Dating of zircons
Studies of lunar zircons have followed this procedure to produce dates from 4.3 billion to 3.9 billion years ago for the late heavy bombardment.
“However, most of the shocked grains do not record the age of the impact but rather the age of the rocks they formed in, which are about 1 billion years older.” The story is different on Earth, says zircon expert John Valley, a professor of geoscience at UW-Madison.
With precise measurements of isotopes scientists can calculate, based on the half life of uranium, how long lead has been accumulating.
If all lead was driven off during asteroid impact, the clock was reset, and the amount of accumulated lead should record exactly how long ago the impact occurred.
Did the giant craters on the Moon form during a brief wave or a steady rain of impacts? “The question of what resets the zircon clock has always been very complicated.
For a long time people have been saying if zircon is really involved in a major impact shock, its age will be reset, so you can date the impact.