A lunar differentiation model in light of new chemical data on Luna 20 and Apollo 16 soils

lb&lmi~~etC&amochlmiccr A&+.107S,Vol.87.pp.O63 to978. PergamonPrem. PrintedinNorthemImland

A

lunar

differentiation model

in light of new

chemical

dataon Luna 20 and Apollo 16 soils DAVID F. NAVA and JOHN A. PHILPOTTS Code 644, Planetology Branch, Goddard Space Flight Center, Greenbelt, Maryland 20771 (Rekved

8 De.cemhw 1972; accepted in re&ed form 9 January 1973)

&&a&-Fines from a Luna 20 soil sample and from three Apollo 16 deep drill core samples have been analyzed for major-minor element abundances by a combined, semi-micro atomic absorption spectrophotometricand calorimetricmethod. Both the major element and large ion lithophile trace element abundancea in these soils, the fist from interior highland sites, are greatly influencedby the very high normative plagioclase content, being distinctly richer in Al and Ca, and poorer in K, P, Cr. Mn, Fe, and Ti, than most bulk soil samples from previous lunar missions. The relatively large compoeitionalvariations in the Apollo 16 core can be ascribed almost entirely to decreasingplagioclacewith increasingdepth. The chemicalcompositionof the Luna 20 soil indicatea leea plagioclaeeand less KBEEP than in the Apollo 16 soils. A lunar differentiation model is presented in which is made the suggestion that KBEEP is the result of a second fusion event in a lunar cruet consisting of early feldapathiccumulates and primary aluminous ‘liquid.

INTRODUCTION LUNA 20 LANDED in the highlands between Mare Fecunditatiaand Mare Crisium. Apollo 16 visited a site near the crater Descartes in the Central highlands. Samples returned on these missions constitute the first lunar materials collected at sites neither in nor immediately adjacent to mare areas. Previous studies, however, had given considerable insight into the composition of highlands material. The Surveyor 7a back-scstter analysis of Tycho ejecta showed higher Al and Ca and lower Fe snd Ti than mare materials (TURKEVICH, 1971). The orbital X-ray experiments on Apollo 16 and 16 showed the highlands to hs,ve higher Al/Si and lower Mg/Si than mare areas (ADLER et al., 1972a, b). The Apermine Mountains highlands bordering Mare Imbrium had been sampled on Apollo 15 and found to be rich in plagioclase (LSPET, 1972). Also petrographic examination of Luna 16 and Apollo 11,12,14 and 16 soils had revealed feldspathic fragments not represented as rocks and they were thought to come from the highlands (e.g. WOOD et al., 1970). In light of these studies it was not very surprising when preliminary investigation of the Luna 20 (VINOGRADOV, 1972) and Apollo 16 (LSPET, 1973) materials revealed high plagioclase contents. The purpose of the present paper is to report major-minor element abundances in a Luna 20 soil sample and in three soil samples from the Apollo 16 deep drill core. These data will be compared to bulk chemical soil compositions of the various lunar sites previously sampled and discussed in conjunction with large-ion lithophile (LIL) tram element concentrations reported previously for these four samples and six other Apollo I6 core-tube soils (PHILPOTTSet aE., 1972a). SAMPLES The Luna 20 sample analyzed in this study (22001,4) is a 48 mg portion of the ~126 pm finea sieve fraction which constituted about 70 per cent by weight of the bulk 2.05 gramme 963

964

DAVID F. NAVA and JOHN A. PHILPOYCS

exchange sample (LSAPT, 1972). It is estimated that the sample came from an average depth of approximately 10 cm. Petrogmphic observations relevant to the Luna 20 material have been made by VINOQRADOV (1972), GLASS (1973), MEYER (1973), PRINZ et aE. (1973), REID et al. (1973), ROEDDER and WEIBLEN (1973) and TAYLOR et al. (1973). The Apollo 16 deep drill core was obtained at Station 10, 175 m southwest of the LM and 26 m south of the ALSEP (APOLLO LUNAR GEOLOQYINVESTIUATION TEAM, 1972). The samples

analyzed in the present study were
16 samples studied in this work were analyzed for their major

