Question about Radiometric Dating?

Question

I have a very simple understanding of how they radiometric date a piece of material. Yet, the problem I always see with this process is wouldn't you have to know how much of the particular radioactive material was there to start with? They test to see how much of the radioactive element has decayed but if half or more of it was decayed when it got there wouldn't that put the age of the object drastically off? How do they account for this when using radiometric dating?

Answer 1

Yes, you would have to know the initial parent-daughter ratio.  There are several ways of doing this.  Typically an isochron diagram is used.  

When you date a sample, you measure the ratio of the parent isotope relative to its other isotope that doesn't decay, and the ratio of the daughter element to its other isotope that isn't a product of decay.

You do this measurement on several samples from the same rock.

When you plot these measured ratios on a diagram, a linear relationship should appear if the system was closed to both elements (if the rock is good to be dated, basically).  The intercept of this line gives you the intitial concentration of daughter element-essentially the case where there would have been no parent element in the rock.

At the time of rock formation, you would have a flat line, all of the samples would have the same ratio of isotopes in the daughter element (no decay has yet occurred to change the ratio).  The line becomes steeper over time as the parent element undergoes decay (those parts of the rock with higher parent element concentrations will reveal a higher ratio of daughter isotopes).  You then calculate the age based on the change in slope.

Look up isochron dating to get a better idea of what I am trying to explain.  There is a lot more to it than I have described, but what I said above is the essentials of the method.

In summary, you can get the initial condition based on measurements of present conditions, and once you know both present ratios and initial ratios, you can calculate the amount of time that has elapsed since the rock was formed.

Answer 2

I hate to just respond with a wikipedia link, but it describes the process very well and answers your question better than any efforts of regurgitation I could come up with.

http://en.wikipedia.org/wiki/Radiocarbon...

Answer 3

<<Yet, the problem I always see with this process is wouldn't you have to know how much of the particular radioactive material was there to start with?>>

Remarkably enough or otherwise, generations of researchers into this subject have already recognized the same potential problem and dealt with it.

In some cases, it's a simplification to say that a radioactive isotope breaks down into a measurable daughter product.  In reality, it breaks down into a chain of daughter products at a regular rate and, also, in known proportions.  Take potassium-argon dating.  Potassium-40 breaks down at a consistent rate and this results in two daughter products; calcium-40 and argon-40.  Not only is the half life consistent.  So are the proportions of the resultant calcium (88.8%) and argon (11.2%).  If you've got precisely those proportions present in a sample, then you can be sure the only possible source was the original amount of potassium-40.

<<How do they account for this when using radiometric dating?>>

With facts such as the one just cited.  If such details can't be established, then you haven't got a method that's useful for dating stuff.  The link is a good introduction to the subject.  Print a copy out and read it.

Answer 4

That's a great question.

Taking the example of radiocarbon dating, carbon in the environment which can be used by plants exists in stable ratio of three isotopes carbon-12. carbon-13 and carbon-14.  Carbon-14 is produced by cosmic radiation of a neutron hitting an N2 molecule. and so is continuously generated and mixed in the atmosphere.  This ratio of C-12, C-13 and C-14 is taken up by plants and used.  C-12 and C-13 are stable, but C-14 has a half life of 5730 years, and decays back to Nitrogen.  Once the plant dies, it stops continuously taking in this ratio of C-12 + C-13  to C-14 and the C-14 begins to decay.  (C-14 is always decaying, but is used by the plant while it's still alive)

By measuring the amount of each of the carbon isotopes, you can find out how long the C-14 has been decaying because you can calculate how much C-14 should have been there to begin with by measuring the total of C12 + C13.  And knowing how much is actually in the sample now compared to what it began with, you can calculate the age based on the decay rate of C-14.

I didn't word that too well, but I hope it's clear enough.

Check out the wikipedia article on radiocarbon dating for a more precise explanation.