36.06-P

Oxygen Isotope Constraints on the Origin of Georgia Tektites

E. F. Albin (Fernbank Science Center)

Georgia tektites ("georgiaites") are North American tektites that occur in east-central Georgia. In this investigation small chips of tektite material was separated from 24 individual specimens for oxygen isotope analysis. Results have an analytical precision of 0.2% based on duplicate analysis of a NIST silicate standard (NBS-28) and tektites. Oxygen isotope ratios (i.e., O - SMOW) range from + 6.9 to + 10.7 parts per mil. The mean O for Georgia tektites is + 9.1 parts per mil. These results are consistent with a O value reported previously on a single georgiaite [1]. It appears that the tektites have oxygen isotope ratios similar to igneous and/or metamorphic rocks. Blum and Chamberlain [2] argue that since sea water has a relatively low O value, its addition to isotopically heavier sedimentary rocks could account for the relatively low oxygen isotope ratios determined for the tektites. Such a model would require vaporization of the target material in order to exchange oxygen between the water and silicates. However, vaporization is not a popular theory of tektite petrogenesis, and since tektites contain very little water, it is difficult to reconcile the low O values due to mixing with sea water. An alternative explanation may be that the low O values are derived from a crystalline basement or it may be that the sedimentary target rocks at the proposed source crater (i.e., Chesapeake Bay crater) have O values similar to that of the tektites. To resolve the issue, it will be necessary to melt samples of the proposed target material and make O measurements on the resulting glass and then compare the results to the tektites. References: [1] Taylor, H.P. and Epstein, S. (1969) J. Geophys. Res., 74, 6834-6844. [2] Blum J.D. and Chamberlain C.D. (1992) Science, 257, 1104-1107.