( > 1 wt. %) and some minor (0.01-l wt. %) element abundances by the combined, semi-micro atomic absorption and calorimetric spectrophotometry method previously developed for our bulk chemical analyses of relatively small quantities of meteoritic and lunar materials (NAVA, 1970; Sc~nxx and NAVA, 1971; SCHNETZLERet al., 1972; PHILPOTTS et al., 1972b). These compositional data are listed in Table 1. Table 1. Chemical composition Constituent

Luna 20

(Weight *%)

22001,4

Apollo 16 80007,114

Apollo 16 60008.7

46.4 0.47 23.44 9.19 13.38 0.29 7.37 0.10 0.06 0.14 99.84

45.2 0.39 29.08 4.43 16.35 0.42 3.60 0.02 0.08 0.05 99.62

45.4 0.52 27.61 6.83 15-42 0.43 4.89 0.04 0.09 0.07 ILOO.20

48.0

47.5

43.2

loom

22 om

67 cm

SiO, TiO, A& 2: Ne,O Fe0 MllO P*% Cr,% Total: Sample weight L-

~

of Luna 20 soil, Apollo 16 deep drill core samples, and USGS reference silicate rocks*

(me): depth (spproximata):

-~

Apollo 16 60001.66At

USGS W-l

USGS BCR-1

45.3 0.56 26.19 6.42 16.38 0.49 6.29 0.08 0.13 0.10 99.92

n.d. 1.06 (1.07) 14.79 (14%) 6.76 (6.62) 11.08 (10.92) 2.16 (2.15) 9.92 (9.98) 0.174 (O-17) 0.133 (0.14) 0.016 (0*016) -

ad. 2.23 (2.23) 13.66 (13.66) 3.27 (3.28) 7.08 (6.96) n.d. n.d. 0.181 (O*i8) 0.366 (0.36) n.d. -

47.4

60.0

60.0

223 cm

-

-

* Ti and P d&arm&d oolorimetrioslly; al1 other elementa detarmined by atomio absorption qwotmphotometry. Total iron expressed ea FeO. U.S.G.S. rooks en&wad. in 60 mg quadrupliosten. oonaurrently with them lunar samples aa aoouraoy and precision monitors. Alao. AGV-1. AlsOa: 17.09 (17*01); PCC-1, 6iOs: 41.8 (41.87); DTS-1, SiO,: 40.9 (4046). Data in perenthestw are recent litamture valuee: W-l from Tsbie 1, oolumn 3b of FLEI~CHEB (1969); other reference silicate rocks from Table 4 of FLANAOAN (1969). n.d. = not determined. Anslyst: D. F. Nave. t Biomedical Consortium sample.

Accuracy and precision of the analysis method were both monitored for the present four highlands samples by simul&neous analyses of U.S. Geological Survey reference (i.e. ‘etandard’) silicate rocks. The relative precision, from quadruplicate 60 mg aliquants of these USGS reference silicates. is f l-2 per cent, or better, for most major elements. Precision for the minor elements is generally within f 6 per cent, relative. Analytical accuracy can be assesr& from the results which are r&o listed in Table 1, along with recent literature values for comparison. COMPARISONS

comparing the Luna 20 and Apollo 16 soils to each other and to other lunar ma&i&, it is worth (a) examining other investig&~~’ results for the Luna 20 Before

Differentiation model in light of

new

chemical data

on

Luna 20 and Apollo 16 soils

966

Apollo 16soilshave not been studied by other analysts) and (b) considering to what extent these materials are representative of each site. Relevant compositional data for Luna 20 soils, which should be directly comparable to our sample (i.e. being from the same sieve fraction), have been reported by BA.NSALet al. (1972), HEIXEE et al. (1973), JAWHOBBANIet al. (1973), and LAUL and SCHMITT(1973). Except for Si by JAWHORBAMet al. (1973) and Na and Cr, the major element data agree to within 10 per cent with our values given in Table 1. The agreement, however, is not really satisfactory from an analytical point of view and this re-emphasizes the problem of sample heterogeneity and other difficulties associated with analysis of small samples. The trace element data also agree to within 10 per cent, in general, with our results for Luna 20 soil (PHILPOTTS et al., 1972a) given in Table 2. Erbium shows a 20 per cent range, as does Zr. In addition there appear to be high reported values for Ce (BAMAL et al., 1972), Gd (HELMEEet al., 1973), and Lu (IAUL and SCHMITT,1973). These few exceptions, however, merely serve to emphasize the high quality of the trace element results obtained by all of the analysts. soil (our

