Aspects of a glassy meteorite from the moon bearing on some problems in extraterrestrial glass-making

Aspects of a glassy meteorite from the moon bearing on some problems in extraterrestrial glass-making

Journal of Non-Crystalline Sohds 67 (1984) 383-396 North-Holland, Amsterdam 383 Sectton VI. L u n a r a n d other rare glasses ASPECTS OF A GLASSY...

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Journal of Non-Crystalline Sohds 67 (1984) 383-396 North-Holland, Amsterdam

383

Sectton VI. L u n a r a n d other rare glasses

ASPECTS

OF A GLASSY

METEORITE

FROM

O N S O M E P R O B L E M S IN E X T R A T E R R E S T R I A L R.F. FUDALI

THE MOON BEARING GLASS-MAKING

and Melame KREUTZBERGER

NHB- 119, Smithsoman Instttution, Washington D C 20560, USA

Gero KURAT

and Franz BRANDST.~TTER

Naturhtstortsches Museum, Postfach 417, A- 1014 Vwnna, Austrta

Antarctic meteorite ALHA 81005 is a glassy piece of lunar highland regohth breccia. Not only is it a unique meteorite but the glasses contained thereto probably come close to spanning the PT range that lunar tmpact-generated glasses have experienced. The glasses that formed near the upper end of this range are particularly instructive. They are optically and chenucally homgeneous, contain no bubbles or rehct nunerals and are perfectly colorless (though there is no obvious chenucal reason why this should be so) Color aside, these are features that have been previously attributed to lunar volcanic glasses. They are also characteristic of most tektites and have been offered as evidence agmnst an impact ongm for these important glasses and, by default, for a lunar volcamc ongm Both these posttions are no longer tenable All the criteria used to distmgmsh lunar volcamc from lunar impact glasses deserve re-evaluatmn And the tenaceously held theory that tektites are products of recent, hyper-explosive lunar volcanism should (but will not) be abandoned

1. Introduction S p e c i m e n A L H A 81005 was s e r e n d i p i t o u s l y d i s c o v e r e d o n t h e M i d d l e W e s t e r n Ice F i e l d , 30 k m w e s t o f A l l a n Hills, S o u t h V i c t o r i a L a n d , A n t a r c t i c a d u r i n g t h e v e r y last s n o w m o b i l e r e c o n n a i s s a n c e o f the 1 9 8 1 - 1 9 8 2 A n t a r c U c s e a r c h for m e t e o r i t e s ( A N S M E T ) . Its white, a n g u l a r clasts set in a j e t b l a c k matrix and partial, light-green, translucent fusion crust immediately caught the a t t e n t i o n o f t h e A N S M E T t e a m in t h e field a n d f l a g g e d it as a v e r y u n u s u a l m e t e o r i t e (fig. 1). A n d i n d e e d it has t u r n e d o u t to b e n o t m e r e l y u n u s u a l b u t a b s o l u t e l y u n i q u e a m o n g all m e t e o r i t e s f o u n d to date, b o t h in A n t a r c t i c a a n d elsewhere. Concerted efforts by a number of investigators have convincingly d e m o n s t r a t e d t h a t 81005 is a p i e c e o f t h e M o o n - m o r e s p e c i f i c a l l y it is a l u n a r h i g h l a n d r e g o l i t h b r e c c i a [ 1 - 5 ] a n d a s o u r c e o f s o m e e m b a r r a s s m e n t to t h o s e w h o h a d " p r o v e n " it was d y n a m i c a l l y i m p o s s i b l e to a c c e l e r a t e l u n a r s u r f a c e m a t e r i a l to l u n a r e s c a p e v e l o c i t y w i t h o u t v a p o r i z i n g it. 81005 is a glassy m e t e o r i t e - b r o w n glass m a t r i x c o m p r i s e s o v e r 50% o f the s e c t i o n s w e h a v e e x a n u n e d . A d d i t i o n a l l y , p e r f e c t l y d e a r , c o l o r l e s s glass o c c u r s b o t h as a n g u l a r f r a g m e n t s a n d as s p h e r u l e s a n d e l l i p s o i d s in t h e m a t r i x . A l l this glass is d e m o n s t r a b l y o f i m p a c t origin. T h e c h a r a c t e r i s t i c s o f t h e s e glasses 0022-3093/84/$03.00 © Elsevier Sctence Pubhshers B V. (North-Holland Physics Publishing Division)

384

R F Fudah et al / A glassy meteorste from the moon

Fig. 1. M e t e o n t e A L H A 81005. C u b e ts o n e c e n t t m e t e r o n a s~de.

bear on the problem of recognizing volcanic versus impact-generated glasses in the lunar regolith. Further, 81005 is a microcosm that has relevance to a well-known macroproblem - the homogeneity of the most important terrestrial, impact-generated glasses (tektites).

