Fragments of terra rock in the Apollo 12 soil samples and a structural model of the moon

IetI~US 16, 462--501 (1972)

Fragments of Terra Rock in the Apollo 12 Soil Samples and a Structural Model of t h e M o o n JOHN A. WOOD Smithsonian Institution, Astrophysical Observatory, Cambridge, 3lassachusetts 02138 R e c e i v e d A u g u s t 22, 1971 ; r e v i s e d O c t o b e r 12, 1971 Samples of l u n a r soil collected b y Apollo 12 in O e e a n u s P r o c e l l a r u m c o n t a i n n o t only b a s a l t f r a g m e n t s , o b v i o u s l y locally derived, b u t also a c o m p o n e n t (~ 10%) of light-colored rock f r a g m e n t s t h a t p r o b a b l y d e r i v e f r o m t h e h m a r terrae. M o s t of t h e s e are n o r i t i e in c o m p o s i t i o n (i.e., consist of plagioclase a n d low-Ca p y r o x e n e s in c o m p a r a b l e a m o u n t s ) , n o t a n o r t h o s i t i c as were t h e light-colored soil particles of Apollo 11. T e x t u r a l l y , t h e y are recrystallized breccias. A n e v o l u t i o n a r y m o d e l is d e v e l o p e d t h a t is c o n s i s t e n t w i t h t h e p r o p e r t i e s of t h i s a n d o t h e r l u n a r rock t.ypes, a n d w i t h t h e g e o p h y s i c a l p r o p e r t i e s of t h e Moon. (I) T h e surface layers of t h e M o o n were m e l t e d to a considerable d e p t h in early times, possibly b y e n e r g y of a c c r e t i o n of t h e Moon. (II) I g n e o u s d i f f e r e n t i a t i o n o c c u r r e d as t h e surface l a y e r cooled; crystallizing plagioclase floated to t h e surface to f o r m a n a n o r t h o s i t i c layer. R e s i d u a l liquid b e n e a t h t h e a n o r t h o s i t e was n o r i t i c in c o m p o s i t i o n . ( I I I ) M a j o r p l a n e t e s i m a l i m p a c t s s t r i p p e d a w a y t h e a n o r t h o s i t e cover locally, a n d liquid n o r i t e welled u p to fill t h e holes (these are n o w t h e n o n m a s c o n m a r i a , i n c l u d i n g O e e a n u s P r o c e l l a r u m ) . (IV) A d d i t i o n a l m a j o r i m p a c t s o c c u r r e d a f t e r c o m p l e t e solidification of t h e c r u s t a l s y s t e m ; (V) isostatic a d j u s t m e n t d r o v e plugs of solid b u t plastic m a n t l e m a t e r i a l u p i n t o t h e s e b a s i n s (now t h e m a s c o n maria). (VI) M u c h later, l a v a g e n e r a t e d a t d e p t h in t h e M o o n b y r a d i o a c t i v e d e c a y e r u p t e d a t t h e surface a n d pooled in t h e t o p o g r a p h i c lows (mare basins). T h e r e l a t i o n s h i p of g r a v i t y a n d t o p o g r a p h y on t h e n e a r side of t h e M o o n c a n be u s e d to c o n s t r u c t models of c r u s t a l s t r u e t u r e ; s u b s t a n t i a l t h i c k n e s s e s ( ~ 2 5 k m ) of n o r i t e b e n e a t h O e e m m s P r o e e l l a r u m , a n d a n o r t h o s i t e b e n e a t h t h e c e n t r a l h i g h l a n d s , a p p e a r to b e r e q u i r e d to reconcile g r a v i t y w i t h topography. Most of t h e Apollo 14 s a m p l e s (crustal roeks e x e a v a t e d b y t h e I m b r i u m i m p a c t ) are n oritie breceias similar to t h e Apollo 12 n orites, e x c e p t t h a t t h e y a p p e a r to h a v e b e e n m e l t e d for a b r i e f time, p r o b a b l y b y t h e I m b r i u m e v e n t itself.

The samples returned from the Moon by the Apollo missions have been collected from the regolith, an impact-generated debris layer t h a t everywhere blankets the solid lunar bedrock. Ahnost all the large rock samples collected by the Apollo ] 1 and 12 missions are basaltic in character; these are undoubtedly debris fragments from the bedrock immediately underfoot, and demonstrate the volcanic character of the basin fill in Mare Tranquillitatis and Oceanus Procellarum. In addition to locally derived debris, however, the mare regolith must also contain a component of fragments t h a t originated far from the collection site. This is because a small Copyright © 1972 by AcademicPress, Inc. rights of reproduction in any form reserved.

All

fraction of the debris excavated by every high-energy impact cratering event on the lunar surface is projected great distances (Gault et a/.,1963); repeated impacts must have promoted a diffusion of fragmental material from each discrete geologic unit on the lunar surface into the regoliths overlying adjacent geologic units. It is interesting to search for obviously exotic materials in the Apollo samples. These are most likely to be found in the "coarse fines" fraction of lunar soil samples, because particles in this sieve fraction are small enough (1-10mm) to be very abundant yet large enough, in most cases, to display all the properties of the

462

T E R R A R O C K ; F R A G M E N T S ][I~ A P O L L O

p a r e n t r o c k masses f r o m which t h e y were spalled. A h a l f - g r a m s a m p l e of coarse fines (typical of t h e size of s a m p l e s allocated) consists of a b o u t 100 particles, so one has a chance of finding t y p e s of r o c k t h a t are p r e s e n t in t h e local regolith at t h e ~ 1 % level. T h e soil is a fruitful source of exotic m a t e r i a l s for a n o t h e r reason: it a p p e a r s t h a t i m p a c t cratering t e n d s to cast small debris f r a g m e n t s f a r t h e r t h a n large ones, so a larger c o m p o n e n t of alien m a t e r i a l a c c u m u l a t e s in t h e soil t h a n in the a s s e m b l a g e of pebbles a n d rocks p r e s e n t on the surface at a given point. Our g r o u p a t t h e S m i t h s o n i a n Astrophysical O b s e r v a t o r y sectioned, e x a m i n e d , a n d classified 1676 coarse particles f r o m t h e Apollo ] ] soil a n d 497 particles f r o m five discrete soil samples collected b y t h e Apollo 12 a s t r o n a u t s . 1 T h e t y p e s of l u n a r r o c k we o b s e r v e d are described, a n d t h e p r o p o r t i o n s of each t y p e p r e s e n t in the soils are given, in W o o d et al. (1970) a n d M a r v i n et al. (1971). T h e m o s t conspicuously exotic c o m p o n e n t in t h e Apollo l l soil consisted of rock f r a g m e n t s similar in chemical a n d mineralogical composition to t e r r e s t r i a l a n o r t h o s i t e s (Wood et al., 1970; S m i t h et al., 1970; K i n g et al., 1970; a n d o t h e r a u t h o r s who p u b l i s h e d their o b s e r v a tions in t h e Proceedings of the Apollo 11 L u n a r Science Conference, P e r g a m o n , 1970). F r o m their light color a n d compositional s i m i l a r i t y to t h e t e r r a m a t e r i a l a n a l y z e d b y S u r v e y o r 7 ( P a t t e r s o n et al., 1970), it a p p e a r s likely t h a t these a n o r t h o s i t i c f r a g m e n t s are samples of t h e lunar terrae. T h e i r a b u n d a n c e in t h e coarse fines fraction o f the Apollo l l soil ( ~ 5 % ) is a p p r o x i m a t e l y t h a t p r e d i c t e d for t e r r a m a t e r i a l 1 Individual particles in our collections are referred to by numbers of the form (102 14), meaning particle No. 14 (as numbered on a photographic map) in thin section No. 102.

12

SOIL

463

b y considerations of cratering mechanics, given t h a t T r a n q u i l l i t y Base lies a b o u t 50 k m n o r t h of a m a j o r t e r r a region ( S h o e m a k e r et al., 1970). W e were m u c h i n t e r e s t e d to learn w h e t h e r f r a g m e n t s of this t y p e also occur in t h e Apollo 12 soils. W e f o u n d t h a t lightcolored particles are indeed p r e s e n t in the Apollo 12 soil samples, at levels ( 4 - 1 9 % ) c o m p a r a b l e to their a b u n d a n c e in t h e Apollo 11 soil, b u t m o s t of t h e m are n o t anorthositie. T h e y r a n g e in composition f r o m gabbroic (or, more specifically, noritic, in view of the low-Ca c h a r a c t e r of their p y r o x e n e s ) to anorthositic, b u t the noritic end of the s p e c t r u m is m o r e densely p o p u l a t e d . T w o properties serve to set these rocks a p a r t f r o m the familiar l u n a r m a r e basalts: their textures, which are usually those of breccias r a t h e r t h a n of p r i m a r y igneous rocks; a n d the lou T Ca c o n t e n t of their pyroxenes. I n addition, t h e y contain less ilmenite t h a n m o s t m a r e basalts a n d are, as a l r e a d y noted, lighter in color. M a n y groups besides our own n o t e d these alien light-colored particles in the Apollo 12 soils: our " n o r i t e s " are e q u i v a l e n t to the o r t h o p y r o x e n i e f r a g m e n t s of F u c h s (1970), the crystalline KREEP of Meyer et al. (1971), the " g r a y m o t t l e d f r a g m e n t s " of A n d e r s o n et al. (1971), a n d also to " L u n y R o c k I " f r o m the Apollo 11 soil described b y Albee a n d Chodos (]970). A m o n g t h e Apollo 12 soil particles of noritie to a n o r t h o s i t i e composition, t w o classes m a y be distinguished. These are sufficiently dissimilar to suggest different sources on the Moon a n d different m o d e s of origin. One ( t e r m e d T y p e A in this paper) consists largely of recrystallized breceias w i t h fairly u n i f o r m l y sized clasts; the o t h e r ( T y p e B), of unrecrystallized breccias in which r e l a t i v e l y large clasts are

FrG. 1. Photomicrographs of thin sections of noritic particles from the Apollo 12 soil. a, b, By plain transmitted light; c, d, by reflected light. Minerals visible are pyroxene (P), plagioclase (F), ilmenite (I), and glass (G). a and c: a Type A norite particle (115-3) of the most common sort. Texture is clearly that of a recrystallized breccia ; angular forms of pyroxene and plagioclase clasts are preserved. Opaque grains are chiefly ihnenite. The rock is perfectly solid; recrystallization has eliminated all pore space between clasts, b: a Type A norite (130 12) with igneous texture. Visible are diverging sheafs of feldspar and pyroxene crystals, d : Igneous (diahasic) texture in another Type A anorthositic gabbro (130 10):laths of plagioclase (F) embedded in pyroxene (P).

464

J O H N A. WOOD

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T E R R A ROCK F R A G M E N T S I N APOLLO 1 2 SOIL

embedded in a matrix of glass and/or mieroerystalline matter. These two classes will be discussed separately.

u n i f o r m l y sized clasts (Fig. la) to clearly igneous rocks of c o m p a r a b l e grain size (Fig. lb). Particles from the breccia end of the s p e c t r u m are more a b u n d a n t . P e t r o graphically, the T y p e A f r a g m e n t s range from norites almost to gabbroic anorthosites, b u t most are noritie in character. Mineral modes of all the coarse (>0.5mm) T y p e A n o r i t e - a n o r t h o s i t e particles in

TYPE A I ~ O R I T E - - A N o R T H O S I T E S The first a n d more n u m e r o u s class of light-colored particles ranges t e x t u r a l l y from recrystallized breccias with fairly 12001

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FIG. 2. Bar graph showing mineral proportions (modes) in 57 Type A and B light-colored particles, from surveys of ~ 100 points on each particle. Distance that plagioclase segment of each bar extends downward, read against the left edge of the diagram, gives rock classifications. * Includes both residual K-rich glasses in Type A particles, and shock-melted glasses in Type B particles.

466

J O H N A. WOOD

o u r s o i l s a m p l e s a r e s h o w n i n F i g . 2. T h e s e are based on a relatively small number of p o i n t c o u n t s ( ~ 1 0 0 p e r p a r t i c l e ) , so m u c h statistical uncertainty attaches to the reports of minor mineral content. The estimates of plagioclase and mafic content

are reliable, however, and the proportions of minor minerals for the grand average of all t y p e A p a r t i c l e s a r e a l s o s t a t i s t i c a l l y valid. Bulk chemical compositions (defocusedbeam microprobe analyses) of eight Apollo

TABLE I BULK CHEMICAL ANALYSES (DEFoCUSED-BEA~ MICROPROBE ANALYSES) AND ~OR.-~S OF LIGHT-CoLORED t)ARTICLES FROSI APOLLO 12 AND A~OLLO 11 SOIL SA]~{PLES

W e i g h t p e r c e n t a g e s o f oxides a Apollo 12 (I2037)

Apollo l l (10085)

Fee MnO Mg0 CaO Na20 K20 NiO P205 S03 Sum

A B 46.7 54.6 2.4 2.0 22.4 19.5 0.1 0.1 6.9 8.5 0.1 0.1 7.1 4.1 12.1 9.3 0.6 1.2 0.5 1.1 0.0 0.0 0.4 1.3 0.2 0.1 97.5101.9

C D E F G 42.7 45.0 50.4 51.1 49.4 4.2 0.4 0.6 0.4 2.8 16.4 32.0 18.3 17.3 18.4 0.1 0.1 0.2 0.2 0.1 ll.5 2.4 9.0 8.3 9.8 0.1 0.0 0.1 0.1 0.1 4.6 6.4 9.7 8.9 8.0 12.2 15.3 10.0 10.0 9.4 0.9 0.6 0.8 0.9 1.1 1.6 0.1 0.6 2.0 0.8 0.0 0.0 0.0 0.0 0.1 4.0 0.1 1.2 0.9 0.5 0.4 0.0 0.1 0.1 0.2 98.7100.6101.0100.2100.8

