American Mineralogist, Volume 59, pages 231-243, Ig74
Lunar Mineralogy:A HeavenlyDetective Story PresidentialAddress, part 11
JosnpnV. Surrs Department ol the GeophysicalSciences, Uniuersity ol Chicago, Chicago,Illircis 60637
Abstract
general A introduction emphasizes the strolg interactions between mineralogy and related sciences,and the need for cooperative programs among the various mineralogical groups. provide To maximum access to the literature, the bulk of lunar rock types are classified into ANT, FETI, and KREEP groups. The Anorthositic-Noritic-Troctolitic compositions mostly occur as metabreccias, and are believed to come from the crust. The basalts rich in Fe and Ti result from late lava flows in mare basins. The KREEP suite of basaltic compositions occurs mostly as glassesand metabreccias rich in K, REE, P, and other large-ion elements; it prob- ably results from a late liquid in primary differentiation of the Moon, or partial melting of ANT rocks, or both. Some lunar rocks are hybrids formed by melting of soils, Metamorphism is widespread. Minor rock types include peridotite, spinel allivalite and troctolite, and other fragments containing Mg-rich minerals. Granitic material occurs rarely. Minor meteoritic fragments occur. Major mineral groups are pyroxene, feldspar, olivine, spinel, pseudobrookite (armalcolites), silica (quartz, tridymite, cristobalite), Fe-rich types (iron, troilite, ilmenite), phosphates (apatite, whitlockite), and Zr-minerals (zircon, baddeleyite). Minor silicates include pyroxfer- roite, amphibole' garnet' and tranquillityite. Minor sulfides are mackinawite, pentlandite, cubanite, chalcopyrite, and sphalerite. Minor oxides are rutile and corundum. Minor metals include copper, brass, and tin. A Zr-rich mineral is either zirkelite or zirconolite. Schreibersite, cohenite, and niningerite probably derive from iron meteorites. Rusty areas probably consist of goethite perhaps associated with hematite and magnetite; presence of Cl suggestspossible terrestrial oxidation of meteoritic lawrencite. Important features of feldspars are: mostly calcic plagioclase, but rare K,Ba-feldspar in residua; correlation of Mg,Fe,K, and Na with rock type giving test of differentiation models; Eu anomaly depends on temperature, oxidation state, etc; solid-solution of ca(Mg,Fe)sLo, id FETI plagioclase; exsolution of pyroxene, silica, and Fe in ANT plagioclase. Important features of olivines are: correlation of mg [- atomic Mg /(Mg + Fe)] with rock giving type test of differentiation models; Ca may distinguish volcanic from plutonic environ- ment; cr enters as divalent ion, and may provide guide to oxidation state; Mg,Fe ordering of some olivines indicates long annealing; melt inclusions give magma composition; high Al in some olivines may correlate with spinel exsolution; melt inclusions give magma cornporitioo.
Introduction presentation many relerencesore giuen, but these are highly selectiue This written address differs considerably lrom the and chosen for conuenienceand oral presentation. Because ol time limitations, the not lor priority or prestige. In particular, relerences general introduction was omitted. Seueral handred tend to concentrate on controaersial items and on min:erdogists and peffologists worked o,n the lunar papers submitted lor publication, Matters of com- program, and most ideas were discouered almo,st mon krwwledge ffe not fally documented. simultaneously by seaeral persons or groups. In the The PresidentialAddress provides an opportunity oral presentation, relerences to indiuidual scientists for the Retiring Presidentto philosophizeor to rv were deliberately omitted to auoid tedious repetition view some aspect of his work or of mineralogy in and inuidious selection of names. In this written general. I shall briefly pontificate, and then use a l PresidentialAddress, Mineralogical Society of America. review of lunar mineralogyas an implicit illustration Delivered at the 54th Annual Meeting of the Society, 13 of my main theme: that the distinctionsbetween November1973. mineralogr and allied sciences(including geochemis- 231 IOSEPH V. SMITH try, geophysics,petrology, and crystallography) are Unquestionably,it is important to develop strong blurring so rapidly that researchmineralogists should cooperativeprograms among the various mineralog- become competent in as many areas as possible, and ical groups (and also among the various geological should collaborate closely with scientists lvith related societies); otherwise prolonged isolation leads to and overlapping interests. distrust and antagonism.It would be tragic if the It is quite impossible to understand the phase world of mineralogy split up into separatecamps, equilibria of (say) plagioclase feldspars without in- each fearful and contemptuousof the others. formation on the following: (1) controlled