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, were determined in twenty-seven tektite and impact glass samples, using an ion microprobe. Samples included tektites from the Australasian, North 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 composition 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 unimportant during tektite formation. This is supported by the observation that B contents and the delta(11)B values of the different samples from the Australasian tektite strewn field are not correlated with each other. 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 compared 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 high B and Li abundances; only clay-rich sediments, such as pelagic and neritic sediments, as well as river and deltaic sediments have B contents (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 thousand). Be-10 and Rb-Sr data indicate continental crustal source rocks and 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 locality close to the coast of SE Indochina in the South China Sea. On the 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 carbonates or evaporites among the source rocks. This finding supports an offshore locality for the North American tektite source crater.