Fo Fa En Fs We Or Ab An Ilm Chr Qtz Cot Apa S

0.0 0.0 18.1 12.7 0.1 3.3 5.4 58.3 0.7 0.2 0.1 0.0 1.1 0.1

0.0 0.0 11.3 14.8 0.0 9.7 8.0 35.0 8.0 0.1 2.7 0.5 9.4 0.2

SiO2 Tie2 A12Oa

Cr203

H I J K L M N 47.4 47.8 46.6 33.9 44.2 46.0 45.4 0.5 0.4 0.1 0.3 0.0 0.3 0.0 18.5 25.5 32.5 32.9 34.6 27.3 33.8 O.1 0.1 0.0 0.1 0.0 0.2 0.0 8.7 6.2 1.6 4.0 1.1 6.2 2.8 0.1 0.1 0.0 0.0 0.0 0.1 0.1 10.5 7.2 2.3 3.8 1.3 7.9 1.7 9.9 14.6 17.7 17.0 18.3 14.1 17.5 1.0 0.4 0.6 0.5 0.7 0.3 0.4 0.3 0.1 0.0 0.1 0.I 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 1.2 . . . . . 0.1 0.2 0.1 0.1 0.0 0.0 0.0 98.3102.6101.5102.8100.3102.4101.7

Norms b 0.0 0.0 10.0 12.2 0.0 6.2 9.8 36.9 3.8 0.1 15.5 2.6 2.9 0.0

5.7 1.5 7.7 1.9 0.0 0.5 5.5 74.8 0.7 0.1 0.0 1.5 0.3 0.0

0.0 0.0 23.9 15.6 0.0 3.7 6.4 41.7 1.1 0.2 3.7 1.0 2.7 0.1

0.0 0.0 22.3 14.7 2.9 12.I 7.9 36.9 0.8 0.2 0.2 0.0 2.0 0.0

0.0 0.0 19.9 13.5 0.0 4.6 8.9 43.0 5.3 0.2 3.3 0.0 1.2 0.1

0.6 0.4 25.8 15.0 0.0 1.9 8.9 42.0 1.0 0.2 0.0 1.4 2.7 0.0

1.1 0.0 6.5 0.7 0.0 5.2 16.1 5.7 0.0 9.6 2.7 0.0 2.0 2.7 0.0 0.5 0.2 0.5 3.0 4.8 3.9 66.1 84.7 82.2 0.8 0.3 0.5 0.2 0.0 0.1 0.0 0.8 0.0 0.0 0.0 1.1 . . . . . . 0.1 0.0 0.0

2.2 4.5 0.0 1.5 2.7 0.0 0.0 12.9 4.2 0.0 7.1 5.2 1.9 0.0 0.0 0.4 0.0 0.0 6.1 2.5 3.3 86.1 68.4 84.5 0.1 0.6 0.0 0.0 0.3 0.0 0.0 0.0 0.5 1.7 1.2 1.3 . . . . . 0.0 0.0 0.0

a A . T y p e A a n o r t h o s i t i e g a b b r o (115-1). B. T y p e A norite (115 2). C. T y p e A norite (115 3). D. T y p e A a n o r t h o s i t i e g a b b r o ( 115 4). E. T y p e A n o r i t e ( 115 5). F. T y p e A norite ( 115 6). G. T y p e A norite (115-7). H . T y p e A norite (115--8). I. H e a v i l y s h o c k e d a n o r t h o s i t i e g a b b r o (19-57). J. U n reerystallized breccia of igneous gabbroie a n o r t h o s i t e (19 1). K . Gabbroie a n o r t h o s i t e , p r o b a b l y reerystallized breccia (37 9). L. A n o r t h o s i t e , igneous or reerystMlized breccia (37-1). M. A n o r t h o s i t i e gabbro, diabasie t e x t u r e (9 8). N. Gabbroie a n o r t h o s i t e , reerystMlized breccia (37-7). b C o m p u t e d w e i g h t p e r c e n t a g e s of forsterite (Mg2SiO4), fayalite (Fe2SiO4), e n s t a t i t e (MgSiO3), ferrosilite (FeSiOs), wollastonite (CaSiOa), orthoelase (KA1Si3Os), albite (NaA1SisOs), a n o r t h i t e (CaA12Si2Os), ilmenite (FeTiOa), c h r o m i t e (FeCr204), q u a r t z (SiO2), c o r u n d u m (A12Os), a p a t i t e (Ca4F2(PO4)2), a n d e l e m e n t a l sulfllr.

T E R R A ROCK FRAGME:NTS I N AI'OLLO 12 SOIL

467

TABLE II REPRESEI~TTATIVE M I N E R A L COMPOSITIONS ( W E I G H T P E R C E N T A G E S , E L E C T R O N MICROPROBE A N A L Y S E S ) IN A P O L L O 12 L I G H T COLORED PARTICLES

A OI Na Mg A1 Si P 1( Ca Ti Cr M~l Fe Stan

47.0 0.3 -19.3 21.4 . 0.0 13.3 0.0 --0.1 101.4

B

C

48.0 44.4 1.0 0.0 -14.0 18.5 0.6 22.9 26.0 . . . 0.1 . 12.2 1.1 0.1 0.5 -0.3 -0.2 0.1 14.2 102.9 101.3

D

E

F

G

H

43.0 0.1 8.4 0.4 25.0 .

41.5 0.0 11.7 0.3 24.0 . . 1.0 0.2 0.2 0.4 19.2 98.5

39.6 -18.6 0.0 17.9

42.7 0.2 1.9 0.1 0.2 21.3 0'0 30.0 0.0 0.0 0.0 0.7 102.9

32.4 -2.1 0.0 0.1 --0.2 32.2 0.3 0.3 31.7 99.3

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14.2 0.5 0.3 0.1 8.4 100.4

0.1 0.0 0.0 0.2 23.9 100.1

A. P l a g i o c l a s e in T y p e A n o r i t e (101-12): Ab4.4An95.4Or0.2. B. 1)lagioclase in T y p e B breccia (133 6): Abl2AAns6.9Orl.0. C. ( ) r t h o p y r o x e ~ e in T y p e A n o r i t e (101 4): Er166.7Fs30.0Wo3.3. D. A u g i t e in T y p e A n o r i t e (101 12): En40.TFSlT.9Wo41.4. E. O r t h o p y r o x e n e i n T y p e B b r e c c i a (102-17) : En56.1Fs40.gWo3.0. F. Olivine in T y p e A n o r i t e (102 1): Fa35.s. G. ~Vhitlockite in T y p e A n o r i t e (129 1), w h i c h c o n t a i n s i n a d d i t i o n 2.0°/) Y, 0 . 5 % L a , 1.3%, Ce, 0 . 3 % Pr, 0 . 8 % N d , 0 . 4 % Sin, 0.5%0 Gd, 0.01 ± 0 . 0 1 % Cl, a n d 0.0 ± 0 . 1 % F. H . l l m e n i t e in T y p e A n o r i t e (101-12). I. O x y g e l l b y s t o i c h i o m e t r y .

ANORTHITE

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FIG. 3. O r t h o c l a s e (KA1SiaOs) v s . a n o r t h i t e (CaA12Si2Os) c o n t e n t of p l a g i o c l a s e g r a i n s m i c r o p r o b e d in l i g h t - c o l o r e d p a r t i c l e s . A n a l y s e s s h o w i n g v e r y h i g h o r t h o c l a s e c o n t e n t are p r o b a b l y e r r o n e o u s , o, T y p e A c r y s t a l l i n e n o r i t e - a n o r t h o s i t e s , Apollo 12; e , T y p e B b r e c c i a t e d n o r i t e a n o r t h o s i t e s , Apollo 12.

468

JOHN A. WOOD

130 - 6 106 -196 ,06-z33 106 - I I 5 106 - t98 106-313 102-17 133-8 I06-15 153-6

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80

MOLE PCT. ANORTHITE F m ~. 4. R e c o r d ofplagioclase c o m p o s i t i o n s e n c o u n t e r e d in e a c h of 26 light-colored particles studied (white vs. black entries as in Fig. 3). " Z o n i n g " b a r shows m a x i m u m c o m p o s i t i o n a l v a r i a b i l i t y present in a n y single plagioelase grain or clast.

12 Type A particles are shown in Table I, together with representative analyses of Apollo ] 1 anorthosites for comparison. The Apollo 12 norms are generally in good agreement with the modes reported in Fig. 2, with the exception of levels of phosphate minerals. Evidently, these were often not recognized during modal surveys.

Properties of the minerals that comprise the Type A particles are discussed in the paragraphs to follow. Our observations are presented in more detail in Wood et al.

(1971). Plagioelase crystals in Type A particles range in form from euhedral laths in the igneous fragments (Fig. ld) to anhedral or

CaMg(Si03) z

CaFe(Si03) 2

o

/ / MgSiO 3

/:o /~

v

co

oi o

'~

°,:o.o 1 o

oo.

o ,ioP OX N v

v FeSiO3

F]c. 5. Compositions of pyroxenes mieroprobed in Apollo ]2 and Apollo 11 light-colored particles, in t e r m s of their relative c o n t e n t ( m o l e % ) of ideal p y r o x e n e end m e m b e r s , o, T y p e A crystalline norite anorthosites, Apollo 12; e , T y p e B breeciated norite-anort.hosites, Apollo 12; *, micrographic granite, Apollo 12 ; A, Apollo 11 anorthosites.

TERRA ROCK FRAGEMNTS I N APOLLO 12 SOIL

t

_Q

4

470

J O H N A. WOOD

s u b h e d r a l grains in the recrystallized breccias (Fig. lc). T h e larger clasts a n d laths display p o l y s y n t h e t i c twinning. No obvious signs of a c u m u l a t e t e x t u r e were seen in the igneous particles. A r e p r e s e n t a tive m i c r o p r o b e analysis of the plagioclase in T y p e A particles is r e p o r t e d in T a b l e I I ; mineral compositions in mole % a n o r t h i t e

and orthoclase for all Type A plagioclase grains microprobed are plotted in Fig. 3. It is likely that the few high K analyses are spurious, having been performed in small plagioclase grains close to regions of K-rich residual glass (described below). Plagioclase in these Apollo 12 Type A particles does not differ significantly in composition from the plagioclase of Apollo 11 anorthosite fragments, either in the distribution of anorthite contents or in the levels of minor elements (K, Fe, Ti). Zoning in plagioclase is relatively feeble. Typically, in going from the center of a feldspar crystal to the outermost position that can be microprobed without impinging on adjacent minerals (a distance of 30tL, say), the anorthite content decreases by 0.2 to 2.0mole % while the orthoclase content rises by 0.i to 0.2mole %. On the other hand, much greater compositional variations were observed between individual plagioclase grains and clasts within a given soil particle (Fig. 4), showing that c o m p l e t e chemical equilibration did n o t a c c o m p a n y the t e x t u r a l recrystallization these rocks h a v e experienced. T h e g r e a t m a j o r i t y of p y r o x e n e grains in T y p e A n o r i t e - a n o r t h o s i t e s are Ca-poor (Fig. 5). Most seem to be o r t h o p y r o x e n e s , judging f r o m their c h e m i s t r y , a n d a smaller n u m b e r are pigeonites; however, because of their small dimensions a n d irregular forms, we c a n n o t confirm this distinction optically for m o s t of t h e individual

p y r o x e n e grains analyzed. W h e r e extinction angles can be observed, the p y r o x e n e is usually monoclinic. B o t h i b r m s o f p y r o x e n e are present, however. R e l a t i v e l y few ( ~ 1 5 % ) of the p y r o x e n e grains in this class of particles are augitic. T h e p y r o x e n e grains are usually anhedral in f o r m (Fig. lc), b u t occasionally n e a r - e u h e d r a l f o r m s appear. T h e y are clear a n d colorless in t h i n section. An e x a m p l e of t h e complex relations existing b e t w e e n p r i m a r y (clast) p y r o x e n e a n d p y r o x e n e s n e w l y f o r m e d during the recrystallization is shown in Fig. 6a. Chemical analyses of r e p r e s e n t a t i v e p y r o x e n e grains are p r e s e n t e d in T a b l e I I . These are s o m e w h a t richer in F e t h a n t h e p y r o x e n e s we a n a l y z e d in Apollo l l anorthosites, as Fig. 5 d e m o n s t r a t e s . P y r o x c n e s in Apollo 12 light-colored f r a g m e n t s display the s a m e relationship b e t w e e n Ti a n d A1 c o n t e n t as was f o u n d in Apollo l l basaltic p y r o x e n e s , i.e., two A1 a t o m s s u b s t i t u t e for Si for e v e r y Ti a t o m t h a t s u b s t i t u t e s for Mg or Fe, in order to m a i n t a i n charge balance. The Cr~O~ contents of the p y r o x e n c s of l u n a r light-colored particles are shown in Fig. 7. The Cr c o n c e n t r a t i o n is a useful criterion b y which the p y r o x e n e s front m a r e basalts a n d Apollo 12 norites can be differentiated. There is no o b v i o u s relationship between Cr c o n t e n t a n d the ortho- or clino-charactcr of the p y r o x e n e (Ca content). Zoning in the Ca-poor p y r o x e n e s of T y p e A n o r i t e - a n o r t h o s i t e particles is n o t p r o n o u n c e d : typically, the r i m of a p y r o x e n e grain of 100tz d i a m e t e r contains ~ 5 mole % more ferrosilite t h a n t h e center. I n c o n t r a s t to the situation with feldspars described above, the v a r i a b i l i t y of p y r o x ene compositions f r o m one grain to a n o t h e r

Fro. 6. a, Effect of recrystallization on a Type A norite (130-6): overgrowths of Ca-poor (P, En56Fs40Wo4) and Ca-rich (R, En33Fs2sWo39) clinopyroxene on an original clast of clinopyroxene (A, En29Fs84Wos7). Transmitted polarized light, b, "Feldspathic" regions (darker colored) in a Type A norite (115 6) by reflected light, showing these to consist actually of blocky plagioclase crystals embedded in glass (appears darkest of all). c, Lunar whitlockite (W) in its most typical form, in Type A norite (115-5). Whitlockite is softer than surrounding silicates, and tends to polish out in relief; these depressions become apparent when the specimen is illuminated by an offset incident light source, d, A complex grain of zircon (Z), of almost 200/z extent, by reflected light. Scale as in Fig. 6a. The crystal is riddled with plagioclase and pyroxene inclusions, giving it tile appearance of Swiss cheese. Type A norite (115 6).