2. Optical and microprobe data The brown matrix glass in 81005 has clearly been formed in-situ by the partial melting of the constituent minerals in the meteorite. Under the microscope this glass is seen to be intimately intermingled with and invasive of the surviving minerals; it contains small relict mineral fragments and tiny bubbles; and in many places it has a strongly developed flow texture (fig. 2). Microprobe analyses show it to be inhomogeneous from spot to spot on a micron scale. Even the most ardent advocate of lunar volcanic glasses would classify the brown matrix glass as an impact glass. Conversely the clear glasses do not display any of these signatures of impact-generation. The angular fragments, spherules and ellipsoids are optically featureless, show no signs of relict minerals or bubbles and, for the most part, are sharply outlined against the matrix. They were obviously incorporated into the regolith as solid bodies and have since remained so. However, virtually all of them do show some signs of reaction with the matrix. These include some corrosion of the clear glass boundaries, minor invasion of extremely fine-grained

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A glassy meteorite from the moon

385

Fig. 2(a, b). Brown, matnx glass showing well-developed flow texture and contmnlng tiny bubbles and rehclt minerals. Long dimension of photormcrographs are 0 6 mm.

material across boundaries and along cracks in the glass, and the development of narrow, light b r o w n glass rinds (fig. 3). Thus this regolith breccia must have been s h o c k - h e a t e d and indurated subsequent to the arrival of the clear glass fragments and spherules/ellipsoids; in the simplest of possible histories this

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R F Fudah et al / A glassy meteorite from the moon

s u b s e q u e n t shock e v e n t w o u l d also b e r e s p o n s i b l e for the f o r m a t i o n of the b r o w n glass. T h i r t y - f o u r clear glass f r a g m e n t s a n d eight s p h e r u l e s / e l l i p s o i d s i n o n e t h i n

F~g. 3. (a) Clear glass fragment (0.54 mm m longest dimension) showing manor corrosion and some mvaswe material along boundaries and cracks. (b) Clear glass spherule (0.3 mm diameter) showing manor corrosion and the development of a hght brown zone just reside the glass surface.

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387

section were analyzed wtth an electron microprobe. Additionally, eighty points in the brown matrix glass were analyzed. The instrument used was a nine spectrometer, ARL microprobe, model SEMQ, utdizing an accelerating potentml of 15 kV, a beam current of 0.15 /tA, a one micron spot size and a 10 s count. Internal standards and updated Bence-Albee procedures [6] were used to correct the raw data. Standards used were an analyzed basalt glass, mlcrochne (for K) and chromite (for Cr) - USNM Nos. 11240/52, 143966 and 117075 respectwely [7]. Both the brown glass and the clear glass show a wide range m major element composition. A major distinction is that the brown glass is inhomogeneous from point to point while the clear glass fragments and spherules/elhpsoids are, individually, homogeneous. Representative analyses are shown in table 1. The elemental ranges of the 81005 glasses match those of the high alumina glasses found m the soils - sometimes in considerable amounts - at most of the lunar sites sampled to date. For example, Reid et al. [8] report glasses of these compositions constitute 17-33% of the glasses found in the soils returned by Apollos 11, 12 and 14 and Lunar 16. It is generally agreed that such glasses formed from anorthositic highland units but there is considerable reluctance on the part of others to draw any conclusions beyond this point. We experience no such reluctance. We cannot visualize either primary or derivative volcanic hquMs with these compositions. The 81005 glasses have cumulate composmons. As such the clear glasses could only have been produced in the same manner as the brown glasses; 1.e. by impact into a crystalline target. Rather surprisingly, there is considerable overlap in the elemental ranges displayed by both glass types and some clear glasses and brown glasses are essentially isochemacal (e.g. analyses 1 and 2 in table 1). All the glasses are strongly depleted in the alkali elements. In the case of the brown glasses, which are unlikely to have suffered unusually high temperatures, this alkali depletion is s~mply a reflection of the (cumulate) parent rock chemistry (the alkalis being partitioned into the residual liqmd). Many of the clear glasses, however, have virtually no Na and K. This can reasonably be attributed to vapor fractionation of a low-alkali target under more extreme shock-melting conditions. The most highly depleted have not only lost virtually all their Na and K but also a fair amount of S1. For example, the spherule composition shown m analysis no. 4 (table 1) has been so depleted in S~ that three different computer programs were unable to calculate a rational norm for it (the best of the three calculated 9% kalsilite even though there is essentially no K in the analysis!). The two glass types occupy distinct, linear-trending fields on variation diagrams plotting the refractory elements (fig. 4). The trends are parallel and the degrees of overlap between the two types are well-displayed. The glasses are characterized by strong positwe correlations between A1 and Ca and between Mg and Fe (with somewhat more scatter of the extreme analyses outside the main envelope of points). Si, the only other element present in major amounts, shows no correlation with any other element. The good

