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ff 653 July 65 a 7

The Chemical Composition and Origin of Moldavites f * J. A. Philpotts and W. H. Pinson, Jr.

Department of Geology and Geophysics Massachusetts Iilstizute of Technology Cimbricgc :" asachuse:ts

Abstract :

Twenty three new ma,cr-eienm: ,nalyads or' moldavites are report&. I 1 The samples include seve-nzean Bohe.Ai<-..i ar.d six Moravian tektites. The 1

ranges in the contents 05 the various oxicies are as follows: Si02, 75.5 - 80.6; A1203, 9.62 - 12.64; TLG 2' 0.253 - 0.460; Fe203, 0.12 -

0.31; FeO, 1.42 - 2.36; MgO, 1.13 - 2.50; CaO, 1.46 - 3.71; Na20, 0.31 - i 1 f 0.67; K20, 3.26 - 3.81.' The Rb and Sr contents and the Rb/Sr ratios are 1 t also reported for the 23 specimens; the ranges are as follows: 1 Rb, I 120 - 160 ppm; Sr, 130 - 156 ppm; Rb/Sr, 0.82 - 1.20. The densities 1 i and refractive index values range from 2.3312 to 2.3718 gm/cm3 and from

1.486 to 1.495, respectively. / !

In contrast to the australites, the mo1davit:s display significant I 1 I negative c~rrelatlonsbetween the alkali metals (Na and Rb) and the I alkaline .earths. The variations in the chemical composition of moldavites ! would seem to be unlike those of sedimentary or igneous rocks. It is

b suggested that the observed variations in composition are largely due to fractional volatilization. The wide range of Rb/Sr ratios in con- i i junction with the unifar;rLty of the Sr isotopic composition supports i

I this theory . I I ! * I Present address: Theoretical Division, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland.

! -2-

ACKNOWLEKEMENTS

This research was sponsored by the NASA Research Grant No. NsG

222-61. The mass spectrometric analyses were financed by the U. S. t i t the supervision of Professor P. M. Hurley. The X-ray fluorescence analyses for rubidium and strontium were performed on equipment granted ! to M. I. T. by a National Science Foundation grant, under the supervision ! of Professor H. W. Fairbairn. Preliminary major element X-ray fluorescence

L analyses were performed at Harvard University under the supervision of !

Professor C1iff ord Frondel and Mrs. Frondel. X-ray fluorescence analyses for SiOp and A1203 were performed in the Geochemistry Laboratory, I 1 I I Goddard Space Flight Center, NASA, Greenbelt, Maryland. To the above individuals and organizations the-authors express their appreciation. I ,--2*- . *: 4'

c IK T?3 33 CTIGii

Published analyses have shorn xht tcktites from a particular

geographic group are qulce sinniiar ii? chercical composition and that

there is some similarity between tg!;tltes Zron different groups. In

general, tektites are recognizrble 2s such by Lheir chemical compositions.

Compilations in the literature, of older analyses by various investigators

using different analytical techniques, have tended to obscure real

variations within and between the tektite groups. There existed a need

for precise analyses of representative numbers of samples by uniform

techniques, preferably utilizing accepted rock standards to monitor

accuracy. This need has been satisfied in the case of the australites , (Taylor, 1960; Taylor --et al, 1961; Cherry and Taylor, 1961; Taylor, 1962; Taylor and Sachs, 19641, the bediasites (Chao, 19631, and various

South East Asian tektites (Schnetzler and Pinson, 1964a). The purpose

I / of this paper is to report a number ofjinternally consistent chemical

analyses of .Czechoslovakian tektites and to discuss theories of origin

of the moldavites in the light of this data. Seventeen moldavites from c the Bohemian "strewn field" and six moldavites from the Moravian field

\ were analyzed for major element contents. The rock standards G-7 and

W-1 were used as analytical monitors. Analyses of the trace elements .. Rb and Sr were also perfoxlied. The refractive indices and sp++-+n- &-&i ti6

gJ%r=.&Se-s gJ%r=.&Se-s of the 23 tektites were also determined.

Previous chemical analyses of moldavites have been reported by

Barnes (19K)), Schnetzler and Pinson (1963~1964~1, 1964b), and Bouska

and PPyondra (1964). *t 4'

'I I' -4-

Further details concerning all aspects of this work have been given

by Philpotts (1965).

, --._--.

\ \* \

.. _- -.. _r - .. -3-

CI dl 4, ANALYTICAL TECIINI QUES Refractive -Index --and Density Determinations Refractive index values for the 23 moldavites were obtained

on powders by theGrnersion oil method. Most powders gave a range of

refractive indices and the reported values are' an estimate of the

mean value for each sample. Because of the estimate involved in the

determinations, accuracy is thought to be about N.002.

