Geochimica 0 Pergamon 0016-7037/82/091591-I er Cosmochimico Acra Vol. 46. pp. 1591 to 1601 Press Ltd. 1982. Printed in U.S.A. 1$03.00/O Lunar anorthosite 60025, the petrogenesis of lunar anorthosites, and the composition of the Moon GRAHAM Lunar Curatorial Laboratory, Northrop Services, RYDER Inc., P.O. Box 34416, Houston, TX. 77034, U.S.A. (Received January 29, 1982; accepted in revised form April 29, 1982) Abstract-Systematic variations of the mineral chemistry of ferroan anorthosite 60025, which is probably a mixture of closely related materials, suggest that lunar anorthosites formed by strong fractional crystallization and near-perfect adcumulate growth, without trapping liquid. The parent liquid for the most primitive samples was saturated with olivine, plagioclase, pigeonite, and chromite, and evolved to one saturated with plagioclase, pigeonite, high-Ca pyroxene, and ilmenite. The parent liquid also had a very low Na20 content, and combined with strong fractional crystallization this explains the steep trend of anorthosites on an Mg* (atomic 100 X Mg/(Mg + Fe)) V. An diagram. The mineral and chemical data for other anorthosites are consistent with such a model. Near-perfect adcumulation can occur if growth takes place at the crystal-liquid interface without the physical accumulation of crystals grown elsewhere, and is encouraged by the shifts in phase boundaries with pressure. Anorthosites are probably the remnants of a crust floating on, and crystallizing at the surface of, a magma ocean originally of bulk Moon composition. Mineralogical and trace element data suggest that the parental liquid for the most primitive anorthosites had previously crystallized no plagioclase and some but perhaps very little pyroxene. Hence the bulk Moon appears to be similar to that proposed by Ringwood (1976) but to have even lower alkalis, a subchondritic Ca/Al ratio, and REE abundances and patterns close to chondritic. The mare basalt sources are not directly complementary to the feldspathic crust, because experimental and trace element data indicate that they are too magnesian and contain too much high-Ca pyroxene. Other crustal rocks, such as the Mg-suite samples, are not closely related to anorthosites; in addition to their chemical differences they have a different crystallization sequence: 01 - plag - px, in contrast with the 01- px - plag inferred for anorthosite parental liquid evolution. LUNAR ANORTHOSITES are an important constituent of the lunar crust, as demonstrated by the individual large samples collected on the Apollo 15 and 16 missions, and by the aluminous nature of highlands soils. However, their genesis is not well understood. The recogn+ion of the highlands as feldspathic (Wood et al., 1970; Smith et al., 1970) led to the generalized concept of a magma ocean from which plagioclase floated to form the crust very early in lunar history. The idea that the mare basalts (negative Eu anomalies) had mantle source regions which are mafic cumulates complementary to the floated anorthosites (positive Eu anomalies) became, and still is, widely accepted by lunar scientists (Taylor, 1982). Lunar anorthosites are cataclasized and they rarely have more than a few percent mafic minerals. They are ferroan (Dowty et al., 1974) and are chemically and mineralogically distinct from other highlands igneous rocks, which compose an Mg-suite (Warner et al., 1976; Warren and Wasson, 1977) (Fig. 1). This paper concentrates on the petrogenesis of the anorthosites, without recourse to, or a requirement to explain, data for the Mg-suite. The genesis of the Mg-suite, at least some members of which probably date back to the earliest differentiation of the Moon, is an important but separate problem. It might ultimately be related to anorthosite genesis in some complicated way, but attempts to show this have so far been without complete success (e.g., LonANORTHOSITES ghi and Boudreau, 1979). In fact the Mg-suite is internally complex (James, 1982; Ryder, 1982) and may constitute several petrogenetic schemes. The model for anorthosite genesis suggested in this paper is neither confirmed nor vitiated by the known characteristics of Mg-suite rocks. On Fig. 1 the anorthosites show a steep trend which has a wide range of mafic mineral compositions but a narrow range of plagioclase compositions (Mg* = atomic 100 X Mg/(Mg + Fe)). However, individua/ anorthosite samples have fairly narrow ranges of mafic mineral compositions. Those which do not have narrow ranges have generally been interpreted to be polymict (McCallum et al., 1975; James, 1980; Kempa and James, 1982). The steep trend has been attributed to abundant cumulus, or suspended and assimilated, plagioclases and varied amounts of both trapped liquid and cumulus mafic minerals (e.g., Wood, 1975; Longhi and Boudreau, 1979; James, 1980). This explanation appears to be capable of explaining a similar trend in anorthosites of the Stillwater intrusion (Raedeke and McCallum, 1979). The validity of this explanation for the lunar anorthosites has been questioned by Norman and Ryder (1979) and Warren and Wasson (1980), in part because several samples contain large mafic crystals which cannot result from trapped liquid. Further, Haskin et al. ( 198 I ) found that the differences in trace elements between splits of individual samples could not be explained by mixtures of cumulate plagioclases and trapped liquid. 