MAJOR ELEMENT CHEMISTRY

Written By Aubrey Whymark 2013-2017
The major chemistry of tektites reveals that tektites have similarities with other impact-related glasses, but are clearly differentiable from volcanic glasses (Koeberl, 1990). Variations in ratios of major elements allows the tektite strewn fields to be differentiated (see Table 5.2). This is because the terrestrial source rock of the tektites was different in each impact event. The type and composition of the source rock is reflected in the composition of the tektite, with allowances for loss of volatiles.

Silica-Rich

All tektites are rich in silicon dioxide. This is a pre-requisite for glass making. At ground zero, source rocks that contain a low percentage of silica will not form tektites.
 
ABOVE: The major chemistry of tektites from four strewn fields. Australasian data from Chapman and Scheiber (1969) – in Koeberl (1990); Ivory Coast data from Koeberl (1986) – in Koeberl (1990); Central European data from Engelhardt and Berthold (2005); North American data from Chao (1963) combined with Cuttita et al. (1967) – in Koeberl (1990). All data in wt%.
Within a single strewn field the tektite composition is not identical and different groups, which often overlap, can be separated out. Table 5.3 shows the typical composition of major goupings of Australasian tektites. Further chemical sub-groups have been identified in Australia (Chapman and Scheiber, 1969). The chemical variability is due to the source rock being heterogeneous. In order to form a tektite (as oppose a spherule) the source rock must necessarily be high in silica. Due to terrestrial weathering processes on Earth, silica is often concentrated in certain common sedimentary rocks. Silica-rich rocks are therefore usually sedimentary or metamorphosed sedimentary rocks. These rocks often vary both laterally and vertically in composition. Resultant tektites, ejected from different areas and depths, in the earliest stages of cratering, will also consequently have slightly different chemical compositions.

Major Element Variation

Major elements vary within a strewnfield as the tektite forming material is not significantly mixed. Thorough mixing is somewhat of a myth and can be disproven based on geochemisty, such as using Be10. Due to lateral variation in the lithology type, ejecta rays may subtly vary in composition. Due to vertical (think stratigraphic) variations in lithology distal (first ejected and proximal (last ejected) ejecta will vary in composition.
 
ABOVE: The mean major chemistry of tektites from the Australasian Strewn Field. Indochinite, billitonite & javanite and philippinite data from O’Keefe (1963). Australite data from Chao (1963) – in Koeberl (1990). All data in wt%.

References