The specific gravities of the moldavites were determined by

weighing the bulk samples in air and in distilled water on a high-

precision chain balance. Duplicate determinations were made on six 3 samples; the mean difference between duplicates was 0.0004 gm/cm .

The precision and accuracy of the density determinations are thought 3 to be Setter than 30.001 gm/cm .

SamDle PreDaration

The moldavites were prepared for chemical analysis in th'e /

following manner. The samples were washed in acetone, in distilled

water, and in hot, six normal HC1; they were broken on a steel plate

z and a portion of each tektite was crushed in a flat-surfaced, hardened

steel', percussion mortar; a hand aagnet was passed over the crushed

fragments in order to remove any incorporated steel; the samples were

powdered in a boron carbide nortar and homogenized.

Rapid Si1icate Procedures

Total Fe,MgO, CaO, Na,O, K.0, Ti0 and P 0 were determined by ,, L 2 2 25 the rapid silicate procedLLLs of Shapirq and Brannock (1956) as .

describe2 by Schnetzler and ?inson (1964a). Two weighing3 of each -6-

r .I

e tektite sample, except T 5296d, were used in the determinations; a

single weighing of T 5296d was used. FeO was determined by the

spectrophotometric method of Shapiro (1960). Only a few determinations

of P 0 were made because the method proved unreliable at the 25 low concentrations found in the moldavite samples. For the same reason,

determinations of MnO, H 0 and CO were not undertaken. 2 2 X-ray Fluorescence I -- It was decided to determine Si02 and A1 0 by X-ray fluorescence 23 techniques because of the number of determinations required to obtain

a reliable Si02 value, and because of the frequently anomalous A1203 i i results obtained by the "rapid silicate" method. This decision was 4

made in light of the successful X-ray fluorescence analyses for light elements by Volborth (19631, Rose --et a1 (1963), Schnetzler and Pinson (1964a1, and Welday et a1 (1964). It was further decided to forego I -- I !

the fusion technique of sample preparation because tektites are I I relatively homogeneous glasses produced by a natural fusion process, ,

and because tektites from the same locality (i.e. Czechoslovakia) 5 are quite similar in chemical composition. \

P,reliminary X-ray fluorescence investigations were done at-the

Hoffman Laboratory, Harvard, on a helium path, Phillips unit, under

the supervision of Dr. J. W. Frondel. In order to determine the

suitability of the moldavite powdered saaples for X-ray fluorescence

analysis, they were first analyzed for K; flame photometric data on

K were available for comparison. The moldavite powders were packed

,, into lucite trays by pressing the surface with a glass slide. The K Ko,

radiation of each sarii;-k was rapidly scanned (1 degree per minute)

0 Tram 40r- to 42O, using a tungsten target and an ADP crystal. A

______--- ,-.------I * -.-..- .+7+7-->------'\ B- .I - e .. .. *% .. 1 %.% \ *. .\ .1 i -7- -5 *I

* comparison of the X-ray K results (obtained from a least squares fit)

and the flame photometric K results for the same 23 moldavites is

presented elsewhere (Pinson --et al, 1965); the mean difference between

the two sets of results was 0.04% K,. It was next decided to analyze

for A1203. A tungsten target and a gypsum crystal were used.

Unfortunately only a few samples could be run because of other

demands upon the X-ray unit at the tine.

The A1203 determinations were continued on a General Electric,

vacuum, X-ray unit in the Geochemistry Laboratory at the Goddard

Space Flight Center. The moldavite samples were prepared for analysis

by briquetting 50% mixture of sample powder and boric acid at 10,000

psi for one minute.: A chromium target, PET crystal and plexiglass

sample holders were used in the A1 0 determinations. Three sets of 23 100 second counts were taken at a background setting and on the

/ A1K peak. This corresponded to a total of about 7500 counts on Q the background and 75,000 counts on the A1KU peak for each sample.

Moldavites previously analyzed for A1 0 spectrophotometrically 23 c were'*used as standards. Si0 was determined using a chromium target 2 \ and a PET crystal. Three sets of 100 second counts were made at

each of the two background settings, and three 20 second counts were

made at the SiK peak. This corresponded to a total of about 3000 a counts on each of the backgrounds and about 90,000 counts on the

SiK peak for each sample briquette. Some samples gave anomalous oc background counts. Nost of these anomalous counts were obtained on

Samples run in the szxe --_....?leholder. An empirical correction factor,

equal to the average oackground count divided by the observed -8-

.)a .I1

b2ckground counG, was applied to the peak counts in these cases. I Moldavites which had been analyzed for Si0 by the "rapid-silicate" 2 method were used as standards.