1591 G. Ryder 1592 mol % Ab in PIW 80m 2 6 5 50. 5 b z _. 0 80- FIG. 1. Mg* of mafics v. An of plagioclase for lunar highlands pristine rocks, adapted from Warren and Wasson (1980). Data for 60025 from this study. Little attempt appears to have been made to establish if there are any systematic variations among anorthosites; most discussions are of individual samples or generalize for the group as a whole. In the present study, I made a new, detailed study of sample 60025, in particular its mineralogy and mineral chemistry, and established that there are systematic variations and relationships among its minerals. Literature data on petrography and chemistry demonstrates that the same features exist for anorthosites as a whole. The data strongly suggest that the anorthosites are samples of a suite of related near-perfect adcumulate rocks, i.e., all crystals in a given unmixed sample crystallized from a liquid of constant composition and temperature and are unzoned; little or no liquid was trapped. The rocks crystallized successively from a magma undergoing strong fractional crystallization. The steep trend on Fig. 1 is a natural result of the low-alkali content of the magma. The data allow strong inferences to be made about the parental liquid and its evolution, and about the bulk composition of the Moon and its evolution. Mare basalt sources are not directly complementary to the anorthosites. a reinspection of sample 60025 was made. lt proved to contain clumps of green mafic minerals up to several millimeters wide and a few centimeters long, as was suggested in the original Apollo 16 Sample Information Catalog (1972). Some of this material was allocated, with plagioclase, for the geochronological study. At the same time samples were taken for petrographic and microprobe analyses to support the geochronological study. 60025 is a large (1836 g) sample of coarse-grained cataclastic anorthosite. A slab was cut from it (Fig. 2) and all of the original allocations for chemistry, petrography, and other studies were made from it. Thin sections from this slab (99% plagioclase) contain iron-rich pyroxenes and lack olivine (Walker ef al., 1973; Kushiro and Hodges, 1973; Dixon and Papike, 1975; Takeda er al., 1979). An undocumented chip, rich in mafic minerals and which was loose in the same sample return container as 60025, was made into thin sections. Apparently from a lack of conviction that this chip was really a part of 60025, no data were reported for it until Warren and Wasson ( 1978) presented mineral analyses. These authors did not allude to the possibility that this sample might not be from 60025 nor to the clear difference in the mafic mineral chemistry (olivines and pyroxenes, more Mg-rich) from the data previously reported for 60025. The green mafic mineral clumps are on the end of the sample opposite from the slab (Fig. 2). For this study, I have made microprobe analyses of mafic minerals, plagioclase, and oxide minerals in the clumps; in a thin section serial to that which was analyzed by Warren and Wasson (1978); and in thin sections from the slab. The immediate objectives were to ascertain the composition of the mafic clots. FEZRROANANORTHOSITE 60025 Lunar anorthosites have extremely primitive 87Sr/ ‘??Grratios and so are believed to have been formed at the very beginning of lunar evolution, approximately 4.5 billion years ago. However, no mineral isochron has thus far been obtained, mainly because the mafic minerals are so small and rare. It was in response to a request for material to obtain an internal Sm-Nd isochron on a ferroan anorthosite that FIG. 2. Original splitting of 60025: the matic clumps were at one end and not sampled. Pieces labelled “T” were made into thin sections, those labelled “G” had grains picked for grain mounts. Lunar anorthosite 60025 the total variation of mineral compositions, and the homogeneity of individual grains. A fuller presentation of the data, including pyroxene minor element chemistry and oxide analyses, will be given elsewhere (in prep.). Photomicrographs of the mafic chip and of picked mafic grains are shown in Figs. 3 and 4. 