No Lead 210 Quantity Assumption

[Gavinchi]

I was once told that the science behind radio dating the Earth is filled with assumptions. Like that it's assumed the Lead 210 content of the Earth was at zero when it was cooling. Why would we assume that the Earth contained zero Lead 210 at the point of cooling and how would we account for preexisting Lead 210 when using Uranium-Lead dating methods?

[Gavinchi]

Self-correction, The uranium isotopes used in Uranium-Lead dating do not decay into 210.

The basis of my question is... how do we know that the lead that is present in igneous rock is or isn't radiogenic lead?

[studiot]

This is a good question and to answer it you need to look at what is well established in astrophysics, as well as a bit of nuclear physics.

 

Here is a simplified diagram showing the important bits.

 

 

This plots energy against atomic number and important elements are positioned along the axis.

You will note that the energy is a minimum at the element iron (which has a lower atomic number than lead).

I have drawn a dashed vertical line at this point.

 

Physical systems tend to the lowest available energy state and the diagram shows that processes to the left of the dashed line that combine elements to form new ones release energy or lower their own energy states.

These are of course, fusion processes and are the ones that occur in ordinary stars.

 

The diagram also shows that to the right of the dashed line, to continue to assemble heavier and heavier elements (ie greater atomic number than iron) requires a supply of energy.

Sufficient is not available in ordinary stars so elements heavier than iron are formed by another process.

 

Such a process happens in a supernova.

Ordinary stars are self sustaining for billions of years, but another important supernova process is the violent explosion they undergo.

This ejects material in the explosion.

So this provides freshly minted uranium for the inclusion in the formation of new planets.

 

Looking further at the diagram you can see that all elements beyond iron have the potential to decay by fission with the release of energy.

However not all elements readily do this. Uranium is the element of choice here and lead is pretty stable and one result of the decay of uranium.

 

A great deal of readable further information is provided here.

 

http://large.stanford.edu/courses/2013/ph241/roberts2/

[John Cuthber]

As I understand it they look at things like zircon crystals.

There may have been lead in the mixture from which they crystallised but, since lead is the wrong size and its ions have the wrong charge, it won't fit properly into a zircon crystal lattice.

So, as the crystals form, any lead present is left behind in the liquid phase.

 

https://en.wikipedia.org/wiki/Recrystallization_(chemistry)

[Gavinchi]

This is a good question and to answer it you need to look at what is well established in astrophysics, as well as a bit of nuclear physics.

 

Here is a simplified diagram showing the important bits.

 

astro1.jpg

 

This plots energy against atomic number and important elements are positioned along the axis.

You will note that the energy is a minimum at the element iron (which has a lower atomic number than lead).

I have drawn a dashed vertical line at this point.

 

Physical systems tend to the lowest available energy state and the diagram shows that processes to the left of the dashed line that combine elements to form new ones release energy or lower their own energy states.

These are of course, fusion processes and are the ones that occur in ordinary stars.

 

The diagram also shows that to the right of the dashed line, to continue to assemble heavier and heavier elements (ie greater atomic number than iron) requires a supply of energy.

Sufficient is not available in ordinary stars so elements heavier than iron are formed by another process.

 

Such a process happens in a supernova.

Ordinary stars are self sustaining for billions of years, but another important supernova process is the violent explosion they undergo.

This ejects material in the explosion.

So this provides freshly minted uranium for the inclusion in the formation of new planets.

 

Looking further at the diagram you can see that all elements beyond iron have the potential to decay by fission with the release of energy.

However not all elements readily do this. Uranium is the element of choice here and lead is pretty stable and one result of the decay of uranium.

 

A great deal of readable further information is provided here.

 

http://large.stanford.edu/courses/2013/ph241/roberts2/

 

Thanks for the reply, Studiot. I had an understanding of where the heavier elements originate in photo-planet formation I just didn't know why you'd assume that there would be zero Lead isotopes homogenized within cooled zircon. If there were lead isotopes present among Uranium isotopes at the time of cooling, wouldn't it give the impression that radio-dated samples are older than they actually are? Thank you for the information regardless! I'll take a look at that article here in a bit! Maybe the link will delve into this.

 

As I understand it they look at things like zircon crystals.

There may have been lead in the mixture from which they crystallised but, since lead is the wrong size and its ions have the wrong charge, it won't fit properly into a zircon crystal lattice.

So, as the crystals form, any lead present is left behind in the liquid phase.

 

https://en.wikipedia.org/wiki/Recrystallization_(chemistry)

 

 

Awesome! Thank you, John. This is what I was looking for. I'll go through this wikipedia article.