Table 2. Compfuison of ‘average’ oompositions * of lunar soils and ratios of model soils/actual soila L-

kutitwn~

Apollo 16 Apollo 11 Apollo 12 Luna 16 Apollo 14 mareApollo16 I,,- 20 m8re mare m8re KREEP highland bigbland highland

SiO, TiO* Al@, z

42.1 7.62 13.81 12.08 7.86

%O Fe0

0.47 16.70

Z cz$, Li K Rb Sr B8 &a Nd 8m z

0.21 0.12 0.29

DY E? Yb Lu

sites

llO0 2.8 160 170 46 40 14 17 1.8 19 12 11 1.6 -

46.3 2.70 13.19 9.71 10.61 0.66 I6.w) 0.22 0.31 0.36 21 2600 7.9 160 460 110 68 IS 2.0 23 26 I6 14 a.2 -

46.1 3.3 16.0 8.3 11.6 0.39 16.4 0.23 0.06 0.33 8.3 870 1.9 260 170 32 26 8.1 2.2 10 10 6.8 6.4 0.83 230 6.9

48.1 1.72 17.60 9.36 10.82 0.69 10.37 0.14 0.61 0.22 31 4300 l&i 180 820 170 110 30 2.6 36 39 23 22 3.3 910 21

46.6 1.64 13.60 Il.06 10.39 0.42 16.21 0.20 0.19 0.42 14 1700 6.6 130 280 66 43 12 1.6 14 I6 9.3 8.4 1.3 360 8.8

46.2 0.62 27.61 6.66 lb.81 0.44 4.80 0.06 0.11 0.07 8.1 940 2.7 170 130 29 19 6.1 1.2 6.2 7.1 4.0 3.8 0.63 170 4.0

46.4 0.47 23.44 Q*lS 13.38 0.29 7.37 0.10 0.06 0.14 6.04 666 1.60 144 93.8 16.1 10.6 2.98 0.94 3.81 4.08 2.40 2.36 0.38

04 2.7

Ration 38% 22001 64.3% 22001 +9% 14306 +13*3x 14306

+63% 16416 +=4% 16416 60007

1 1.08 I.01 0.88 1.09 2.50 1.08 1.40 O-90 0.93 1.02 I-14

1 0.07 0.99 0.88 0.97 1.07 1.06 0.97 0.93 0.98 -

60001

1 1.01 0.98

O-78 1.04 I.20 0.92 1.10 0.97 la) 0.91

1.11

1 0.90 ;:: 1.06 0.97 1.00 0.90 la0 0.96 -

l Oxidtuaclweight pea cent: data for Apollo 11everngedfrom snalyseaof Wiik. Compston,Maxwell.Peak, Rose in ApoD&12 Lunlw Sci. Proo.; Apollo 12 from Comp&m. Cuttitta in rsCwndtUnar Sci. Proc.; Luna 16 from Gillum. Hubbard in Earth Pkwt. Sci. lieit. Luna I6 issue: Apollo 14 t%omNews, Rose in Thizd Lumw Sci. Pmt.: Apollo 16 and 16 from our d&a end

MSU; Luna 20 our data. Elements aa ppm; all our data.

