Comparison of the chemical variation in a flanged australite with the chemical variation among “normal” Australasian microtektites

EARTH AND PLANETARY SCIENCE LETTERS 9 (1970) 240-246 NORTH-HOLLAND PUBLISHING COMPANY

COMPARISON OF THE CHEMICAL VARIATION WITH THE CHEMICAL VARIATION AUSTRALASIAN

IN A FLANGED

AUSTRALITE

AMONG "NORMAL"

MICROTEKTITES

Billy P. GLASS Planetology Branch Goddarcl Space Fhght Center, Greenbelt, Md. 20771, USA Recewed 30 Aprd 1970 Revtsed version recetved 27 June 1970

A smgle flanged austrahte was found by electron mtcroprobe analysts to have a wtde range in chemistry The sfltca content vanes from 60 to 85% The AI, Fe, Mg, Ca, Na, Tl, Mn and Ba contents vary reversely, and K and P vary directly wtth the sthca content The chemical vartatlons and trends for the austrahte are sunflar to those found among the Austrahan Basm deep-sea mtcrotektttes

1. Introduction Mlcrotekhtes have been found m deep-sea sediments adjacent to the Australasmn and Ivory Coast tektite strewn fields [1,2] Based on the physical and chemical properties of s~xty Australasian m~crotektites, Cassldy et al [3] were able to dlv:de them into two groups. One group is composed of microtektites which have a charactenshc corroded appearance and bottle-green color. These m~crotekt~tes, called bottle-green m~crotekt~tes because of their color, have low s:hca contents and high magnesium content. The second group is composed o f microtektites with physical and chemical properhes slmdar to Australasmn tekntes and are thus called normal mlcrotektltes. The chem:cal compositions of the normal m~crotekhtes are stmtlar to the composmons of the Australasian tekhtes except that about one-third have slhca contents less than 68% which was the lower lmalt of slhca content most often quoted for Australasmn tektites before 1969. Cassldy et al [3] concluded from their study of Australasian mlcrotektltes that Australasmn tektites must have a wider range in chemistry than previously thought and that there must be a high Mg (bottlegreen) variety of Australasian tekhte. Both of these conclusions have been at least partially confirmed by

the work of Chapman and Schelber [4] who have determined the chemical composition of 530 Australasian tekhtes. They define a high Mg variety of Australas:an tekhtes which seem to be related to the tugh Mg m:crotektltes In addmon, they found individual specimens with sdlca contents as low as 62%. If 62% is used as a lower hmlt for the sfl:ca content of Australasian tektites, then all but four mlcrotektlte specimens, designated as normal mlcrotekhtes by Cassldy et al., would have sdlca contents within the slhca range for Australasxan tekhtes. In order to obtain further evidence that the Australasmn tektites have a greater range m chemistry than is generally given, the chem:cal vanat:on of a single flanged australlte was studied with an X-ray microprobe analyzer

2. Method of analysis The tektite used for thts invest:gabon was an austrahte button gwen to Dr. O'Keefe by Dr. Baker. Two thm sections were cut from the tektite. One included only the flange, while the other one included both the flange and the core (fig. I). Both were cut at right angles to the posterior and anterior surfaces and to the outs:de edge o f the flange. During a prehmmary

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Fig 1 Photomicrographs of australite thin sections. (A) Thin section of flange and core Sohd arrows indicate the location of the two low silica inclusions (B) Thin section of flange Sohd arrow indicates location of low silica inclusion Open arrow indicates location of sIhca-nch inclusions investigation the thin sections were scanned m order to locate the areas with the hxghest and lowest sdica content In order to obtain a continuous range m composition, traverses were made across the low and high sdlca inclusions. The percent abundance of the following elements was determined m groups of three S1, A1, Fe, Mg, Ca, Na, K, T1, Mn, P and Ba. Sdlcon was analyzed along with two other elements in each set of analyses, so that the varmhon of each element could be compared directly with the sdlca content. The standards, method of analysis and computer program for converting the raw data to percent oxides were the same as those used by Cassldy et al. [3] for determining the chemical composition of the Australasmn m~crotekhtes.

3. Presentation of data A total of three low sdaca and two high sdlca inclusions were found m the two thm sections (fig. 1). Two of the low sdlca inclusions are m the core area.

