Geochemical Journal, Vol. 9, pp. 25 to 41, 1975 25

Some thoughts on the origin of lunar ANT-KREEP and mare basalts

HIROSHI WAKITA*, J. C. LAUL and R. A. SCHMITT

Department of Chemistry and The Radiation Center, Oregon State University, Corvallis, Oregon, 97331 U.S.A.

(Received March 14, 1975)

Abstract-A regular correlation between the Sm abundances and the Eu anomalies in all kinds of lunar samples strongly suggests that a series of ANT-KREEPtype rocks and the source material for the 11, 12, 15 and 17 mare basalts may have been derived from a systematic and common magmatic differentiation. In such a differentiation, significant quantities of trapped liquid were occluded with the cumulates and upon partial melting of these source materials, the trapped liquids played a dominant role in the derivation of mare basalts. After increasingthe concentration of LIL trace elements to --I OXchondrites by crystallization of Mg-rich mafic minerals such as , both plagioclase and olivine began to crystallize simultaneously. Varying amounts of melt were trapped with pure anorthosites and yielded overall positive Eu anomalies. These anorthosites remained near the lunar surface. Simultaneously,mafic cumulates and any melt.inclusions plus significant amounts of trapped liquid in the cumulate layers settled to yield sourcematter for the basaltic rocks with minimum Eu anomalies. With increasing plagioclase and mafic mineral crystallization, the magma changed its chemical composition and the Sm/Eu ratio. Toward the end of the crystallization sequence, the plagioclase differentiation series became KREEP material. The settling mafic cumulates and their trapped liquids account for the sequence of the Apollo 15, 12, 17 and 11 mare basalt source materials. Appreciable amounts of apatite may have settled out with the and 17 mafic cumulates. Subsequent partial melting of the solidified melt inclusions and trapped liquid, accessory minerals and a fraction of the major mafic minerals from the source materials yielded the respective mare basalts. The melt at the last stage probably had a chemical composition similar to the ilmenite microgabbros.

INTRODUCTION

Positive and negative Eu anomalies observed in almost all lunar samples are known to be one of the most striking chemical features of the surface materials of the moon. In a sense, various lunar samples may be grouped in two categories, "mare basalt" and "ANT -KREEP" materials. Dark mare basalts are highly enriched in Fe and Ti . Light colored aluminous and calcic "ANT (anorthosite-norite-)-KREEP" materials are well distributed all over the surface of the moon by cratering events. Since the finding of large Eu anomalies in Apollo 11 basalts, it has been postulated that plagioclase minerals segregated from the primordial lunar magma and yielded highland materials (WOOD et al., 1970; SMITH et al., 1970; KING et al., 1970), while partial melting of cumulates resulted in the generation of "mare basalt" (PHILPOTTSand SCHNETZLER,1970;

* Present address: Department of Chemistry, University of Tokyo, Tokyo, Japan. 26 H. WAKITA et al.

PHILPOTTSet al., 1974; LONGHIet al., 1974). Other investigators(e.g., GASTet al., 1970; HUBBARDand GAST,1971; HASKIN et al., 1973;WEILL et al., 1974;TAYLOR and J AKES,1974) have invoked partial melting mechanisms on a variety of source materials for generation of KREEP and mare basalts. From studies of rocks, KREEP material was inferred for a common component of the lunar highlands (HUBBARD and GAST, 1971; MEYER et al., 1971). Anorthositic fragments were found in the Apollo 11 soil and whole anorthositic rocks such as 15415 were picked up at the Apollo 15 site, and these samples showed large positive Eu anomalies (WAKITA and SCHMITT, 1970; HUBBARD et al., 1971). Highland breccias recovered mostly from the and 17 and Luna 20 sites had various degrees of positive and negative Eu anomalies. A large volume of precise bulk and trace chemical data permits us to hypothesize a cogenetic relationship between two distinct differentiation series "ANT-KREEP" and "mare basalt".