It is di&ult to estimate, at this time, how representative,our Luna 20 fines are of the site. Microprobe major element analyses of glasses and lithic fragments in the soil(G~~ss, 1973; Pnr~etal., 1973; R~~~etal., 1973; ROEDDERaBdWEIBLEN,1973) tend to show a wide and fairly uniform scatter around the soil composition. Soil

966

DAVID F. NAVA

and JOHN A. PHILPOTTS

fragments (HELIVIKEet al., 1973; LAUL and SCHMITT, 1973) have both higher and lower trace element concentrations than do the bulk fines. Analyses of other Luna 20 soils (VINOGRADOV, 1972) tend to agree with ours to within about 5 per cent of the amount reported for SiO,, AlaO,, MgO and FeO. However, values for Na,O, TiO,, CaO, and trace elements tend to show large differences. Without detailed knowledge of the petrography of the Soviet samples and their preparation, and without data on samples analyzed in common, it is difficult to interpret their results. The Apollo 16 core samples (Table 1; PHILPOTTS et al., 1972a) appear to cover essentially the compositional range for soils from that site. Unpublished analytical data that we have obtained to date for nine other Apollo 16 soils extend the range only a small amount. Compared to published analyses (LSPET, 1973) core sample 60007, 114 has higher Al and lower Ti, Fe, Mn, and Sr, whereas 60001,55A has the lowest Al and highest Mg, K and Rb. The fact that more elements are not at extreme values might reflect the nature of ‘grab’ samples of multi-component mixtures or analytical error. In any case it appears likely that our core samples, and more particularly the ‘average’ soil listed in Table 2, can be considered representative of soils at the Apollo 16 site. Indeed, judging from the preliminary analyses (LSPET, 1973) and assuming random selection, this soil composition might be similar to the average for Apollo 16 rocks. The most remarkable bulk compositional features of these Apollo 16 core-soil and Luna 20 soil samples are that (1) the Apollo 16 samples exhibit the most distinctive correlation of chemical composition with depth (Tablel) compared, for example, to deep drill core samples from Apollo 15, which show relatively slight increases of model KREEP components with depth (SCHNETZLERet al., 1973); and (2) the chemical abundances of the Apollo 16 and Luna 20 soil samples reported in this study and by LSPET (1973) greatly extend the compositional range defined by analyses of soils from previous lunar sites (Table 2). Comparison of Luna 20 and Apollo 16 soils to each other and to soils from other lunar sites is facilitated by Table 2 in which ‘average’ soil compositions, based largely on our own analyses, are given. The disadvantage with this form of presentation is that ranges are not shown. The soils collected on Apollo 12 and 15, for example, show a wide range in large ion lithophile trace element concentrations compared to that for Apollo 16 soils. Nevertheless, the Table is useful for indicating general differences between the sites. It is apparent that the Luna 20 and Apollo 16 highland soils have higher Al and Ca contents (indicating higher normative plagioclase) than have soils from previous missions. The Apollo 16 samples are more extreme in this respect than the Luna 20 soil. These highland soils also have distinctly lower Fe, Ti, Mn and Cr and, except for Apollo 11 and Luna 16 soils, Mg and P. Although there is considerable overlap with Luna 16 and the most depleted Apollo 15 soils ( SCHNETZLER et al., 1973), the large ion lithophile trace elements tends to be less abundant in Apollo 16 soils relative to other soils ; they are even less concentrated in the Luna 20 soil. Although in this section we have been comparing lunar soils, it is of interest to note that the Luna 20 soil is very well bracketed in composition by two similar Apollo 16 crystalline rocks 61156 and 66095 (LSPET, 1973) for many constituents. In addition to Si and Sr, the elements not bracketed are Ti, Na, P, K and Zr, which are

DiEerentiation model in light of new chemioe;ld&a on Luna, 20 and Apollo 16 soils