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c o r r e l a t i o n s a m o n g refractory elements a n d the lack of such correlations with m o r e volatile elements has previously been n o t e d for l u n a r i m p a c t glasses [9]. T h e o n l y negative c o r r e l a t i o n p l o t t e d herein is that b e t w e e n C a a n d Mg. But we r e m i n d the r e a d e r that the strong positive correlations d i s p l a y e d d e m a n d that there also be g o o d negative c o r r e l a t i o n s between M g a n d A1 a n d b e t w e e n F e a n d b o t h C a a n d A1. T h e m o s t s t r a i g h t f o r w a r d e x p l a n a t i o n for these o b s e r v e d correlations ( b o t h positive a n d negative) is that the clear glass in 81005, as well as the b r o w n glass, was p r o d u c e d b y a h y p e r v e l o c i t y i m p a c t i n t o a crystalline target d o m i n a t e d b y o n l y two m i n e r a l s - a C a - A 1 silicate a n d an M g - F e silicate. C o n s i s t e n t with this e x p l a n a t i o n , the c a l c u l a t e d n o r m s for the glasses are d o m i n a n t l y ( b u t n o t entirely) a n o r t h i t e a n d c l i n o p y r o x e n e o r olivine. A n interesting, a n d h e r e t o f o r e u n r e m a r k e d , aspect of the glasses in 81005 ( a n d also o t h e r l u n a r glasses) is that some are d e e p l y c o l o r e d a n d some are entirely colorless even t h o u g h they are essentially isochemlcal *. F u r t h e r , s o m e * To be sure we are only viewing the clear glasses m tlun sectaon. But it is obvious they would show httle if any color with increasing tbackness because 81005's fusion crust, which is only lightly colored in hand specimen still shows some color in the same than section.

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of the clear glass is higher, by a factor of 2 to 3, m Fe - the obvious candidate element for the coloring agent in the brown glass. The brown and clear glasses in 81005 are all impact glasses and were undoubtedly produced from crystalhne targets with very similar llthologies. Why, then are they so different? We suggest the answer lies in the far greater seventy of the shock event experienced by the clear glasses. Specifically we suggest the clear glasses were subjected to temperatures high enough to reduce most or all of the Fe 2+ in the target rock to Fe °, which is uniformly dispersed throughout the glass in metal droplets only tens to hundreds of angstroms in diameter. The brown rinds, that some clear glass spherules and fragments exhibit, resulted from the re-oxidation of this metallic iron during the subsequent, milder thermal event that was responsible for the brown matrix glass. A recent ferromagnetic resonance and SEM study [10] of lunar glass beads has demonstrated that metallic Fe in tiny droplets is indeed present in many glass beads of impact origin. However, the details of that study, which utilized variously colored and colorless beads, are not obviously supportive of the above speculations. We hope to have additional evidence bearing on this problem shortly.