labora- Finally, I want to mention the practicaluses of tory synthesesat high temperature with X-ray dif- minerals.Agriculture dependson the proper inter- fraction and electron-optical characterization of the action between the inorganic minerals and the or- products; (2) chemical, X-ray diffraction, and elec- ganiccontent of soils.Many chemicaland all metal- tron-optical characterization of plagioclases from lurgical industriesdepend on extraction of minerals, natural rocks ranging from volcanic to metamor- some of which are becoming scarce.Novel uses of phic; (3) heating studies of natural specimens. In minerals, such as the employmentof zeolites as addition, calorimetry and resonancetechniques, plus catalystsin the petroleum industry and the appliea- many others, provide further knowledge. A crystal- tion of mineral analogs as lasers and solid-state lographer without a thermodynamic background devices,should attain increasingimportance; for might misinterpret the phase relations, while a geo- example,several rare mineralswith strong pleochro- chemist might overlook the importance of domain ism may havepotential electronic uses' For this rea- texture in the occurrence of trace elements. The son, mineralogicalsocieties should also collaborate electrical conductivity and diffusion coefficients of closely with chemical and physical societies. ferromagnesian minerals depend strongly on struc- No mineralogistcan rest on the knorvledgegained tural defects and the oxidation state, thereby having in his formal education.One of the greatadvantages a profound effect on geophysical interpretation of of the lunar program was the opportunity for the phenomena such as creep in the mantle and the tem- scientiststo meet closely with practitioners in other perature profile of the Moon. The routine use of the disciplines. Some mineralogists gained a working electron microprobe (hopefully to be joined by the knowledgeof such apparentesoterica as a europium ion microprobe for trace elements and isotopic ra- deficiencyand the time relation of BABI and ADOR. tios) brings geochemical data within the immediate It is extremely important to provide opportunities control of the mineralogist and petrologist. The or- for mineralogiststo expandtheir knowledge,and the ganization of research is beginning to utilize the Mineralogical Societyof America should becomeac- power of coordinated studies, as in the mineralogy- tive in organizing short coursesand symposia,and petrology-geochemistry consortia for lunar speci- in publishing critical monographsand treatises' mens. Many mineralogical problems must be studied So much for the pontificating;now for lunar min- by groups of investigators, often geographically dis- eralogy with its illustration of the strong intercon- persed; such investigators should be diplomats as nectionsbetween mineralogy, petrology, geochemis- well as scientists. try, andgeophysics. Fortunately,not all mineralogyrequires the big battalionswith the kilo- or mega-buck.A perennial Lunar Mineralogy joy of mineralogyis the scopefor the collectorand Introduction the artist, for the field man and the museumcurator. Somemodern artists examine the world for existing The completionof the Apollo missionsprovides a natural beauty; collectors of minerals have been suitabletime to evaluatethe statusof lunar miner- doing this foi centuries.Mining operationscontin- alogy.An article"How the Apollo programchanged ually reveal new mineral deposiisof spectacularin- the geology of the Moon: suggestionsfor future terest,such as the one at St. Hilaire. Somemineral studies"by Smith and Steele(1973) givesa general museumsexpand their collectionsand put on mag- background.Briefly, the Moon was in orbit around nificent dispiays;others deteriorate.Probably most the Earth at least 4 giga-yearsago; it underwent mineralogistsrecognize the value of a pluralisticap- major chemicaldifferentiation at least3'9 giga-years proach,*a uppr.iiute the mutual interestsand dif- ago (and perhapsas early as 4.5 giga-yearsago) ierent stylesof the various types of mineralogists. producinga crust, a mantle,and perhapsa core; in- LUNAR MINERALOGY 233
