Chlorine-36
Chlorine-36 is a cosmic-ray-produced isotope used to measure how long a meteorite has been on Earth.
Chlorine-36 is a radioactive isotope with a half-life of about 301,000 years. In meteoroids, cosmic rays produce it mainly from iron, calcium and potassium. Once a meteorite lands and production stops, the chlorine-36 decays at a steady rate.
This makes it a useful clock for terrestrial ages between about 100,000 and a few million years, the time-scale of many Antarctic finds. Carbon-14, with its much shorter half-life, covers ages up to about 40,000 years, mostly for hot desert meteorites.
Chlorine-36 dating showed that some Antarctic meteorites have been preserved in ice for more than a million years, and a few for over two million. Such ages reveal how ice sheets carry meteorites to stranding zones and hold them for long periods. Iron meteorites are particularly suited to chlorine-36 measurement, because they produce it from iron at a predictable rate and are not affected by chlorine contamination from soil if carefully cleaned.
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