BORON CONTENT AND ISOTOPIC COMPOSITION OF TEKTITES AND IMPACT GLASSES- CONSTRAINTS ON SOURCE REGIONS

Abundances of Li, Be, and B, as well as boron isotopic compositions, w ere determined in twenty-seven tektite and impact glass samples, using an ion microprobe. Samples included tektites from the Australasian, N orth American, and Ivory Coast strewn fields, and Aouelloul and Darwin impact glasses. Variations of B abundance and isotopic composition in a Banged australite were also studied. delta(11)B variations of only a few permil were found within the australite flange. The isotopic com position shows no correlation with the B contents or with the distance from the rim of the flange. The mean delta(11)B value for the Banged australite is very similar to that of Muong-Nong type tektites (-1.9 /- 1.9 parts per thousand). Thus, vapor fractionation has been unimpor tant during tektite formation. This is supported by the observation th at B contents and the delta(11)B values of the different samples from the Australasian tektite strewn field are not correlated with each oth er. Most tektites show a rather limited range of delta(11)B values (-9 .3 +/- 1.5 to +2.7 +/- 1.5 parts per thousand), which is small compare d to the range observed for common terrestrial rocks (-30 to +40 parts per thousand). The B abundance and isotopic data can be used to place constraints on the tektite source rocks. Australasian tektites have h igh B and Li abundances; only clay-rich sediments, such as pelagic and neritic sediments, as well as river and deltaic sediments have B cont ents (up to 100 ppm) and delta(11)B values that are in agreement with the range shown by Australasian tektites (-4.9 to +1.4 parts per thous and). Be-10 and Rb-Sr data indicate continental crustal source rocks a nd exclude pelagic and neritic sediments. However, deltaic sediments, e.g., from the Mekong river, which are of continental crustal origin, agree with Be-10, Rb-Sr, and B data, and support a possible source loc ality close to the coast of SE Indochina in the South China Sea. On th e other hand, one bediasite sample has a very high delta(11)B value of +15.1 +/- 2.1 parts per thousand, requiring the presence of marine ca rbonates or evaporites among the source rocks. This finding supports a n offshore locality for the North American tektite source crater.