New Rb, Sr and Sr87 Sr86 determinations are reported for seventy-three tektites from the Southeast Australasian strewn-field. The specimens represent several of the new chemical types of Chapman and Scheiber (1969) within this field (High-Ca, High-Mg, High-Cu and B, Normal, Low- Ca Al and High- Na K). They were selected for maximum dispersion of Rb Sr from knowledge of their K and Ca contents. Except for the Low- Ca Al philippinites, each type gives an isochron which is fitted to within experimental error. The High- Na K type has no dispersion in Rb Sr, the lowest Rb (20 ppm), high-Sr (400 ppm) and lowest Sr87 Sr86 (0.704); it is quite separate to the others on the isochrondiagram. The High-Mg type (here represented by javanites, one australite and one philippinite) defines an apparent isochron of age 100 ± 30 m.y. and initial Sr87 Sr86 of 0.7154 ± 0.0007. Rb is positively correlated with SiO2, inversely with Mg and inversely with Sr, probably due to igneous differentiation in the source-rock to which the isochron would thus refer. The High-Ca type defines a different isochron, of age 265 ± 20 m.y. and initial Sr87 Sr86 0.7101 ± 0.0004 and the High-Cu, High-B type is different again at 200 ± 25 m.y., 0.7117 ± 0.0010. For all types except Low- Ca Al, there are different and strong Sr-Ca correlations and characteristic trends or fields of Rb vs. SiO2, which parallel the definition of types by major-element compositions and which support the non-pooling of their data on the isochron-diagram. The components of layered tektites (Muong-Nong type) disperse widely in Rb Sr but fit the isochrons of their particular types. The coherence within types of Rb with K, and of Sr with Ca shows that the principal dispersion of Rb Sr is not due to selective loss of Rb during melting and cooling following tektiteformation. The Rb-Sr data do not distinguish between a lunar or a terrestrial origin. If each of the different chemical types of tektite represent separate differentiated igneous suites from the Moon, as proposed by Chapman and Scheiber (1969) then the Rb-Sr isochrons would measure their ages of differentiation. Lunar vulcanism as recent as 100 ± 30 m.y. would be indicated. On the other hand, if the major- and trace-element chemistry of the several types can be interpreted as due to mixing of lunar rocks of different ages or as signifying terrestrial sedimentary rooks, the apparent isochrons may have no direct time-significance. © 1969.
