Crystallization of plagioclase in lunar basalts and its significance

EARTH AND PLANETARY SCIENCE LETTERS 14 (1972) 14-18. NORTH-HOLLAND PUBLISHING COMPANY

CRYSTALLIZATION

OF PLAGIOCLASE

IN LUNAR BASALTS

AND ITS SIGNIFICANCE A E RINGWOOD and D H. GREEN Department o f Geochemistry, A ustrahan National University, Canberra, A ustraha

Received 15 November 1971

A new series of crystalhzatlon experiments upon an average high K, Rb type Apollo 11 basalt has been carried out in order to check the suggestion [ 7] that earlier results by several groups of investigators were invalid because of failure to achleve equilibrium The new experiments on samples which Initially contained plagmclase confirm the earher conclusion that plagloclase is not a hquIdus phase in this magma, and Indeed, does not separate until more than 30% of the magma has crystallized as ohvlne, pyroxene and ore minerals The magma is thus demonstrated to be of a decidedly non-cotectlc character A suggestion that departure from the plagmclase-pyroxene cotectlc is a consequence of loss of alkalis by volatlhZatlon IS also checked experimentally and found to be Invalid Imphcatmns of the noncotectlc character of Apollo 11 and 12 magmas are discussed

1. Introduction Petrologists have recently offered differing interpretations o f the experimental crystalllzatmn behav1our of maria basalts returned by the Apollo 11 and 12 mlssmns. O'Hara et al. [1] studied Apollo 11 hasalts and stated that they "have compositions smatlar to those of hqmds in equlhbrmm with crystals of ohvme, two chnopyroxenes, feldspar and lron-tltamum oxide" They concluded from the inferred near-cotectlc nature of the Apollo 11 basalts that the latter represented "the residual hqulds of advanced nearsurface crystal fractionatxon, most probably m a vast lava lake" On the other hand, Rmgwood and Essene [2], Roedder and Welblen [3], Smith et al [4] and Wed et al [5] found that average Apollo 11 basalts were n o t saturated with plagloclase, which did not begm to crystalhze until 30 to 50% of the magma had already crystallized as olivine, pyroxene and ore minerals The Apollo 11 magmas were thus found to exhibit strong departures from the low-pressure cotectlc compositions In which plagloclase would necessarily appear on the llqmdus From this behavlour, and also from other geochemical characteristics, it was concluded that Apollo 11 magmas had been performed deep in the lunar interior by a small degree of

partial melting and had n o t undergone extensive nearsurface crystalhzatlon differentiation [2, 6]. It is important that these differences be finally resolved if further progress is to be made towards an understanding of the origins of the maria Blggar et al [7] have recently claimed that the source of the conflict was that other workers had not achieved equdlbrlum in their experiments These authors [7] also qualified their position by suggesting that if perhaps some Apollo 11 magmas indeed displayed a hmlted departure from cotectlc compositions, this may have been the result of loss of alkalis by volatilization (p 871)

2. Crystalhzation of Apollo 11 basalts The above proposition are readily amenable to testing by direct experiments Blggar et 21 [7] claim firstly that the results o f three other groups o f workers [2, 4, 5] are lnvahd because they used synthetic glasses as starting materials in their experiments which resulted m the delayed non-equahbrlum crystalhzatlon of plagloclase On the other hand, Blggar et al [7] used natural crystalhne basalts in their experiments in which the presence o f plagloclase was believed to

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A E Rmgwood, D.H. Green, Crystalhzatton ofplagtoclase m lunar basalts

Table 1 Composltmns of Apollo 11 basalts and synthetic analogues on which experimental crystalhzatlon studies have been carried out a

b

c

d

e

f

g

S:O2 TIO2 A1203 FeO MgO CaO Na20 K20 P205

40 7 11 6 88 19 7 77 105 0 52 0 30 0 18

39 8 11 3 86 19 2 75 102 2 46 0 78 0 18

40 7 11 9 78 19 5 75 108 0 51 0 30 0 18

41 6 10 3 106 17 2 80 106 0 51 0 15

40 8 98 98 20 7 70 111

43 0 10 0 100 19 0 65 97

43 4 11 2 78 21 3 65 90

0 5

0 5 0 2

Quartz Orthoclase Alblte Anorthlte Nephelme DmpsMe Hypersthene Olwme llmenlte Apat~te

