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Re: 129I/129Xe
Apparently there is still controversy over whether I/Xe is really even a clock.
The I/Xe in meteorites could have been affected by aqueous alteration,
reset by shock or confused by a nonhomogeneous distribution. Ugh.
So there could be more to this dating game than I/Xe. In any event, as I
mentioned, it is only good for relative dating since 129I did its thing in
the first 100 m.y. (Supernova Time) and then vanished. Whatever was forming
at the time (chondrites, chondrules) would have picked up the resultant
129Xe. It should be noted that 129Xe is present in the Earth's atmosphere,
but chondrites have an amount in excess of this, due (it is thought), to
the supernova explosion in our vicinity way back then.
Joel