Anomalous xenon in archean cherts from Pilbara Craton, Western Australia

We report the discovery of anomalies in the xenon isotopic composition of A rchean cherts belonging to the metasedimentary sequences of Pilbara Craton, Western Australia. The cherts show contemporary excesses of Xe and Xe131-1 36, compared to atmospheric Xe. The Xe-129 excesses are comparable to those observed in mantle-derived samples, U-238 fissiogenic-produced such as MOR Bs, xenoliths, diamonds and CO2-rich well gases and correlate with excesses of Xe-136. close to the mantle ratio. However, isotope Xe-131 is in excess of U-238 fissiogenic-Xe production. The Xe-129/Xe-131 ratio ranges from 0. 7 to 3.2 and it is consistent with production of these two isotopes by neut ron-capture reactions with tellurium. These anomalies are analogous to thos e measured in metamorphic rocks (gneisses and amphibolites) in the KTB bore hole in Germany and probably were produced by reactions with Te. (C) 2001 E lsevier Science B.V. All rights reserved.