Origin of lunar feldspathic rocks

EARTH AND PLANETARY SCIENCE LETTERS 20 (1973) 325-336. NORTH-HOLLAND PUBLISHING COMPANY

ORIGIN OF LUNAR FELDSPATHIC

ROCKS

D. WALKER, T.L. GROVE, J. LONGHI, E.M. STOLPER and J.F. HAYS Hoffman Laboratory, Harvard University, Cambridge, Mass. 02138 {USA) Received 20 April 1973 Revised version received 6 September 1973 Melting experiments and petrographic studies of lunar feldspathic rocks reveal possible genetic relationships among several compositionally and mineralogically distinct groups of lunar rocks and soil fragments. Dry, low PO2 partial melting of crustal anorthositic norites of the anorthositic-noritic-troctolitic (ANT) suite produces liquids of the KREEP-Fra Mauro basalt type; dry, low PO2 partial melting of pink spinel troctolite (PST) produces liquids of the "very high alumina basalt" or microtroctolite type. Both ANT and PST are probable components of the primitive terra crust. If crystal fractionation in a cooling basaltic liquid could have produced such a crust, it would also produce a mafic interior capable of yielding mare basalts by later remelting at depth.

1. Introduction

2. Relevant phase equilibria

Several groups investigating lunar soil glasses and lithic fragments have identified statistically preferred compositions. Of particular interest in reconstructing the early history and crustal evolution of the moon are the feldspathic varieties with Fe/Fe+Mg~0.4. These types are found in all lunar soils but predominate at lunar highland landing sites where they presumably are mechanical and thermal degradation products o f the bedrock. Their occurrence in mare soils where mare basalt detritus predominates is the result of transport from the highlands by meteorite impacts. The preferred compositions of the feldspathic types can be related to points and curves of multiple crystal saturation in the relevant experimental systems at low pressure. Hence the possible genetic relationship among the various types must be constrained by equilibrium crystal-liquid processes. It is the purpose of this paper to identify these processes and attempt to state the constraints, when there are any, that are placed on the bulk compositions of the source materials from which the feldspathic types are derived. If ambiquities of time and scale inherent in the reasoning can be resolved, it will be possible to characterize the evolution of the lunar crust.

The feldspathic lunar compositions can be approximately fitted into the system SiO2-CaA12Si208Fe2SiO4-Mg2SiO4. We have previously presented a pseudo-ternary liquidus section through tile pseudoquarternary liquidus tetrahedron of this system for conditions of low Cr203 and PO2 an order of magnitude or two below the Fe/FeO buffer [1,2]. This section was constructed on the basis of our experimental work on lunar materials with reference to the end-member systems presented by Andersen [3] and Roeder and Osborn [4]. The phase boundaries are projected on a section through SiO2 and CaA12Si2Os and having a molar Fe/Fe +Mg = 0.3. This particular section is useful for the feldspathic lunar rocks which have molar Fe/Fe+Mg in the range 0.2-0.4. A composition may be plotted in this section by recalculating its SiO2, A1203, CaO, FeO and MgO in terms of molar SiO2, CaA12Si2Os, CaSiO3, Fe2SiO4, and Mg2SiO4. Since these lunar compositions have a Ca/A1 ratio essentially determined by calcic plagioclase, the CaSiO3 component is small and may be neglected. The olivine formulas are combined and the resultant plotted on the pseudo-ternary with SiO2 and CaA12Si2Os. An important shortcoming of this projection is that F e - M g variation is suppressed,

326

D. Walker et al., Origin o f lunar feMspathic rocks

TABLE 1 Microprobe analyses used to locate boundaries in fig. 1

SILICA

A

B

C

45.9 0.91 18.8 0.15 7.28 12.8 13.6 0.14 0.51

49.1 2.36 15.9 0.31 9.26 9.19 12.1 0.67 0.69

55.4 5.19 12.5 0.13 7.71 4.59 9.70 1.55 0.60

100.09

99.58

97.37

UNITS E =0.3 +MG LOW PO2 AND CReO3

/

/

/ "i,,

PLAGqOCLASE ~ \

OL,~,.~ \ '

/ OLIVINE

/

A ""

\"