987

lower in the Luna 20 soil. These l&ter elements are all concentrated in KREEP. The ~mpo~tion&l scanty of these rocks a;nd the soil suggests that the lunar highlands at the Apollo 16 and Luna 20 sites m&y have & good deal in common. This point is expanded upon in the next section. MODELSort MIXTURES As noted above, the Apollo 16 soils are richer in both normative pl~gioclase and LIL trace elements than the Luna 20 soil. This sugg~ts that the Apollo 16 soils might contain greater amounts of anorthosite and KREEP. We have attempted on the basis of aluminum and cerium concentrations to model the two Apollo 16 soiIs for which we have major and LIL trace element data in terms of three arbitrary, but not unreasonable, components of Luna 20 soil (Table 2), Apollo 14 KREEP-rich breccia. 14305 (PHIIZOTTSet at., 1972b), and Apollo 15 snorthosite 15415 (LSPET, 1972; HUBBA_RD et al., 1971aJ. It should be noted here that there sre not sufficient data svailrtble at the present time to permit rigorous modelling of the Apollo I6 soils in terms of modal components. Instead, we are assuming that the Luna,20 and Apollo 16 soils msy have much in common, and checking to see whether the differences can be reconciled in terms of the ‘pure’ components KREEP and anorthosite. For 60007 the estimated propo~ions are 38 per cent 22001, 9 per cent 14305 and 53 per cent 15415; and for 60001, 54 per cent 22001, 13 per cent 14305 and 33 per cent 15415. That these proportions work quite well for the other elements may be seen in Table 2 where model to real soil ratios are presented. Ratios wering by more than 10 per cent from unity have been italicized. It will be noted that except for Cr, the deviant elements are the same for both cases. Titanium is low in the model soils. It is contributed about equally by 22001 and 14305. This suggests either that the KREEP component in the Apollo 16 soils may have higher Ti than 14305 or th& there is an additional Ti bearing component (cf. BANSALet al., 1972). Sodium is Jso low in the model soils. It is contributed by all three components. A possible means of building up Na in the model would be by using plagioclase more sodic than 15415. Manganese, which is contributed largely by 22001, gives the worst match, being higher in the model soils. The situation is similar for Cr. Given the relatively small sample sizes analyzed, heterogeneous distribution of spine1 might account for some of the discrepancy. Barium is slightly high in the model soils. The dominant contribution is from 14305. The variable K/Ba and Ba/Ce ratios for samples of the KREEP-rich Apollo 12 breccia 12013 (SCHNETZLER et al., 1970) suggest that this is not a serious The final discrepancy is for Eu which is sightly low in the model soils. discrep~~. Europium is contributed by all three components but one possible explan&ion, as for Ns, would be a somewhat later stage feldspar than 15415. It should be noted at this time that this approach to modelling is decidedly crude. No attempt has been made to optimize the components selected, in terms of compositional or modal data. Also certain subtleties of the data have been ignored. Thus the slightly higher coning of K and Rb relstive to rsre-earths in 60001 compared to 60007 might be explsined in terms of a component similar to 12013 light fraction (SCHNETZLER et al., 1970) which would also affect Na, Eu, etc. Nevertheless the match between the model soils and actual soils is quite remarkable and warrants further consideration.