Rb and Sr were determiried D:~ the 2:ar~k- herican Phillips X-ray

fluorescence unit at M.1.;'. uiidir the supkryT-sLori of Professor H. W.

Fairbairn. Powdered sas!.j:es 'iu'zre used. .4 molybdenuz target, topaz

crystal, and scinti1:ation countzr .dere em~-oyd. Three sets of counts

were registered on -,tree Lackgrcunc: settir,;s, *:he R5R peak, and the CY

SrK peak. A total of 6000 coufi~swere taker, ZE 2;cn af r;he background o!

settings and a total of 12,090 cocn'cs tAyzre tcker, ori soth 02 che peaks, '

for each sample. Each run on four samples included a standard; the 1

Rb and Sr contents of the three "unknowns" were determined by comparison

of peak heights with those of the standard that was run with them.

The standards used were mo1davite.s for which Rb and Sr contents had

t been (or were later) determined by mass spectrometric stable isotope

dilution analyses (Schnetzler and Pinson 1964b; Pinson --et a1 , 1965). Each moldavite sample was packed and run two or three times in the X-ray 5 fluorescence analyses for Rb and Sr.

Precision and Accuracy of the Chemical Analyses - -- I

The precision and accuracy of the rapid silicate procedures employed I

in this investigation havc been discussed by Schnetzler and Pinson

(1964a). The most mzaningr^ul e:i_?ressions of precision and accuracy of

the analyses are derive; Zrom T;he results oE -aplicste analyses of the I

rock standards G-1 an2 1.4-1 which were prep:.:ed a?.;. r--&with the tektites.

Results of the analyses of G-1 and W-1 arc: -,Lvsn Li. 2cle 1. Tne

accepted values for the monitors are takeil from Fleischer and Stevens (1962). -9 -

co e4 h I+ 0 4 9 9 9 0 0 0 0

ro \D ro W

h el In d v.4 tn

ro N 0 r(

N h 4- h v1 0 W 9 \o N 0 Fl

r( I c3 h W In h =J- h 0 0 In 0 9 9 9 v) 0 0 a, 0 0 0 v) h rl C a 0 C' .A c W \o \D ro ro c, a .r(> a, a cv \D rl m 4- m N m m cv :. 2 cd N 0 cr) m 0 a h C (D c, m a, N m 5 .s s m =t e4 .. N r*) VI 0 h 0

N ,e4 0 0 - II *A El a Ocv .A Cn XI x M H b - 10 -

-1 .I The precision and the accuracy of the tektite analyses are believed

I to be as good (for comparable concentrations) as those of the analyses

of G-1 and W-1, if not better, because of the ease with which tektite

glass goes into solution.

The accuracy of the X-ray fluorescence determinations of Si0 2

an^ Ai U aepends upon the accuracy of the spectrophotometric deter- 23 minations of these constituents in the standard moldavites. The

average difference between the sFZcirophotometric values and the X-ray

fluorescence values (obtained fron 3 least squares fit) for A1 0 23 in the four standard moldavites n~as 'less than O.l%Al 0 The average 2 3' difference between the spec~i-o~ho~~~rl~tr_-and the X-ray fluorescence

values for Si02 in the, five standards w2.s 0.5% Si02.

The Rb X-ray fluorescence analyses of tile moldavites have a

precision (c) of about 3~2%; the Sr X-ray znalyses have a precision of

about 3~3%. These conclusions are bzsed upon numerous replicate analyses '

of G-1 and W-1 by Professor H. W. Fairbairn (M.I.T., 1964, unpublished). The accuracy of the X-ray fluorescence analyses for- Rb and Sr depends upon 'the accuracy of the mass spectrpmetric determinations of these

constituents in the standard moldavites. Results obtained on G-1 and

W-1 in the M.I.T. Geochronology Laboratory, when compared with results

obtained elsewhere by various reliable methods of analysis (Fleischer

and Stevens, 1962), suggest an accuracy of better than *5% for the mass

spectrometric determinzL -:I& dL ,Pb and Sr.

Direct evidence 0: .:.z overall accuracy of the moldavite analyses

is given by the fact thhi sunaatio.is of t:A-values of the constituent . -11-

oxides fall between 99% and 101% for 20 out of the 23 samples analyzed;

it should be noted that only the CaO and Mg-0 and the FeO and Fe 0 23 determinations are not independent. A conservative estimate of the

0 overall precision (c = -X (100)) of the analyses is as follows:

Si02, *l%; A1203, f3%; Ti02,'i3%; MgO, *5%; CaO, ~54%;Na20, &15%; !:

K20, 12%; totai Fe as FeO; *2%; Rb, &2%; Sr, &3%.