•

\

SiO2 TiO2 A1203 Cr203 FeO MgO CaO K20 Na20 Sum

ANORTHITE

Fig. 1. Pseudo-ternary liquidus section in the system SiO2CaAI2Si208-Fe2SiO4-Mg2SiO 4 at a molar Fe/Fe+Mg of %0.3 after Walker et al. [2]. Curves in the neighborhood of point A differ slightly from those shown in Walker et al. [1] which were extrapolated from the end-member systems [3, 4]. Filled circles show "dirty system" experiments used to plot and bracket the curves [1,2]. See table 1.

necessitating explicit notation of Fe/Fe+Mg or concomitant use of some other projection such as SiO2Fe2SiO4-Mg2SiO4. However since the position of the curves is rather insensitive to F e - M g variation in the limited range of Fe/Fe+Mg ratios in this projection, the pseudo-ternary liquidus is a useful device for illustrating compositional and thermal relations. For other purposes other projection schemes may prove more useful. To facilitate the location of points A, B, and C from fig. 1 in other projections, representative analyses of these liquids are provided in table 1. Fig. 1 reproduces this diagram. The primary phase volumes sectioned correspond to crystalline phases occurring within the system with the exception of the spinel primary phase volume. For this reason the uppermost curve bounding the spinel primary phase volume (which appears as a point in this section) is a peritectic or " o d d " curve [5]. Spinel-consuming reactions take place along this curve. Piercing point A is the reaction spinel + liquid = olivine + plagioclase, near 1275°C in this section. By the same reasoning the surface separating the pyroxene and olivine primary phase volumes is "odd". Peritectic pointsB (the intersection of the section with a curve separating the

olivine, pyroxene and plagioclase primary phase volumes) is the reaction olivine + liquid = pyroxene + plagioclase near 1200°C in this section. Since the curve representing the junction of the silica, pyroxene, and plagioclase primary phase volumes in compositionally interior to those phases the corresponding cotectic point in this section is "even". Point C is located below 1150°C. Crystallization paths in this section have been previously discussed, and thermal arrows indicate the direction of falling temperature on the curves.

3. Fra Mauro basaltic types We have previously examined and remarked upon [1] the close approach of the soil at the Fra Mauro Apollo 14 landing site to peritectic B. We also took note of the fact that preferred compositions of glasses reported b y the Apollo 14 soil survey appeared to be controlled by the plagioclase saturation curves emanating from point B. Since that time several further soil glass and lithic fragment surveys have been conducted by various groups [ 6 - 1 3 ] on Apollo 14, 15, 16 and Luna 20 soils, and each has found preferred compositional groups which lie near B or on the adjacent plagioclase saturation curves. The nomenclature of the types is somewhat more diversified than the compositional varieties. Fra Mauro basalts (low, moderate, and high K), norite, glasses B-C-D, alkalic high-alumina basalt, KREEP basalt, and KREEP-Iess KREEP basalt are varietal names of this chemical type which reflect differences in alkali and trace

D. Walker et aL, Origin o f lunar CeMspathic rocks

element abundances, nature of material surveyed (glass vs. lithic fragments), and the persuasions of the various investigators. We prefer to use the name Fra Mauro basalt after the type locality which has produced the only hand-specimen sized representatives with basaltic texture. There is some structure to the chemical variation within this type. Fig. 2 summarizes the results of the various surveys on the pseudoternary liquidus diagram. The compositions appear to be controlled by the plagioclase saturation curves emanating from B, with the bulk soil composition at Fra Mauro being close to B. If these compositions are generated by crystal-liquid equilibrium processes, partial melting is clearly favored to fractional crystallization on several lines of evidence. Hubbard et al. [16] among others have noted that the variable enrichment in trace LIL elements as opposed to the "primitive" Fe/Fe+Mg ratio favors a partial melting origin. J.A. Wood (personal communication) among others has noted that the probable source rocks are not enriched enough in KREEP components to produce the high concentrations of KREEP occasionally observed in this chemical type by reasonable amounts of fractional crystallization. We have previously noted [1] the difficulty in producing substantial amounts of this chemical type due to the delicate chemical and thermal balance at a peritectic in crystallization processes as opposed to melting processes. These arguments are most applicable to generating Fra Mauro material near point B by small amounts of partial melting. Degrees of melting substantial enough to exhaust pyroxene from an appropriate source region before exhaustion of olivine and plagioclase would produce liquids along the B to A curve. Under conditions of fractional fusion one might expect a gap between the compositions at B and A [36]. Trace element enrichments produced by incipient melting at B are diluted and Fe/Fe+Mg of the liquid is lowered by extensive melting as the liquid approaches A. LowK Fra Mauro material near A (also called low-alkali high-alumina basalt) has all these characteristics and can be understood most easily as the product of more extensive melting of the appropriate source region than the moderate-K Fra Mauro material near B (also called alkalic high-alumina basalt and KREEP basalt). Differentiation of moderate-K Fra Mauro material near B by crystallization and separation of pyroxene and plagioclase would be expected to produce silica-

