Chemical systematics among the moldavite tektites

Gacchimica et Cosmochimica Acta Vol. 46, pp. 2447-2452

0016-7037/82/122447-06503.00/0

© Pergamon Press Ltd. 1982. Printed in U,S.A.

Chemical systematics among the moldavite tektites J. W. DELANO* and D. H. LINDSLEY Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794 (Received December 3, 1981; accepted in revised form August 20, 1982) Abstract--The compositional variations that occur among the moldavite tektites are caused principally by incomplete mixing of two components during fusion. With the possible exception of silica, there is no evidence for significant losses of volatile species by fractional vaporization. Chemical constraints have been calculated for the two source-materials that contributed to the moldavites. If these tektites were formed by impact fusion, as is commonly believed, then the compositional systematics preserved within the moldavites suggest that hypersonicflow and ejection of impact melts are orderly processes. Insights gained from the study of tektites should prove useful in interpreting the chemistries of impact glasses from other bodies in the solar system. 1. INTRODUCFION TEKTITES are either impact glasses produced during meteoritic collisions with the Earth (e.g., King, 1977; Taylor, 1973) or volcanic glasses ejected from the Moon (O'Keefe, 1978). Although the former view is presently held by most investigators, the physics involved in forming and transporting tektites has remained a subject of enduring interest and debate (e.g., Chapman, 1971; Chapman et al., 1962; Hawkins and Wolfson, 1960; Jones and Sanford, 1977; Kieffer, 1977; Lieske and Shirer, 1964; O'Keefe, 1976, 1978; O'Keefe and Ahrens, 1982). The present paper is an attempt to contribute to this subject of tektite formation. The moldavites are a group of tektites occurring in two Czechoslovakian strewnfields (Fig. 1) separated by a 60-km hiatus (Bougka, 1964, 1968, 1972; Bougka et al., 1968). The western strewnfield is located in Bohemia, while the eastern one is in Moravia. Since the Ries crater in Germany and the moldavites have similar ages (14.7 +__0.7 m.y.; Fleischer et aL, 1965; Gentner et al., 1963, 1967; ZAhringer, 1963), there is a commonly held view that they are genetically related, despite the fact that moldavites are situated at distances of 260 km to 415 km from that crater. One hypothesis is that the Ries crater was the source of these tektites by fusion of materials at the Ries site (e.g., Bougka, 1968; Bou~ka et al., 1973; Engelhardt, 1967; Gentner et al., 1967; H6rz, 1981; Jones and Sandford, 1977; Konta and Mrfiz, 1969; Konta, 1972; Pohl et al., 1977). This view was, however, confronted by the fact that impact glasses occurring at the Ries crater are chemically unlike the moldavites (e.g., Bou~ka, 1968; Bou~ka et al., 1973; Bougka and Randa, 1976; Engelhardt and HiSrz, 1965; Engelhardt, 1967; Haskin et al., 1980; Philpotts and Pinson, 1966; Schnetzler et al., 1969). This problem has received considerable attention from investigators, so that now there is agreement that the Ries * Department of Geological Sciences, State University of New York, Albany, New York 12222.