18 18 44 20 8 00 24 8 24 0 00 22 0 04

00 46 12 2 10 1 47 32 4 00 14 2 21 5 04

28 18 44 18 3 00 28 5 21 1 00 22 8 04

25 09 4.4 26 5 00 21 8 24 2 00 19 8 00

18 00 17 26 0 00 24 5 27 3 00 18 7 00

0 2

0 16

58 10 43 24 9 00 19 8 25 1 00 19 2 00

70 12 42 18 5 00 21 9 25 9 00 21 3 00

a Average composition (recalculated to 100%) of K,Rb-nch Apollo 11 basalt type (from Hubbard and Gast [9] ) b Composition derwed from a by the addltmn of 2 0% Na20, 0 5% K~O and recalculatmn to 100% c Composltmn of basalt 10017 (Compston et al [8] ) This is the basalt used by O'Hara et a l m thetr experimental stu&es [1] d Average composition of Apollo 11 basalt used by Rmgwood and Essene [2] e Composltmn of mclusmn in ohwne of rock 10020 Crystalhzatlon behavlour studwd experimentally by Roedder and Welblen [3] f SyntheUc analogue of Apollo 11 basalt studied expenmentaUy by Smith et al [4] g Synthetic analogue of Apollo 11 basalt studied experimentally by Wefll et al [5] prevent the delayed nucleation which they attribute to other experimenters To place the issue beyond dispute, we have carried out a further series of melting experiments upon a previously crystallized, plagloclase-bearlng sample o f hlgh-Rb ApoUo 11 basalt [8, 9] A glass of this composition (table 1) was first prepared from Analar reagents using standard procedures A 100 mg sample of this was then crystallized at 1050°C, 1 atm, 24 hrs m an iron capsule sealed in an evacuated silica tube to obtain a fine-grained assemblage of pyroxenes, plagloclase and llmenIte The plagioclase occurred in grains up to 2 - / a m in size, In intimate lntergrowth vclth pyroxene and llmenlte Fifteen mtlhgram charges of this material were then sealed In small Iron capsules placed m evacuated silica tubes and re-run at various temperatures for 3 hours to determine the position of plagloclase In the crystallization sequence The results are presented m table 2 Plagioclase is a

major phase at 1120°C and is present in minor amounts ( < 10%) at 1130°C, accompanied by about 40% of chnopyroxene and dmemte At 1140°C, plagxoclase is absent, the rock consisting of about 30% crystalline phases (mainly chnopyroxene) and 70% glass These results thus collfirm those o f earlier workers [2, 4, 5] who used glass starting materials with compositions slightly different from the present (table 1) The concluslon that plagloclase does not begin to crystalhze from most Apollo ! 1 magmas unttl about 30 to 50% of the magmas has already crystalhzed as olivine, pyroxene and ore minerals IS also in agreement with the results o f Roedder and Welblen [3] on the crystalhzatlon of natural Apollo l 1 magmas containing nuclei o f all major phases, Including plagloclase Blggar et al [7] ignored the significance o f this latter work The above experiments thus demonstrate that under the experimental conditions employed, there is no

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A E Rmgwood, D H. Green, Crystalhzatton o f plagtoclase m lunar basalts

Table 2 Expertmental results on average Apollo 11 basalt (high K,Rb type) (table 1, column a) and an alkah-ennched basalt (table 1, column b) Length of all runs was 3 hours Run no

Temp [° C]

Starting material

Run products

Est % glass

1160 1150 1140 1130 1120

Cpx Cpx Cpx Cpx Cpx

G1 + O1 + Arm + rare Cpx G1 + minor O1 + Arm + major Cpx G1 + Arm + major Cpx + Ilm G1 + Arm + Cpx + Ilm + minor P1 G1 + Cpx + llm + major P1