471

TERRA ROCK FRAGMENTS IN APOLLO 12 SOIL A I

0.8

o * .,Ye 0.6

o~

I

|i

i i i I I

/

I

I

\ \ x \

0 000

o

o

I-:

~

\

C~cD

0.4

\

'~ °o\

o

\

i-:

o

8o~

?

ob

\

APOLLO

0.2

o~

BASALTS

O

I

0

12

0.2

0 4

0.6

0.8

F e / ( F e +Mn + Mg}(MOLAR}

:FIG. 7. Chromium content (as Cr203) of pyroxenes in light-colored particles, as a function of major element composition (:Fe/(Fe + Mn + Mg)). o, Type A norite-anorthosites, Apollo 12; e, Type B norite-anorthosite, Apollo I2; A, Apollo 11 anorthosites; *, orthopyroxene monoerystalline fragment (110-1).

I

I 12'9-3 130-6 102 -13 106-163 106-125 106-Z33 102-10 106-198 129-1 106 - 3 1 3 106 - 2 8 4 102 - I 115 - 6 130 - 7 106-115 IOI - 1 2 106-312 106-15 I01 - - 4

0.2

ZONING

I

I 0 CO 0

0

0

0 0 0

oo-8 O O O O--O O---O O-O CO O O O

o

0:~-o 0 o

O

0

oo

0o8o-o

o

I

I

0.3

0.4

Fe/(Fe+Mg}(MOLAR)

IN C a - P O O R

PYROXENES

FIG. 8. Record of low-Ca pyroxene compositions encountered in each of 19 Type A noriteanorthosites studied. "Zoning" bar shows m a x i m u m compositional variability present in any single pyroxene grain or clast.

472

JOHN

in T y p e A f r a g m e n t s is n o t m u c h g r e a t e r t h a n t h i s (Fig. 8). T h e h i g h - F e c l u s t e r s o f d a t a p o i n t s f o r p a r t i c l e s (101 5) a n d (101-12) (which were most thoroughly s t u d i e d ) in F i g . 8 w e r e t a k e n in t h e s m a l l e s t pyroxene grains, those presumably most affected by recrystallization ; the divergent low-Fe measurements c a m e fl'om t h e centers of relatively large clasts. Except for t h e s e c e n t e r s , i t a p p e a r s t h a t t h e p y r o x e n e s in t h e s e r o c k s a r e f a i r l y well equilibrated; the individual pyroxene grains must have "communicated" more successfully with one another than did the feldspar grains during the thermal event that recrystallized textures. Type A norite-anorthosites contain abundant glassy material along grain boundaries between the major minerals. T h i s is u s u a l l y v i s i b l e o n l y b y r e f l e c t e d l i g h t (Fig. lc). S o m e t i m e s close e x a m i n a t i o n s h o w s t h a t w h a t s e e m e d t o b e a single p l a g i o c l a s e g r a i n is a c t u a l l y a c o m p l e x o f e u h e d r a l p l a g i o c l a s e c r y s t a l s e m b e d d e d in g l a s s (Fig. 6b). T h e g l a s s m a y b e f e a t u r e l e s s a n d clear, o r i t m a y h a v e t h e m o t t l e d appearance characteristic of partly microc r y s t a l l i n e m a t e r i M . I t is r i c h in K a n d A1 ( T a b l e I I I ) ; o r t h o e l a s e is t h e d o m i n a n t normative mineral. These glasses are generally similar to the areas of K-rich r e s i d u a l m a t e r i a l f o u n d in s o m e A p o l l o 11 basalts (Roedder and Weiblen, !970); principal differences are that the noriteanorthosite glasses contain substantially m o r e A1 a n d less F e t h a n t h e b a s a l t i c residua. Approximately 5% of the marie mineral g r a i n s in T y p e A n o r i t e - a n o r t h o s i t e p a r t i c l e s c o n s i s t o f o l i v i n e . T h e s e o c c u r as elasts or grains mingled with the pyroxene a n d p l a g i o c l a s e ; t h e r e is r a r e l y i f e v e r a n obvious reaction relationship with pyroxeiie. R e p r e s e n t a t i v e compositions are s h o w n in T a b l e I I . The Type A norite anorthosites contain approximately 3% by volume of phosphate m i n e r a l s ; all t h e g r a i n s m i c r o p r o b e d a r e of whitlockite. Occasionally this appears in l a r g e c l a s t s , b u t u s u a l l y as s m a l l , anhedral grains that would be inconspicuous if it were not for their hollowed-out a p p e a r a n c e b y offset r e f l e c t e d i l l u m i n a t i o n

A. WOOD

TABLE III COMPOSITIONS AND N O R M S OF G L A S S E S IN L I G H T - C o L O R E D P A R T I C L E S (A, B) AND O T H E R L U N A R K - R I C H S U B S T A N C E S (C E ) (WEIGHT PERCENTAGES, MICROPROBE ANALYSES)

A

B

C

D

E

SiO2 Tie2 AlcOa

61).7 0.3 20.5

46.8 3.1 14.4

70.8 0.6 12.7

73.5 0.5 12.2

74.5 0.5 11.5

Cr203

--

0.2

0.0

--

--

FeO MnO MgO CaO BaO Na20 K20

0.2 0.0 0.2 3.5

9.8 0.1 8.6 12.1

6.3 0.1 0.4 1.0

0.9 -0.4 1.2

2.5 -0.3 1.8

P205

-

-

1.1 13 1 -

-

Sum

99.6

Fo Fay En Fs We Or Ab An Ihn Chr Qtz Cor Ap~

0.4 0.0 0.0 0.0 6.9 77.8 9.3 1.0 0.4 0.0 0.0 4.2 --

-

-

1.2 O6 -

-

96.9

-

-

1.1 7.4

0.9

-

-

101.1

97.9

0.4 6.5 --98.9

0.0 0.0 1.1 10.6 0.0 43.9 9.1) 0.1 1.1 0.0 30.5 2.9 1.6

O.0 0.0 1.0 0.9 0.0 42.1 12.2 6.1 1.0 0.0 36.5 0.3 --

0.0 0.0 0.8 3.8 0.0 38.9 3.4 9.0 1.0 0.0 42.6 0.5 --

0.7

1.4 6.9 --

~'OFnlS

2.2 1.5 19. I 11.5 12.I 3.8 10.5 33.1 6.0 0.2 O.0 0.0 --

A. Apollo 12 Type A norite anorthosites: K-rich residual glass (average of four analyses). B. 1Representative brown interstitial glass, in Type B breccia (133 8). C. Defoeused-beam analysis of mierographie granite (102- 5). D. tloek 12013: salie components in ovoid fillings of dark lithology (average of eigh~ fillings) (Drake et al, 1970). E. Apollo 11 basalb: K-rich residual glass (average of 33 oeeurrenees) (lloedder and ~Veiblen, 1970). (Fig. 6c). B e i n g a s o f t e r m i n e r a l t h a n t h e s u r r o u n d i n g s i l i c a t e s , w h i t l o c k i t e on a section surface tends to polish away faster and leave low-lying areas. The lunar whitlockite contains astonishing amounts of yttrium and rare-earth

TERRA

ROCK

FRAGME:NTS

2

2

I:N APOLLO

2

2

12

473

SOIL

2

2

2

[

I00

CPS

20 kv, O.OIFA PET, Ar FPC I 1.8

1

I 2.0

I

I 2.2 WAVELENGTH,

I

I 2.4

I

I 2.6

I

~,NGSTROMS

FIe. 9. Microprobe data on whitlockite grain in a lunar norite : spectral scan through the wavelength range of rare-earth element Lu x-radiation. P E T - - t y p e of diffracting crystal used in microprobe analysis (pentaerythritol). FPC--flow proportional counter. e l e m e n t s (Fig. 9). A t y p i c a l m i c r o p r o b e a n a l y s i s o f this m i n e r a l is p r e s e n t e d in T a b l e I I . W e h a v e n o t c o n f i r m e d its

1.0

I--

0.5

i d e n t i f i c a t i o n b y x - r a y diffraction, b u t the low h a l o g e n c o n t e n t o f t h e a n a l y s e s m a d e seems t o rule o u t t h e p o s s i b i l i t y t h a t it m i g h t be a p a t i t e . R a r e - e a r t h - r i c h whirl i lockite w a s first r e p o r t e d in this t y p e ot l u n a r m a t e r i a l b y Albee a n d C h o d o s (1970), O w h o o b s e r v e d it in t h e C a l t e c h g r o u p ' s "Luny Rock," 10085-LR-l. The average o of our analyses reduces to the approximate f o r m u l a 2.22 C a 0 . 0 . 2 3 M g O . 0 . 0 1 N a 2 0 . o .........~:~ 0 . 0 6 F e O - 0 . 0 9 Y 2 0 3 . 0 . 0 8 r a r e - e a r t h oxides ...........~:~:;~i~i:i:i:iiiii~i~i~i~i~!~i~i~i."P.~05, w h i c h is s o m e w h a t s h o r t o f t h e o ..:.::::::::.:.:.:.:.:.:.,............ o ~i~ii~i~i~i~i...... ~i~i~i~i~i~i~i~i~i~i~i~i~i~i~i~ ideal w h i t l o c k i t e f o r m u l a , 3 C a O . PeO 5i~:~i:i:iii!::~i:~i:i:i:i:iii~iii~i!~!!~i!~i~i~i~i~ii~i~i~iii~i~!~!iii~!~Z i r c o n is r e m a r k a b l y a b u n d a n t in t h e : (~i![~!~ii!~iii~i~iiiiiiiii:ii~i!i~::::;:::""

..........

o O o

o 5 Ni, WT. PCT.

I ~o

FIG. 10. Ni vs. Co contents of metal grains microprobed in Type A norite-anorthosites. Shaded area shows compositional range of iron meteorites (Moore et al., 1969).

T y p e A n o r i t e - a n o r t h o s i t e s . I t o c c u r s in

irregular a n h e d r a l g r a i n s , u s u a l l y enclosing several e u h e d r a l p y r o x e n e a n d feldspar crystals, w h i c h give t h e s u r r o u n d i n g zircon a Swiss-cheese-like a p p e a r a n c e (Fig. 6d). T h e s e zircon c o m p l e x e s ( a p p a r e n t l y m o n o crystals, in spite o f t h e i r t o r t u o u s shape) c a n be q u i t e large, m o r e t h a n 100 F in d i a m e t e r . T h e y are m o r e easily r e c o g n i z e d b y reflected t h a n b y t r a n s m i t t e d light. I l m e n i t e is t h e m o s t a b u n d a n t o p a q u e p h a s e in t h e T y p e A n o r i t e - a n o r t h o s i t e s ,

FIG. 11. Photomicrographs of Type B breccias, by plain transmitted (a-c) and reflected (d) illumination. a, Plagioclase-rich breccia (ll0 33). All visible clasts are plagioclase. These are separated by red-brown glass; where the glass occurs in sizeable volumes, it has precipitated plagioclase microlites (Q). b, Heavily shocked Type B breccia (102-17) containing abundant clasts of pyroxene as well as plagioclase. Most of particle is shock-melted glass. Clinopyroxene clasts (C) contain submicron inclusions in parallel planar array, producing striped appearance, c and d, Red-brown glassy (to microcrystalline) mesotasis (M) separating plagioclase clasts in Type B breccia (133-8). Blocky form of plagioclase surfaces is at least in part constructional ; red-brown glass is residual after precipitation of some plagioclase on surfaces of original clasts. Glass analysis appears in Table I I I .

474

Jo~N

A. WOOD

TERRA ROCK FRAGMENTS IN APOLLO 12 SOIL

comprising a b o u t 1.5% of t h e m b y volume. I t occurs as anhedral to subhedral grains, usually of small size (typically 10 to 20/~; see Fig. lc). A r e p r e s e n t a t i v e microprobe analysis is shown in Table II. T h e r e is some substitution of the geikielite molecule (MgTiO3) for F e T i 0 3 in the ilmenite of these rocks. Nickel-iron metal is present in v e r y small a m o u n t s as m i n u t e grains (usually 1 to 2/~). The Ni and Co contents of grains analyzed are shown in Fig. 10. T h e y are quite similar to meteoritic k a m a c i t e in composition; it seems likely t h e y were c o n t r i b u t e d to the lunar rock b y meteoritic infall, t h o u g h their compositions m a y have been modified b y reaction with one a n o t h e r and with the lunar material during recrystallization (meteoritic k a m a c i t e does n o t contain more t h a n a b o u t 7% Ni). The T y p e A n o r i t e - a n o r t h o s i t e s also contain trace a m o u n t s of troilite, chromite, spinels of unusual composition, armalcolite, and silica minerals. T Y P E B BRECCIAS

A second, less n u m e r o u s class of plagioclase-rich particles was found, which are distinguishable t e x t u r a l l y and mineralogically from the T y p e A n o r i t e - a n o r t h o s i t e series. I t is s o m e w h a t misleading to lump these with the T y p e A f r a g m e n t s as "lightcolored particles," because t h e y are not conspicuously light in macroscopic color. T h e y eluded our a t t e m p t s to hand-pick n o r i t e - a n o r t h o s i t e particles, often ending up in the batches we t o o k to be basaltic in character. T h e T y p e B particles are unrecrystallized breccias, assemblages of relatively large mineral f r a g m e n t s e m b e d d e d in glass or a finely crystalline mesostasis (Fig. 11). The close-fitting relationships of m a n y a d j a c e n t

475

mineral fragments and the small proportion of interstitial glass in most T y p e B particles indicate t h a t the rock was brecciated in situ, not m i x e d and gardened as in a regolith; some of the particles are heavily shocked (Fig. l l b ) . Most of the T y p e B fragments are conspicuously more anorthositic, i.e., contain a higher proportion of plagioclase feldspar, t h a n t h e T y p e A n o r i t e - a n o r t h o s i t e s (Fig. 2). Compositions of minerals in T y p e B breccia particles are listed in Table I I and p l o t t e d in Figs. 3, 4, 5, and 7. I t appears t h a t there are systematic differences between T y p e s A and B mineralogy, b u t we h a v e n o t a c c u m u l a t e d enough d a t a for the l a t t e r class of particles to say this conclusively. 1. T y p e B p y r o x e n e s are richer in F e t h a n are those of T y p e A or Apollo 11 anorthosite particles. I t appears t h a t their Mn c o n t e n t m a y be substantially higher (Table II), b u t more analyses would be needed to confirm this. 2. T y p e B plagioclase is somewhat more sodic t h a n T y p e A plagioclase. 3. Opaque minerals are almost t o t a l l y lacking. R a r e occurrences of ilmenite, armalcolite, whitlockite, and silica were observed, b u t no zircon. The interstitial glass in T y p e B particles is u n r e l a t e d to the K - r i c h residual glass in T y p e A particles. I t is brown basaltic glass (Table III), of composition similar to t h e T y p e A n o r i t e - a n o r t h o s i t e s (Table I) and b r o w n glass fragments t h a t are a b u n d a n t in the Apollo 12 soil ( H u b b a r d et al., 1971 ; Marvin et al., 1971). This glass was und o u b t e d l y shock-melted and injected into the pore space of s h a t t e r e d anorthositic minerals, where it cooled rapidly. I t does n o t seem to have been g e n e r a t e d locally; its high Ti content ( ~ 5 % n o r m a t i v e ilmenite, higher t h a n t h a t of most T y p e A n o r i t e - a n o r t h o s i t e s ) contrasts sharply with