968

DAVIDF. NAVAand JOHNA. PHIISOTTS

Several conclusions may be drawn from this exercise in mode&g. One is that the Apollo 16 soils differ from each other almost entirely only in terms of the amount of the anorthosite component. 22001 and 14305 are in the proportions of 4.2 to 1 and 4-l to 1 in the 60007 and 60001 model soils. For the deep drill core itself, this means a decrease in the anorthosite content with depth. The essentially constant proportions of 22001 and 14305 in the model soils indicate that the Apollo 16 soils can be modelled largely in terms of anorthosite and another component. BAN~ALet al. (1972) have arrived at a similar conclusion. The composition of this component in our model corresponds to the Apollo 14 crystalline rock 14310 (PHILPOTTS et aE., 1972b) to within about 10 per cent for the major elements; the LIL trace elements abundances are, in general, two to three times higher in 1.4310 than those indicated. There is nothing special about 14310, however, and the second component in the Apollo 16 soil model could have considerably lower normative plagioclase. Perhaps a more attractive model for the major elements would be a mixture of anorthosite and the peritectic liquid in equilibrium with plagioclase-olivinepyroxene (WALKERet al., 1972). The trace elements will be discussed further below. We now turn to a consideration of the Luna 20 soil. This was used as a component in modelling the Apollo 16 soils. This soil itself,however, is a mixture and an attempt can be made to model it in terms of reasonable components such as 14306 KREEPrich breccia, 15415 anorthosite, and component X. Again no attempt is made here to model the soil in terms of modal components. Rather we are attempting to oharacterize the Luna 20 soil in terms of components that our genetic prejudices (see below) lead us to believe are important. It is hoped that any surficial lunar material will have modal components that can be modelled in terms of our ‘pure’ components. (These are termed pure but in fact all, whether they be primary cumulate minerals or primary or secondary liquids, are expected to show compositional ranges.). It might also be noted that we are ignoring a mare-basalt component that might amount to 10 per cent of the sample (REID et al., 1973). The LIL trace elements in 22001 show very similar relative concentrations, excluding Sr and Eu, to those in the Apollo 16 soils (PHILPOTTS et al., 1972a) and in KREEP. It seems entirely possible that a considerable part of these traczeelements in the Luna 20 soil is ultimately contributed by a KREEP component. The Luna 20 soil could contain only about 9 per cent of 14306 KREEP-rich breccia. Subtracting this amount of 14305 leaves a hypothetical material which could consist almost entirely of 69 per cent 15415 anorthosite and 31 per cent Mg,Fe silicate. The indicated composition of this latter component is 26 per cent MgO, 22 per cent Fe0 and 47 per cent SiO,. This suggests low Ca pyroxene but the uncertainties in the calculation are such that this must be taken only as an indication of a possible component. In view of the possibility mentioned in connection with the Apollo 16 soils of a ‘KREEP’ component with lower LIL trace element concentrations (e.g. low alkali high alumina basalt), and the paucity of KREEP fragments in the Luna 20 soil (GLASS,1973; PRINZet al., 1973; REID et al., 1973; ROEDDERand WEIBLEN, 1973; TAYLORet aZ., 1973), it is of interest to attempt to put limits on the amount of ‘KREEP’ in terms of the major element abundances. One method of doing this is to assume that the component left after extracting ‘KREEP’ from the soil is not likely to have CaO/AI,O, less than O-555, the value for 15415 anorthosite (LSPET,

Difbrentiation model in light of new uhemioaldata on LU.IU 20 and Apollo 16 soila 969

1972). This method yields a maximum of 22 per cent 14306 in the Luna 20 soil. As in the case using 14306 trace element data, the ‘KREEP’-free eminent contributes most of the MgO, FeO, etc., to the mixture and has high MgO/FeO (1.35). One final point might be made concerning these mixiug models. Although the choice of components is open to debate, there is little doubt that these materials can be modelled to a &at approximation in terms of a few simple components, In light of this, the current tendency to consider each constituent identified in a soil or breccia as a f~damental rock-type, worse still to give them names, worst yet names with unproven genetic or selenographic connotations, is unfortunate to say the least. LUNAR DIFFEREN~TION MODEL Inasmuch sa the Luna 20 and Apollo I6 samples constitute the first samples returned from highland sites remote from mare aress it seems app~p~ate in light of these new data to discuss briefly some of what is known at the present time about the major and LIL trace element geochemistry of the Moon. To a first approximation, lunar samples may be grouped in terms of composition into three kindreds which for oonvtmience may be called ‘mare basalt’, ‘KREEP’ and ‘anorthosite’. Mam basalt ~onstitu~s a major rock-type returned on the Apollo 11, 12 and 15 and Luna 16 missions. In general, these rocks are characterized by relatively high Fe and low Al. Perhaps no less important are the facts that mare basalt is younger than other lunar materials, and almost certainly formed from magma rising from depth within the Moon. The second type of material to be returued as a rook was found on the Apollo 12 mission as the breccia 12013, pa~ie~arly the dark portion, It was shown that this type of material constituted a major component in the Apollo 12 soils (SCHNET~LER et aE., 1970, 1971; HUBBARDet al., 1971b). This type of material was variousIy termed KREEP and nonmare basalt (HUBBARD and GAST,1971). The term KREEP is appropriate enough in view of the high concentration of K, rare-e&h elements, and P (in addition to other LIL trace elements). The term nonmare basalt, whereas app~p~a~ enough in that this material is quite distinct from mare basalt, might be confusiug in that this material is obviously abundant at the Apollo 12 mare site and indeed the Apollo 16 and 16 orbital y-ray experiments, supported by ground truth at the landing sites, show K-U-Th (most likely KREEP) to be highest in the regolith of Mare Imbrium and Oceanus Procellarum (METZQERet al., 1972). The characteristics of KREEP, besides high trace element abundances, are relatively low Fe and intermediate Al. Complicating the nomenclature, but of much potential significance, is the apparent occurrence of rocks having similar major element compositions to KREEP but lower trace element abundances (REID el al., 1972). The ‘anorthositic’ rocks were first returned on the Apollo 16 and 16 missions (LSPET, 1972; 1973). It seems likely that feldspar accumulation was involved in the genesis of these rocks. Their ~h~ac~~tics include relatively low Fe and high Al. One of the major problems in lunar geochemistry at the present time concerns the relationships between these three types of materials. There appears to be a close connection between ‘KREEP’ and the anorthositic rooks. They occur intimately

