Tektites and Their Origin - O'Keefe - 1976

 

 

 

 

This result is surprising, in a way, because the  SiO₂ content is 70.4% for the central tektite, and 72.6% for G-1, while for AGV-1 it is 59.0%.  We might thus have expected the tektite to be close to the granite than to the andesite, because it is well known that the chemical composition of igneous rocks is largely determined by the silica content.  However, it was pointed out by Mueller (1915) that tektites could be distinguished from terrestrial igneous rocks because the ratio (FeO + MgO)/Na₂O + K₂O) was higher for a given silica content.  This ratio decreases with increasing silica content, so that Mueller was finding the above-mentioned result, namely that tektite composition resembles (for most elements) that of terrestrial rocks of lower silica content.  (We refer here to the left side of the periodic table.)  Suess (1933) comments similarly; so do Loewinson-Lessing (1935).  Barnes (1939) used this fact as the basis for his conclusion that tektites are for the most part formed from terrestrial sedimentary rocks.  Urey (1958a, b) commented similarly.  The same idea was expressed by Cherry and his collaborators (Cherry et al., 1960; Taylor, 1960, 1962c; Cherry and Taylor, 1961; Taylor and Sachs, 1961; Taylor et al., 1961).

Later, Taylor suggested that sedimentary processes could enhance the silica content in the way required.  In particular, Taylor studied the Henbury sandstone (see Fig. 31); here it is clear that sedimentary processes are enhancing the silica content, because some of the sandstone has as much as 94% SiO₂ (Taylor and Kolbe, 1965).  By proper choice of sandstone, Taylor was able to come close to the australite composition, both in silica content and in other non-volatile elements (Taylor, 1966).

The discrepancy in hydrogen (i.e. water content) was commented on by Suess (1900, p. 247); in fact, the field test to distinguish a tektite from an obsidian is to heat it with a blowtorch or blowpipe; the tektite melts with at most a few bubbles, while the obsidian foams (La Paz, 1948).  Terrestrial igneous rocks tend to have about 5000 ppm water, and sedimentary rocks several times as much.  In an impact, however, the water may escape, at the price of turning the rock into a mass of bubbles (see e.g. Taylor and Kolbe, 1965).

On the right side of the periodic table, there are major discrepancies in many of the elements which are volatile at temperatures of the order of 1000° C.  These discrepancies are difficult to measure by the usual methods of spectrochemical analysis.  The elements are scarce; they volatilize at temperatures below those which are optimum for the metals on the left side of the diagram; and their spectra often have the important lines in relatively inaccessible parts of the ultraviolet.  Hence these discrepancies tend to be overlooked.  Nevertheless Preuss (1935) and Heide (1936b) noted that tektites are lower in copper, germanium, tin, and lead than their suggested terrestrial comparison material (Norwegian loam); similarly Taylor (1966) remarked on the deficiencies in copper, lead, tin, thallium, iridium, and bismuth compared with the Henbury impact glasses.

Similar discrepancies are conspicuous when lunar rocks are compared with terrestrial rocks of similar type.  In Fig. 32 a basaltic clast, 14321.223 from Apollo 14 (Wänke et al., 1972) is compared with W-1.  In Fig. 33 the central tektite composition is compared with lunar sample 12013, the only lunar sample of acidic composition for which substantial trace element data exist.  The comparison is unsatisfactory in several respects; but it suggests that the systematic discrepancy in the volatile elements is removed.

There is also a marked difference in the ratio of ferrous to ferric iron, which does not appear on the charts.  Terrestrial acid igneous rocks or terrestrial sandstones typically have ferric/ferrous ratios of the order of 1, or even more.  For tektites, as mentioned, the ratio is generally less than 0.15.

The chemical parameter which underlies the ferric/ferrous ratio is the oxygen fugacity, which is numerically equal to the equilibrium partial pressure of oxygen, P_O2, that is, to the value of the partial pressure which would be in equilibrium with the glass at the given temperature.  Walter and Doan (1969) report preliminary values as follows:

where P₀₂ is in atmospheres, and T is the Kelvin temperature.  The corresponding number in N/m² is greater by a factor of 10⁵.  The relation yields 10^-9.6 N/m² (or 10^-14.6 atmosphere) at 1100° C, about six orders of magnitude below terrestrial rocks at this temperature (Walter and Doan's value of 10^-17.6 atmosphere for 800° C is a slip; it should be about 10^-21.3 according to their unpublished charts).  More measurements are needed. 

Vorob'yev (1959a) found magnetite spherules, often hollow, in surface cavities of Philippine tektites.  Kleinman (1967) found magnetite inclusions some of which had nickel-free iron cores, as if the tektites had formed in equilibrium with metallic iron, which later oxidized (verbal suggestion by L.S. Walter).  This would again suggest a low oxygen fugacity.

Brett (1967) has noted a relation between the nickel content of metallic spherules in tektites and impact glasses, and the presence or absence of a halo of iron oxide in the surrounding glass.  It appears that for impact glasses, a portion of the iron oxidizes and dissolves in the glass, leaving the spherule nickel-rich.  For tektites, on the other hand, the nickel enrichment and the iron oxide halo are both missing.  This result seems to imply that the oxygen fugacity in tektites is much lower than in impact glasses, so that they do not tend to oxidize the iron.

THE PRINCIPAL FAMILIES OF TEKTITES

When one is confronted with a hand specimen of a tektite, it is usually possible to find out where it has come from by analyzing it.  Most tektite analyses are carried out in weight percent of major oxides.  By comparison with the central composition (Table V), the North American tektites (see Table VI, cols. 15-19) are systemtically low in CaO and MgO.  Moldavites tend to have high silica (over 75%) and high K₂O (over 2.5%).  Ivory Coast tektites usually have low silica (under 70%) and often have Na₂O>K₂0.  Although Aouelloul glass is broadly similar to Darwin glass, both being in the range near 85% SiO₂, the Aouelloul glass has higher CaO and MgO.

When careful studies are made, including microtektites and rare types of macrotektites, it is seen that the range within one strewn field is often much greater than the range from one strewn field to another.  In Table VI are given the major element compositions of tektites from various strewn fields.  In the major fields, the attempt is made to choose examples which show the range in silica content, and the corresponding changes in the other major elements.  Table VII lists the corresponding minor and trace elements, where these have been determined.

The variety of tektite types is such that some kind of overview is needed.  In Figs. 34 and 35, the numbers of the points correspond to numbers of the columns in Tables VI and VII.  Fig. 34 diagrams the more important types of tektites within the Australasian strewn field.  To construct the figure, the weight percent MgO was taken from the analysis, added to CaO and 0.05 SiO₂; call the sum s.  Then MgO/s is plotted vertically upward from the base of the triangle, and similarly for the other two directions.  The central composition of Table V is that of the normal australite, No. 1, near the middle of Fig. 34.  Normal philippinites (10) and normal indochinites (2) are nearby.