3. Lunar volcanic glasses Early in the Apollo program, wrtually all of the glasses found on the lunar surface were considered to be of impact origin. Subsequently a number of workers characterized Apollo 17 orange glass and Apollo 15 green glass as volcanic and postulated an explosive, fire-fountaining mechanism for their formation (e.g. [11-13]). This spurred a re-examination of the glasses from all the Apollo sites and, currently, Delano and co-workers claim to have identified 23 different volcanic glass groups in Apollo 11, 14, 15, 16 and 17 soils [14,15]. The concept of lunar volcanic glasses ~s very appealing to selenologists, especially in view of the frustrations encountered in trying to identify primary lunar magmas (magmas whose compositions have not changed subsequent to separation from their source regions) among the lunar crystalline rocks [16]. As Delano and Livi [14] have stated: "Since it is generally agreed that lunar volcanic glasses are the result of explosive fire-fountaining.., there is reason to believe that the magmas ascended rapidly from their source regions m the lunar mantle with little opportunity for significant heat loss and crystal/liquid fractionation... [and thus] may reflect compositional characteristics of the deep lunar interior." This is an exciting hypothesis that should, however, be approached with great caution. At the very least, verification of the hypothesis is critically dependent upon being able to distinguish the volcanic glasses from impact glasses (which simple common sense tells us must greatly predominate in the lunar regolith) with some certainty. The 81005 clear glasses illustrate the need for such caution. They are optically featureless, without relict minerals and

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R F Fudah et al / A glassy meteorite from the moon

chemically homogeneous. Heretofore, these characteristics have been considered diagnostic of lunar volcanic glasses but the 81005 glasses are demonstrably of impact origin. A further cautionary note is introduced when the other "diagnostic" features of lunar volcanic glasses are examined. Table 2 lists the principal criteria that have been used to distinguished between lunar volcanic and impact glasses. In light of the 81005 clear glasses, criteria nos. 1 and 2 are obviously no longer valid. We have not measured Ni in these glasses but the other elements are so uniformly distributed that we would be very surprised not to find Ni uniformly dispersed too. Moreover, the presumption underlying this distinction is that the Ni is a randomly added meteoritic contaminant. There is no good reason this need be so. If the Ni in the glass is due solely to the target material its intraspherule distribution may be as uniform as are the major element distributions. No. 4 is a negative crtterion. While the presence of a meteoritic component is good evidence against a volcanic origin, the absence of such a component is not evidence for a volcanic origin. We do not understand no. 5 - if there are areas on or near the lunar surface with M g / A I ratios > 1.5, and there is orbital ewdence that such areas are widespread [17], then there must be glasses with Mg/A1 > 1.5 that have formed from impacts into these units. Similarly we do not understand why the concentration of a large number of exotic, volatile elements on the glass surfaces precludes an impact origin. On the contrary, vapor fractionation and subsequent surface condensation seems a

Table 2 Some suggested criteria for dlstmgmshlng lunar volcamc glass from lunar impact glass Volcanic glass characteristics

Impact glass characteristics

1. Absence of schheren and exotic inclusions 2. Intra-sphere chemical homogenetty 3 U m f o r m Nt distribution 4. < 2 × 10 - 4 u m e s c h o n d n u c abundance of any of the refractory siderophfle elements (e.g. Au, Ir) 5. Hagh M g / A I ratto (generally > 1.5) 6. Surface concentrations of volatde elements 7. Clustenng of inter-sphere chemical analyses

presence of schheren and exotic inclusions mtra-sphere chemical heteorgenetty non-uniform N1 d~stnbutlon > 2 × 10 - 4 Umes chondntlc abundances etc.

or

Element correlation trends indicative of crystal fractlonatlon or partml melting

low M g / A I ratio (generally < 1.5) no surface concentrations of volatile elements random spread of rater-sphere chermcal analyses or preservation of the refractory element correlations in the target or vapor fract~onat~on trends

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393

plausible mechanism to account for such volatile concentrations. In any case only a few undisputed impact glass surfaces have been examined for the presence or absence of exotic volatile elements to date. Criterion no. 7 is interesting. Twelve of the reputed 23 volcanic glass groups exhibit inter-spherule clustering [18]. But tight lnter-spherule clustering on variation diagrams is also occasionally observed in undisputed impact glass populations (see, for example ref [14] fig. 2). The other eleven reputed volcanic groups exhibit systematic Inter-sphere chemical variations that are attributed to the effects of either fractional crystallization, (e.g. ref. [14]) or partial melting (e.g. ref. [19]). It may be difficult to distinguish such variations from the variations known to occur in impact glass populations. But a much more serious objection to this criterion is that it appears, in fact, to be incompatible with a volcanic origin. Those who have advanced It have either forgotten or ignored the crucial fact that the observed chemical variations are found in submillimeter glass spherules in small soil samples; m many cases in the same thin sections. Clearly these spheres had to be closely related, m time and space, at their source. We can see no way that crystal fractionation, partial melting, or any other conceivable mechanism of differentiation could generate or maintain these intimately associated millimeter to submillimeter regions of variable chemistries in an all-liquid magma. Such systematic inhomogenelties are, however, an inevitable consequence of crystallization to a polymineralic rock. If such a target rock is shock-melted and quickly disrupted, the inter-sphere variations are not surprising. (As a corollary, if, in some cases, the shock-melts are not so quickly disrupted, then inter-sphere chemical clustering is conceivable.) It remains to explain the dispersion of minor elements, such as Ti and Cr in the 81005 glasses, into all the impact glass if they were only present in rare accessory minerals in the crystalline target. A reasonable answer is that these elements were present both in rare grains that occasionally perturb the chemistry of small spheres (e.g. analysis no. 6, Table 1) a n d as impurities in the lunar pyroxenes, (see, for example, the analyses of lunar pyroxenes in ref. [16]) so they were ubiquitously distributed, at a low concentration, prior to impact melting. In sum, we do not believe that the criteria developed to date can clearly distinguish between lunar volcanic and impact glasses and that, therefore, the case for lunar explosive fire fountainxng as a glass-making process has yet to be convincingly made.

