NEODYMIUM AND STRONTIUM ISOTOPIC STUDY OF AUSTRALASIAN TEKTITES - NEW CONSTRAINTS ON THE PROVENANCE AND AGE OF TARGET MATERIALS

Nd and Sr isotopic studies of Austalasian tektites provide information on the age and provenance of the target materials and allow us to characterize the target area and the impact process leading to tektite formation. Epsilon(Nd) values of australites, splash-form indochinites, and Muong Nong-type indochinites are indistinguishable within analytical uncertainty and average -11.5 epsilon-u. Depleted mantle Nd model ages fall within the narrow range of 1490-1620 Ma, indicating that the source material was derived dominantly from a Proterozoic crustal terrane. Epsilon(Sr) values are variable and are correlated with the geographic location of the tektite samples. Analyses of four Muong Nong-type (or layered) indochinites from a single locality in eastern Thailand yield an isochron age of 167 +/- 12 Ma (2-sigma). A correlation of Rb/Sr fractionation with Sr model ages indicates that the last major Rb/Sr fractionation event experienced by the target materials occurred 175 +/- 15 Ma ago. We interpret this age as the time of deposition of sedimentary target rocks and consider the compositional layering observed in Muong Nong-type tektites to reflect compositional variability inherited from Jurassic sediments. The Nd and Sr isotopic data provide evidence that all Australasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 Ma. We suggest that all of the Australasian tektites were derived from a single impact event, and that the australites represent the upper part of a melt sheet ejected at high velocity, whereas the indochinites represent melts formed at a lower level in the target material which were distributed closer to the area of impact. The impact site is inferred to be within an area of Jurassic sedimentary bedrock, which spans the geopolitical boundaries between northern Cambodia, southern Laos, and south-eastern Thailand.