ISOTHERMAL HEATING EXPERIMENTS ON BJURBOLE - IMPLICATIONS FOR THE RELEASE MECHANISMS OF RADIOGENIC XE-129

To understand the response of the I-Xe system to thermal events, isoth ermal heating experiments were performed on the Bjurbole (L4) standard used for I-Xe dating. The time dependence of the release of radiogeni c Xe-129 was determined by analyzing Bjurbole heated for various times at temperatures of 1000, 1100, and 1200 degrees C using noble gas mas s spectrometry. Results indicate that a single domain (site of radioge nic Xe-129) model is insufficient to describe the release. At least tw o, and possibly more, domains are present in Bjurbole. At least one do main outgasses in one hour at a temperature less than or equal to 1000 degrees C by a mechanism which is not indicative of volume diffusion. For the 1100 degrees C heating one domain containing the majority of radiogenic Xe-129 in Bjurbole is seen to outgas in a manner suggestive of volume diffusion. Release characteristics at 1200 degrees C indica te the presence of at least one high temperature domain. In domains wh ich 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 Bjurbole. Based on diffusion parameters, the diffusion length of radiogenic Xe-129, a t peak metamorphic temperatures, is much less than a characteristic do main size of 10(-3) cm. Therefore, if the release is governed by volum e diffusion, then the I-Xe system has remained unaltered by thermal me tamorphism. Loss of radiogenic Xe-129 by mechanisms other than volume diffusion, such as a phase transformation or the melting of a host min eral phase, is possible, but is not indicated in the release of the ma jority of the gas. If the release is by melting of highly retentive do mains then the I-Xe system has remained unaltered by thermal metamorph ism.