970

DAVID F. NAVA

and JOHNA. PHILPOTTS

mixed in rocks from Apollo 15 (LSPET, 1972) and Apollo 16 (LSPET, 1973). In addition they are both feldspathic. The problems arise in trying to relate these rocks to the mare basalts. In general, mare basalts have relatively low MgO/FeO and lack feldspar on the liquidus (RIN~WOOD and GREEN, 1972). KREEP, on the other hand, has high MgO/FeO, plagioclase on the liquidus (GREEN et al., 1972) and high LIL trace elements abundances. In general, MgO/FeO is expected to decrease with extent of differentiation, and LIL trace element abundances to increase; also, plagioclase, once on the liquidus, is not expected to disappear. The problems are manifest. An easy way around these problems, perhaps the correct way, is to assume no igneous connection between mare basalt and KREEP. Thus HUBBARD and GAST (1971) and GREEN et al. (1972) propose an initially layered, heterogeneously accreted Moon. Arguments against heterogeneous accretion have been advanced (PHILPOTTY et al., 1972b). Further the remarkable isotopic similarity of lunar rocks (EPSTEIN and TAYLOR, 1972; CLAYTON, 1973) and the MnO-Fe0 correlation found by LAUL and SCHMITT (1973) lend no support to the heterogeneous accretion model. Another factor that tends to support an igneous relationship between mare basalts and other lunar materials is their complementary aspect. An example of this is that mare basalts, in general, fall on the calcic pyroxene side of the meteoritic Ca-Al correlation (WILLIS et aZ., 1971) and KREEP and anorthosite falling off, from about the same For these reasons, attempts to relate mare concentrations, towards plagioclase. basalts to the other materials still appear to be worthwhile. It appears possible, due to the involvement of oxide or, in light of the reducing oonditions, even metal phases, that the MgO/FeO variation with differentiation is opposite to that expected. However, there is little evidence that supports this suggestion, and it will not be explored further here. The lack of plagioclase on the mare basalt liquidus can be explained in terms of the model that was put forward as early as Apollo 11 (PHILPOTTS and SCHNETZLER, 1979) involving fusion during a second igneous event of mafic (including Ca pyroxene) rich cumulates or residues. Thus the only apparent problem with the initially homogeneous Moon model is the conjunction of high MgO/FeO and LIL trace elements in KREEP. The facts that most highland rocks appear to be mixtures (LSPET, 1972; 1973) and that there occurs material similar to KREEP in major elements but deficient in trace elements, suggest a model in which the LIL trace elements and the major elements (and Cr) in KREEP are not genetically related (i.e. KREEP is a mixture). In this model the high MgO/FeO of KREEP would reflect its early position in the differentiation. The LIL trace elements in such a mixture might be contributed by a highly enriched late liquid of unknown major element composition. One problem with this model is that a priori there would appear to be no reason why the trace elements should be mixed with KREEP any more so than with anorthosite (although reasons can be contrived), yet the Apollo 16 rock data (LSPET, 1973), for example, show an inverse correlation between anorthosite and LIL trace element abundances. A preferable model involves the fact that the major element composition of a liquid tends to remain fixed during partial fusion until a major solid phase is exhausted whereas the trace element conoentrations are a function of the modally dependent bulk solid-liquid partition coefficients. It has been shown (GRREN et al., 1972 ; WALKER ct al., 1972) that the KREEP composition corresponds to that of the

