How Does Uranium-Lead Dating Determine the Age of Rocks?

[Esas Shakeel]

I just recently read about how we got to know the true age of the Earth but I am a bit confused and curious about one thing.

The mineral or rock is initially going to be a uranium right? If it is, isn't all of it going to decay at the same rate?
I know that a radioactive reaction is spontaneous but if it is, is it even possible to figure out the age of the rock with the same idea? Please help me out here and correct me if I am wrong. Thanks

 

[Bandersnatch]

Are you familiar with the concept of half-life?

 

[Esas Shakeel]

Are you familiar with the concept of half-life?

Oh! How could i possibly not relate! So the half life of uranium-238 is 4.5 billion years. What does it change itself to? Lead? or any other radioactive element?

 

[Borg]

I think that what you're missing is that radioactive elements often decay into other radioactive elements which then have their own decay rates. By examining the various percentages of elements, it is possible to date the object.

 

[Bandersnatch]

What does it change itself to? Lead? or any other radioactive element?

It decays to Thorium-234. But that is highly radioactive itself. The final, non-radioactive result of its decay chains is Lead.

 

[Esas Shakeel]

@Bandersnatch @Borg
So by knowing how much lead is present and all the other percentage of elements in the rock, We'd add up all those years and find out?

 

[Bandersnatch]

In order to do proper dating, you need to be sure that the mineral had a specific uranium content when it was formed.
How this particular method is done, is you take zirconium crystals, which can incorporate uranium atoms in their lattice when they grow, but will not incorporate lead. Thus a freshly-formed zirconium crystal will have some uranium content, but never any lead.
In the process of radioactive decay, uranium transmutes into lead. But since now the crystal is already grown and rigid, the transmuted lead atoms remain trapped in the lattice.
If you then take a zirconium crystal, and measure how much uranium and lead it contains, the proportion will tell you how long ago the mineral was formed. E.g., if of the total lead+uranium atoms you find in your sample half is lead, then it tells you that ~4.5 billion years must have passed, since that's the half-life of uranium-238 (i.e. time after which half of its original atoms decay).

 

[Esas Shakeel]

In order to do proper dating, you need to be sure that the mineral had a specific uranium content when it was formed.
How this particular method is done, is you take zirconium crystals, which can incorporate uranium atoms in their lattice when they grow, but will not incorporate lead. Thus a freshly-formed zirconium crystal will have some uranium content, but never any lead.
In the process of radioactive decay, uranium transmutes into lead. But since now the crystal is already grown and rigid, the transmuted lead atoms remain trapped in the lattice.
If you then take a zirconium crystal, and measure how much uranium and lead it contains, the proportion will tell you how long ago the mineral was formed. E.g., if of the total lead+uranium atoms you find in your sample half is lead, then it tells you that ~4.5 billion years must have passed, since that's the half-life of uranium-238 (i.e. time after which half of its original atoms decay).

WOW! Very well-explained! Thanks a lot for all of this. Much appreciated

 

[davenn]

In order to do proper dating, you need to be sure that the mineral had a specific uranium content when it was formed.
How this particular method is done, is you take zirconium crystals, which can incorporate uranium atoms in their lattice when they grow, but will not incorporate lead. Thus a freshly-formed zirconium crystal will have some uranium content, but never any lead.
In the process of radioactive decay, uranium transmutes into lead. But since now the crystal is already grown and rigid, the transmuted lead atoms remain trapped in the lattice.
If you then take a zirconium crystal, and measure how much uranium and lead it contains, the proportion will tell you how long ago the mineral was formed. E.g., if of the total lead+uranium atoms you find in your sample half is lead, then it tells you that ~4.5 billion years must have passed, since that's the half-life of uranium-238 (i.e. time after which half of its original atoms decay).

nice response mate

That's the way I learned it doing geology at uniDave

 

[Drakkith]

Can the proportion of other elements in the decay chain give you any more information or help you when looking at other minerals besides zirconium?

 

[anorlunda]

In order to do proper dating, you need to be sure that the mineral had a specific uranium content when it was formed.
How this particular method is done, is you take zirconium crystals, which can incorporate uranium atoms in their lattice when they grow, but will not incorporate lead. Thus a freshly-formed zirconium crystal will have some uranium content, but never any lead.
In the process of radioactive decay, uranium transmutes into lead. But since now the crystal is already grown and rigid, the transmuted lead atoms remain trapped in the lattice.
If you then take a zirconium crystal, and measure how much uranium and lead it contains, the proportion will tell you how long ago the mineral was formed. E.g., if of the total lead+uranium atoms you find in your sample half is lead, then it tells you that ~4.5 billion years must have passed, since that's the half-life of uranium-238 (i.e. time after which half of its original atoms decay).

That's a clever way to get the age of the crystal, but the crystal could have been formed billions of years afte Earth.

Come to think of it, how do we define the age of Earth? The process of proto bits clumping together might have taken a billion years. Is there a definition for when a protoplanet becomes a planet?

 

[Drakkith]

Come to think of it, how do we define the age of Earth? The process of proto bits clumping together might have taken a billion years.

Current models predict something on the order of a hundred million years I believe.

Is there a definition for when a protoplanet becomes a planet?

I'd guess that it's when it meets the criteria of a planet and clears its own orbit and all that. Not sure to be honest. I assume there's some "wiggle room" in the estimates.

 

[Bandersnatch]

Can the proportion of other elements in the decay chain give you any more information or help you when looking at other minerals besides zirconium?

I think these will act mostly as sources of errors, unless accounted for. For example, if you only count lead and uranium, and ignore all the intermediaries, then you'll end up underestimating the initial uranium content and the age estimate will be too high. But the magnitude of the error goes down the shorter the half-lives of intermediaries (there's less of them present at any given time), so it might be acceptable to ignore some/all.

The details of the dating process are less clean-and-easy than the outline provided earlier would suggest. For one, there are two primordial isotopes of uranium, each with its own decay chain (leading to different isotopes of lead). Another omission is that zirconium can also accommodate thorium in its lattice at formation (but its half life is measured in hours or days, depending on the parent isotope, so it's one of those intermediaries that you can ignore). Yet another issue is isolating homogeneous samples (and not e.g. minerals that were partially melted and then reformed, or contaminated).
I'm sure @davenn can give you more nitty-gritty details, since he seems to have had some hands-on experience.

but the crystal could have been formed billions of years afte Earth.

Sure. In isolation it only gives you the lower bound for the age of the planet, or more specifically for the earliest time its surface cooled down sufficiently for crystals of this particular mineral to grow.