tensebombardment devastated the surfaceto depths ficult geochemicaland geophysicalproblems, espe- of severaltens of kilometers,perhaps with significant cially concerning(1) the sourcesof energyand the late accretionof the incoming projectiles;lava flows timing of igneousevents on the Moon, and (2) the coveredimpact basinswith the last activity occurring ultimate origin of the bulk chemistry of the Moon. near 3 giga-years ago (though some unexplored The mineralogy and petrology of the Moon pro- areasof the Moon may havelater extrusions);and vides a fascinatingchallenge to the lunar detective. continuing bombardmentby relatively small projec- Few samplesreturned to Earth are simple igneous tilesproduced a surfacecovered by debris(regolith). rocks. Most consist of complex breccias or fine- Geophysical and geological data show that the grainedsoils. In spite of careful selectionof prospect- presentcrust of the Moon consistsof a looseregolith ing sites and detaileddocumentation (too detailed some meters thick lying on some tens of kilometers for the temper of many lunar investigatorsfaced with of material with low seismic velocity. In the mare sampledescription forms!), it is very difficult to pin regions, the upper parts are dominated by basalts down the actual starting point of most lunar samples. rich in Fe and Ti, whereasin the highland regions, Fortunately the mineralogy of the Moon is rather the dominant rocks are probably rich in calbic simple compared to that on the Earth, comprising plagioclasewith subordinate pyroxene and olivine. only some few dozens of speciescompared to sev- The nature of the hypothetical mantle is controver- eral thousand (ultimately, of course, the full range sial, but many workers believe that it is dominated of meteoritic minerals should be found on the by magnesianolivines and pyroxenes.Figure I, a Moon). There are.few rock types (though one model of the Moon proposed after the Apollo 11 might not think so from the confusingnomenclatures mission, may provide a crude description for a time in the literature!). In general,it is possibleto ar- prior to 4 giga-yearsago. There are some very dif- range most of the information on lunar rocks and minerals into plausible sequencesconsistent with Thin crusl (5-2Okm) Plogioclose - rich crystal-liquiddifterentiation, metamorphism, hybridi- underloinby liquid rock ond bosolfs zation.The major uncertaintiesrelate to geochemical Impocls creote =2.8-l.t P gm/cmJ lorge lovo lokes and geophysicalproblems: what were the time se- (seos quencesof crystal-liquid fractionation; what was the extent of late accretion; how important were re- melting and metamorphism;what are the implica- ._ ei?,r'- Forside tions for the internal structure and the bulk com- ick Crust, liftle or no position of the Moon? In attemptingto answerthese underlyingliquid questions, the mineralogic detective must bear in Impoclscreote mind that the regolith is dominated by crustal ma- deep crolers terial,and that deepseatedmaterial should be sparse Liquid feirobosotl wilh liff le"or p = 3.3 gm/cm3 no lovo. and found mainly as fragments in breccias from major impacts. (drownto neorside t'thin loyerof sulfide by Eorth's grovity ond ilmeniie- rich due to high densily rock.p:5 Generalstatement on lunar rock types comporedlo overlyingcrusf') The nomenclature of the lunar rocks is in a sham- Ftc. 1. A possiblemodel for the Moon during primary bles. Unfortunately many lunar rocks are actually differentiationprior to 4 giga-yearsago. This m9!-e] was metamorphosed breccias or remelted regolith, ani proposedin l97o beforedetailed characterizatio" ,o,n" ,ru.". are given on the basis of bulk chemistry rocks. Major reworkingof the outer part of "l-*fP!"the Moon "".:'- ":"'- would occur in the neit giga-year,and the FEn-(;;.;; rather than the petrology and mineralogy' Further- basaltswould appearin this period.The ANT group more, the chemistry of the minerals is biased towards '"plagioclase-rich might consistof the rock and basalts"plus fur- alkali-poor compositions and reduced states of transi- ther differentiatesfrom the ferrobasalt.The KREEp group tion metals with respect to terrestrial analogs, thereby might arise from both the late "ferrobasalt"and reme.lting causing'* tf,oUt"11, when terrestrial names are applied of the ANT group.The FETI basaltsmight result from --l; " to lunar remelting of cumulatesof pyroxene and oxide ;";;; rocks. followed by complexfractional crystallization.Minor ultra- The following nomenclature and description of basicrocks might be cumulatesfrom the uppermantle and the major rock types are chosen to maximize initial crust.From Smither al (1970). accessto the literature: 234 IOSEPH V. SMITH
ANT suite: (name from Anorthositic-Noritic- tures (e.g.,Anderson et al, 1972;Chao, Minkin, and Troctolitic; Keil el aI, 1972) comprised mainly of Best, 1972 Warner, 1972), and it is not always calcicplagioclase, orthopyroxene, and olivine; rang- obvious what rocks and minerals composedthe ing from anorthositic to noritic and troctolitic in original breccia. (3) The applicabilityof the terms bulk composition.Mostly occurs as metamorphosed high-aluminabasalt and very high-aluminabasalt breccias (often monomict), fragments,or glasses. (e.g.,Bansal et al, 1973) with respectto other rock Commonlyassigned to crust.Believed to resultfrom descriptionsis not fully clear at the moment (see early differentiationwith gravity separationof crys- later); whethersuch rocks representa distinctigne- tals and liquid. Termslike gabbro,anorthositic gab- ous suite, or whether they are