Using Aluminum-26 as a Clock for Early Solar System Events

Written by Ernst Zinner Washington University, St. Louis, MO Our solar system formed 4.6 billion years ago. Primitive meteorites provide samples that were formed in its earliest days and thus can give us information about this period. To establish the sequence of events during solar system formation on a time scale of a million years radioactive isotopes that decay with half-lives comparable to this time scale can potentially serve as clocks for dating these events. 26Al, which has a half-life of 0.73 million years appeared to be an ideal chronometer. However, for this to be the case, 26Al had to be uniformly distributed in the early solar system and this fact had not been clearly established. Comparison measurements with two different clocks, 26Al and the decay of uranium isotopes, in refractory Ca-Al-rich inclusions (CAIs) and in feldspar crystals from ordinary chondrites indicate that both techniques give the same ages. It appears that 26Al can indeed be used as a fine-scale chronometer for early solar system events.