#asses were formed by impact melting of lithologies occurring in the crystalline basement (Fig. 2) at depths > 600 meters (Bou~ka, 1968; Dennis, 1971; Engelhardt and Htrz, 1965; Engelhardt, 1967, 1972; H6rz, 1965, 1981; Schnetzler et aL, 1969; Stable, 1972). The source-materials for the moldavites may have been sedimentary rocks (Fig. 2) unconformably overlying the crystalline basement at Ries (BouSska, 1968, 1972; Bougka et al., 1973; Bougka and 15,anda, 1976; Engelhardt, 1967; H6rz, 1981; King and Bougka, 1968). A second view for the source of the moldavites is that they are related indirectly to the Ries event by the simultaneous impact of another projectile near the present strewnfields (Barnes, 1964, 1969; Rost, 1972). This scenario would be analogous to the Darwin glass which is (a) coincident in time with the Australasian tektites (Fleischer and Price, 1964; Fleischer et al., 1969; Gentner et aL, 1973) and (b) associated with a small ( ~ 1 km) crater in Tasmania (Ford, 1972; Fudali and Ford, 1979). This implies that the Australasian 'event' may have been characterized by the impact of at least two projectiles (Fudali and Ford, 1979; Gentner et aL, 1973). The problem confronting this hypothesis for the moldavites is that no crater has yet been found in the vicinity of the strewnfields, except for the Ries. The third view for the source of the moldavites (and other tektites) is that they are lunar volcanic glasses (O'Keefe, 1969, 1976, 1978, 1980). The absence of identifiable meteoritic contamination within the moldavites and Ries glasses (Morgan et al., 1979) could be used as evidence in favor of a volcanic rather than an impact origin. A lunar origin for tektites in general has, however, been made more complicated since the Apollo explorations of the Moon (King, 1977; Taylor, 1973; Taylor and McLennan, 1980). 2. COMPOSITIONAL VARIATIONS Moldavites, as well as tektites in general, display significant compositional variations within individual specimens (Glass, 1970; King and Bou~ka, 1968)

2447

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A f t e r Pohl et al. ( 1 9 7 7 ) FIG. 1. Map showing the locations of the Ries Crater in Germany and the moldavite strewnfields in Czechoslovakia. The Bohemian moldavites occur in the western strewnfieid (left) and the Moravian moldavites are in the east. Figure has been modified after Pohl et al. (1977). and among samples belonging to the same strewnfield (e.g., Bougka and Povondra, 1964; Chapman and Scheiber, 1969; Philpotts and Pinson, 1966; Schnetzler and Pinson, 1964; Taylor and Kaye, 1969). These variations have been attributed to fractional vaporization of volatile species during fusion (e.g., Bou~ka, 1968; Cassidy et al., 1969; Cohen, 1963; Konta, 1972; Philpotts and Pinson, 1966; Sehnetzler et al., 1969; Walter, 1967; Walter and Clayton, 1967) and/or heterogeneous source-materials (e.g., Bou~ka, 1968, 1972; Bou~ka et al., 1973; Cherry and Taylor, 1961; Engelhardt, 1967; King and Bou~ka, 1968; HSrz, 1981; Sehnetzler and Pinson, 1964. I f fractional vaporization had contributed to the chemical variations among the moldavites, then the simple approach of plotting the analyses as ratios of

refractory lithophile elements (or oxides) would be useful in diminishing the effects of vaporization in order to better distinguish the variations caused by incomplete mixing of heterogeneous source-materials. This approach has already been applied with some success to lunar impact glasses (Delano et al., 1981) and Australasian tektites (Delano et aL, 1982a). The moldavite analyses of Philpotts and Pinson (1966) have been selected for special consideration because those authors investigated the largest number of samples of any research group. Other sets of contemporary data (Bougka and Povondra, 1964; Konta and Mrfiz, 1969; Rost, 1972; Sehnetzler and Pinson, 1963, 1964) are plotted on separate diagrams. That approach has two benefits. Firstly, it can furnish confirmation of trends identified in the data from Philpotts and Pinson (1966). Secondly, it avoids the blurMOLDAVITES I2

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F[o. 3. Analyses of moldavites by Philpotts and Pinson (1966) are plotted against ratios of refractory lithophile oxides. This approach would tend to minimize the variations caused by fractional vaporization,/fthat process occurred during formation of the moldavites. These tektites display systematic variations indicative of two-component mixing. The arrows at each end of the curves show the locations of the "end-members" used to construct the mixing lines by the method of Langmuir et al. (1978). The curves are

not least squares fits to the data. Note in Fig. 3(a) that the Ries impact glasses do not lie along the moldavite mixingcurve. The data for the Ries glasses are from Engelhardt (1967), Engelhardt and H~rz (1965), and St/~le (1972). The Moravian moldavites are shown as triangles, while the Bohemian moidavites are circles. Equations of the curves are provided in Table 1.

