It's a good question, I can only guess. The sun certainly has K40, in the fraction that was present in the original nebula out of which the solar
system condensed, minus the amount that has decayed. My guess is that when the earth formed, a small rocky planet near the sun, it was naturally
mostly rock and little gas. So presumably it had little of the primordial argon present in the nebula, the argon that shows up as Ar36 in the sun
today. But it did have K40, as a part of the rocks, which has since decayed to produce Ar40. That's my guess.
There are other differences in isotope abundances between the earth and the solar nebula. Jupiter and the outer layers of the sun should reflect
fairly well the abundances in the solar nebula, but the small planets don't. This is certainly true for the deuterium abundance on earth.
Any other SF Bay chemists?
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