DISCUSSION

1. Sm-Eu correlation diagram A clear correlation between the abundances of Sm and the relative Eu anomalies is observed for almost all lunar materials taken from all eight lunar landing sites. These include the ANT rock suite, mare basalts, breccias, soils, KREEP materials and glasses from widespread areas of the lunar surface. A correlation plot between the abundances of Sin and the ratio of Sm/Eu in the sample to Sm/Eu in chondrites is shown for pure anorthosites, noritic and troctolitic anorthosites, norites, the "VHA" very high alumina basalts (HUBBARDet al., 1973a) and KREEP materials. Abundance data for Sin and Eu for 170 individual ANT-KREEP materials from all eight lunar sites are plotted in Fig. 1. HASKINet al. (1970) first introduced a linear plot of Sm/Eu v. Sm for Apollo 11 samples. They pointed out that a 1n Sm/Eu v. 1n Sin was also linear for the Apollo 11 samples. The ranges in Sm/Eu ratios and Sm abundances were from 5 to 13 and 8 to 27, respectively. Similar linear plots for both Apollo 11 and 12 samples were shown by HASKINet al. (1971) and WAKITA et al. (1971). In the lower left corner of Fig. 1, cataclastic anorthosites from Apollo 16 and 15 (61016, 60015 and 15415) are shown with the largest positive Eu anomalies. As the Sm abundance increases, the correlation line passes through the noritic and troctolitic anortho site regions up to 2.8 ppm Sm with no associated Eu anomaly. All of these samples were obtained from the six Apollo and the Luna 20 sites. The apparent hiatus between Sm. = 0.15 to 0.35 ppm has been filled by one sample, 67075; we believe that further analyses of anorthositic rock will populate the entire sequence. For Sin abundances greater than 2.8 ppm, that is 14X chondrites, negative Eu anom alies are found for practically all samples. Almost all Apollo 16 highland breccias and samples from the boulder-2 and the Luna 20 sites fall on the line. With Sm abundances of approximately 8 ppm, the Apollo 16 "VHA basalt" or microtroctolites fall a little off the line but the trend continues to the field of high-K KREEP materials from various sites. Between the VHA and high-K KREEP rocks fall most of the Apollo 17 boulder-2 breccias, and 15 breccias. From pure anorthosites to high-K KREEP, the trend is continuous and varies in Sm abundances over three orders of magnitude. Lunar ANT-KREEP and mare basalts 27

I0 • Ap 11 77115 0 Ap 12 012003 x Ap 15 BASALTFIELDS /•X • Ap17 ANT-KREEP Ap 15, /2, 17, X O Luna 16 060636 A Ap 14 72335 169051 0 O Ap 16 644 A14063 Luna 20 Ti o X06-40 a NEGATIVE Eu ANOMALY 1.0 77135 166516 770'7 ml 16 014063 18155 X15459 N 724/7 DUNITE 77135 • 67915 POSITIVE Eu ANOMALY O) 10085 A14321 a 60626 O 67 E 031 0 N C 15416 60017 0 t 765435 64435 ~g 0 06335 674 83335 7 U 55 0065 W 15416x 64423 7 850 14161 X15455 W 100 E 64422 012033 E 1n 670750 0.1

67075 0 15362 69955 64435 O O 60023 15314 X O 60025 61016 80015

81016 6001515415 060015

0.01 0.01 0.1 I 10 100 Sm (PPM)

Fig. 1. Ratios of Sm to Eu abundances in the ANT-KREEPmaterials relative to those of chondrites (2.67) have been plotted v. the abundances of Sm. These 170 data points were obtained from groups who published their results in the five Proceedings of The Lunar Science Conferences(1970-1974) and literature sources cited in the reference section. For convenience of sample identification, sample numbers are attached for severalsamples. It is noted that K content of 12033 (HUBBARDet al., 1971) is significantly higher by a factor of ~60 compared to other ANT rocks with similar Sm abundances and that samples 15455 (TAYLORet al., 1973) and 14321 (TAYLORet al., 1972) have markedlylower FeO/MgO values of 0.39 and 0.3, respectively,compared to the range (0.7-1.4) of rocks with similar Sm abundances. The seven dunite 72417 points are taken from LA UL and SCHMITT(1975) .

Only the 72417 dunite points, which represent the values of nine small fragments , 69 to 135mg each, are strikingly displaced from the Sm/Eu v Sm curve (LAUL and SCHMITT,1975). Extrapolation of the dunite line to the correlation line suggests that the dunite sample may be genetically related to a residual magma from which the gabbroic anorthosites such as 60626 (78% pl), 67031 and 60017 (83% pl) may have been derived (LAUL and SCHMITT,1975). Fields for mare basaltic rocks from the four sites, Apollo 11 , 12, 15 and 17, are also shown in Fig. 1. Note that the line passes through the Apollo 15 and 12 mare basalt fields. The Apollo 17 and 11 mare basalt regions are a little off the line but these areas practically overlap the trend found in the ANT-KREEP rock suites. In Fig. 2 , the same Sm-Eu correlation diagram is shown for Apollo 11 , 12, 15 and 17 mare basaltic rocks. The fields for the Apollo 15 and 12 mare basalts overlap each other and the ANT-KREEP line passes through the middle of these fields. With larger negative Eu anomalies , the Apollo 17 and 11 mare basalts follow the ANT-KREEP trend . The data plotted in Figs. I and 2 are taken from many literature sources in the five Proceedings of the Lunar Science 28 H. WAKITA et al.