- . -12 -

RESULTS

The results of the chemical analyses of the 23 moldavites are

< I.< '2.1. given in Table 2 along with the -spscif-id[?a.vity and refractive index values. The results are presented in order of sample occurrence from west to east in the "strewn fields". The najor elesmt coritents are reported in Table 2 as weight % of the oxides; a surmratio3 of the constituent contents is included. Tutal Fe contents, as % FeO, are given. The contents of Rb and Sr in parts per million, are also reported. The results of the P 0 acalyses are not given because 25 these analyses proved to be of poor quality; the 15 samples analyzed for P 0 gave results ranging I'rom 0.00 to 0.06 with a mean value of 25 0.03.

The values reported in Table 2 for the various constituents show the following ranges: Si02, 75.5 - 80.6; A1203, 9.62 - 12.64; Ti02,

I 0.268 - 0.460; Fe203, 0.12 - 0.31; FeO, 1.42 - 2.36; MgO, 1.13 - 2.50; i, : GO, 1.46 - 3.71; Na20, 0.31 - 0.67; K20, 3.26 - 3.81; total Fe as FeO, c 1.53 - 2.61; Rb, 120 - 160 ppm; Sr, 130 - 156 ppm. All of these values

I fall within the ranges given in the literature (Barnes, 1940; Schnetzler 4

-t and Pinson, 1963, 1964a, 1964b; Bouska and Povondra, 1964) with the 41 1 exception of one K20 analysis and four Rb analyses. The K20 analyses ! b r. all fall in tke upper regioi-. i,f xb.2 range reported in the literature. r . .. 'l< ./ The s@&=g--&vi.-ty and iefractive index vai,c=s of zhz 23 moldavites range from 2.3312 to 2.3718 gm/crdd and erorn 1.486 to 1.495, respectively, /

! I ! 3 3 m d *m In hl hl 0 9 a3 0 0 0 m l-4 h d m rl

0 m 0 03 In ‘9 h m rl m m 0 0 rl h 2 m 0 l+

m hl 74 m h In d m m m In rl 0 0 0 h d 3 rl

m \D 0 c\1 hl h m rl m 03 0 0 0 \D h rl m 0 rl

tll c, aa, 3 1 * a d a3 u3 h 0 m cy co In rl 0 0 0 0 h rl cy r( d (d 0

rl a3 m 0 h hl m cy

rl u3 0 0 3 r( h rl a d

a m m r-i 3 m cn c) m o\ h 0 0 r-l h 2 0 zr-l

h u3 m 0 W rl a3 hl m m rl rl 3 m 0 0 a3 rl rl h 2 =f rl

E m 0 a N cy a 0 Ohl 0 ON .A rl -A P li cn c !+ r2 p: p: co N 4 a cx) 0 N rl co In m 3 rl N rl In 8-l h m '9 a3 0 0 d N c) 0 m 0 h 41

VI K m co 3 m m 0 rl =I. N d a N d 3 0 '4 m 3 c9 h 0 0 0 d N c) 0 m 0 h rr

'-" h U1 =f co m In m \o 0 rl In m N rl In '9 l-4 u) h 9 rr m m 0 0 rr r( cv 0 m I h

h co cn \D m m 0 In m a, 0 N N rl * d In m '9 a, h 0 0 rr N m 0 m 0 h $1

h a a, 3 In C c cn 0 N N a3 0 In a3 el .-I \o N l-i 3 m 3 In 4 4J 9 9 rr C 0 0 0 d rr N 0 m u0 03 2 W el a, 0 0 m m 0 h In =I. h z 0 h m d =f m m rl u3 m 9 3 03 0 0 d rr 0 0 m 0 h $1

rl m m rl m N 74 In u3 W rr 0 N h 41 rl In h 0 h 4 0 0 rl N m 0 c) 0 ,h rr

h m h 0 \o \3 a LT, hl Q\ \o h m c.: 3- m =t u7 9 '? + a3 0 0 rr P-l N 0 m t! $1

/" m k ?n u53 c4

.. . .