Moldavite tektites ring of trends caused by interlaboratory bias within the analyses. Figure 3 shows the results of plotting ratios of refractory lithophile elements (or oxides). Since these plots should have minimized the effects, tf any, of fractional vaporization among the moldavites, the trends evident in Fig. 3 are indicative of multiple source-materials. The curves in Fig. 3(a, b, c) are not least-square fits to the data, but rather are two-component mixing lines calculated using the approach of Langmuir et al. (1978). The two end-members used in the arithmetic are indicated by the arrows at the ends of each curve, and were employed consistently throughout all diagrams (end-member #1 = average of samples T5320 and T5325 from Philports and Pinson; end-member #2 -- average of sampies T5317 and T5318 from Philpotts and Pinson). The satisfactory agreement between these curves and the moldaviws suggests that incomplete mixing between at least two components occurred during the melting event. In addition, since the mixing curve does not intersect the compositional field of the impact glasses occurring at the Ries crater (Fig. 3a), the moldavites must have been produced from different materials than the Ries glasses (Bougka, 1968; EnMOLDAVITES •

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RG. 5. Analyses of moldavites by Philpotts and Pinson (1966) are plotted using volatile/refractory ratios along the Y-axis and a refractory/refractory ratio along the X-axis. The moldavites display systematic variations indicative of two-component mixing. These trends would have been disrupted if significant fractional vaporization had occurred among the volatile alkalis during formation of the moldavires. The curves axe two-component mixing curves that were constructed by using the two "end-members" indicated by the arrows. Same symbols as in Fig. 3. Equations of the curves are provided in Table 1.

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FIG. 4. Moldavite analyses from other investigators are also consistent with the two-component mixing evident in Fig. 3. The dashed line is the mixing curve from Fig. 3(a) using the data from Philpotts and Pinson (1966). Displacements from this reference curve may in part be due to interlaboratory bias among the various investigators.

gelhardt, 1967, 1972; Engelhardt and HSrz, 1965; Schnetzler et aL, 1969; StAhle, 1972). The Ries #asses originated by fusion of the crystalline basement rocks (Fig. 2) located at depths > 600 meters below the preimpact surface (e.g., Engelhardt and HSrz, 1965; St~hle, 1972). Figure 4 demonstrates that this mixing relationship among the moldavites is a real feature and not merely an analytical artifact of one investigative group. However, displacements of the data in Fig. 4 from the reference curve are, in some instances, substantial. Although we prefer to believe that these displacements are a consequence of inteflaboratory bias among the various investigators, this may not necessarily be the whole explanation. Volatile/refractory ratios are plotted along the yaxis in Fig. 5. The fact that two-component mixing is also readily apparent in these diagrams demonstrates that fractional vaporization of K20, Na20, and Rb during fusion was not significant. That is, partial- and variable-losses of these volatiles due to vaporization would have caused the points to scatter, which is not observed. As in Fig. 3, the curves shown in Fig. 5 are two-component mixing lines calculated using the end-members indicated by the arrows. The agreement between these curves and the moldavites is strong evidence for incomplete mixing between two components with no significant vapor-loss of the volatile constituents.

2450

J.W. Delano and D. H. Lindsley MOLDAVITES

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terrestrial atmosphere). This is compatible with the moldavites (Figs. 5, 6). The second hypothesis requires that the sourcematerials for the moldavites consisted of at least three components. The third component would have been enriched in SiO2. In addition, it must have been strongly depleted in all of the other elements (Al, Mg, Fe, Ca, Ti, Rb, Sr, Na, K) in order not to disrupt the two-component mixing evident in Figs. 3 and 5. At present, we are unable to eliminate either of these two hypotheses.