90 75 65 50-60 30-40

Apollo 11 basalt A253 A256 A252 A255 A254

+ P1 + Ilm + P1 + Ilm + P1 + Ilm + P1 + llm + P1 + Ilm

Alkah-enrlched basalt A264 A260 A258 A263 A259 A262 A261

1170 1150 1130 1120 1110 1100 1090

Cpx + P1 + Ilm + minor Cpx + P1 + Ilm + minor Cpx + P1 + Ilm + minor Cpx + PI + Ilm + minor Cpx + P1 + Ilm + minor Cpx + P1 + Ilm + minor Cpx + P1 + Ilm + minor

O1 O1 O1 O1 O1 O1 O1

G1 + O1 + rare Arm G1 + O1 + Arm G1 + minor O1 + Arm + major Cpx + Ilm G1 + minor O1 + Arm + major Cpx + Ilm G1 + possible O1 + Arm + Cpx + Ilm + P1 G1 + possible O1 +*Arm + Cpx + Ilm + P1 G1 + Cpx + llm + P1 (very fine-grained run)

95 95 70 60 50 30 10

Abbrevlataons Cpx = chnopyroxene, Ilm = llmemte, Arm = armalcohte, P1 = plagloclase, O1 = ohvme, G1 = glass slgmficant difference b e t w e e n results o b t a i n e d using glasses, or finely crystalhne starting materials. There is a simple reason for tins. The glass sample reqmres a couple o f m m u t e s to reach the equilibrium temperature after being placed In the furnace and we have found that it devitrifies to a very finely crystalline plagloclase-bearmg assemblage whilst it is warming up to chosen run t e m p e r a t u r e b e t w e e n sohdus and hquidus. Crystalline nuclei o f major phases are therefore present at the begcnnmg o f the e x p e r i m e n t and there is no reason to d o u b t that equilibrium is finally achieved Ironically, the e x p e r i m e n t a l results o f O'Hara et al [1] on the crystallizatmn o f Apollo 11 basalt 10017 (table 1) actually agree w i t h the findings o f all o t h e r workers [ 2 - 5 ] that plagmclase is not a hquidus phase in average Apollo 11 basalt. O'Hara et al [1] table 1, p. 696, report oh~ane + m i n o r c h n o p y r o x e n e + o p a q u e phases at 1162°C (1 a t m , lron-Wfistlte buffer, Mo capsule), m i n o r ohvlne + major c h n o p y r o x e n e + llmerote at 1133°C and c h n o p y r o x e n e , plagtoclase and flmemte at 1099°C. Thus, plagloclase appeared between 1133°C and 1099°C c o m p a r e d to the occurrence o f m i n o r plagloclase at 1130°C and m a j o r plagmclase at 1120°C m our lbresent e x p e r u n e n t s In fig. 1 o f

O'Hara et al [1] the key run at 1 1 6 2 ° C is n o t illustrated and the b o u n d a r y markang the i n c o m i n g o f d m o p y r o x e n e is drawn t o o low in t e m p e r a t u r e

3. Alkali volatilization A l t h o u g h t h e y repeatedly make the unqualified generalization that Apollo 11 basalts lie o n the plagmclase-pyroxene-llmemte -+ o h w n e c o t e c n c [ 1 , 7 ] Blggar et al [7] elsewhere l m p h c l t l y admit that tins m a y n o t h o l d strictly for some specific basalts They suggest that the deviation o f these basalts f r o m the cotectic c o m p o s m o n s m a y be c o n n e c t e d w i t h loss o f alkalis through volatilization at the lunar surface They h y p o t h e s i z e d that the primary Apollo 11 m a g m a m a y have resembled terrestrial alkah-ohvlne basalts or tholeutes and only a c q m r e d their characteristic highlyreduced and alkah-depleted c o m p o s i t i o n on e r u p t i o n [1 ] Loss o f alkalis was claimed to cause an expansion m the primary fields o f crystalhzation o f marie minerals. " A n y volatilization losses will &splace the e r u p t e d m a g m a c o m p o s i t i o n towards these (marie) mineral compositions rather than away f r o m t h e m These are precisely the types o f departure f r o m atmospheric

