Gas retention age
A gas retention age is the time since a rock last became cold enough to hold radiogenic helium or argon.
A gas retention age measures how long a rock has been retaining a gas produced by radioactive decay, usually helium-4 from uranium and thorium, or argon-40 from potassium-40. Because gases escape easily from warm rocks, these ages often record the last time a meteorite was strongly heated.
In ordinary chondrites, gas retention ages vary widely. Many H chondrites have argon ages near 4.4 to 4.5 billion years, showing they cooled undisturbed. Many L chondrites have ages near 470 million years, revealing that their parent body was broken up by a large collision then. Some meteorites have very young helium ages, meaning they lost helium even later, perhaps when passing close to the Sun or in a minor impact.
Gas retention ages thus act as an archive of collisions in the asteroid belt. Comparing them with fossil meteorites and impact craters on Earth allows researchers to connect events in space with events on our planet.
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