1 N o\ h Pic m N I4 co m hr 3 c: 3 ri ? 2 0 r(r 4 0 rc a-cn rl N 2 rc rc

(u z h h?! 3 rc 0 cn m am W w < cy 3 0 0 rc OF) cn N mrc rc -(Y m m

h C W ma N l-4 h W d (u> m rc m m m 3 w h 03 ;f

0 0 N 0 rc N .-( (Y cn N so rc rl

w mm 0 h d com ln cn

0 0 drc rl rl hrl rl c4 m .-I

m h m a- =J.m co rnh h h CL F) d wN w ln-f hl h r.4 9 0 0 rlrc rc om 0 rc mrc rc N mco m 2 rl rc

h om 0 W N VI ww W --.cv 0 0 Ndrc om hl rc , I

-t 3 w rc a3 0 N 03 h (u N 2* =J. v) -.w 9 '9 rc) '_ 0 0 N 0 2 (v

/' m 4 d a am ON c1 Q) 0 P F E-c Table 2 continued * spectrophotometric determination of Si02

Amass spectrometric isotope dilution analysis by Professors

W. H. Pinson, Jr., and H. W. Fairbairn. -Note: Tektites bZY6a - 331Y are from Bohemia, tektites 5320 - 5325 are ZrOm Moravia. Note:- The weights and shapes of the smples, and the results of each duplicate analysis may be found elsewhere (Philpotts, 1965).

~ --

--. ._

\, I \

I - 17 -

DISCUSSION

The results reported in Table 2 indicate variations in the

contents of all constituents. The observed scatter of the moldavite

analyses, expressed in terms of the same function C as was used to

evaluate precisioii, is as follows: SiO,, f2%; A1203, i4%; Ti02, *13%; L total Fe as FeO, *13%; MgO, 123%; CaO, *29%; Na20, *19%; K20, 54%;

Rb, f7%; Sr, i4%. It is evident khat the observed scatter is greater

than the analytical error for all constituents, and indeed, it is

considerably greater for most constituents. It is therefore concluded

that real differences in chemical composition exist between the moldavites

analyzed in this investigation. 4

The Moravian tektites were found to have higher average contents

Of Si02, A1203, Ti02, FeO and Na2G, and lower average contents of .dc 4.5 f he s MgO , CaO , and Sr, than the Bohemian tektites. The epe&5-kz=gzav-W / of the Moravian samples were lower, in general, than those of the

' Bohemian samples; this is in accord with the recults of the specific

gravity study by Chapman --et a1 (1964). The major distinction between

moldavites from the two localities is that the Moravian samples>have

high and variable FeO and TiG2 contents with low and relatively constant

CaO, MgO, and Sr, whereas the Bohemian samples have high and variable

alkaline earth contents with low and consKant FeO and Ti02. This

difference is Lllustiated for the pair C& - FcO in Figure 1. The

overall similarity between tektites from the two groups, however, suggests

I that a common origin is highly probable. The data do not indicate any - 18 -

clear regional trends in composition within either of the "strewn

fields". Considerable differences in chemical composition were found between tektites from the same localities. This.is not surprising in view of the facts that moldavites of ail coioui.s UCCU~Ln =vat lccclitiez

(Faul and Bouska, 1963) and that .tektites from Habri showed a wide range of specific gravities (Chapman --et all 1964).

Certain relat'ionships exist between the various constituents of moldavites. In Table 3, correlation coefficients and degree of significance are presented for various pairs of constituents. The alkalis show positive correlations amongst themselves, as do the alkaline I earths. Na 0 and Rb are inversely correlated with the alkaline d 2 earths. Si0 shows negative correlations of various significances 2 JtLij, $wry. with the alkaline earths, A1203, refractive index and speGi-f

FeO (actually total Fe as FeO) has significant positive correlations with Ti0 and with. Na20. 2 The variations in the chemical composition of moldavites reflect variations in the parent material and/or differential ciiiiiiges Lii composition brought about during fusion. Taylor (19621, partly on the basis of significant negative correlations of aamost all constituents with Si02, concluded that the australite parent material consisted of a mixture of 75% honogeneous shale and 25% quartz. Such a L-iixture is not suggested by t>.c rr,ol:,.cLze dara becacse Si0 is not inversely 2

correlated with the alkalis or vich FeO. in fact, I the nonlinear correlations of certain pairs of constituents (e.g. CaO-FeG, Figure 1;

Na 0-FeO) cannot explained in terms of any two-phase mixing process, 2 be .