FIc. 12. a, Apollo l l anorthosite (37-17) with recrystallized breccia texture similar to that of Type A Apollo 12 particles, by polarized transmitted light ; numerous minute crystals of pigeonite (elongated) and olivine (equant) are visible. Ilmenite is absent, b, Heavily shocked coarse anorthositic breccia fragment (40-7), Apollo 11, by polarized transmitted light. Similar to Apollo 12 Type B particles. O, olivine; remainder of particle is distorted plagioclase, c and d, Micrographic granite particle (102-5) by, respectively, transmitted polarized light and reflected light. Most of the particle consists of a fine, uniform intergrowth of K feldspar (K) and a silica mineral (S) but minor amounts of Fe-rich olivine (O1, FasT), ferroaugite, ihnenite, and apatitc (A) are also present. The silica displays a curious crystallographically oriented pattern of elongate voids or soft inclusions. 17

476

JOH~

A. WOOD

y

TERRA

ROCK

FRAGMENTS

the near-zero Ti c o n t e n t of the mineral f r a g m e n t s it intrudes. APOLLO 11 REVISITED According to our p a p e r describing the Apollo l l sample (Wood et al., 1970), the light-colored particles in the soil from T r a n q u i l l i t y Base are of a v e r y different character t h a n the Apollo 12 particles discussed a b o v e : we i n t e r p r e t e d t h e m as igneous cumulates and r e p o r t e d higher plagioclase contents t h a n in the particles described above. I t seemed desirable to r e e x a m i n e the Apollo 11 anorthosites in the light of our current observations to see w h e t h e r or not the two groups of particles grade into one a n o t h e r in properties, a n d if not, how f u n d a m e n t a l is the gap between them. T h i r t y - f o u r anorthositic particles in our Apollo 11 sections were studied. The largest single category t h a t emerged (12 particles) L

I

I

IN

12

APOLLO

477

SOIL

consists of rocks t h a t are clearly recrystallized breccias (from the angular clasts still visible in their t e x t u r e s ) and equivocal rocks t h a t can be i n t e r p r e t e d either as breccias in an a d v a n c e d stage of recrystallization or as v e r y fine-grained igneous rocks. All degrees of obliteration of the p r i m a r y breccia t e x t u r e can be observed. An example is shown in Fig. 12a. The mineralogical differences we r e m e m b e r e d were confirmed. The plagioclase contents of these recrystallized breccias is m u c h higher t h a n t h a t of the Apollo 12 n o r i t e - a n o r t h o s i t e s ; the f o r m e r are t r u e anorthosites. T h e y contain little or no phosphates, zircon, or residual glass. This is t r u e with a single exception: one of the 34 particles e x a m i n e d is a t r u e Apollo 12 T y p e A norite, complete with whitlockite, Swiss-cheese-like zircon, residual glass, and a b u n d a n t ilmenite in u n c o m m o n l y large grains. I t contains ~ 6 0 % plagioclase. T h e n e x t largest category of particles (9

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478

JOHN A. WOOD

examples) consists of heavily shocked, unrecrystallized, coarse plagioclase breecias (Fig. 12b). These are similar to the Apollo 12 Type B breecias, except that glass interstitial to the clasts is colorless (probably feldspathic) rather than redbrown. Of the remaining particles surveyed, six have been very heavily shocked and either isotropized or completely melted (with subsequent precipitation of plagioclase microlites) ; four have been melted (presumably by shock), but afterward have crystallized or devitrified to a large extent, to a dense assemblage of plagioclase fibers and microlites ; one is an unequivocally igneous rock; and two are unclassifiable. Clearly, the Apollo 11 sample contains light-colored rocks that are texturally the equivalents of the Apollo 12 Type A and Type B materials. We minimized the importance of these classes in our earlier report and overplayed the significance of igneous processes. However, there are significant differences in the chemistry and mineralogy of light-colored particles from the two sites. From the bulk chemical compositions of Apollo 11 and 12 particles in Table I, the comparisons shown in Fig. 13 were drawn to see if the differences are clear cut or if there is not a smooth progression of rock types and compositions from one site to the other. The question is not very satisfactorily answered. When the analyses are arrayed according to their plagioclase content (i.e., how "anorthositic" they are), their contents of Ti and K and the proportion of olivine in their normative marie minerals vary, but not smoothly. There is an abrupt break at about 50% plagioclase, suggesting that two discrete populations of rock types are involved. Yet the break is not a step function, only an abrupt steepening of the relationship ; and there is a smooth if limited chemical overlap of the two populations (entries between 65 and 85 mole % plagioclase). Petrographic examination of the three particles in this range of overlap shows them to have an appropriately intermediate or equivocal character. On balance, it seems most likely t h a t the ranges of properties of Apollo 11 and Apollo

12 light-colored particles overlap and grade into one another and t h a t the two groups of particles are genetically related. In summary, the two groups have in common the following : 1. Each clan has anorthositic affinities (at least some members contain substantially more than 50% plagioclase). 2. The pyroxene, where present, is dominantly Ca-poor (pigeonite and/or orthopyroxene). 3. The same two textural types are present among light-colored rocks from both sites : recrystallized breccia (Type A), and coarser unrecrystallized breccia (Type B). 4. Metallic minerals of meteoritic composition are present in Type A rocks from both sites. Differences are the following: I. The average plagioclase content of Type A particles is higher in the Apollo i l soil (they are more anorthositic). 2. The Apollo 11 Type A particles contain no whitlockite, zircon, or residual K-rich glass, and little or no ilmenite; all these are present at significant levels in most Apollo 12 Type A particles. 3. The ratio of Type B to Type A particles is higher in the Apollo 11 material; considering that the very heavily shocked (isotropized, melted) Apollo I t particles probably have Type B rather than Type A affinities, Type B particles may actually form the bulk of the Apollo 11 light-colored soil fragments. 4. Type B particles appear to be more heavily shocked in the Apollo 11 sample than in the Apollo 12. COMPARISONS ~NITH THE SURVEYOR 7 ANALYSIS OF TERRA MATERIAL

The compositions of Apollo 11 anorthosite particles, as determined by defocusedbeam microprobe analysis, are strikingly similar to the composition of ejecta from the great crater Tycho in the lunar highlands, as determined by the Surveyor 7 alpha back-scattering experiment (Patterson et al., 1970; Wood, 1970). Results ot this comparison are crucial to the interpreration t h a t anorthosite particles in the soil

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480

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at T r a n q u i l l i t y Base derive from the lunar highlands. However, the comparison has not been drawn previously for two elements, F and P. The alpha back-scattering e x p e r i m e n t d e t e c t e d a surprisingly high level of F in T y c h o ejecta, several t e n t h s of an atomic percent; F (which is n o r m a l l y a trace element in rocks) had not been measured in our defocused-beam analyses. We felt confident t h a t it was not present at t e n t h percent levels, as we had not observed fluorine minerals in the anorthosites. To complete the comparison, we recently m e a s u r e d bulk F and P levels in a n u m b e r o f Apollo 11 and 12 light-colored particles b y electron microprobe analysis (Wood et al., 1971). Results for particles from b o t h sites are p l o t t e d in Fig. 14, which is a g r a n d comparison of analytical results for all elements from S u r v e y o r 7, Apollo 11 anorthosites, and Apollo 12 T y p e A n o r i t e anorthosites. As expected, F is an order of m a g n i t u d e less a b u n d a n t in Apollo 11 anorthosites t h a n the levels d e t e c t e d b y S u r v e y o r 7. The F content in Apollo 12 particles is somewhat higher b u t still falls short of the T y c h o values. Levels of P and S in light-colored particles from b o t h sites are seen to be in a g r e e m e n t with the S u r v e y o r 7 values. B o t h F and P are more a b u n d a n t in Apollo 12 particles t h a n in Apollo 11, which is consistent with the greater abundance of visible p h o s p h a t e minerals in the Apollo 12 particles and with the supposition t h a t at least p a r t of the F in the particles occurs in the phosphates. I n spite of the a p p a r e n t fluorine discrepancy, we feel t h a t the compositional similarity between T y c h o ejecta and the Apollo light-colored particles is v e r y striking and convincing. B o t h classes of particles are a good m a t c h to the T y c h o highlands material, b u t it might be argued (on the basis of the three heaviest element

groups) t h a t the Apollo 11 anorthosites provide the closest fit. To split hairs even finer, a material i n t e r m e d i a t e in composition between the Apollo 11 and 12 particles would furnish the closest m a t c h of all. MICROGRAPHIC GRANITE One of the 497 particles we e x a m i n e d is unique: (102-5) consists chiefly of a silica mineral and potassium feldspar, finely intergrown in a fabric reminiscent of p e r t h i t e or m y r m e k i t e (Figs. 12c, d). W e have n o t identified the p o l y m o r p h s of silica and feldspar: the fine grain size of the particle p r e v e n t e d optical determinations, and we did not w a n t to d e s t r o y this unique thin section b y making it into an x - r a y diffraction mount. A microprobe analysis of the feldspar is shown in Table II. Olivine, clinopyroxene, ilmenite, whitlockite, and apatite arc present in minor a m o u n t s (Figs. 12c, d). The mafic minerals are m u c h enriched in Fe: the olivine contains 87 mole % fayalite, and the p y r o x e n e is ferroaugite (Fig. 5). Of course, the observed coexistence of olivine and free silica would not be possible unless the olivine were Fe-rich; all b u t the Fe-rich olivines t e n d to react with silica to form pyroxene. The ilmenite in particle (102-5) is more nearly pure FeTiO~ (contains less Mg) t h a n the ihnenites of n o r i t e anorthosites. A defocused-beam analysis of (102-5) is shown in Table I I I , where it is c o m p a r e d with other possibly related lunar K - r i c h substances. The n o r m for (102-5) also appears in Table I I I ; here all the Mg and F e have been made to appear in pyroxenes, because our p r o g r a m for computing norms forbids the coexistence of olivine and quartz. This rock has no terrestrial counterp a r t ; it seems closer in its properties to a micrographic granite t h a n to a n y t h i n g else.

FIG. 14. Comparison of compositions of Apollo 11 and Apollo 12 light-colored particles (defoeusedbeam microprobe surveys) with the composition of crater ejecta in the lunar highlands (Surveyor 7 alpha back-scattering analyses; Patterson et al., 1970). Error bars shown (except where they are smaller than circles representing probe surveys) represent two-sigma counting statistics. Apollo 11 F and P measurements were made on different particles than those surveyed for other elements.

T E R R A ROCK Jt'RAGME~TS I N APOLLO

PETROLOGICAL I:NTERPRETATIO:NS

Type A Recrystallized Breccias As n o t e d above, a large p r o p o r t i o n of t h e light-colored lithic f r a g m e n t s f r o m b o t h Apollo 11 a n d Apollo 12 soil samples are t e x t u r a l l y t h e same k i n d of r o c k : an a s s e m b l a g e of b r o k e n mineral f r a g m e n t s (breccia) t h a t has been healed into massive rock, p r e s u m a b l y b y t h e r m a l recrystallization. (At a later time, of course, this r o c k was f r a g m e n t e d into the soil particles we h a v e described; b u t relict t e x t u r e s of the earlier f r a g m e n t a l stage m a y still be o b s e r v e d within each of the p r e s e n t soil particles.) Apollo l l T y p e A particles are on an a v e r a g e m u c h m o r e feldspathic (anorthositic) t h a n Apollo 12 T y p e A particles, b u t p r o p e r t i e s of the two sets of particles t e n d to g r a d e into one a n o t h e r (Fig. 13): it a p p e a r s t h a t T y p e A rocks f r o m b o t h sites are genetically related, i.e., a c o m m o n m a g m a s y s t e m g a v e rise to a r a n g e of p r i m a r y igneous r o c k t y p e s t h a t were s u b s e q u e n t l y b r e c c i a t e d to f o r m the

12

481

SOIL

v a r i o u s T y p e A particle compositions we observe. I n c o m m o n w i t h o t h e r writers, W o o d et al. (1970) h a v e a r g u e d t h a t crystal fractionation m u s t h a v e p r o d u c e d the l u n a r anorthosites. (When a m a g m a cools, crystals t h a t f o r m in it do not in general h a v e the s a m e specific g r a v i t y as t h e residual liquid s u r r o u n d i n g t h e m ; t h e y t e n d to float or sink in the liquid, d e p e n d i n g on t h e n a t u r e of the d e n s i t y contrast, a n d a c c u m u l a t e at t h e t o p or b o t t o m of the m a g m a system. L u n a r plagioclase is less dense t h a n the melts of k n o w n lunar mafic rocks (see T a b l e IV), so crystallizing plagioclase p r o b a b l y t e n d e d to float upw a r d in e a r l y lunar m a g m a systems. A p p a r e n t l y , a n o r t h o s i t e f o r m s m u c h of t h e lunar surface (in highland terrains); a c c u m u l a t i o n of plagioclase a t the surface implies a p a r e n t m a g m a s y s t e m t h a t was itself a w a s h on the surface of t h e Moon, n o t deeply buried as were the k n o w n terrestrial m a g m a s y s t e m s t h a t p r o d u c e d a n o r t h o s i t e s w h e n t h e y crystallized.)

TABLE IV SPECIFIC GRAVITIES OF ]:~EPRESENTATIVE LUNAR MINERALS AND ROCKS, IN SOLID AND LIQUID STATES a

p (g/tIn3) Mineral A. B. C. D. E.

