Isothermal heating experiments on Bjurböle: Implications for the release mechanisms of radiogenic 129Xe
To understand the response of the IXe system to thermal events, isothermal heating experiments were performed on the Bjurböle (L4) standard used for IXe dating. The time dependence of the release of radiogenic 129Xe was determined by analyzing Bjurbole heated for various times at temperatures of I000, 1100, and 1200°°C using noble gas mass spectrometry. Results indicate that a single domain (site of radiogenic 129Xe) model is insufficient to describe the release. At least two, and possibly more, domains are present in Bjurböle. At least one domain outgasses in one hour at a temperature ≤ 1000°C by a mechanism which is not indicative of volume diffusion. For the 1100°C heating one domain containing the majority of radiogenic 129Xe in Bjurböle is seen to outgas in a manner suggestive of volume diffusion. Release characteristics at 1200°C indicate the presence of at least one high temperature domain. In domains which contain I-correlated Xe, assuming loss by volume diffusion, Dodson closure temperatures are at least as high as, and perhaps much higher than, the peak metamorphic temperature experienced by Bjurböle. Based on diffusion parameters, the diffusion length of radiogenic 129Xe, at peak metamorphic temperatures, is much less than a characteristic domain size of 10 -3 cm. Therefore, if the release is governed by volume diffusion, then the IXe system has remained unaltered by thermal metamorphism. Loss of radiogenic 129Xe by mechanisms other than volume diffusion, such as a phase transformation or the melting of a host mineral phase, is possible, but is not indicated in the release of the majority of the gas. If the release is by melting of highly retentive domains then the IXe system has remained unaltered by thermal metamorphism.