Two decades ago, Robert Gentry and his colleagues at Oak Ridge National Laboratory reported surprisingly high amounts of nuclear-decay-generated helium in tiny radioactive zircons from Precambrian rock.Up to 58% of the helium
(that radioactivity would have generated
during the alleged 1.5 billion year
age of the granodiorite) was still in the zircons. Yet the zircons were so small that they should not have retained the helium for even a tiny fraction of that time.
The helium should have
diffused out of the zircons over such a long time period.