RB-SR STUDY OF IMPACT GLASS AND COUNTRY ROCKS FROM HENBURY METEORITE CRATER FIELD

Rb-Sr data for the Henbury country rock (Winnall Beds) and the laterally equivalent Pertatataka Formation 95 miles ENE of Henbury, fall close to an isochron with age 730 m.y. ± 45 m.y. and an Sr87 Sr86 initial ratio of 0.724. The scatter about the isochron is interpreted as variation in initial Sr87 Sr86 reflecting small changes in Sr provenance. The impact glass data are particularly well-aligned over a 15% range in Rb Sr, and independently define an isochron of 675 ± 100 m.y. Certain of the sediments will fit the glass alignment to within experimental error. These data show that no selective loss of Rb relative to Sr occurred during melting. Comparison of Rb, Sr abundances of glasses and sediments suggests that, although similar, none of the analyzed subgreywackes are identical with the inferred parent material of the glasses, which would contain a greater clay fraction. Alternatively the composition of the glasses was biassed by selective melting of the clay matrix. © 1969.