Plagioclase Low-Ca pyroxene Augite Olivine Ilnlenite

Composition (mole %) AbsAn92 EnToFsu0 En40Fs20"Wo40 Fo70Fa30

25°C

1400°C (solid)

2.75 3.40 b 3.48 3.58 4.79

2.70 3.26 3.33 3.39 4.55

p (g/cm s) (computed) Model Rock Tranquillitatis basalt Procellarum basalt Anorthosite Norite

Composition (wt. %) A37C44E19 A32C59E9 As7BsD5 A63B 34E3

25°C

1400°C (solid)

1400°C (liquid) c

3.32 3.28 2.81 2.98

3.22 3.17 2.76 2.90

3.17 3.04 -2.89

a Data from Clark (1966), unless otherwise indicated. b Average of densities for orthopyroxene and pigeonite containing 30 mole % ferrosilite (Deer et al., 1963). c Computed from bulk chemical composition of model rock, by the method of Bottinga and Weill (1970).

482

JOHN

A. WOOD

Norites of the type described in this paper can be related to anorthosites of the Apollo 11 type in one of two ways : 1. l~orite may be representative of the original composition of an early lunar surface magma system, from which anorthosites (and also sunken cumulates of mafic minerals) were to form by crystal fractionation. 2. Norite may be representative of the composition of a residual magma t h a t was left between layers of floating anorthosite and sunken ultramafics, after extensive crystal fractionation had occurred in an early lunar surface magma system. The second possibility seems more likely, for several reasons. Its high content of trace and minor elements (K, Y, rare earths, Zr, U) t hat are riot easily accommodated in plagioclase, pyroxene, or olivine suggests t hat lunar norite was a late residual liquid, from which a large volume of "clean" crystals had been removed. The more magnesian (Fig. 5) and silica-poor (undersaturated) character of mafic minerals in lunar anorthosites than in norites is also important. I f norite represented the starting composition of the lunar surface magma system, the deviant composition of anorthosite mafic minerals would have to be attributed to rafting upward of early-crystallized mafics along with floating plagioclase, to lack of large-scale uniformity in the composition of the Moon and its early surface magma system, or to mixing of appropriately different meteoritic silicate minerals into lunar surface rocks at various times and places on the Moon. None of these are very satisfying explanations. I f anorthosite is an early differentiate and norite a later residuum, on the other hand, the differences in character of mafic minerals are those t h a t silicate phase diagrams predict. [Laboratory experiments have established t hat in systems of appropriate bulk composition, as crystallization proceeds more siliceous minerals appear (olivine gives way to pyroxene), and the equilibrium Fe/Mg ratio in these minerals increases.] B u t Type A norites and anorthosites do not have the coarse igneous textures (Fig. 15a) th at should develop in rocks crystal-

lizing in magma systems as extensive as the hypothetical lunar surface magma system (the scale of which will be discussed later); indeed, most Type A particles do not have igneous textures at all (Figs. la, 12a). They are recrystallized breccias, and their textures (distribution of clast sizes, absence of juxtaposed fragments t h a t fit together) are suggestive of origin by repeated fragmentation and mixing as in a regolith, not by one-impact in situ shock brecciation of igneous rock types. E xcept for the effects of thermal recrystallization, t hey are texturally similar to the known lunar soil breccias. The postulated early-formed anorthositic layer at the surface of the Moon would have been exposed to meteoroid bom bardm ent through all of geologic time, and an anorthositic regolith inevitably would have been created; this probably accounts for the fact t hat much of the anorthositic material now available at the lunar surface has the character of a soil breccia. The existence of noritic soil breccia as well makes it appear t hat this rock t ype was not everywhere shielded by an overlying layer of anorthosite: locally on the Moon norite rather than anorthosite was exposed at the surface, and a noritic regolith was created. The great majority of Type A breccias have been healed into massive rock. Pore space (such as can be seen in lunar basaltic soil breccias) has been completely eliminated. Wholesale remelting of the breccia cannot be responsible for this healing, or the angularity of clasts would have been lost. I f these rocks were terrestrial, the healing effect of aqueous solutions might be appealed to, but on the all-but-waterfree Moon this is not possible. The massive nature of Type A breccias can best be understood as an effect of thermal recrystallization. Polycrystalline substances, when held at high temperature for protracted times, tend to reorder their crystal fabrics via solid-state diffusion of individual atoms through crystal lattices and along crystal grain boundaries. Changes wrought are elimination of the smallest crystals (their substance is added to larger crystals in the system), decrease in the amount of surface

TERRA ROCK FRAGMENTS IN APOLLO 12 SOIL

48

O

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~

5O

FIG. 15. a, Outcrop of anorthositic rock in the Adirondack Mountains, New York, showing eoars crystallinity of terrestrial massif anorthosites. Plagioclase crystals (dark) are typically 1 to 2 cm i dimension, b, Noritic breccia particle (229 10) from the Apollo 14 soil b y plain t r a n s m i t t e d light showing resemblance to Apollo 12 T y p e A breceias (Fig. la). c, Same particle by reflected lighl Coffin-shaped euhedral forms of p y r o x e n e crystals (light g r a y m a j o r constituent), in contrast t irregular forms of p y r o x e n e crystals in Apollo 12 breccias (Fig. lc), indicate t h a t Apollo 14 pyroxen crystallized from a melt.

area (both crystal-crystal and crystalpore interfaces) in the system, elimination of phases unstable at the recrystallization temperature, and homogenization of

chemical composition of each discret phase or mineral in the system. The procee is utilized by industry to manufactur sintered ceramic and metal products.

484

J O H N A. WOOD

However, in the case of the lunar Type A breccias, the existence in at least some particles of zones of glassy residual material of composite composition (Figs. lc, 6b; norm of analysis A, Table III) indicates t h a t recrystallization occurred somewhat above the eutectic temperatures of these rocks and t h a t incipient melting occurred. The presence of even a small amount of silicate melt in the pore space of the original breccias would have greatly facilitated and speeded their recrystallization into massive rocks. Breccia recrystallization was not pervasive enough to eliminate all unstable phases, as is shown by the coexistence of three pyroxenes (orthopyroxenc, pigeonite, and augite) in many if not all the Type A norite particles, whereas only two pyroxcnes can stably coexist (see e.g., Deer et al., 1963, p. 129). Nor were mineral compositions homogenized within the volumes represented by the particles studied (Figs. 4 and 8). Plagioclase and pyroxene compositions were nonuniform in the original breccias, probably because the clasts of these minerals were derived from nonuniform (zoned) crystals in igneous rocks t h a t predated the breccias, and also because mixing of minerals from widely separated sources occurred in the regoliths that gave rise to the breccias; these heterogeneities (at least in part) survived the recrystallization. Whitlockite and zircon are normally early-formed minerals in igneous rocks and preserve euhedral outlines; but in the Type A norites, it is clear from their complex anhedral forms (Figs. 6c and 6d) that they were emplaced late in the crystallization sequence. Presumably they assumed these forms during the thermal recrystaliization of the breccias. It appears that sustained high temperatures in early noritic and anorthositic regoliths were required to produce the Type A breccias.

Type A Igneous Rocks A minority of Type A particles are unquestionably igneous (Fig. lb), but like the breccias they are too fine grained go be samples of primary igneous rock

from the early lunar surface magm~ system. I t seems more likely t h a t these are counterparts of the Type A breccias thai were completely melted by the hightemperature events that recrystallized and partly melted the breccias.

Type B UncrystaUized Breccias The light-colored particles from both the Apollo 11 and the Apollo 12 sites also have in common a second textural type: coarse breccias of angular clasts, the latter sometimes not far separated from other fragments with matching surfaces, from which they were obviously sundered. Between the clasts is glass of noritic composition (at least in the Apollo 12 Type B particles), not a K-rich residuum. These are obviously shock brcccias, containing shock-melted glass, and are not recrystallized soil breccias, as the Type A particles appear to be. Clasts in the Type B breccias are almost invariably monocrystalline; the parent rocks t h a t were brecciated to form this type were relatively coarse grained. An upper limit cannot be put on the grain size of the parent rock, but a lower limit of 200t~ can be set (from Type B clast sizes), much coarser than the prevalent grain size in Type A recrystallized breccias. Type A breccias can be excluded as a parent material on textural grounds alone. Type B and Type A breccias also differ in the proportions and compositions of their major minerals. Apollo 12 Type B particles, unlike their Type A counterparts, are often genuine anorthosites, though there are also noritic or gabbroic Type B particles in the Apollo 12 samples (Fig. 2). The plagioclase is more sodic (Fig. 3) and the iron content of pyroxene is higher (Fig. 5) in Type B than Type A particles, which points to derivation of the Type B breccias from parent rocks t h a t formed during a later stage of magmatic evolution than the Type A norites. (Remember t h a t the norites in turn seem to represent a later stage of magmatic evolution than the Apollo 11 anorthosites.) It appears t h a t early in the history of Oeeanus Proecllarum (a) the parent magma of Type A norites erupted and crystallized at the surface (where it was subsequently bombarded by

TERRA ROCK ~RAGME:NTS I N APOLLO

meteorites, and ground into a regolith), and (b) later (at a later stage of magmatic evolution, at any rate; probably also later in time), at a position not exposed to the meteorite bombardment, the parent rock of Type B breccias crystallized, under circumstances t h a t permitted growth and gravity differentiation of relatively coarse crystals. It is hard to escape the conclusion t h a t stage (b) occurred under the noritic surface layer of stage (a). A rapidly chilled surface layer of undifferentiated norite would thermally insulate underlying magma. During slow cooling of this insulated magma, coarse crystals would have time to grow and separate into floating anorthositic and sunken marie layers. The anorthosite would consist of accumulated plagioclase crystals interspersed with residual noritic rock. It seems likely t h a t a subsurface anorthosite of this type was the source rock from which Type B breccias were derived. Although it was too deeply sited to be affected by the day-to-day regolithforming meteorite bombardment, occasional very large cratering events (perhaps the Imbrium impact itself) penetrated to this layer, crushing the plagioclase crystals and fusing the lower-melting constituents of the rock (in general, the noritic intercumulus material). Masses of mixed plagioclase clasts and noritic melt (later glass) were ejected from the crater and eventually found their way into the Apollo 12 soil samples. Where anorthosite occurs at the lunar surface (source of the Apollo 11 Type A anorthosites), Type B breccia probably represents shock brecciated samples of the same cumulate layer taken from depths t h a t were not affected by the regolithforming bombardment. Micrographic Granite The eutectic composition for many rock systems is enriched in K, Si, and Fe; t h a t is, the last liquid to crystallize from a cooling melt, or the first to appear as a rock is heated, contains these elements in larger proportions than the melt or rock as a whole. The lunar basalts contain patches of K-rich glass (Roedder and Weiblen,

12 SOIL

485

1970) that are obviously solidified volumes of eutectic liquid residual after crystallization of the major rock-forming minerals. Lunar rhyolite or micrographic granite [such as our particle (102-5)} is compositionally very similar to the K-rich glasses in basalts (see Table III) and must have formed in the same manner, except t h a t in this case the eutectic liquid segregated into larger volumes than have been observed in lunar basalts and crystallized instead of quenching to glass. An extreme degree of igneous fractionation (via crystallization at super-eutectic temperatures) was required to produce a residual liquid with such a high Fe/Mg ratio as t h a t in (102-5): 9.6, as against an average of 4.7 for the basaltic K-rich residua of Roedder and Weiblen. An even more highly differentiated granitic particle, containing nearly pure fayalite (FeeSi04) as the mafic mineral, has been observed by Mason et al. (1971) in the Apollo 12 soil. Rock 12013, one of the most intensively studied of all lunar samples, is a complex noritic breccia containing veins and ovoid inclusions of granitic material (Drake et al., 1970). The latter is chemically very similar to our particle (102-5) in almost all respects; compare columns C and D in Table III. However, Fc/Mg is not particularly high (1.3) in the 12013 granitic material; this value comes close to Fe/Mg for pyroxenes in the noritic portions of 12013, but falls far short of Fe/Mg in (102-5). Particle (102-5) and the granitic zones of 12013 also differ texturally, the former consisting of a micrographic intergrowth of silica and K-feldspar minerals (Fig. 12c) while the latter contain randomly oriented stumpy phenocrysts of feldspar and plates of silica. The lunar rock unit from which particle (102-5) was broken must have been derived by magmatic fractionation from some more maflc parent magma, but was this of the noritic or the mare basalt kindred? We cannot say for certain. In favor of a noritic heritage is the observed association of granitic material similar to (102-5) with norite in 12013; against it is the fact t h a t residual K-rich glasses in the norites we studied are not compositionally

486

JOHN it. WOOD

v e r y similar to (102-5): residual K-rich glasses in the mare basalts provide a much closer m a t c h (Table III). Nor is it possible to say w h e t h e r segregation of granitic liquids in the Moon produced major bodies of granitic or rhyolitic rock, or if this lithology occurs only as small-scale veins and inclusions in other rock types, as is the case with 12013. The evidence seems to be against m a j o r a m o u n t s of granite on the Moon: Granitic rock samples were not r e t u r n e d b y the Apollo II, 12, or 14 missions, and lithic fragments of granitic composition are v e r y rare in the soils of the three missions. The micrographic t e x t u r e of (102-5) is characteristic of the crystallization of eutectic liquids, suggesting the coexistence of the (102-5) liquid in small a m o u n t s with large proportions of crystalline material, which would h~ve hindered mobility of the liquid and made its segregation into large masses unlikely. Marvin et al. (1971) have not a t t e m p t e d to identify (102-5) with a n y of the mapp~ble geologic units on the Moon.