Bohemian 0 r Moravian X

€3

0 0 0

c X X X XX

I I .ol 1 I I I I 2.0 3.0

wt. o/o total Fe as FeO

Fig. I CaO vs. FeO for 23 moldavites. Table 3

Correlation coefficients (r) and levels of significance, for various pairs of constituents in moldavites. . Moravia Bo hem i a Combined data r signi- r signi- r signi- f icance f ance f icance

Na20 -K 2O +0.96 <1% +O .63 +O -47

Na2O -Rb +o. 99

K20-Rb +O .94 <1 +O .75 +O. 72

MgO-CaO +o .91 +o-81 +O .96 1 ! MgO-Sr +O .98 a.1 +0.72 - i CaO-Sr +0.91 1 +O. 56 - i

Si02-A1 203 Not. sig. -0.65 -

S io 2-Mg0 -0.93 <5" -0.83 -0.88

FeO x- Ti0 +0.99 Q .1 +O -85 +O .96

FeOX-Na20 +O .83 <5 +O -70 +O. 79

I Na20-Mg0 -0.91 1 not sig. -0.53 Na20-CaO -0 -83 <5 not sig . - 0.64 Rb-CaO -0.83 <5 -0.73 43.1 - i Rb-Sr -0.90 <5 -0.50 i- -0.61 .

-ir combined data not applicable

I " n 5 ?

"r, 22

X Total Fe as PeO . / n 16

i - 21 - including the mixing of immiscible liquids. The data could be explained in terms of a complicated mixing process but the significant correlations between many pairs of constituents indicate that a simple cause was responsible for the observed variations in chemical composition. In ariy case, sedimentary parent materials would seem to be ruled out by the oxygen isotope data. Taylor anc 2psLei.n (1952, 1963) concluded on the basis of the oxygen data that tektitzs are either extraterrestrial or they are fused terrestrial graniric rocks with changed chemical compo sition.

Superficially, the variations in chemical composition of the moldavites resemble the variations in acLd igneous rock sequences. In ' such sequences, however, some of the alkalis have positive correiations with Si02, and iron is positively correlated with the alkaline earths and with A1203. These correlations are not displayed by the moldavites.

In addition, there is no evidence for thc erstwhile existence of discrete phases in tektites, with the excc?tio;i o? a SLG, phase. Tne L presence of lechatelierite is of LnLerest becase melts of moldavite composition would presumably lie within the Si0 field and differentiaticin 2 would occur by Si02 phase separation. Products of this Si0 subtraction 2 would exhibit inverse correlations of all constituents with Si0 2' This is not the case for moldavites. Another objection to the origin of the observed variations in chemical composition by igneous differentiation is the fact that these varL~;:c;.s also occur over small distanczs within individual i, L 13s. A 5 Lainary electron microprobe - 22 - study of moldavites indicated that variations in composition, comparable to those between bulk samples, occur over distances of 100~or so, within individual samples. This is supported by the refractive j-ndex data of this present work and of that ~y c;oherl <;;;2:.

In view of the failure of other processes to satisfactorily explain the variations in chemical composition of the moldavites, it is suggested that these variations are, for the most part, the result of fractional volatilization. Many investigators (e.g. Cohen, 1960; Lovering, 1960;

Loman, 1962; Greenland and Lovering, 1962; Chao, 1963) have appealed to fractional volatilization in order to account for various features of the chemical compositions of tektites. Other evidence of extensive 4 .- +,

1964b).

+,\& tir'.i i Experimental data concerning s&eet-iw+ volatilization are meager. Experiments by Lovering (19601, Friedman --et a1 @960), and Walter and

Carron (1964) ~ and optical spectrographic studies (tiiii-eiis aiid Taylor,

1961) on silicate materials have indicated the foilowing order of volatility mder zest of the experimental conditions, from most to least voiatile: Alkalis 2 Si, Fe > Al, alkaline earths. It therefore

-- c. / , ,. . . L :, .i(L L seem probably that sc;-zct-ii&c vc;.,;ilization ot tektite material would reduce the concectraLions of al:i-?Fs and increase hose of the alkaline earths. The content of a particular alkali or aikaline earth constituent could be used as an index of the extent of the process' action. There is the possibility, however, that some of the variation in concentration .

of the constituent could be inherited from the parent material. For example, the addition or subtraction of Si0 from an otherwise homogeneous 2 material would affect the concentration of another constituent; it would not, however, affect the Weight ratio of ar~ytwv CGES~L~Z~Z~S

(excluding SiO,). A ratio of two constituents would therefore serve L as a better index of volatilization.