hybrids formed by bro, and highlandbasalt are also used. meltingof regolith,or both, is unclear. FETI suite: (new name referring to high Fe and On the whole the abovethree-fold grouping covers Ti content) comprisedof basaltsrich in Fe and Ti; many of the lunar rocks, correlatesfairly well with dominated by zoned pyroxenes (mostly augitic), the chemistry of the component minerals, and per- calcicplagioclase, and ilmenite.Olivine may or may haps also correlateswith chronologicaldevelopment not occur. The final residuumcontains several rare of lunar rocks.It certainlyallows a preliminaryex- mineralsand has a granitic component.Mostly oc- amination of the petrogeneticsignificance of min- curs as rock fragments,but some glassesoccur. In- eralogicalproperties. terpreted as the result of late lava flows in mare The nature and extentof the minor rock typesis basins,probably mostly frorn the tops of lava col- rather uncertain. Crystal-liquid differentiation of umns severalkilometers thick.' Origin controversial basalticcompositions leaves a final immiscibleliquid but perhaps from remelting of cumulates rich in with granitic affinity (e.g., Roedder and Weiblen, pyroxenesand opaque minerals, modified by near- 1970). Minor glassyor crystallinerepresentatives surfacedifferentiation in lava suites.Usually called were found in the lunar samples,but whetherlarge marebasalt in literature. rock bodies of granitic composition occur is uncer- KREEP suite: (name is acronym given by Hub- tain. Grains of olivine and pyroxene with very Mg- bard et aI, l97la) a description introduced for ba- rich compositionsoccur as a minor constituentin all salticcompositions rich in K, REE, P,Zr,Ba, U, and the lunar soils (seemany papers),and spinelalliva- Th. Mostly occursas glassesand meta-brecciasbut lite and troctolite samples(e.g., Agrell et al, 1973; a few crystalline rocks with basaltic texture occur. Prinz et aI, 1973a), a peridotite clast (Anderson, Dominated by plagioclase,low-Ca pyroxene and 1.973) and ultrabasic fragments (e.9., Steele and mineralssuch as apatite and zircon which contain Smith, 7972\ have been described.All thesemate- the abovecharacteristic elements. Origin controver- rials are more Mg-rich than equivalents from the sial but perhapsresults from either liquid produced broad ANT suite. A possible source is the upper at the crust-mantleinterface during primary difter- mantle, but crustal processesor accretionmust be entiation, or liquid from partial melting of ANT considered.Rare fragmentsof iron meteoriteshave crustal rocks, or both. Other terms includinghigh- been found with characteristictextures, and other aluminabasalt are also used. iron specimensare believedto repfesentmelted or There are some problems with this three-fold metamorphosedmeteorites on chemical evidence. nomenclature:(1) There is evidenceof both low- Contributions of stony meteoritesto the lunar fines and high-K anorthosites (e.9., Hubbard et al., have been recognizedfrom bulk chemistry (e.9.' l97lb); perhapsthe former wereproduced by flota- Ganapathyet al, l97O), and an enstatitechondrite tion of plagioclaseduring formation of the crust,and has been found (HaggefiY, 1972a). the latter from crystal-liquid differentiationof a K-rich liquid produced by partial melting of the Types ol Lunqr Minerals crust; if so, the former might be assignedto ANT, The major mineralsto be describedin detail later (2) and the latter to KREEP. Somerocks such as are: 14310 almostcertainly were producedby remelting of regolith (e.g.,Dence and Plant, 1972), and there- Pyroxene (Mg, Ca, Fe, Al, Ti, Cr, etc) fore have a bulk compositionderived from many (Si, elc)O3.Very low in Na and K. componentsincluding meteoritic infall (Morgan Feldspar Mostly anorthiteand bytownite,but et aI, 7972); many brecciasshow metamorphictex- rare K, Ba-feldsparin residua. LUNAR MINERALOGY 235
Olivine Mostly Mg-rich but rare fayalite in rocks, e.g. Taylor, Mao, and Bell, 1973), and mag- residua.Divalent Cr. netite?(in "rusty" rocks). Spinel N MgAlrOn in Mg-rich rocks. Hydroxylated mineral: "rusty" areas on some Chromite zoned to ulviispinel in rocks contain goethite? or another iron oxide hy- FETI basalts. droxide (Agrell et al, 1972), perhapswith hematite Pseudobrookite Armalcolite group with divalent and magnetite;presence of Cl suggestspossibility of iron. Low Cr and Zr in FETI terrestrial oxidation of lawrencite,originally derived basalts and high Cr and Zr in from cometary or meteoritic contamination on the metamorphosedMg-rich rocks. Moon (El Goresy et al, 1973; Taylor, Mao, and Silica Tridymite, cristobalite,and quartz. Bell, 1973), but lead isotopedata suggestthat the Fe-rich Iron, troilite, and ilmenite occur, material is indigenousto the Moon (Nunes and especiallyin FETI basalts. Tatsumoto, 1973). Phosphates Apatite and whitlockite, especially Minor minerals,probably