And when I look at the Wikipedia article, the discussion is so technical and defensive that I can't actually picture what is going on. You are right: there is no decay of 40-Ar into 39-Ar. The difference between measured 40-Ar and 40-Ar at formation is used in the procedure.
But then, different passages in the Wikipedia article contradict each other (first section: 39-K is converted into 39-Ar by neutron bombardment; but "age equation" section: 40-K is bombarded; I think it should be 39-Ar).
The age of meteorites and volcanic stones are measured by this method.
His crown was a simple circlet of Valyrian steel, set with big square-cut rubies. He was a solitary person whose only friend was Orys Baratheon.
But since floods jumble materials of different origins and ages together, that meant the scientists had to date dozens of different minerals.
The youngest crystal in the footprint layer would represent the oldest possible age for the prints; the oldest crystal in the layer above it would represent the youngest they could be.
The Argon race is nothing more and nothing less than descendants of early Earth settlers from the days before the Terraformer/Xenon wars – from before the only jumpgate the lead back to Earth was destroyed. The sun's name, however, remained "Sonra" to the present day.
After it became more and more clear, that no return to Earth will be possible, ever, the government of the exiled humans agreed to purge Earth from the immediate history books to start with a new era. The Argon colonies grouped themselves into "The United Planetary Federation", later called "The Argon Federal Government", which was controlled from Argon Prime.