10

Ap II

V-7 Ap 12

a Ap 15 e Ap 17 v ~n ~ 8 4 K .M G 0 E C o NEGATIVE Eu ANOMALY b 0 r I U POSITIVE Eu ANOMALY: 3 w 3 11W E Q b U,E U)

//

0.1 0.5 I 2 5 10 20 50 Sm (PPM)

Fig. 2. Ratios of Sm to Eu abundances in the Apollo 11 (p), 12(X), 15 (-) and 17 (a) and Luna 16 (0) mare basalts relative to those of chondrites (2.67) have been plotted v, the Sm abundances. Fields for the Apollo 11, 12, 15 and 17 basalts include the following number of rock analyses, 13, 20, 27 and 27, respectively. The straight line is obtained from the Sin-Eu correlation diagram for the ANT-KREEP materials. Literature sources are those cited in Fig. 1.

Conferences and other sources, cited in the footnote of Fig. 1.

2. Magmatic differentiation This regular correlation strongly suggests that a series of ANT-KREEP type rocks and the source materials for Apollo 11, 12, 15 and 17 mare basalts may have been derived from a systematic and common magmatic differentiation. We believe that the fields for the mare basalts not fortuitously overlap the trend obtained for the highland ANT type rock suites and the associated KREEP materials. In such a chemical differentiation, we suggest that significant amounts of trapped liquid were occluded with the cumulate minerals. During crystal-liquid differentiation the magma changed its chemical composition progressively. Overall concentrations of the LIL trace elements in cumulate layers would have been controlled largely by the LIL trace element content of the trapped liquid and of any melt inclusions within the cumulate minerals. With time such trapped liquids and inclusions in the cumulates would solidify as adcumulus and orthocumulus materials. Upon partial melting of such source materials, the solidified trapped liquid and inclusions and the cumulated accessory minerals would play a dominant role for producing both ANT-KREEP and mare basaltic compositions. The occurrence and significance of silicate melt inclusions found in extrusive lunar basaltic rocks have been studied from many aspects (see e.g., ROEDDER and WEIBLEN, 1971); however, partial melting of solidified trapped liquids and inclusions with the primary cumulate minerals has not previously been hypothesized as a dominant factor for lunar LIL patterns. Observations by A LBEE et al. (1974) and estimation of significant amounts of melt inclusions in the Apollo 17 dunite 72417 (-.-93% olivine) support this hypothesis (LAUL

P Lunar ANT-KREEP and mare basalts 29 and SCHMITT, 1975). This hypothesis is also supported by the estimation of 8 to 16% of trapped liquid in the Apollo 17 troctolite 76535 (HASKIN et al., 1974). It is worthwhile to mention that these two rocks are originally cumulates of olivine, and plagioclase plus olivine, respectively. Textural studies indicate that these rocks crystallized very slowly at a depth between about 10 and 40km (ALBEE et al., 1974; GOOLEY et al., 1974) and were excavated by subsequent cratering events. For a fractional crystallization sequence or partial melting sequence, HASKIN et al. (1970) first showed that the following equation would apply:

Darn DEu CA, Sm (DSm 1) / (DEu 1) I n CR, Sm In CR ,Sm + In -) CR , Eu Dsm 1 CA, Eu where CR,sm and CR,Eu are the Sm and Eu abundances in the residual phase (magma in a fractional crystallization sequence); CA,Sm and CA,Eu are the Sm and Eu abundances in the original magma; and DSm and DEu are the partition coefficients for Sm and Eu (solid to liquid for fractional crystallization). Based on only Apollo 11 samples, HASKINet al. obtained a slope of --0.70 from their 1 n Sm/Eu v. 1 n Sm plot and they deduced that DEu exceeded 0.70 for Ds. ;z:50. From Fig. 1, we obtain a slope of 0.33 which is converted to a slope of 0.89 for a 1nSm/Eu v. lnSm plot. If we assume that plagioclase crystallization dominantly controls the Sm/Eu ratio, then DEu 0.89 agrees well with the average experimental DE„ 1.07 (McKAY and WEILL,person. com., 1975) for plagioclase which crystallized at 1,200°C and 1,340°C, under low f02 of lunar condition, from a magma along the olivine-plagioclase cotectic line of the pseudo-ternary diagram in WALKERet al. (1973). Such agreement strengthens the argument that plagioclase crystallization was large ly responsible for the variable Sm/Eu ratios over a large range of Sm (REE) abundances. For Sm Z 5 ppm, the observed Sm/Eu ratios fall below the correlation line on Fig. 1; in fact, with increasing Sm abundances, the Sm/Eu ratios increase at a slower rate. In terms of our model, we propose that an ever increasing fraction of trapped liquid is occluded on the major crystallizing minerals as the degree of fractional crystallization increases and reaches the terminal stage, i.e., as Sm increases. A few stragglers such as 77115 are expected to deviate from such a general trend. 3. A proposed model We assume that during the early lunar history the outer shell of 100 to 200km of primordial material was completely melted by accretional energy. The mineral composition of the primordial substance is not known; however, the chemical com position of the LIL (Large Ion Lithophile) trace elements was assumed to be about 5 to 7X chondrites (TAYLORand JAKE , 1974). A flat REE distribution pattern is also assumed. From this magma, Mg-rich ferromagnesian minerals such as olivine (and orthopyroxene) be gan to crystallize. The LIL trace elements remained largely in the residual liquid. After the LIL trace element concentration reached the level of l OX chondrites, olivine and plagio clase cumulates began to crystallize simultaneously. Because of a difference in specific gravity, these two crystal phases will separate. Plagioclase cumulates remained at the surface of the magmatic shell while the mafic cumulates sank to form mafic layers. Since the partition coefficient of divalent Eu for plagioclase is significantly higher than the trivalent REE, the plagioclase cumulates that crystallized first possessed the higher positive 30 H. W AKITA et al.