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3. SOURCE-MATERIALS OF MOLDAVITES 0 t~

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Co0/Ti0 z FIG. 6. (a) At any given value for the CaO/TiO2 ratio, the moldavites have a wide range in abundances of SiO2 (wt.

%). This scattering of SiO~ suggests that some process, in addition to the two-component mixing indicated in Figs. 3 and 5, has occurred. Three-component mixing and/or partial vola"tflizationof SiO2 are possibilities discussed in the text. The open symbols are averages of 51 moldavite analyses compiled by Rost (1972). Philpotts and Pinson (1966) are the source of the solid symbols, as in Figs. 3 and 5. (b) A two-component mixing curve has been constructed. The equation given in Table 1 must be regarded as tentative due to the scatter of SiO2 values in moldavites (i.e., for SiO2, two-component mixing is not a good approximation).

The abundance of SiO2 among the moldavites ranges from 75.1 wt. % to 84.5 wt. % (Rost, 1972). The large variation (Fig. 6a) is not compatible with the two-component mixing evident in Figs. 3 and 5. However, mixing is not entirely obscured by the SiO2variations (Fig. 6b). We suggest that this characteristic of silica can be interpreted by either one of two hypotheses: (1) fractional vaporization of silica; or (2) mixing of a third component that had a high abundance of SiO2 (e.g., >75 wt. %). According to the experimental results of Walter and Carron (1964) and Walter (1967), partial vaporization of SiO2 can occur without significant loss of the alkalis (Fig. 5) under oxidizing conditions (e.g.,

We have shown that the chemical variation among the moldavites (with the possible exception of SiO2) is due to incomplete mixing of at least two components at the site of fusion. Equations for the mixingcurves in Figs. 3, 5, and 6 are listed in Table 1. Constraints on the two end-members are provided in Table 2. These are new limitations that can easily be applied by investigators attempting to identify the source-materials of the moldavites. For example, if the moldavites originated by impact fusion of the unconsolidated Tertiary sediments at pies, as proposed by Bou~ka (1968, 1972), Bou~ka etal. (1973), and H6rz (1981), then analyses of those sediments must fall within the limits specified in Tables 1 and 2. Since a chemical investigation of the Tertiary sediments is currently in progress (Engelhardt, pers. comm., 1982), a diagnostic test of the Pies-source model for the moldavites appears to be near. The constraints listed in Table 2 are consistent, but not uniquely so, with the end-members having been sedimentary in nature, as suggested by Bou~ka ( 1968, 1972), Bou~ka and l~anda (1976), Engelhardt (1967), Haskin eta/. (1980), H6rz (1981), and Konta and Mrfiz (1969). One component was argillaceous (e.g., shale), and the second was more calcareous. If the variation in SiO2 is due to the occurrence of a third component, then it would have been rich in quartz (e.g., sandstone). If the Pies crater was the source of the moldavites, then cratering mechanics (H6rz, 1981; Pohl et al., 1977), in conjunction with the relative distances of the two strewnfields from the pies crater (Fig. 1), would imply that the argillaceous

Table

Table

I.

Two component mixing curves calculated using the method o f Langmulr e t a l . (1978). In a l l equations, x = CaO/TiO2~at-To (by weight). These curves are s h o w n i n Figs. 3-6.

2.

Compositional l i m i t s on the two components furnished by the equations in Table I . Component #1 appears to be argillaceous, while component #2 is mere calcareous. The parentheses around the Si02/MgO ratio indicate that the l i m i t i n g values are regarded as tentative.