STRUCTURE AND EVOLUTION OF THE ~:[OON

Major Types of Lun(~r Rock Studies of lithic fragments from soils collected at two points on the lunar surface (and preliminary observations on material from a third site) have revealed the compositions of several different types of nearsurface lunar rock. Since our group has e x a m i n e d a large n u m b e r of fragments (several thousand), a few percent of which are not locally derived b u t came from sources scattered over the near face of the Moon, it is possible t h a t b y now we have seen examples of most or all of the major rock types t h a t comprise the o u t e r m o s t layer or " c r u s t " of the Moon. These are the following: mare basalts, anorthosite, and norite. The lunar soils also contain rare granitic fragments, b u t there is no evidence y e t t h a t this rock t y p e is a b u n d a n t on the Moon. These materials are certainly underlain b y some altogether different t y p e or types o f mantle rock. A lunar model in which mare basalts, anorthosite, a n d / o r norite

e x t e n d to the center of the Moon would fail to reproduce the m e a n mass density ot the Moon, or would be completely molten at the present time (because of the high levels of r a d i o a c t i v i t y in norite and mare basalt), or both. The three rock types n a m e d are " l i g h t " igneous differentiates, and elsewhere in the Moon c o m p l e m e n t a r y " h e a v y " differentiates must exist, rocks composed of minerals t h a t sank or were left behind when the " l i g h t " differentiates rose to the lunar surface. B y analogy with terrestrial igneous systems, it seems likely t h a t these " h e a v y " differentiates are composed largely of p y r o x e n e s or olivine or both. This would be consistent with the composition of the primordial solar nebula out of which the Moon and planets colldensed; Mg, Si, and F e were p r e s u m a b l y the most a b u n d a n t lithophile elements in the nebula, as t h e y are ill the present solar atmosphere.

A Model Relating Lunar Anortl~osites and ~¥orites I have suggested above t h a t two of the lunar crystal rock types, anorthosite and norite, are p r o b a b l y genetically related; b u t it seems clear ii'om differences in chemical and mineralogical compositions and radiometric ages t h a t the mare basalts are unrelated (or only d i s t a n t l y related) to the light-colored rocks. Since the latter appear to comprise the lunar terrae t h a t surround, underlie, and predate the maria, their petrological history can be considered a p a r t from t h a t of the (later) mare basalts. As n o t e d earlier, the p y r o x e n e s of T y p e A anorthosites contain less iron (lower Fe/Mg) t h a n those of T y p e A norite, which in t u r n contains less iron t h a n T y p e B anorthosite pyroxenes. The situation is summarized on the left of Fig. 16, where fields representing these three classes of pyroxenes are labeled I, II, and I I I , respectively, in a p y r o x e n e quadrilateral. It' the assumption t h a t these three h m a r rock types are genetically related is correct, t h e n the T y p e A anorthositic pyroxenes (I) must h a v e crystallized first from the p a r e n t m a g m a system; the norites with their p y r o x e n e s (II) crystallized later, after crystal fractionation had increased Fe/Mg

TERRA ROCK FRAGMENTS IN APOLLO

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FIG. 16. Left: Summary of compositional differences between pyroxenes in Type A anorthosites (I), Type A norites (II), and Type B anorthosites (III); from Fig. 5. Right: four-step petrological model that would account for compositionM and textural differences between hmar light-colored rock types ; discussed in text. in the residual magma; and Type B A cooling lunar surface magma system is anorthosites (pyroxenes III) crystallized pictured, in which continued crystallizalast of all. Plagioelase compositions (Fig. 3) tion of plagioelase and marie minerals are also consistent with the crystallization causes Fe/Mg and Na/Ca in the residual of norites at an earlier stage than Type B liquid (and in minerals separating from it anorthosites, though they prove nothing at any given time) to increase with time. about the relative positions of Type A A floating layer of plagioclase crystals anorthosite and norite in the erystallization (anorthosite) forms, with low-Fe/Mg liquid sequence, since the mean plagioelase in its interstices; crystallization of this compositions in these two rock types are liquid produces pyroxene (I). Major crateressentially identical. ing impacts strip the anorthositie layer I t was argued in the preceding section away locally, and noritic magma wells up. t h a t lunar anorthosites must have been At its surface, the liquid crystallizes formed by crystal fraetionation (flotation) ; abruptly, much like the chill zone in a t h a t the norites probably derive from magma intrusion, to form noritic rock with magma residual after the crystallization pyroxene (II) whose composition reflects and floatation t h a t formed a surface a more evolved state of the magma. Under anorthositie layer; and t h a t Type A anor- the noritie "chill zone," crystallization thositie and noritie rock (but not Type B proceeds more slowly and fractional anorthosite) was exposed on the lunar crystallization resumes its effect, so t h a t surface and crushed by the meteorite floating plagioelase crystals again accumubombardment into a fragmental layer or late into anorthosite, this time beneath regolith. A model ineorporating these the norite surface layer. Pyroxenes (III) conditions and the crystallization sequence in this latest anorthosite layer would have discussed above is sketched at the right in a higher Fe/Mg ratio than those in the Fig. 16. previously formed units. Meteorite bom-

488

JOHN A. WOOD

bardment would form regoliths on the oldest anorthosite and norite exposures (the source of Type A materials) but would not often affect the deeper-seated, younger anorthosite; this could be reached and excavated (producing Type B particles) only by especially deep cratering events. Obviously this is a greatly simplified model, and the chaotic state of affairs on the lunar surface during the early intense bombardment of meteorites and/or planetesimals would have included remelting and mixing in various proportions of the three rock types named, at many points on the lunar surface. However, I believe the model has value as a tentative framework in which the chemical and age differences between early lunar rocks might be interpreted. A Lunar Crustal Model Based on Gravity and Topography I t is possible to deduce something about the placement and extent of various types of rock on the Moon from considerations of gravity and topography. This approach

has been employed by Runeorn and Shrubsall (1968) and O'Keefe (1968). Since inhomogeneities of lunar gravity do not correspond to topographic irregularities (i.e., topographic highs do not give rise to positive gravity anomalies), the lunar surface must be underlain by bodies of rock of diverse mass density. I f we can assume t h a t masses of mare basalt, anorthosite, and norite are the principal building blocks that comprise the lunar crust, we can use the mass densities of these three rock types to conclude where each lies in the lunar crust, and what thickness of it must be present to reconcile lunar gravity with lunary topography. Consider the hypothetical profile across the Moon shown in Fig. 17, extending from Mare Imbrium to Mare Nectaris. This profile, straightened out, appears at the bottoms of Figs. 18-20. Available topographic data are summarized in Fig. 18 ; these are all the lunar elevations within 150km of the line of profile t h a t were measured by the U.S. Air Force A.C.I.C. (Meyer and guffin, 1965). The topographic

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l~IO. 19. Comparison of topography (above : summarized from Fig. 18) and variations in the acceleration due to gravity (below : Muller and Sjogren, 1969) along the traverse shown in Fig. 17. Lack of similarity of the two curves means that materials having different mass densities underlie the traverse. f o r m of the profile is s u m m a r i z e d in Fig. 19 a n d c o m p a r e d with v a r i a t i o n s in g r a v i t a tional acceleration along the s a m e line (Muller a n d Sjogren, ]969). The lack of correspondence of g r a v i t y to t o p o g r a p h y is obvious. I f the lunar surface were underlain b y a h o m o g e n e o u s m a t e r i a l of uniform density, the t w o curves should be similar in shape. We can s a y q u a l i t a t i v e l y t h a t t h e lunar highlands are underlain b y a less dense m a t e r i a l t h a n Oceanus P r o c e l l a r u m a n d Mare N u b i u m , since the v a l u e o f g r a v i t y is essentially u n i f o r m in all these regions in spite of the presence of excess t o p o g r a p h y in t h e highlands. F u r t h e r , Oeeanus P r o e e l l a r u m a n d Mare N u b i u m are underlain b y less dense m a t e r i a l s t h a n Mare I m b r i u m a n d Mare Neetaris, since t h e l a t t e r are t o p o g r a p h i c a l l y lower a n d y e t e x e r t more g r a v i t a t i o n a l effect t h a n P r o c e l l a r u m / N u b i u m . Thus, Phighlands ~ PProccllarum and .~ P l m b r i u m and rock 2qubium rock .Nectaris rock.

Mass densities h a v e not been m e a s u r e d for m o s t t y p e s of lunar r o c k ; c o m p u t e d values for model l u n a r rocks are shown in

T a b l e IV. (Note t h a t lunar plagioclase a t 1400°C is less dense t h a n norite liquid, t h o u g h n o t b y a wide m a r g i n ; this is consistent with the idea t h a t a plagioclase c u m u l a t e would float to the surface, n o t sink, as is usually the case in terrestrial basic s t r a t i f o r m bodies.) A n o r t h o s i t e is seen to be the lightest r o c k t y p e , followed b y norite, a n d t h e n m a r e b a s a l t (the d e n s i t y difference b e t w e e n Tranquillitatis a n d P r o c e l l a r u m basalts a p p e a r s to be insignificant). I propose t h a t these three rock t y p e s , respectively, fill the roles of three progressively less dense lunar m a t e r i a l s required b y l u n a r g r a v i t y a n d t o p o g r a p h y (inequalities above). This a s s i g n m e n t of roles is consistent w i t h the n a t u r e of Apollo samples r e t u r n e d to date. The basaltic c h a r a c t e r of l u n a r m a r i a is n o w established, so it is not unreasonable to p o s t u l a t e thicknesses of it in Mare I m b r i u m and Nectaris sufficient to account for positive gravitational anomalies (mascons). On the o t h e r hand, we m u s t suppose t h a t the basaltic layer in Oceanus P r o e e l l a r u m is thin, affecting t h e local value of g r a v i t a t i o n a l acceleration

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]~IG. 20. Two models of lunar subsurface structure that would reconcile the differences in topography and gravity shown in Fig. 19. Model 1 (above) assumes no relief on the crust mantle interface; model 2 (below) minimizes the amount of norite in the system. Basalt/mantle interfaces in model 2 are drawn arbitrarily and can be readjusted to suit the reader. m u c h less t h a n does a t h i c k layer of norite t h a t underlies it. T h e presence of a b u n d a n t norite in the Apollo 12 soils s u p p o r t s the idea t h a t this r o c k t y p e is n e a r at h a n d ; in addition, the Copernicus i m p a c t a p p e a r s to h a v e p e n e t r a t e d t h e b a s a l t l a y e r in Oceanus P r o c e l l a r u m a n d e x c a v a t e d norite, which was m e l t e d a n d ejected along t h e Copernicus r a y s ( H u b b a r d et al., 1971; M a r v i n et al., 1971). Finally, w i t h rare exceptions a n o r t h o s i t e has been f o u n d only in soils collected n e a r ancient h i g h l a n d t e r r a i n s [Apollo 11, L u n a 16 (Vinogradov, 1971)]. Using t h e f o r m u l a for g r a v i t a t i o n a l a t t r a c t i o n o v e r an infinite slab gz(mgal) = 4.19 pL, 2 2 Milligal is the unit of gravitational acceleration (equals 10 3em/see2).

(where L is t h e slab thickness in k m ) a n d mass densities of t h e three k e y r o c k types, it is possible to c o m p u t e a p p r o x i m a t e l y the thicknesses of r o c k units needed to a c c o u n t for the d i s p a r i t y in lunar g r a v i t y a n d t o p o g r a p h y , t h o u g h no unique model can be derived. T w o possible models of lunar s t r u c t u r e t h a t reconcile g r a v i t y w i t h t o p o g r a p h y are shown in Fig. 20. T h e u p p e r model m a k e s the simplifying a s s u m p t i o n t h a t t h e r e is no relief on the lunar " m o h o , " i.e., t h a t t h e interface b e t w e e n crustal rocks a n d the lunar m a n t l e (taken to h a v e a m a s s d e n s i t y of 3.3g/cm 3, the m e a n d e n s i t y of the Moon) is a spherical surface. I t also involves unspecified b u t s u b s t a n t i a l thicknesses of norite. This model is p r o b a b l y unrealistic because it involves g r e a t thicknesses of rocks t h a t are r e l a t i v e l y rich in radioa c t i v i t y (basalt: ~ 1 5 0 0 p p m K , ~ 0 . 5 p p m

492

JOHN A. WOOD lain b y norite as shown in model 1. A t the base of a 3 0 k m norite layer, t h e t e m p e r a t u r e after l09 years would be ~600 °. These t e m p e r a t u r e s are i m p o r t a n t because t h e y affect t h e s t r e n g t h of t h e Moon. T h e m a r i a t h a t contain m a s c o n s are out of isostatic equilibrium a n d h a v e been p r e v e n t e d f r o m sinking to an equilibrium level only b y the physical s t r e n g t h of the r o c k u n d e r l y i n g t h e m . T h e higher the t e m p e r a t u r e s are u n d e r the m a r i a (and therefore the higher t h e t e m p e r a t u r e s are in the m a n t l e beneath), the w e a k e r the s y s t e m is a n d the less likely t h a t the mascons would h a v e been s u p p o r t e d t h r o u g h m o s t of geologic t i m e (Urey, 1968). A model of lunar crustal s t r u c t u r e gains in plausibility b y minimizing t h e thickness of K , U, Th-rich r o c k in or u n d e r t h e m a s c o n m a r i a ; a m o r e credible model is shown in the lower h a l f of Fig. 20. H e r e it is a s s u m e d t h a t plastic flow in

U, ~ 2 p p m T h ; norite: ~ 5 8 0 0 p p m K , ~ 2 p p m U ( H u b b a r d et al., 1971), ~ 7 p p m T h (if T h / U is a s s u m e d to be t h e s a m e as in basalt). As G. J. W a s s e r b u r g (personal c o m m u n i c a t i o n ) has stressed r e p e a t e d l y , r a d i o a c t i v e d e c a y in such a s y s t e m would drive s u b c r u s t a l t e m p e r a t u r e s to v e r y high values. T h e r m a l histories at the base of a crustal l a y e r of norite, or m a r e basalt, or inert material, for layer thicknesses of 20, 40, a n d 60km, are displayed in Fig. 21. These a s s u m e a Moon t h a t is otherwise devoid of r a d i o a c t i v i t y , b u t which b e g a n a t a u n i f o r m initial t e m p e r a t u r e of ]200°C. T h u s we are able to see h o w m u c h difference basaltic or noritic r a d i o a c t i v i t y m a k e s in the cooling h i s t o r y of a l u n a r crust t h a t overlies an initially hot Moon. B e n e a t h 3 0 k m of m a r e basalt, t h e a p p r o x i m a t e a m o u n t required b y model 1 of Fig. 20, the t e m p e r a t u r e would s t a n d a t ~250°C after 109 years, or more if the b a s a l t were underi

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TERRA I~OCK I~RAGME~TS

the lunar mantle has forced plugs of m a n t l e material up into the mascon mare basins as suggested b y Wise and Yates (1970). Levels of r a d i o a c t i v i t y in the mantle would p r e s u m a b l y be low, so this is the best possible material with which to t r y to s u p p o r t the mascons. T h e d e p t h to the mare b a s a l t / m a n t l e interface cannot be deduced from gravitational considerations, since the mass densities of the two substances are essentially identical. The basalt thicknesses shown in model 2 were chosen arbitrarily. I f m a n t l e plugs rose to levels of isostatic equilibrium in the mare basins before basalt filling began, only ~ l k m of (superisostatic) basalt would have to be deposited to create the observed gravitational anomalies (mascons). The heating effect of r a d i o a c t i v i t y in such a thin layer of basalt would be negligible. Model 2 still involves a substantial thickness of norite b e n e a t h Oceanus 1)ro -

I~T A P O L L O

12 S O I L

493

cellarum (and, presumably, the other n o n m a s c o n maria). The high t e m p e r a t u r e s this would entail do the model no harm, however, since these areas do not have to have strength to support loads t h a t are out of isostatic equilibrium. Indeed, w a r m t h and weakness would make it easier to understand the r e m a r k a b l y constant elevation of the Oceanus P r o c e l l a r u m - M a r e N u b i u m plain (Fig. 18).