Eu anomalies. The residual magma would have been depleted in Eu, thereby resulting in a corresponding negative Eu anomaly. Since olivine cumulates essentially did not incorporate the LIL trace elements to any significant degree, including the REE and Eu, progressive crystallization of olivine cumulates resulted in an increase of the REE content in the residual magma. At each stage of differentiation, we suggest that small amounts of trapped liquid and melt inclusions are primarily responsible for the overall concentration of LIL trace elements (REE included) and the Sm/Eu ratios of the solidified cumulate layers. 4. REE distribution patterns of the plagioclase differentiation series In Fig. 3 the REE distribution patterns of the feldspathic rock series are shown. At the bottom of the figure are found the patterns for "pure" anorthosites such as 61016, 60015 and 15415 with the largest positive Eu anomalies, while at the top, the high-K KREEP pattern is indicated with the largest negative Eu anomaly. In between these extremes, REE patterns for ANT-KREEP materials have been plotted. It is noted that the sequence is apparently continuous and that all patterns are quite similar to each other. We also emphasize that all samples in the continuous sequence represent widespread areas of the front face of the moon.

500

h'KKR~_ 14 200 ~ 3 -~ 6 oVbi6 603/5 U) 100 72375 72355.72395 63 W 355.72 /~ Fig. 3. REE distribution patterns of the ANT /50/5 I KREEP rock suite. Literature sources are Ir 50 0 G% 01 z 9 3 those cited in Fig. 1. The lowest anorthositic O e/s.200 z 20 REE pattern was found by PHILPOTTSet al. U 11 79x5 (1973) for 61016 anorthosite, which is slightly Io w 600/7 6703/ lower than the pattern for anorthosites 60015 0 s 5 7670- and 15415. Q 64435 67455

2

I 67075 600 555 0.5

srOrs 600r5 _r5 9/5 0.2

0.1 La Ce Nd SmEuGd Tb Dy Yb Lu REE

With continuing differentiation of the magma, the overall REE pattern and especially the Sm/Eu ratio of the plagioclase minerals and the trapped liquid will change progressively. With progressive crystallization, the degree of the positive Eu anomaly decreases in plagio clase minerals. Furthermore, the addition to the plagioclase cumulates of small amounts of trapped liquid and melt inclusions, which d re depleted in Eu compared to other REE, will increase in the cumulate layers the total REE abundances and decrease relatively the

6 Lunar ANT-KREEP and mare basalts 31

Eu abundances. With further crystallization of plagioclase from the magma, a stage of crystallization is reached, at which no Eu anomaly is produced in the feldspathic cumulates. After this stage, further crystallization of plagioclase cumulates will exhibit negative Eu anomalies. In a quantitative argument for the crystal-liquid differentiation for this case, partition coefficients are important factors for modeling and testing the whole system. Excellent approaches in this area have been performed by many researchers (see e.g., HASKINet al., 1970; NAVA and PHILPOTTS,1973; PASTERet al., 1974). Also a favorable model has been presented which considers probable variations in mineral/liquid distribution coef ficients as a function of lunar temperature and oxygen fugacity (WEILL et al., 1974). In our opinion, however, all previous studies have not lead to a complete quantitative understanding of the differentiation of the whole lunar system. In this paper, our model is discussed qualitatively and hopefully in the future, quantitative models for such a complex system may be advanced. 5. REE distribution patterns of the mafic differentiation series While plagioclase minerals crystallized and remained at the magma surface, mafic minerals crystallized simultaneously and settled to the bottom of the magma. Small amounts of trapped liquid and melt inclusions were also occluded with these mafic cumulates. After attaining the LIL trace element concentrations of lox chondrites in the magma, the Apollo 15 mare basalt source materials crystallized from the magma with trapped liquid. The REE patterns of the Apollo 15, 12, 17 and 11 mare basalts are shown in Fig. 4. Among these patterns, those for the Apollo 15 basalts are the lowest in the REE abundances at about 10 to 15X chondrites and exhibit nearly flat REE patterns with minimum Eu anomalies. Such flat patterns may reflect the insignificant degree of Eu anomaly in the solidified trapped liquids and melt inclusions of their source materials.