The greatest range found m the sdlca c o n t e n t was from approxmaately 60 to 85% (table 1), however the low sdlca and high sdlca areas represent less than one percent of the thin section. The bulk of the glass has an average composition of about 76% $IO 2 (table I).

Table 1 Range of composition m a single flanged austrahte as determined by electron rmeroprobe analysis

SIO2 AI203 FeO MgO CaO Na20 K20 TIO2 MnO

Low silica inclusion (%)

High s I h c a inclusion (%)

Average (%)

60 23 60 40 49 07 10 10 0 10

85 7.5 26 07 09 02 17 04 0 04

76 12 39 19 25 07 18 06 0 07

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No systematic difference was observed between the core and the flange areas. As can be seen in table 1, the average compostt~on of the tektite is what one would expect for an australlte except for the somewhat lower N ~ a n d K values Harker dmgrams o f the var~ous oxides versus sdlca (figs. 2 - 1 1 ) show that there is a strong reverse relationship between silica and A120 3, iron (as FeO), MgO, C a • , TiO 2, MnO~and BaO. Soda also seems to vary reversely with slhca, especially between 76 and 85% sdlca, but below 76% silica the Na20 content seems to remain almost constant. However, scatter of the N a 2 0 data is qmte large. Phosphorus, on the other hand, vanes &rectly with the slhca content. The potash also vanes directly with the sd]ca content up to approximately 76% (which Is the average composRlon of the tektite) Above 76% sd]ca, there is a shght reverse correlation between K 2 0 and $10 2. Australasmn normal m~crotektlte data from Cassldy et al. [3] and more recent unpublished Australasmn normal mlcrotekhte data are plotted on the Harker dmgrams for comparison with the austrahte data. Cassldy et al. did not determine the Ba or P content of the Australasmn mlcrotektltes, thus only data obtained during thxs invest~gatlon are plotted on the Harker diagrams for BaO vs. $10 2 and P205 vs. S~O2. In addlhon, only Mn data obtained for this investigation are plotted on the MnO vs. $102 din-

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gram since Cassldy et al. rounded all the MnO values to 0.1%. Although there is greater scatter m the mlcrotektlte data, the sirmlarlty between the Australian Basra rmcrotektltes and the australlte data is obvious. (The FeO vs. S~O2 dmgram, fig. 3, shows the most d~sagreement because of three low FeO values at the low slhca end of the dmgram.) As would be expected, m]crotekhtes from other parts of the strewn field have composlhons which do not agree as well with the austrahte data. This ~s especmlly obwous m the Na20 vs. $102 and K 2 0 vs. $10 2 plots (figs 6 and 7).

4. Discussion

The szmflanty between the normal Austrahan Basra mlcrotektltes and the australlte is even more striking when ~t is considered that the austrahte data are from a single spectmen and the m~crotekhte data are of indwldual mlcrotekhtes from four d-fferent locahons. Analysis of different austrahte specimens would certainly result in a greater range of values and thus m better agreement with the microtektlte data. Several tektite investigators have questioned the identification of the Australasian mlcrotektltes because of their greater range in chemistry than reported for Australas]an tektRes. However, the ranges

COMPARISON OF THE CHEMICAL VARIATION

245

Table 2 Comparison of the chemical variation among normal Australian Basra mlcrotektltes and prewously reported austrahtes w~th the varmtlon m a single austrahte determined by electron mlcroprobe analysis.

S102 AIzOa FeO MgO CaO Na20 K20 T102 MnO

Normal Austrahan Basra mlcrotekt~tes [21] Cassldy et al [ 3 ]

Austrahtes [23] Schnetzler and Pmson [71

592 - 7 6 2 121 - 2 2 1 30 - 5 8 2358 24 - 5 8 04 - 14 03 - 2.5 07 - 1.0 0 0 8 - 01