60025 MAFIC MINERAL VARIATIONS The pyroxene analyses were mainly for nine elements, with deadtime, background, and Bence-Albee matrix corrections. Some were for three elements (Ca, Mg, and Fe) with an empirical correction procedure, and these agreed with the former for the same grains to about half mole percent En. The olivine data were almost all three elements (Ca, Mg, Fe) with similar correction procedures; a few were re-analyzed for nine elements and agreed to within one mole percent Fo. The major element data are shown in Figs. 5 to 7. The mafic clumps consist of olivine and low-Ca pyroxene, individually at least 2 mm across (the size of the picked fragments). Olivine is apparently slightly more abundant, but the distinction of olivine and pyroxene cannot be made macroscopically. The pyroxenes are inverted pigeonites: the augite lamellae are in two directions (Fig. 4) and the bulk compositions of the grains contain about 8 mol % Wo. The olivines are homogeneous, Fo~.~-~,.,,.The pyroxenes are individually homogeneous (except for the exsolution lamellae) but show significant small differences between grains (Fig. S), wtth a range of En68-72. The mafic chip which was also analyzed by Warren and Wasson (1978) contains mafic grains (fractured aggregates) nearly a centimeter across (Fig. 3), with individual relics up to 4 mm across. They are olivines and pyroxenes which have compositions identical with those in the mafic clumps, hence the chip is undoubtedly a part of 60025. (A systematic difference of a few mol percent in both En and Fo between the analyses reported here and those of Warren and Wasson (1978) is apparent.) In addition, the chip contains small pyroxenes and a few olivines which are more Fe-rich. The pyroxenes include both high-Ca and low-Ca varieties, and the compositional range extends to that of pyroxenes in the slab (Fig. 6). Olivine has a much more restricted range. Individual grains of both olivine (Fig. 7) and pyroxene are quite homogeneous. Thus 60025 contains clumps of Mg-rich pyroxenes and olivines and small grains of more Fe-rich pyroxenes, both high-Ca and low-Ca. The total range is close to that reported for the ferroan anorthosite group as a whole. While each grain is homogeneous, the differences between grains show that metamorphic equilibration has not taken place over a distance of more than a millimeter, at least since the rock was cataclasized. Because of their grain size, at least the Mg-rich mafics did not crystallize from trapped liquid or exsolve from plagioclase; they must be cumulate or adcumulate. 60025 PLAGIOCLASE VARIATIONS A survey was made for Na, K, and Ca in many plagioclases, and several grains were separately analyzed for Si, Al, Ca, Na, K, Fe, and Mg. Probes were operated at 15 Kv accelerating potential and 20-30 nA sample current, with the beam focused at approximately 3 pm. Backgrounds were taken on each point in the seven-element analyses. Standards were feldspars except for Fe and Mg, for which a clinopyroxene was used. Data were corrected for deadtime, backgrounds, and matrix effects (Bence-Albee). To avoid secondary fluorescence (Longhi et al., 1976), no analyses were made within 100 pm of any malic or oxide mineral in the seven-element analyses. In the seven-element analyses counting for 180 seconds gave 300-1000 total counts above 1593 background for Fe, Mg, and K, and about 2000 counts above background for Na. Consistency of the data for Mg for the slab sample with that of Hansen et al. (1979) indicates that the third order CaK, peak is not contributing to the Mg count, and this conclusion is supported by analyses of the anorthite glass standard which have MgO - 0.01%. On each grain or area of a grain analyzed, up to six such counts were made within a few hundred microns and averaged for the data plotted in Fig. 8. For such point sets, precisions (1 u) are less than 5% for Na, 20% for Fe and Mg, and 30% for K. On a given point, individual 30 second counts have 1crof 5- 10% for all elements. The albite content is calculated directly from Na as was done by Hansen et al. (1979). Most of the analyses were made on the thin section of the undocumented mafic chip, the mafic mineral study having shown it to be a part of 60025. Plagioclases from the slab and from the malic clumps were also analyzed. The total range of compositions for single analyses in the threeelement survey is Ab1.77-4.35r with the majority in the range Ab3.7-4.0.Individual grains are homogeneous. There is a conspicuous correlation of both Fe0 and Mg* in plagioclase with the Ab content (Fig. 8). In contrast, MgO shows scatter around a fairly constant abundance, and K20 (not shown) has no systematic variation. The data from the slab samples analyzed are consistent with those of Hansen et al. (1979) except that their Mg* value is lower. There