A E Rmgwood, D.H Green, Crystalhzatton ofplagtoclase m lunar basalts

pressure cotectic character which are displayed by Apollo 11 and Apollo 12 lavas" ([7], p 871) We have tested this postulate by adding 2% Na20 and 0 5% K20 to the average K,Rb-rich Apollo 11 basalt type This composition was crystalhzed to a subsohdus assemblage (plagloclase + pyroxene + ohvme + opaque oxides) at 1050°C, 1 atm. The fine-gramed plagloclase-rich assemblage was then crystallized at various temperatures to determine the melting relationships. The comparison of results given in table 2 demonstrates that addition of Na20 + K20 suppressed rather than enhanced the initial appearance of plagloclase The alkah-enriched composition is further from cotectic character than the natural, low alkali lunar basalt These experiments effectively refute the argument that alkali-loss IS responsible as a hqmdus phase Inspection of the norms of table 1 dlustrates that the addmon of alkalis enriches the rocks m normative ohvlne and &opsxde, decreases the normative anorthlte content and changes the normatwe plagloclase to andesme rather than bytowrote. These changes would be expected to result in depression of the temperature of plagtoclase appearance relative to ferromagnesian minerals

4. Non-cotectic character of Apollo 12 basalts An analogous controversy also developed when Apollo 12 rocks were studied Interpretations in this case were complicated by possible segregation of ohvme and pyroxene phenocrysts Blggar et al [7] claimed that all devaatlons from simple cotectic behav1our could be explained by the assumption of variable degrees of segregation of pyroxene and ohvlne within a cotectic liquid On the other hand, Green et al [10, 11 ] compared the experimental crystallization behav1our of five different basalt specimens under controlled laboratory conditions with the record of crystalhzation preserved in the minerals of the natural rocks Whilst noting the important role of ohvlne extraction and accumulation in specific Apollo 12 basalts, these investigations succeeded In demonstrating that none of the basalts was crystalhzmg plagioclase prior to the onset of rapid quenching, and that therefore, these basalts could not be derwatlve hquids from prolonged crystal fractionatlon revolving plagloclase at or near the lunar surface The experimental data on which

17

these conclusions were based were crltlclsed by Biggar et al [7] on the assumed basis that use of glass startlng material gave non-equilibrium metastable assemblages under experimental conditions Their assumption was without foundation however since key runs relating to plagloclase occurrence had been repeated using previously crystalhzed specimens containing well-crystalhzed plagloclase As with Apollo 11 specimens, these confirmed that use of glass starting materials under the particular experimental conditions employed yielded equilibrium results. So far, only one of the Apollo 12 rocks (12038) seems, on the basis of experiments by Biggar et al [7] to represent a hquld crystallizing at the plagloclase-pyroxene cotectlc This specimen was not studied by Green et al [10]

5. Conclusion Experimental Investigations have clearly estabhshed that the majority of Apollo 11 and 12 magmas were not crystalhzmg at the plagloclase-pyroxene cotectlc, and indeed, were highly undersaturated with respect to plagloclase when erupted at the lunar surface If these hqulds represented the end-products of extensive near-surface crystalhzatlon differentiation involving the separation of vast amounts of plagloclase, pyroxene and ore minerals [1, 7], then the basalts would necessardy have been saturated with plagloclase which should have appeared on the hquidus Likewise, if Apollo 11 and 12 basalts had been formed by partial melting in the lunar interior from a source material which contained plagloclase (and some residual plagioclase remained behind in the source after extraction of the basalt) then the magmas which reached the surface would necessarily also have been saturated with plagloclase The effect of pressure in the lunar interior serves only to enhance this effect [2, 6] Likewise, loss of alkalis by volatdlzatlon can only contribute m the direction of oversaturatlon of plagloclase, as demonstrated in this paper The absence of plagloclase on the hquldus of most Apollo 11 and 12 rocks is therefore taken to imply that these magmas are not the products of extenswe near-surface crystal fractlonatlon'in huge, lava lakes nor have they been formed by partial melting of a plagloclase-bearlng region of the lunar interior On the contrary, it lmphes that Apollo 11 and 12 magmas