largely from iron meteo- common in KREEP rocks. High in rites, are: schreibersite,cohenite, and niningerite. REE, Cl, and F. The nature of the lunar mineral variouslycom- Zr-minerals Zircon and baddeleyitetypical of pared with terrestrial zirconolite or zirkelite is con- KREEP rocks; also in residua of troversial, and X-ray structural data are needed to FETI basalts. provide a control for assignmentof atoms: either (Ca, Fe, etc) (Zr, etc)zOzfor zirconoliteor 1:1:5 Minor silicateminerals are: pyroxferroite(analog for zirkelite (Wark et aI, 1973;Busche et al, 1972; of pyroxmangite, common in residua of FETI ba- Peckett, Phillips, and Brown, 1972; Brown et aI, salts;Chao et al,1970; Burnham,1971), amphiboles 1972). Other Zr, Ti-rich mineralsmay occur (e.9., (single occurrencesof alumino-tschermakite(Dence Zl: Haggerty.1972b). et al, l97l), and magnesio-arfvedsonite-richterite Minor metallicminerals are: copper (tiny grains (Gay, Bancroft,and Bown, 1970) in FETI basalts; near Fe metal, ilmenite,or troilite), brass (variable high in halogens), garnet (almandine-richgrains in compositionextending outside terrestrial range), tin a FETI basalt; Traill, Plant, and Douglas, l97O), (embeddedin Fe metal), and indium (contaminant tranquillityite (residua of basalts; Lovering et al, from rock box). I97l), melilite? (uniaxial grainsin fines,no X-ray Aragonite(Gay, Bancroft,and Bown, 1970), SiC or chemicaldata; Massonet al, 1972), phyllosili (Jedwab,t973), and graphitemay be contaminants. cates??(in fines,probably terrestrialcontaminants; Dreveret al, l97O), sphene??(perhaps in devitrified Important Aspectsol Feldspars spherule,tentative X-ray powder data; Gay, Ban- In this and succeedingsections, some aspectsof croft, and Bown, l97O\. the major mineral groups important for understand- Minor sulfide minerals are: mackinawite,pent- ing the genesisof lunar rocks and minerals are out- landite, cubanite,chalcopyrite (all associatedwith lined. troilite at marginsor as inclusionsconsistent with The great majority of lunar feldsparsare calcic exsolution or inversion; optical and chemical iden- plagioclases.K,Ba-feldspars crystallize in the residua tification but no X-ray data; e.g., Taylor and Wil- of FETI basaltsand occur in KREEP basalts;they liams, 1973), sphalerite(e.g., El Goresy,Taylor, may occur with silica minerals as a granitic rock andRamdohr, 1973). typ @.g., fragment of hedenbergitegranophyre, Minor oxide mineralsare: rutile (associatedwith Masonet al, l97L). Sodicplagioclases are very rare ilmenite as lamellae and inclusions indicative of and may resultfrom minor meteoriticcontamination. exsolution,e.9., Haggerty,1972b; as distinctgrains The plagioclasesin the FETI basaltsrange from in a peridotite,Anderson, 1973; and as blue grains sector-zonedhollow tubes to anhedral grains inter- in soils, Jedwab, 1973), corundum (in fines as gro\rynwith pyroxene. Plagioclasesfrom ANT rocks aggregates,Kleinmann and Ramdohr, 1971; and usually show textures ranging from fine-grained as large crystal twinned on (0001), Christophe- "equilibrium" mosaicsto irregular shock textures; Michel-Levy bt al, 1973; vapor deposit?,meteoritic? probably most textures result from one or more or terrestrial contaminant?),hematite? (lamellae episodesof metamorphism(both thermaland shock in ilmenite,and associatedwith goethite?in "rusty" types) following an earlierigneous stage. |OSEPH V. SMITH
Electronmicroprobe analyses of ANT plagioclase KREEP plagioclasesappear to overlap.The Mg and are consistentwith the MTaOs formula, but inter- K contentsappear to be distinctivefor the threetypes pretation of analysesof FETI plagioclaseis unclear. of plagioclase,but the data are probablyconfused by The location of the Mg and Fe atomsis uncertain, relativelylow accuracyof electronmicroprobe analy- and the closestfit to an MT4O8formula is obtained sesfor theseelements. Smith (1971,)noted that the if theseelements are mostly in the tetrahedralsites. compositionbands seemedto divergefrom a com- Weill el ol (1971,) outlinedthe problem,and Wenk positionnear Ab3; K 0.03,Fe 0.1, Mg 0.01 wt per- and Wilde (1973\ coordinatedthe availabledata. cent, and this turnedout to be a commoncomposi- Mcissbauerand electron-spinresonance data show tion for plagioclasein anorthosites(e.9., Steeleand that a small part of the iron is ferric while most is Smith, l97l). Plagioclasesfrom a peridotiteand a ferrous,and that the iron enterstwo structuralsites, pink-spineltroctolite fall near the main concentra- one of which is tetrahedral(e.9., Schiirmannand tion rangefor ANT rocks.As an illustrationthat the Hafner, 1972a;Hafner, Niebuhr, and Zeka, 1973); situation is not simple, data points are shown in see also optical absorptiondata (Bell and Mao, Figure2 for a feldspathicbasalt with igneoustexture 1973). but Fe contenteven lower than for KREEP basalts The compositionof