MAGNESIUM DIFFUSION IN MINERLAS IN CAIs: NEW EXPERIMENTAL DATA FOR MELILITES AND IMPLICATIONS FOR THE Al-Mg CHRONOMETER AND THERMAL HISTORY OF CAIs

Introduction: Ca-Al-rich inclusions (CAIs) consist of high temperature minerals that represent the oldest known solid objects in the solar system. The CAIs are known to have contained the short-lived ra-dionuclide 26 Al, which decays to 26 Mg, and has a half-life of 0.72 Myr. The 26 Al-26 Mg decay system provides a high-resolution relative chronometer for the CAIs [e.g., 1]. However, the chronometer could have been partly disturbed by Mg diffusion in the minerals in CAIs during parent body and/or nebular processes [e.g., 1-3]. It has also been suggested that the non-zero initial δ 26 Mg* of 0.7 ‰ in Efremovka (E44) melilite is a consequence of diffusive exchange of Mg isotopes between melilite and anorthite during a period of 10