Glass occurrence in chondrules as evidence for formation by rapid cooling of liquid droplets - chondritic meteorite

Fredriksson, K.

Reid, A. M.

English

NASA Technical Reports Server

* Smithsanian Insti tution, Washington, D.C. 
I + Scrippe Inst i tut ion of Oceanography, La Jolla, California 
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i bbstract: The occurrence of glass as a major constituent i n  a chondrule * 
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I l l  i '5 ! from the Chainpur meteorite provides further evidence that the '  
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chondrules farmed by rapid cooling of l iquid droplets. The I I 
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distribution of nickel and iron between xul3-ta.l and s i l i ca t e s  
shows tha t  iron was present when theIchondrules solidified, 
and that the metal formed by r e d u c t l a  i n  the molten &age, %hug 
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extracting virtually a l l  nickel, 
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The origin of chondrules has long been one of the most intriguing 
problems i n  the study of meteorites. Many diverse hypotheses were advanced 
t o  explain these strange spherical objects before the examination of th in  
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sections under the microscope placed severe.limitationrr on possible modes . 
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;,of formation. Early investightors of chondrites recognized their  similarity/ 
' to  volcanic rocks and conciuded that t he i r  internal s t r k t u r e  indicated t h a t .  I I ~I I - 1  .I I 
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the chondrules had been quenched from high temperatures. This point of View 
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was very clearly stated by Sorby in 1877 who summarized his  conclusions by 
s ta t ing  that "the conditions under which meteorites were formed mst have 
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. i  been such that the temperature was urgu ~;UULW~ ---**A. t= &.Itaa a.yyI a+.nnv ---- rQa8ses in to  I 
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hcandescent atmasphere, subdect t o  violent mec al dI8turbances; that 
the particlee could exist independently one of the other in an 2 %  
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*.' the force of gravitation wag great enough t o  collect  these fine par t ic les  t , -  
together into sol id  masses, and that these were i n  such a situation that 
they could be metamcrrphosed, further bmken up into fragments, and again 
collected togethe." At ahhe when mphisticated techniques have allowed I 
the study of many of the more esoteric properties of meteorites It i e  
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perhaps instructive to review Sorby's conclusions, the val idi ty  of which 
remain unchanged. 
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Alternatives t o  the 'liquid droplet' hypothesis have been praposed by 
several authors. !&e formation of chondrules by metamorphic recrystall ization 
of sol id  matter has been advocated by Levin (1958) and by Mason (1960) . 
chondrule which i s  described here displays several features which cannot be 
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, exglained by metambrphic recrystaLlizatian. Rather if provides unusually 
clear evidence of having formed by quenching &f a p&lyy o r  more l ikely,  a 
wholly liquid droplet* 
The Chainpur meteorite f e l l  as a shower of stones on May 9, 1907 by %he 
vi l lage of Chainpur (lati tude Z051 'N,  Longitude 83O29'E) in India. 
meteorite was described by Cotter (1912) and, i n  greater detai l ,  by Keil, 
' et  al, (1964) who demondrated the unusual var iab i l i ty  i n  composition of 
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olivine and pyroxene i n  the chondrules. Keil, et al, cmcludedthat indiVld-/ 
ual s i l i ca t e  grains within dingle chondrules m e  not i n  equilibrium as a 
, I  consequence either of rapid crystallization o r  of crystall ization at.*-- 
peratures too low t o  allow dimsion t o  elimlnake the corqorritional variations. 
Tk&xi sectime of the Chainpur meteorite show B h a r p l y  delineated chondrules 
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of widel$ Varying internal 1Structure9 mostly 1-2 mm. in diameter (Figure I). 
Figure 2 is a ph&omicrograpb of the ChOndnSle which baa been studied in ame 
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straddle the chondrule-matrix border. A protrusion of the matrix in to  the 
chondrule can be seen in the bottom right hand corner of Figure 2. 
indentation was apparently produced before the chondrule had completely 
solidified as  the olivine cry&aUites in  the chondrule are aligned parUe3. 
It i s  circular i n  section except where metal and sulphide grains 
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t o  the indented'rnargin'. Either the matrix m t e r i a l  intruded the Still 
plastic chondrule or, p r h a p s  more likely, this Indentation formedby 
shrinkage on cooling. 
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Within the chondrule crystals o f  olivine and pyroxene a r e  se t  i n  8 c lear  
colorless glass* 
forms elongate crystals, some in paral le l  growth patterns, with well developed 
The major crystalline phase is  a fo r s t e r i t i c  olivine which 
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--: crysteit facsa. No evidance .could be seen of any reaction between the olivine . *  
i I ' . I .  . and the glass (!bible 1) . I n  the plane of the th in  section some olivine crystals ' 
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: . .  completely enclose small areas of glass. Olivine I s  more abundant a t  the out--- - 