The moldavite data was interpreted in terms of Rb/Sr ratio for the following reasons: 1. Rb and Sr are inversely correlated and therefore their ratio shows wide variation; 2. Rb and Sr were determined separately from all other constituents and therefore relationships between the Rb/Sr ratio and other constituents cannot be due to analytical idiosyncracies; 3. The Rb/Sr ratio is believed to be as good if not better than the individual Rb and Sr determinations with respect to precisivn and accuracy. In addition, even though the Rb/Sr

I ratio varies from 0.77 to 1-20, eight out of nine Sr87/Sr86 analyses of moldavites fell within a range of only O.OOll,-which is two standard deviations of a single analysis (Pinson --et ai, i965j. ELLS sitcatis:: cannot have long existed. Within the last 30 m.y. either Rb and Sr have been fractionated or Sr has been isotopically homogenized. It seems most probable that the change was effected during the thennai event dated by the i(-A age. Tie parent material night have beer, honogeneous, chenica1;j ..xi Lsacoiically, or homogenization could have occu-*red dci.-ing the thermal event. Tne survival of the lechatelierite incl:?sions leads us to favor a homogeneous lparent. ;n either case, the range of Rb/Sr ratios is best explained in terms of fractional volatil ization. * - 24 -

It is assumed that the Rb/Sr ratio decreased with increasing temper-

ature and duration of heating. Bohemian tektites with lower Rb/Sr ratios

have lower Si0 Na 0, and Rb contents, and higher alkaline earth contents. 2' 2 A1203, Ti02, FeO, and K20 do not vary appreciably with decreasing Rb/Sr.

Moravian tektites with lower Rb/Sr ratios have lower Si0 Ti02, FeO, and 2' alkali contents, and highcr alkaline earth contents. Sample 5324 is

exceptional in that it has the highest IZb/Sr ratio (1.20) and yet has lower '

Si02 and higher A1 0 than the other 5 Moravian samples. If the assumptions 23 made concerning fractional volatilization are valid, then intercepts of

trend lines in plots of pairs of constituents indicate relative volatilities.

Thus in Figure 2 the trend line gives ar, intercept of about 3% K 0 and this 2 indicates that Na 0 was more volatile than K20. Another method of deter-' 2 mining relative volatilities i3 Lyl examination of constituent ratio changes with changing Rb/Sr. K/Rb was found to increase with decreLsing Rb/Sr

(Figure 3), and this indicates that Rb was inore volatil:: than K. The / relative volatilities of the chemical constituents,- as indicated by the correlations, were as follows, from most to least volatile: Na20, Rb,

K20, Si02, A1203, Sr, MgO, and CaO. This order is in essential .agreement with the available experimental evidence. Tne relationships of iron with

other constituents sqgest chat initiially Lron was the most volatile

constitueni and was rapidly lost untll ap, e~~uillbiiumvalae of about 1.7%

toral Fa as FeO was attained (Fig. L, Fig. 4).

It is difEicult to account for the relationships among the chemical

constituents 02 soldavites by means other then fractional volatilization.

This process might have masked preexisting variations. The authors

feel, however, that such initial variations would involve the addition

-- .

0.7 I I X

8 Bohemian Eb 0.6 x Moravian X X

X X 0.5

0.4 X 0 N 0 0 Z d o\" 0 t 0.3 3

0.2

0.I

2 .o 3 .O 4.0 w t. O/o K 2 0

Fig. 2 Na20 vs. K20 for 23 moldavites '*20 x2 -E 8 Bohemian X Moravian

0 8 X 0 X

X 0 X 8 6

OQ 8 Q 0 -a

X G I

1 I 2 00 2;o 220 230 240 I K/Rb

Fig.3 Rb/Sr vs. K/Rb for 23 moldavites I

4 8 Bohemian X Moravian

8 0 6 x X

c 0.90

0.8 5

X 0 1 llll IIII Ill1 Ill 1.0 1. 5 2 .o 2.5

wt. O/O total Fe as FeO

FIG. 4 Rb/Sr vs. FeO for 23 moldavites. I

- 23 -

or subtraction of Si0 The inverse correlations of the alkalis and 2' alkaline earths in moldavites suggests that variations of Si0 in the 2 E parent material of moldavites were very limited. The presence of It lechatelierite inclusions might indicate r;nai the p;lrxt mi+erinl I '*'6 consisted of an aphanitic or glassy matrix containing a few phenocrysts i i; of a pure Si02 phase. Differences between the Bohemian and the

Moravian tektites probably reflect different histories during the thermal 1 ;' 1' event. ! I. t: tektites from other geographical groups. A brief discussion of :