4. Origin of tektites The problem of the origin of tektites has been an intriguing one. Four distinct strewnfields containing macro and micro glass bodies whose total mass is estimated to be billions of tons have been emplaced on the Earth's surface during the past 35 million years. The chemical evidence is compellingly in

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favor of large meteorites impacting terrestrial sedimentary units. However, the physical evidence appears to preclude an impact origin. Basically the physical argument advanced by O'Keefe [20,21] is a negative one - that, because of the very short times necessarily involved, it is impossible to homogenize and fine (remove the bubbles) glasses made by shock-melting polymmeralic, watersaturated targets. Since most tektites are both homogeneous and water-free, they cannot be of impact origin. O'Keefe concludes, by default, that tektites must therefore be the products of ultra-explosive lunar volcanism. This argument collapses if we find indisputable impact glasses that have been homogenized and fined. Such are the clear glasses in 81005. They are homogeneous and, compared to the bubble-laden 81005 brown glass (whatever caused the bubbles - obviously water is not a prime candidate), they have been fined. Clearly there is much we do not yet understand about the conditions attendant upon hypervelocity impact events. Two objections may be raised to a direct comparison between the 81005 clear glasses and tektites. The 81005 glass spheres and fragments are much smaller and less siliceous than tektites, leading to shorter diffusion distances and faster diffusion rates than in tektites, all else being equal. But, as O'Keefe [20] has pointed out, if the target is a reasonably fine-grained sediment, diffusion distances of only a few millimeters may be sufficient to homogenize the melt. And most of the 81005 clear glasses are fragments of formerly larger pieces. So necessary diffusion distances for homogenization of these glasses and tektites may be reasonably comparable. Further, calculated viscosity (diffusion rate-controlling) differences are not that large (table 3). The 81005 glass is 2 (plus) orders of magnitude less viscous at 1400 o and only one (plus) order of magnitude lower at 2000 o C. This is a consequence of the asymptotic nature of the viscosity versus temperature curves and, if these calculations are approximately correct, the viscosity differences should continue to narrow at temperatures beyond 2000 °C. We have not attempted calculations beyond 2000 o C because the extrapolation from experimental data is already large at this temperature. Finally, any homogenization advantages the 81005 clear glasses may have because of viscosity and homogenization distance differences are at least partially compensated by the much shorter cooling time such tiny droplets (very high surface to volume ratio) must experience relative to tektites. Table 3 Vlscosmes of 81005 clear glass (analysis no 2, table 1) and an austrahte from Pindera, NSW (22, analyses no. 57), calculated by the method of Shaw [23] Temperature ( o C)

1400 1600 1800 2000

Viscosity (P) 81005

Austrahte

31 5 7.6 2.4 0.9

6003 692 120 28

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W e are therefore c o n f i d e n t that the c o m p a r i s o n with tektites is valid. W e believe it is also p e r t i n e n t to p o i n t out that tektites are c h a r a c t e r i z e d b y c o n t i n u u m s of s y s t e m a t i c i n t e r - s p e c i m e n chemical trends that, as we have a l r e a d y noted, are so very difficult to generate a n d m a i n t a i n in an all-liquid m a g m a b u t which are readily e x p l a i n e d b y shock-melting, either as preserved target heterogeneities or v a p o r f r a c t i o n a t i o n trends or both.