COMPONENT

#1

3.5 • CaO/TiO2 z 0

C O M P O N E N T #2

55 > CaO/TiO2 ~ 14

y = (AI203/MgO);

0.81x - 0.52Xy - 1.02y + 26.0 = 0

y = (zFeO/MgO);

O.05x - 0.52xy - 1.02y + 5.3 = 0

y = (Sr/MgO);

O.O02x - O.52xy - 1.02y + 0.028 = 0

y = (K20/MgO);

0.36x - O.52xy - 1.02y + 8.0 = 0

7.9 ~ K20/MgO • 3.5

1.5 _> K20/MgO • 0.9

y = (Na20/MgO);

-0.03x - 0.52xy - 1.02y + 1.7 = 0

1.6 >- Na20/MgO z 0.5

0.15 • Na20/MgO -> 0

y = (Rb/Sr);

7.78x - Ig.Oxy - 274y + 357 = 0

y = (Si02/MgO);

7.60x - O.52xy - l.OZy + 17g = 0 (tentative)

25 z AI203/MgO -> I I

4.5 ~ Al203/MgO a 2.4

5.l z zFeO/MgO z 2.0

0.7 -> zFeO/MgO ~ 0.3

27 X lO-3 z Sr/MgO ~ 12 x lO-3

1.3 -> Rb/Sr ~ 1.2 075 -> Si02/MgO ~ 70)

6.6 x lO-3 -> Sr/MgO -> 4.5 x lO "3

0.85 • Rb/Sr z 0.60 (30 -> Si02/MgO ~ 20)

Moldavite tektites component # 1 was shallower in the target than the more calcareous component #2 (Table 2). Isotopic evidence indicates that both components were formed <100 m.y. ago (Schnetzler et al., 1969). 4. CONCLUSIONS The following conclusions can be offered from this study: (1) The compositional variations among the moldavites, with the possible exception of SIP2, are due to incomplete mixing of at least two components at the site of melting. Limits on the compositions of those components have been calculated and will be useful in evaluating models for the source of moldavites. (2) For those elements considered in this study (Si, Ti, AI, ICe, Mg, Ca, Na, K, Rb, St), fractional vaporization played no significant role in causing the chemical variation among the moidavites, with the possible exception of silica. This result appears to invalidate the model proposed by Konta (1972). If, as is commonly believed, the moldavites were formed by impact fusion, then the following points can be made: (3) Large scale homogenization of multi-component targets does not necessarily occur during fusion by hypervelocity impacts. Consequently, moldavites possess a memory of the detailed chemical and spatial characteristics of their source-materials. This indicates that hypersonic flow and ejection of impact melts are orderly processes even on a planet containing a substantial atmosphere. (4) We expect that principles derived from further work on tektites will not only furnish additional constraints on the nature and provenance of their sourcematerials but also will make impact glasses, which are collected from other celestial objects in our solar system, valuable samples for regional exploration (e.g., Delano et al., 1982b). Acknowledgments--The comments and criticisms made by V. Bou~ka, W. yon Engelhardt, F. H0rz, J. A. O'Keefe, and an anonymous reviewer on an earlier version of this manuscript are gratefully acknowledged. This research was supported by NASA Grant NGL33-015-130. REFERENCF~

Barnes V. E. (1964) Variation of petrographic and chemical characteristics of indochinite tektites within their strewnfield. Geochim. Cosmochim. Acta 28, 893-913. Barnes V. E. (1969) Petrology of moldavites. Geochim. Cosmochim. Acta 33, 1121-1134. Bou~a V. (1964) Geology and stratigraphy of moldavite occurrences. Geochim. Cosmochim. Acta 28, 921-930. Bou~
2451