327

SILICA

"

OLIVINE

ANORTHITE

Fig. 2. A compilation of average glass and lithic fragment compositions reported in the literature [ 6 - 1 5 ] are plotted on the liquidus diagram of fig. 1. Solid symbols represent material of the Fra Mauro type from all landing sites. Representatives near A on the A B curve have the lowest K and F e / F e + M g of the suite. Open symbols are the average of (spinel) troctolite types from Luna 20 [8] and Apollo 16 [15]. The spinel micro-troctolite nearest A (rock 62295) appears near the low-K Fra Mauro as an artifact of this projection which suppresses F e / F e + M g variation.

enriched liquids of higher Fe/Fe+Mg such as the high-K Fra Mauro material between B and C. If we interpret this suite as the product of partial melting starting at B with compositional variation being the result of variable degrees of melting and subsequent fractional crystallization, then we can set constraints on the depth and mineralogy of the source region. These anhydrous equilibria are relatively insensitive to pressure up to 5 kbar and so the source regions could conceivably be as much as 100 km deep but are almost certainly within the lunar crust. As has been demonstrated by Ford et al. [17] and Kushiro et al. [18] the hydrous, alkali-enriched phase boundaries at modest pressures are shifted toward plagioclase. We do not feel that such experiments are relevant because of the lack of evidence for these constituents ever having been present in quantities sufficient to shift the phase relations appreciably. Experiments and petrography [1,19,20] suggest that Brown and Peckett [21] have over-estimated the amount of alkalies which have demonstrably been lost from 14310.

328

D. Walker et aL, Origin of lunar feldspathic rocks

It should be noted that chemical variation among Fra Mauro preferred analyses appears to be controlled by low pressure, low PO2, anhydrous phase relations and that since the hydrous relations or relations at PO2 as high as Fe/FeO are appreciably different, we must conclude that the crustal partial melting process was dry and occurred at low PO2. The mineralogy of the source region follows from the diagram as being plagioclase, olivine, and lowcalcium pyroxene (of one or more sorts). The Fe/Fe+ Mg of the ferromagnesian silicates would be expected to be in the neighborhood of 0.25. These criteria do not uniquely define the source region but serve to restrict the candidates found as lunar rock types which could be acceptable in terms of mineralogy. The relative proportions of the required phases is unspecified. Several possible candidates have been found but in terms of widespread distribution and relative abundance, the anorthositic norite suite is the most appealing source material. 3.1. R o c k 14310

As perhaps the only large specimen of the Fra Mauro basalt chemical type which exhibits basaltic texture, rock 14310 occupies a singular niche in lunar petrology. It has been most satisfactorily and exhaustively described by James [20] and several previous investigators. The interpretation of this rock has also been exhaustive but not as equally satisfying. Petrographically it appears to be the crystallization product of a melt of composition differing only from the bulk composition of the rock by some amount of plagioclase. The estimates of cumulus plagioclase vary from 3-17% [19-21 ]. The range in these estimates arises both from the heterogeneous nature of the rock and from differences of opinion as to which plagioclase is cumulus. The lower abundance estimates are based upon the amount of large, ragged or granulated, texturally peculiar and chemically distinct plagioclases or material in cores of large grains; while the higher abundance estimates also include a portion of the not so obviously xenocrystic material. James [20] has discounted the hypothesis that substantially more than 4% of the plagioclase is cumulus because there is no clear separation into groups of microphenocryst and ground mass plagioclase, but rather a continuous transition. We feel that this criterion does not exclude the