is possibly some systematic error in the data presented here, and I cannot claim the accuracy of Hansen et al. (1979), who used different and more appropriate standards for Mg and Fe in plagioclase. SPATIAL ASSOCIATIONS AND MINERAL CHEMISTRY CORRELATIONS Although there is a wide variation of mineral compositions, they are not randomly distributed in the rock. In the slab sample, all plagioclase grains are sodic (-Ab,) and the mafic grains, lacking olivine, are iron-rich (Fig. 6). In contrast, the most Mg-rich mafic grains are adjacent to the most calcic plagioclases: in the one multimineralic picked grain analyzed (depicted in Fig. 4b) pyroxene En,*, olivine Foe,, and plagioclase Abz are present. In the mafic chip, several textural zones occur (Fig. 3). A consists of cataclasized olivine (FOG,)in coarse grains, with some pyroxene (En,,). C consists mainly of a single 4 mm pyroxene (En,), with minor olivine (Fo& and some plagioclase (Abz-&. D is similar to the slab and consists mainly of coarse plagioclase (Ab,) with minor Fe-rich pyroxenes, and lacks olivine. B, in contrast, mainly consists of finely ground-up plagioclases and mafic minerals, with a variety of compositions. Therefore, with the exception of zone B, there are correlations among the types and compositions of mafic minerals, and between the compositions of mafic minerals and plagioclases. These are plotted on Fig. 1, using data from the slab, the analyzed multimineralic picked grain, and zones C and D from the mafic chip. The trend is positive as in normal igneous rocks, but is steep, not quite vertical (Fig. I). That this slope is positive has not previously been demonstrated for a lunar anorthosite, although it was suggested by Dixon and Papike (1975) for a group of several anorthosites. The scatter for anorthosites in Fig. 1 probably results from a combination of varied-quality analyses, different methods of calculating the albite content of plagioclases, and the unrecognized polymict nature of some of the samples. (There is also the possibility that different anorthosites fall on different, roughly parallel, lines.) A further correlation, if the spatial inferences are correct, is that of Mg* plagioclase with Mg* mafic minerals. PETROCENESIS OF 60025 The mafic minerals in 60025 are not accidental inclusions, because their compositions vary system- FIG. 3. Thin section 60025,21 from matic chip. a) Crossed polarizers. b) Plane light, with textura: zones outlined. Dark pattern in zone D is plucked plagioclase. not mafic mineral. c) Map showing maltcharacteristics of the four textural zones. atically with plagioclase compositions. The range in compositions suggests that the assemblage was not produced by metamorphism, and that 60025 has not significantly equilibrated between grains in the subsolidus. At least the large mafic mineral grains did not exsolve from the plagioclases during exsolution. Lunar anorthosite 60025 mainly -z .2mm/ FIG. 3. (Continued). and there is no evidence that the small grains did. Hansen et al. (1979) suggested re-equilibration to be the cause of the calculated KEi”s (Fig. 9) being less than the 0.49 experimentally determined for lowTi mare basalts by Longhi et al. (1976). In contrast, I conclude that the low values approaching those of are those of igneous processes. The only sigK&i:LMg nificant subsolidus mineralogical changes were inversion and exsolution of pigeonite. Given an igneous origin and the inferred mineralogical correlations, the phase variations show a response to an evolving silicate liquid. That liquid was multiply saturated: plagioclase crystallized continuously, and at first (Ab,) was accompanied by pigeonite, olivine, and chromite, and later (Ab,) by pigeonite, high-Ca pyroxene, and traces of silica and ilmenite. Olivine, which ceased to crystallize, must 1595 have been, or become, in a reaction relationship with the liquid. The data are consistent with those from other anorthosites in which primary high-Ca pyroxene is much later and in minor quantities compared with low-Ca pyroxene and olivine; olivine much m;re Fe-rich than Faso has only rarely been reported. The mafic mineral clumps in 60025 are of a size such that they are unlikely to be crystallized trapped liquid, but similar-sized grains appear to have been produced by cumulate and adcumulate growth in terrestrial intrusions. The clumps also contain aluminous chromite; their mafic mineral grains have small compositional differences; and they are associated with the fenst sodic plagioclases. None of these characteristics is consistent with trapped liquid, nor indeed with much reaction with the melt at all. The large mafic minerals would have crystallized from a liquid with Mg* - .40, so even the more Fe-rich mafic grains (Mg* crystals - .50) are too magnesian