the calcic plagioclasescor- (Steeleand Smith, 1,973). relateswith the host rock. Figure 2 is a plot of wt The well-knownpreference of Na for liquid over percentFe os mole percentAb inferred from the Na coexistingplagioclase (e.9., Kudo and Weill, 1970) content. The plagioclasesfrom the FETI basalts allowsthe Na contentof lunar plagioclaseto be used have the highestFe contents,and there is a definite as a test of differentiationmodels. The low Ab con- tendencyfor the Fe contentto increasefrom about tent of ANT plagioclaseis consistentwith their origin 0.5 wt percentat Ab5 to 1 wt percentnear Abrz. as cumulatesfrom liquids which later produced The plagioclasesfrom the KREEP basaltstend to be KREEP plagioclase(note the very calcicplagioclase more sodic,mostly occurring in the rangefrom Abro in the pink-spineltroctolite); alternatively,partial to Abzs: again the Fe contentcorrelates with Ab, melting of ANT rocks could yield liquids which give but the concentrationis lower than for FETI basalts. rise to KREEP plagioclase.If FETI basaltsresult The third main groupof plagioclases,that from ANT from partial melting of a plagioclase-bearingsource rocks, tendsto be highly calcic with mcst compos! material,that plagioclasewould be extremelylow in tions nearAbs. The compositionranges of ANT and Ab content,assuming that equilibrium occurred. Unfortunatelythere are few laboratory data on the partition of Fe, Mg, K, etc betweenplagioclase t.2 and basalticliquids, though one might expectthat the substitutionof Mg and Fe in plagioclasewould de- creasewith temperature,and that the Mg/Fe ratio - 0.8 r will dependboth on the oxidation state and the Mg/ KREEP Fe ratio of the coexistingliquid. Greenet al (1972) d-S bosolls and Akella and Boyd (1972) reported 0.3-0.6 = ------MgO and 0.5-1.0 wt percent FeO in plagioclases x --\__Ilori--tltz- xxx x synthesizedin the presenceof liquid from lunar "I i--J-b-,x___) 0-20r00 basalticcompositions, which valuesare comparable mol.% Ab with those observedin FETI basalts.Systematic data are badly neededto allow use of these ele- plagioclases. Frc. 2. Fe us Na content of lunar Schematic ments for testing difierentiationmodels. The mg principal for FETI basalts,KREEP drawing showing ranges (where : basalts, and ANT materials. P shows the plagioclase in ratio of the plagioclase mg atomic Mg/ peridotite clast 15445,10 (Anderson, 1973) and PST that (Mg + Fe) ) may prove valuable in giving an for pink-spinel troctolite (Prinz et al, 1973a). The ranges indication of the parent liquid (e.g., Walket et aI, for plagioclasesfrom KREEP and ANT rocks probably 1973); sparsedata of low accuracysuggest that overlap. The crossesshow data for 67747 feldspathicbasalt lng tends to be higher for KREEP than for FETI large poikilitic olivines feldspar which contains enclosing the laths and intergranularpyroxenes (Steele and Smith, 1973). plagioclase,as expectedby comparisonwith Data from many sources.Data for Mg, Mg/Fe, and K to be data for pyroxenes.Crawford (1973) found that publishedelsewhere. mg of zoned FETI plagioclasewas the same as LUNAR MINERALOGY 237 for pyroxene crystallizing simultaneously.Detailed that the europium anomaliesin lunar rocks can be measurementsof the alkali and alkaline-earthele- ascribed largely to plagioclaseJiquidfractionation. ments are badly neededas tests for differentiation There are many electron-diffraction and X-ray models; unfortunatelythe concentrationsof these difiraction studiesof lunar plagioclaseswhich show elements, except for Na and K, are too low for complex domain textures indicative of exsolution accuratemeasurement with an electronmicroprobe, and order-disorder. Unfortunately, the phenomena but hopefully detailed ion-microprobe data will be depend in a complex way on the bulk chemical forthcoming (see Andersen,Hinthorne, and Fred- composition and on the crystallization and post- riksson, 1970, for indicativedata). crystallizationhistory. At the moment the data are Interpretation of the minor element content of of more value in providing evidenceon the phase plagioclases from ANT rocks, metamorphosed relationsof plagioclasein generalthan on the spe- breccias,and fines must take into account the effect cific problems of lunar mineralogy. of solid-statereactions which change the content It is quite certain that lunar plagioclasecan of minor and trace elements.Steele and Smith provide many clues to the discerninglunar de- (1974) pointed out that the pyroxene and silica tective. minerals occurring as inclusionsinside ANT plagio- clase,and as grains at plagioclaseboundaries (e.g., Important Aspectsol Oliaines James, 1972), may result from solid-state expul- The mg value of lunar olivines provides a valu- sion from plagioclasewhich originally showed high able guide to crystal-liquiddifferentiation. Figure 3 contentsof Fe and Mg, perhapscomparable to those