An Evolutionary Model A sequence of events is suggested in Fig. 22 t h a t would lead to the crustal s t r u c t u r e discussed above. The process ends (Stage 6) with a s t r u c t u r a l cross section t h a t is simply a more stylized version of model 2, Fig. 20. Stage 1. The fact t h a t rocks (anorthosite, norite, granite) which are older t h a n the mare basalts (from radiometric dating and a p p a r e n t geologic relationships) are igneous

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494

J O H N A. W O O D

differentiates, requires t h a t the Moon was at least partially melted in v e r y early times (within 1 to 2 × 10Syr of the origin of the solar system). The m i n i m u m fraction of the Moon required to be melted and differentiated to produce structure on the scale of Fig. 20 can be e s t i m a t e d from mass balance considerations, for a n y assumed overall lunar composition. E a r l y heating m a y have been due to kinetic energy deposited during v e r y rapid accretion, dissipation of tidal energy during close interaction with the E a r t h , electrical interactions with an early intense solar wind (T Tauri phase of the solar system), or absorption of r a d i a n t energy during a high-luminosity phase of the Sun's evolution. Most. of these h y p o t h e t i c a l processes would have c o n c e n t r a t e d their effect near the surface of the Moon. Tidal heating would be more intense at d e p t h in the Moon (Kaula, 1963), unless surface melting h'~d been initiated b y one of the other mechanisms; in this case, tidal energy dissipation would p r o b a b l y be concentrated in the surface m a g m a "oceans," possibly causing additional melting. Stage 1 in Fig. 22 assumes t h a t partial melting of the o u t e r m o s t layers of the Moon has generated a layer of gabbroie, possibly noritic, magma. Stage 2. The m a g m a system has cooled and largely crystallized; flotation of plagioclase crystals has produced an anorthosite layer at the surface, and marie minerals have sunk to form untramafic eumulate~. Between these, a residual layer of noritic liquid remains. Stage 3. Major planetesimal impacts have stripped a w a y the anorthositic layer locally (as in the present Oceanus Proeellarum and nonmaseon maria), and underlying noritic liquid has welled up to fill t h e m to levels of isostatic equilibrium. Obviously this involves some lateral movem e n t of noritic liquid, from u n d e r areas of i n t a c t anorthosite to the holes in the crust. Stage 4. The system has cooled and solidified completely; additional major impacts create mare basins (the present mascon mare basins) t h a t are not filled b y liquid. Stage 5. Dense mantle material, p r o b a b l y

still relatively w a r m and weak from the early h i g h - t e m p e r a t u r e stage of the Moon's outer layers or from energy deposited b y the recent basin-forming collisions or both, has forced its way up into the late mare basins, in an effort to a t t a i n isostatic equilibrium. This phase of lunar history was first suggested b y Wise and Yates (1970). The illustration for Stage 5 assumes t h a t mantle plugs rose to positions of isostatie equilibrium, and in fact t h a t the entire lunar landscape is isostatically equilibrated at this time. Note t h a t there are highlands and lowlands in spite of the state of isostasy ; the former are underlain b y low-density rock, the latter b y highdensity material. Stage 6. At a much later time the outer layers of the Moon have cooled and become stronger, while the interior of the Moon has been h e a t e d b y K, U, and T h decay. L a v a generated at d e p t h in the Moon has e r u p t e d on the surface. I t does not cover the lunar surface uniformly b u t fills the low places preferentially. Since the lunar surface was isostatically equilibrated before this latest lava eruption, the addition of lava at only a few localities on the surface must give rise to positive g r a v i t y anomalies (mascons) in those areas (the late mare basins). I t is necessary to suppose t h a t the subbasin mantle material had cooled sufficiently to be able to support these mascons once formed, and t h a t lava flowed into the basins in such a way (through onedimensional eonduits) as not to w a r m and weaken the basin floors significantly. This sequence of events, based on the preceding geophysical model of crustal structure, agrees with and embraces the model of petrological evolution discussed earlier in this section and pictured in Fig. 16. Regolith f o r m a t i o n (breceiation) would have occurred on anorthositie surfaces from Stage 2 onward, and on noritie surfaces during Stages 4 and 5. I t appears t h a t w h a t e v e r energy source melted the lunar surface m a g m a system in the first place did not halt a b r u p t l y b u t tapered off in intensity, so t h a t for some time after a solid surface layer had crystallized, it and its regolith were m a i n t a i n e d at high temperatures. At this time, m u c h regolith

TERRA ROCK FRAGME:NTS Ilq APOLLO

material was recrystallized to form the massive Ty p e A anorthositic and noritic breccias. I t is not difficult to picture accretional activity at the lunar surface, and heat generated by it, tapering off in this fashion. One apparent discrepancy between the two evolutionary models sketched in this section is th at massive norite underlies Oceanus Procellarum in Fig. 22, while in Fig. 16 this material has further differentiated. However, the differentiation involves only a vertical rearrangement of densities, so would produce no significant change in the value of gravity over Oceanus Procellarum. Thus, the differentiated structure shown under Oceanus Procellarum in Fig. 16 could have been specified in Figs. 20 and 22 instead of uniform norite, and on petrological grounds probably should have been. Again it is essential to recognize the high degree of simplification in this model and to be prepared for m a ny situations where the Moon has been affected in contrary and more complicated ways. For example, the (presumably anorthositic) highlands contain man y major craters filled with lightcolored plains-forming material (the Cayley Formation). I would interpret this as noritic lava th at flowed into the craters from beneath the anorthositic layer during Stages 2 and 3 of Fig. 22. Because of the very substantial percentage of highland surface t h a t consists of Cayley Crater fill, I predict th at gamma-ray measurements of K, U, and Th levels over the lunar highlands will reveal neither the low concentrations appropriate to lunar anorthosite nor the high concentrations t h a t would denote norite, but something in between. Since lunar norite is rich in certain trace elements (such as U, Ba, rare earth elements), the question of mass balance must be considered: does this postulated thick layer of norite at the lunar surface contain impossibly large amounts of these elements, amounts t h a t igneous differentiation could not have supplied even if the whole Moon's complement of U, etc. were channeled into this layer.+ Lunar norite contains ~400 times as much Ba and Ce and ~200 times as much Sm, Gd, and U as

12 S O I L

495

ehondritic meteorites (Hubbard and Gast, 1971). Although a 25km layer of norite is shown in model 2 of Fig. 20, this does not surround the Moon ; according to the model developed in this paper, such thicknesses of norite were created only where major impacts stripped away an overlying anorthosite layer and liquid norite flowed laterally from beneath the surrounding anorthosite layer to fill the hole. The mean thickness of the subcrustal norite layer can have been substantially less than 25 kin, probably less than 10kin. A 10kin surface layer comprises ~ 2 % of the volume of the Moon, so the Moon overall would have to contain at least eight times the chondritic levels of Ba and Ce and four times as much Sin, Gd, and U as chondrites, to generate such a layer. This, however, is probably not an unreasonable requirement. It is clear by now t h a t the Moon is not chondritic in composition; volatile elements are depleted in the moon, and refractory elements are present at considerably greater than the chondritic levels (LSPET, 1969). The elements we are concerned with (U, Ba, rare earths) fall in this latter category.

Generation of the Mare Basalts All t h a t we have learned from the Apollo samples has tended to confirm Baldwin's (1970) conception of the Moon as a planet t hat was once thermally active, and experienced extensive melting. The evolutionary model in Fig. 22 suggests t hat two great episodes of igneous activity occurred at the lunar surface: first, extensive melting and differentiation occurred at the time when the Moon was formed or soon after; and substantially later, an epoch of volcanic activity filled the mare basins with basaltic lava. Radiometric dating on the admittedly very limited suite of lunar samples collected to date tends to support this picture. Model ages for the lunar soils and for L u n y Rock I and the time of the first magmatic event involved in the history of Rock 12013 (Albee et al., 1970) fall in the range 4.4 to 4.6 × 109 yr; isochron ages of the basaltic rocks from the Apollo 11 and Apollo 12 samples are generally 3.4 to 3.6 × 109 yr. The "episodes" postu-

496

JOHN A. WOOD

lated were not thermal spikes, but spanned substantial periods of time ; however, there is as yet no evidence of extensive lunar igneous activity in the period between them (3.6 to 4.4 × 109yr), or earlier than 3.4 x 109yr ago. (Ages of the Apollo 1 4 rocks, which fall in the interval between "episodes," are discussed below.) Some insight into the thermal history of the Moon can be gained by carrying out model heat-flow calculations. Results of a series of such computations are shown in Figs. 23-25 (Wood, 1971). These figures do not show the detailed evolution of lunar temperature profiles, but only the times and places in the Moon where temperatures would have been above the melting point of basalt and where the lavas t h a t filled the mare basins might have been generated. Parameters used in the calculations are shown in Table V. Model A (Fig. 23) shows the appearance of a melting zone at depth (>500kin) in the simplest possible model, one t ha t starts off uniformly cold (0°C) and contains chondritic levels of radioactivity. Model B assumes higher initial temperatures, but is otherwise identical to A. (All energy of accretion of the Moon is assumed to be conserved as heat in B, and this accretional temperature is superimposed on the 0°C of initial temperature assumed for A. This results in highest temperatures in the outermost layers of the Moon, since the more massive the Moon grew, the more strongly (SURFACE)

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FLOW CALCULATIONS REPORTED IN FIGS. 23--25 Radial interval between computation points: 20kin T i m e i n t e r v a l b e t w e e n successive c o m p u t a t i o n s : 10~yr Surface t e m p e r a t u r e of t h e M o o n : 0°C T h e r m a l p r o p e r t i e s of l u n a r m a t e r i a l : h e a t c a p a c i t y , 1.2 joules/g °C l a t t i c e c o n d u c t i v i t y , 10 2cal/cm see °C m a s s d e n s i t y , 3.35 g / c m a opacity, 1 0 c m - I r e f r a c t i v e index, 1.7 T e m p e r a t u r e a t w h i c h b a s a l t b e g i n s to m e l t : l l 5 0 ° C (surface of Moon) 1627°C (center of Moon) :Present-day a b u n d a n c e s of r a d i o a c t i v e e l e m e n t s in t h e Moon ( c h o n d r i t i c levels) : K, 1000ppm U, 0.01 p p m Th, 0.04ppm

it attracted accreting objects and the more energetically t hey would have impacted. As seen in model B, 100% efficient retention of aecretional energy would have heated the outermost 500km of the Moon to above the melting temperature of basalt.) Model B shows t h a t the effects of an early epoch of surface melting, and later melting at depth due to radioactive decay, would have blended smoothly together. The net effect would have been a zone of I

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F I G . 2 4 . Melting diagrams for lunar m o d e l s in w h i c h m a g m a m i g r a t i o n redistributes r a d i o a c t i v i t y , Several different transfer efficiencies are tried, a n d several v a l u e s for c o m p l e t e n e s s of r e t e n t i o n o ! accretional energy. T h e s e m o d e l s d i s p l a y " h u m p s " 1.5 to 2 x 109 y r after the lunar origin, w h e n the z o n e of m e l t i n g rises in the Moon.

melting that migrated downward in the Moon. The depth to the top of the melting zone increases monotonically with time, and one supposes that lava would erupt to the surface less and less frequently. A resurgence of volcanic activity one billion years after the Moon was formed is not predicted. However, the above models omit one important aspect of the high-temperature behavior of planets. It is an accident of geochemistry that the heat-generating elements K, U, and Th tend to concentrate

in the first eutectic liquid that appears whet a rock begins to melt. If the melt is les~, dense than the residual solids, as is generally the case, and tends to be driver toward the surface of the planet by density differences, it carries part of the heatgenerating potential of the planet with it This can affect the thermal history of th( planet profoundly (Reynolds et al., 1966) This effect can be at least crudely imitated in heat-flow calculations. On( specifies in the program that, once a poinl in the Moon rises above the meltin~

498

A. W O O D

JOHN

temperature of basalt, the radioactivity (or some appropriate fraction of it) at that point is translated outward along the lunar radius until it reaches a point that is below the basalt melting temperature; there it is deposited. The heat-flow models in Fig. 24 include this effect. A matrix of models is shown, for several assumed values of efficiency of transfer of radioactivity and several percentages of aeeretional energy retained as heat. Interestingly, in most eases the melting diagram displays a "hump." The zone of melting rises closer to the surface about two billion years after the Moon was formed. This effect was first noted by McConnell et al. (1967), and is due to the accumulation of very high concentrations of radioactivity (released by O

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melting at depth in the Moon) at the top of the melting zone. It seems plausible that this temporary rise in the boundary of the melting zone would be accompanied by increased volcanic activity at the hmar surface. B y assuming higher initial temperatures (Fig. 25) or higher levels of radioactivity in the Moon, interior melting can be made to occur sooner, bringing radioactivity to the top of the melting zone at an earlier date. In this way the hump in the melting zone can be shifted to a position only one million years after the formation of the Moon, in correspondence with the episode of volcanic activity that filled the lunar maria. The use of ehondritic proportions of radioactive elements in computing the 200 °

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Fro. 25. C o m p a r i s o n o f m o d e l G (Fig. 24) w i t h m o d e l s t h a t s t a r t at h i g h e r b a s e i n i t i a l t e m p e r a t u r e s b u t are o t h e r w i s e s i m i l a r to G. N i g h e r b a s e initial t e m p e r a t u r e s c a u s e t h e " h u m p " to o c c u r earlier. F i g s . 23 25 f r o m W o o d (1971).