possibility of a significant amount of cumulus plagioclase. We quite agree that there is not 17% of clearly xenocrystic plagioclase, but we must consider it to be extremely probable that a liquid composition near the peritectic in equilibrium with plagioclase would not have the plagioclase crystals homogeneously distributed throughout. A liquid aliquot bearing somewhat more plagioclase than precipitated locally need not appear cumulate and could not necessarily be distinguished on the basis of texture from the crystallization product of the original peritectic liquid. It could only be distinguished on the basis of bulk composition. We feel that since 14310 differs from the peritectic by having extra plagioclase, and since some of the plagioclase present clearly is exotic and some is likely to be cumulate, that 14310 is a partial cumulate. We cannot exclude the alternate hypothesis that 14310 crystallized from an impact-melted soil, but we note that its composition differs from average Apollo 14 soil in the direction of excess plagioclase. In either case the 14310 melt would have been derived either directly or indirectly through an intermediate episode ofcomminution and impact melting by the process advanced for the origin of the Fra Mauro basalt suite. 3. 2. K R E E P

The Fra Mauro basalt type comes in varieties enriched in the KREEP component as at the Apollo 14 landing site and in varieties which are not so enriched. A possible objection to the origin of the KREEP-rich Fra Mauro basalt by partial melting of lunar crustal rocks is that the most acceptable source rocks in terms of major element chemistry have low trace element abundance levels which require such small degrees of partial melting to produce the observed enrichments, that segregation of the resultant liquids is unlikely. This dilemma may be resolved in several ways. One argument is that the variable enrichment in trace elements coupled with the static major element chemistry constitutes firm proof of a partial melting history and hence there must have existed suitably enriched source regions. Alternately Wood [22], taking note of the non-uniform distribution of the KREEP-rich material determined from orbital 3'-ray experiments [23], has suggested that the abnormal KREEP

D. Walker et al., Origin of lunar feMspathic rocks

enrichments may be related to specific peculiarities of the Oceanus Procellarum region. The fact that KREEP enriched material generally has Fra Mauro basalt major element chemistry is most easily understood in terms of the partial melting hypothesis, with or without extra enrichments. Of particular interest in this connection are recent studies of partially molten noritic breccias and anorthositic norites with KREEP-like interstitial glasses [24,25].

4. Microtroctolites (62295 type) The Apollo 16 and Luna 20 samples contain a statistically significant rock type which has been only sparingly observed at other sites [26]. For a rock type so recently added to the lunar pantheon, the nomenclatural diversity is astonishing but may not necessarily be a true reflection of the rock's importance. This rock type has been most clearly circumscribed by Prinz et al. [8] and named (spinel) troctolite. This rock type may include members of the VHA (very high alumina) basaltic chemical type of [27] and [28]. VHA also appears to contain members of the Fra Mauro type as described earlier herein or the lowalkali, high-alumina type of Prinz et al. [8]. Hand specimen sized representatives of the group have received less distinctive names which will not be reproduced here. Rock 62295 is the hand specimen representative of this suite most extensively studied [29, 30,2]. This suite is characterized mineralogically by magnesian olivine and anorthite, often with Mg-A1 spinel. Chemically it is characterized by A1203 2 0 25% and Fe/Fe+Mg in the neighborhood of 0 . 1 5 0.20. The latter characteristic distinguishes this type from the low-K Fra Mauro type which has Fe/Fe+Mg 0.3 (see fig. 2). Compilation of lithic fragments of this type from Luna 20 and Apollo 16 [8,15] on the pseudo-ternary liquidus diagram shows that they plot in the spinel primary phase volume. The fact that this material commonly bears spinel but has a bulk composition with substantial normative pyroxene, incompatible with subsolidus spinel, merely reflects a cooling rate of the parental liquid which exceeded the cooling rate necessary to complete reaction equilibria. Experimental and petrographic work on 62295 [30, 2] have shown that 62295 is very close to spinel

