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Opaque Oxide Mineral Crystallization in Lunar High-Titanium Mare Basalts

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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
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