and color distribution of moldavites. Acta Universitatis Carolinae-Geologica 4, 277-286. Bou~ka V., Benada J., Randa Z. and Kuncif J. (1973) Geochemical evidence for the orion of moldavites. Geochim. Cosmochim. Acta 37, 121-13 I. Bou~ka V. and I~anda Z. (1976) Rare earth elements in tektites. Geochim. Cosmochim. Acta 40, 486--488. Cassidy W. A., Glass B. and Heezen B. C. (1969) Physical and chemical properties of Australasian microtektites. J. Geophys. Res. 74, 1008-1025. Chapman D. R. (1971) Australasian tektite geographic pattern, crater and ray of origin, and theory of tektite events. J. Geophys. Res. 76, 6309-6338. Chapman D. R., Larson H. K. and Anderson L. A. (1962) Aerodynamic evidence pertaining to the entry of tektites into the earth's atmosphere. NASA TR-134. Chapman D. R. and Scheiber L. C. (1969) Chemical investigation of Australasian tektites. J. Geophys. Res. 74, 6737-6776. Cherry R. D. and Taylor S. R. (1961) Studies of tektite composition. II. Derivation from a quartz-shale mixture. Geochim. Cosmochim. Acta 22, 164-168. Cohen A. J. (1963) Asteroid- or comet-impact hypothesis of tektite origin: The moldavite strewn-fields.In Tektites (ed. J. A. O'Keefe), 189-211. Univ. Chicago Press. Delano J. W., Lindsley D. H. and Rudowski R. (1981) Glasses of impact origin from Apollo 11, 12, 15, and 16: Evidence for fractional vaporization and mare/highland mixing. Proc. Lunar Planet Sci. Conf. 12th, p. 339-370. Delano J. W., Lindsley D. H. and Glass B. P. (1982a) Nickel, chromium, and phosphorus abundances in HMg and bottle-green microtektites from the Australasian and Ivory Coast strewnfields (abstr). Lunar Planet. Sci. XIII, p. 164-165. Delano J. W., Lindsley D. H., Ma M.-S. and Schmitt R. A. (1982b) The Apollo 15 yellow impact glasses: Chemistry, petrology, and exotic origin. Proc. Lunar Planet. Sci. Conf. 13th (in press). Dennis J. G. (1971) Ries structure, southern Germany, a review. J. Geophys. Res. 76, 5394-5406. Engelhardt W. yon (1967) Chemical composit4on of Ries glass bombs. Geochim. Cosmochim. Acta 31, 1677-1689. Engelhardt W. yon (1972) Shock-produced rock glasses from the Ries Crater. Contrib. Mineral. Petrol. 36, 265292. Engelhardt W. von and Htrz F. (1965) R i ~ r und Moldavite. Geochim. Cosmochim. Acta 29, 609-620. Fleischer R. L. and Price P. B. (1964)Fission track evidence for the simultaneous origin of tektites and other natural glasses. Geochim. Cosmochim. Acta 28, 755-760. Fleischer R. L., Price P. B. and Walker R. M. (1965) On the simultaneous origin of tektites and other natural glasses. Geochim. Cosmochim. Acta 29, 161-166. Fleischer R. L., Price P. B., Viertl J. R. M. and Woods R. T. (1969) Ages of Darwin glass, Macedon glass, and Far Eastern tektites. Geochim. Cosmochim. Acta 33, 1071-1074. Ford R. J. (1972) A possible impact crater associated with Darwin glass. Earth Planet. Sci. Left. 16, 228-230. Fudali R. F. and Ford R. J. (1979) Darwin glass and Darwin crater: A progress report. Meteoritics 14, 283-296. Gentner W., Lippolt H. J. and Schaeffer O. A. (1963) Argonbestimmungen an Kaliummineralien XI: Die Kalium-Argon-alter der Gl~ser des N6rdlinger Rieses und der B~hmisch-M~ihrischen Tektite. Geochim. Cosmochim. Acta 27, 191-200. Gentner W., KleinmannB., and Wagner G. A. (1967) New K-Ar and fission track ages of impact glasses and tektites. Earth Planet. Sci. Lett. 2, 83-86. Gentner W., Kirsten T., Storzcr D. and Wagner G. A. (1973) K-Ar and fission track dating of Darwin Crater glass. Earth Planet. Sci. Lett. 20, 204-210. Glass B. P. (1970) Comparison of the chemical variation

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