to be simply trapped liquid; and silica, phosphates, and other minor phases are absent or extremely rare. Haskin et al. (1981) concluded that the rare earth chemistry of ferroan anorthosites, including that of several samples from the 60025 slab, is consistent with only extremely small quantities of trapped liquid, and the mafic minerals are more abundant than can be accounted for by these small quantities (this is so even if 60025 is metamorphosed as long as it did not melt). Thus, the mineral compositions in 60025 were a product of adcumulus growth in response to a changing liquid composition, but that liquid was never trapped very much, and re-equilibration between liquid and crystals did not take place. FIG. 4. a) Picked low-Ca pyroxene grain showing exsolution lamellae. Crossed polarizers. Width - 1.2 mm. b) Picked multimineralic grain. Crossed polarizers. Width -0.7 mm. 1596 G. Ryder \r l 0 0 z 2 . ‘.\ ..w l Y Y Y Y 72 Yol% Y \ 68 10 En might provide a means for partial homogenizatron (James, pers. comm.), a means lacking in the lunar case. However, individual plutonic samples of the Mg-suite have homogeneous mineral compositions. including those samples which contain evidence (zucon, phosphates, etc.) for trapped liquid. At least one class of lunar rocks, the feldspathic impactites ur granulites (Warner et al., 1977) was substantially equilibrated in the subsolidus. It seems likely that 60025 is polymict, but not generally mixed on a fine scale, and all the pieces are related magmatically. Longhi ( 1977) calculated mineral fractiunul crystallization paths for several estimated bulk Moon compositions. For those with low alkali contents, the trend on an Mg* (mafic) is. .4b (plagioclase) dia gram following plagioclase crystallization 1s steep and similar to that observed in 60025 (and indeed the whole anorthosite suite). This is perfectfy logical: in this sequence, there is twice as much Na in the latest plagioclases (Ab > 4 mole %) as in the earliest ones (Ab < 2 mole %). Assuming a constant DNa for plagioclase-liquid, the liquid must have undergone more than 50% crystallization, enough to allow the mafic minerals to evolve from Mg* approximately 70 to approximately 50. Thus, the steep slope is 2 natural consequence of the low alkali content, and does not require trapped liquid. The similarity of the Stillwater and lunar trends is illusory, or at least the trends result from different mechanisms. Longhi and Boudreau (1979) have invoked the suspension and assimilation of plagioclase to create the steep slope for anorthosites on the Mg* 11.An diagram. This model effectively uncouples plagioclase (equilibrium) from mafic mineral (fractional) crystallization and would not necessarily produce the mineralogical car., relations observed in 60025. That model invokes trapped liquid which is not apparent in lunar anorthosites. FIG. 5. Homogeneity and distinction of magnesian pyroxenes in 60025 analyzed in this study a) dashed lines encompass analyses of individual grains picked from mafic clumps b) each portion represents a single grain from a thin section of the undocumented chip. Dashed circle represents the size of an individual point in Fig. 6. Individual hand samples from the Stillwater intrusion show much less variation of mafic mineral chemistry than 60025, usually only a few mol percent En or Fo, although plagioclase shows variations of up to 10 mol percent An (Raedeke and McCallum, 1980). While it is possible that this homogeneity in part results from metamorphism, McCallum (pers. comm.) believes it to be primary. A greater variation of up to approximately 10 mol percent Fo, En, and An exists in those individual samples of the Skaergaard which show independent evidence for trapped liquid (Hoover, pers. comm., 1981). Trapped liquid can produce large ranges of mineral composition over very small distances, but in the terrestrial case, a hydrous phase present during subsolidus cooling FIG. 6. Compositions of pyroxenes in 60025. Circles (thus study): solid, host phase; open, exsolved phase. Tie lines connect points from individual grains. Dashed areas enclose previous data from the slab published by Walker et cri. (1973). Hodges and Kushiro (1973), Dixon and F’apikc ( 1975), and Takeda et al. (1979). Lunar anorthosite 1597 60025 FIG. 7. Compositions of olivines in 60025 analyzed in this study. Each vertical stroke is a point analysis; each horizontal band is an individual grain. Replicates are analyses on a single grain taken at different times in the study. Marjalahti (the standard) analyses shown to same scale to compare homogeneity. ADCUMULUS GROWTH AND CHEMISTRY For adcumulate growth to be the case of lunar anorthosites in general, a plot of the calculated liquid Mg* (henceforth Mg:) against calculated liquid Sm