i - .  - . I  - ,  ' margins of the  chondrule, which the elongate crystals tend t o  parallel .  Pyroxene is prominent i n  the core o f  the chondrule where it forms elongate, 
paral le l ,  feathery c rys ta l l i t es  (Fi@;ure 3) . gome of the crys ta l l i t es  are 
made up of nkerous smal l  distinctively shaped crystall ine dtf3 with a 
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'common orientation but separated by areas of c lear  glass. The glass itself i s  
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colorless, isotropic and homogenous. .Metal occurs as th in  seams w i t h i n  some; - - -  -- 
of the larger olivines, Figure 5 ;  and 8s a few globules i n  the glass, Figure 6. 
. Phase compositions determined with an ART electron probe microamdyzeY are 
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t i sham i n  Tables 1 and 2. The dietributkon of selected elements among the i 
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was found that  their  skeletal  habit made it impossible t o  avoid I 
overlapping the microprobe beam into the adjacent glass. 
reason only minimum values of calcium and magnesium could be obtained. 
Figure 4 shows that the pyroxene contains l e s s  iron than the ol ivine '  
or the glass. The variation in calcium and magnesium content across 
the pyroxene c rys ta l l i t es  l a  i l lust rated i n  Figure 7 where the peak 
values attained correspond t o  6 weight percent for  calcium and 10 . 
weight percent f o r  magnesium. These values correspond t o  a very 
low iron pigeonite with a calcium content higher than most of the 
For this  
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pyroxene. i n  Chabpur,. (Keil e t  al, 1964). Boyd and Schairer (1%) I .  
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have shown that homogeneoua pyroxenes with compositions midway between 
ens ta t i t s  and diopside can be formed by quenching but that they are t -  
/ .  stable 'even a t  elevated temperatures and unmix t o  yield two 
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. pyroxenes of contras%ing magnesium t o  calcium ra t ios  on prolonged I .  
heating o r  on slow cooling. The unusual composition of the ppoxene I I 
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c rys ta l l i t es  can only be explained by rapid cooling from high tempera- 
tures, an interpretation f U l y  in accord with the skeletal  habit of 
the crystal l i tes .  , L 
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Rapid cooling has also allowed the preservation of fo r s t e r i t i c  
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I 1 olivine i n  intimate contact with a s i l i c a  r ich  e;lass. 
constituent of all but the youngest ternefttrial rocks. 
meteorites is of particular interest  as most meteorites appear t o  have 
Glass i s  a rare 
Ulass in 
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' formed early i n  the history of the solar system. True glass i s  not . 
common i n  meteorites but the textures of many fine-grained aggregates 
i n  chondrules, in barred chondrules ror exanrpie, s i &  tkt %hey 
have formed by the devltrlf icatian of glass. 
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The glalre in Chaixrpur has ,. 
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not devitrif ied because the meteorite has not been subjected t o  any 
significant thermal metamorphic effects  and because of the composition 
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of the glass. 3 
The first column i n  Table I represents the composition of the 
glass as analyzedwith a n a  electron probe microanalyzer. Sodium 
i s  not included. Sodium i n  the glass presents a special problem as 
the sodium content of the m a l l  valume excited by the electron beam 
decreases t o  about 25 percent of i t s  original value i n  20 seconds. 
The first analysis i n  Table l i s  thus an analysis of the glass aFter 
it has l o s t  most the original sodium. 
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The t rue soda content of the 
glass i s  approximately 8 percent. 
and the LoV eotnrtiag n t e s  make this figure less reliable than the 
'other data presented. 
Escape of sodium from the glass, 
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Examination of data obtahed under a wuliety of 
instrumental conditions does, however, indicate that t h i s  value is 
substantially correct. 
composition, recalculated t o  lOO'percent, fo r  a value of Na20=8 weight 
pencent, 
The second c o l m  i n  Table 1 shows the glass 
The glass camposition represents an arrested stage i n  the continuum 
of compositions embraced by the l iquid portion of the chondrule as 
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crystall ization progressed. If th i s  particular chondrule i s  not atypical 
i n  bulk composition then the glass composition can indicate one direction 
i n  which differentiation of chondritic material can proceed i f  governed by 
crystal-liquid fractionation. Seeregation of a l iquid f'raction with the 
composition of the k i n p u r  gla88, T&3e I, c c d d  probably o&y be achieved ; 
by a rapid, catastrophic process. Recalculation of the glass analysis in  
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terms of normative minerals, Table 3 shows 68 percent albite and 1 1 
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- * -  i percent quartz. The glass in this respect resembles the glass-like f 
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material in barred olivine chondrules, 
L- -- The presence of glam in Chrrinpur, the agsociation of forsterite 
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with silica-rich glass, the composition of the pyroxene and the skeletal 
habit of the crystallites all argue strongly that the chondrule formed 