329

peritectic A on the pseudo-ternary diagram and can be interpreted as an anhydrous, low PO2 partial melt of an anorthite-olivine-spinel assemblage at low pressure. Estimates of the degree of partial melting and accumulation required to produce the bulk compositions of 62295 and the fragment suite are somewhat flexible as a result of the uncertainty in the location of point A as a function of Fe/Fe +Mg, PO2, CrO2 content, and total pressure. If the partial melting hypothesis is correct then the source region can be constrained. Experimental work on 62295 to 30 kbar [2] shows that no source region is as reasonable in the framework of a partial melting hypothesis as the near surface region. The spinel + liquid field for 62295 expands with increasing pressure [2], making the bulk compositions progressively more remote from compositions of multiple crystal saturation. Mineralogically the source region can be constrained to consist of the assemblage anorthite, olivine, and spinel. Chemically the olivine must have Fe/Fe+ Mg< 0.1. Among lunar materials returned so far there is only one candidate, the pink-spinel troctolite (PST) described by LSPET [31] and Prinz et al. [32]. That such a rock constitutes the source region for 62295 is very evident from the fragmental xenocrystic inclusions it bears of An97, Fo93 and chromian pink pleonaste which duplicate the mineralogy of PST [32]. These xenocrysts have the chemistry of the equilibrium crystal assemblage observed at point A for experiments on 62295. This strongly suggests that the fragments are not random unmelted relicts of an impact melted soil. These xenocrysts show shock effects whereas the ground mass olivine and plagioclase do not. This suggests a mechanical history for these crystals before entering the liquid parental to 62295 and hence they are not phenocrysts grown from that liquid. They are most easily interpreted as crystal fragments of the PST source region. It has been suggested [8] that the fragmental (spinel) troctolite suite represents cumulate material produced during the fractional crystallization of liquids from A to B along the olivine-plagioclase curve on the pseudo-ternary liquidus. This hypothesis has some merit and may be the explanation of the composition of some of the members of the (spinel) troctolite suite. However, there are certain problems and contradictions inherent. Such cumulates should not be spinel bearing unless they have been thermally

330

D. Walker et aL, Origin o f lunar feMspathic rocks

reprocessed later (ad hoc). Appeal to non-reaction of spinel to explain its preservation as a cumulate mineral conflicts with the observation that the ANT rocks which are held to be cumulates derived from the same liquid, do not bear Mg-A1 spinel. Although the PST has unambiguous cumulate texture, this fragment is unlike all other members of the (spinel) troctolite group both chemically and mineralogically. Its Fe/Fe+ Mg ratio of 0.09 contrasts with values of about 0.15 to 0.20 in the (spinel) troctolite suite, and PST has neither modal nor normative pyroxene. Cumulates of anorthite and olivine and spinel must plot below the olivine-anorthite line as does PST. However, none of the (spinel) troctolites plot below this line [8]. The cumulate nature of PST, therefore, canno~ be used to infer a cumulate origin for other members of the (spinel) troctolite suite. We do not totally exclude a cumulate origin for some (spinel) troctolites, but prefer to accept the evidence of 62295 for an origin by partial melting of a rock very similar to PST.

5. Summary of products of partial melting We have argued that there are two suites of lunar feldspathic rocks (Fra Mauro and 62295-type microtroctolite) which can best be explained as anhydrous, low pressure, low PO2 partial melts of contrasting types of lunar crustal materials (ANT and PST). Our arguments have not considered the time, scale, or mechanism of these processes, nor are they suited to doing so. Before these problems can be addressed we shall try to set constraints on the mode of origin of the feldspathic rock types which compose the lunar crust and serve as source materials for younger feldspathic lunar rocks.

6. Materials of the lunar crust Feldspathic components recognized in surveys of glass and lithic fragments in non-highland lunar soils, other than local detritus, were postulated to come from the highlands [33,34]. These felspathic components were indeed found to be the volumetrically important detritus, presumably locally derived, at the highland landing sites. Rudimentary seismic evidence suggests that the moon has a layered structure in its

