| In this study, we conduct high-pressure (HP) high-temperature (HT) diffusion experiments in anhydrous rhyolite near 1000 °C and 1100 °C at 100 MPa using the cold seal pressure apparatus to diffuse argon into natural obsidian charges. We use electron microprobe measurements of the argon concentration gradients to calculate the argon diffusivity (DAr) as a function of temperature and pressure. The range and mean of DAr calculated from these measurements are in good agreement with previous studies (Carroll 1991, Behrens and Zhang 2001). Even so, we observe significant variability in our DAr results, beyond what is likely due to analytical or experimental uncertainty; nearly as much variability as Carroll (1991) and Behrens and Zhang (2001) reported. In particular, we notice a small systematic geometric bias in the distribution of argon in our samples, which appears to correlate with sample deformation. We attribute this effect to distortion of concentration gradients by viscous advection. |