from individual samples should show a correlation. The Sm in the liquid should be proportional to that in the bulk rock if most of the Sm is one mineral *20 ,,,,,,,,, - ,,,,,,,,, FeOwt% I,,, ’ *l5- a’. : 00 40 - * : .I V I phase, as it is in most analyzed splits of lunar anorthosites. Some anorthosites, e.g., 15415, are certainly metamorphosed (James, 1980) but this should not affect the bulk chemistry unless partial melting and migration of the melt took place. Figure 10 plots Mgf, calculated from the mafic mineral compositions, against bulk rock Sm, for all samples for which there are appropriate data. (Ideally, this exercise should be for pure plagioclase separates or for a constant pyroxene content.) For samples larger than 10 g there is a good correlation. The most aberrant samples are 62236 and 62237, which contain significant amounts of mafic minerals which have been shown to contain significant Sm (Haskin et al., 1981). The correlation slope is appropriate for the adcumulate model: for Mgt to evolve from 36 to 20 requires approximately 60% of a cotectic liquid to have crystallized. Therefore, incompatible Sm should have increased approximately 2.5 times, which is close to MgOWt% .15 0 c 0 8 0 0 VV 0 0 0 .60- l 6 .o *50 - V l - : MS* V 40s ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 3.0 4.0 mol % Ab 26 ’ ’ ’ ’ ’ ’ ’ ’ ’ FIG. 8. Minor elements in plagioclases analyzed in this study, except open symbol from Hansen et al. ( 1979). Triangles from the slab, all others from undocumented chip. 40 wl* 60 60 Plagioclase FIG. 9. Mg* of parent liquid, calculated from mafic mineral compositions, v. Mg* in plagioclase for several lunar anorthosite samples. X’s are end-members Adapted from Hansen from this study. et al. ( 1979). 1598 G. Ryder -10 BULK 15 ROCK Sm ppm FIG. 10. Mg* of parent liquid, calculated from mafic mineral compositions, v. Sm of rock for lunar anorthosite samples, Solid symbols, samples >lO g, open symbols, samples ~10 g. All Mg* calculated from low-Ca pyroxene except squares, from olivine. Dashed lines connect olivine and pyroxene calculations from a single sample. ?, scatter in data base. Data sources in Ryder and Norman ( 1979, 1980) and Haskin et al. (1981). I) 65327 2) 65315 3) 65325 4) 60055 5) 64819 6) 60015 7) 15415 8) 1529:i 9)67455,30 IO)67035 11)62237 12)67635 13)62236 14)67636 15)60025 16)62255 17)67455.;i 18) 15362 19) 67637 20) 67455.32 what the bulk rock data show to have happened. Because Eu is almost constant in the suite, Mgf also correlates with Sm/Eu, and the increase of Sm/Eu from the most primitive to the most evolved sample requires substantial plagioclase separation. A trapped liquid model, in contrast, does not require a correlation of Mgt with Sm, unless either there are no cumulate mafics, and liquids evolved to different stages are trapped in different rocks or the same amount of cumulate mafics but different amounts of a single liquid are trapped in different rocks. Neither are likely or compatible with the data of Haskin et al. (1981). Bulk rock NazO abundances (almost exclusively in plagioclase) have a wide range for rocks larger than 10 g (Fig. I 1, which is a re-expression of the Mg* v. Ab diagram), but trapped liquids would produce trivial changes in bulk Na,O. For the central part of the diagram there is a reasonable correlation of Na20 and Mgt; the mafic rocks 62236 and 62231 are aberrant and fall closer to the correlation line when allowance is made for the mafic minerals. The three samples to the right of the diagram are truly aberrant (plagioclase analyses by Hansen et ul.. 1979, confirm their sodic nature). They are either a separate type of anorthosite or have been altered in some way. A refinement to these tests could be made by analyzing pure plagioclase separates, or by obtaining pure plagioclase trace element analyses by extrapolation for those samples for which more than one split has been analyzed (e.g., SC v. Sm; SC v. Y b). If the strong fractional crystallization with near-perfect adcumulate growth model is correct, the COTrelations (such as in Figs. 10 and 11) should then be very strong. In calculating parental liquids and their evolution from bulk rock data, it is clear that the presence of a few percent mafic minerals, which may ” * -20 -30 BULK FIG. 11. 