1 i i possible volatilization effects in australites has been presented 1 t elsewhere (Philpotts and Pinson, 1965). ,I Four major tektite groups are generally recognized, namely North i American, Czechoslovakian, Ivory Coast, and South East Asian. The i ! ! / probability of fusing terrestrial material that is similar to tektites i in chemicai composition, oxygen isotopic composition (Taylor and i i Epstein, 1962, 1963) and strontium isotopic composition (Schetzler i

sa,: K-A ige (Zahringer,,l963), within the experimental error, as the

Nordlingen Ries impact crater, which is about 300 km west of the strewn field. "=his K-A data has been interpreted to mean that the moldavites . - 30 - were blasted out of the Ries crater (Cohen, 1963). The diverse rock types found rimming this crater (Shoemaker and Chao, 1961), however, would seem to be unsuitable parents, chemically and isotopically

(Taylor and Epstein, 1963), for the moldavices. Ft=L->Lsps, C'Keefe

(1963) has suggested on the basis of stratigraphic relations, the moldavites and the Ries Srater are not of identical age. If they are of the same age it could be fortuitous but this is unlikely in view of the similarity of ages of other tektite groups and various impactites (Fleischer,et-- a1 , 1965). Perhaps the Ries crater was formed by the impact of the moldavite parent-body. Such an origin might best explain

\ the alignment of the moldavite strewn field with the Ries crater. In the case of the Ivory Coast tektites, ,however, there is ancillary evidence to support a hypothesis of terrestrial origin. Not. only do the Ivory Coast tektites have the same K-A age as the Bosumtwi crater I' (Gentner --et al, 1964), but they would also seem to have the same Rb-Sr age as rocks in that part of Africa (Schnebzler --et al, 1965).

The interpretation of the moldavite data in terms of fraztizna? volatilization throws no further light on the problem of terrestrial or lunar oqigin of tektites, although it does necessitate some modification in the explanation of the Rb-Sr ages of tektites. The oxygen isotope data indicate that acid igneous rocks are the only possible terrestrial parent-material fo: tektites (Taylor and Epstein, 1962, 1963). Such rocks, however, do not have contents of the rare earths similar to those possessed by tektites (Haskin and Gehl, 1963). This might be construed as evidence of the extra-terrestrial origin of tektites.

Tlh.~Luwc.-L, .ntro 7- the overall similarity in chemical composition, including the rare earths, of tektites and the "crustal average" indicates terrestrial I - 31 - origin (Taylor and Sachs, 1964). In view of the conflicting evidence, however, it would seem that if tektites are of terrestrial origin then.:

SEP fiindamental point would so far seem to have,been overlooked.

_-___ - . - 32 -

SWRY

New analyses for nine major elements and for the trace elements

Rb and Sr in 17 Bohemian and 6 Moravian moldavites have been reported. cfc~tisi tT; es The refractive indices and bulk SnP_r-i-ic-grawi4&e6-of the 23 moldavites

have a1 so been reported. Real variations iri c;ZazLc.zl cnm?osition and

physical properties were found.

The Moravian tektites that were analyzed had higher average

contents of Si02, A1203, Ti02, FeO and Na2 0, and lower average contents

of MgO, CaO and Sr than had the Bohemian tektites. The major distinction

between samples from the two strewn fields is that Moravian tektites

4 have high and variable FeO and Ti02 contents with low and relatively

constant MgO, CaO, and Sr, whereas Bohemian tektites have high and

variable alkaline earth contents, with low and constant FeO and Ti02.

The overall similarity between tektites from the two groups suggests /

that a common origin is highly probably. No clear regional trends , in composition within the strewn fields were found.

'The relationships between the concentratians of the various.

constituents were examined. It was concluded that the variations in

chemical composition within and between individual moldavites are best

explained in terms of fractional volatilization that occurred during

the brief thermal event dated by the K-A age. The wide range of Rb/Sr

ratios and the uniformity of the Sr isotopic composition support

this conclusion. The variations in chemical composition were re-examined.

using ~~.ii?Rb/Sr ratio as an index of the extent of the proposed

fractional volatil;iLxtion process. The data indiaate the following

order of volatility from most to least volatile: Na20, Rb, K20, Si02, . - 33 -

The efficacy of !the proposed fractional volatilization process

might be demonstrated if (1) the occurrence of coesite (Walter, 1965)

or the diffusion gradient around lechatelierite inclusions or any other

indicator of thermal history could be related to bulk cuutpUsltL~: cf

tektites, (2) U-Pb or Rb-Sr ages of tektites from a specific geographic

group were found to be the same as the K-A or fission track ages, or

(3) tne observed variations in chemical composition were duplicated by experimental volatilization studies.

It is concluded that if tektites are of terrestrial origin then

some fundamental point would so far seem to have been overlooked. .;

I - 34 -

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