from a 'liquid droplet'. Recrystallization by thermal metamorphism in 
the solid state cannot account f o r  the occurrence of glass nor the lack 
of equilibrium in this high temperature assemblage. 
one chondrule *om the Chainpur meteorite but Chainpur contains a great 
diversity of chondrule types. The few occurrences of glass studied to 
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This paper discusses 
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~ R W S Y C !  all In tb-niare reduced chondrules $.e. those in which there 
is little iron in the silicates. 
are  low in Iron and also in manganese, some of which is present in the 
sulphides. 
chondrules may reflect slight variations in redox conditions, composition, 
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The silicates in the glassy chondrule 
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The varying texture and phase compositions of the different . 
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cooling rate and iolidification temperature, all of which are intimately' 
related. 
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L The manganese content of the olivine is related to the iron to 
magnesium ratio. As shown by Keil and Fredriksson, (1963), manganese 
appears in the sulphide phase under strongly reducing'conditions. The , 
distribution of nickel is quite different from that of manganese. 
irm&cible U q q d  me&, Table 2 and Fig. 6, formed ;by reduction:. before 
Olivines of widely varying iron content in Chainpur a l l  contain little 
or no nickel, Table 2. Ajqzeztly zickt?l Y=B effectively removed as 
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crystallization of  the' silicates. The f inal  distribution of nickel ! 
and iron is  undoubtedly influenced by th i s  early segregation of some 
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of the iron and virtually a l l  of the nickel. Experimental studies 
'presently. In progress together with textural studies of chondrites 
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(e.g., see Figure 8) kggest that iron, nickel and sulphur nay be 
redistributed in EC chondrite at temperatures well below the c-ry&aJJA- 
zation temperature of the silicates. The distribution of iron, nickel 
and sulphur in chondrites may be controlled by both a high temperature---- 
fractionation of contrasting phases and a low,temperature redistribu- 
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t ion i n  the solid. I 
The contrast i n  nickel content between the chondrule s i l i ca tes  
and the matrix s&rounding the chondrules is shown in Figure 5. 
Figure 9 shows that a similar pattern exists i n  the carbonaceous 
chondriie Murray, The nickel distribution cannot be explahed by 
an i n  s i t u  transformation of either matrFx into chondrules by solid- 
s ta te  recrystallization (Mason, 1960) o r  chondrules into matrix by 
weathering ( M e s n e  and Anders, 1962). The contrast i n  composition . 
between matrix and chondrules i n  these meteorites implies that  they 
are mechanical mixtures of materials formed under'different Cmditiofis 
(fiedriksson and Keil, 1964). / 
On the subject of meteoritic chondrules Merrill i n  1929 wrote - 
"8uch interesting and peculiar forms are now known t b  be due t o  the 
cooling and par t ia l  crystallization of molten drops of stony matter; 
* - - - t he i r  or ig in  has been iii~de the wt5?e& of much discussion and 
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wordy warfare q o n g  atudentrr, but the matter need not be gone into 
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further here." Concurrence with MerrU's conclusions and With the 
fact that iron was present when the chondrules 8olidified allows 
advance to the w e n  greater problem of the conditions under which 
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ACKN-S 
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We thank R. Fitzgerald for his advice and assistance in the electron 
microprobe analysis. G .  Moreland prepared the polished thin sections. The ' I  
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generous support for this work from the National Aeronautics and Space Agency, 
numbers NsO=688 ie grate- acknowledged. 
RxPERENw 
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' Cotter, 0. P., ,1912, Notes on Indian aerolites recorded since 196: Rec. 
Feel. S u r v .  India, 42, 65-27'". 
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DuFresne, E. R., and'hders, E., 1$2, On the chemical. evolution of the 
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-. carbonaceous chondritesr Geochim, et Cosmochim. Acta, 26, 1085-lll4. 
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distribution in'the M u r r a y  carbonaceous chondrite: Meteoritics, 
2, 201-2l7. 
Ken, K., and Fredriksson, K., 1.963, Electron microprobe analysis of some rare 
minerals in the Norton County achondrite2 Geochim. et Cosmochim, 
Acta, 27, 939-948. 
ail, K., Mason, B., Wiik, H. B., and Fredriksson, KO, 1964,) The Chaispur , 
meteorite: Am. Museum Novitates, 2l73, 1-28, I 
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Levin, B. J., 1958, Uber den Ursprmg der Meteoriten: Chem. Erde, 19, 286-295,: 
I Mason, B., 1960, Origin of' chondrules and chondritic meteorites: Nature, 186, 
230-231 
Merrill, 0. E'.,* 1929, The s t o r y  of meteorites: in Minerals from earth and sky, 
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The Smithsonian Series, volume 3, Pari; L, The B ~ t ~ s o ~ ~ ~ . ' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ U ~  
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N0,Y.P p. 75. * 
Sorby, H. C., 1877, On the structure and origin of meteorites: Nature, 15, 495-498. 
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A chondrule in the Chainpur meteorite

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