10. -40 ROCK Na20 50 -60 wt% Mg* of parent liquid v. NarO of rock, analogous to Fig. 10. Symbols, data sources as Fig Lunar anorthosite 60025 have only a small effect on the light REEs, may have a large effect on the heavy REEs. If the distribution coefficient for Yb between pigeonite and liquid is similar to that proposed by Schnetzler and Philpotts (1970) then the presence of 2% pyroxene in a rock will increase Yb approximately 30% over than that of pure plagioclase. Detailed calculations are not possible without a better knowledge of the relevant distribution coefficients, and mineral separates data for ferroan anorthosites are highly desirable. All in all, however, it is clear that the chemical variations between anorthosites are more systematic than the analysis of Haskin et al. (1981) suggests, and are consistent with an adcumulate model. Indeed, for the anorthosite chemical data, “Relative amounts of FeO, Co, SC, Cr, and REE vary as would be expected for different proportions of plagioclase, pyroxene, olivine, chromite, . . . , if rather random proportions of each phase were present instead of representative portions of residual liquid or of accumulated minerals” (Haskin et al., 198 1). In other words, an adcumulate model is compatible with the data. ADCUMULUS GROWTH, PHYSICAL SElTING, AND PHASE EQUILIBRIA Adcumulus growth for lunar anorthosites has been questioned (e.g., Haskin et al., 1981) because terrestrial intrusions rarely contain rocks devoid of trapped liquid products, and because the elimination of such trapped liquids from a cumulate pile is difficult. However, these objections more or less assume that the crystals occur like a pile of dropped bricks with liquid in between. This need not be appropriate in a magma ocean, in which, once plagioclase had started to crystallize and float, slow growth might take place dominantly at the crystal-liquid interface; hence there is no “cumulus” pile or trapped liquid to start with. Growth insulates already existing rocks from reaction even though growth is slow. Further, if a liquid is crystallizing plagioclase on the cotectic at even a shallow depth and convects upwards, phase boundary shifts due to pressure put it slightly into the plagioclase liquidus field, thus interface growth is only of plagioclase, at least at first. The phase shifts also stabilize olivine instead of pyroxene once the cotectic is reached again; hence for a triply saturated liquid, slight oscillations to and from the olivine-pyroxene-plagioclase point resulting from shallow convection could explain the persistence of olivine over several mol percent Fo rather than its reacting away. However, only a small amount of olivine and pigeonite would crystallize at the interface, and most might crystallize directly on the floor of the magma system. Morse (1982) discusses the physical possibilities for adcumulate growth of lunar anorthosites. ORIGIN OF PARENTAL LIQUID The most primitive anorthosites (highest Mg*) crystallized from a liquid at the olivine-plagioclase- 1599 pyroxene point (R in Morse, 1980; and hereafter) in the 01-Qu-An phase diagram. How did the liquid get to R? The liquid probably had not crystallized plagioclase prior to reaching R because calculated liquids do not show negative Eu anomalies. In fact they show positive anomalies, which are explained by Longhi (1980) as resulting from plagioclase being resorbed into the liquid, but are possibly artifacts resulting from our inadequate knowledge of the Eu distribution coefficient. If the adcumulate model is more or less correct, then the constancy of Eu among anorthosites (see Haskin et al., 1981) suggests that the partition coefficient DEU (wt. concentration of Eu in plagioclase/wt. concentration of Eu in liquid) is actually - 1S, otherwise Eu would increase in successive rocks, assuming cotectic proportions of plagioclase and mafic minerals crystallizing. This DE” is substantially higher than the 1.0-l .2 value normally assumed, but is compatible with some experimental data for low oxygen fugacities (Drake, 1975) and is probably within estimated errors of the 1.2 value (McKay, 1982). The higher D-value reduces or eliminates positive Eu anomalies calculated for anorthosite parent liquids. However, to allow negative Eu anomalies in the parent liquid would require even higher D-values. Thus the most primitive anorthosites probably formed as soon as plagioclase started to crystallize from a magma ocean, if an ocean is indeed the correct origin (see below). We are more likely to have sampled thefirst anorthosites to form than any others, because of flotation. New studies by McKay (1982, and pers. comm.) on the relevant crystal-liquid distribution coefficients and their application to anorthosite genesis suggest only a small, perhaps no, heavy REE depletion in the parent liquid of primitive anorthosites if no trapped liquid is present (a small amount of trapped liquid does produce a calculated heavy REE depletion). However, the data are suggestive of a heavy REE depletion, which presumably indicates the crystallization of some pigeonite prior to the liquid arriving at point R, assuming chondritic proportions in the original system. If so, the liquid travelled along the olivine-pyroxene peritectic (or cotectic?). However, this argument