H Ap Il ®Ap/2

200 El Ap/5

U) H Apl7 w I 100

0 Fig. 4. The ranges of REE distribution Z 50 0 patterns for the Apollo 11, 12, 15 and U

17 mare basalts are taken from many w J 20 sources cited in the reference section; a

see Fig. 1. a 10

5

La Ce Nd SmEuGd Tb Dy Yb Lu REE 32 H. WAKITA et al.

The Apollo 12 basalts are superimposed over Apollo 15 basalts with respect to REE patterns and show similar but slightly higher REE abundances. Apollo 11 and 17 basalts are much higher in their REE abundances and show smooth, but significant light REE depletion and appreciable heavy REE depletion patterns. These Apollo 11 and 17 patterns reflect mainly the cumulated accessory minerals and the trapped liquid and melt inclusions in their source rocks. After the crystallization of cumulate mineral layers with trapped liquid and melt inclusions that later became the Apollo 12 basalt source materials, additional cumulates crystallized from the magma and concentrated the light REE to 20 to 30X chondrites. At this stage of crystallization with La at --20 to 30X chondrites in the magma, CaO con tent of the magma may have been > 12%. We suggest that at this stage appreciable amounts of apatite crystallized with the major cumulate minerals, such as olivine, ortho pyroxene and clinopyroxene, and these cumulates also had, appreciable amounts of oc cluded trapped liquid and melt inclusions. It is noted that the REE distribution pattern of apatite in terrestrial rhyolitic rocks (N AGASAWA,1970) is quite similar to those of Apollo 11 and 17 basalts with negative Eu anomalies. At the same stage, significant quantities of ilmenite crystallized and/or Ti was highly enriched in the trapped liquid. Partial melting of the solidified Apollo 11 and 17 source materials resulted in the melting of apatite, solidified trapped liquids and melt inclusions and a small fraction of the cumulated host minerals to yield the present Apollo 11 and 17 mare basalts. Parenthetically, we note that REE and phosphate contents of the Apollo 11 high alkali basalts are approximately twice higher than those of the low alkali basalts. It is noted that apatite cumulates were not occluded on the plagioclase cumulates because of the appreciable difference in specific gravities between plagioclase (2.8) and apatite (3.2) minerals. Two Apollo 17 breccias 77017 and 79035 (LAUL and SCHMITT,1973b) are significantly enriched in Ti02 (5.3 and 6.5%) by a factor of 10 compared to the other Apollo 16 and 17 breccias with similar Sm abundances. Since the REE patterns in the above two breccias are similar to those of the Apollo 11 basalts, it is suggested that an apatite effect may have been involved in the genesis of these breccias.

6. Changes in chemical composition during differentiation In these two main dif ferentiation sequences, that is the plagioclase and mafic series, the magma changes its chemical composition progressively as plagioclase and mafic cumulates crystallize from it. In Figs. 5 and 6, abundances of A1203, CaO and Ti02, and MgO, FeO and Na2O v. Sm for the plagioclase differentiation series are plotted, respectively. Only data were plotted for which both Sm and these bulk elements were measured in the same sample or alquots thereof. Generally the.. trends are all consistent and systematic for this series of rocks. With increasing Sm content, the abundances of A12O3 and CaO decrease progres sively and those of FeO, MgO, TiO2 and Na2O increase. The first plagioclase to crystallize is essentially pure anorthite at about 36% A1203 and 20% CaO. These two bulk oxides changes continuously to high-K KREEP with about 16% A1203 and 10% CaO. On the other hand, values of less than 0.1 % of Ti02 in. the first plagioclase cumulates are compared with significantly enriched values of more than 2% in high-K KREEP. Lunar ANT-KREEP and mare basalts 33

Y 0 A1203 50 • Coo 14 M • Ti 02 20 o FeO 2.0 A Mg0 A 12 S • Na20 40 LA A O o°• 1.5 0 10 A d` a~ 1.5 0 A 0 0 ,~ 0 0 of 30 OD 0 8 0 0 0 Al A AAAA A 0 12 i A o A 40 0 o O A a J . 1.0 1.0 0 N 0 Ao ° M 0 0 0 0 6 z 0 0 C13 H 01 U a 00 00 08 20 0 8 A Y 0~ r • • o 4 • I 0 0 A 0.5 • o y0 0.5 • 0 IN No • %* • •1 i ~• .w• 111 2 • • M •• O • 10 NO m

0 0 0 0.1 1.0 10 0. I I.0 10 50 Sm (ppm) Sm (ppm)

Fig. 5. Abundances of A1203, CaO and Ti02 v' Fig. 6. Abundances of FeO, MgO and Na20 those of Sm for the plagioclase differentiation v. those of Sm for the plagioclase differentia series. Literature sources are those cited in tion series. Literature sources are those cited Fig. 1. in Fig. 1.

