I Isn't planetary rock dating really supernova dating?

  • Thread starter swampwiz
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The age of the oldest rocks on Earth & Moon are about 4.5 Gy old. But doesn't the material that made the Earth come from the supernova that had occurred prior to the formation of the Solar System, and doesn't the radioactivity dating measure the age of the creation of that material in that supernova, and not simply when that material had coalesced into the Earth & Moon?

Perhaps the formation of the Solar System was a short time after that supernova, so it would essentially be the same age?
 

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But doesn't the material that made the Earth come from the supernova that had occurred prior to the formation of the Solar System
Yes.
and doesn't the radioactivity dating measure the age of the creation of that material in that supernova,
No. It would if the original radioactive atoms and all of the daughter products stayed together after the initial nucleosynthesis, but that's not generally the case. For example, the mineral zircon readily accepts uranium into its crystal structure but not lead. Thus, when zircon forms it contains no lead at all, and we can reasonably assume that all the lead present in a zircon sample came from the decay of uranium after it was formed.
 
Yes.No. It would if the original radioactive atoms and all of the daughter products stayed together after the initial nucleosynthesis, but that's not generally the case. For example, the mineral zircon readily accepts uranium into its crystal structure but not lead. Thus, when zircon forms it contains no lead at all, and we can reasonably assume that all the lead present in a zircon sample came from the decay of uranium after it was formed.
OK, I see; it's like the way that an organism collects Carbon-14 from its food (or CO2, for plants) only during its life. Zircon collects its "food" of Uranium during its crystallization phase.
 
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Some of the atoms came from supernova. But not the compounds in the Earth's rocks. The dating method determine the age of the rocks and not the age of the atoms. If you could determine the age of the heavy atoms you could tell how long ago they were formed in a supernova. This would be interesting but not possible I believe.
 
swampwiz said:
...doesn't the radioactive dating measure the age of the creation of that material?
In order to date a sample of radioactive material, you need to compare the amount of material left to the amount of decay products in the sample, in order to see just how much of the original sample has decayed. This can only be done using a solid sample, i.e. rock. So you have to wait until the sample has solidified, i.e. until the Earth has formed.
 
Whenever the material melts and then solidifies, it does so by crystallization. Crystals for with specific composition, which separates out selective elements from the bulk. As an example, consider calcium aluminium inclusions, which are some of the oldest solids, and formed over several million years at the beginning. Now, if they were to contain 26Al when it formed there would be 26Mg in it now, and provided we knew how much 26Al was in the original nebula, we could date them. If we don't, we can still date them relatively and rank samples according to age. Suppose the sample was contaminated with magnesium? There is a known distribution of the Mg isotopes in any sample of Mg, so if there were an excess of 26Mg, we would know it had to come from 26Al. Now obviously it is a bit more complicated than this, but that shows the general principle. We know the distribution of natural isotopes, so if there is an enhancement of one, the decay of something else is a possibility. Unfortunately, it gets a bit more complicated than that because there are also physical ways of altering isotope distributions, so as usual, care and analysis is required.
 

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