Actually plutonium belongs to the so-called primordial elements, which existed in the matter from which the Earth and the rest of the Solar System were made.
Among the primordial element isotopes, a large number are radioactive, including e.g. certain isotopes of the potassium and calcium of our body.
All the radioactive primordial element isotopes have partially decayed since the formation of the Earth, so they can be found now in smaller quantities than in the beginning.
Among the isotopes that can still be found in large quantities, U235 has the smallest half-life and it has decreased about one hundred times.
The next isotopes with the longest half-life, but lower than U235 are plutonium 244 and one isotope of samarium, which have half-time close to one hundred million years.
So these 2 have decreased about 2^45 times. This is a lot so you would not be able to extract a visible quantity of plutonium from a terrestrial rock, but due to the huge number of atoms that existed in the entire Earth, there are still a few atoms of plutonium 244 that have survived from the formation of the Earth.
A few years ago, it was announced that a team succeeded to detect some atoms of primordial plutonium 244 in some rocks, so the primordiality of plutonium can be considered as proven, even if that does not matter from a practical point of view, as there is too little primordial plutonium left, to be useful.
On the other hand, during the formation of the planets, in the early Solar System, plutonium 244 and many other short-lived isotopes, with half-lives of a few million years, could have played a non-negligible role, because they were a significant source of internal heat.
Has anybody detected any actual plutonium there, rather than its fission byproducts? It 'reactor' event itself took place nearly 2 billion years ago, a long time for most of us, including isotopes of plutonium.
That would be interesting, I wonder what other elements will eventually join it. Also, I'm sure they know what they are doing, but it came out of the ocean floor off the coast of Japan, how did they rule out Fukushima? Is the way it was incorporated into the rock make it certain?
They said a Japanese oil exploration company, not that it was near Japan. It could have been anywhere in the pacific, and it was as gifted to researchers in Australia not Japan.
Radionucleotide contamination is an issue though. Apparently carbon dating will be useless to date anything since the 1950s in the future due to carbon isotope emissions from nuclear testing. So they must have taken care to ensure they were examining clean samples.
Pretty sure it was not an isotope producible in an Earth fission reactor. A stable plutonium you could date back to only 10 million years old would be a different animal.
