164-168. The Lunar Science C. E. Nehru and G. J. Taylor (1978)Catalogue of Institute,Houston. Apollol7 RakeSamples from StationsI A, 2,7 and8. Spec.Publ. -, G. E. Lofgren,C. H. Donaldsonand R. J. Williams(1975) No. 18,University of New Mexico-lnstituteof Meteoritics. Experimentallyreproduced textures and mineralchemistry of Wechsler,B. A., C. T. Prewittand J. J. Papike(1976) Chemistry pLC, high-titaniummare basalts. 6, 997-1020. and structureof lunar and syntheticarmalcolite. Earth Planet. Walker,D., J. Longhi,E. N. Stolper,T. L. Croveand J. F. Hays Sci.Lett., 29.9l-103. (1975)Origin of titaniferouslunar basalts. Geochim. Cosmochim. Williams, K. L. and L. A. Taylor (1974)Optical propertiesand Acta,39, 1219-1235. chemicafcompositions ofApollo l7 armalcolites.Geol., 2,S-8. Warner,R. D., K. Keit, M. Prinz,J.C. Laul, A. V. Murali and Wright,T. L. and P. C. Doherty(1970) A linearprogramming and R. A. Schmitt(1975a) Mineralogy, petrology, and chemistryof least squarescomputer method for solvingpetrologic mixing mare basaltsfrom Apollo l7 rake samples.pLC,6, 193-220. problems.Geol. Soc Am. Bull.,81, 1995-2008. A. V. Murali and R. A. Schmirr(1975b) petroge- neticrelationships among Apollo l7 basalts(abstr.).ln Origins of Mare Basaltsand Their Implicationsfor Lunar Euolution,179- Manuscript receiued, December 5, 1977; accepted 183.The Lunar ScienceInstitute. Houston. for publication, May 22, 1978.