is not strong, and without a firm knowledge of the distribution of heavy REEs between liquid, plagioclase, and pyroxene, the exact magnitude of the slope of the heavy REEs for the liquid cannot be determined. With strong fractional crystallization, olivine could not have persisted long after any pyroxene started to crystallize, so perhaps little pyroxene did crystallize prior to R. The total amount of olivine crystallized cannot be deduced from the anorthosite data alone. The crystallization sequence of the anorthosite suite is then 01 opx? - pig - plag, which is different from that of the Mg-suite samples (definitely 01 - plag - px and which also includes spinel). In both suites high-Ca pyroxene is very late. Was the parent liquid of anorthosites a magma 1600 G. Ryder ocean? Two considerations suggest a positive answer (Longhi, 1980; and pers. comm.). First, the required complementary mafic cumulates have not been identified among samples, thus they are deeply buried, implying a large scale system. Second, the anorthosites seem to be global in extent, although more sampling, especially of the farside (approximately 30% Al,03 from remote sensing data) is needed to verify this. Anorthosites may also be locally missing. e.g., Apollo 17 site (Ryder, i 98 1). If the anorthosites are from a magma ocean, its original major element composition (and presumably that of the bulk Moon) must be more similar to that proposed by Ringwood (1976) than most other proposed compositions, i.e., low alkalis, and low A1203. It could, of course, be richer or poorer in olivine than proposed by Ringwood ( 1976) which does, however, produce mafic minerals of appropriate Mg*. However, other details of the composition must be changed, in particular lowering its alkali content. According to the anorthosite data, the bulk composition of the Moon certainly could have, and is likely to have, a chondritic REE pattern. It could also have chondritic REE abundances, according to how much olivine was crystallized before the point R was reached. However, the late crystallization of high-Ca pyroxene, and its rare occurrence among highlands rocks, suggests a distinctly subchondritic Ca/Al ratio of unknown cause. COMPLEMENTARY MAFIC CUMULATES AND MARE BASALT SOURCE REGIONS Anorthosites crystallized from a cotectic liquid, hence accumulations of olivine and pigeonite must exist somewhere in the Moon. This cumulate sequence must have Mg* values ranging from approximately 70 to 50; more magnesian cumulates will not have crystallized plagioclase (previous section). The sequence must also lack high-Ca pyroxene except for minor and very iron-rich ones. Most of the mare basalt sources, according to interpretations of highpressure experimental data, have Mg* 75-85 (unless the basalts are produced by -30% partial melting), and are close to a three-phase olivine + low-Ca cpx. + high-Ca cpx. field (e.g., Green et al., 1975; Walker et al., 1975). At least the high-Ti mare sources have high-Ca clinopyroxene as a discrete phase, in some cases residual, although in low-Ti mare sources the high-Ca component is generally dissolved in a pigeonitic phase at multiple saturation. Trace elements, particularly the rare earths, strongly suggest the influence of high-Ca clinopyroxene in many mare sources (e.g., Nyquist et al., 1977). Hence, even though mare sources have an intrinsic negative Eu anomaly, they cannot be directly or simply complementary to the ferroan anorthosites. Because they are also required to contain cumulus ilmenite in some cases, they may have had a complex origin. CONCLUSIONS 1) 60025 is a mixture of pieces from a related sequence of anorthosites. 2) This sequence was generated by near-perfect ad- 3) 4) 5) 6) 7) cumulate growth during strong fractional crystallization. The parent liquid of the most primtttve anorthosites was saturated with olivine ( -Fo,~), plagioclase ( -Ab2), pigeonite (-EnT2), and chromite, and evolved to one saturated with plagioclase (-AbJ, pigeonite, high-Ca clinopyroxene. and ilmenite. The steep slope of anorthosites on an Mg* (mafits) v. Ab (plagioclase) diagram is a result of the very low alkali content of the magma and of the original magma ocean. The bulk Moon had low A1203, a sub-chondritic Ca/Al ratio, and REE abundances and patterns which were probably close to chondritic. Mare basalt sources are too magnesian and some contain too much high-Ca clinopyroxene to be directly or simply complementary to a floated anorthosite crust. Mg-suite plutonic pristine rocks are not closely related to anorthosites; their crystallization sequence of 01 - plag - px contrasts with the an orthosites 01 - px -+ plag. Acknowledgments-Reviews by J. LonghI, ! Steele. G. J. Taylor, D. Walker, and 1. 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