TERRA ROCK FRAGMENTS I ~ APOLLO

above heat-flow models is probably very unrealistic (G. J. Wasserburg, personal communication), but experimental runs using other elemental proportions have shown t h a t this is relatively unimportant. The absolute amount of heat-generating potential in the system is what counts, and a level of K + U + Th (in the true lunar proportions) can be found t h a t produces melting diagrams closely approximating those of Figs. 23-25. These melting diagrams seem to indicate t h a t the lunar interior is molten at present. The actual meaning of the diagrams is t h a t temperatures are above the melting point of basalt, but this would promote only partial melting of the lunar material; and it is not unreasonable to assume that most or all of the liquid migrated surfaceward, leaving hot but solid refractory minerals in the lunar interior. Even so, the final internal temperatures of the Moon are at odds with those deduced for the lunar interior from interactions of the Moon with the interplanetary magnetic field (Sonnett et al., 1971), unless solid-state convection has operated to cool the lunar interior more efficiently than the conductive and radiative heat transfer that were assumed for the heat-flow calculations reported above (Turcotte and Oxburgh, 1969). It is worth noting that lava erupted at the lunar surface in connection with the appearance of a " h u m p " in the melting diagram would not have had a simple history. I t would not have been partially melted from virgin lunar material and translated straightway to the lunar surface. Magma generated in the lunar interior would have solidified at the top of the melting zone, to be subsequently remobilized when the local radioactivity concentration and temperature grew high enough. The evolutionary history of mare basalts was at least this complicated, a fact t h a t needs to be kept in mind when constructing models of trace -element fractionation. APOLLO 14 AND TH]~ F R A MACRO FORMATION: PRELIMINARY OBSERVATIONS

Geologic considerations indicate that the Fra Mauro Formation is a deposit of mare

12 SOIL

499

basin e]ecta, excavated from 1 to 5km beneath the lunar surface by the monumental impact t h a t created the Imbrium basin (Wilhelms, 1965). The Apollo 14 mission was designed to provide samples of this deep crustal material. Everything about the nature of the Apollo 14 samples is consistent with such an origin. The great majority of them are breceias: aggregations of broken rock fragments, exactly the type of debris one would expect a major cratering event to produce. All but a few of the rocks are noritic in composition. Some have straightforward igneous textures, but most are aggregations of fragments each of which is superficially similar to the Type A breccias of Apollo 12 (compare Fig. 15b with Fig. la). However, detailed microscopic examination reveals an important difference. Pyroxene crystals in the matrices of virtually all Apollo 14 Type A breccias tend to be euhedral (Fig. 15c) rather than anhedral (Fig. lc), a distinction t h a t probably means the Apollo 14 breccia matrices were largely or completely remelted, though they may have remained liquid for only a very short time. Compositions of the Apollo 14 matrix pyroxenes are consistent with such a history; they tend to have intermediate contents of Ca rather than a bimodal distribution of Ca contents (low-Ca orthopyroxene and pigeonite vs. high-Ca augitcs), such as are found in Apollo 12 norites (Fig. 5). Intermediate Ca contents are characteristic of high-temperature (igneous) pyroxcnes ; while polarized Ca contents are found in lower temperature pyroxenes, such as would be created during solid state reerystallization. The fact that angular clasts are preserved in the Apollo 14 breccias, and have not been absorbed by the melts t h a t surrounded them, indicates that the melting event was transitory, as would be the case if it had been associated with the explosive excavation and ejection of the rock from the Imbrium basin. The Apollo 14 rocks t h a t have been dated are surprisingly young, 3.8-3.9 × 109 years old (Wasserburg, 1971). This age falls between the igneous "episodes" discussed earlier. It could mean t h a t

500

JOHN A. WOOD

igneous activity on the Moon was not episodic after all, but continuous ; and t h a t the lunar norites in general are not the immediate products of melting that occurred when the Moon was formed (as is suggested by Fig. 22), but had instead a l e n g t h y and complicated geologic history. On the other hand, it is possible t h a t 3.8-3.9 × 109 years old is nothing more than the date of the Imbrium impact, and t h a t this event heated and remelted the ejected fragments to such an e x t e n t that their radiometric clocks were "reset." In this case, the chemical fractionation that created the norites could still have occurred soon after the formation of the Moon. As noted above, the textures and mineralogy of Apollo 14 breccias do seem to indicate t h a t t h e y were briefly remelted. The fact t h a t Apollo 14 soil consists largely of norite, not anorthosite, appears to mean that the Imbrium impact occurred in terrain t h a t was akin to (perhaps an extension of) the Oceanus Procellarum basin, n o t in a highlands area. ACKNO~'LEDGMENT This work was supported in part by a grant from the NASA Apollo Lunar Research Program, NGL 09-015-150. REFERENCES ALBEE, A. L., BURNETT, D. S., CHODOS, A. A. I~AINES, E. L., ~IUNEKE, J. C,, PAI°ANASTASSlOU, D. A., PODOSEI~, F. A., RUSS, G. P., AND VCASSERBURG, G. J. (1970). Mineralogie and isotopic investigations on lunar rock 12013. E a r t h Planet. Sci. Lett. 9, 137-163. ALBEE, A. L., AND CHODOS, A. A. (1970). Microprobe investigations on Apollo 11 samples. Proc. Apollo l l L u n a r Sci. Conf., Geochim. Cosmochim. A c t a S u p p l . 1, 135-157. ANDERSON, A. T., JR., AND SMI~H, J. V. (1971). Nature, occurrence, and exotic origin of "gray m o t t l e d " (Luny Rock) basalts in Apollo 12 soils and brcccias. Proc. Second L u n a r Sci. Conf. 1,431-438. The M.I.T. Press. BALD~,VIN, R. B. (1970). Summary of arguments for a hot Moon. Science 170, 1297 1309. BOTTINGA, Y., AND ~VEILL, D. F. (1970). Densities of liquid silicate systems calculated from partial molar volumes of oxide components. A mer. J . Sci. 269, 169-182. CLARK, S. P., Jiz. (1966). " H a n d b o o k of Physical Constants." Geol. Soc. A m e r . Mere. 97.

DEER, W. A., HO~VIE, R. A., AND ZUSSMAN, J. (1963). "Chain Silicates." Vol. 2 of "RockForming Minerals." Longmans, Green, London. DRAKE, M. J., McCALLU~, I. S., MCKAv, G. A., AND VVEILL, D. F. (1970). Mineralogy and petrology of Apollo 12 sample no. 12013: a progress report. E a r t h ]~lanet. Sci. Lett. 9, 103-123. FucHs, L. H. (1970). Orthopyroxcne-plagioclase fragments in the lunar soil from Apollo 12. Science 169, 866-868. GAULT, D. E., SItOE~IAKER, E. M., AND MOORE, H. J. (1963). Spray ejected fl'om the lunar surface by meteoroid impact. NASA Tech. Note D-1767, National Aeronautics and Space Administration, V~'ashblgton, D.C. HUBBARD, N. J., AND GAST, P. ~V. (1971). Chemical composition and origin of nonmaro lunar basalts. Proc. Second L u n a r Sci. Conf. 2, 999 1020. The M.I.T. Press. HUBBARD, N. J., MEYER, C., JR., GAST, P. W., AND WE1SMANN, H. (1971). The composition and derivation of Apollo 12 soils. E a r t h Planet. Sci. Lett. 1O, 341-350. KAULA, ~V. M. (1963). Tidal dissipation in the Moon. J . Geophys. Res. 68, 4959 4965. KING, E. A., JR., CARMAN,M. F., AND BUTLER, J. C. (1970). Mineralogy and petrology of coarse particulate material from t h e hmar surface at Tranquillity Base. Proc. Apollo 11 L u n a r Sci. Conf. LSPET, (1969). Preliminary examination of lunar samples fl'om Apollo 11. Science 165, 1211 1227. MARVIN, U. B., WOOD, J. A., TAYLOR, G. J., REID, J. B., JR., PO~,VELL, B. N., DICKEY, J. S., JR., AND BOWER, J. F. (1971). Relative proportions and probable sources of rock fragments in the Apollo 12 soil samples. Proc. Second L u n a r Sci. Conf. 1, 679 699. The M.I.T. Press. MASON, B., NELSON, ~V. G., HENDERSON, ]~]. P., JAROSEWICH, E., AND NELEN, J. Mineralogy m~d petrology of some Apollo 12 samples. (1971). Apollo 12 L u n a r ~b'ci. Co~f. (unpublished proceedings). McCONNELL, R. S . , MCCLAINE,L. A., LEE, D. W., ARONSON, J. R., AND ALLEN, R. V. (1967). A model fi)r planetary igneous differentiation. Rev. Geophys. 5, 121-172. MEYER, C., JR., BRETT, ]:~., HUBBARD, N. J., MORmSON, D. A., McKAY, D. S., AI~KEN, F. K., TAKEDA, H., AND SCHONFIELD, U. (1971). Mineralogy, chemistry, and origin of the K R E E P component in soil samples from t h e Ocean of Storms. Proc. Second L u n a r Sci. Conf. 1,393 411. The M.I.T. Press.

TERRA ROCK FRAGMENTS IN APOLLO 12 SOIL MEYER, D. L., AND RUFFIN, B. W. (1965). Coordinates of lunar features : Group I and I I solutions. Aeronautical Chart and Information Center Tech. Paper No. 15. MOORE, C. B., LEWIS, C. F., AND NAVA, D. (1969). Superior analyses of iron meteorites. I n "Meteorite Research" (P. M. Millman, Ed.), pp. 738-748. Springer-Vcrlag, New York. MULLER, P. M., AND SJOGREN, W. L. (1969). Lunar gravity map. Distributed at Conference on Recent Development in Lunar Studies, Goddard Institute for Space Studies, New York. O'KEEFE, J. A. (1968). Isostasy on the Moon. Science 162, 1405-1406. PATTERSON, J. H., TURKEVICH, A. L., FRANZGROTE, E. J., ECONOMOU, T. E., AND SOWINSKI,K. P. (1970). Chemical compositions of the lunar surface in a terra region near the crater Tycho. Science 163, 825 828. REYNOLDS, R. T., FRICKER, P. E., AND SUMMER, A. L. (1966). Effects of melting upon thermal models of the earth. J. Geophys. t~es. 71, 573-582. ROEDDER, E., AND W~EIBLEN, P. W. (1970). Lunar petrology of silicate melt inclusions, Apollo 11 rocks. Proc. Apollo 11 Lunar Sci. Conf. Geochim. Cosmochim. Acta, Suppl. 1, 801 837. RUNCORN, S. K., AND SHRUBSALL, M. H. (1968). The figure of the Moon. Phys. Earth Planet. Interiors 1, 317. SHOEMAKER, E. M., HAIT, M. H., SWANN, G. A., SCHLEICHER, D. L., SCHABER, G. G., SUTTON, R. L., DAHLEM, D. H., GODDARD, E. N., AND WATERS, A. C. (1970). Origin of the lunar regolith at Tranquillity Base. Proc. Apollo 11 Lunar Sci. Conf. Geochim. Cosmochim. Acta, Suppl. 3, 2399-2412. SMITH, J. V., ANDERSON, A. W., NEWTON, R. C., OLSEN, E. J., WYLLIE, P. J., CREWE, A. V., ISAACSON, M. S., AND JOHNSON, D. (1970). Petrologic history of the moon inferred from petrography, mineralogy, and petrogenesis of Apollo 11 rocks. Proc. Apollo 11 Lunar Sci. Conf. Geochim. Cosmochim. Acta, Suppl. 1, 897-925.

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SONETT, C. P., SCYIUBERT, G., SMITH, B. F., SCHWA~Z, K., AND COLBURN, D. S. (1971). Lunar electrical conductivity from Apollo 12 magnetometer measurements : compositional and thermal inferences. Proc. Second Lunar Sci. Conf. 3, 2415-2431. The M.I.T. Press. TURCOTTE, D. L., AND OXBURGH, E. R. (1969). Implications of convection within the Moon. Nature 223, 250-251. UREY, H. C. (1968). Mascons and the history of the Moon. Science 162, 1408 1410. VINOGRADOV, A. P. (1971). Preliminary data on lunar ground brought to E a r t h by automatic probe "Luna-16." Proc. Second Lunar Sci. Conf. 1, 1-16. The M.I.T. Press. WASSERBURG, G. J. (1971). Ages and irradiation history of Apollo 14 site. Paper presented at 14th Annual COSPAR Meeting, Seattle, U.S.A., J u n e 18-July 2. VVILHELMS, D. E. (1965). F r a Mauro and Cayley ]?ormations in the Mare Vaporum and Julius Caesar quadrangles. Astrogeol. Stud. Annu. Progr. Rept., J u l y 1964-July 1965, Pt. A: U.S. Geol. Survey open-file report, pp. 1328. W'ISE, D. U., AND YATES, M. T. (1970). Mascons "~s structural relief on a lunar "Moho." J. Geophys. Res. 75, 261-268. WOOD, J. A. (1970). Petrology of the lunar soil and geophysical implications. J. Geophys Res. 75, 6497-6513. WOOD, J. A. {1971). Thermal history and early magnetism in the Moon. Preprint; submitted to Icarus. WOOD, J. A., DICKEY, J. S., JR., MARVIN, V. B. AND P OWELL, B. N. (1970). Lunar anorthosite., and a geophysical model of the Moon. Proc Apollo 11 Lunar Sci. Conf. Geochim. Cosm ochim Acta, Suppl. 1, 965 988. WOOD, J. A., MARVIN, U. B., REID, J. B., JR. TAYLOR, G. J., BOWER, J. F., POWELL, n . N. AND DICKEY, J. S., JR. (1971). Mineralog~ and petrology of the Apollo 12 lunar sample Smithsonian Astrophys. Obs. Spec. Rept No. 333.