Question about Radio-dating of Rocks?

Question

Hi

Basically, I'm not entirely sure I've got the right idea on Radiodating of rocks. This is how I think it works, was wondering if someone could either correct me or confirm I'm right?
- Some rocks (Generally Igneous) contain radioactive elements. Take Uranium as an example
- These Radioactive elements decay over time, and have 'Half-lives', namely the time taken for 50% of the Uranium to decay (700 Million Years for Uranium 238)

- So, the first half life is how long it takes for 50% of the original amount of Uranium 238 (Call that quantity X) to decay. So after 700 Million years, the quantity of Uranium will be 1 half of X. The 2nd half life is the amount taken for half of the remaining Uranium to decay. So after the next 700 Million Years, the quantity of Uranium 238 present will be 75% of X and so on...

- This means we can sort of take multiple half lives from any given point and they will be constant. So if we record the quantity of Uranium when it is 75%  of X, then 700 Million years later, half of that will have decayed, leaving a quantity of 35% of X.

Is this correct?

Also, does anyone know how "long-lived" Uranium is, as in how old does a rock have to be before the quantity is so minute that it is no longer useful in radiodating (as in too small to be measured effectively etc). I seem to remember reading it can be used for up to about 2 Billion years, but I think that might be wrong. Can Uranium be used to test rocks of over 4 Billion years (i.e. from the formation of earth), and if it can't, which elements are/were used to establish this age?

? · Accepted Answer

Your idea is basically correct.  But the problem is that generally you do not know how much of the original element was present.  So you can't tell whether the amount you have is 20% or 80% of the original.

So in any radiometric dating technique, you must know one of a few things:
1) A rock which forms in a way that the daughter element is excluded (none present initially)
2) A rock where the ratio of parent to daughter element is known.  1) is a special case of 2).  

There are several forms of dating that use uranium in one form or another.  The half-life of U238 is 4.5 billion years, so it makes possible the dating of extremely long timescales.  The technique that is useful over the longest scale is uranium-lead dating.  
http://en.wikipedia.org/wiki/Uranium-lead_dating

Most techniques are useful through a length of about 8 half-lives.  After that time, there is usually too little of the parent element remaining for use.  That means that carbon-14 dating isn't very useful after 8 * 5700 or much more than 50000 years.

With 4.5 billion of U238, there's a lot of time to play with.

The main technique of U-Pb dating relies on zircons.  During the formation of a zircon, lead is excluded.  There should be zero lead atoms in it initially.  So the ratio of uranium to lead in the sample should be related to the time since formation.

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Question about Radio-dating of Rocks?

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