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I want my planet's atmosphere to have roughly 10% argon so that its inhabitants can easily use it to color their lights purple. But I'm not sure how to explain the cause of this high argon content. Does anyone have any ideas?
Some of my ideas include argon warfare and argon-breathing plants.
Would this atmosphere even be feasible?
Edit: Thanks for answering, it seems that it would be unfeasible. I'll ditch the argon idea and keep an Earth-like atmosphere.
Argon (in particly, $^{40}\text{Ar}$) in terrestrial planet atmospheres tends to come from the decay of Potassium-40 in rocks. Therefore, higher crustal concentrations of potassium would likely lead to higher amounts of argon. It's possible that a planet not subject to impacts early in its life would retain more lighter elements in its crust, like potassium, and therefore would produce more argon over the course of its life, leading to an atmosphere with a higher fraction of argon - particularly if it initially had more quite a bit more potassium than Earth. Tectonic activity might be a reasonable pathway to actually releasing this argon.
It's also worth noting that Argon is slightly heavier than oxygen and nitrogen. This means that a slightly lower escape velocity could lead to the planet losing more lighter gases while still retaining all of its argon. So pick a potassium-rich, relatively unscathed, tectonically-active, low-escape velocity planet!
On the note about plants using argon: As L.Dutch mentioned, argon is an inert gas, and as such does not easily undergo chemical reactions. Therefore, I'd be rather surprised if it played a key role in some modified photosynthetic process. On Earth, it doesn't seem like argon significantly inhibits metabolic processes provided that enough oxygen is present, but it's unlikely that it would actually be a key part of exotic reactions.
Argon is a noble gas, as its name suggests
The name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive", as a reference to the fact that the element undergoes almost no chemical reactions.
Because of this it cannot be produced by any biological activity.
Nearly all of the argon in the Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most easily produced by stellar nucleosynthesis in supernovas.
If you want to have large amounts of argon, you need to have large amounts of potassium-40
Potassium-40 is a radioactive isotope of potassium which has a long half-life of $1.251×10^9$ years. It makes up 0.012% (120 ppm) of the total amount of potassium found in nature.
which doesn't seem that plausible, considering the relatively low abundance it has.
Argon wells.
Gases produced by radioactive decay can accumulate under impervious strata. Helium used to be a byproduct of natural gas recovery. Recently, helium prospectors deduced where helium would accumulate, and drilled the first helium well.
So too argon. Both in our world and yours, it should be knowable where argon would accumulate because of the geologic features of the ground. Your people have argon wells, the same way we now have helium wells.