The FeO/MgO ratios in these rocks reflect those of the magma at each differentiation stage. The ratio of about 1.5 observed in the first plagioclase cumulates may reflect either the ratio in the primordial magma or significant amounts of Mg-rich ferromagnesian mineral separation had occurred prior to plagioclase crystallization. Previous estimates (TAYLORand TAKES,1974; GANAPATHYand ANDERS,1974) for the initial bulk moon indicate FeO and MgO contents of 11 and 31 %, respectively. After about 75 % crystalliza tion of Mg-rich olivine from such a compositional melt, the FeO and MgO ratio will be close to 1.5. With the initiation of plagioclase crystallization, Fe-rich ferromagnesian minerals crystallized and changed the FeO/MgO ratio from 1.5 to 0.6. This ratio then increased 'to an average FeO/MgO ratio of about 1.0, which was maintained during the last stages of crystallization of the ANT-KREEPmaterial. We are unable to interpret adequate ly such reversals in the FeO/MgO ratios. The basaltic rock series are quite different in chemical composition. In Figs. 7 and 8, abundances of A1203, MgO and Ti02, and FeO, CaO and Na20 V. Sm are plotted, respec tively. Markedly significant enrichments of Ti02 are observed in later differentiated rock suites. Approximately 1' 3% Ti02 in Apollo 15 mare basalts contrasts with that of about 12% in Apollo 11 high alkali rocks. Abundances of A1203 and FeO are rather constant at about 9 and 21 %, respectively. The CaO abundances show a slightly increas ing trend and the MgO, a decreasing trend with the differentiation index of Sm. The FeO/MgO ratios of approximately 2 in Apollo 15 and 12 basalts are compared with those

A0 34 H. WAKITA et al.

17 A1203

A Mg0 24 CD 0 0 o m 15 • Ti02 ~0CD 22 0 0 00 ° 0 6 0 0 0 20 000p 0 A 0 ® A • 0 o0 0 0 9A O 18 0 10 0 00 00 12 • • 0.6 0 • r ED 0 RP 0 • LT 10 ° 0 II 0 man 0% 0.5 p 0 0 • • • A U • •• ° °° • z 8 a 5 • ° 04

6 0.3 o Fe O • COO 4 0.2 A No20

0 2 I 2 5 10 20 50 1 2 5 10 20 50 Sm (ppm) Sm (ppm)

Fig. 7. Abundances of Ale 03, MgO and TiO2 v. Fig. 8. Abundances of FeO, Ca0 and Na20 v. those of Sm for the mafic differentiation series. those of Sm for the mafic differentiation series. Literature sources are cited in Fig. 1. Literature sources are those cited in Fig. 1.

of more than 2.5 in Apollo 17 and 11 rocks.

7. Chemical composition of the magma at the onset of plagioclase crystallization From the chemical data for pure anorthosites such as 61016 with the lowest REE abundances, we may speculate what the chemical composition of the associated magma was. We will assume that such a "primitive" plagioclase had the composition of CaA12Si2O8and that TiO2 (0.01 %) FeO (0.35%) and MgO (0.20%) in this anorthosite are attributed to the trapped liquid and any melt inclusions. Values for these latter three oxides were obtained from a variety of literature sources. From these values and the composition of anorthite, we estimate the trapped liquid to be about 2.2% by weight with a composition of about 0.5% Ti02, 18% FeO and 10% MgO.

