Anomalous xenon in Archean cherts from Pilbara Craton, Western Australia

We report the discovery of anomalies in the xenon isotopic composition of Archean cherts belonging to the metasedimentary sequences of Pilbara Craton, Western Australia. The cherts show contemporary excesses of 129Xe and 131–136Xe, compared to atmospheric Xe. The 129Xe excesses are comparable to those observed in mantle-derived samples, such as MORBs, xenoliths, diamonds and CO2-rich well gases and correlate with excesses of 238U fissiogenic-produced 136Xe, close to the mantle ratio. However, isotope 131Xe is in excess of 238U fissiogenic-Xe production. The 129Xe/131Xe ratio ranges from 0.7 to 3.2 and it is consistent with production of these two isotopes by neutron-capture reactions with tellurium. These anomalies are analogous to those measured in metamorphic rocks (gneisses and amphibolites) in the KTB borehole in Germany and probably were produced by reactions with Te.