Some Thoughts on the Origin of Lunar ANT-KREEP and Mare Basalts
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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 Apollo 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 olivine, 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 Apollo 15 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 Apollo 11 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-troctolite)-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 Apollo 12 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 Apollo 16 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 Apollo 17 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, Apollo 14 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 76543564435 ~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.