summarizesthe principal ranges of mg for lunar for FETT plagioclase. They proposed that with olivines togetherwith correspondingdata for pyrox- falling temperaturethe componentCa( Fe,Mg) SisOe enes (seenext section). broke down into pyroxene Ca(Fe,Mg)Si2O6and The ANT group of rocks carries olivines mostly silica SiOe. If this is true, the pyroxenesin some in the range of mg = 0.85 - 0.65 whereasthe lunar anorthositesconsist partly or perhapswholely FETI group carries olivines ranging mostly from of material exsolvedfrom plagioclase.Nevertheless 0.7 to 0.3 with rare fayalitesin the final residua. the mg content of the pyroxene may provide a guide to that of the liquid from which the plagio. clase crystallized.The best evidencefor exsolution KREEP is that of Lally (1972) et al who obtainedelectron (mol.%) micrographsof pyroxenegrains lying in twin bound- ariesof plagioclase.Brett et al (1973), and others, , reported tiny oriented rods of FeeeNi, metal in plagioclase grains from Luna 20, and preferred sub-solidusreduction as the explanation. En Fs The europium anomaly has figured in many geo- Fo Fo chemical discussionsof lunar rocks. Unfortunately r00 80 60 40 (v all accurate data obtained to date are on bulk Ftc. 3. Schematic drawing of major-element concentra- samplesusing neutron activationor mass spectro- tions of lunar olivine and pyroxene. Note that considerable metric analyses.Under reducing conditions,euro- zoning occurs and that the ranges cover the principal but pium becomesdivalent, whereas its neighboring not the total ranges of composition. The pyroxenes of FETI rare earths remain trivalent. In the divalent state. basalts show final zoning to pyroxferroite or Fe-rich pyroxenes, europium tends to enter the feldspar structure while the olivines zone into fayalite. Both KREEP pyroxenes give preferentially, and FETI compositions which thus giving a positive anomalyin a tend to concentrateat the Mg-rich side of the ranges.Data feldspar and a negative anomaly in a coexisting from many sources. The data for ultrabasic fragments basalticliquid. Unfortunately,the europiumanomaly (filled circles) are from Steele and Smith (1972). P and dependsin a complex way on the temperature, PST refer to peridotite and pink-spinel trocolite (see Fig. oxygenfugacity, and bulk composition(e.g., Weill 2). The rectangle shows the composition range of large crystals of orthopyroxene in Apollo 17 fines which may and Drake, 1973) and carefulstudy will be needed result from a true norite with coarse texture: veins of before quantitativeevaluation of lunar rocks can diopside have the same composition as the filled circle be made. Neverthelessit seemsreasonablv certain (Irving et al,1974). 238 TOSEPH V. SMITH
Ultrabasic rocks and fragments carry olivines with of mg for the bulk compositionof mostANT rocks. mg = 0.93 - 0.83; sevencompositions found by If the FETI basalts result from partial melting of Steeleand Smith (1972) for clasts and fragments a rock dominatedby olivines and pyroxenes,the are shown in Figure 3 along with that of 0.91 for mg rangeof the latter should be about 0.6 to 0.8. the peridotite clast of Anderson (1973) and of Partial meltingof the ANT suite shouldgive liquids 0.92 for the pink-spinel troctolite of Pinz et al with mg mostly from 0.4 to 0.6, which overlaps (1973a). Other data in the literature for frag- the rangesfor the bulk valuesof FETI and KREEP ments, clasts, and single grains fall in the same basalts. range. Olivine is rare or absentin KREEP rocks. The minor elementsin olivinesvary considerably, Figure 4 is a simplified versionof Figure 6 of Steele and have petrogeneticvalue. Smith (1977) cau- and Smith (1973\ which shows data on mg of tioned about problemsarising from inaccurateelec- olivines, Ca-poor pyroxenes,and basalticliquids. tron-microprobeanalyses, especially for secondary Roeder and Emslie (1970) determinedthe frac- fluorescenceof transition metals. Review of the tionation of Mg between olivine and terrestrial extensivedata in the literatureshows the following: basalticliquids, and Medaris(1969) evaluatedthe ( 1) Mn correlatespositively with Fe and has fractionation of Mg between olivine and low-Ca no petrogeneticsignificance (indeed major devia- pyroxene. Electron microprobe analysesof syn- tions from the correlation probably indicate an in- thetic lunar samplesyield a single band displaced correct analysis). to the left of the one for terrestrialsamples, and (2) Ca tends to increasewith Fa content rang- there is no clear distinction betweenolivines and ing from 0. 