690-788 94 - 1 5 3 37 - 5 7 14 - 25 21 - 5 1 09 - 18 17 - 26 05 - 08 0.08- 012

in the major oxide contents m a single flanged austrahte, reported on m th~s paper, are not only greater than the ranges reported among 23 austrahte analyses, but are even greater than the ranges among 21 normal Austrahan Basra mlcrotekhtes (table 2) The K 2 0 concentrahon m austrahtes shows httle variation with $102 content and for most pubhshed data there seems to be a slight reverse relahonshlp between K 2 0 and SIO 2. This is in contrast to the K20-$102 trend for the austrahte specLmen reported on m this paper. Otherwise, however, within the same $102 range the chemical trends obtamed for the single australlte specimen are slmdar to those among australltes and other Australasmn tektites. Some of the chemical variations observed for Australasian tektites, especially between the various groups (australltes, mdochlnltes, phlhppimtes, etc.), is u n d o u b t e d l y due to heterogeneity of the parent material. However, it is difficult to understand how incomplete mtxmg of a heterogeneous parent material could produce such well-defined trends, over such a wide range in concentrations, as is found for the single austrahte specimen reported on in thls paper. On the other hand, the trends observed for this australlte are strndar to those produced by vapor fractlonatlon m an oxldlzmg environment as proposed by Walter [5] and such a process is capable o f producing welldefined trends. Unhke those o f the other oxides, the Na20-S102 and K20-$102 plots do not appear to be straight lines. Below approximately 76% $102, the N a 2 0 -

Austrahte thin section

59 -85 7 -23 2 -59 06 - 4.1 0846 000.96 1022 0 4 3 - 107 0 0 2 - 012

$102 slope decreases and is almost horizontal and above approximately 76% SIO2, K 2 0 appears to be reversely correlated with S102 rather than directly correlated. These changes m slope m a y be due to an increase m volatdlty o f the alkalis at lower $102 contents. (In fact, Walter and G m t r o m c h ' s [6] vapor fractlonahon data. can be interpreted as showing the same changes in slope for Na20-SIO 2 and K20-$102 at approximately 76% SiO 2 ) An alternative explanation for the reverse correlation between Na and $1 and K and Sl above 76% $102 may be that the trends are the result o f mtxmg with lechateherlte ($IO 2 glass). However, the extrapolated trends do not seem to pass close to 100% SIO 2 at 0% N a 2 0 or 0% K 2 0 .

5. Summary and conclusions Microprobe analys~s o f than sections taken from a flanged austrahte shows that the austrahte has a contmuous range m $102 content from at least 60% up to 85% (excluding lechatehente). Thas range in silica content ~s greater than that observed among all the normal Australian Basin microtektites No systematic difference m composition was observed between the core and the flange o f the austrahte. Aluminum, Fe, Mg, Ca, T1, Mn, Ba, and to a lesser extent, Na, vary inversely with the silica content, whde P and K vary darectly with the sdlca content. The chemtcal variations and trends observed for the austrahte are smadar to those observed among Australasian mlcrotektltes

246

B P. GLASS

- especmlly the Austrahan Basra mlcrotekhtes - and the chemical trends for both the austrahte and microtektites are sLrnflar to those produced by vapor fractionatlon m an oxidizing enwronment. The data presented m th~s paper are interpreted as further evidence that the Australasmn tektites and m~crotektltes have a common origin.

Acknowledgements I am grateful to Dr. J.A. O'Keefe of Goddard Space Fhght Center for suggesting that I look for chemical variations m a thin section of an austrahte flange which was prepared for ham by W. Kouns. F Wood, Jr. helped w~th the mxcroprobe analys~s and discussions with Dr. L.S. Walter were helpful. Thas research was performed whale the author was on actwe duty with U S. Army Corps of Engineers assigned to Goddard Space Flight Center. The author is presently a National Research Councd Postdoctoral

Resident Research Associate At Goddard Space Fhght Center.

References [ 1] B.P Glass, Mlcrotektltes m deep-sea sediments, Nature 214 (1967) 372. [2] B P Glass, Glassy objects (mlcrotektltes 9) from deep-sea sedtments near the Ivory Coast, Science 161 (1968) 891 [3] W A Cassldy, B P Glass and B C Heezen, Physical and chemical properties of Australasian mlcrotektltes, J Geophys. Res. 74 (1969) 1008 [4] D.R. Chapman and L C. Schelber, Chemical mvestlgatlon of Australasian tektites, J Geophys Res. 74 (1969) 6737. [5] L S Walter, Tektite compositional trends and expertmental vapor fraetlonatlon of sdlcates, Geochma Cosmochtm Acta 31 (1967) 2043 [6] L S Walter and J F. Gmtromeh, Vapor fractlonatlon of slhcate melts at high temperatures and atmospheric pressure, Solar Energy J. 11 (1967) 163. [71 C C. Schnetzler and W H. Pmson, Jr., The chemical composition of tektttes, m Tektites, edited by J A O'Keefe (Unwerslty of Chicago Press, Chicago, 1963) p 95