outer 100 km [35] and that these feldspathic materials are the obvious candidates in terms of density and seismic velocity for the layer extending down to about 65 kin. This feldspathic crustal material is not monolithologic but appears to be a broad continuum. This continuum has been conveniently characterized [8] as the anorthosite-norite -troctolite (ANT) suite. In terms of abundance and widespread distribution, several groups have made a case for the anorthositic norite type as most representative of the bulk of the lunar crust while clearly recognizing that variations exist in the suite of which the anorthositic norite is a part [37,8,12]. Since the anorthositic norite type appears to be the most abundant and since the Fe/Fe+Mg of its ferromagnesian minerals best satisfy the criteria necessary for the source material of the Fra Mauro type, we favor the anorthositic norite type as the source material of the Fra Mauro type while recognizing the non-uniqueness of this solution. As a corollary of the abundance argument we must conclude that due to the paucity of PST this type may be volumetrically unimportant on a global scale although the genetic implications are important. Any satisfactory explanation of the lunar crust, which will lead to an understanding of its evolution must recognize the chemical variatiens present in the rock types of the lunar crust. The variations appear to be systematic from the anorthosites to the anorthositic norites. Fig. 3 shows this series. The most obvious variation which distinguishes one group from the next is the plagioclase content, the anorthosites being the most rich in plagioclase. The other variations can be related to the plagioclase abundance parameter. Fe/Fe+Mg ratios of the rocks increase with increasing plagioclase content. Normative silica increases with increasing plagioclase; the anorthositic norites are olivine bearing and normative, while the anorthosites are quartz normative and silica minerals have been reported. Steele and Smith [38] have noted that the mineral chemistry of lithic fragments in this suite shows a positive correlation between the fayalite content of olivines and the anorthite content of plagioclase. This curious observation suggests contradicting differentiation indices. The coherence of these variables is not perfect but strong enough to require explanation; the deviations may simply reflect minor modifications, by subordinate processes. A further trend which is suggested by abundance data is that

D. Walker et aL, Origin o f lunar feMspathic rocks

SILICA

c

ANT SUITE

OLIVINE

To,/PST

ANORTHITE

Fig. 3. A compilation of average glass and lithic fragment compositions reported for anorthosite-anorthositic norite material [6-14,37] are plotted on the liquidus diagram of fig. 1. The dashed line separates quartz-normative from olivine-normative compositions. Shaded area includes liquids which could produce both PST and ANT rocks. PST does not properly plot in this section because spinel is not described on the system, however, the arrow indicates the direction irl which the recalculation of PST projects on the section. there are fewer representatives of the more plagioclase enriched varieties. The formation o f and chemical variation in the lunar crust can be discussed in terms o f at least three different frameworks: direct accretion, partial melting, and crystal accumulation. The first o f these processes has been suggested as a mechanism of uncertain importance in lunar evolution. Anderson [39] has recently explored the possibility that the whole moon may be composed o f high temperature condensates from the primeval nebula, with the implication that the CaO and A1203-rich crustal rocks are not anomalous in terms o f the chemistry o f the whole moon. Gast and McConnell [40] have made a slightly less extreme suggestion that only the crust is composed o f this condensate, curiously enough for a high temperature condensate, forming the outside rather than the inside o f the moon. Unfortunately it is rather unproductive to examine these stimulating possibilities in terms o f our immediate problem, the chemical variation within the crust. The present state of the condensation calculations only allow gross characterizations o f the condensates to be made and are not

331

suited to discussion of fine scale variations. Furthermore the assumption of equilibrium in these processes is somewhat insecure, metastability especially requires more examination. In this connection there is no assurance that processes involving liquid would not occur subsequent to condensation. Walker et al. [2] have shown that material intermediate between anorthositic norite and noritic anorthosite could be produced by partial melting of a plagioclase-spinel corundum assemblage at a depth of about 300 km. Such a residue would not be inconsistent with Anderson's lunar model. However, this interpretation was rejected since it was inconsistent with the results of Grove et al. [41] and Green et al. [42] for the source regions o f the Apollo 12 basalts. Partial melting at depth is not able to produce directly the chemical variations in the crust because fractionation trends in this regime are more nearly radial to silica than to plagioclase. Even if partial melting at depth were responsible for the anorthositic norites, surface fractionation would still be necessary to produce the rest of the series, and furthermore surface fractionation of such material will not be able to produce PST as a crustal rock type. We are led to conclude that even if direct accretion or partial melting have been partly responsible for the crust, we must still have recourse to surface fractionation processes. This obvious conclusion was preceeded by a perhaps overlengthy discussion o f the principle alternative (not modifying) hypotheses because it will be seen that simple schemes o f fractionation are not sufficient to explain the variation in lunar crustal rocks.