8. Other trace element correlations and the heat source for melting In Table I we have compared the average abundances of selected trace elements and MgO in the low alkali Apollo 11, 17, 12 and 15 mare basalts; the data are taken from reliable literature sources. The general similarity in the K and Rb contents of these mare basalts suggests that K and Rb contents of the source cumulates and trapped liquid material were rather similar to each other if the degree of partial melting was the same for generation of all these mare basalts. Moreover, this implies that K and Rb contents of the trapped liquid fraction in the above cumulates would also be rather similar to each other. The above conclusions are also supported by the Cs data as well, although the average Cs value (0.022) for Apollo 17 basalts is based on only two values. These three heavy LIL alkali elements are expected to be concentrated in the trapped liquid component and will be contained in the first partial melt. Sodium, concentrated in plagioclase cumulate, would be expected not to

0 Lunar ANT-KREEP and mare basalts 35

Table 1. Average abundances in low alkali lunar mare basalts

Ap 11 Ap 17 Ap 12 Ap 15 Na (ppm) 3100 3000 1700 2100 K (ppm) 600 500 500 400 Rb (ppm) 0.7 0.6 0.8 0.8 Cs (ppm) 0.03 0.02 0.05 0.03 Ba (ppm) 60-120 40-90 65 50 Sm (ppm) 8-16 7-18 5 1-5 Th (ppm) 1.0 0.4 0.9 0.5 Sc (ppm) 94 75 50 40 V (ppm) 75 90 170 240 Cr (ppm) 1780 1640 3600 2900 Ni (p 3 -1 35 64 MgO p~pm) (°/o) 7.1 7.4 11-17 10 follow the trends exhibited by the heavy alkalis. PHILPOTTS et al. (1974) has also com mented on the relationship between the degree of partial melting of the cumulates and the uniform Rb content of the mare basalts. The generally higher REE and TiO2 (ilmenite indicator) abundances in the Apollo 11 and 17 mare basalts relative to the Apollo 12 and 15 mare basalts would seem to show that in such a postulated lunar fractionation sequence, the cumulates for the subsequent generation of the Apollo 11 and 17 basalts settled at a higher stratigraphic level relative to the cumulates for the Apollo 12 and 15 basalts - see Figs. 4 and 9. If apatite cumulates are not invoked for the REE patterns in the Apollo 11 and 17 mare basalts, then a reduction in the crystallizing lunar primordial magma of at least a factor of -2 must have occurred between the settling of the Apollo 12 and 15 cumulates and the Apollo 11 and 17 cumulate source materials. Obviously heavy alkali contents of the trapped liquids with the latter cumulates would be higher by 2 relative to the trapped liquid in the former cumulates. Consequently, a larger fraction of the Apollo 11 and 17 cumulates must have

partial melting Lunar surface impact remelting brecciation C PURE ANORTHOSITIES 0 W 0 a ANT U c 0 a)) a "KREEP 0' a ) (FeO/Mg0"I) a APATITE nr,r,nn~ninr ZIRCON FeO/MgO > I not sampled cn K-FELDSPAR (LMENITE Uvvvv v c Fig. 9. A concept of the proposed lunar Ap II 0 c) outer shell before bombardment by an 0 -20X Ap 17 basalt cient meteors. a a) z source a) Ap 12 =o J rocks Ap 15 U -lox 0

