| Abstract: | Variations in the major cation ratios in the (micro-) tektites of each of the different strewn fields were studied with the help of Niggli parameters. There is a high degree of variability in almost each individual strewn field which, however, is not random. Invariably, as SiO2 increases the proportion of alumina and the alkalies among the cations increases, that ofMgO + ΣFeO (total iron as FeO) and CaO decreases; simultaneouslyK2O/Na2O increases whileMgO/FeO decreases. This is entirely analogous to magmatic differentiations and is incompatible with an explanation by mechanical mixing of sedimentary parent rocks. The picture can best be explained by partial melting, in analogy to fulgurites and combustion ultrametamorphic glasses.In some strewn fields (e.g. javaites, georgiaites, bediasites), all tektites formed from the same set of parent rocks. In these fields each cation parameter, when plotted against silica, follow a linear slightly curved path with a very small scatter (simple type). In others (e.g. moldavites, philippinites, australites) some or all cation parameters show a strong scatter which sometimes almost obliterates the differentiation trends. These complex diagrams can be reduced to 2–5 diagrams (lineages) of the simple type, each reflecting a different combination of parent rocks. Most lineages again show the influence of partial melting differentiation.If the chemical variations of tektites are caused by partial melting, the silica-poor end of each variation diagram (case of total melting) and not the average composition corresponds to the parent rock combination involved. Thus, neglecting losses by evaporation (which are minor), the chemical compositions of 25 starting materials were established. All except one of these can very closely be matched by a combination, in appropriate proportions, of only two of the most common sedimentary rocks. The only group which cannot be duplicated by a combination of any two sedimentary rocks are the bottle-green microtektites. |