7. Differentiation of the lunar crust It has been reasonably clear to most investigators that the anorthosites represent the thermally and mechanically disturbed remains of magmatic cumulates. The principal uncertainties are on what scale and when did the accumulation occur, from what liquid was the plagioclase collected, and are the less feldspathic members of the suite simply related? We may address the latter two related questions to the pseudo-ternary diagram o f fig. 1. One possibility is that the liquid parental to the lunar crust had a composition somewhere within the series now composing

332

D. Walker et al., Origin of lunar feMspathic rocks

the crust. Such a liquid would be in the plagioclase liquidus field where the earliest crystallizing plagioclases are the most calcic. The opportunity to segregate efficiently the early anorthites into anorthosite bodies while the liquid is not cluttered with ferromagnesian debris makes this scheme suitable for explaining the extremely calcic plagioclase found in the anorthosites. However, simple plagioclase fractionation is unable to produce Fe/Fe+Mg or normative silica variations. To produce Fe/Fe+Mg and silica saturation changes, the parental liquid from which plagioclase is collected must be saturated with a ferromagnesian phase as well as plagioclase. The anorthositic norite to anorthosite series may be interpreted as a series of cumulates derived from liquids along the A B curve. Accumulation in high Fe/Fe+Mg liquids near B would give the anorthosites while less fractionated liquids near A would be parental to the anorthositic norites. This scheme is suitable for explaining Fe/Fe+Mg increases with silica excess as a result of olivine and pyroxene fractionation and also explains the lesser abundance of anorthosites as a consequence of there being less liquid remaining at the end of fractionation than there was at the start. Unfortunately this scheme would also require the anorthosite's plagioclase to be more sodic rather than less sodic than the anorthositic norite's. There is also no obvious reason why the residual liquids should be more efficient at segregating plagioclase to form the more plagioclase-rich anorthosites. In fact the opposite might be expected in terms of the viscosity and potential untidyness of the residual liquid. If we wish to retain the hypothesis that the crustal rocks are a related series of cumulates, then we must modify the hypothesis or discount the more troublesome observations. It is more advantageous to retain a modified accumulation hypothesis. Accumulation in a parental magma with composition somewhere within the crustal series leaves no provision for forming a PST crustal rock type. Accumulation of plagioclase in liquids along A B requires a parental liquid at A, and liquid A would be a perfectly natural consequence of a more primitive liquid having fractionated and precipitated a PST cumulate. The principal modifications necessary are with respect to the Na anomaly. If the anomaly is real there are several possible explanations. There may be more than one magma system involved as suggested by Taylor and Reid [43] or Na may have

been differentially devolatilized from some members of the suite. If the hypothesis can be successfully sustained, the phase relations discussed in this paper allow a minimum estimate to be made of the amount of complementary cumulus material which must be stored somewhere in the moon.

8. Formation of the lunar crust and implications If the hypothesis that crustal rocks represent a series of related cumulates is not without flaws, it remains the most appealing hypothesis and is adopted for the remainder of the discussion. Inclusion of PST as a related crustal cumulate sets constraints on original liquids which could be parental to the crustal suite. Such liquids must lie in the shaded region of fig. 3 in order that the crystallization history be consistent with the texture of PST. The PST would be an early cumulate leaving a residual liquid parental to the anorthosite-norite suite which would form by some complex set of accumulation processes. In this connection the PST more nearly deserves the title "genesis rock" than does the anorthosite 15415. In view of the superbly preserved cumulate texture of the PST in contrast to the common granulation of the anorthosites, a radiometric age determination on the PST would be most desirable even if technically difficult. In this framework for the origin of the lunar crust one can set lower limits for the amount of complementary ferromagnesian cumulate produced in generating 65 km of crustal ANT rocks with an average composition near anorthositic norite. Making the most-restrictive assumptions that PST cumulate has negligible volumetric significance (so that, the parent lies near A) and that the ferromagnesian cumulate is totally devoid of plagioclase and aluminous spinel, one calculates with reference to fig. 3 that a m i n i m u m of 50 km of complementary ferromagnesian cumulate must exist beneath the crust. If any of these assumptions are relaxed, the estimate can increase dramatically. A more primitive liquid than A which accumulated Cr, Ti, A1 spinel along with olivine and pyroxene could easily generate 2 0 0 - 3 0 0 km of ferromagnesian cumulate for 65 km of crust. Such a hypothetical cumulate spatially encroaches upon and could mineralogically satisfy the constraints on the Apollo 12