W CUMULATE MINERALS a' J MAFIC LAYER wI a-z as MELT :D 5 INCLUSIONS 36 H. W AKITA et al. partially melted in order to preserve the near constancy of the heavy alkalis in the mare basalts. But a larger degree of partial melting of the Apollo 11 and 17 cumulates relative to the frozen trapped liquid, which we assume is melted first, will also dilute the REE abundances. Such reasoning has necessitated the postulate of cumulate apatite in the Apollo 11 and 17 cumulate source material. If this postulate is valid, then the Apollo 12 and 15 cumulates and their associated trapped liquids are displaced not too far in the stratigraphic level; consequently, it would appear that titaniferrous-rich cumulates began to crystallize in copious quantities shortly after the Apollo 12 and 15 cumulates were laid down. The large variations in the REE abundances of Apollo 17 mare basalts have been interpreted by SHIN et al. (1975) in terms of near surface fractionation of ilmenite, olivine, plagioclase and clinopyroxene. An alternative interpretation, consistent with our model, suggests that REE content of cumulate apatite varied with stratigraphic level, i.e., at lower levels REE were more enriched in apatite. Partial melting of such levels could account for the observed differences. However, this mechanism does preclude some near surface fractionation as suggested by SHIH et al. Since the Sc partition coefficients in clinopyroxene are appreciable, e.g. 1.6 at 1,200°C (GOLES, HERING and LEEMAN, person. com. 1974), partial melting of the Apollo 11 and 17 relative to the Apollo 12 and 15 source cumulates at different temperatures could explain the different Sc abundances in the above two groups of mare basalts (LAUL et al., 1974). High V abundances in Apollo 12 and 15 mare basalts relative to the Apollo 11 and 17 are sympathetically tied to the Cr contents of these two groups and this relfects the general lunar V-Cr correlation first observed by LAUL et al. (1972). From some recent V studies on pallasitic meteorites in our laboratory, the approximate partition coefficient of V in olivine is estimated at -0.15. Therefore, we suggest that the Apollo 12 and 15 cumulates have a larger fraction of cumulate chromite than do the Apollo 11 and 17 cumulates. Higher Ni abundances in Apollo 12 and 15 mare basalts are probably associated with large Ni contents of the Apollo 12 and 15 source cumulates, which may have had higher olivine content. The heat source for partial melting of the cumulates in the generation of mare basaltic magma has always been a serious problem. Meteoritic impact that induced melting does not seem plausible in view of the ages of mare basalt crystallization of -3.1-3.9 AE ago. From the low partition coefficients Q 0.01) of the heat producing elements K, Th and U in the principal cumulate minerals of olivine, pyroxene, plagioclase and ilmenite, it is easily shown that insufficient amounts of K, Th and U are present to provide the necessary energy required to heat the cumulates above their base temperatures to their melting points. However, we suggest that modest amounts, say ^•5%, of trapped liquid in cumulate layers will provide the necessary levels of K, Th and U required for melting the cumulates in 0.7-1.5 AE after lunar accretion. For example, if we assume that Th and U contents were -20X and K was IX chondrites in the magma from which the cumulates crystallized and that '' 5 % of trapped liquid was retained in the cumulate layers, then the contributions of K, Th and U in the frozen trapped liquid component are -5 times more than those in the cumulate minerals. Calculations show that K is the principal heat producer and that such amounts of these three elements are sufficient to Lunar ANT-KREEP and mare basalts 37 provide sufficient heat for partial melting of the cumulate-frozen trapped liquid layers. SUMMARY In the early history of the moon, extensive melting occurred in the outer lunar shell and a magma layer of 100-200km was formed. Upon cooling, Mg-rich ferromagnesian minerals sank to form the lunar lower-crust. During ferromagnesian mineral crystalliza tion the concentration of LIL trace elements, such as the REE, and FeO/MgO ratio in residual melt continued to increase. After the chemical composition of the magma reached the stage of about 0.5% TiO2, 18% FeO, 10% MgO, high concentrations of CaO and A1203, and about lox chondrites in the LIL trace element abundances, both plagioclase and Fe-rich ferro magnesian minerals began to crystallize simultaneously. A difference in specific gravity for these minerals separates them from the magma. Varying amounts of melt of about 2 to 10% will be occluded on both cumulates. Plagioclase formed in the early stages was more enriched in Eu compared to other REE. The chemical composition of the residual liquid became more depleted in Eu but more enriched in other REE. Subsequently formed plagioclase possessed higher relative REE abundances and smaller positive Eu anomalies. Addition of small amounts of trapped liquid and melt inclusions with indigenous negative Eu anomalies to the plagioclase cumulates decreases continuously the positive Eu anomaly in the ANT rocks. Simultaneously, the Fe-rich ferromagnesian cumulates which occluded small amounts of trapped liquid and melt inclusions continued to crystallize to yield the source rock for the Apollo 15 basaltic rocks. The REE distribution pattern of this source rock does show the minimum negative Eu anomaly. With both plagioclase and ferromagnesian mineral crystallization the magma progressively changed its chemical composition and the Sm/Eu ratio. Subsequently, noritic rocks and Apollo 12 basaltic source rocks are formed (Fig. 9). Toward the end magma changed its chemical composition to yield apatite cumulates with the major mineral cumulates. It is noted that the REE distribution pattern for apatite in terrestrial rocks is very similar to those of Apollo 11 and 17 basalts. Except for a few cases, these apatites are not included in the plagioclase cumulates because of specific gravity differences for these two minerals. Finally, the chemical differentiation as exhibited by the plagioclase series reached the high-K KREEP stage. Separation of apatite minerals drastically changed the REE distribution pattern in the residual liquid. The V-shaped patterns observed in 12013 light materials (WAKITA and S CHMITT,1970a) with large negative Eu anomalies may reflect those of the residual liquid. At different degrees of partial melting of the mafic cumulate source materials, the trapped liquid and melt inclusions played a dominant role for producing the mare basaltic compositions. Subsequent early and extensive impacts by ancient meteors induced remelt ing, partial melting and. brecciation processes primarily on the plagioclase-rich cumulates and these phenomena were responsible for the final complex history of the lunar surface. Between high-K KREEP and the Apollo 11 basalt source materials, we suspect the presence of a residual liquid which has not been sampled and which has a FeO/MgO ratio greater than 1 and which is significantly enriched in apatite, zircon, K-feldspar, ilmenite minerals, etc. The "ilmenite microgabbros" found in the Apollo 17 breccia 72275 reported by MARVINet al. (1974) may be a probable candidate. 38 H. WAKITA et al.

ACKNOWLEDGMENT

We thank Dr. N. ONUMA for helpful discussions. This work was supported by NASA Grant 38 002-039.

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