1-0.4 wt percent CaO near forsterite low-Ca pyroxenes.At liquidus temperatures,mg is to 0.4-0.9 wt percentfor the rare fayalitesin residua similar for low-Ca and high-Ca pyroxene,but at of FETI basalts(Fig. 5). The rangesfor the FETI low temperature,mg is higher for the low-Ca pyrox- and ANT groups overlap, but the latter tend to be ene. In general,for crystalJiquid fractionationon richer in Mg and poorer in Ca. Note that many the Moon this diagram provides a good test of specimensin the ANT group are breccias, often differentiation models, irrespective of the precise recrystallized.Simkin and Smith (1970) found that details of temperatureand pressure. the Ca-content provides a distinction between ter- Consider the olivines of the ultrabasic group. restrial olivines from plutonic rocks and those from If theseare phenocrystswhich accumulatedfrom a volcanic or hypabyssalrocks. In Figure 5 all the basaltic liquid, that liquid must have mg ranging olivines from FETI basalts lie in the region for from about 0.6 to 0.8. This is actually the range volcanic and hypabyssalterrestrial olivines; further- more, the dominantrange of FETI olivinesis close to the data points found by several investigators -^folivine "'ellow- Co px for olivines synthesizedin the presence of liquid
intered: 0.6 from lunar basalticcompositions. The ANT group olivines ! the plutonic and the volcanic-hypa- 0n0pxs : overlaps both of sourceI byssalranges suggesting that this group derivesboth rocksof : 0.8 tL | | DOSdI$ from volcanic and plutonic rocks. Smith (1971) olivinesofT ultrobosicl noted a distinction betweena high-Ca and a low-Ca frogments- group on the basisof just Apollo 11 and 12 data, ANT--- mg liquid but examinationof all availabledata shows that KREEP FETI there is a continuousgradation-e.9., the data by Prinz al (1973b) for ANT fragments in the Ftc. 4. Relation between mg for coexisting liquid and et either olivine or low-Ca pyroxene. The shaded area shows Luna 20 sample.The effect of metamorphismon the range of synthetic data for lunar basaltic compositions the Ca content of olivine has not been quantified obtained by various workers including Biggar et aI (1971), so far, but availabledata on diffusion rates (e.9., Green et aI (1971, 1972) and Kushiro (1972). The ranges Buening and Buseck, 1973) suggestthat meta- below the abscissashow the bulk composition for the three morphism in a thick ejecta blanket may cause sig- major rock types. For KREEP-type material, the mg value drops to zero for extremely-differentiated residua. See text nificant migration of Ca from the olivine. Hence for further explanation. [Modified from Steele and Smith a low Ca content may indicate either a plutonic (1973,Fie.6)1. origin, or metamorphism,or both. The possibility LUNAR MINERALOGY 239
r.0 relatesnicely with data obtained by severalworkers for olivines synthesizedfrom melts of lunar basaltic composition.The band for ANT olivines is displaced 0.8 FETI,, to higher Mg values and lower Cr concentrations. Particularly noticeable are the low Cr concentra- tions for the peridotite and pink-spinel troctolite. o nA o Currently there is no definitive explanationfor the O ANT wide range of Cr-contents (when normalized for he the Mg correlation). A simple explanation A \? | might 0.4 A ltt be that outgassingof the Moon resultedin reduction ;/ E t^ A lr/ of Cr3* to Cr2*. Alternatively one might invoke IPST l-/ efiectsfrom the bulk compositionof the rock, from -'i 0.2 ---;;L-rffiyt___ - volconicg the temperatureof equilibration,and from the effects \---__ of simultaneouscrystallization of minerals such as n XP | | spinel and pyroxene (e.9., Butler, 1973). Dodd Fo 80 60 40 20 Fo (1973) reported Cr contentsin olivines from the Sharps (H-3) chondrite. These occupy the range mol.% Fo from 0.02-0.2 wt percentCr2O3 and 100-70 mole FIc. 5. Relation between wt p€rcent CaO and mole per- percent Fo, without any correlation between Mg cent Fo for lunar olivines. The two ringed areas show the and Cr. Dodd found a weak correlationbetween ranges of electron microprobe analyses for ANT and FETI Cr and Al. and favored the occurrenceof some of olivines. Most specimens fall in the regions of Fo marked the Cr as trivalent with a coupled substitution of by the continuous lines. P and PST stand for a peridotite - and a pink-spinel troctolite (see Fig. 3). The dashed line cr3* * Al3* (Mg,Fe)r- + Si4.. marks the boundary between terrestrial olivines from (4) Al is a puzzle.Accurate analysisof Al by plutonic us volcanic and hypabyssal rocks. The triangles electron microprobe methods is difficult because show olivines crystallized from liquids of lunar rock com- positions (Biggar et al, l97l; Green er al, 1971, 1972: Kushiro, 1972). n PFI of metastablecrystallization with interaction between olivine and other Ca-bearing minerals must be evalu- ated; furthermore the bulk Ca content of the host .6o n rock must be considered. Butler (1972) gave de- O 0.6 Gj (J tailed data on the zoning trends of four FETI O basalts, and found a correlation of Ca content with 0.4 bs d-e 0.4 texture indicating the effect of temperature of
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