5. Summary and conclusions T h e glasses in A L H A 81005 a p p e a r to r e p r e s e n t s o m e t h i n g a p p r o a c h i n g two e n d m e m b e r s of a s p e c t r u m of l u n a r i m p a c t glasses. T h e b r o w n glasses were p r o b a b l y g e n e r a t e d at close to m i n i m u m P T c o n d i t i o n s a n d the clear glasses were p r o b a b l y g e n e r a t e d m u c h closer to m a x i m u m PT conditions. T h e " m i n i m a l l y " p r o d u c e d b r o w n glasses have all the characteristics of i m p a c t glasses. T h e " m a x i m a l l y " p r o d u c e d clear glasses d o not. T h e y have been melted, fined, h o m o g e n i z e d a n d p r o b a b l y r e n d e r e d colorless b y the i m p a c t process. It would, of course, be of c o n s i d e r a b l e interest tf colors, or lack thereof, of l u n a r glasses could be related to the p r e s s u r e / t e m p e r a t u r e a n d r e d o x c o n d i tions of f o r m a t i o n . Such d e t e r m i n a t i o n s d o n o t seem b e y o n d the capabilities of existing e x p e r i m e n t a l e q u i p m e n t a n d techniques. Be that as it may, the existence of these " m a x i m u m " i m p a c t glasses confuses the effort to identify true lunar volcanic glasses. It also negates virtually all the a r g u m e n t s a d v a n c e d for a l u n a r origin of tektites. W e suggest: (1) the c o n t e n t i o n that s u b s t a n t i a l a m o u n t s of glass in the l u n a r regolith are r e c o g n i z a b l y volcanic deserves re-evaluation; a n d (2) the c o n t e n t i o n that tektites were p r o d u c e d b y geologically recent, hyper-explosive, l u n a r volcanoes deserves to be a b a n d o n e d .

References [1] D D Bogard and P Johnson, Lunar Planet. SCL XIV (1983) p 1 [2] U.B. Marvin, Lunar Planet Scl XIV (1983) p. 18. [3] G. Kurat and F. Brandstatten, Lunar Planet. Scl XIV (1983) p 14 [4] J.C. Laul, Lunar Planet. SCL XIV (1983) p 16. [51 T.K Mayeda and R.N. Clayton, Lunar Planet Scl XIV (1983) p 20. [6] A.L. Albee and L Ray, Anal. Chem 42 (12) (1970) 1408. [7] E Jarosewlch, J.A. Nelen and J A Norberg, Snuthsoman Contr. Earth SCL 22 (1979) 68. [8] A.M. Reid, W.I. Radley, R.S Harmon, J Warner, R Brett, P Jakes and R W Brown, Geochlm Cosmoclum. Acta 36 (1972) 903 [9] J W. Delano, D H. Lmdsley and R. Rudowskt, Proc Lunar Planet. Scl. 12B (1981) 339 [10] C D Stone, L A Taylor, D S. McKay and R V Morns, Proc. 13th Lunar Planet. Conf (1982) A182. [11] D.S McKay, U S Clanton and G. Ladle, Proc. 4th Lunar So Conf (1973) 225. [12] G.H Helken, D.S. McKay and R W. Brown, Geoclum. Cosmochlm Acta 38 (1974) 1703

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R F Fudah et a L / A glassy meteorite from the moon

[13] C. Meyer Jr., D.S McKay, D.H. Anderson and P Butler Jr, Proc 6th Lunar ScL Conf. (1975) 1673. [14] J.W Delano and K LlVl, Geoctum Cosmochlm Acta 45 (1981) 2137 [15] J.W. Delano and D H Lindsley, Lunar Planet. SCL XIV (1983) 156. [16] Basaltic Voicamsm Study Project (Pergamon, New York, 1981) p. 399 [17] C.G Andre, R.W. Wolfe and I. Adler, Proc 10th Lunar Planet Scl Conf. (1979) 1739. [18] J.W. Delano, personal commumcatlon. [19] A.B Binder, Proc 13th Lunar Planet. Conf (1982) A37 [20] J.A O'Keefe, Tektites and Their Ongm (Elsevier, Amsterdam, 1976) p. 180. [21] J.A O'Keefe, these Proceedings (Natural Glasses), p. 1. [22] B. Mason, Snuthsoman Contnb. Earth So. 22 (1979) 14 [23] H.R. Shaw, Am J. Scs 272 (1972) 870.