18

I A E Rmgwood, D.H Green, Crystalhzanon o f plagloclase m lunar basalts

were formed at depths below those at which plagloclase was present as a stable phase m the lunar interior On this, and on other ground, it was concluded that Apollo 11 and 12 magmas were formed by small and varying degrees of partial melting from a pyroxenerich source region at depths of 2 0 0 - 5 0 0 km [2, 6, 10, 11 ]. Perhaps this is true of maria basalts generally. A possible explanation of the europium anomaly m Apollo 11 and 12 basalts m terms of a mechanism wtuch does not involve plagioclase has recently been offered [12] On the other hand, our prehmmary studies on Apollo 14 and 15 highland rock compositions show that these magmas indeed have plagloclase on their hquldl at atmospheric pressure, and furthermore, are not parental to or dwlded from Apollo 11 and 12 type basalts by any process of low-pressure crystal fractlonatlon. Other geochemical evidence indicates that they were formed by partial melting rather than by fractional crystalhzatlon [9] It appears therefore that Apollo 14 or 15 rocks (and perhaps lughland basalts generally) represent magmas formed by partial melting m a region of the moon where plagloclase was a stable phase. This suggests that the source region of tughland basalts (plagloclase present) was shallower than the source region of maria basalts (plagxoclase absent). Since the maria are beheved to be younger than the lughlands, this suggests that on the average, the depth of magma generation on the moon increased with time [6, 9]

Acknowledgements The able assistance of Mr W. I-hbberson m carrying out the experimental runs is gratefully acknowledged

References [1] M O'Hara, G Blggar, S Richardson, C Ford and B Jamleson, The nature of mascons, seas, and the lunar interior in the hght of expermaental studies, ]?roe Apollo 11 Lunar Science Conference 1 (1970) 695 [2] A.E Rmgwood and E. Essene, Petrogenesls of lunar basalts and the internal constitution and ongm of the moon, Proc. Apollo 11 Lunar Science Conference 1 (1970) 769 [3] E Roedder and P Welblen, Lunar petrology of sthcate melt inclusion, Apollo 11 rocks, Proc. Apollo 11 Lunar Science Conference 1 (1970) 801 [4] J V Smith, A.T. Anderson, R Newton, E. Olsen, P Wyllie, A. Crewe, M. Isaacson and D Johnson, Petrologic model for the moon referred from petrography, mineralogy and petrogenesls of Apollo 11 rocks, Proc Apollo 11 Lunar Science Conference 1 (1970) 897 [5] D Weft, I McCallum, Y Bottlnga, M. Drake and G. McKay, Mineralogy and petrology of some Apollo 11 igneous rocks, Proc. Apollo 11 Lunar Science Conference 1 (1970) 937. [6] A E Rmgwood, Petrogenesls of Apollo 11 basalts and tmphcatlons for lunar ortgm, J Geophys. Res 75 (1970) 6453. [7] G.M Blggar, M J O'Hara, A Peckett and D. Humphrles, Lunar lavas and the achondntes Petrogenesls of protohypersthene basalts m the marm lava lakes, Proc. Second Lunar Science Conference 1 (1971) 853 [8] W. Compston, B Chappell, P. Arlens and M Vernon, The chemistry and age of Apollo 11 lunar material, Proc Apollo 11 Lunar Science Conference 2 (1970) 1007 [9] N J Hubbard and P W Gast, Chemical composition and ongm of nonmare lunar basalts, Proc Second Lunar Science Conference 2 (1971) m press [10] D.H Green, A.E Rmgwood, N G Ware, W.O Hlbberson, A Major and E Kiss,Expertmental petrology and petrogenesis of Apollo 12 basalts, Proc Second Lunar Science Conference 1 (1971) m press. [11] D.H Green, Experimental petrology of Apollo 12 basalts, Earth Planet. Scl Letters 13 (1971) 85 [12] A L Graham and A E. Rmgwood, Lunar basalt genesls The origin of the europmm anomaly, Earth Planet SOl Letters 13 (1971) 105