333

D. Walker et al., Origin o f lunar feMspathic rocks

TABLE 2 Summary of relationships among lunar rocks (numbers in parentheses are Fe/Fe + Mg) Primitive magma differentiation (outer few hundred km of moon). See fig. 4. cumulate _

pleonaste + olivine + anorthite Pink Spinel Troctolite (0.08)

cumulate

some (spinel) troctolite

cumulate--

plag + low-Ca pyroxene + olivine Anorthosit e-Norite-Troctolite suite (0.25 -0.60) Anorthositic Norite (%0.25)

partial melting

62295 -type spinel microtroctolite or some (spinel) troctolite (0.15-0.20) or Very High Alumina basalt

extensive partial melting

low-K Fra Mauro

mall degree of partial melting cffmulate

olivine + pyroxene (variation in time and depth of Fe/Fe+Mg)

residue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

basalt source region described by Grove et al. [41]. This hypothetical cumulate, as pointed out by several others, makes an ideal source region for mare basalts in terms of the Eu anomaly since all the Eu has been previously stored in the crustal plagioclase. The existence of such a cumulate derived from the crystallization of an original liquid outer shell of the moon has been an important element in the models of Wood et al. [33] and Anderson et al. [34]. Aspects of this model have been discussed by Philpotts and Schnetzler [44] and Wakita and Schmitt [45]. The possible sequence of events outlined above cannot be established as having occurred unless the scale on which the crustal generating process acted can be established. Was it acting only in local magma basins or was it an event of truly global proportions? Phase equilibrium data only outline permissible interactions but not the scale on which they occur. If the scale of the event is resolved the time is constrained. A global event is presumably unique while local events

late partial melting of the lunar interior

or

low-alkali, high-alumina basalt (0.30-0.35) Fra Mauro or KREEP basalt (0.35-0.45) Mare Basalts (0.45-0.60) some Apollo 12 picrites surface fractionation other mare basalts

need not be simultaneous. While we know that the crust existed before 4.1 by ago, much of the fine structure of the radiometric record before, if there was any, has been disturbed by intense meteorite bombardment in the neighborhood of 4.0 by ago [46]. Interpretation of the formation of the crust as a global event is appealing in its simplicity and its ability to explain observables. As a hypothesis it may enjoy only a short life in the face of hard evidence as to the scales involved.

9. Summary It is suggested that dry, low PO2 partial melting relates younger derivative rock types to more primitive feldspathic types in the lunar crust. The rock type produced is a function of the source rock and the degree of melting. The scale on which this process operated and the heat sources which activated this

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1

2

Fig. 4. Schematic lunar cross-section: 1 = partial or extensive melting of lunar crust by an external heat source such as an impact; 2 = partial melting of lunar crust by an internal thermal anomaly; 3 = ringed mare basin filled with lava generated by partial melting of the lunar interior; 4 = lunar crust composed largely of plagioclase, low-calcium pyroxene, and olivine with minor amounts of spinel-bearing troctolite; 5 = lunar interior of pyroxene and olivine, possibly a cumulate complementary to the lunar crust.

process are a m a t t e r o f speculation. If the origin o f the lunar crust, which itself served as a source region for later melts, involved magmatic differentiation on a moon-wide scale, then the phase relations discussed here allow a m i n i m u m estimate to be made o f the a m o u n t c o m p l e m e n t a r y mafic cumulate w h i c h must be stored at depth in the m o o n . It is seen that this h y p o t h e t i c a l cumulate spatially encroaches u p o n and could mineralogically satisfy the constraints on the source region o f the Apollo 12 picrites. Table 2 summarizes these points in outline form and fig. 4 offers a schematic lunar cross-section.

Acknowledgements We have benefited from discussions with E. D o w t y , M.J. Drake, O. James, M.J. O'Hara, M. Prinz, G.J. Taylor, J. Warner and J.A. Wood. We thank O. James, M. Prinz, A.M. Reid and G.J. Taylor for furnishing preprints o f their work. This study was supported by N A S A grants NGR-22-007-175 and NGL-22-007-247 and the C o m m i t t e e on E x p e r i m e n t a l G e o l o g y and Geophysics o f Harvard University.

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