Dif%rentiation model in light of new ohemical data on LUXN% 20 and Apollo 16 soils

971

peritectic liquid in equilibrium with plagioclase and pyroxene (and olivine). The high &&O/Fe0 and LIL trace element inanition preclude a direct relatio~~p between KREEP liquid and the mare basalts. It is therefore proposed that =EEP basalt was produced by partial fusion of near surface rocks during a second igneous event. The high LIL trace element content of KREEP requires that its parental material also contained appreciable concentrations of these elements. This can be accounted for if the parent material, in addition to consisting of early primary feldspathic cumulates, also contained considerable Arnold of the primary alu~nous liquid (with =EEP-like major element compositions) from which the feldspathic cumulates formed. That KREEP might be such a secondary liquid, rather than a primary liquid, appears to be supported by the orbital y-ray data (METZUETCet al., 1972). These data relate to the present day soils but it appears likely that the pie-mare basalt rocks will be found to have a similar asymmetric distribution. In summa~, a simple sta~ment of the model is as follows : (1) igneous differentiation of at least the outer portions of a homogeneously accreted Moon. The liquid was aluminous and gave rise to feldspathic cumulates now represented in the highlands, and at a later stage to the more iron rich cumulates parental to the mare basalts. (2) Limited partial fusion of feldspathic cumulates and primary aluminous ‘liquid’ (basalt, gabbro) during a rare, if not unique, major second fusion of crustal rocks, selenograp~ca~y restricted, giving rise to KREEP basalt. (3) Derivation of mare basalt from depth. The compositional data for Luna 20 and Apollo 16 highland soils discussed in this paper appear to be entirely consistent with this model. The important components for modelling surficial lunar rocks and soils would appear to be (a) primary liquid, (b) cumulate minerals, (c) KREEP, and (d) mare basalt. AcknowEe@nen&-We are grateful to the Academy of Soiencee of the USSR for making it possible for us to participate in the analysis of the Luna 20 sample. We sincerely appreciate the confidenoe whioh the Lunar Sample Anelysia Planning Team has shown in the atomic absorptioncalorimetric analytical method herein described to entrust a portion of the exchanged Luns 20 sample for this research. We &IOthank our respective families for their patience during the course of this work andthe prep~tion of the manueeript.

ADLER I., TROMBKAJ., GERARDJ., LO-N P., SCH~DSBEUKR., BLOD~ETH., ELLERE., YIN L., LAMOTEER., &RIENBTEMP. and BJO~OLM P. (1972a) Apollo 16 geochemical X-

ray fluorescence experiment: preliminiuy report. Se&me13 175,430-440. ADLERI., TRonasgh J., GERARDJ., LO~MA~ P., SCHNADEBECK R., BLOD~~ H., ELIJZRE., YIN L., LAMOTEER., OSSWALD G., GORENSTEIN P., BJORKHOLM P., GVRSKYH. and HaRRrs B. (19728) Apollo 16 geoohemical X-ray fluorescence experiment: preliminary report. Sca&ce 1’7’7,256-268. APOLLOLUNARGEOLOGY IN~ESTIGIATION TEAM(1972) Documentation and environment of the Apollo 16 samples: a preliminary report. U.S. Geol. Surv. Interagency Report: As&oGary

51.

BANSALB. M., Cmo~ S. E., GASTP. W., HUB-D pii.J., RHODESJ. M. and WISEH. (1972) The chemical composition of soil from the Apollo 16 and Luna 20 sites. Earth Planet. Sci. Lett. 17, 29-35. CLAYTONR. N. (1973) Oxygen isotopic composition of the Luna 20 soil. Cleochim. Cosmochim. Acta 87, 8114313.

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Differentiation model in light of new chemical data on Luna 20 and Apollo I6 soils

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